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PRIKCIPLES OF HISTOLOGY

A. H. TUTIXE

THHHi

THE PRINCIPLES OF HISTOLOGY

DHSCKIPTIVIC AND PRACTICAL

BOOK I.

DESCRIPTIVE HISTOLOGY.

BY

ALBERT H. TUTTLE.

PUBLISHED BY ANDERSON BROS.

UNrV'ERSlTY OF VIRGINIA.

1898.

Entered according to Act of Congress by Albert H. Tuttle,
University o£ Virginia, J 898,

Press of Prout the Printer,

Charlottesville, Va.

ft

^

PREFACE.

This manual represents an effort to state the most im-
])ortant facts of descriptive histology in a manner adapt-
ed to the wants of my own classes of students, both aca-
demic and medical. It lays no claim to originality save
in the arrangement and mode of presentation, and ac-
knowledgement is here made of the extensive use of the
])est modern treatises and monographs accessible in its
preparation. In the portions dealing with the nervous
system I am under special obligations to the writings
of Cajal and Van Gehucten among others. I am in-
debted to Dr. Lyman J. Skeen for frequent and valuable
aid in the preparation of the book.

This volume will be followed by a second, now in course
of preparation, dealing with Practical Histology.

ALBERT H. TUTTLE.
University of Virginia, Mav, 1898.

CONTENTS OF BOOK 1.

PART 1: THE CELL AND THE TISSUES.

CHAPTER L

Page

Introductory: Definitions: The Cell, - - 14— IG

CHAPTER n.
The Epithelia : Endothelium: Epithelioid
AND Endothelioid Structures, 17— 22

CHAPTER HL
The Cartilage Group: Cartilages, - - - 23— 30

CHAPTER IV.
The Fibrous Tissues: Areolar Tissue : Adi-
pose Tissue: Retiform Tissue : Membranes:
Tendons, 31— 4-6

CHAPTER V.
The Lamellated Tissues : Corneal Tissue :
Osseous Tissue : Periosteum : Marrow :
Bones, 47— 58

CHAPTER VL
Ossification : of Membranes ; in Cartilage, 59— 72

YI CONTENTS.

CHAPTER VII.
The Blood, - 73— 82

CHAPTER VIII.
The Contractile Tissues: Muscles, - - - 83 — 90

• CHAPTER IX.

The Small Vessels: Bloodvessels; Lym-
phatics: Serous Cavities, - 91— 98

CHAPTER X.

The Nervous Tissues: Nerve Fibres; Cor-
puscles : Terminals : Nerves ; Ganglia :
Neuroglia, ._----_- 99 — ]14

CHAPTER XI.

The Structure of the Cell: Nuclear
Division, --------.----- 115—123

PART II: HISTOLOGICAL ANATOMY.

CHAPTER XII.

Introductory: The Embryonic Tissue Lay-
ers: Systems of Organs, ------- 127—134

CONTENTS. VII

CHAI'TBK XIII.
Thk Skin and Appendages: Epidermis: Dermal
Glands: Hairs: Nails, 13r)— 152

CHAPTER XIV.
The Mouth and its Contents: Glands of
THE M-wcous AND Serous Type;s: Salivary-
Glands: the Teeth: the Tongue, - - - 153—170

CHAPTER XV.
The Alimentary Canal: Component Stra-
ta: The Pharynx: The Oesophagus: The
Stomach : The Duodenum : The Small In-
testine: The Colon: The Rectum: The
Pancreas: The Liver, - - - 153—190

CHAPTER XVI.
The Respiratory Apparatus: The Trachea:
The Bronchi.: The Lungs: The Pulmon-
ary Blood Supply, - 191—202

CHAPTER XVn.

The Urinary Organs; The Kidneys: The
Bladder: The Urethra. 203 — 216

CHAPTER XVin.
The Male Reproductive Organs: The Scro-
tum ; The Testes: The Spermatic Ducts
and Seminal Vesicles: The Male Ure-
thra : The Penis : The Prostate : Cow-
per's Glands, 217—232

YIII CONTENTS.

CHAPTER XIX.

The Female Reproductive Organs: The
Ovaries: The Oviducts: The Uterus: The
Vagina: The Vulva: The Mammary
Glands: The Homologies of the Urino-
Gexital Organs, __... 233—258

CHAPTER XX.

The Vascular System; The Arteries; The
Heart; TheVeins: The Lymphatic Trunks;
The Serous Membranes: The Synovial
Membranes, .- 259—268

CHAPTER XXI.

The Ductless Bodies: Lymphatic Nodules
AND Nodes: The Spleen: The Thymus: The
Thyroid: The Parathyroids, Carotid
Glands, AND Coccygeal Gland: The Adre-
nal Bodies: The Pituitary Body: The
Pineal Body, - 269—286

CHAPTER XXII.

The Nervous System : The Meninges : The
Spinal Cokd: The Cerebellar Cortex:
The Cerebral Cortex, 287—320

CHAPTER XXIII.

The Organs of Special Sense; The Taste
Buds; The Organs of Smell; The Eye;
The Ear, - - 321—360

BOOK I.

DESCRIPTIVE HISTOLOGY,

PART I.

THE CELL AND THE TLSSUES.

\-\-{^yt<YJ^v~,

CHAPTER I.
INTRODUCTORY.

Histology is the science that treats of tissues; their
structure, their components, their development luul modi-
fication, and their arrangement to form the organs of the
body. We may distinguish between Animal and Vege-
table and between Normal and Pathological Histology,
as well as between Human and Comparative Histology ;
expressions which define themselves. As the term Histol-
ogy is most commonly used in medical literature it signi-
fies the normal histology of man.

In its most limited sense the term Histology is applied
to the description of the tissues alone. This description is
also sometimes called General Histology. The discussion
of the arrangement of the tissues to form the organs of
the body is distinguished as Histological Anatomy ; or,
(less appropriately) Physiological Anatomy.

A Tissue may be defined as a mass of similar structural
elements (cells or cell-derivatives) having similar prop-
erties and functions; together with such substances as
are characteristicalh'^ present between the elements (and
are, as a rule, formed by their action), serving to unite
them together.

14 PART I. THE TISSUES.

This definition, while true of the great majority of tis-
sues, cannot be universally applied in a literal sense.
There are some tissues (e. g., adenoid tissue) regularly
involving more than one kind of element. Such may with
propriety be called Compound Tissues.

The combination of tissues gives rise to organs of defi-
nite structure and function; and, conversely, every organ
is ultimately resolvable into its component tissues.
There are, however, certain tissue-aggregates which, while
themselves entitled to be regarded as organs, sustain a
similar relation to the structure of the larger and more
complex bodies to which that term is applied as do the
tissues themselves. Among such are the smaller blood and
lymph vessels, small glands, adenoid masses, etc. The con-
sideration of these compound factors of structure may
properly accompanj'- that of the tissues.

There also occur in the body substances (notably the
blood) which, while they lack that coherence properly as-
sociated with the idea of a tissue, contain cellular or cor-
puscular elements analogous to those forming the basis
of tissue-structure. While they can hardly be classed as
tissues, their study is clearly within the province of His-
tology.

The cells (or, more strictly, the corpuscles) to which
reference has been made, although of exceedingly various
form and size, are fundamentally similar in that each is a
mass of (more or less modified) protoplasm provided for a

CHAPTER I. INTRODUCTORY. 15

portion if not the whole of its life with a nucleus. In
some cases the outer layer of the protoplasm ^.^ives rise to
or is modified to form a more or less well-defined mem-
brane or w^all of varying composition, thus forming a true
cell ; but the constant presence of a cell-wall of definite
and approximately uniform composition characteristic of
the tissue elements of plants cannot be affirmed of those ot
animals, the converse being more generally true.

The study of cells as living beings, their internal struc-
ture, activities and life-histories, or Cytology, is an im-
portant and rapidly growing branch of Biology. We are
here concerned with them as components of the tissues
and with their structure and activities as related thereto.
Such discussion thereof as this implies, while it logically
precedes an account of the tissues, is on the whole best
deferred until some practical familiarity with them and
with the elements that compose them has been acquired.

It is sufficient at this time to state that the term proto-
plasm is applied to what has until recently been regarded
as one substance, of exceedingly complex chemical compo-
sition and apparently devoid of structure; clear or slighth'
granular in appearance under ordinary powers of the
microscope; and viscid or semi-solid in consistency: im-
proved methods and appliances of research have, how-
ever, shown that it is neither homogeneous nor structure-
less. It is the seat of the processes which make up physi-
cal life; and it and its products compose the living body,
which in its earliest stages consists of a number of appa-
rentl}' similar minute nucleated masses of protoplasm

16 PART I. THE TISSUES.

(embryonic cells): the specialization in each of these masses
of some one function, and the associated modification in
the form of the mass and of its solid products, if any,
give rise to tissue-differentiation.

The nucleus is a body usually spheroidal in form, both
physically and optically denser than the protoplasm by
which it is surrounded. It is jclearly seen, even with
ordinary microscopes, to be more complex in structure
than the latter : a nuclear membrane, a more or less defi-
nite intranuclear network, and occasional granules, the
largest of which, when distinctly spheroidal in form, are
known as nucleoli, being in many cases readily discerni-
ble. During cell-multiplication the nucleus is the seat of
important changes, which will be discussed in a subse-
quent chapter, together with its structure and that of the
surrounding protoplasm.

While we may recognize a large number of tissue-ele-
ments differing from each other in form and size, as, of
course, in function, they can all be included in a very few
primary groups, as follows.

Epithelial, those lining or investing a free surface.

Skeletal, those forming an investing, supporting or pro-
tective framework for an organ or for the whole body.

Contractile or Muscular, those producing by their com-
bined action definite movements.

Irritable or Nervous, those acting as reservoirs of
energy, or as channels for its discharge, or as receivers and
distributors of stimuli.

Reproductive (modified epithelial) elements.

CHAPTER II. THE EIMTHELIA. 17

CHAPTER II.
THE EIMTHELIA.

Epithelium may be defined as a continuous layer of
cells always (a) disposed on a free surface; (b) united by an
intercellular cement substance; (c) devoid of blood-ves-
sels, though not necessarily of nerve-terminals.

Epithelium of one or another form is normally present
on all free surfaces, save some of those known as synovial ;
it invests the skin and the mucous membranes with their
various diverticula, lines the cavities of the nervous axis,
occurs in the organs of special sense, in the cavities of the
th\'roid and similar bodies, and (as endothelium) lines the
serous surfaces and the cavities of the heart and vessels.
There are also found in the structure of various solid
organs (e. g., the thymus) masses of cells, which, while
they no longer line free surfaces, are evidently epithelial in
character, comparative and embryological studies demon-
strating their epithelial origin. The same may be said of
the cells which compose the greater portion of the sub-
stance of the liver.

The reproductive elements have already been mentioned
as epithelial in origin. They are derived from epithelial
layers by processes diametrically opposed in character;
those of the male being set free from the layer in which

18 PART I. THE TISSUES.

they are formed, those of the female sinking (for a time)
into the subjacent tissues. Further consideration of their
formation will be deferred until the discussion of the organs
in which they occur.

The classification of the epitheha is based on the form,
arrangement, or special modification of some or all of the
constituent cells.
As regards the form of the cells, epithelia may be

A. Flattened, with flattened nuclei, comprising

1. Squamous ; flattened or scale-like, with bevelled

edges ; if with vertical borders, then more properly

known as

, 2. Pavement; having the form and disposition of

!'Lvjuir~-<^'p' tiles. The terms squamous and pavement are often

"^ ' ^^T^T^ tivUv' confounded; they are here used to distinguish

lUh Ly-xH--'' clearly recognizable differences of form.

B. Isodiametric, or with nearly equal dimensions in
"various directions, the nuclei central and spherical or
nearly so. Known as

J-vv 3. Polyhedral; also called Cuboidal, Spheroidal,

"'* ' and, from its most frequent occurrence. Glandular;

where found on curved surfaces the cells are not un-

frequentl}' wedge-shaped or pyramidal through

compression,

C. Vertically elongated ; the nucleus undergoing a simi-
lar change of form, and in some cases situated nearer the
base than the free end of the cell. Termed

4. Columnar: the various modifications of form
"^ ,^r, may be distinguished as cylindrical, prismatic,
club-shaped, etc.

CHAI'TEK II. THE KI'lTlIELIA. 19

As regards the arrangement of cells, epithelia ma}-^ be

1 . Simple ; composed of a single layer of pavement,
cuboidal, or columnar cells.

2. Transitional; composed of a layer a few cells
in depth, the constituent cells varying but slightly
in form. ' ^

3. Stratified: several cells deep, and more or less
distinctly definable into layers . called Stratified
Squamous or Stratified Columnar, according to
the form of the most superficial cells.

There are numerous special modifications of epithelial
cells occurring in particular localities, as in the organs of
special sense : such will be described in their proper con-
nections. Of more general occurrence are the following:

A. Ciliated epithelium ; usualh' columnar, occasionally
cuboidal, rarely flattened ; the free surface of the cell is be-
set with hair-like or lash-like prolongations of its proto-
plasm (cilia), capable of vigorous flexion in one direction.

B. Goblet-cells: Columnar (rarely polyhedral) mucus-
secreting cells in which the undissolved mucigen accumu-
lates in the distal extremity of the cell, forming a viscid
transparent mass, while the protoplasm, together with
the contained nucleus, is crowded down to the base of the
cell. The extremity of the cell is finally forced off b}- the
escaping mucigen, leaving a chalice-shaped structure.

C. Prickle-cells: the polyhedral cells in the deeper por-
tion of stratified squamous epithelium have their surfaces
beset with fine immobile processes (prickles) connecting
adjacent cells; the latter being separated from each other

20 PART I. THE TISSUES.

by intercellular spaces and channels. By some these pro-
cesses are regarded as continuations of the protoplasm,
by others of the modified surface of the cell. They will be
discussed more fully in connection with the skin.

The term Endothelium is applied to the pavement epi-
thelium lining the vascular, serous and to some extent the
synovial surfaces. Embryological considerations led the
earlier histologists to regard it as quite distinct in origin
from other epithelia; more extended knowledge has shown
this distinction to be doubtful, and some have urged
that the use of the term endothelium should be abandoned
altogether. There are reasons why it appears to be well
to retain the term with the significance above indicated :
as thus applied, it can also be defined as a layer of con-
nective tissue cells on a free surface, a definition that will
receive further consideration in another chapter.

Serous endothelium, when viewed from above, presents
a mosaic of polygons whose various dimensions are nearly
equal, and whose boundary lines are straight and short.
Vascular endothelium is made up of cells elongated in
the direction of the vessel which ihev line, and tapering at
each extremity. The boundary lines are long and sinuous.

The terms epithelioid and (more commonly) endotheli-
oid are applied to layers of connective tissue-corpuscles
which resemble epithelium in their regularity, but are not
found upon a free surface. The description of these layers
does not, however, properly belong to the discussion of
the epithelia.

CIIAPTKR II. THE KPITIIKLIA. 21

The statements above made concerning the form and ar-
rangement of epithelial cells are purposely quite general in
character: it will rejidilv be understood that within the
groups indicated we may have great variety of detail,
the epithelia of different localities having in almost all
cases their own individual characteristics, by which they
can in many instances be clearly identified, as will be
pointed out from time to time in the discussion of the or-
gans containing them. It should also be clearly under-
stood that the differences in form described are separated
by no hard and fast lines ; flattened, isodiametric, and
elongated cells passing into each other by gradations at
times so slight that it is often difficult if not impossible to
say of a particular cell whether, for instance, it should be
termed columnar or poK-hedral ; and the continuation of
the same simple layer affording in some instances exam-
ples of each in turn of the three fundamental forms.

As regards the arrangement of the cells, epithelia can
never be said to pass into each other so gradually, since a
layer of cells must always be either one cell deep or more
than one cell deep. The alternative is usually between a
simple epithelium and one composed of more or less dis-
tinct strata, the cells on the surface differing markedly
from those at the base : an arrangement transitional be-
tween these is usually defined, as stated on a preceding
page. Practicalh' the term transitional is by most histol-
ogists applied only to the epithelium, peculiar alike in struct-
ure and function, which lines the urinar}' tract, in connec-
tion with which it will be discussed.

22 Part i. the TrsstJEis.

The various epithelia of the body are of exceedingly di-
Terse embryonic origin, as will be pointed out in a subse-
quent chapter: but however diverse their origin, and how-
ever much the\' may differ in the form and arrangement of
the elements of the adult structure, they in each case origin-
ate as a single la^-er of isodiametric cells resting upon the
subjacent membrane or its precursor. Their subsequent
multiplication, if by cleavage in a vertical plane only, gives
rise to a simple Xaryer of polyhedral, or, by lateral pressure,
columnar cells: if cleavages occur also in a horizontal
direction, or irregularh', a stratified epithelium is the re-
sult.

The power of multiplication is probabh' retained
throughout life b}' all epithelia : by some, however, it may
be manifested at times only as a process of repair, or, if
normally constant, may proceed but slowly: b}' others,
however, the constant and more or less rapid production
of new^ cells is a part of its chief function. This is nota-
bly the case with the stratified squamous epithelia, whose
outermost cells are continually being discharged from the
surface, either by exfoliation or by their union to form
such solid masses as the hairs and the nails.

CHAPTKR III. THE CARTILAGE GROI'P. 23

CHAPTER III.
THE CARTILAGE GROUP.

The presence of an Intercellular cement-substance has
been mentioned as characteristic of the structure of the
epithelia : this substance, is, however, ahvavs small in
quantity, and mayor may not be the product of the activ-
ity of the cells themselves. Where the epithelial elements
give rise to formed products of any considerable extent,
thej' are either deposited in a solid form as a direct invest-
ment of the cell itself (e. g., the keratin layer forming the
wall of a squamous cell from the outer surface of a strati-
fied epithelium), or are discharged on the free surface as
secretions either in a semi-solid form (e. g., the mucigen of
the goblet cells), or as a more complete solution (e. g., the
products of most glands).

The tissues of the group noAv to be considered, on the
other hand, have it as their chief if not as their sole com-
mon characteristic that, however much the\^ may differ
in appearance and consistency (from transparent, color-
less, almost semi-fluid gelatinous tissue to hard, white,
opaque dentine), they consist in every case of cells or cor-
puscles, which, as their chief activity, give rise to a rela-
tively large amount of formed products deposited as an
intercellular matrix.

24 Part i, the tissues.

The matrix thus formed is at first of slight consistency,
and homogeneous in structure. It may become strength-
ened by subsequent direct modification in density and ten-
acity; by calcification, lamination, or fibrillation; or by
various combinations of these methods. The various
changes in the structure of the matrix thus produced, with
the concomitant changes in the form, number and arrange-
ment of the corpuscles, give rise to a variety of tissues
which have an essential community of function directly
associated with their fundamental community of struc-
ture. The former is illustrated by the facts of compara-
tive anatomy, w^hich show that these tissues replace each
other to a very great extent in different vertebrates ; and
the latter by the substitutions and adventitious growths
that occur abnormally in the human body. As will readily
be supposed, they have a common embryonic origin.

The term Skeletal Tissues is here applied to the mem-
bers of this group because of their chief function. Thej'
are the framework tissues of the body, investing and pen-
etrating every organ, and supporting and protecting every
other tissue. On account of their continuity, and the part
they play in binding the organs of the body to each other
they are also widely known as the Connective Tissues ;
a term, however, more appropriately used in its original
significance, as applied to one of the principal divisions of
the group.

As will be readily inferred from what has already been
stated, the classification of the skeletal tissues is based in
part on differences in the form and disposition of the cor-

CIIAPTKR III. CARTILAGE. 25

puscles, and in part on ditit'erences in the structure of the
rnatrix. It is characteristic of the group as a whole that
the corpuscles tend to branch irregularly and to remain or
become united b}- the prolongation of their branches into
more or less extensive protoplasmic networks whose nodes
are the nucleated bodies of the corpuscles. It is within
the meshes of this network that the matrix, whether sim-
ple or complex in structure, is deposited.

While, however, the branching and intercommunication
of the corpuscles just mentioned is shown by the evidence
alike of comparative anatomy, of embryology, and of path-
ology' to be characteristic of each of the skeletal tissues in
some animals or in some stages and conditions, we find in
the healthy adult bod}' of man and the mammals generallv
differences in this and associated respects which divide the
tissues in question into two principal groups quite sharply
distinguished from each other as regards the primary
structure of the forms included in each, although in some
cases the two types are secondarih^ intermingled.

In the first, or Cartilage Group, the corpuscles, which
are always of one kind only, (fixed corpuscles) are usually
either spheroidal in form, or, as the result of pressure,
polyhedral or flattened : in either case, however, they are
simple in outline; and are isolated or at least discon-
nected : in rare cases they are sparingU' branched and
connected. The matrix is firm, elastic, primarily homo-
geneous and finely granular, apparent^ structureless
(though some facts indicate an internal structure not yet
clearly demonstrated): whether more or less dense, it is

26 PART I. THE TISSUES.

alwa^'-s permeable b}' diffusion to the nutrient plasma on
which the corpuscles depend for sustena ice, Ij'-mph-chan-
nels being absent (or very doubtfully present), and a regu-
lar blood-supply wanting, although large masses are some-
times sparingly penetrated by blood-vessels: it is some-
times secondarily reinforced by the intermingling of fibrous
bujodles, or the deposition of lime salts.

According to the extent to which the matrix is devel-
oped, and, to the character of its reinforcement, when this
occurs, the various members of the cartilage groLip may
be classified as follows:
■ A. Matrix simple, or not reinforced b\' fibres :

1. Cellular Cartilage: Matnx very scanty, con-
sisting only of thin laj-ers deposited around the
corpuscles, which are numerous and relatively
large: sometimes called parenchymatous carti-
lage from its resemblance to the parenchyma of
plants : occurs in the embr\^os of man and many
vertebrates and in the auricular cartilages of small
mammals, as well as elsewhere in the permanent
skeleton of some of the lower vertebrates.

2. Hyaline Cartilage: Matrix abundant, though
varying in quantity, the corpuscles solitary' or
gathered into small groups as the result of recent
subdivisions : translucent, white or bluish-white in
color, brittle, firm and elastic : the typical form of
cartilage. Occurs in the encrusting cartilages of all
freely movable joints (the corpuscles in them being
numerous, small and near the surface flattened
verticalh'); in the laryngeal cartilages, with two

CHAPTFK III. CARTII.ACK. 27

exceptions to be noted later; in the nasal, costal,
tracheal and bronchial cartilages ; and in nearly
all toetal cartilages.

3. Calcified Cartilage: Hyaline cartilage is fre-
(luently reinforced in old age, both in man and in
the mammals generally, by the regular deposition
in the matrix of nodules of lime salts. This pro-
cess occurs regularly in some of the lower verte-
brates to such an extent as to give rise to a tissue
almost bonelike in density and forming the princi-
pal framework of the body.
B. Matrix reinforced by the intermingling with it in
smaller or larger proportions of fibrous bundles:

4-. Reticular Cartilage : Matrix continuous, pene-
trated irregularly by a network of i^ellow, elastic
fibres; the corpuscles relatively large and near
together, approaching cellular cartilage in this re-
spect. Occurs where great flexibility and toughness
combined with elasticity are called for ; inthecartil-
ages of the external ear, in the Eustachian tube, in
the epiglottis and in the cartilages of Wrisberg and
of Santorini in the larjmx. On account of its color
and structure this tissue is sometimes spoken of as
yellow fibro- cartilage, and on account of its phy-
sical properties as elastic cartilage.

5. Fibro-Cartilage proper: Matrix largeh- replaced
by bundles of white fibres ; the corpuscles small
and few in number, resembling those of h^'aline
cartilage in appearance. Occurs where great ten-
acity combined with elasticity and moderate flexi-

28 PART I. THE TISSUES.

bilitA'' are needed ; in the intervertebral disks, in in-
terarticular masses, at the margins of ball-and-
socket joints, in the sacro-iliac articulations, in the
sx^mph^'sls pubis. As distinguished from the pre-
ceding it is sometimes termed white fibro-cartil-
age.

Fibro-cartilage may^ also be described as consist-
ing of masses of interwoven bundles of fibrous tis-
sue with small nodules of hyaline cartilage inter-
spersed sparingly in the meshes. So considered, it
may be regarded as a mixture of fibrous and cartil-
age tissues.

Cartilage always originates as a mass of contiguous
spheroidal or polyhedral cells. As development proceeds
the cells are seen to be separated by thin layers of a color-
less substance, which is formed b}- the deposition about
each cell of a la\'er of matrix substance known as the
capsule of the cell ; cellular cartilage never proceeds be-
3'ond this stage: in the case of h\'aline cartilage the mat-
rix substance accumulates between the capsules bx'^ exter-
nal deposition, or else is formed b}' the gradual transfor-
mation and removal of the capsules. Reticular cartilage is
always pre-formed as h^^aline cartilage, the elastic fibres
afterwards appearing in the matrix. In fibro-cartilage the
cartilaginous substance and the fibrous tissue are said to
appear simultaneousl3\

Cartilage grows by cell-division, which can without diffi-
cult}^ be seen to have been in progress during life in any-
good section of hj^aline cartilage, the corpuscles being

CHAPTER III. CARTILAGE. 29

tbund in «jroups of two, four or more, so related as to
clearly indicate their recent origin : in some instances two
cells each with a proper capsule can be found within the
capsule of the cell from whose division they were devired.
Such interstitial growth doubtless proceeds more rapidly
in most cases near the surface than in the deeper portions
of the cartilage : it may suffice merel r for the constant re-
newal of the tissue, or may proceed with sufficient rapid-
ity to give rise to actual increase in size. Growth in this
sense is believed by some to take place chiefly by apposi-
tion : that is by the deposition upon the surface of new
cartilage substance.

It is customary to mention in connection with t1ie de-
scription of cartilage that the matrix consists chiefly of a
substance frequently called chondrogen, and said to yield
chondrin on boiling; the latter is defined as a member of
the gelatin group of compounds. Gelatin is itself ob-
tained chiefly by boiling the fibrous tissues, which are rich
in its antecedent, collagen. By some chemists the matrix
of cartilage is regarded as also consisting largely of colla-
gen, the so-called chondrin being regarded as only an im-
pure or slightly modified gelatin. The matter is one that
has no direct bearing upon the structure of tlie tissues in
question (as far as our present knowledge goes) but it is
well for the student to understand what is meant by the
terms mentioned.

A cartilage, in the anatomical sense of the word, is an
organ : that is to say, a particular part of the body hav-
ing a definite form and function. As such, its description

30 PART I. THE TISSUES.

might with propriety be deferred to the second part of this
book : in the case of this, however, as of some other organs
consisting chiefly (though not solely) of a single tissue, it
will be for various reasons desirable to discuss its struct-
ure in connection with that of its prevalent tissue.

A cartilage, then, may be defined as a mass of cartilage
tissue having a definite and reg'ular form. It is, especially
where composed of hyaline, calcified, or reticular cartilage,
usually covered by the perichondrium, a thin fibrous mem-
brane moderately rich in blood vessels, which are the sole
or chief source of nutriment for the mass : the elastic fibres
of reticular cartilage are continuous with this membrane.
A cartilage is always devoid of bloodvessels, though large
cartilages are sometimes excavated by spaces of greater
or less extent, through which bloodvessels pass, accom-
panied by lymph-vessels and sometimes by fat, forming
what is sometimes termed a "cartilage marrow." It is
always devoid of nerves and insensitive to pain.

CHAI'TKK IV. FIHROl'S TISSUES. 31

CHAPTER IV.
THE FIBROUS TISSUES.

In the second of the two groups of skeletal tissues above
indicated, the Fibrous Tissue Group, in addition to
the corpuscles primarily associated with the formation ot
the tissue and permanently located in it (hence called fixed
corpuscles), there ma\' be present, in some members of the
group at least, characteristic accessor}' or adventitious
corpuscles of various kinds. The fixed corpuscles are
always irregular in form, with lamellar or filamentojus
branches, the latter frequently connecting with similar
processes from adjacent cells, thus forming a more or less
continuous network. The matrix isalwaj's homogeneous,
transparent and yielding in the embryonic state, but very
early becomes penetrated by fine fibrillae running irregu-
larly in various directions : the fibrillation is in most cases
extensive, the matrix finally consisting chiefly of a mass of
fibres variously disposed in bundles, in more or less closely
felted layers, or in clearly defined laminae. In some cases the
fibrillation of the matrix is regularly followed by calcifi-
cation.

The number of tissues which agree in having the general
structure indicated as characteristic of the Fibrous Tissue
group is larger than that of all the other tissues of the

32 PART I. THE TISSUES.

body put together. Three of them, namely, corneal tis-
sue, bone tissue and dentine, resemble each other and differ
from all the rest in the fact that the fibres formed by the
union of the fibrillae are always very minute and are closely
felted together to form definite lamellae between which or
exterior to which the fixed corpuscles are situated : the
first of these is remarkable for its extreme transparency ;
the other two are normally and extensively calcified, form-
ing tissues of great density and firmness. Their further
description will best be deferred to a subsequent chapter.

The remaining members of the group constitute the Fi-
brous Tissues proper or the Connective Tissues in the
more limited sense in which the term may best be used.
Of these one is chiefly if not entirely embryonic, existing in
the adult human body only in an extremely modified form.
As it is an essential constituent of an important foetal
structure, it merits a description as a distinct form of con-
nective tissue: and since it is the precursor of most of the
others, its discussion may properly precede their classifica-
tion and description.

Mucous Tissue (or, as it is also called, gelatinous tis-
sue) : the matrix is at first homogeneous, transparent and
semi-fluid in consistency ; it is described as albuminous in
composition, with the addition of mucin: fibres very early
begin to appear in it, their mode of formation being not
yet fully determined : the fixed corpuscles are irregular,
branching, connected by their slender processes into a net-
work : in addition there are to be seen here and there in
thejnatrix scattered isolated corpuscles which in the fresh

CHAI'TKR IV. FIHROUS TISSUES. 33

tissue ma}^ be seen to move through the jelly-Hke sub-
stance with an irregular or amoeboid motion ; these are
the migratory corpuscles or leucocytes which, as we
shall see, are characteristic of the connective tissues as a
group. Mucous tissue constitutes an important factor of
the umbilical cord, where it forms, under the name of the
jelly of Wharton, the largest portion of the mass lying be-
tween the epithelium upon the surface and the bloodvessels
in the centre, up to the fifth month ; later the fibrillation'
which has already' begun about the vessels and near the
surface penetrates the w^hole mass mo^-e and more exten-
siveh' : but a certain amount of sparingly fibrillated mu-
cous tissue always persists. While mucous tissue is a fre-
quent constituent of the skeletal framework of some of the
lower animals, it is represented in the adult human body
(save as a constituent of morbid growths) by the following
structures, if at all.

The vitreous body (or so-called vitreous humor) of the
eye may best be regarded as a modified form of mucous
tissue. In the embr\'o it possesses for a time all the char-
acteristics of that structure ; but in the adult the matrix
undergoes watery' degeneration, and fibres are extremely
rare; fixed corpuscles are altogether wanting, the only
corpuscular elements present being a few leucocytes. The
centre of the intervertebral djscs of fibro-cartilage con-
tains a soft and yielding mass sometimes regarded as a
form of mucous tissue: it lacks, however, some of the
essential features of that structure and may best be re-
garded as the remains of the notochord, the embryonic
precursor of the vertebral column, which, while it differs

34 PART I. THE TISSUES.

in some respects from cellular cartilage, approaches more
nearly to it than to mucous tissue as here defined. The
pulp of the teeth also consists in part of a modified form
of gelatinous tissue, which will be more fully described in
connection with those organs.

Passing now to the consideration of the fibrous tissues
proper, as defined by the limitations recently indicated, it
is important to note at the outset that the fibres which in
every case enter so largel}^ into their composition are of
two kinds, both of which are present in most of the tis-
sues in question, though their proportions may vary ex-
ceedingly ; the characters of the tissues included in the
group being in great measure based upon the proportion
of the two kinds of fibres, and the modes of their disposi-
tion. These are known respectively as white fibres and
yellow or elastic fibres.

The former are exceedingly delicate unbranching fila-
ments of collagen (which, as has been stated, is converti-
ble into gelatin by boiling), rarely more than a micron in
diameter, and often much less; they are almost always
united into bundles of varying size ; when so united the
fibres have a silky appearance, and tend to assume a char-
acteristic waviness in which all the fibres of the bundle par-
ticipate in such manner as to retain their almost strictly
parallel arrangement : the bundles are again frequently
united together in larger aggregates sometimes termed
trabeculae: white fibres are very tenacious and entireh'
devoid of elasticity.

The elastic fibres are coarser than the white and more
variable in thickness, being from one to six micra in diam-

CHAPTER IV. FIBROUS TISSUES. 35

eter in man, and in some animals as much as fifteen micra:
thev are pxismatic in form, and under some circumstances
appear to be tninsverselv striated : they branch occasion-
allv, and not unfrequently anastomose: when not on the
stretch they tend to Jissume large sweeping curves, and
the free ends, which break square across, curl up in a ehar-
a£|er|stic manner : like the white fibres, they are often as-
sociated in bundles. They are, as their name implies, emi-
nently elastic, but are of only moderate tenacity, a bundle
of them being far more easily broken across than a bundle
of \yhite fibres of the same size. They are composed of a
substance known as elastin, a complex nitrogenous com-
pound which is not converted into gelatin on boiling. The
elastic fibres are not readily affected by weak acids, as are
the white fibres.

But little is as yet known of the mode of formation of
either kind cf fibres. It is b\^ some held that they are in
all cases formed by the transformation of a portion of the
protoplasm of embryonic cells, the remainders of which,
either with or without subsequent increase in size, become
the fixed corjjuscles of the tissue in which they are found ;
this view, however, is urged not so much on account of
observations directly supporting it as of the conviction on
the part of its most positive adherents that the entire liv-
ing body consists and must consist onh^ of protoplasm
and the immediate products of protoplasmic changes:
there are, moreover, some facts very difficult of explana-
tion from this standpoint. On the other hand, it is claimed
that fibrillation ma}' and does result from a chemical
and physical change in a part of the homogeneous ground-

36 PART L THE TISSUES,

substance of the matrix along lines not in actual contact
with any mass of protoplasm ; although it is freely ad-
mitted that the proximity of such masses (the corpuscles)
may have a decided influence in initiating and determin-
ing such a process. In the case of the elastic fibres, there
is good reason for the view (first proposed by Ranvier)
that the elastin is deposited in the matrix in the form of
small globules, which later fuse together to form fibres :
this, if true, will account for the transverse striation
already mentioned.

The corpuscles of the fibrous tissues also demand pre-
liminary consideration. It has already been indicated
that each tissue is characterized by the presence of a spe-
cific form of fixed corpuscle peculiar to it ; in the case of
mucous tissue the presence of migratory corpuscles, or, as
they are very frequently termed, leucocytes has been men-
tioned as a feature in which this tissue may be taken as
a type of the group : and reference has been made to other
accessory or adventitious corpuscles. The fixed corpus-
cles proper to each tissue may best be described in connec-
tion with its definition: the nature and origin of leuco-
cytes will be considered later in connection with the de-
scription of the lymph and the blood : the accessory cor-
puscles now call for discussion. They are often spoken of
as modified fixed corpuscles : but while one form is cer-
tainly and others are possibly derived from these bodies,
there is reason to question whether those of one (if not of
more than one) kind are not modified leucocytes: omit-
ting further discussion of their origin (save in one instance)
they may be described as follows.

CHAPTER IV. FIBROUS TISSUES. 37

What are known as fat cells arc fornied by the fatty
transformation of the protoplasm of certain of the fixed
corpuscles of one or more of the fibrous tissues: small
droplets of oil at first appear scattered in the cell-body;
these become more numerous or larger, finally fusing in
one large mass, the nucleus being crowded to one side, and
'the residual protoplasm forming a thin pellicle or cell-wall.

Under the name of plasma cells arc included certain
corpuscles having elongated and sometimes slightly
branching bodies with central oval nuclei, the protoplasm
of which contains a large number of small vacuoles (of
varying size) which contain a clear fluid probabh' similar
in composition to the lymph or blood-plasma, whence the
name of these corpuscles.

Plasma cells were first described by Waldeyer. He in-
cluded under that term not only vacuolated cells, but also
what are now known as granule cells: these are usually
spheroidal in form and devoid of branches ; their proto-
plasm is highly granular : on account of the marked
afiinity of the granules for eosin (as well as for many ani-
line dyes) they are sometimes termed eosinophile cells; the
same term has been otherwise applied in connection with
the blood, as will be indicated later.

The name of pigment cells has been given to connective
tissue corpuscles (and to some epithelial cells as well)
characterized by the presence in their protoplasm of numer-
ous rounded brown or black granules of a substance
termed melanin. Pigment cells are of very irregular form,
commonl}' branched, and often exhibiting amoeboid mo-
tion when examined in a living condition : the pigment

38 PART I. THE TISSUES,

granules are often so numerous and so closely packed that
the nucleus and other structural features are entirely hid-
den; in some cases, however, they are much less abundant.
The following classification of the fibrous tissues is based
upon the general disposition of the fibres present, their
abundance, the proportionate amount of the two kinds of
fibres, the details of their distribution, and on the kinds
and relative quantities of the associated corpuscles.

A. Fibres varying in abundance, solitary or aggregated
into bundles and trabeculae running irregularly in various
directions, loosely interwoven or more or less closely felted
together.

1. Areolar Tissue: called by the older histologists
cellular tissue: found beneath the fibrous layer
of the skin as subcutaneous, beneath the mucous
membranes as submucous, and the serous mem-
branes as subserous, in the interspaces between
adjacent organs as intermediate, upon their sur-
faces as investing and forming their internal frame-
work as penetrating areolar tissue, it is well nigh
continuous throughout the entire body and merits
in the strictest sense the name of connective tis-
sue. The matrix consists in part of a semi-solid
homogeneous ground-substance, penetrated in
every direction by interlacing bundles and trabecu-
lae of white fibres and by elastic fibres either soli-
tary or in bundles : the bundles of fibres may vary
greatly both in their total amount and in the pro-
portion of the two varieties ; but they are never
so numerous but that irregular spaces of varying

CHAPTER IV. FinROrS TISSUES, 39

A. Fibres varying in (juantity, loosely interwoven, or
felted ( continued) .

size termed areolae occur in so great numbers as
to be practically continuous ; a fact of great im-
portance in the history of dropsical and other effu-
sions. The whole structure is penetrated by blood
vessels, lymphatics, and by nerves passing through
it : it is colorless or whitish, filmy in texture and
of but slight tenacity.

The fixed corpuscles, or areolar tissue corpuscles
proper, are quite numerous : they are frequently
flattened upon the surface of bundles of fibres or.
situated in the angles where two or more bundles
come together, in either case the processes extend-
ing along the bundles and their subdivisions : leu-
cocytes are of frequent occurrence : plasma cells
and granule cells less so, except in particular local-
ities. Pigment cells are not common in areolar
tissue proper in the human body except in certain
places: fat-cells are very common here and there in
the areolar tissue of well nourished individuals:
where they accumulate in particular localities they
give rise to

2. Adipose Tissue: this, which may under favorable
conditions be formed wherever areolar tissue
occurs, but which is most common in connection
with the subcutaneous, subserous, and interme-
diate regions, is in effect little else than areolar tis-
sue in which the fixed corpuscles in particular lo-
calities become greatly increased in number, filling

40 PART I. THE TISSUES.

A. Fibres var\nng in quantity, loosely interwoven or
felted {continued).

up the areolae, and undergo fatty transformation,
giving rise to small fat lobules which are gathered
together to form the fat masses visible to the naked
eye: the blood supply of these localities is always
greatly increased, each lobule having a capillary
system of its own, while the fibres between the cor-
puscles undergo no corresponding increase in num-
ber, and in some cases are exceedingly scanty.

3. Retiform Tissue, or, as it is also called, reticu-
lar tissue: this, as its name implies, consists
chiefly of a network of fibres, or rather of fibre bun-
dles and trabeculae composed chiefly of what are
in all probability most nearly allied to white fibres ;
true elastic fibres are very sparingly present or are
wanting altogether, as is the homogeneous ground
substance characteristic of areolar tissue. The
fixed corpuscles are flattened, adhering closely to
the surfaces of the bundles and trabeculae, and are
often so numerous as to form an endothelioid in-
vestment. Retiform tissue may perhaps be re-
garded as a modification in the direction of greater
stability of areolar tissue (with which it is often di-
rectly continuous), and forms the internal frame-
work of some organs, as well as the basis of the
two compound tissues known respectively as ade-
noid tissue and marrow. The former of these will
be described in connection with the lymphatic sa's-
tem, and the latter in connection with bone.

CHAPTER IV. FIMKorS TISSUES. 41

A. Fibres varying in (juantity, loosely interwoven, or
felted {continued).

4. Fibrous Membrane: this differs from areolar tis-
sue chiefly in the fact that the bundles and trabecu-
lae of fibres, both white and elastic, are far more
numerous and closely felted together, obliterating
the areolae and leaving small space for the inter-
fascicular ground substance : the fixed corpuscles
are quite numerous, but are, as a rule, smaller than
those of areolar tissue, and, with their nuclei, gen-
erally flattened in the direction of the membrane
in which they lie ; both plasma and granule cells
may be occasionally present, and in some cases
pigment cells occur in great numbers : fat cells but
rarely.

The principal membranous tracts of the body
{e. g., the mucous membranes) which support epi-
thelial or endothelial layers are sometimes more or
less clearly divisible into a stroma, which makes up
by far the greatest part of the layer, and a delicate
film situated just beneath the epithelium and
known as the basement membrane, ormembrana
propria ; this is sometimes an endothelioid layer o £
corpuscles, and in other instances a special conden-
sation of the fibres, either white or elastic. Such
differentiation does not occur in those investing
membranes such as the perichondrium alread\' re-
ferred to in connection with cartilages, the similar
periosteum of bones, etc., which are not associated
with a free surface. Elastic membranes consist

42 PART I. THE TISSUES.

A. Fibres varying in quantity, loosely interwoven, or
felted (continued).

chiefly or entirely of elastic fibres, or, in some cases,
of continuous layers of elastin.
5. Fenestrated membrane: this form of membrane
is produced where, in an otherwise normally formed
fibrous membrane, there are at iatervals of greater
or less extent neither fibre bundles nor ground-sub-
stance, thus leaving rounded openings of various
size and frequency, as in the omentum of man and
of numerous mammals. Fenestrated membranes
may also consist chiefly, if not entirely, of elastic
tissue, in which the fibrous structure at times dis-
appears in great measure, as in the fenestrated
elastic membranes of the blood vessels.

B. Fibres exceedingly abundant, one or the other kind
predominating, aggregated into smaller and larger bun-
dles, which are in a generally way disposed parallel wise :
ground-substance very scanty. Fixed corpuscles few, flat-
tened between the bundles of fibres.

6. Tendon tissue or white fibrous tissue : as areo-
lar tissue merits in the strictest sense the name of
connective tissue, so this, above all others, merits
that of fibrous tissue. It consists almost exclu-
sively of white fibres : these are united together
in small bundles by a small quantity of ground-
substance, each being covered by an endothelioid
layer of flattened corpuscles : the smaller bundles
are gathered together with occasional anastomo-

CHAPTKK IV. FIBROUS TISSUES. 4.'i

R. F'ibres abundant, chiefly of one kind, in parallel bun-
dles {continued).

ses into larger ones of varying size, which are sep-
arated by interstitial areolar tissue continuous
with the investing sheath of the whole mass: elas-
tic fibres are very sparingly present, chiefly in the
penetrating areolar tissue. The characteristic fixed
corpuscles or tendon cells are generally found in
rows whiclj occupy the spaces between two or
three contiguous bundles, their branches taking
the form of interfascicular lamellae : leucocytes are
rarely present, and other forms of corpuscles are
wanting. Blood vessels and lymphatics, and, in
some cases at least, nerve fibres follow the intersti-
tial areolar tissue. White fibrous tissue is found
in tendons and ligaments and also in tendinous
aponeuroses and fasciae, which form a transition
to fascicular investing membranes.

7. Elastic tissue : this, as its name implies, is com-
posed chiefly of elastic or yellow fibres, which are
arranged in bundles of varying size and complex-
ity, the interstitial connective tissue penetrating
not only the larger but also the smaller bundles,
and in some cases separating individual fibres:
small bundles of white fibres run in the connective
tissue in various directions, and in some cases at
least there are well defined bundles arranged in
groups as in tendon tissue and running throughout
the whole length of the structure. Elastic tissue
is generally regarded as differing from all other

44 PART I. THE TISSUES.

B, Fibres abundant, chiefly of one kind, in parallel bun-
dles {continued).

forms of skeletal tissue in having no characteristic
fixed corpuscles, none being found in it that can
with certainty be regarded as such : some authors
have described in this connection flattened corpus-
cles found scattered in the ground-substance be-
tween the fibres and in close apposition with the
latter; it is not, however, certain that these do
not more properly belong with the interstitial con-
nective tissue. The term elastic tissue is by some
histologists applied alike to the chief constituent
of elastic ligaments (in which sense it is here de-
fined), and to that of elastic membranes.

We may now with profit revert to the definition of endo-
thelium given in a previous chapter, which described it as
a layer of connective tissue corpuscles on a free surface.
We have seen that the association of corpuscles with
fibrous structures is characteristic of the connective tis-
sues : embryological evidence shows that the serous and
vascular cavities are alike formed by cleavages or excava-
tions of the mass of embryonic cells from which the skele-
tal tissues are derived : the tendency of connective-tissue
corpuscles to arrange themselves in layers (properly
termed endothelioid) has been mentioned in connection
with such structures as basement membranes, the investing
layers of small tendons and tendon-bundles, etc. : and, as
we shall see later, the direct continuity between endothe-
lioid cells and connective tissue corpuscles can be observed

CHAPTER IV. FIUKOUS TISSUES. 45

at the free extremities of lymphatics and at the margins
of synovial surfaces: it can, therefore, be readily under-
stood how connective tissue corpuscles should form a con-
tinuous layer under such exceptionally favorable condi-
tions as those found on the free surface of a membrane.
Some characteristic activities of the corpuscles found upon
serous surfaces will be described in connection with the
lymph and the blood.

We know at present but little of the duration of an^' of
the fibrous tissues, or, indeed, of the skeletal tissues in
general. Consisting largely of the constituents of the
matrix, which some histologists regard as formed and in
a certain sense not-living substances, it has been held that,
once established, they may endure as long as the body
lasts. It is possible that this is the case, to some extent
at least, and that the tendon fibres, for example, of our
old age are the identical tendon fibres of our childhood :
on the other hand, the tendons of the child certainly in-
crease both in thickness and in length toward manhood;
and the mechanism by which interstitial increase takes
place is certainly adequate for interstitial replacement as
well ; we have, however, at present no certain evidence of
any such mechanism of absorption or removal of worn-
out fibres (if such there be) as a method of replacement
would imply. Whether the matrix of a fibrous tissue is to
be reg£irded as living or not depends entirely on what we
mean by a word for which no generally accepted defini-
tion has 3'et been given : it is certain, however, that under
certain conditions of defective nutrition such structures

4!6 PART I. THE TISSUES.

undergo marked changes to which the name of death is
certainly not inappropriate.

These changes are generally regarded as due primarily
to the death of the corpuscles : but this merely shifts the
problem. It is difficult to conceive that any single nucle-
ated mass of protoplasm should retain its powers una-
bated for half a century or more ; but if it may, what are
its probable activities in the case, for example, of a tendon
cell? If, on the contrary (as seems more probable), it is
constantly renewed, what are the activities of the succes-
sive generations ? The phenomena of tissue-repair in the
case of injuries throw some light on the problem, but it is,
on the whole, at present unsolved. It is desirable, how-
ever for the student to know^ of its existence.

Before dismissing the discussion of the fibrous tissues it
is proper briefly to mention a form of tissue at first sup-
posed to be a member of this group, and described as a
modification of retiform tissue: subsequent investigations
have shown, however, that it differs in important re-
spects from any of the true connective tissues in structure,
and is of widely differing embryonic origin. It is the sus-
tentacular tissue of the brain and spinal cord, and is
known at present by the name of neuroglia. It is com-
posed entirely of branching corpuscles and their fibrillar
processes, known as glia-cells: their full description can
best be given in connection with the nervous tissues with
which they are associated.

CHAPTER V. I.AMKLLATKl) TISSUES. 47

CHAPTER V.

THE LAMELLATED TISSUES.

As was stated in the preceding chapter, there are three
members of the skeletal tissue group (namely, corneal tis-
sue, bone tissue and dentine), which, like most of the
fibrous tissues described in that chapter, are character-
ized bv extensive fibrillation of the matrix, the fibrillae
being even more closely intermingled than are those of
ordinary membranes: they differ from these structures,
however, as already indicated, in the facts that the fibril-
lae, which are always exceedingK^ fine and resemble most
nearly those composing white fibres, are never aggregated
into the bundles and trabeculae which are interwoven to
form membranes, but are felted together to form d_ense
layers, having in each case the characteristic fixed corpus-
cles situated between them or exterior to them.

These lamellar structures have evidently much in com-
mon with and are originally derived from the modification
of membranes, as the facts of comparative anatomy and
embryology plainly demonstrate (two of them being
clearly dermal in origin in man and the lower animals
alike) : the characters above stated, are, nevertheless, of
such importance as to warrant their separate considera-
tion. The name at the head of this chapter is, therefore,

48 PART I. THE TISSUES.

proposed for the group: but it should always be borne
clearly in mind that they are more nearly related to the
fibrous tissue group than are either of them to the car-
tilages : this is the more important because of the fact
that one of them, osseous tissue, largely replaces cartilage
in the formation of most of the bones of the body, thus
giving rise to the impression that these two tissues are
closely allied, and that the latter is in some way trans-
formable into the former, an error which leads to much
unnecessary confusion.

^^^ The formation of dentine, the most peculiar member of

j^^^^^^^^jj^he group, is so intimately related to that of the other tis-
io^^JaJm;:^' gygg q£ ^YiQ teeth, (of which it forms the largest part), and
especially with that of the pulp contained in the tooth-
cavity that it cannot well be discussed apart from these
associated structures: they will be considered together
when the teeth are described in connection with the other
organs of the region in which they occur.

g=rr Since corneal tissue is found only in the structure from

^r which it derives its name, its consideration might in like

Ai,^^^^ manner with propriety be deferred until the description of

"^ the eye. There are, however, points of resemblance be-

I tween it and bone tissue which make its stud}' desirable

as a preliminary to that of the latter, particularly as the

absence of calcification renders far easier the recognition

of important details.

The transparent lamellae which form almost its entire
bulk are quite uniform in thickness throughout the entire
cornea : they are composed of white fibres running parallel

CllAPTEK V. IwVMKLLATKI) TISSUES. 49

to each other in each himella, scj disposed that those of
()ne hunella cross those of the nex^t at rij^ht angles, or
nearly so, in the centre of the cornea ; toward the margins
thev cross at varying degrees of oblitiuity : the lamellae
near the outer surface of the cornea are traversed oblit|uely
by occasional bundles of fibres, which thus unite them
together. Adjacent lamellae are separated here and there
by shallow lens-shaped spaces which occur frequently, but
at irregular intervals; these may be designated as lacunae:
those lying between the same two lamellae are connected
with each other by numerous branching channels, which
may with equal propriety be termed canaliculi: there is
thus formed a continuous system of canals and spaces
across the entire cornea between each two lamellae, the
lacunae of one such system having no definite relation in
position or otherwise to those of the next.

In the lacunae lie the corneal corpuscles, apparently ad-
hering to the surface of one or the other of the two adja-
cent lamellae. They are flattened, with flattened nuclei,
and irregular in outline; they branch freely, the branches
extending into the canaliculi and in many cases connecting
with those from adjacent corpuscles, thus forming a pro-
toplasmic network, which is possibl}' coextensive with
the canal-system, but does not completely fill it. The re-
mainder of the space forms a means of distribution of
plasma from the blood vessels at the margin of the cor-
nea; leucocytes also wander through the larger canals
from lacuna to lacuna. Neither blood vessels nor lymph-
atics penetrate the substance of the cornea, nor are there
transverse canals which bring into communication the

50 PART I. THE TISSUES.

canal-systems separated by the lamellae. The surfaces of
the cornea, both anterior and posterior, are invested by
membranes in direct contact with the most superficial
lamellae; but there is no genetic relationship between the
membranes and the lamellae beneath. Farther description
of these membranes, and of the epithelia supported by
them, w^ill be deferred to a subsequent portion of this
work.

It should be stated before leaving the description of cor-
neal tissue that the use of the terms lacunae and canaliculi,
as here applied to the corneal spaces and lymph channels,
is unusual. It is warranted by the resemblance between
these spaces and channels and those occurring in osseous
tissue to which these names are commonly applied ; and
is offered here from the conviction that a clear idea of the
resemblances (and also of the differences) of the two tis-
sues will aid materially in a clearer understanding of the
structure of the latter.

The terms bone tissue and OSSeous tissue are applied
indifferently to the chief constituent of the organs well
known as bones : like all skeletal tissues, it can be defined
by the structure of the matrix and the tornis and relations
of the fixed corpuscles.
„,jjJ^!^iiir*A The matrix consists of t^irn lamejjae composed in part
of fijires (the mode of whose arrangement is not clearly
demonstrable), and in part of a hjomogeneous ground-
substance which is strongl}' impregnated with lime and
other salts, calcium phosphate being the chief Prolonged
boiling converts the fibres into a substance which has been

UJU^I'Aj)

CHAPTER Y. LAMELLATKD TISSUES. 51

called ossein, but which is probably an iinjjurc form of
j^clatin : the fibres are therefore allied to if not identical
with white fi lyres.

Between adjacent lamellae are found frequent lenticular
spaces of exceedingly irregular outline, the lacunae ( for-
merly erroneously termed the bone-cells); like the similar
spaces in corneal tissue, these are connected together be-
tween the lamellae by branching channels, or canaliculi:
in bone, however, such canaliculi are not only present be-
tween the lamellae, but also penetrate them in great num-
bers, thus bringing into free communication lacunae of
diflerent systems; the transverse canaliculi in some cases
traversing two or more lamellae before their termination.
The transverse canaliculi are both larger and more numer-
ous than the interlamellar, in accordance, as will be seen,
with the mode of nutrition characteristic of osseous tissue.

Within the lacunae are found the characteristic bone
corpuscles: these, like the cavities which contain them,
are tlattened and irregular in outline, with flattened nuclei;
they can be seen to branch in some cases, but this occurs
far less freely than in the case of the corneal corpuscles ;
nor is there good evidenceof the connection of the branches
through the canaliculi to any considerable extent, as has
been supposed. Leucocytes do not traverse the Ivmph
channels of bone, nor are any other corpuscles present.

The lamellated and calcified fibrous matrix, with its
characteristic lacunae and canaliculi, and the enclosed cor-
puscles, are the essential structural factors of osseous tis-
sue wherever found : and in some of the lower vertebrates

52 PART I. THE TISSUES.

bones occur which consist simph- of a few parallel lamel-
lae. In man and the higher animals, how^ever, these fac-
tors are always arranged in one of two methods which
are so constant as to constitute two distinct forms of
osseous tissue called respectiveh' dense and spongy bone.

In the former the great majority of the lamellae are ar-
ranged concentrically to narrow tubular spaces known as
Haversian canals; these contain blood vessels, l^^mphatics
and a small quantity of another tissue presently to be de-
scribed ; they are of varying length, running in a general
way parallel to the surface of the mass in which they oc-
cur, and anastomosing frequently : Each is surrounded by
several lamellae whose lacunae are put into communica-
tion with it by means of transverse canaliculi. A Haver-
sian canal and its surrounding lamellae constitute a Ha-
versian system. In the irregular spaces between tbe
Haversian systems of dense bone are found discontinuous
lamellae also as a rule lying in a general way parallel to
the surface of the mass : these are known as interstitial
lamellae; and beneath the outer surface are always found
more or fewer lamellae parallel therewnth and exterior to
the Haversian systems : these are known as circumferen-
tial lamellae.

The circumferential lamellae in all large masses of dense
bone are pierced here and there by oblique canals for
blood vessels, etc., which communicate with the Haver-
sion canals, but differ from them in having no surround-
ing lamellae: these are known as Volkmann's canals.
They are also penetrated transversely by bundles of
white fibres (which have undergone calcification) proceed-

\

LAMELLATED TISSUES. 53

ing from witliout in\vards, atid l)y occasional bundles of
elastic fibres. The name of Sharpey's fibres is applied to
both, hut more especially to the former. Dense bone con-
stitutes the shafts of all long bones and forms a layer of
varvingf thickness on the surface of the flat and the short
bones.

In spongy bone the structure is more open, as the name
implies. The solid portion consists of a meshwork of tra-
beculae, or bars and plates of varying width and of great
irregularity of form, each of which is several lamellae in
thickness, the lamellae being in a general way parallel to
the siirface and the lacunae communicating therewith by
means of the transverse canaliculi. The cavities between
the trabeculae, which are quite irregular in form and size,
are known as Haversian spaces. Spongy bone fills the
interior of the short and the flat bones and the ends of
the long bones.

Just arc cartilage tissue is found in masses of definite
form and function (organs), which are known as cartil-
ages, so dense and spongy osseous tissues are combined in
the definite masses which, with certain associated tissues,
make up the organs well known as bones. As in the case
of the cartilages just referred to, so in this instance it will
be expedient to describe the organs in connection with the
tissues of which they are chiefl\' composed.

A bone may be defined as a mass of osseous tissue sur-
rounded by a proper investing and genetic membrane
known as the periosteum; permeated by blood vessels and
lymphatics : and containing the tissue called marrow.

/

54 PART I. THE TISSUES.

The periosteum, as found upon the surface of the bones
of adult and particularly of elderly individuals, is a thin,
tough, closelv felted membrane which adheres tenaciously
to the structure beneath ; it can, nevertheless, be in most
cases resolved into two distinct layers. In adolescence,
childhood, and notably in foetal life these are clearly dis-
tinguishable. The outer or fibrous layer has the structure
of an ordinary membrane, the constituent bundles (chiefly
of white fibres) being clo^lv^inJ:er woven, and the fixed
corpuscles flattened, with flattened nuclei : here and there
occasional fat^cells may be seen, with numerous blood ves-
sels and lymphatics, as well as delicate nerves : the outer
surface frequently exhibits extensive areas having an en-
dothelioid investment.

The inner or osteogenetic layer is more loosely felted,
and contains a larger proportion of elastic fibres : toward
the surface of the osseous tissue it contains numerous cor-
puscles which (particularly in young bone) are larger and
more irregular in form than those of the outer layer ; the
protoplasm is granular and the nuclei are spheroidal : these
are know as osteoblasts, and in youth are directly con-
cerned in the formation of new bone, as their name and
that of the layer which includes them implies. In adult
bone they become flattened and inactive, forming a layer
just without the osseous tissue.

The complex tissue known as marrow fills the internal
cavities of all bones : those in the shafts of the long bones
are filled with yellow marrow ; the Haversian spaces of
spongy bone, in most places where it occurs, with red

CHAPTER V. LAMELLATED TISSUES. 55

marrow : the larger Haversian eanals on the inner side of
dense bone are lined with a layer of modified marrow,
which is proronged in the smaller canals by a peculiar form
of connective tissue cont£iining osteoblasts, and finally
continuous on the outer surface with the osteogenetic
layer of the periosteum ; the marrow, as will be shown
later, being originally derived from an ingrowth of that
layer. The two kinds of marrow are closely related in
structure, the yellow being derived from the modification
of the red : the latter will therefore be described first.

The basis of structure of red marrow is a delicate frame-
work of retiform tissue associated with an abundant vas-
cular network, which will be described more fully in an-
other connection: the interstices contain corpuscles of at
least three different kinds. Throughout the mass, and
particularly near the surface, are the marrow cells, prop-
erly so called : these are relatively large, with faintly gran-
ular protoplasm and oval nuclei ; those upon the surface
of the mass have the appearance and perform the function
of osteoblasts in young bone. The interior of the marrow
contains numerous smaller cells with granular nuclei, to
which the name of erythroblasts has been given, on ac-
count of the part they play in the formation of colored
blood corpuscles: associated with these are great num-
bers of the immature blood corpuscles themselves ; to these
the color of the marrow is largely due.

Upon the surface and more rarely in the interior of the
marrow are found occasional masses of protoplasm, sev-
eral times larger than the ordinary marrow cells, best
known as giant cells: they were called myeloplaxes by

V

56 PART L THE TISSUES.

Robin, who believed them to be peculiar to marrow ; but
bodies to all appearances similar to them have since been
found in other tissues. It was proposed by Kolliker to
call them osteoclasts, from the belief that they were agents
in the absorption of bone; where, as sometimes occurs in
embryonic bone, the marrow lies in contact with tempor-
ary cartilage which has begun to be absorbed, it has been
proposed to call them chondroclasts for a similar reason :
this view of their function is supported by the fact that they
not infrequently (but by no means invariably) lie in little
pits or depressions of the surface of the bone or cartilage
undergoing absorption : these pits have been called How-
ship's lacunae or foveolae. Nothing, however, has yet been
certainly proven concerning the function of these bodies :
the name first given is therefore for the present at least the
most desirable.

The giant cells are of two kinds, differing chiefly as to
their nuclei. Some show a large number of these bodies ;
others have but a single nucleus, which is always very
large, and frequently of an exceedingly irregular shape : the
former are termed multinuclearand the latter uninuclear
giant cells. It has been supposed that the former are
derived from the latter by the fragmentation of the large
irregular nucleus; but Howell has shown that this view
is certainly very doubtful and probably erroneous.

From the description given it will be seen that the red
marrow is a very important tissue of quite complex func-
tion, having important relations not only to the surround-
ing bone, but also to the elaboration of the blood. The
yellow marrow, on the other hand, is, as far as our pres-

CHAPTKK V. LA.MHLLATEI) TISSUES. 57

ent knowledge u^oes, one of the most pjissive tissues ot the
hodv. It differs from the retl marrow, from which it is de-
rived, chietly by the facts that theerythroblastsand imma-
ture blood corpuscles arealike wantinj^, and that the great
majority of marrow cells in the interior ol the mass have
undergone fatty transformation. Upon and near the
surface both marrow cells and giant cells are found.

From the description above given it will be seen that
the nutrition of dense bone is maintained by the vessels of
the ])eriosteum directly for the corpuscles of circumferen-
tial lamellae, and by the vessels of the Haversian canals
for the concentric lamellae, and thus indirectly by the peri-
osteum, from which these vessels are derived ; the perips-
teum is thus seen to be ndt only a protective, but also a
nutrient investing membrane: this is conspicuously
shown by the fact that whenever the ])eriosteum is re-
moved by accident or disease from any considerable area
of bone the subjacent osseous tissue perishes. In like man-
ner the corpuscles of lamellae which make up the spicules
and trabeculaeof s_pongy bone depend upon the blood
vessels of the ad[acent marrow for their food supply, the
Haversian spaces occupying the same relation to them as
the Haversian canals to the concentric lamellae of the
dense bone : these spaces are in a certain sense the expan-
sions of the canals, as the niiirrow which they contain is
the continuation of their lining, and thus, in a roundabout
way, of the inner la\'erof the periosteum. It is also to be
noted that the lamellae in each case depend for their nutri-
tion upon supplies drawn from a surface to which thev are

58 PART I. THE TISSUES.

parallel : the importance of the transverse canaliculi and
the reason for their number and extent thus becomes evi-
dent.

In speaking of tendon tissue it was stated that while a
mechanism of interstitial increase was known to exist, we
had no clear evidence of any continuous process of inter-
stitial removal and replacement. The case of bone is dif-
ferent : no one can examine a transverse section ol dense
bone, and note the manner in which what appear to be re-
cent Haversian systems cut into the territory of what are
probably older, and how both the interstitial and the cir-
cumferential lamellae are interrupted by both, without
the conviction that a process of replacement is involved:
and this conviction is confirmed when we compare the
cross section of the femur (for example) of a child with
that of an adult, into the central marrow space of which
it could be thrust, each with its Haversian systems, its in-
terstitial and its circumferential lamellae. Dense bone
shows us, however, no tissue in process of removal, and
no Haversian systems in process of formation : and the
means by which both removal and replacement take place
are yet to be discovered.

CIlArTKK VI. OSSIFICATION. 59

CHAPTER VI.

OSSIFICATION

Attention has already been called to the fact that the
periosteum is at once an investing and nutrient, and a gen-
etic membrane; and to the terms applied to its inner layer
and the corpuscles contained therein. It has also been
pointed out that the marrow cells upon and near the sur-
face of the red marrow agree with osteoblasts in form and
also in function. The function of an osteoblast may now
be briefly stated by saying that it is its normal destiny to
become a bone corpuscle. Bone is laid down in lamellae by
the activitv, j)rol)ablv periodical in character, of an osteo-
genetic layer, the osteoblasts nearest the lamella just pre-
viouslv formed being enclosed by the new layer of fibril-
lated and calcified matrix, and thus converted into bone
cor])uscles.

It has already been stated and should always be born
ill mind that the marrow, as will shortly be seen, is a de-
velopment from the osteogenetic layer of the periosteum:
its outer surface, with its layer of osteoblasts, is as truly
an ostcojrenetic laver as that from which it is derived :
attempts have, indeed, been made to define it as a distinct
membrane under the name of the endosteum; but this is
unnecessary: and is not advisable, if for no other reason.

60 PART I. THE TISSUES.

on account of th^ fact that it cannot bs anatomically sep-
arated from the tissue below, nor its boundary on that
side at all clearly defined.

The formation of osseous tissue by the enclosure of os-
teoblasts in a fibrillated and calcified matrix is called ossi-
fication, wherever it occurs, as, for instance, in the bones
of a growing child or young animal. The term is more
especialh' applied, however, to the original formation and
early development of bone as it takes place in the embryo.
Used in this sense, the name is applied to two processes
often regarded as essential!}' different, and designated re-
spectiveh-- as intramembranous ossification, or the devel-
opment of bone in previously existing connective tissue,
and ossification in cartilage, or the development of bone
in previousl}^ existing cartilage. It is of the highest im-
portance, how^ever, to keep clearl}' in mind the fact just
stated, that bone is always formed in a peculiar kind of
connective tissue, the osteogenetic layer already defined as
in every instance derived directly or indirectly from a mem-
brane. Bone is therefore alwa^'s derived from the modifi-
cation of a membrane: and the facts of comparative anat-
omy and of pathology, on the other hand, show that
almost any membrane may under certain circumstances
become osteogenetic.

The so-called ossification in cartilage, or, as it is often
termed (and the expression is still more apt to mislead ),
the ossification of cartilage, is in reality the replacement
of cartilage by bone, the cartilage itself being absorbed
and disappearing in great measure before bone forma-
tion takes place. It is particularly important to avoid

lA^'

CHAI'TI;K VI. OSSIFICATION. <)1

the en oiicous impression that is sometimes caused by the
exjjression iti (|uestion, to the effect that the previously ex-
istinjj cartilage is in some manner transformed into hone:
it" this l)c borne constantly in mind, there is no objection
to the use ot the term in either form : nor, with this quali-
fication, to the use of the terms "membrane bone" and
''cartilage bone," frequently employed for convenience to
desi<j:nate briefly bones formed respectively in the two
methods above defined.

Since bone is to be regarded as the result of a develop-
ment originating in connection with a membrane, we may
properlv first consider the process of intramembranous
ossification : this takes place in the human body and in
that of the higher vertebrates generalh' in a very small
number of bones, the great majority of those entering into
the structure of the adult skeleton being preceded b}' car-
tilage: the tegmental bones of the skull, the squamous por-
tion of the temporal bone, the bones of the face and jaws
(excepting a small portion of the lower jaw), and the clav-
icle are the only bones not so preceded, or, in other words,
the only so called membrane bones.

A studv of the development in the embryo of any one
of these bones gives substantially the following results.
The place of the future bone is at first occupied by a mass
of embryonic connective tissue not yet possessing the den-
sity and structure of membrane, permeated by a network
of blood-vessels. At one or more places, known as centres
of ossification, bundles of stout fibres are formed, radia-
ting outward from a point midway between the adjacent

62 PART I. THE TISSUES.

vessels : these, which are known as OSteogenetic fibres,
resemble white fibres in appearance, but are less distinctly
fibrillated : between them, and crowded upon the surfaces
of the bundles, are large numbers of connective corpuscles,
modified to form osteoblasts. The homogeneous sub-
stance which forms their matrix now begins to undergo
calcification, the salts being at first deposited in the form
of minute globules; as these become more and more nu-
merous, they fuse together forming a continuous and ap-
parently homogenous mass : the osteoblasts imbedded
therein becoming the bone corpuscles, and the modified,
bundles of fibres becoming spicules of bone.

The formation of new fibres, Avith their associated cor-
puscles, continues to go on in advance of calcification at
the extremities of the spicules, the tufts of bundles diverg-
ing in such manner as to anastomose with those of adja-
cent spicules, their growth taking such direction as to pass
between the meshes of the vascular network. As a result^
there is formed a spongy network of bone interlacing with
the network of blood vessels, the interstitial embryonic
connective tissue forming the basis of the primar^^ mar-
row which fills the spaces and in which the blood vessels
are imbedded. While this is taking place in the interior
of the mass, the embryonic connective tissue upon its sur-
face becomes converted into a well defined la3^er of fibrous
membrane, the fibrous layer of the periosteum : from the
OSteogenetic tissue immediately beneath dense periosteal
bone is formed. The ossification thus set up is continued
Outw^ard from the centre or centres of ossification until the
whole territory involved is converted into osseous tissue,

CHAPTKK VI. OSSIFICATION. 63

with the associated periosteum, maiTow and blood ves-
sels: as the botie increases in thickness new layers of dense
bone are deposited by the periosteum, while that first
formed is absorbed and replaced by the spongy bone of
the interior.

In the replacement of cartilage by bone, or, as it has
been sometimes termed, intracartilaginous ossification,
blood vessels also play a conspicuous part, as we shall
presently see; the centres of ossification in this, as in the
preceding case, arising by the development of highly vas-
cular areas of osteogenetic tissue : the formation of these
areas and their subsequent extension, known as the vas-
cularization of cartilage, is the immediate preliminary to
the deposition of bone. The process is rendered ;nore
complex by the fact that the previously existing cartilage
must necessarily be removed, at least in great measure,
before bone can be deposited in its stead ; and the addi-
tional fact that the receding cartilage is itself the seat of
noteworthy changes which invariably take place, although
their direct relation to the formation of bone is by no
means clear. It should also be mentioned, before begin-
ning a detailed discussion of the process in question, that
the replacement proper of the cartilage is also accom-
panied or followed at a very early stage by its investment
with bone formed by the newly developed periosteum.:
and that this takes place at the outset in a manner some-
what resembling the process of intramembranous ossifica-
tion just described; whereas, at a later stage, the torraa-
tion of dense bone takes place.

64 PART 1. THE TISSUES,

We ma}', therefore, recognize four distinct and definite
stages in the process of intracartilaginous ossification, so-
called (to the second of which alone, however, the term is
strictl^v applicable) : these stages are in the main succes-
sive for any particular point; though all may be in pro-
gress at the same time at points adjacent to each other ;
they are definable as follows.

The first, which may be called the transformation of
cartilage, includes all those changes which take place in
that tissue from the first disturbance of its normal condi-
tion to its final dissolution in great measure. The second
is the development of spongy bone in the spaces formed by
the dissolution of the cartilage and upon its remains, or
the formation of endochondral bone. The deposition of
spong}^ bone beneath tbe newly developed periosteum as
an 'investment of the endochondral bone and the trans-
forming cartilage is the third : from the position where it
occurs this is designated the formation of perichondral
bone, or, as it is sometimes called, primary periosteal bone-
The fourth and last stage is the formation of dense bone
surrounding the perichondral bone, through the continued
activity of the periosteum ; the transition from the one to
the other being in some cases exceedingl}' gradual.

We may now enter upon a discussion of the changes
which take place in the formation of the shaft of one of
the long bones, such as the humerus, the tibia, or one of
the metatarsal bones. The bone is preformed, to use a
current expression, in cartilage: that is to say, its future
place is occupied by a mass of hyaline cartilage having in
a general way the form and relations of the bone that is

CHAPTER VI. OSSIFICATION. 65

to replace it : the mass of cartilage is covered by a single
and closely adhering fibrous layer, the perichondrium,
which is moderately rich in blood vessels. The transfor-
mation of the cartilage involves four recognizable changes,
which may be designated as the rearrangement of the
corpuscles, the calcification of the matrix, the forma-
tion of primary areolae, accompanied by the degenera-
tion of the corpuscles, and the formation of secondary
areolae by the partial dissolution of the matrix.

At a point near the centre of the mass the corpuscles be-
gin to multiply and to increase in size, and (in a manner
not 3'et clearly' understood) to arrange themselves in
columns or rows which at first radiate from the central
point, with intervening regions consisting of matrix only,
thus forming a spheroidal region of transformation which
continues to increase: when the surface of the mass is
reached on the adjacent sides the region of transformation
is of course restricted to the cartilage lying in the direc-
tion of the ends : and as it advances toward them in both
directions the columns of rearranged corpuscles and the
intervening regions of matrix soon take on a direction
parallel to the axis of the bone.

Very shortly after the rearrangement of the corpuscles
just described, the deposition of lime salts in the matrix
takes place, particularly in those tracts which lie between
the corpuscular rows, bars and plates of calcified cartilage
thus being formed. At the same time the cavities in which
the cartilage corpuscles lie begin to be enlarged, while the
corpuscles themselves undergo degeneration, becoming
shrunken and irregular in form, and lying in the enlarged

66 PART I. THE TISSUES.

cavities, which are now known as the primary areolae
above referred to. In some cases the areolae consist of two
or three cavities in the same row, united by the dissolution
of the thin lamella of matrix lying between them. The
farther dissolution of the matrix, and the formation of the
secondary areolae, in which the deposition of endochon-
dral bone begins, depends upon the development of other
structures presently' to be described.

About the time that the transformation of cartilage is
set up in the centre of the mass important changes take
place in the surrounding membrane, beginning in a zone
immediately adjacent, and proceeding thence toward either
extremit}' simultaneously with the advancing transfor-
mation of the mass within. The fibrous layer becomes
thicker and more highly vascular: its inner portion be-
comes looser, and the corpuscles rapidly increase in num-
ber and in size, with associated changes in form and
structure: in other words, the perichondrium of the em-
bryonic cartilage becomes converted into the periosteum
of the future bone, with an outer fibrous and inner osteo-
genetic la^^er.

By the time the central spheroidal area of transform-
ing cartilage has reached the lateral surfaces of the mass,
the adjacent zone of periosteum is fully formed ; and the
deposition of a delicate network of spongy bone may have
already taken place. As the two regions of activity come
in contact, one or more loops of blood vessels grow out
toward the transformed cartilage, carrying with them an
investment of osteogenetic tissue: before their advance the
transformed cartilage is absorbed, and they quickly reach

CHAPTHR VI. OSSIKICATION. 07

the centre of the mass. This is sometimes termed the
primary vascular invasion. From the centre the forma-
tion of vascular loops and the associated development of
the investing osteogenetic tissue is directed toward the
ends of the bone, following the advancing areas of trans-
formation of cartilage ; the thin lamellae of matrix lying
between the primary areolae in the longitudinal rows are
aljsorbed, as well as (to some extent) those lying between
adjacent rows: there is thus formed a network of elon-
gated spaces, the secondary areolae, separated by inter-
communicating bars and plates (trabeculae) of calcified
cartilage matrix, and opening toward the centre of the
bone.

These spaces are rapidly filled by the advancing blood
vessels and associated osteogenetic tissue, which together
form the primary marrow: the surface of the mass is in-
vested with numerous osteoblasts, which are brought
directly in contact with the trabeculae of calcified cartilage
matrix : upon the surfaces of the latter the deposition of
thin lamellae of true bone substance begins. There is thus
formed a spongy mass of bone trabeculae having a small
quantit}' of calcified cartilage included in their structure,
and forming an area which advances towards the end of
the l)ody just behind the area in which the last steps in
the transformation of the cartilage are taking place: this
is the endochondral bone. In the long bones with hol-
low shafts it is itself a more or less temporary structure,
being absorbed from behind almost as rapidly as it is
formed along the area of advance, its place beingtaken by
the permanent marrow. This is formed from the modifi-

68 PART I. THE TISSUES.

cation of the primary marrow, which at first consists
only of the blood vessels and the associated osteogen-
etic tissue, which is quite delicate and almost gelatinous
in structure : later, its retiform framework becomes more
fulh' developed, giant cells and erythroblasts appear, and
the characters of ordinary red marrow^ are assumed; these
in turn (in the region referred to) giving place to those of
the yellow marrow which finally occupies the cavity.

It has already been stated that the newly developed zone
of periosteum begins shortly after its formation to de-
posit a layer of spongy bone upon the surface of the trans-
forming cartilage in the middle of theshaft, similar to that
first formed where the seat of ossification is a membrane.
Along with the progress of the areas of the transforming
cartilage toward the extremities of the bone, and the im-
mediately successive development of endochondral bone in
the interior of the mass, there is a corresponding advance
in both directions of the change which converts the fibrous
membrane investing the cartilage into periosteum. This,
again, is immediately followed by the deposition of a layer
of spongy bone, or, rather, by the extension of the layer
already begun. While the layer so formed is in this man-
ner steadily increasing in extent, it also undergoes increase
in thickness in the region where it was first formed.
There is thus formed a sheath of spongy bone, shaped like
a dice box open at both ends, thickest in the middle and
gradually becoming thinner towards its extremities, which
contains in its middle the newly formed permanent mar-
row : its ends surround the masses of endochondral bone
previously described ; immediatel}' beyond lies the area of

CHAPTER VI. OSSIFICATION. 69

transforminjj^ cartila<i^c: and still beyond (and invested as
yet by perichondrium) the hyalinecartilage which has thus
far underg^one no chan<ye. This sheath of spongy bone is
the perichondral bone, or as it is sometimes called, the
primary periosteal bone.

As the periosteum extends its borders toward the ends
of the future bone, its newly formed osteogenetic la3'er
making the deposit of perichondral bone just described, a
change, more or less gradual, takes place in the activity of
its central and older portion. The meshes of the vascular
network within its inner layer become more and more
elongated in the direction of the long axis of the bone :
the osteogenetic tissue which surrounds the vessels, instead
of continuing to occupy a large portion of the intervascu-
lar spaces as marrow, deposits successive lamellae of bone
substance concentric to the vessels, until the spaces are re-
duced to slender canals. Haversian systems are thus
developed; while from the immediate surface of the perios-
teum interstitial and circumferential lamellae are pro-
duced, and the sheath of perichondral bone is itself
invested with a layer of dense bone, or permanent peri-
osteal bone. The perichondral bone is but little more en-
during than the endochondral bone which it for a time
surrounds : it has already been stated that as the latter
advances toward the degenerating cartilage, it is itself, in
the long bones, sooner or later absorbed and replaced bv
the permanent marrow : the perichondral bone suffers a
like fate, the mass of marrow becoming enlarged in diameter
as well as in length, and the surrounding spongy bone being
absorbed about it until the marrow lies directly beneath

70 PART I. THE TISSUES.

the tube of dense bone which forms the permanent shaft
of the femur or tibia, as the case may be.

It will be remembered that the mass of cartilage which
in the foetus represents one of the long bones under con-
sideration, is very much smaller than the bone which we
find in its place even in the newborn child : the increase in
thickness will be readily understood. It should be borne
in mind, however, that this increase in thickness is for
quite a while due to the formation of bone of the peri-
chondral type only : it is not until some months after birth
that dense bone of the ordinary type is deposited, the
shafts of the long bones remaining up to that time more
or less spongy throughout.

The increase in length is due to a continuous growth of
the primary cartilaginous mass itself, which up to birth
and for some time after continues to increase in length and
also in thickness in the part which has not as yet begun
to undergo transformation. This growth continues at a
constantly decreasing rate, the region of untransformed
cartilage thus becoming smaller and smaller. It is en-
croached upon alike by the advancing endochondral bone
of the shaft, and the similarlj^ formed but more permanent
mass of spongy bone in the epiphyssis, until finally the two
areas of ossification meet, and the union of the epiphysis
with the shaft is accomplished, the two masses of spongy
bone and their investing layers of dense bone alike becom-
ing confluent.

The table on the opposite page presents a summary of
the various processes which have been described above as
taking place in the formation of the shaft of a long bone.

REPLACEMENT OF CARTILAGE TO FORM THE SHAFT OF A
LONG DONE (c. g. FEMUR.)

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72 PaUT I. THE TISSUES.

In the short bones, and in the epiphyses of the longbones,
the replacement of cartilage takes place in an essentially
similar manner, the vascular invasions reaching the cen-
tre of the mass of transforming cartilage, and the develop-
ment of spongy endochondral bone taking place on all
sides. Some of the trabeculae thus formed are shortly
afterwards absorbed, while others become larger, and
form part of the permanent framework of the interior of
the bone. It is probable even here, however, that there is
in the earlier part of adult life at least more or less of ab-
sorption and rebuilding continuously going on. An invest-
ing layer of perichondral bone is formed, and contributes
to the permanent spongy mass : and later a thin layer of
dense bone is superimposed. There are some very inter-
esting variations in detail as to the manner in which ossifi-
cation arises in some of the bones, but their consideration
would be foreign to the purpose of this work.

CHAPTER VII. THE BLOOD^ 73

CHAPTER VII
THE BLOOD,

The blood consists of a fluid portion, the plasma, and
numerous corpuscles found floating therein. It has some-
times been described as a tissue with a fluid matrix. It is
perhaps a straining of the use of these terms to apply
them to the blood : it is certain, however, that a descrip-
tion of the structural elements of the body would be in-
complete without an account of those contained in that
fluid. While they cannot be strictly defined as connective
tissue elements, their origin is such as to warrant their de-
scription at this time.

The plasma is during life and in health a perfect fluid as
long as it is contained in the blood vessels. When allowed
to escape therefrom, or more rarely under certain abnor-
mal conditions while contained therein, it undergoes coag-
ulation, an important process, which concerns us chiefly
because of the formation of a solid substance whose ap-
pearance should be familiar to the student of histology.
It is known as fibrin, and when present in small quanti-
ties can be plainly seen to consist of exceedingly' delicate
filaments interlacing in every direction ; as the quantity
increases, the meshes of the network thus formed become
filled with fibres, until a practically continuous solid mass

74 PART I. THE TISSUES.

is formed : when hardened and stained such a mass pre-
sents in section a granular appearance due to the cross-
section of the fibres.

The corpuscular elements contained in the plasma and
constituting normally a little over one-third of the vol-
ume of the blood, are chiefly of two kinds, the colored (or
so-called red) corpuscles, or, as they have been termed,
the erythrocytes, and the colorless (or so-called white)
corpuscles, which are also commonly known as leuco-
cytes. There are also present in normal blood smaller or
larger quantities of minute bodies known by various
names, the most common of which is the blood platelets.

The colored corpuscles are by far the most numerous,
about five millions being contained in a cubic millimetre of
the blood of a healthy man : the number in the blood of
women is about ten per cent, less : in certain forms of dis-
ease their number may be very greatly reduced. When
seen singly under the microscope each colored corpuscle is
seen to be of a yellowish green color, the former tint pre-
vailing in arterial, the latter in venous blood. The form
of each is that of a biconcave circular disk, the central
portion being slightly hollowed on each side and the mar-
gin rounded.

The size of the colored corpuscles varies within rather
wide limits, the same sample of human blood exhibiting
individual corpuscles whose transverse diameter is as
great as ten micra, and others that are but half as broad :
by far the great majority, however, will be found to be

CHAPTER VII. THE BLOOD. 7.'

from seven to eight niicra in breadth : the average meas-
urement has been variously stated by different observers
after a very large number of measurements ; it is not far
from the truth to say that it is about seven and three-
fourths micra. Little diflerence, if any, is found in this
respect in the blood of persons of different age or sex, nor
is any noticeable in that of the various races of mankind.
The greatest thickness of the blood corpuscle is about
one-fourth its breadth. The smallest corpuscles found are
sometimes distinguished as a separate form of elements
under the name of microcy tes ; but this distinction is of
doubtful significance.

«

The colored corpuscles of the blood of all mammals save
those of the camel family resemble those of human blood
in form and general appearance: they vary, however,
greatly in size in different species, the differences that are
found to occur having no relation to the differences in size
of the animals themselves ; those of the mouse, for exam-
ple, being distinctly larger than those of the horse. The
largest known are those of the elephant (between nine and
ten micra in diameter) and the smallest those of the musk
deer (about two and one-half micra). There is, also, no
necessary close resemblance in this respect between nearly
allied animals ; thus, while the colored corpuscles of the
blood of oxen, sheep and goats are very nearly of the
same size, those of dogs and cats differ greatly.

While the colored corpuscles of the blood of many mam-
mals differ in size so decidedly from those of human blood
asto make their distinction a matter of absolute certainty,
there are several species of mammals whose colored blood

76 PART I. THE TISSUES.

corpuscles approximate so closely in size to those of man
as to render impossible a certain distinction between them.
While, therefore, it might be possible under certain circum-
stances to testify that a given stain was not caused by
human blood, the converse is not justifiable in the present
state of our knowledge.

The structure of the colored corpuscles has been and still
is a matter of much debate. By some the disk is regarded
as consisting of a denser colorless portion, spongy in struct-
ure, the stroma, whose meshes contain the fluid colored
portion, a solution of the substance known as haemoglob-
in: its behavior towards certain reagents (notably to-
ward water) leads others to the conviction that what we
have is realh^ a closed sac (or cell in the true sense of the
word) filled with the colored fluid. It is at least certain
that the corpuscle is far from being a mass of protoplasm,
being as greatly modified therefrom as a fat cell. It should
also be noted that a nucleus is wanting in the ordinary
colored blood corpuscles of all mammalia: in which re-
spect they differ from those of all other vertebrates.

When blood is drawn from the vesvsels, or when, from
any cause, it stagnates for a time within them, the colored
corpuscles show a tendency to adhere together by their
sides in masses resembling piles of coin ; this formation of
rouleaux, as it is termed, was once supposed to be pecul-
iar to blood corpuscles; but it has been shown that under
certain circumstances the same process may be caused to
take place with floating disks of cork or other substances
under conditions that are purely mechanical. The rou-
leaux thus formed not unfrequently arrange themselves in

CHAPTER VII. THK lU.OOD. 7<

a coarse network in whose meshes the colorless corpus-
cles may be seen, and the first formed filaments of fibrin
detected as coaj^ulation begins. The formation of rou-
leaux does not take ])laee in the blood of vertebrates other
than mammals, the thickening of the centre of the cor-
puscles due to the presence of the nucleus acting as an
obstacle thereto.

If sodium chloride or any other salt, sugar, glycerine, or
any other reagent that tends to increase the specific grav-
ity of the plasma be added to fresh blood, or if the same
effect is produced by circumstances favoring rapid evapo-
ration, the colored corpuscles undergo a characteristic
change of form known as crenation, the corpuscle becom-
ing shrivelled and the surface studded with minute projec-
tions, the whole bcingin appearance not unlike the fruit of
a thorn-ap])le or a horse-chestnut. This change may pos-
sibly take place in the vessels themselves under circum-
stances temporarily causing an appreciable difference in
the proportion of water in the blood.

The colorless corpuscles, or leucocytes, are nucleated
masses of protoplasm which may be regarded as tvpical
animal cells. When at rest they are s])heroidal in form,
but, as will be seen later, are capable of very great modi-
fication in this respect. The}' are far less numerous than
the colored, a cubic millimetre of healthy human blood
containing on an average about ten thousand : the number
varies greatly, however, not only in different individuals,
but in the same individual under different conditions;
thus, the number is greatly increased shortly after eating

78 PART I. THE TISSUES.

and marked!}' diminished by prolonged fasting: we may
therefore find in the blood of a healthy person at one time
twice as many to the cubic millimetre, at another even less
than half as many as the average above given. Since the
number of the colored corpuscles is not subject to such
marked fluctuation, the ratio between the two varies
therewith ; and the number of colorless corpuscles is often
stated in terms of that ratio as ranging between one col-
orless to two hundred and fifty colored and one colorless
to over one thousand colored corpuscles: the average
being about one colorless to five hundred colored corpus-
cles. This mode of statement is, however, unsatisfactory,
for the reason that, as will be readily seen, a like change
of ratio would result from a marked diminution in the
number of colored corpuscles or an undue increase in the
number of the colorless, changes in each case of very great
importance, but of quite dift'erent significance.

The colorless corpuscles vary also in size to a great ex-
tent: their average diameter, when in the spheroidal
state, may be stated as about ten micra. The smallest,
sometimes distinguished specifically as small leucocytes,
have large spheroidal nuclei, surrounded by a small
amount of protoplasm : they may be regarded as newly
formed elements that have only recently entered the blood
stream ; they are somewhat smaller than the average col-
ored corpuscles. When fully developed they may become
as much as twice as large, the increase being chiefly in the
amount of protoplasm : they are then sometimes termed
large leucocytes; still retaining the single spherical nu-
cleus.

CHAPTER YH. THK BI.OOD. 70

The smaller and lar<?cr uninuclear elements make uj) in
all about tvvciity-fivc per cent, of the leucocytes of the
blood. Almost all of the remainder (about seventy per
cent, ot the whole number) are what are sometimes
termed multinuclear leucocytes: these, which are com-
paratively uniform in size and somewhat larger than the
colored corpuscles, have either two or three small nuclei
or one lart^^e nucleus of irregular form and apparently
about to undergo fragmentation. In addition there may
be found (occurring but sparsely in normal blood) a small
number of leucocytes with pale nuclei and highly granular
protoplasm, the contained granules staining deeply with
eosin and similar reagents. These are sometimes known
as granular or (from their affinity for the substance just
mentioned) eosinophilous leucocytes.

When human blood or that of any manmal is examined
at an ordinary temperature the colorless corpuscles are
spheroidal and motionless : if, however, the stage of the
microscope be so heated as to maintain the temperature
of the blood at that of the living body, the spheroidal
form is no longer maintained. The living corpuscle puts
out one or more stout lobular processes, thus assuming an
irregular form bounded by sweeping curves : these curves
are vseen to change slowly but eonstantl}', the outline not
remaining the same for any length of time. This shifting
of form may go on about equally in all directions, the cor-
puscles as a whole remaining stationary ; or there may be
a flowing movement of the protoplasmic body toward one
of the lobular processes, and a consequent change of posi-
tion, as irregidar and indefinite as the movements by

so PART I. THE TISSUES.

which it is caused. This mode of motion, character-
istic of leucocytes wherever found (and of some other
elements as well) is identical in its nature with that seen
to take place in the members of the genus amoeba, a
group of very lowly organized unicellular animals: it is,
herefore, termed amoeboid.

All the other forms of colorless corpuscles are probably
derived from the modification of the small unicellular ele-
ments. These, in turn, are brought into the blood stream by
the Wmphatics, where, under the name of lymphocytes,
they constitute the corpuscular elements of the h-mph,
some, hovk^ever, attaining their full size while in that fluid.

The origin of the lymphocytes is well known. They are
formed chiefl}', if not solely, in those organs which consist
in great measure of w^hat is known as lymphoid or ade-
noid tissue: those known as lymphatic glands, or, more
properly, as lymphatic nodes, having the formation of
Ivmphocytes as their principal if not their sole function.
The histological anatomv of these organs will be described
in a subsequent chapter: it is sufficient here to say that
they consist in the main of masses of adenoid tissue well
supplied with blood vessels and enclosed in each instance
in a capsule, into which several small Kmphatic vessels
enter and from which a single larger h'mphatic vessel
leads, the current of lymph which thus passes through the
mass, carrying with it the newly formed Wmph corpuscles.

What is here termed adenoid tissue is, like the marrow
of bone, a compound structure of which retiform tissue is
the basis. In addition to the flattened connective tissue
corpuscles which form an endothelioid layer upon the reti-

CHAI'TKR VII. TMK BLOOD, 81

culuin, ami which have been described in a previous chap-
ter as the fixed corpuscles proper of relitonn tissue, the
trabeculae which make up the framework of tlie juass are : -MMri-r»*~4A^
1^ in most Ccises so numerous as to entirely fill the meshes of e*r^^Ha,'jLuc
the network. These cells, properly known as lympho- '*^^*'
blasts, niultij)ly rapidly by cell division, the older gradu-
ally falling into the lymph channels which penetrate the
mass and becoming young lymph corpuscles or lympho-
cytes. At this stage they are spheroidal, with kirge nuclei
and a very small amount of investing protoplasm : the
latter substance increases in quantity as they are carried
to the heart; but, as we have already seen, many enter
the blood stream before they are fully matured.

The origin of the colored corpuscles of the blood is now
also well established. During the early stages of embry-
onic life large nucleated colored corpuscles are formed in
the newh^ forming blood vessels. Later, colored corpus-
cles of the ordinary type ar^ formed first in the liver, later
in the spleen, and fimilly in the red niarrow. After birth
tile last named structure is the chief and probably the sole
])lace of their formation in most if not all mammals, at
least during health ; it is possible that their formation
may be temporarily resumed by the s])leen if not bv the
liver under certain abnormal conditions.

In the description of the marrow already given it wiis
stated that the interior of that structure contains numer-
ous small cells with granular nuclei known as erythl'O-
blasts : these are formed by cell division from larger cells
not distinguishable from ordinary' marrow cells. The
erythroblasts so formed multiply rapidly by the ordinarv

82 PART I. THE TISSUES.

method of cell division, the cells thus formed being sphe-
roidal and nucleated and gradually becoming converted
into immature blood corpuscles by the formation within
them of haemoglobin. Later the nucleus is extruded from
the corpuscle and the spheroidal mass becomes converted
into the biconcave disk which we find in the blood stream.

Reference has been made to the blood platelets. These
are minute rounded and colorless bodies (from one-third
to one-fourth the diameter of a colored corpuscle) which
are found in the blood either singly or adhering together
in masses of considerable size: their total number in nor-
mal human blood is but small. They have been described
under a number of names, as the elementary particles
of Zimmermann, the granules of Osier, the haematc-
blasts of Hayem. The name of blood platelets was pro-
posed for them b}' Bizzozero. They have been regarded
by Hayem and others as concerned in the formation of red
corpuscles within the blood stream, but the evidence for
this view is not conclusive. Perhaps the most probable
explanation of them is that they are the disintegrating
fragments of broken down colorless corpuscles. Our
knowledge of them is, however, exceedingly imperfect.

CJIMTKK VIM. CoNTUACni.E TISSUES. H.'i

i

CHAPTER VIII. :.«^««»..

THE COXTKACTILK TISSUES,

What is in physiological language termed contractility
consists in a change of form but not of volume. Irregu
lar contractility, or change of form in any direction and
in an indefinite manner, is one of the powers which are
inherent in living matter and may be manifested bv anv
cell which is still in an embryonic condition : that speciali-
zation of function and structure which con verts the embry-
xm\c cell into the tissue dementis in many cases accom pan- ?*.£>. yJ^*^
ied by the disappearance of this power: it is retained, how-
ever, in some cases, as in the contractility of pigment cor-
puscles in many vertebrates, and notably in the amoeboid
movements of leucocytes which has been described in the
preceding chapter.

In one group of elements, however, this j)ower is special-
ized and becomes their distinctive function. This speciali-
zation is of more than one kind : there is probably an in-
crease in the actual amount of contraction, and certainlv
an increase in its rate; but the most important feature is its
detiniteness of direction, one axis of the mass (the long-
est) always becoming shorter, while the mass as a whole
becomes thicker. From the fact that elements belonging
to this group are the essentials of structure of the organs
known as muscles, the tissues formed of them are known

84 . PART I. THE TISSUES.

as muscular tissues, although the elements of which they
are composed sometimes occur singl}^ and are frequently
found in masses that cannot with propriety be called mus-
cles.

There are three distinct kinds of muscular elements or
fibres (as they are commonly termed, from the elongation
generalh' characteristic of them) the smooth, the cardiac,
and the striped or striated muscular fibres. The elements
of the first two kinds have in each instance a single nu-
cleus, and may therefore be regarded as resulting from the
direct modification of a single embrj'-onic cell. Those of
the third kind are much longer as well as thicker than the
others, and while they arise in each instance from a single
embryonic cell, this becomes greatly elongated and the
nucleus divides repeatedly; the resulting strand of proto-
plasm thus becoming multmuclear.

What is variously called smooth, unstriped, plain or
involuntary muscular tissue is composed of spindle
shaped cells or fibres whose protoplasmic bodies show at
times quite distinct evidences of longitudinal striation,
but are at other times perfectly plain. The existence of a
very delicate investing membrane or sheath has been dem-
onstrated. The nucleus is elongated, sometimes oval but
in many cases distinctly rod shaped, and is situated in the
centre of the mass. Smooth muscular fibres vary some-
what in size and particularly in length : the transverse
diameter usually ranging between five and ten micra,
while the length maybe less than tenormorc than twenty
times the diameter. When so situated as to escape lateral

CHAPTER Vm. CONTRACTILE TISSl'ES. 80

[jress^ure the smoolh fibres are cjrculnr in cross section :
when, however, as is frequently the case, thev are pressed
together, the sides become flattened and their cross sec-
tions polygonal. When seen in transverse section, there-
fore, the investing membrane forms a circle or polygon,
within which is seen the protoplasm of the body of the
fii^rc, devoid of any clearly discernible structure, and in the
centre the circular section of the nucleus.

Smooth muscular fibres are usually associated in bun-
dles, the tapering extremities (which are sometimes forked )
overlapping upon the bodies of adjacent fibres and adher-
ing closely thereto; a small amountof intercellular cement
substance intervenes, as can be demonstrated by the use
of silver nitrate. Little if any skeletal tissue pervades the
bundles. The latter are in some cases more or less loosely
interwoven : their most common arrangement, however,
is in more or less extensive layers; as, for instance, in the
muscular wall of the intestine: such la3'ers are penetrated
by areolar tissue accompanying the blood vessels and nerv-
ous supply of the muscle fibres. It is very rarely the case
that smooth muscular fibres are aggregated together into
definite masses that can with propriety be called muscles,
their most common occurrence being in the blood vessels
and the viscera. In no case are they under the control of
the will.

The cardiac fibres are found, as their name implies, in
the muscular substance of the heart, both the auricles and
the ventricles beingchiefl}' composed of them : they also con-
stitute an important portion of the walls of the ]ndmon-

86 PART I. THE TISSUES,

ary veins and the superior and inferior venae cavae for a
short distance previous to their openings into the auricles.
What are known anatomically as fibres are in this case as
in others aggregates of structures not visible to the naked
eve. In this instance, however, a confusion sometimes
arises from the application of the term fibre to bodies
which are thus compared with ordinarj' striped fibres:
the}' are in realitj' rows of shorter elements more nearh'
comparable to the smooth muscular fibres.

The cardiac muscular elements are short, stout, irreg-
ularly prismatic bodies, intermediate in diameter between
smooth and striped fibres and three or four times as long
as wnde. Their ends in some cases terminate squarel}^ in
others are quite jagged and irregular. They are sometimes
of uniform diameter throughout, but many give off short
branches which unite with those from adjacent elements,
Nothing like an investing membrane or sarcolemhia has
been demonstrated. The elements are faintly striated
longitudinall}' and more distinctly transversely. Each
element has a single nucleus, surrrounded b\' a compara-
tively large amount of protoplasm showing no trace of
the structure which in the superficial portion gives rise to
the appearance of striation.

The c.ardiac elements are joined together by their ends
to form the cardiac fibres to which reference has been
made, a larger or smaller quantity of intervening cement
substance being clearly demonstrable. As many of the
elements branch and anastomose with those of adjacent
fibres, the appearance presented is that of a network with
elongated and narrow meshes.

CHAPTER VIII. CONTRACTILE TISSUES S7

The various names of striped, striated, voluntary or
skeletal muscular fibres are applied to those niultinu-
eleated fibres whieh form the organs ordinarily called
muscles and usually attached to the bony or cartilaginous
skeleton, the majority of them being under the control of
the will. They are much larger than the smooth or thecar-
diac muscular elements, their transverse diameter ranging
from ten to seventy micra, while they are in some cases as
much as three or four centimetres long: they are prismatic
in form, the ends tapering more or less graduall}-. Each
fibre is invested by a thin homogeneous membrane known
as the sarcolemma: within this is the mass of modified
protoplasm which is the seat of the function of contrac-
tion : its most conspicuous feature is the transverse strip-
ing or striation which gives to the fibres the name most
commonlv applied. This, when seen b\' moderate powers
of the microscope, presents to the eye the appearance of
alternating dim and clear bands; while through the mid-
dle of the clear band may be seen a narrow black line: a
longitudinal striation may also be seen, but usually less
distincth'.

Beneath the sarcolemma, lying between it and the con-
tractile substance, may be seen here and there elongated
oval nuclei: these may be shown to be surrounded with a
small amount of granular protoplasm which extends as a
thin disk for a short distance around the nucleus. The
protoplasm and the nucleus together make up what is
known as a muscle corpuscle: these, like the similar
masses in the centre of the cardiac elements, may be re-
garded as the residuum after the formation of the contrac-

88 CHAPTER Vlir. CONTRACTILE TISSUES.

tile substance. When a cross section of a bundle of striped
muscular fibres is examined the nuclei are seen between the
sarcolemmaand the contractile substance, the latter being
subdivided into small polygonal areas, the fields or areas
of Cohnheim.

The appearances above described are easily seen : the
explanation of the structure of the contractile substance
upon which most of them depend is still a matter of dis-
pute. The following facts are, however, quite generally
conceded, and will probably form the basis of any further
positive addition to our knowledge. The contractile sub-
stance may be regarded as made up of a clear viscid or
semi-fluid portion, the sarcoplasm : imbedded in this are
great numbers of elongated or rod-like bodies (whose ex-
act form is not yet certainly demonstrated, and probably
varies with different animals) ; these are know^n as the
sarcous elements. They are quite uniform in length and
lie in disk-like groups w^hich compose the dim bands or
zones of the fibre, the clear zones being filled chiefly bv
sarcoplasm : the exact cause of the dark line in the middle
of the clear zone (known as Dobie's line or as the mem-
brane of Krause) is not yet certainly known. The sar-
cous elements are not only regularly grouped across the
fibre, but also succeed each other regularly along its length,
and are possibly united end to end ; the rows of sarcous
elements constituting the fibrillae.

The sarcous elements are not uniformly distributed
across the fibre, the fibrillae which thev compose being
grouped together in strands known as muscle columns:
These are separated from each other by sarcoplasm, and

CHAPTER VIII. CONTK.VCTILK TISSUES. 89

the columns themselves arc somewhat irregularly aggre-
gatcd in a similar manner. This may best be seen in the
cross section of a fibre, where the arcfis of Cohnheim are
the cross sections of the columns, the finely granular ap-
pearance of their interiors representing the ends of the
fibrillae; the lines which bound the areas are composed of
sarcoplasm, those which are thickest separating the groups
of columns abt)ve mentioned.

The conversion of the whole of the interior of the fibre
into sarcoplasm and sarcous elements and the consequent
lateral position of the muscle corpuscles is characteristic
of most striped fibres of adult mammals, if not of all. In
the young of most mammals, however, and particularly
in the embr\'o, this conversion is not entire, and the nuclei
are still found in the interior of the fibre : a condition that
is permanent for many of the lower vertebrates. Such
fibres have also been described in certain muscles of some
species of mammals when fully grown.

A muscle, in the ordinary sense of the term, is an organ
consisting substantially of a mass of striped muscular
fibre and its associated skeletal structures. It will there-
fore be convenient in this case, as in those of the cartilages
and the bones, to describe the histological anatomy of the
muscles in connection with their single characteristic tis-
sue.

When seen with the naked eye a muscle appears to be
made up of readily distinguishable fibres of varying fine-
ness : these, which are the anatomical fibres, are bundles
or fasciculi of the elements which are termed fibres in the

90 PART I. THE TISSUES.

histological sense. Each fasciculus is invested b\^ a layer
of areolar tissue continuous upon its outer surface with
that of those adjacent and giving off from its inner surface
delicate septa which lie between the individual fibres. The
investing layer of the fasciculus is known as the perimys-
ium; the internal skeletal tissue as the endomysium.
The aggregated fasciculi which make up the body of the
muscle are invested as a whole by a layer of connective
tissue continuous with the outer perimysial layers, termed
the epimysium. The arteries and veins proper to the mus-
cle are chiefly located in the perimysium, while the capil-
lary network, whose meshes are as a rule greatly elon-
gated in the direction of the fasculi, are situated in the en-
domysium in such a manner as to be in close proximity to
every fibre.

The striped muscular fibres terminate by rounded or
obliquely truncated ends, which are closely applied to the
correspondingly shaped extremities of white fibre bundles,
the sarcolemma of the muscular fibre being directly contin-
uous therewith : these bundles are in some cases almost
immediately connected with the periosteum of a bone or
some similar place of attachment: in other instances they
are prolonged beyond the muscle in a fibrous mass, the
tendon of origin or of insertion, as the case may be.

Cn.VrTKK IX. SMALL VKSSKLS. 91

chapti:r IX.

TUB SMALL VLSSlvLS.

Mention was made in the introductorv ehapter of the
iaet that eertain tissue aggregates, while themselves de-
serving to rank as organs, sustain the same relation to
larger and more complex organs as do the tissues them-
selves. Among these compound factors of structure, as
they were there termed, the most important are the small
blood and lymph vessels, particularly the former. As they
are built up of endothelial, skeletal and muscular tissues,
their structure may now properly be described. The fol-
lowing statements apply, however, only to those smaller
vessels which enter the structure of other organs. The
larger vascular trunks will be described in a subsequent
chapter in connection with the other organs of the circu-
latory system.

The blood vessels are commonly distinguished as arter-
ies, which carry the blood from the heart, veins, which
return it to the heart, and capillaries, which intervene
between the arteries and the veins, and in which the blood
is brought into the closest proximit}'^ to the tissues possi-
ble in a closed system of vessels. The lymph vessels origi-
nate in the interstitial spaces of the tissues (chiefly if not
solely the connective tissues), these communicating directly
with the open mouths of very small and thin-walled

92 PART I. THE TISSUES.

vessels known as the lymph capillaries: these unite to
to form the larger vessels sometimes called lymph veins
since they convey their contents toward the heart, but
more commonly spoken of as lymphatics.

The interior of a small artery, such, for example, as can
be just distinguished with the unaided eye, is lined with a
laj^er of endothelium, whose cells are as a rule greatly
elongated in the direction of the vessel ; the nuclei also
being elongated. Beneath this is a layer of elastic tissue
usually in the form of a membrane, either hotnogeneous
or fenestrated, but occasionally composed of reticulated
fibres. The endothelium and the elastic layer make up
what is usually called the intima, or inner coat ; the term
is, however, applied by some histologists to the elastic
layer, to the exclusion of the endothelium.

Beneath the intima is the middle coat, or media; this,
in the vessels under consideration, consists almostentirelv
of smooth muscular fibres, arranged in a la^^er several cells
deep, the long axes of the fibres crossing the vessel at
right angles or nearly so. Like all layers of muscular tis-
sue, this is highly elastic; during life it is always upon the
stretch ; and the contraction which usually takes place in
it after death throws the intima into longitudinal folds,
which, when seen in cross section, give an undulating out-
line to the interior of the arter}^ which is highly charac-
teristic.

External to the media is the adventitia, or outer coat :
this consists in the smaller arteries of a la3'er of connective
tissue in most cases clearly definable on the one hand as

CIIAPTEK IX. SMALL VESSELS. 93

pertaining to the artery, on the other passing over more
or less gradually into the adjacent interstitial tissue.
/ As the small arteries divide and subdivide, finally becom-
ing lost in the ca|)illaries, there is a gradual reduction
alike of the adventitia, the media and the intima. The for-
mer becomes reduced to a layer of extreme thinness : the
media diminishes until it is reduced to a single layer of j
/transverse smooth muscular fibres and later to scattered V
^fibres not in contact and not forming a continuous layer: i
the elastic layer of the intima is similarh' reduced in ex-
tent, and finally disappears; the last and least of the ves-
sels that may with propriety be called arterial consisting
merely of the endothelial lining and an imperfectly contin-
iuous layer of smooth muscular fibres, surrounded more or
less definitely by a few branched connective tissue corpus-"'
les.

The capillaries are the direct continuation of the arter-
ies, arising either by the farther subdivision of the struct-
ures just described or springing directK% as in some cases,
from the sides of vessels still distinctly arterial in their
structure. In either case they branch freely, forming a net-
work, whose meshes have a size, form and direction in
direct relation with the structure of the organ in which
they occur. Thc}^ do not, like the arteries, become smaller
as they branch, those of any one network being approxi-
mately of the same size, though they may vary consider-
ably in different parts of the same organ. Their size in
life is not easily determined, but the majority of them are
probably not over ten micra in diameter, though in some

94 PART I, THE TISSUES.

tissues, notably in the marrow, they may be as much as
twice as large.

" In structure the capillaries are simple tubes (usually
cylindricEil in form) of elongated endothelial cells of such
width that from two to four may be seen in the cross sec-
tion of a single capillary. As in other endothelial layers,
the cells are united by an intercellular cement substance:
here and there patches of this substance may be demon-
strated by the silver method which may be regarded as
indicating gaps between the cells ; these have been termed
stigmata; and it is probable that such places offer favor-
able opportunity for that migration of the leucocytes
from the blood-stream into the tissues which is known to
be normal to them. Capillaries are almost always situ-
ated in interstitial areolar tissue or its equivalent, and are
in direct relation with the connective tissue corpuscles : in
some organs branched corpuscles appear to form an almost
continuous layer investing the capillaries : such a layer has
been termed an adventitia capiilaris.

As the capillaries originate by the subdivision of the
arteries, so their union forms the origin of the veins. A
small vein resembles a small artery in its general struct-
ure, its wall, like that of the latter, being distinguishable
into an intima, a media, and an adventitia : the chief differ-
ences between them may be stated briefly as follows. The
endothelium of the veins is, as a rule, composed of shorter
and broader cells than that of the arteries, and the elas-
tic layer of the intima is thinner: the media is very much
thinner, the amount of muscular tissue being very much

CIIAPTKK IX. SMALL VESSELS. 95

reduced: the advcntiLia, which is the principal coat of tlic
veins, is, if anythiiij^, thicker than that of the correspond-
ing artery, and is composed largely of fibrous tissue. The
veins are as a rule of a greater diameter than the arter-
ies which they accompany.

Unlike tlic arteries, the veins arc not during life continu-
allv on the stretch; and the recoil mentioned in connection
with the former vessels does not take place after death :
this reaction on the part of the arterial wall tends to
drive the contained blood through the capillaries and into
the veins: as a rule, therefore, the arteries are empty after
death, but retain their patency on account of the thickness
and elasticity of their walls. The veins, on the other hand,
are usually filled with blood; or if empty become col-
lapsed and flattened on account of the thinness and inelas-
ticity of their walls.

A small artery and a small vein may therefore usually
be distinguished when seen in cross section by the follow-
ing differences : The artery is cjrcular inform : it is in most
cases empty, or contains but a small amount of blood
dot: the intima is thrown^ into folds, giving the elastic
layer a sinuous contour, upon whicli the nuclei of the endo-
thelial cells are often peculiarly conspicuous : the media is
quite thick, consisting chiefly of numerous muscular fibres :
the adventitia is comparatively weak, and the least con-
spicuous of the three coats. The vein may be either circular
or more or less irreg^ular in forni : if thejormer, it is usually
filled with blood clot, readilv recognizable from the numer-
ous colored corpuscles, devoid of nuclei and appearing as
clear circles, and the few leucocytes with distinct nuclei:

96 PART I. THE TISSUES.

the intima is thin and siraj)le_injc:ontour : the media is also
thin, containing but few smooth muscular fibres : the ad-
ventitia is often the thickest and most conspicuous^ the
three coats. Where the artery and vein He side_bj5?_side,
the latter is usuall}^ the larger.

A similar application of the principles of structure above
described w^ill enable the student to interpret the different
appearances seen when a vessel is cut obliquely or longi-
tudinally, and with practice to recognize an artery or a
vein wherever met with in a section passing through one
of the organs of the body.

The interstitial spaces of the tissues form in many cases
an irregular network, in others definite lacunae or chan-
nels for the lymph, a rather indefinite terra applied to the
fluid originally derived from the blood and destined to be
returned to it again by way of the lyraphatic vessels. The
sraallest of these, the lymph capillaries, arise from the
lymph-spaces of the tissues in an exceedingly irregular
manner, the connective tissue corpuscles in many cases pass-
ing by an insensible transition into their endothelium.
When fully formed they consist of tubes which resemble
blood capillaries in consisting solely of endothelial cells,
but differ from those structures in the fact that they are
rarely if ever cylindrical, being exceedingly irregular in
form, though usually flattened, and very often quite vari'
able in diameter: they anastomose freely, forming net-
works of very irregular meshes. The cells of which they
are composed are usually about as long as broad, but are
usually characterized by a distinct sinuosity of outline.

CMAPTKK IX. SMALL VESSELS. 97

The Iviiiph c.'ipillaries unite to I'nnu the lar;.,a'r vessels
known as llie lymphatics. These do not differ materially
from the former exee|)t in size and in the presence of valvu-
lar folds. Like the capillaries, they consist almost entirely
of endothelial cells, which are, however, more elongated in
the direction of the vessel than those forming the capilla-
ries, but having the same sinuous outline. It should be
stated here that bv some histologists it is held that the
characteristic irregular outlines seen in preparations of
the lymphatic endothelium by the silver method are due
solely to shrinkage of the tissues after death, the outlines
of the cells in the living tissues being far simpler.

A lymphatic when seen in cross section in the interior of
an organ appears as an opening of irregular form, its sides
usually approaching each other (due to the flatness of the
vessel) ; it can in many cases hardly be distinguished from
a simple tear or fissure in the connective tissue, save by
the definiteness of its outline: this, except in cases where
the vessel is quite collapsed, is made up of simple curves;
and in it may occasionally be seen the nuclei of the endothe-
lial cells of which it is composed. In some cases the open-
ingis larger and more nearly equal in its various diameters,
resembling the cross section of a vein : it can then be dis-
tinguished from the latter b^-^the greater thinness and sim-
plicity of its walls and by the fact that any clot which ma}'
be present consists almost entirely of fibrin, appearing
granular in the section, with here and there a few lymph-
ocytes with large nuclei and a small amount of surround-
ing protoplasm.

98 PART I. THE TISSUES.

The structure of the larger lymphatic vessels v/ill be de-
scribed iti connection with the circulatory system.

The lymphatics have a distribution closely related to
that of the blood vessels : and it is not unusual to see a
small artery, its companion vein, and one or more lym-
phatics in close proximity. In some cases a blood vessel
may be situated within a lymphatic vessel. Such lym-
phatics are termed perivascular.

The serous cavities of the body are in reality enlarged
lymph cavities. They are lined by the serous membranes,
which are known by distinctive names (e. g., pleura), ac-
cording to their position. These consist in each case of
connjective tissue which contains a more or less well de-
veloped network of fine elastic fibres, surmounted by a
homogeneous basement membrane. On this rests the
serous endothelium, a layer of polygonal pavement cells
of varying size. Here and there ma^- be found small open-
ings called stomata, which put the serous cavities in com-
munication with the lymph channels of the membrane and
thus with the lymphatic system : they are surrounded by
cells which are usually more tumid and granular than
those adjacent.

In addition to the tumid cells which surround the sto-
mata there may sometimes be found on the serous surfaces
patches of granular cells, which may be cuboidal if not
columnar in form: these give evidence of rapid cell divi-
sion, and may be regarded as local centres for the forma-
tion of the leucocytes which may be found in the serous
cavities.

A^ ha^jik^

CHAI'TFR X. NERVOUS TISSUES. 99

CHAPTER X.

THE NERVOUS TISSUES.

The nervous tissues have for their characteristic function
the reception, conduction, distribution, and discharge of
impulses or stimuli. The stimuli received may be such as,
when manifested in consciousness, we know as sensations ;
or the\' may be such as are never reported to our con-
sciousness : in either case they may be either rnechanical,
physical, or chemical in their origin: the distance to which
they are transmitted may vary greatl}^ in different cases,
as may also the extent and nature of their distribution.
The impulses which are discharged (with or without pre-
vious distribution) must be regarded as liberations of
energy due to changes set up in the central nervous ele-
ments by the ingoing stimuli : the\' ma^' or may not re-
sult in associated and more conspicuous liberations of
energy in the muscular tissues.

The nervous elements, are, then, like the muscular ele-
ments, reservoirs of energy : they differ from them both
functionally and structurally by what has been termed
their polarity, by which is expressed the fact that the im-
pulses received enter in a more or less definite direction,
while the ensuing discharge takes place along a definite
line which is in a general way in an opposite direction to
that along which the incoming stimulus enters. A nervous

100 PART I. THE TISSUES.

element of the simplest type may be conceived of as com-
prising a spheroidal or spindle-shap^ed rnass of protoplasm
provided with a nucleus, each of whose extremities is con-
tinued into a filament of more or less length. Along one
of these filaments stimiuli travel toward the central corpus-
cle, and the extremity of the filament is modified for their
reception: along the other, nervous impulses pass outward
to their appropriate place of discharge, its extremity show-
ing corresponding modification. The whole structure,
from one set of terminals to the other (and including both)
is the result of the modification of a single embryonic cell :
on account of the delicacy of the various parts, however,
and the great length in many cases of the conducting fila-
ment, it is rarely (if ever) possible to isolate such a nervous
element in its entirety, as we can isolate a smooth muscu-
lar fibre or an epithelial cell: nor can nervous elements
(with a few exceptions) be so prepared that the\^ can be
seen in their entirety in a single section : in most cases our
study must at any one time be chiefly directed to one or
the other of the regions above indicated. Practicallv, it
has been customary to distinguish, as the structural fac-
tors of the nervous system, the conducting strands of
greater or less length along which impulses are transmit-
ted ; the central corpuscular masses ; and the terminals of
reception and discharge: and although these are now known
not to be independent parts, it is convenient to retain
this distinction as a basis for their description: according,
therefore, to the form and associated function of the vari-
ous components of the nervous system, it is customary
to classify them as follows.

CHArTKK X. NKRVors TISSUKS. 101

A. ComlucLiiiL,^ elements, or nerve fibres : ihese consist
in every case of continuonsev linilrical ( or rounded) strands
w hieh run without interruption Ironi the central cor|3us-
cles to the terminal modifications ol their free extremities.
The presence or absence of certain investing structures dis-
tinguishes two kinds of nerve fibres, known as

1. Non-medullated fibres: also called gray or gel-
atinous fibres, or (from their discoverer) the fibres
of Remak: the axis consists of a grayish cylinder
or a band with rounded edges, showing delicate
longitudinal striations : they are invested, if at all,
by a very delicate primitive sheath whose exist-
ence is questioned by many histologists. Upon the
surface of the fibre are seen numerous nuclei. The
gray fibres not unfrequently branch and anasto-
mose with adjacent fibres. ^

2. Medullated fibres: also called white fibres: the O^^^
•axis-cylinder is grayish and longitudinally striat-
ed : it is surrounded by a layer of a fatty substance

termed myelin, the layer being called the medul-
lary sheath or white substance of Schwann:
outside of this is the clearl}^ defined primitive
sheath or neurilemma. The medullary sheath is
interrupted at frequent intervals, the points where
this occurs being termed the nodes of Ranvier ;
the intervening segments are called internodes:
each exhibits one or two so-called nerve-fibre cor-
puscles situated between the medullary sheath and
the neurilemma and consisting of oval nuclei spar-
ingly invested with protoplasm. Medullated fibres
rarely branch, except near their extremities.

102 PART I. THE TISSUES.

B. Central elements, or nerve corpuscles: these are
variously shaped bodies with conspicuous nuclei: they
vary greatly in size also, some being among the smallest
and others among the largest of the tissue elements. Their
surfaces always give off one or more processes termed
poles: according to the number of these processes they
are commonly distinguished as unipolar, bipolar, and
multipolar corpuscles. Most nerve corpuscles (in the
higher vertebrates, at least) are multipolar; and in the
great majority of instances one of the poles of a multipol-
ar corpuscle can be distinguished from the x)thers as the
process of Deiters, or, better, as its axis-cylinder process :
the rest are then known as protoplasmic processes and
rapidly divide into irregular branches termed dendrites :
in some multipolar corpuscles no axis-cylinder process can
be distinguished ; such corpuscles are termed amacrine.
Other features will be discussed in a subsequent para-
graph.

C. Peripheral elements, or nerve terminals, formed by
the modification of the extremities of nerve fibres with or
without epithelial or skeletal elements. According as they
are situated at the extremities of fibres which conduct im-
pulses toward the nerve centres (afferent fibres) or away
from them (efferent fibres) they are distinguished as

1. Receiving terminals: of these various kinds are
known, composing two groups which differ from
each other alike in the form and in the arrangement
of the structures included. In one group large num-
bers of similar terminals are associated with modi-
fied epithelial cells to form the essential structures
of the three great organs of special sense, the nose.

f)

CHAPTER X. NKKVnCS TISSUI-S. 103

Fcri|)luM'al cleinenls or nerve terminals (continued).
the eye, and the ear: these will be described in a
subsequent chapter, together with the less highly
sjjecialized organs of tiiste. In the other the term-
inals are always either solitary or in groups of two
or three, though they mav be more or less al)und-
ant. Some of these forms of terminals have been
regarded as associated with specific modes of sen-
sation, but the function of most of them is alto-
gether unknown : they are therefore usually dis-
tinguished by peculiarities of form or of location,
or by the names of their discoverers. The principal
forms ma}' conveniently be described here.

a. The simplest form of receiving terminal is seen
in what are known as free endings. In these the
nerve fibre first looses the investing white sub-
stance of Schwann, and later the neurilemma,
though the nerve-fibre corpuscles are still seen for
a while before the axis-cylinder becomes entireh-
naked. When the latter stage is reached, the fibrils
are rapidly split up into small bundles, and finally
form a tuft or pencil of delicate more or less vari-
cose filaments which ramify among the cells of the
epithelium in the case of the skin, a mucous mem-
brane, or the substance of a gland ; or among the
elements of other organs. Branching terminals,
essentially similar, found in tendons, are known as
organs of Golgi.

b. Closely allied to the preceding are the terminals
associated with what are known as tactile cells :
these are spheroidal cells found in the deeper por-
tion of the epidermis and probabl}' epithelial in
character. The nerve fibre in relation with a group

104 PART I. THE TISSUES.

C. Peripheral elements or nerve terminals [continued).
of them looses its investments and breaks up into
a number of slender branches, each of which ends
in a saucer-shaped disk, the tactile meniscus,
which embraces the proximal surface of the tactile
cell, so called. There is no positive evidence that
these, rather than other cutaneous terminals, are
the special organs of touch.

C. The bodies known as compound tactile cells,
also sometimes called the corpuscles of Grandry,
found in the skin of certain birds, and also de-
scribed as occurring in mammals, may be regarded
as composed of two or more layers of tactile cells,
and having between adjacent layers terminal ex-
pansions known as tactile disks. The neurilemma
of the nerve fibre of which the tactile disks are the
termination becomes continuous with a connective
tissue capsule that invests the whole structure.

d. The name of tactile corpuscles has long been
given to large ovoid bodies found in the papillae of
the skin of the hand and foot and in other places
whore the sense of touch is well developed : they are
also called the corpuscles of Meissner. Each is com -
posed of a mass of connective tissue about which a
medullated fibre winds spirally once or twice, the
sheaths of the fibre then becoming merged in the
mass or continuous with its capsule: the axis-
C3dinder passes as a nonmedullated fibre into the
interior, where it branches more or less freely, the
branches becoming varicose. Two, three, or even
four medullated fibres may be connected with a
single large corpuscle of Meissner.

e. What are termed end-bulbs are spheroidal or
cylindroidal bodies of simple structure in which the
axis cylinder of a medullated fibre enters the proxi-

CIIAPTKR X. NICKVOUS TISSUES. 105

C. Peripheral elements or nerve terminals (continued).
mal end of a mass of connectivctissue which is con-
tinuous with the sheaths of the fibre, and extends
throughout its length, branching but little if at all.
They are found in the conjunctiva of the eye, and
in other modified dermal structures, as well as in-
ternally. The articular corpuscles found in the
vicinity of joints, as well as the genital corpuscles
of the male and the female sexual organs are prob-
ably to be regarded as modified end-bulbs, though
both are held by some histologis ts to approach more
nearly in structure to the corpuscles of Meissner.

f. The Pacinian bodies, or, as thev are sometimes
called, the corpuscles of Vater, are the largest of
the terminals, being easily visible to the naked eye
in man\'^ cases. They are irregularly^ ovoid in form,
each having for its axis the axis-cylinder of a med-
ullated nerve-fibre, which may terminate near the
extremity of the corpuscle by a bulbous enlarge-
ment or may divide near the end into short irregu-
lar branches wnth pyriform extremities. The axial
structure is imbedded in a cylindrical core of doubt-
ful nature: it is faintly granular and contains scat-
tered nuclei, and is possibly homologous with the
principal mass of an end-bulb. Surrounding this
is a series of concentric tunics which maybefiftyor
more in number, each consisting of a fibrous layer
and an endothelial investment: the whole may be
regarded as a highh^ specialized modification of
the capsule of the end-bulb. Pacinian bodies are
found widely distributed throughout the bodv. As
is the case with the other forms of receiving termi-
nals, we have as yet no certain knowledge of their
specific function.

lOG PART I. THE TISSUES.

C. Peripheral elements or nerve terminals {continued).
2. Discharging terminals : Impulses sent out from
the nerve centres may give rise to muscular, glan-
dular, or other activities. The terminals by which
discharge is made upon the elements of the tissues
involved have in every case, as far as known, the
form of ramifications or arborizations of the ex-
tremities of efferent fibres. In the case of glands
or other secretorv structures the terminal subdivi-
sions are situated among the epithelial cells. In
the case of smooth muscular tissue, nonmedullated
fibres from an adjacent ganglion or ganglionic
plexus enter the muscular layer, between whose
elements the ramifications of the fibre are situated.
The mechanism of discharge in the case of striped
muscular tissue is somewhat more complex. Me-
dullated fibres from the intramuscular plexus, fol-
lowing the endomysium, divide in each instance
into two or more branches, each branch passing
to a single muscular fibre. The medullary sheath
disappears, the neurilemma apparentU^ becomes
continuous with the sarcolemma of the muscular
fibre, and the axis-cylinder breaks up into a
number of fine varicose branches : the latter rest
upon or are imbedded in the sole-plate, a flat-
tened granular mass of protoplasm containing
several nuclei and lying between the sarcolemma
and the body of the fibre: the whole structure is
termed a motor end-plate. By some histologists
the end-plates are believed to be situated altogether
outside of the sarcolemma.

CHAPTKR X. NERVOI'S Tl«Sl'ES. 1(>7

Attention has l)ccn called to the delicate longitudinal
striation of the axis cvlinder of the medullated fibre and
of the corresponding portion of the gray fibre. This is
the expression of a distinct fibrillation, the primitive
fibrillae l)eing imbedded in an intervening homogeneous
substance, the neuroplasm (the resemblance to the fibril-
lae and sarcoplasm of striated muscular fibre is note-
worthy). The terminal subdivision common to medul-
lated and nonmedullated fibres consists of a breaking up
of the axis into smaller bundles of these fibrillae and even-
tually, in some cases, to the separation of each individual
fibrilla.

The gray or nonmedullated fibres consist of little more
than bundles of fibrils and neuroplasm. Those which
compose the branches of the olfactory nerves have a well-
defined and nucleated primitive sheath: in most gray
fibres no such sheath can be demonstrated. Scattered
nuclei are seen upon the surface of the gray fibres: these,
like the sheath, when present must be regarded as skeletal
rather than nervous in character. Gray fibres show well
marked varicosities, w'hich are possibU' due to local accu-
mulations of neuroplasm, the fibrillae being correspond-
ingly separated at such points. Where gray fibres branch
and anastomose bundles of fibrillae accompanied by neu-
roplasm pass over from one fibre to another: the angles
formed by the branches are frequently filled for a short
distance with neuroplasm, and investing nuclei are often
relatively abundant at such points.

The axis cylinders of white fibres show under certain
methods of treatment transverse striation curiouslv like

108 PART I. THE TISSUES.

that of a striped muscular fibre: if a reality, and not due
merely to the reagents used for its demonstration, this must
be catised b\' regular variations in the size of the fibril-
lae. The axis cylinder may also be shown to be suddenly
thickened at the nodes of Ranvier, the spindle-shaped en-
largement being apparently due to an increase in the
amount of neuroplasm with an accompanying separation
of the fibrillae. Some histologists maintain that the axis
cylinder is invested with a delicate structureless sheath,
for which Kuehne has proposed the name of the axlleaiina :
whether this structure exists in the living fibre is still a
matter of question.

' The medullary sheath also presents (under certain treat-
ment) apparent evidence of a structure that would hardly
be inferred from its semifluid character as seen in the fresh
nerve. A reticular framework of a substance of a horny
nature known as neurokeratin can be demonstrated,
whose meshes and filaments vary greatly in size in differ-
ent parts of the same fibre. That the substance in ques-
tion exists as a component of myelin is probably true: but
the solid framework described is quite possibly due to its
coagulation by the reagents employed. Far more con-
spicuous are the oblique clefts seen in the medullary sheath
after treatment with certain reagents, notably osmic acid :
these are evidently the view in section of conical cleavage
spaces running from the primitive sheath to the axis
cylinder, and dividing the medullary sheath into the me-
dullary segments of Schmidt and Lantermann, a number
of which may be found ineachinternode: whether these are
real or artificial must, however, be regarded as still unset-
tled.

,-A.I, f*^'

CHAPTKH X. NKRVOTTS TISSIKS. 109

The iiciirik'iiinia exhibits no special sLniclur.'il features
worthy of remark. It should he noted that when a me-
dullnted fibre joins the brain or cord, while the medullary
sheath is continued within the axial structure as far as
the gray matter, the neurilemma disappears: thus the
columns of the cord are made up in f^^reat measure of me-
dullated fibresdev^oid of neurilemma. At the distal extremi-
ties of the fibres the medullary sheath is the first to disap-
pear, the neurilemma bcinj^ continued for some distance
toward the terminal.

Nerve fibres, both gray and meduUated, var}' considera-
bly in size, their diameters ranging from two to twenty
micra ; the difference appears to be associated with a cor-
responding difference in the length of the fibres.

Corpuscles, fibres, and terminals are now known to
be continuous structures and components of what may
properly be called true tissue elements, meaning by that
term in each case the result of the modification of a single
embr3''onic cell. As indicated at the outset, such an element
may consist of a receiving terminal, an afferent fibre
(medullated or nonmedullated), a central corpuscle, an
efferent fibre (of either kind) and a discharging terminal.
The simplest form of terminal is in either case a tuft of
fibrillae: if the subdivisions of the receiving terminal are
called dendrites, and the discharging cluster an arboriza-
tion, the two can readily be distinguished by these terms.
A corpuscle so situated would be essentially bipolar; such
corpuscles exist, though not in great numbers, in the ner-
vous tissues of the higher vertebrates : more frequently
the points of attachment of the two fibres become approx-

110 PARTI. THE TISSUES.

iniated and finally consolidated for a short distance, form-
ing what is apparently a unipolar corpuscle with what is
termed either a Y- or a T-connection according to the
mode of separation of the two fibres. In certain super-
ficially situated elements of a sensory character in some of
the lower animals (and possibly in higher forms as well)
the receiving terminal and afferent filament become so
shortened and condensed as to form a mere eminence onl\'
on the body of the corpuscle: such elements may be said
to be in form (but even then not in function) unipolar.
What have been called in the past apolar corpuscles prob-
ably do not exist.

In the ganglia of the sympathetic system corpuscles are
found with more than two processes, each of which be-
comes an axis cylinder (or a gray fibre) : such corpuscles
are in the strictest sense multipolar: whether the majority
of the poles are afferent or efferent is unknown : both
conditions may possibly occur.

The term multipolar has long been applied to the cor-
puscles found chiefly in the brain and spinal cord in which
a distinction can be made, as has been pointed out, be-
tween a single axis-cylinder process and a number of so
called protoplasmic processes which subdivide into a
group of dendrites. It has been suggested that the latter
have for their function some connection with the nutrition
of the corpuscle: but a more reasonable interpretation is
one which regards such a corpuscle as resulting from the
disappearance of the afferent fibre, its primary subdivi-
sions thus becoming processes of the corpuscle itself. The
axis cylinder process may in its course give off one or more

ClIAPTKK X. NKUVOUS TISSITRS. Ill

slciidcr branches ; these leave the process at well marked
angles, but soon after bend str()n<^ly to become ajjproxi-
matcly parallel to it In most cases: they are known as
collaterals, and like the processes from which they arise
terminate in arborizations.

Axis-cylinder processes which pass from the jj^ray into the
white matter of the cord become invested with a medul-
lary sheath and are then true axis cylinders: elements in
which this is the case are known as corpuscles of the first
type: in other cases the efferent process is (piite short, the
terminal arborization beinr^ situated in the gray matter:
such elements are called corpuscles of the second type.
The disappearance of the process altogether, making the
arborization sessile, like the dendrites, gives rise to the
amacrine corpuscles of Cajal.

Nerve corj)uscles alwa\^s have large and conspicuous
nuclei, in the vicinity of which a patch of pigment granules
is very commonly present. The iibrillae of the processes
may be traced into the interior of the corpuscles, but their
internal distribution is as yet unknown. The corpuscles
are almost always situated in well defined lymph spaces
which agree closely with them in contour. The forms of
the corpuscles of the brain and cord will be described in
the chapter devoted to those organs.

The nerves are definite aggregates of nerve fibres : like
the blood vessels, the\' penetrate the organs of the bod\'
and are consequently to be regarded among the factors of
structure thereof: the same is true of manv sfanjrlia: both
will therefore be described at this time.

112 PART I. THE TISSUES.

A nerve is a bundle of nerve fibres or an aggregate of
such bundles. Each bundle is termed a funiculus, and is
composed of a number of fibres surrounded by a cjUndrical
sheath called the perineurium. The latter is lamellated
in structure, the number of lamellae never being less than
three save in the smallest branches of the ner v^es : they are
separated by distinct lymph spaces lined with endothelioid
corpuscles. The inner lamella is continued into the funi-
culus by the connective tissue which lies between the fibres
and supports their capillaries, called the endoneurium.
In small funiculi this connective tissue is homogeneous in
composition, approaching gelatinous tissue in consistency :
such funiculi are termed simple: larger funiculi, called
compound, show here and there in the endoneurium con-
nective tissue septa which divide the funiculus irregularly.

In small nerves, consisting of but a single funiculus, the
outer lamella of the perineurium is continuous with the
adjacent areolartissue: where several or more bundles are
associated, however, as in the larger nerves, a definite
mass of connective tissue, containing more or less fat, and
definitely compacted on its outer surface, invests and sup-
ports the funiculi, becoming continuous with their outer
lamellae: this is known as the epineurium. Within it
the associated funiculi divide and anastomose from time
to time, each large nerve being thus in reality a greatly
elongated plexus.

As the funiculi divide into small groups of fibres and
finally into single fibres in the vicinity of their destination,
the perineurium becomes greatly reduced, being finally
continued for a short distance on the single fibres either as

CIIAPTI^R X. Ni;UV()l'S TISSUES. 113

a single lamella or as a mere layer of etidothelioid eclls :
sneli an investment is known by the name of Henle's
sheath.

A ganglion is a mass of nerve corpuscles invested with
a definite sheath or capsule of connective tissuecontinuous
with the e|)ineurium of the nerv^cs with which it is asso-
ciated; or, in the case of nerves consisting of single funi-
culi, with the perineurium. These ma}^ be but two in
number, the ganglion in such cases being practically seated
upon a nerve trunk; or there maybe three or more, the
!2fanorli<jn being situated at their intersection. In almost
all the larger ganglia there may be clearly distinguished a
cortical portion, consisting chiefl}'^ of nerve corpuscles, and
a central portion, consisting largely of nerve fibres. Of
these some pass directly through the ganglion, while others
pass into or out from thecortical portion, being connected
with the cori)uscles. Each corpuscle is, as a rule, con-
tained in a delicate capsule continuous with the neuri-
lemma of the associated fibre or fibres and enclosing, as
has already been stated, a pericorpuscular lymph space.

The neuroglia, or sustentacular tissue of the brain and
cord, has been referred to in the chapter devoted to the
fibrous tissues. It differs from all the tissues of that group
in its origin and in the absence of anythinglike the matrix
characteristic of them ; consisting, as was stated, entirely
of peculiar branched corpuscles known as glia-cells.
These are stellate or irregularly shaped cells with large
nuclei, which stain conspicuously with some reagents.
Their branches, which are quite numerous, terminate in

11 4 PART I. THE TISSUES.

long slender processes : these are stated bv Ranvier to be
fibrillated, the fibrillae passing through the body of the
cell from one process to another: the\' are varioush^ ar-
ranged upon glia-cells from different parts of the cerebro-
spinal axis, those of the gray matter of the cord, for exam-
ple, differing from those of the white in the number and
disposition of their processes. In a general way it may
be said that the latter form a dense reticulum of closely
interwoven fibres: this in sections has a fineh^ granular
appearance conspicuously seen in the gray matter..

The glia-cells are found in the cerebrospinal axis and are
derived from the same embryonic la^^er as the nervous ele-
ments themselves. While, therefore, their function is
probably purely mechanical, or, in a sense, skeletal, they
must be regarded as closelv related to the nervous tissues
rather than to the skeletal tissues proper. These latter
also penetrate the cerebrospinal axis, in the form of con-
nective-tissue trabeculae which compose a proper skeletal
framework : and although they gradually diminish by
subdivision and become reduced to delicate fibrils inter-
mingled with those of the neuroglia, nothing like a transi-
tion from one tissue to the other has ever been observed.

^"^ ^ ^ II. /^ - / t

V

OLyw|-^^

en APTHR XI. STRUCTURE OF TIIH CKLL. 115

CHAPTER XI.
THE STRUCTURE OF THE CELL.

\Vc have now passed briefly in review the various tissue
elements, considering both their form and characters and
their union to compose the tissues of the body; as also the
structure of some of the simpler aggregates of tissues, or
organs. The elements of the tissues were at the outset
defined as cells or as derived from the modification of cells;
and a cell was defined as a nucleated mass of protoplasm.
It is important now for us to consider the structure of the
protoplasm itself, and of the nucleus as well; and to learn
something of the process by means of which new cells are
formed.

It was stated in the opening chapter that the protoplasm
which makes up the body of the cell is neither homogene-
ous or structureless, as it was once supposed to be. The
delicate granulation generally characteristic of its appear-
ance as seen by ordinary' powers proves with more im-
proved means of research to be the expression of a delicate
reticulum or network of a somewhat denser substance
which has been given the name of spongioplasm ; its
meshes are filled with a less dense or semifluid substance
designated as hyaloplasm: the proportion between the
amounts of these two substances may var^^ greatly: as a

116 PART ]. THE TISSUES.

i^^enerjil rule the relative amount of the former increases
with the age of the cell. The meshes of the spongioplasm
may var^y greatly in size and in the coarseness or fineness
of their constituent fibrils: the accumulations at their in-
tersections are the granules most readily seen.

In cells which become surrounded b}' a cell wall com-
posed of some formed product (e. g., an epidermal cell),
the reticulum becomes quite close and dense near the sur-
face: but in man\' cases (e. g., a leucocyte) the converse is
the case, the exterior of the cell consisting almost, if not
quite wholly, of hyaloplasm. Such a clear outer portion is
sometimes termed ectoplasm, in distinction from the gran-
ular inner portion known as endoplasm : the distinction
is, however, of questionable value, since in some of the
lowest animals the conditions are reversed, the same
terms being applied to a denser outer and a more fluid
inner portion of the cell. The terms paraplasm and deu-
toplasm are also sometimes made use of in connection
with the structure of the cell body to designate granules
imbedded in the protoplasm, and consisting either of sub-
stances taken up by the protoplasm in aj^olid form, or of
formed products temporarily stored in the cell ; such, for
example, as yolk granules in the ovum.

The nucleus gives evidence of a reticular structure even
with ordinary powers ; and this structure is also clearly
seen to vary in the different nuclei of adjacent cells or in the
same cell at different times if watched while still living.
Under ordinary conditions, however, the nucleus when
seen in what is usually designated the "resting" condi-

CHAI'THR XI. STUrCTl'KK OF TUT CKI-L. 117

lion is ri splKToicial vt'siciilar body hounded \)y a well de-
fnicd wall, and eontainiiiLi the network al)ove referred to:
this is sometimes fnie and close meshed; at others com-
posed of but a few coarse fibrils, which are in some cases
quite irrcf^^ularly disposed ; in some cases the fibrils form
a continuous filamen^which is arran^red in a tangled
skein. The nodes of the network in many cases form
coarse granules, which are cpiite conspicuous : in addition
there are often seen in the nuclei distinct spheroidal bodies
apparently different in composition from the network:
such a bod}' is called a nucleolus.

The meshes of the network are filled with a clear semi-
fluid substance which is not readily colored by the stain-
ing fluids wdiich render the network and nucleoli conspicu-
ous. This difference between the two principal substances
of the nucleus led Flemming to propose for the substance
composing the filaments the name of chromatin, and for
the clear substance that of achromatin. More recent re-
searches have made clear, however, the fact that the
denser portion of the nucleus itself consists in part of a
substance which does not stain any more readily than the
more fluid portion: while the latter, now usually termed
the nuclear matrix, is regarded as possibly similar in com-
position to the hyaloplasm of the cell-body. The name of
chromoplasm has therefore been proposed by Carnoy for
the substance which forms the filaments, that portion
which is readily stained being designated by the term
chromatin, and that which resists ordinary stains by the
term achromatin; a different application of these terms
from that originally i)roposed by Flemming.

lis PART I. THE TISSUES.

The nuclear wall is composed of chromoplasm : by some
it is regarded as consisting merely of a fine and close net-
work of that substance; b\' others as a definite and con-
tinuous layer. Its continuity with the network is evident
from its comportment at the time of nuclear division.
Whether the nucleoli consist of chromoplasm must for the
present be regarded as an open question.

While the arrangement of the filaments of the nuclear
network is often exceedingly irregular, especially in the
resting stao^e, it can often if not alwavs be seen at the
time of its greatest development to have a definite plan,
whose basis is the formation of a larger or smaller num-
ber of elongated loops having a meridian-like arrange-
ment in relation to a definite axis. The turns of the loops
are directed toward one end of the axis, termed the pole
of the nucleus: the free extremities meet (and frequently
interdigitate)in the region around the opposite end of the
axis, the anti-pole. The sides of the loops are often ex-
ceedingly irregular in their course, and may in addition
branch frequently, the branches anastomosing and thus
forming the irregular netw^ork commonly seen. Where
the ends of adjacent loops become continuous the convo-
luted filament sometimes seen is produced.

The division of older cells to form new ones is preceded
in man and the higher animals generally by nuclear
division. The older observers (whose imperfect micro-
scopes showed them in the interior of the nucleus only the
nucleoli and the coarser nodal granules) believed this pro-
cess to be quite simple in its nature, consisting merely in

CHAPTKK XI. STRrCTVRE OF THK CELL. 119

the passage of a clcavaijc-planc throuijjh the nucleus in the
same wav as is seen in the cell body itself. This, which is
called direct division, may possibly sometimes occur; but
the constant advance of our knowledge makes it yearly
more and more evident that the common mode of nuclear
division as it occurs in jilants and animals, and in normal
and i)athological changes alike, is an exceedingly' complex
process, to which the rather unfortunate name of indirect
division is commonly applied : and it is quite possible, if
not probable, that this is the sole method. The series of
changes involved in this process has been termed karyo-
kinesis. The name of mitotic division has also been ap-
plied to the process, the successive stages being called
mitoses: the so-called direct division being distinguished
as amitotic.

The successive steps that can be recognized in what is
really one continuous process have been designated by
special names, based on the appearances presented b\' the
nucleus from time to time. The first step is the formation
of what has been variousl}' termed the spirem, or close
skein : the secondary filaments are retracted into the
primary filaments or loops; the nuclear membrane is also
absorbed, as are the nucleoli; the latter fact indicating the
possible identity of these bodies with the chromoplasm of
the network. The next step is the formation of the open
skein, or wreath : the j^rimary filaments contract, be-
coming shorter and stouter, and having a less tortuous
course: they gradually' assume the form of an equatorial
wreath of loops with, as has been stated, their flexures

120 PART I. THE TISSUES.

turned toward the region of the pole. To these loops the
name of chromosomes has been given.

While the-chromosomes are thus being defined, there ap-
pears in the polar area a group of fibres of achromatin,
known froin its form by the name of the achromatic spin-
dle: this gradually moves toward thecentreof thenucleus,
taking an axial position with its extremities directed
toward the pole and the anti-pole respectively. The chro-
mosomes having b\^ this time become quite short and
stout, and V-shaped from the divergence of the limbs of
the loops, attach themselves to the spindle, eventually as-
suming a radial position at its equator. When the nucleus
is viewed at this stage from a polar or an anti-polar direc-
tion the radiating arms of the chromosomes together form
a starlike figure to which the name of the aster has been
given.

During the formation of the aster or immediateK^ there-
after, the cleavage of the chromosomes take place, each
loop being split into two similar (but of course more
slender) loops by a plane passmg through them all equa-
torially. This is the central process in the division of the
nucleus: the changes which follow have therefore been
sometimes designated by the term metakinesis: they are
in a certain sense the retracing of the processes already de-
scribed. In its more limited sense the term metakinesis is
applied only to the cleavage of the chromosomes, and the
changes immediately connected therewith.

The daughter-loops formed by the cleavage of the chro-
mosomes begin to separate first at their apices, these being
turned toward the extremities of the spindle: they then

CHAPTER XI. STKICTIKE OF THE CELL. 121

travel slowly along the achromatic til)rcs in each direc-
tion, two sets of chromosomes thus being formed; these
gradually arrange themselves about the poles of the spin-
dle, which have now become the polar areas of the two
daughter-nuclei. The limbs of associated loops remain for
some time connected together by delicate achromatic
uniting filaments: the whole figure seen from the side has
a resultant barrel-shaped appearance ; at either extremity
the chromosomes have a stellate arrangement, and this
stage is therefore designated the dyaster.

As the dyaster is formed, a cleavage plane passes through
the body of the cell, whose course in the nuclear region is
sometime marked by nodal points on the uniting filaments.
As their separation is completed, the free extremities of the
chromosomes of the daughter-nuclei bend inward toward
the new antipolar areas, which face the plane of cleavage
and therefore, also, each other. Changes now ensue in
the inverse order of those described as taking place at the
beginning of the process, the chromosomes becoming first
converted into open skeins, and later into the closed
skeins whose farther modification gives rise in each to a
reticulum b\' the formation of anastomosing secondary-
filaments, accompanied with or followed by the formation
of a nuclear membrane, the appearance in some cases of
nucleoli, and the final assumption of the resting stage.
The stage in which two adjacent and parallel skeins are
seen is sometimes termed the double skein or dispirem :
but it is evident that w'e are here dealing with structures
which, though genetically associated, are now parts of
Separate and distinct nuclei.

122 PART I, THE TISSUES.

The number of the chromosomes varies considerably in
different plants and animals, but is probably constant for
the same tissues in each species : their form may also vary
greatly, particularly as regards the length of the branches
of the loops. The changes above described may in many
cases be followed step by step with a good microscope of
ordinary powers, either as they take place in living cells,
or as thc}^ may be found in adjacent cells of suitably pre-
pared tissues, each of the characteristic figures mentioned
being clearly recognizable : in some instances, however,
owing to the irregularity in the form of the chromosomes,
or to variations in the rate of their transformation, some
of the phases may be so far modified as to be no longer
distinguishable. The essential features of the process are,
nevertheless, always to be discerned, and should be dis-
tinctly borne in mind : they are, in their order, as follows :
first, the collection of the chromatin of the nuclear net-
work into chromosomes ; second, the equatorial arrange-
ment of the latter in what has been termed the nuclear
plate; third, their metakinetic cleavage ; fourth, the sepa-
ration of the two sets of chromosomes thusformed ; and
finally their resolution into the nuclear networks of the
two resultant daughter-nuclei, whose formation b}'^ this
method is accompanied by the cleavage of the protoplas-
mic body of the parent cell, thus completing the forma-
tion of new cells.

It has recently been made fully evident that the changes
taking place in the chromatin of the nucleus during karyo-
kinesis are accompanied and preceded by other equally
complicated changes in structures made up wholly of ach-

CHAPTKK XI. STKICTI'KK OF THK CKLL

12vS

roniatin, the formation of the achromatic spituUc hcin<( a
jiortion thereof. While these changes are doubtless as im-
portant as those already described, their nature is as yet
far less clearly understood. A description of them ww
therefore not be necessary at this time.

END OF PART I.

' e'

Hi

9 I

PART II.

HISTOLOGICAL ANATOMY.

CHAPTER XII. INTRODT'CTORY. 127

CHAPTER XII.
INTRODUCTORY.

HistolDgical Anatomy has already been defined as the
study of the arrangement of the tissues to form the organs
of the body : an organ has also been defined as a particu-
lar part of the body having a definite form and function :
and it is a familiar fact that organs having common or
essentially similar functions are associated together under
the name of a system, whether they are continuous, as in
the case of the nervous, or discontinuous, as in that of the
muscular system.

While the study of the histological anatom\' of the
organs can in most cases be pursued most naturally by
considering them in their relations as components of the
various physiological systems, on account of the commu-
nity of structure which, in most cases, characterizes asso-
ciated organs, and while structure and function are with-
out question closely (though not always evidenth-) re-
lated, it should always be kept clearly in mind that we are
here concerned with structure only ; and particularly with
structure as composed of tissues : our constant endeavor
should be, in the first place, to analyze the organs into the
tissues of which they are composed, and to determine the
relations of each to the others ; and in the second place to
note any characteristic pecidiarities exhibited by any of

128 PART II. HISTOLOr.ICAL ANATOMY.

the tissues present ; this should be accompanied in each in-
stance by a careful consideration of the disposition and
characters of the compound factors of structure present,
such as the blood, lymph, and nervous supply.

Tissues have alread}'- been defined as masses of cells or
of cell-derivatives; and since tissues compose the organs,
and organs make up the whole body, it follows that the
body is to be regarded as a mass of more or less modified
cells. The innumerable cellular elements which make up
the adult human organism are in every case derived from
the division of previously existing cells, and are therefore
necessarily the descendents of a single ancestral cell. That
cell is the fertilized, ovum or, as it is sometimes termed,
the oosperm. The study of fertilization, of the segmenta-
tion of the oosperm, and of the subsequent development of
the tissues and organs of the body lie strictly within the
province of the science of Embryology' ; but a brief state-
ment of the origin of the various tissues may with advan-
tage be given here, as throwing light on the structure and
relations of the organs of-the body.

Repeated cell division or segmentation in a short time
divides the oosperm into a spheroidal mass of apparently
similar cells : these soon arrange themselves in two dis-
tinct layers, from w^hich a third intermediate layer is
shortly afterward derived: the outer of these layers is
called the epiblast or ectoderm, the middle the mesoblast
or mesoderm, and the inner the hypoblast or entoderm;
the whole trilaminar structure receiving the name of the
blastoderm.

CHAPTKR XII. INTRODUCTORY. 129

From the cells which compose the epiblast are derived
the following structures :

The epidermis, and its appendages the hairs and the
nails, and the epithelium lining the tegumentary
glands (sweat glands, sebaceous glands, mammary
glands).

The epithelium of the nasal passages and the asso-
ciated cavities and glands.

The epithelium of the mouth and of the glands con-
tinuous therewith, and of a portion of the tongue:
the taste organs : the enamel of the teeth.

The epithelium of the conjunctiva and of the glands
of the eyelid, and of the front of the cornea: the
lens of the eye: the retina (secondarih- as an out-
growth from the brain).

The epithelium of the membranous lab\'rinth of the
ear.

The epithelium lining the cavities contained in the
cerebrospinal axis : the nervous tissues : the neuro-
glia : the pineal body : the pituitary body.

From the cells which compose the middle layer or meso-
blast are derived the following structures:

The epithelium of the urinary and genital organs
(with the exceptions of the epithelium of the blad-
der and urethra), including the reproductive ele-
ments of both sexes.

All the muscular tissues of the body, with the excep-
tions of the cells (doubtfully muscular) found in
the sweat glands.

The skeletal tissues of all sorts throug^hout the bod v.

130 PART II, HISTOLOGICAL ANATOMY.

The bloo(J-vascular and lymph-vascular system :
the serous membranes : the spleen and other adenoid
bodies: the blood and lymph corpuscles.
It should be stated that by some histologists the cells
which give rise to the tissues of the first two groups are
regarded as having an origin somewhat different from
those giving rise to the last two : the name mesoblast has
been retained by them as a collective title for the former,
while for the latter the name of parablast was proposed
bj^ His, and later that of mesenchyma by the Hertwigs.
While this is probably true of the tissues of birds, it has
not yet been proven for any mammal: and there are spe-
cial reasons why it might be true in one case and not in
the other.

From the cells which compose the hypoblast are derived
the following structures :

The epithelium of the back of the tongue, the lower
part of the pharynx, the oesophagus, stomach and
intestines : that of all the glandular appendages of
the alimentary canal.

The epithelium of the Eustachian tube and middle
ear.

The epithelium of the larynx, the trachea, the bronchi,
the bronchial tubes and the air sacs of the lungs.

The epithelium lining the urinary bladder and the
urethra.

The epithelium lining the vesicular alveoli of the thy-
roid body.

The concentric corpuscles or epithelial nests of the
thymus.

CHAPTER XII. INTRODUCTORY. 131

Even a brief study of the tabularstatementof the origin
of the tissues of the body above given will make clear the
facts that most organs are made up of tissues derived from
more than one of the primary tissue-layers, and that in
some cases at least, tissues which are structurally continu-
ous and to all appearances similar arc of difterent embry-
onic origin : this is notably the case with the transitional
epithelium found in the ureters and the bladder; and other
instances might be mentioned.

The order of study pursued in acquiring a knowledge of
the histological anatom\' of the various organs and sys-
tems of the body is plainly' a matter of convenience. That
which will be here pursued is one shown by experience to be
desirable on some accounts : but it should be understood
that it may be readily varied at will. Omitting from far-
ther consideration the simpler organs already described in
the preceding part in connection with the tissues which
chiefl\' compose them, such as the cartilages, the bones,
the muscles, etc., the various regions, systems or groups
of organs will be successively described as follows:

The tegumentary system will first receive attention :
this includes not onh- the skin, but also those solid append-
ages, the hairs and the nails, which are derived from the
special modifications of its outer layer; and those in-
growths of the same la\'er which constitute the sudor-
iparous, sebaceous and mammary glands.

132 PART II. HISTOLOGICAL ANATOMY.

The skin upon the enter surface of the lips is continuous
with the so-called mucous membrane of its inner surface :
a transition which brings us naturally to the mouth and
its contents: this includes the study of the lining mem-
brane above referred to; the buccal and other glands
which open thereon ; the teeth and the tongue.

The mouth is the antechamber of the alimentary canal,

though it is often regarded as a part of it. Beginning with
the pharynx, we naturally consider in their order the
oesophagus, the stomach, the small intestine in its various
regions, and thelargeintestine (including the rectum ) ; and
also the glandular appendages of the canal, the liver and
the pancreas.

The pharynx is not only a portion of the alimentary
canal, but of the respiratory tract as well : the latter
being, as embryology shows, an outgrowth of the diges-
tive tube ; its study includes that of the larynx, the trachea
and bronchi, the bronchial tubes in their various ramifica-
tions, and the terminal sacs which make up with them the
proper substance of the lungs.

The bladder and the urethra are parts of an outgrowth
from the posterior region of the alimentary canal, as the
respiratory tract is of the anterior. With them as median
structures are closely associated the paired organs which
complete the urinary apparatus: and intimately related
therewith are the male and female reproductive glands,
and the accessory organs connected with reproduction
and micturition. This group will next be studied.

CIIAPTKR XII. INTROI^rCTORY. 133

Following the groups of organs above indicated, the
circulatory system may next receive attention. This in-
cludes the study of the heart, the larger blood vessels, and
the greater lymphatic trunks, the smaller blood vessels
and lymphatics having been already considered. Since
the great serous cavities, such as that of the thorax or of
the abdomen, may best be regarded as lymph spaces, the
special discussion of their lining membranes may appro-
priateK' be considered here.

In addition to the lymph nodes, there are found in the
body larger organs apparently allied to them or derived
from their modification, such as the spleen and the
thymus. The name of ductless glands has long been ap-
plied to these and to other soft organs of uncertain func-
tion, such as the thyroid, the pituitary body, and the
adrenals (the so-called suprarenal capsules). The group
(to which the name of the adenoid bodies is sometimes ap-
plied) is a heterogeneous one, its members in some cases
having little in common : but they ma}' for convenience be
considered together.

The structure of the central nervous system is at pres-
ent being worked out chiefly b\' histological methods: it
is not as yet alwaj's easy to distinguish between what
may be regarded as physiological, and what as histologi-
cal anatomy in some cases. Imperfect as our knowledge
is at present, the briefest statement of its details would
transcend the limits of an elementarj^ course in histo-
logy ; but some knowlede of itgs most salient features is

134 PART II. HISTOLOGICAL AXATOMY.

essential: the structure of the spinal cord will be discussed,
together with that of the principal regions of the brain.
On account of their intimate relation to the cerebrospinal
axis, the membranes w^hich invest it and which form the
lining of the spinal canal will be taken up in this con-
nection.

Finall}'^ the study of the central nervous axis ma}^ prop-
erh^ be followed by that of the complex outlj'ing struc-
tures which are the essential parts of the organs of special
sense, such as the eye, the ear and the nose. Beginning
our course with the common investment of the body, we
close it with organs w^hich are in a great measure speciali-
zations thereof.

'-ci

'^"

AXA trr

CHAPTER XIII. SKIN AND APPENDAGES. 135

CHAPTER XIII.

THE SKIN AND APPENDAGES.

The skin, which is the investing and protecting mem-
brane of the bod}^ is, like other membranes found upon
free surfaces, composed of two primary layers, one of which
is epithelial, the other skeletal : each of them being capable
of division into more or less well marked secondary layers
or strata. The outer or epithelial la^'^er is known variously
as the epidermis, cuticle, or scarf skin : the inner or skeletal
layer as the derma, corium, or cutis (or less properly the
cutis vera or "true" skin).

The epidermis can under favorable circumstances be
seen even with the naked eye to be made up of two distinct
layers, an inner moist or mucous layer (sometimes called
the rete mucosum), and an outer dry or horny layer;
and each of these layers can be resolved by the microscope
into strata characterized by differences in the form and
arrangement of the component cells : it may therefore be
regarded as the best example of a stratified squamous epi-
thelium in the whole body. Beginning with the mucous
layer, we find next the corium cells which are columnar in
form, and which are constantl}' undergoing division ; the
new cells formed at their free extremities are at first verti-
cally elongated, then polyhedral in shape, and later some-
what flattened vertically : they form with the basal cells

136 PART II. HISTOLOGICAL ANATOMY.

the first or lowermost stratum of the epidermis, known as
the stratum Malpighii : the elements composing this
stratum have the form of prickle-cells, their numerous
short processes preventing the actual contact of the sur-
faces of adjacent cells, thus forming channels for the circu-
lation of lymph and the nutrition of the elements : all the
cells of this stratum may be regarded as living. Here and
there leucocytes may sometimes be found ; they have wan-
dered into the epithelium from below and occupy irregular
intercellular spaces. The cells of the deepest portions of the
stratum Malpighii contain pjgment^ranules, the color of
the skin in different races depending chiefly on the relative
abundance and color of the pigment. Delicate nerve fibrils
enter this stratum of the epidermis, and, as stated in a
previous chapter, Merkel has described special terminals
thereto under the name of tactile cells.

As new cells are constantly formed in the deeper por-
tions of the stratum Malpighii, the older cells are as con-
stantly pushed farther and farther from the blood vessels
of the corium which constitute their basis of nutritive sup-
ply through the agency of the lymph channels already
mentioned. At a certain distance, which varies in differ-
ent portions of the body, they begin to yield more con-
spicuously to the mechanical pressure from without, be-
coming more flattened in form, and at the same time to
undergo degenerative changes, granules of a fat-like com-
pound termed eleidin appearing in their substance in great
numbers. There is thus formed a definite layer never more
than a few cells deep to which the name of stratum gran-
ulosum is applied: the stratum granujosum and the

CHAPTER XlII. SKIN AM) APPENDAGKS. 137

Stratum Malpigliii together make up the mucous layer,
and in most parts of the body the greater portion of the
epidermis.

ImmL'diately above the stratum granulosum and sharply
distinguished from it is a thin layer of cells which resemble
those of that stratum in b:ing compressed in form, but
differ from them in greater homogeneity of substance and
therefore in transluccncy : this is the stratum lucidum,
the lower of the strata of the horny layer. According to
Ranvier the formation of eleidin is followed by its trans-
formation into keratin, the characteristic substance of
horn, nails, claws, etc., which are, as we shall see, in the
main developments of the stratum lucidum.

While the stratum lucidum is constantly receiving acces-
sions from the cells of the stratum granulosum upon its
lower or inner side, it is as constantlv undergoing mod-
ifications on its upper or outer surface ; and the line which
marks this transformation is equall}' well defined in either
case, the stratum lucidum remaining like the stratum
granulosum of nearly constant thickness and definite lim-
itations. Thej:ells as they pass from its outer surface be-
come somewhat swollen and more loosely disposed, form-
ing a layer in which the outlines of the individual cells
may be clearly discerned : the nuclei have disappeared and
all traces of protoplasmic structure. This layer is some-
times designated the stratum corneimi: in places where
there are no hairs upon the surfacp it can be divided into
two layers, the lower, or stratum epitrichium, consisting
of thicker cells, more loosely disposed, and the outer or

138 PART II. HISTOLOGICAL ANATOMY.

stratum Stiuamosum of thin closely oppressed scales,
which are eventually and constantly cast oflP. The stra-
tum lucidum and stratum corneum (with the subdivisions
of the latter) together make up the horny layer.

The epidermis varies greatly in thickness in different
parts of the body: it may be no more than a tenth of a
millimetre in thickness, or may be as much as a millimetre
or more in places where the pressure ai»d friction upon the
surface is greatest, even under ordinary circumstances :
and under special conditions the cells of the stratum squa-
mosum of the palms of the hand or the soles of the feet,
instead of being exfoliated, may become impacted to form
la^-'ers two or three millimetres in thickness. The external
surface shows irregularities which in some measure corres-
pond to the conformation of the corium below, but as a
rule differ therefrom by their less extent : the surface of the
corium is, as we shall see, covered by projections known
as papillae, into the spaces between which the stratum
Malpighii descends : but this stratum varies correspond-
ingly in thickness to such an extent that the papillary
elevations are reduced on its upper surface to mere un-
dulations, to which the remaining strata conform.
The deeper lines upon the surface of the epidermis corre-
spond to definite folds in the corium.

Like the epidermis, the corium can be divided into strata,
which are not, however, so clearly defined in the case of
the latter as of the former, consisting as they do in modifica-
tions of a fibrous layer. Upon the surface the fibres are

CHAPTER Xm. SKIN' AND APPENDAGES. 139

fine and very eloscly felted, forming a thin, homogeneous
and almost translucent stratum, which may be distin-
guished as the basement membrane of the epidermis. Its
surface is closely beset with minute projections, which
interlock with corresponding irregularities at the lower
extremities of the columnar cells which lie at the base of
the stratum Malpighii.

Beneath the basement membrane above referred to the
corium consists of a felted mass of rather coarse bundles
of white fibrous tissue, reinforced by elastic fibres in vary-
ing quantity, and containing in some localities a greater
or less amount of smooth muscular fibres, notably in con-
nection with the hair-follicles. The outer portion is denser,
the bundles being smaller and more closely felted and
h'ing in the main parallel to the surface: it bears the
papillae already- referred to in another connection, and has
therefore been variously designated from its structure and
conformation as the dense stratum or the stratum papil-
lare: below it passes rather abruptU', but with no well-
defined line of demarkation, into a region in which the
bundles are coarser and less numerous, being more looseh'
and irregularly disposed : from its structure this is known
as the stratum reticulare. The papillae are conical or
club-shaped projections upward of the dense layer: in
man)' cases they are more or less subdivided at their free
extremities, in which case the}' are known as compound
papillae: they are most abundant in the regions of the
skin where the sense of touch is most acute, and also in a
modified form make up the dermal portion of the nail-
bed. In some places, notably on the tips of the fingers,

140 PART 11. HISTOLOGICAL ANATOMY.

they are arranged in single or double rows along ridges of
the corium, forming the familiar patterns readily seen with
the naked eye: these patterns remain constant for each
digit of each individual throughout life, and have therefore
been used as marks of identification. Where best devel-
oped the papillae are from an eighth to a fourth of a milli-
metre in height.

The connective tissue corpuscles of the corium, like those
of other fibrous membranes, are small, and as a rule com-
pressed, their long axes lying parallel to the direction of the
bundles with which they are associated : leucocytes are
also present, as are pigment cells in moderate numbers.

' The mcvshes of the stratum reticulare not unfrequently
contain clusters of fat-cells of varying extent : immedi-
ately beneath, in most portions of the body, we come to
a la^^er of larger or smaller ovoid or polj^hedral fat-lobules
separated by a coarse meshwork of fibre bundles. This
layer, frequently termed the panniculus adiposus, is often
spoken of as subcutaneous : it is, however, in many cases
no more clearly marked off from the stratum reticulare
than is the latter from the stratum papillare : it may with
good reason be regarded as a portion of the skin, and as
such be distinguished as the stratum adiposum: whether
it shall be regarded as cutaneous or subcutaneous is
largely a matter of definition ; the fact of its association
with the other structures of the skin as a part of the tem-
perature regulating mechanism of the body is important.
Below it passes into the loose layer of subcutaneous areo-
lar tissue, which, save in a very few localities, intervenes

CHAPTER XIII. SKIN AND A ITKNDACES. 14-1

between the skin and the structures beneath, permitting
of its more or less free movement upon them.

The skin, exclusive of the stratum adiposum, varies in
thickness from h^lf a millimetre to two or three milli-
metres: and may even occasionally be as much as twice the
latter quantity in thickness. It is thickest upon the shoul-
ders and back.

The arteries which pass to the skin branch and subdivide
in the subcutaneous tissue, the small vessels thus formed
proceeding toward the surface : on their way they give off
twigs which supply the fat-lobes above mentioned, and the
hair follicles, sweat glands, etc., presently to be described.
As they approach thesurface they branch and anastomose,
and finally break up into a meshwork of capillaries, situ-
ated j ust below the basement membrane, suppK'ing the pap-
illae together with the other portions of the dense layer. The
capillaries unite to form a superficial venous network, the
larger veins arising therefrom passing to the deeper por-
tions of the skin in such a way as to accompany the ar-
teries in large measure.

Lymphatics arise in the spaces between the bundles of
fibres which make up the dense layer. These are so dis-
posed as to form a lymphatic network just below the
superficial capillary network just mentioned : lymphatics
have also been demonstrated in some of the larger papil-
lae. A second network is said to exist in the deeper por-
tion of the corium, the two communicating freely with
each other and with the subcutaneous lymphatics.

142 PART 11. HISTOLOGICAL ANATOMY,

The nerve supply of the skin varies greatly in different pot'
tions of the bod\'. Like the blood vessels, the nerves form
plexuses in the papillary region, the meshes immediately
beneath the epidermis becoming ver^^ fine and close : from
the fibres composing them fibrillae are sent up into the
stratum Malpighii in the manner alread\^ described. As
stated in a previous chapter, it is not yet certain how
these fibrils terminate.

In certain (if not all) portions of the bod}', more or
fewer of the papillae are supplied with nerve fibres which
there terminate in the so-called tactile corpuscles of Meiss-
ner: these are most abundant in the papillae of the fingers
and toes. Other fibres terminate in end bulbs, while man}'
others are distributed to the hair follicles and the s-weat
glands. In the deeper or subcutaneous region fibres are
found which end in Pacinian bodies.

The glands of the skin are chiefly of two kinds: the
sudoriparous or sweat glands, distributed in Yarjmg
abundance over the whole surface of the body ; and the
sebaceous glands, found chiefl}' at the bases of the hairs.
In certain localities specialized glands are found which maj'
be regarded as modifications of one or the other of these
two types.

The sudoriparous glands are situated in the reticular
stratum and the outer portion of the adipose layer : they
are spheroidal bodies, from one-half of a millimetre to
two millimetres in diameter, each consisting of a small
tube coiled into a ball : from this the tube proceeds (as the
duct of the gland) with slight deviations from a direct

CHAPTER XIII. SKIN AND A PrHNDAC.ES. 143

course through the coriuni, at whose surface it becomes
continuous with a closely coiled spiral opening through
the epidermis ; the whole structure consisting, therefore, of
a tubular depression of the surface, whose basement mem-
brane is a continuation of that which forms elsewhere the
outer limit of the cprium, while its lining epithelium is a
direct continuation of the epidermis.

The glandular (or the greater part of thecoiled portion)
is considerably larger than the rest of the tube, being
sixty or seventy micra in diameter: immediately upon the
basement membrane in this region is found a simple layer
of elongated elements resembling smooth muscular fibres
in appearance, and commonly regarded as such ; their long
axes are parallel with the direction of the tube. Upon this,
and surrounding the lumen of the tube, is a layer of col-
umnar glandular cells about fifteen micra in diameter,
which are frequently pigmented. The duct is from twenty
to thirty micra in diameter: it consists of a basement
membrane upon which is seated a stratified epithelium of
a very simple order: immediately next the membrane is a
layer usually two cells thick, composed of polvhedral cells ;
while upon this is a single layer ol flattened cells often
designated the cuticle. On reaching the epidermis the
basement membrane is, of course, continued into that of
the surrounding region : the deeper portion ot the epithe-
lium becomes continuous with the stratum Malpighii : the
cuticular layer lines the passage through the epidermis,
which is from the first over twice the diameter of the
lumen of the duct, and in the stratum corneum flares to
form a trumpet-shaped opening at the surface.

144 PART II. HISTOLO '.ICAL ANATOMY.

Each sweat gland has a network of capillaries which
penetrates the coil, following the interstitial connective
tissue ; and is also provided with a proper nerve supply.
The number of the glands varies in different portions of
the bod}' from four or five hundred to the square inch (or
more than a millimetre apart) upon the back of the neck
and trunk to five or six time as many (or less than half a
millimetre apart) on the palm of the hand or the sole of
the foot. Those of the armpits are quite large, as are those
at the root of the penis in the male and on the labia
majores in the female. About the anus are found glands
identical with the sweat glands in structure, but still
larger than those just mentioned : they are sometimes dis-
tinguished as the circumanal glands. In the larger sudor-
iparous glands (and sometimes in the smaller) branching
of the coiled portion occasionally occurs : and in some
cases the duct is bifurcated, either before or after it enters
the epidermis. The ceruminous glands of the external
auditory meatus of the ear resemble the sweat glands
in their structure, though differing from them to a marked
degree in the nature of their secretion.

The sebaceous glands occur, as has been stated, at the
roots of the hairs : they are also found upon the labia
minores and the prepuce and occasionally in other hairless
localities. Each consists of a short duct usually opening
into the outer portion of a hair follicle and connecting in-
ternally with the adjacent secreting portion: this commonly
consists of from three or four to eighteen or twenty sac-
cules, or, rarely, a single spheroidal sac. The basement
membrane, which is continuous with that of the corium,

CIIAPTHR XIII. SKIN AM) APPENDAGES. 14-')

supports a conipouiul layer of polyhedral cells, wliich in
the glandular portion are constantly being renewed by the
division of the cells next the basement membrane. The
cells thus cast off undergo a sort of fatty degeneration,
the interior of. the gland thus becoming filled with a semi-
Huid mass consisting of oil droplets, fragments of broken-
down cells, etc., which is discharged upon the surface
through the duct. The Meibomian glands of the eyelid
are to be regarded as enlarged and modified sebaceous
glands. The mammary glands are also to be regarded as
originally derived from the modification of bodies essen-
tiallv similar to sebaceous glands and are properly to be
considered as tegumentary organs. On account of their
size and complexity, however, their structure will be bet-
ter understood after thediscussionof the general structure
of glands in a subsequent chapter. They will, therefore,
be discussed later in connection with the female reproduc-
tive organs.

Both sudoriparous and sebaceous glands are formed by
solid ingrowths of the epidermis which in the case of the
former penetrate the corium, the cavity being for-med in
the centre of the club-shaped mass and later communicat-
ing with the surface, while the inner end becomes coiled
into the shape characteristic of the adult structure. In a
similar manner the mass of cells which is the precursor of
the sebaceous gland becomes divided into lobules, the fatty
transformation of the central cells already described be-
ginning in them and advancing along the axis of the pedi-
cle, which thus becomes converted into the duct.

l-iG PART II. HISTOLO ICAL ANATOMY.

While the hairs are practicalh^ outgrowths from the
skin thej^ are in reality derived from the modification of
ingrowths resembling in some respects those which give
rise to the sebaceous and sudoriparous glands. A hair is
a mass of epidermal cells which cohere together to form a
cylindrical or more or less flattened rod : these cells are
formed upon and around a papilla situated at the bottom
of a depression of the corium known as a hair-foUicle,
whose sides are lined with a specially modified layer of epi-
dermal cells, continuing the Malpighian layer of the epi-
dermis and the stratum' lucidum to the region around the
papilla at its base. The portion of the hair which pro-
jects beyond the surface is termed the shaft: that which is
contained in the follicle is known as the root: the epider-
mic portion of the follicle surrounding the root is com-
monly called the root-sheath.

The fully formed hair, whether within or without the
follicle, has on its surface a layer of imbricated scales, with
their free edges directed toward the outer end of the hair ;
this layer is known as the cuticle: on the form and ar-
rangement of the free margin of thecuticular cells depends
the pattern of the superficial markings seen on the hairs of
man and of different animals. Immediately beneath the
cuticle lies the cortical substance of the hair: this is com-
posed of greatly elongated fusiform cells in which traces
of the nucleus are still visible, though the body of the cell
has largely imdergone horny transformation : these cells
are so closely united that their limits are not ordinarily
distinguishable, the cortex appearing to be made up of
elongated fibres ; it is to the color of the cortical cells that

IokJx.IjU.^-'^-'^^^ "

ClIArTKK XIII. SKIN AND AI'I'HNDAGIiS. 14-7

the color of the hair is largely tlue. The cortical substance
in some hairs extends clear to the centre: in others it sur-
rounds (as its name imj)lies) an axial mass ofcuboidal or
polyhedral cells, the medulla: the bodies of these cells
usually contain small air-vesicles, rendering the medulla
white by reflected light. Such air s])aces may also be
present in the elongated cells of the cortex.

The formation of the hair at the papilla will be more
clearly understood if its description is preceded b\' that of
the follicle with whose epidermis it is continuous: this, like
the structure of which it is a modification, consists of a
fijjrous or dermal portion and an epithelial or epidermal por-
tion. The dermal portion maybe resolved into three layers:
of these the outerfibrouslayerconsistsof bundles of white
fibrous tissue running chiefly in the direction of the follicle
and containing numerous connective-tissue corpuscles ; it
resembles the corium in most respects, save in the absence of
elastic fibres : the middle layer, sometimes called the
muscular layer, consists of connective tissue whose fibres
run transversely, and of transversely-disposed elongated
cells with rod-shaped nuclei which much resemble smooth
muscular fibres and have been described as such, but which
may perhaps be regarded as modified connective-tissue
corpuscles; this layer extends from the bottom of the fol-
licle to the point where the sebaceous gland opens, beyond
which it is wanting: the inner or hyaline layer, or glassy
membrane, as it is sometimes termed, is thin, homogene-
ous and transparent ; it corresponds to the basement
membrane of the corium.

148 PART II. HISTOLOGICAL ANATOMY.

The epithelial portion of the follicle, while it is structur-
ally free from the hair within, is generally brought awa\'-
with the latter if it be pulled out of the skin during life:
it is therefore commonly termed the root-sheath of the
hair. It consists of two layers, comparable in a general
way to the mucous and the horny layers of the epidermis.
The outer of these, or outer root-sheath, is much thicker
than the inner, being a layer several cells deep ; of these
the cells next the hyaline layer (or basement membrane)
are columnar, like those at the base of the stratum Mal-
pighii ; while those within are polyhedral prickle cells like
the majority of those in the stratum just mentioned.

The inner layer of the epithelium, or inner root-sheath,
may be divided into three strata: the outer of these,
known as Henle's layer, is a single layer of flattened cells
of a horny appearance, in which nuclei are not distinguish-
able : wnthin this is Huxley's layer, composed of polyhe-
dral cells with small nuclei, the layer being two or three
cells deep: this is lined on the inside by the cuticle of the
root-sheath a single layer of flattened cells which are im-
bricated in a manner similar to that of the cells of the
cuticle of the hair, but in the opposite direction, the free
edges of the cells in question being directed toward the
bottom of the follicle ; as a consequence, their edges inter-
lock with those of the cells on the surface of the hair. The
name of inner root-sheath is sometimes restricted to the
part which comprises Henle's and Huxley's layers, the
two becoming confluent at base of the follicle ; the cuticle
of the root-sheath is in such cases described as third layer
thereof; but this is entirely a question of names: no satis-

CHAPTHK XIII. SKIN AM) A I'I'K.\I)A(iKS. 14-9

factory attem])t has been made to identify either of these
layers with the various strata of the skin in the way that
the outer root-sheath can l)e identified with the stratum
Malpighii.

As we pass to the base of the follicle the outer fibrous
layer is continued around its curving extremity" to be con-
tinued into the hair papilla, a large club-shaped papilla
which projects upward into the lower end of the follicle:
the middle layer becomes thinner and terminates near the
lower end of the follicle : the hyaline layer, now resting on
the fibrous layer, invests the surface of the papilla as a
basement membrane. The whole follicle is slightly enlarged
or bulbous at the base; the thickening being chiefl\'due to
an enlargement of the epithelial portion of the follicle and
of the lower end of the hair itself The outer root-sheath
is continued downward with little change, the columnar
cells at its base passing around the curve of the valley
which surrounds the base of the papilla and over the sur-
face of that body, acting here as elsewhere as the genera-
tive layer of the epithelium : the poU'hedral cells become
confluent with those which form the layers of Huxle>^ and
of Henle, these latter having previously become merged
into one, and with the cells of the cuticle of the root-sheath.
The valley at the base of the papilla is thus filled with a
mass of newly formed cells, which, as they are rapidlv
multiplied, are pushed off as a cylindrical mass, the hair,
from around the papilla. The medulla of the hair is formed
from the cells developed upon the upper end of the papilla
itself, and is in a certain sense the continuation of the col-

150 PART II. HISTOLOGICAL ANATOMY.

umnar layer of the outer root-sheath : the cortex of the
hair represents the polyhedral layer and the la^^ers of Hux-
ley and Henle ; while the cuticle of the hair corresponds to
that of the root-sheath.

The outer fibrous layer of the hair follicles is richly sup-
plied with blood vessels and nerves : some fibres of the lat-
ter pass to the outer root-sheath, where they appear to
terminate among the epithelial cells in a manner similar
to that found in the stratum Malpighii of the epidermis,
chiefly in the immediate vicinity of the sebaceous glands.
In some of the lower animals large hairs, chiefly about the
face, are provided with special forms of nerve terminals :
such hairs are termed tactile hairs.

The hair follicles are rarely vertical to the surface of the
skin, the degree of their obliquity varying in different local-
ities and, in consequence, the position of the hair upon the
surface. Many hairs have small bundles of smooth mus-
cular fibres passing from a point on the papillary layer of
the corium near the opening of the follicle and on the side
toward which the hair is inclined, to be inserted in the
outer fibrous layer of the follicle near the bulb. The con-
traction of these muscles, known as the arrectores pili,
tends to erect the hairs.

Hairs are formed as solid club-shaped downgrowths of
the stratum Malpighii of the epidermis, which meet with
specially formed papillae around which the hair-bulb is
moulded : the young hair is developed as a conical mass

CHAPTKK XIII. SKIN AND APrKNDAOES. 151

above the pa])illa, the solid epithelial plug first formed
undergoing sebaceous degeneration in its centre and thus
permitting the escape of the hair: its lateral portions be-
come the root-sheath, outgrowths therefrom giving rise
to the sebaceous glands. When a hair ceases to grow, the
papilla gradually disappears and the hair finally drops
out of the follicle: this may or may not have been pre-
ceded by the formation of a new downgrowth from the
bottom of the follicle and the development of a new pa-
llia, thus giving rise to a replacing hair.

The nails, like hairs, are masses of epidermal cells, con-
sisting chiefly of a thickened and otherwise modified ex-
tension of the stratum lucidum. Each nail can be regarded
as composed of three portions : the free margin, in which
growth has entireh- ceased, the nail-body, which consti-
tutes its greater portion, but which receives but slight ad-
ditions to its under surface, and the nail-root, which is the
region of greatest increase. The body of the nail is con-
tinuous below with a modification of the stratum Mal-
pighii, which rests upon a modified portion of the corium,
called the nail-bed: laterally this fibrous layer is folded
upward to form the lateral nail-grooves, and posteriorly
upward and forward to form the posterior nail-groove ;
the lower portion of which is termed the nail-matrix, in-
cluding the whitish curved area at the base of the nail
known as the lunula.

The nail-bed and nail-matrix are continuations of the
corium which has become highly vascular and is well sup-

152 PART II, HISTOLOGICAL ANATOMY.

plied with nerves : the papillae upon its surface are simple
and closelv crowded tog-ether: as far as the outer marmn
of the lunula thev show no definite arrangement, but
throughput the nail-bed proper are arranged in longitud-
inal row^s, their extremities inclining toward the free end
of the nail : the^-are so closeh'set in the rows as to appear
to be confluent in ridges, which are sometimes said to re-
place them. Below, the nail-bed is connected with the dis-
tal extremit}' of the last phalangeal bone b\' numerous
strong bands of fibrous tissue: as it passes around the mar-
gin of the nail to enter the walls of the nail-grooves, it
assumes the structure commonh^ characteristic of it.

The stratum Malpighii is by some histologists defined
as a part of the nail-bed : it is a question of names merely,
but it is perhaps better on the whole to divide the two re-
gions by the natural boundary between dermal and epi-
dermal structures. The columnar cells of this stratum
are close]}'- packed together, and multiply rapidly, partic-
ularh'- in the region of the matrix: very few polyhedral
cells are to be seen, the newly formed cells passing over
rapidly into the substance of the nail without an inter-
vening stratum granulosum. Like the stratum lucidum,
the bod}^ of the nail consists of flattened horny cells, in
which traces of nuclei can be seen after dissociation. Dur-
ing foetal life the nail is invested by the stratum epitrich-
ium, traces of which overlie its margins at birth.

CIIAPTHR XIY. MOl^TH AND CONTENTS. 153

CHAPTER XIV.
THE MOUTH AND ITS CONTENTS.

The mouth is formed b}^ an ingrowth from the surface
of the head ; its lining is therefore epiblastic in origin and
directl\' continuous with the epidermis: the cavity of the
mouth does not at first communicate with that of the phar-
3'^nx, but the two are connected later by the perforation of
their common wall at the fauces. Within the cavity of the
mouth are found the jaw arches, covered by the gums and
bearing the teeth, and the tongue, which rises from its
floor; and in the so-called mucous membrane which lines
it throughout are found the labial, buccal, palatal and lin-
gual glands : while other and larger glands more remoteh-
situated discharge their secretion into the cavity of the
mouth b}'^ means of ducts.

As we pass from the skin upon the outer surface of the
lip to the mucous membrane which is found upon its inner
surface marked changes are to be noted in both the epithe-
lial and the fibrous layer. Near the margin of the lip the
hair follicles are wanting, though sebaceous glands are
present ; the derma becomes thinner and highly vascular ;
while the epidermis becomes much more transparent, al-
lowing the red color of the blood in the dermal capillaries
to shine through. As the transition is made from a sur-
face constantly dr}- from exposure to the air to one con-

154 PART 11. HISTOLOGICAL ANATOMY.

stantly moist, the stratification of the egithelium becomes
less distinct: and on the inside of the mouth is found a
la^^er of protoplasmic cells corresponding to the stratum
Malpighii passing by insensible gradations into a layer of
hornj^ cells, flattened, and with small nudei, that are con-
stantly being exfoliated, corresponding to the stratum cor-
neum : all trace of the inter veniiig stratum granulqsum
and stratum lucidum disappearing The cells at the base
of the layer are columnar, like those in the correspond-
ing portion of the epidermis, and, like them, are con-
stantly forming new cells to replace those lost from the
outer surface.

Underneath the epithelium is the fibrous J[ayer, corres-
ponding to the corium, to w^hich the name of mucosa or
mucous membrane is sometime restricted : it is in most
cases thinner than the corium, but bears upon its surface
numerous papillae : below it breaks up into looser connec-
tive tissue as a rule, the meshes being occupied by the
glands of the mucosa and by fat lobules : to bhis looser
subjacent tissue the names of submucosa is applied: in
the region of the fauces, the soft palate, and the uvula ad-
enoid tissue is present in great abundance in the raucous
membrane ; a feature never found in connection with the
corium of the skin.

Where the mucous membrane invests the hard palate,
and where it passes over the arch of either jaw to form
the gums, the fibrous layer becomes firm and dense, glands
and fat lobules alike being absent or very sparingly pres-
ent, and the deeper portion becomes directly continuous
with the periosteum of the subjacent bone. The papillae
and the investing epidermis of the dorsal surface of the

CHAPTKK XTV. MOITII AM) CONTKNTS. 155

tongue undergo special modifications best described in
connection with that organ.

The term mucosa, or its equivalent, mucous membrane,
above applied to the lining of the mouth, is also used
to designatethe lining of all those cavities which communi-
cate directly or indirectly with the outside of the body
(with the exception of the abdominal cavity of the female).
It thus includes the investment of the nasal passages and
the associated sinuses, the respirator\' tract, the oral cav-
ity and the alimentary canal, the urinogenital tract, and
the middle ear with the Eustachian tube. B\' some histol-
ogiststhe term is even applied to the lining of the hair folli-
cles and the dermal glands. Excluding the latter, it may in
general terms be described as consisting, like the skin, and
as seen in the lining of the mouth, of an epithelial and a
skeletal portion. The epithelium of the mouth and of the
nasal passages is epiblastic in its origin : that of the kid-
neys and genital glands, and of their proper ducts, meso-
blastic : that of all the rest of the surfaces named hypo-
blastic. It is to the regions lined with h3'poblastic epithe-
lium that the term mucosa is chiefly applied.

The epithelium of a mucous surface ma\' be simple and
flattened, polyhedral, or columnar; or transitional; or
stratified. The skeletal layer consists usnallv of a base-
ment membrane (sometimes termed a membrana propria,)
which may be either a homogeneous layer of closely felted
fibres, or an endothelioid layer of connective tissue cor-
puscles ; beneath this is the felted fibrous layer termed va-
riously the corium, the stroma, or the tunica propria, or,
by some, the mucosti in a limited sense. It consists of a

156 PART II. HISTOLOGICAL ANATOMY.

thicker or thinner layer of rather loosely felted bundles of
white fibres, rich in blood vessels and in some cases having
its surface raised into papillae. Elastic fibres are some-
times present in such great numbers as to form a definite
elastic layer, and at other times are almost entirely want-
ing. Adenoid tissue may be present in varying quantity,
sometimes forming definite nodules or clusters of nodules.
Smooth muscular fibres may also be present, forming a
stratum in the deeper part of the membrane one or more
layers in thickness and known as the muscularis mucosae.
In exceptional cases, such as that above described in con-
nection with the hard palate and the jaw^ arches, the mu-
cosa is firmh' united to the subjacent structures : as a rule
the deeper portion passes over into a la^^er of loose areolar
tissue knowm as the submucosa, thus permitting of the
free movement of the mucosa on the structures beneath.

Where the epithelium of a mucous surface is columnar a
varying proportion of the elements have the form of gob-
let cells, as described in a previous chapter : these are per-
haps the simplest form of special secreting organs found
in the human body ; and are sometimes spoken of as uni-
cellular glands : the term gland being often loosely em-
ployed to designate any secreting organ and, indeed, some
organs that are not at all secretory in function.

Increase of secreting surface is obtained by ingrowths
of the epithelium which penetrate the fibrous layer to a
greater or less extent, in a manner similar to that de-
scribed in connection with the skin ; like those referred to
these ingrowths may be either tubular or saccular. Where

CHAPTKR XIY. MOl TH AND CONTENTS. 157

such ingrowths are not sub-divided internally they are
commonly called simple tubular or simple saccular
glands, as the case may be. A distinction of importance,
both structural and physiological, ought, however, to be
noted in this connection. Some such simple secreting or-
gans are lined with cells that are similar in form and alike
secretory in function throughout their whole extent :
while in others the work of secretion is restricted to
specialized cells in the epithelium of the deeper portions,
that which is found upon the portion next the surface
having lost its secretorj' activity' and become modified to
form the lining of a conducting tube through which the
secretion of the deeper portion is discharged. This dis-
tinction can be kept in view if we always apph' to the
structure which secretes throughout its whole extent the
name of a follicle, and define a gland as a secreting
organ provided with a duct.

Glands, as thus defined, may be either tubular or sac-
cular, and either simple or, by the subdivision of the se-
creting portion, multiple or compound: the secreting por-
of a simple gland is called the fundus: those of a com-
pound gland are called acini or alveoli. Where the final
divisions of a compound gland are saccular in form, the
gland is frequently designated as acinous or racemose ;
where they are elongated, the gland is called compound
tubular; and where both forms of alveolus are present the
term acino-tubular is applied. B\' many histologists,
however, the distinctions which these terms imply are re-
garded as of questionable value.

Follicles, simple glands, and the smaller compound glands

158 PART II. HISTOLOGICAL ANATOMY.

rareh' penetrate deeper than the submucous or subcutane-
ous tissue of the membrane Irom which they are derived.
Large compound glands, on the contrar3^ are usually sit-
uated at some distance from the surface where their secre-
tions are discharged. Glands which are thus situated out-
side of the organs to w^ich their ducts lead are called ex-
trinsic, the term intrinsic being applied to those contained
Avithin the organs in question.

With the exception of the tegumentary glands discussed
or mentioned in the preceding chapter, all the glands of
the body open upon mucous surfaces ; and with the far-
ther exception of the kidne3'-s and the genital glands are
derived from the modification of a mucous membrane.
The secretions discharged b}^ them have always one or
both of two functions : to lubricate and preserve the sur_
face in question, which is a general function, or to act as
ferments upon food stuffs taken into the body, which is a
special function restricted to the alimentary tract. Two
principal types of glands are therefore distinguishable
both structurally and functionally. Those of the first sort
are known as mucous glands; those of the second (from
the more water}^ character of their secretions, but less
properly) as serous glands. In the case of some com-
pound glands some of the alveoli are of the mucous and
some of the serous t3^pe: such are designated mixed,
glands.

In glands of the mucous type the acini or alveoli are
lined with polj^hedral cells which do not extend quite to
the centre of the cavity, thus leaving a well defined central

CHAPTER XIV. MOl'TH AND CONTEXTS. '[~>9

opening or lumen: the free extremity of the eell is trans-
parent and does not stain readily with most reagents,
while the ])rotoplasm and the somewhat flattened nu-
cleus which it surrounds are crowded down to the base*
of the cell; ijn other words, the general appearance is like
that previously described as characteristic of goblet cells,
which, it should be recalled, are mucigenous in function.
In addition, there may in some cases be seen between the
glandularcells and the basement membranecrescent shai)ed
groups of granular cells which stain deeply : these were
described independently by two observers, who gave them
names associated with their form ; they are therefore
called the crescents of Gianuzzi, or the demilunes of
Heidenhain: their nature and functions are not yet fully
understood: the constituent cells (sometimes solitary ) are
called marginal cells.

In glands of the serous type the secreting epithelium
consists of polyhedral cells which when at rest extend clear
to the centre of the alveolus: a distinct lumen cannot,
therefore, be recognized. The whole body of the cell is
granular, the substance which is to become the character-
istic secretion being thus stored up in the protoplasm :
the nucleus is spherical, and situated near thecentre of the
mass ; and the whole cell stains readily. No trace of any-
thing like the crescentic cell-masses above mentioned or of
marginal cells is to be found in any serous gland.

The appearances above described are those seen in sec-
tions of glands previousK'hardenedby reagents, and taken

160 PART II. HISTOLOGICAL ANATOMY.

trom organs which had not been actively stimulated im-
mediately before the preparation was made. If a piece of
the fresh gland be examined in blood serum the mucigen
is seen in the case of the mucigenous cells to be present in
the form of very large granules : while the ferment secre-
ting cells are so swollen that not only the lumen of the
acinus but also the outlines of the cells are obliterated :
otherwise the appearances are much as described above.
As the result of prolonged stimulation the mucigenous
cells discharge their secretion, and the nuclei approach a
central position : while the cells of the serous type become
smaller after discharge, and a distinct lumen becomes vis-
ible in the acinus. The two types of alveoli, become
therefore much more nearly alike, though not so much so
as to prevent their distinct recognition.

It is -in the mouth that we first meet with a mucosa ;
and its discussion has therefore been deferred until after a
description of the lining membrane of that cavity. The
glands of the mouth also furnish usw^ith examples of both
mucous and serous, and both intrinsic and extrinsic
glands.

The intrinsic glands of the mouth are the labial, buc-
cal, palatal (including those of the uvula), and lingual.
These are all racemose glands situated in the submucosa,
with the exception of the lingual, which lie between the
muscular bundles of the tongue. They are all of the mu-
cous t3^pe save those on the posterior part of the tongue,
which are serous. Their secretion contributes to form the
saliva of the mouth.

CHAPTER XIV. MOUTir A.NT) CONTENTS. 101

The extrinsic glands of the mouth are those commonh'
referred to under the name of the salivary glands: they
are dcslpiatcd, from their positions as tlic sublingual,
the submaxillary, and the parotid glands: the first-
named are doubtfully to be called extrinsic, on account of
their structure and position, since they consist in each in-
stance of a group of small glands opening by several
ducts, and are situated just beneath the mucous membrane
on either side of the base of the frenum of the tongue.
The nature of the salivary glands differs in different mam-
mals: in man the sublingual and submaxillary are mixed
glands, and the parotid is of the serous type.

The acini or alveoli of one of the larger or extrinsic
salivary glands vary greatl^^ in form from flask-shaped
sacs to wavy, contorted, or even branched tubules: the
basement membrane is reticulated and the epithelium
varies with the type of gland in question. Each alveolus
leads into a tubule smaller than itself known variously as
the ductule, the connecting or intermediate tubule or
the intercalary duct: the basement membrane of the duc-
tule is continuous with that of the alveolus; its epithelium
consists of a sjngle layer of flattened cells. The ductules
of a number of alveoli enter a common tube known as an
intralobular duct or salivary tube of Pflueger, the
ajveoli and ductules with the duct just mentioned together
making up a lobule. An intralobular duct consists of a
basement membrane continuous with that of the ductules,
and a single la3^er of columnar epithelial cells : the latter
have spherical nuclei situated near the centre of the cell ;

h^UJJrOJr di^cli r (^J ducJr

162 PART II. HISTOLOGICAL ANATOMY.

the outer extremit}^ of the cell is finely granular, while
that next the basement membrane is vertically striated.
The basement membrane of the alveoli, the ductules and
the intralobular duct alike rest upon the interstitial con-
nective tissue, which contains rich networks of capillaries
surrounding the alveoli.

The intralobular ducts lead into larger conducting tubes
known as interlobular ducts, around which the lobules are
aggregated into lobes, their boundaries within the lobe
being marked by septa of fibrous tissue. The interlobular
lead into the interlobar ducts, which finally enter the
chief duct of the gland. The larger ducts have beneath
the basement membrane a definite fibrous layer which in
some cases contains smooth niuscular fibres : the epithe-
lium is columnar and simple, save in the largest ducts, in
which a layer of polyhedral cells lies betw^een the colum-
nar cells and the basement membrane.

The interstitial connective tissue which fills the spaces
between the alveoli passes into thin laminae of fibrous tis-
sue which separate the lobules : these septa are again con-
tinuous with stouter structures of the same nature which
lie between the lobes of the gland. The interlobar septa
are continuous internally with the stroma of connective
tissue which immediatelj^ surrounds the proper fibrous
tunic of the chief duct and its principal subdivisions, and
externalh^ with a membranous layer which surrounds the
whole gland and is known as its capsule. The place
where the chief duct leaves the interior of the gland is
known as the hilum: the nerves, arteries, veins, and
lymphatics of the gland also enter or leave at this point,

CHAPTER XIV. MOITH AND CONTENTS. 1 Orj

the capsule here becomino^ continuous with the stroma of
fibrous tissue (above mentioned as surrounding the duct
and its branches) in which they He, and in which occa-
sional small nervous ganglia may be found.

The saliva contained in the mouth is a mixture of the
secretions of the various intrinsic and extrinsic glands.
With the nature and properties of the fluid itself we are not
here concerned : it constantly contains, however, certain
histological elements which may with propriety be men-
tioned in this connection. The most abundant of these
are the scpiamous cells which are constantly being exfol-
iated from the surface of the stratified epithelium of the
mouth : the}' occur singlj^and in patches; and when found
in the latter form the overlapping of their bevelled edges
can be plainU'seen: the nuclei arc small and flattened, and
stain readih'. Less numerous, but quite abundant in the
saliva from the back part of the mouth are the so-called
salivary corpuscles, spheroidal bodies but little larger
than colorless blood corpuscles, each containing one or
two spheroidal nuclei and numerous minute granules which
exhibit a constant dancing movement within the interior
of the cell. The salivary corpuscles are really modified
leucocj'tes that have escaped into the oral cavity from the
tonsils or similar adjacent structures, and have become
swollen by the imbibition of the water}' saliva; the gran-
ules of the protoplasm being suspended in the imbibed
fluid and exhibiting Brownian motion in consequence.

The mouth contains certain organs by means of which

164 PART II. HISTOLO UCAL ANATOMY.

the food is masticated and prepared for swallowing by the
thorough admixture of the saliva. These are the teeth
and the tongue. A tooth is a vertical]}^ elongated mass of
the tissue mentioned in a previous chapter by the name of
dentine, having a solid free portion, the crown, project-
ing above the gum and covered with a layer of enamel:
and a hollow portion, the fang, imbedded in a socket in
the jaw known as an alveolus; the outer surface of the
fang is covered with a layer of cementum, and the inter-
nal cavit}^ occupied by the pulp. The slightly constricted
region where the crown and the fang meet and the tooth
pierces the gum is called the neck of the tooth.

The pulp which fills the central cavity of the tooth is a
mass of connective tissue approaching in character more
nearly to the gelatinous tissue of the embryo than any
other structure in the adult human body, its soft and spar-
ingly fibrillated matrix containing numerous branching
corpuscles whose processes are connected. It contains a
rich network of bloodvessels, and a small bundle of nerve
fibres, entering its substance through the minute canal at
the tip of the fang. Toward its surface the corpuscles
rapidly increase in number and in size, forming a superfi-
cial layer of crowded cells much resembling a columnar
epithelium : these are the odontoblasts ; they are in all
probability directly associated with the formation of den-
tine.

Like osseous tissue, dentine is characterized by a lamel-
lated and calcified matrix traversed by canaliculi : it
differs essentially from the substance of bone, however, in
the mode of calcification and the definiteness of the lamel-

CHAPTKR XIY. MOl'TH AND CONTEXTS. 165

lae, and particularly in that the characteristic corpuscles
associated with its formation, the odontoblasts, do not,
like the osteoblasts of bone tissue, become imbedded be-
t\veen the lamellae, but remain situated upon the inner
surface of the mass, their long and sparingly branched
processes, the fibres of Tomes, occupying the canals, called
dentinal tubules, which traverse the dentinal lamellae.
Calcification takes place, at least in the outer portion of
the mass, by the deposition of globular nodules of lime
salts; these at first do not occupy the whole of the matrix,
leaving numerous irregularly stellate spaces, the inter-
globular spaces, the outer layer in which they abound
being known as the granular layer. The interglobular
spaces communicate outwardly with the surface of the
dentine and inwardK' with the dentinal tubules : corpus-
cles have been described as contained in them and commu-
nicating with the extremities of the fibres of Tomes.
Deeper the calcification becomes more nearh' continuous ;
at certain intervals it is, however, incomplete, irregular
lines in a general way parallel to the surface being seen :
these, known as the incremental lines of Salter, may be
regarded as marking the boundaries of the lamellae. The
dentinal tubules are lined b}- a condensation of the fibres
of the matrix sometimes described as a distinct membrane;
they have a wavy or spiral course across the lamellae and
give off branches occasionally at acute angles which pur-
sue a similar course; their waviness gives rise to a striated
appearance when a tooth is seen in longitudinal section
with the naked eye, or a low pow-er of the microscope :
the alternating dark bands seen are know^n as the lines of

166 PART II. HISTOLOGICAL ANATOMY,

Schreger. The general direction of the tubules is in the
fang at right angles to the lamellae ; in the crown they
pass obliquely upward.

The enamel consists of a layer of elongated calcified
prisms, usually hexagonal in cross section, set in a general
way vertical to the surface of the dentine of the crown of
the tooth ; they are alternated lighter and darker through-
out their extent, giving to the enamel as a whole a banded
appearance. In addition, occasional dark brov\rn lines
may be seen crossing the enamel columns parallel-wise to
the surface; these, known as the stripes of Retzius, may
indicate lines of growth. Here and there vertical spaces
may be seen slightly separating the enamel columns near
the dentine, with which the interglobular spaces of the
latter possibly communicate. At the time of irruption
the surface of the enamel is covered with a thin cuti-
cular layer know^n as Nasmyth's membrane; it is rapidly
worn away as soon as the teeth are put to active use.

The cementum, cement-substance, or crusta petrosa, as
it is variously termed, investing the surface of the fang, is
composed of tissue essentially similar in structure to that
of dense bone : the lamellae are few and irregularly ar-
ranged, the lacunae varying in size and form ; the canali-
culi of the latter are said to communicate with the inter-
globular spaces of the adjacent dentine: there are no
clearly defined Haversian systems. The layer is formed by
the so-called periodontal membrane, which is practically
continuous outwardly with the periosteum lining the
alveolus which contains the tooth.

CHAPTER XIV. MOITII AND CONTENTS. 107

The enamel is epithelial in its origin, being derived from
the calcification of a layer of cells or formed by their secre-
tion. The dentine and ccmcntum are true skeletal struc-
tures. The margin of the foetal jaw early shows a thick-
ening of the stratified epithelium which grov/s downward
into a groove into the mesoblastic tissue beneath, the
dental groove: the curved rod of epithelial cells thus
formed is known as the primary enamel germ. At regu-
lar intervals along the under side (jf this rod further prolif-
erations of the e])ithelial cells occur, with localized down-
growths, the special enamel germs, at first flask-shaped.
At the same time a condensation in the mesoblastic tissue
beneath each gives rise to a conical dental papilla which
grows upward to meet the epithelial downgrowth. The lat-
ter grows down about the papilla, thus becoming converted
into a cap-like mass, the enamel organ, connected with
the epithelial ridge above b}- a slender stalk: the stalk is
attached to one side of the enamel organ, owing to the
mode of growth of the latter. A condensation of the
mesoblastic tissues surrounding the newly formed enamel
organ and papilla gives rise to a membrane which
soon becomes rich in bloodvessels, and is known as the
dental sac.

The papilla, thus invested by the enamel organ, assumes
the general shape of the crown of the future tooth : the
corpuscles near the surface become more numerous and
larger than those of the interior, and the layer of odonto-
blasts is formed, after which the deposition of dentine is
begun. Important changes have in the meantime taken
place in the enamel organ : the cells upon the concave sur-

168 PART II. HISTOLOGICAL ANATOMY.

face next the papilla become greatly elongated verticall}'',
and b}^ their activity begin the deposition of the enamel ;
whether b}'- a process of secretion or by the transforma-
tion of their own substance is still a matter of debate ;
the layer of enamel-forming cells is known as the inner
layer of the enamel organ : the margin of this layer is
continued over the convex surface of the enamel organ b}^
the outer layer of cuboidal epithelial cells, which is in turn
continuous through the stalk with the epithelium of the
jaw, and which lies in contact with the dental sac: the
interior of the enamel organ contains a mass of cells, the
so-called middle layer, which becomes converted into
branching corpuscles, their interspaces being filled with a
water}' fluid. From the side of the enamel organ or from
the adjacent stalk a bud arises early in the history of the
tooth, and grows downward to form a second enamel
organ in the case of those teeth which are succeeded by
others.

As the time of irruption approaches, the middle la^^er of
the enamel organ becomes greatly compressed, and the
enamel forming cells of the inner layer reduced in depths
to flattened scale-like bodies, forming the membrane of
Nasmyth. The papilla rapidly elongates, forming the
fang, thus pushing the first formed crown upward to-
ward its final position. After the dentinal portion of the
fang is fully formed the cementum is deposited upon its
outer surface in a manner practically the same as that of
the formation of bone elsewhere.

The tongue is a mass of striped muscular tissue in-

CttAl'TlvR XIV. MOITH AND CONTEMTS. 100

Vested with the niueous njembrane of tlic mouth, which
upon its upper surface is specially modified. The muscu-
lar fibres which make up its bulk are divided into two
symmetrical masses by a median vertical partition of
fibrous tissue, the lingual septum, which is better devel-
oped below than above: right and left of this the}' lie in
interwoven bundles some of which run kHigitudinall}^
others transversely, and a third set vertically; their
arrangement to form the lingual muscles is a matter
anatomical rather than of histological discussion.

The mucous membrane of the lower surface and sides of
the tongue does not differ essentiall)' from that of the rest
of the mouth. At the margin of the upper or dorsal sur-
face, however, it suddenly becomes modified, the majority
of the papillae becoming greatly enlarged to form the pro-
jections of the surface visible to the naked ey^ and known
as the lingual papillae. Of these there are three kinds :
those most abundant, and covering the whole surface of
the tongue with a velvety layer, are known as conical or
filiform from their shape; they are tapering upgrowths
of the corium, covered with a corresponding layer of epi-
thelium, and are frequently bifid at their tips: scattered
here and there among them are spheroidal elevations of
the surface about the size of a small pin's head ; these are
termed fungiform^ and are invested with a thin layer of
stratified epithelium. Near the root of the tongue, and
arranged in a V-shaped figure whose apex is directed back-
ward, are eight to twelve large papillae, roughly cylindrical
in form, which are sunk in depressions of the surface and
surrounded by circular trenches or grooves ; these are the

170 PART II. HISTOLOGICAL ANATOMY,

cir cum vallate papillae: the stratified epithelium of their
sides forms a deep la^'er in which are imbedded the taste-
buds or gustatory organs. The description of these struc-
tures will be given in the chapter devoted to the organs of
special sense.

The papillae all contain capillary networks, the fungi-
form and circumvallate papillae having a specially abun-
dant blood supply. The mucous membrane of the pos-
terior part of the tongue contains diffuse adenoid tissue
with here and there occasional nodules of the same.
The lingual glands of the anterior part of the tongue are
all of the mjLicous type : some of those of the posterior
part, known as the glands of Ebner, are of the serous
type, their ducts opening into the grooves which surround
the circumvallate papillae. The tongue is formed in part by
an upgrowth from the floor of the mouth, in part by a
growth forward from the ventral wall of the pharynx :
its epithelium is therefore in part epiblastic and in part
hypoblastic in origin ; but the two regions cannot be dis-
tinguished in the adult

iJo^^^Uii^ M^^^ vhiwvt

CHAPTER XV. ALIMENTARY CANAL. 171

CHAPTER XV.
THE ALIMENTARY CANAL.

As we pass from the mouth through the opening of the
fauces we enter the tube lined with h vpoblastic epitheHum
of which the mouth is the antechamber, and to which the
name of the alimentary canal is strictly applicable. The
anterior and posterior pillars of the fauces are folds of the
mucous membrane rich in mucous glands and containing
some diffuse adenoid tissue : between them on either side
of the mouth is situated a tonsil: a rounded body of va-
riable size consisting of a mass of fibrous tissue richly in-
filtrated with adenoid tissue and containing a variable
number (from ten to twenty) well-defined nodules of the
same structure, like those found in the mucous membrane
of the rest of the tongue. The surface of the adenoid mass
-thus formed is covered with stratified epithelium continu-
ous with that adjacent, which is in places invaded by
numbers of leucocytes passing through it into the oral
cavity to become the bodies described above as salivar\'
corpuscles. Deep folds or depressions of the surface occur,
lined with stratified epithelium : these are termed crypts,
and receive the secretions of subjacent mucous glands.

The alimentary canal, p'taperWW^c^lVeiil is a tube which,
beginning at the pharynx and ending with the rectum.

172 PART II. HISTOLOGICAL ANATOMY.

varies in size and form in different portions. Its structure
varies correspondingly in detail, as will be stated in the
descriptions of the various regions. In general plan it is
a tube having essentially a double wall whose divisions
are connected by a stratum of areolar tissue, and invested
throughout a portion of its extent b}' the serous membrane
which lines the abdominal cavity.

The inner or glandular layer, the mucosa, is a mucous
membrane whose epithelium varies greatly in different
parts : the fibrous portion is in all the various regions more
or less infiltrated with adenoid tissue which is in places
quite scanty in amount or wanting altogether, in places
so abundant as to form conspicuous nodular masses.
Smooth muscular fibres are also present in all the regions,
usualh'- in such quantity as to form a well-defined muscu-
laris mucosae, which may consist of two or even three
distinct layers.

The stratum of areolar tissue into which the loose and
folded mucosa passes, the submucosa, varies in extent in
different regions, but always permits of free movement of
the folds of the mucosa. It is characterized throughout
the greater portion of its extent b}' the presence of a net-
w^ork of nonmedullated nervous fibres with small ganglia
at the nodes, the plexus of Meissner: and contains the
larger blood and lymph vessels from w^hich those of the
mucosa are derived.

Beyond the submucosa, which is continuous with its in-
ternal skeletal framework, is the outer principal layer of
the tube, the musculosa, or muscular layer. The anterior
portion is composed of striped fibres; the remainder (and

chaptp:r XV. ali.mkntary canal. 173

greater portion) of sniooth filires. Except in the pharynx,
where definite muscles are found, the bundles of fibres,
whether smooth or striped, are arranp^cd in two continu-
ous strata, the inner of which is composed of circular and
the outer of longitudinal fibres: between them is a thin
layer of connective tissue containing blood vessels and a
second and larger nervous network known as the plexus
of Auerbach.

In the neck atid thorax the connective tissue adjacent
forms a more or less well defined membranous layer just
without the musculosa sometimes described as a separate
layer of the canal under the name of the fibrosa; while the
term serosa is commonly applied to the peritoneal fold
which rests upon the musculosa throughout the abdomi-
nal cavity.

The pharnyx is the first region of the alimentary canal.
Its u_pper or res^giratory portion is lined with ciliated col-
umnar epithelium ; while that of the lower and larger por-
tion is stratified squamous: the latter is throughout a large
part of its extent more or less infiltrated with leucocytes.
The fibrous membrane contains a large amount of adenoid
tissue, which in the upper and posterior part of the phar-
ynx is collected into a large patch containing adenoid nod-
ules and crypt-like depressions, to which the name of the
pharyngeal tonsil has been given : gjan^s of the rnucous
type are abundant. Beneath the submucosa is found a
dense layer of fibrous tissue, the pharyngeal aponeurosis,
separating the mucous membrane from the muscular coat.
Without are the well defined pharyngeal muscles, com-

u 0 IkAj^ ^JLCuf4- H odj^\AMM}LL{--uJ (lMy\^i.nA

174 PART II. HISTOLOGICAL ANATOMY.

posed of striped muscular fibres. The structure of the
pharynx ma}^ perhaps be best understood if we regard the
musculosa proper of the aHmentary tract as entirely defi-
cient, the aponeurosis representing the fibrosa, and the
constrictor muscles as external additions thereto.

The oesophagus, like the lower part of the pharynx, is
lined with stratified squamous epithelium. The mucous
membrane, which is thrown into longitudinal folds, bears
numerous small papillae upon the surface, covered by a
homogeneous basement membrane. Adenoid tissue is very
sparingly present in the mucosa, but occasional solitary
nodules may be found. The muscularis mucosae is want-
ing in the uppermost part of the oesophagus : lower it at
-first appears in the form of scattered longitudinal bun-
dles, which near the lower end of the oesophagus become
so numerous as to form a continuous layer of longitudinal
fibres.

The submucosa, which is well developed, contains the
intrinsic glands of the oesophagus, which are of the
mucous type; their ducts traverse the mucosa : blood ves-
sels and lymphatics are numerous : the plexus of Meissner
is wanting or very scantily developed throughout the
greater part of the oesophagus.

The musculosa consists throughout the uj22er_third of a
transverse and a longitudinal layer of striped muscular
fibres ; lower down the layers are made up wholly of
smooth fibres. The plexus of Auerbach is present, though
its ganglia are smaller and less numerous than those of
the rest of the alimentarv canal. There is a more^iDi^ less

CHAPTER XV. ALIMENTARY CANAL. 175

well defined fibrosa just without the external muscular
layer.

The oesophagus opens into the stomach, a saccular dila-
tion of the gilimentary canal whose wall is greatly modi-
fied, chiefly in the mucosa: thjs layer is much thjckened,
owing to the development of long tubular glands which
open upon its surface, are imbedded in it, and make up the
greater part of its substance ; it is thrown into numerous
folds, or rugae, and is covered with a simple la\'er of
columnar epithelium containing nujnerous goblet cells, the
transition between which and the stratified epithelium of
the oesophagus is abrupt : its surface is dotted all over with
the minute openings of the gastric glands previously
mentioned.

The gastric glands are distinguished by their structure
into two kinds, designated according to their position as
cardiac (also called peptic), occupying the region extend-
ing from the oesophageal opening to beyond the middle of
the stomach, and the pyloric, which occup\^ the Ipw'er
third. The former have short ducts lined with columncir
epithelium similar to that of the surface of the stomach :
the glandular portion is a long wavy or slightly coiled
tubule, whose end is sometimes strongly bent ; from two
to four such secreting tubules commonl}^ opening into a
single duct : lining the tubule throughout its whole extent
is a single layer of polyhedral cells, while scattered along
the tubule here and there between the la\'er of cells just
mentioned and the basement membrane are larger spher-
oidal or ovoidal cells. The cells first mentioned are called

UvVMi:*^VAJ^^vyUL\, «,4^M0U-tUA^ J^ Jm^^UOMU^-^J,^^^^ ithM4^i

176 PART II. HISTOLOGICAL ANATOMY.

the chief or from their position the central cells, or, from
their probable ferment-secreting function, the peptic cells:
the others are known as the accessory or parietal cells ;
they have also been called the oxyntic cells, and are sup-
posed to produce the acid of the gastric juice.

The pyloric glands differ from the cardiac in the propor-
tions of their parts, the ducts being much longer and the
tubules shorter: in the form of the secreting tubules,
which are much more contorted; and in the absence of
parietal cells. Between the region of well defined cardiac
and pyloric glands is a narrow intermediate zone of
transition in which the ducts become longer and the
tubules shorter than in the cardiac region, while the pari-
etal cells become less numerous and finally are wanting
altogether.

The interstices between the gastric glands are filled with
interstitial areolar tissue containing a rich network of
capillaries and lymphatics and a small amount of adenoid
tissue, with occasional slender bundles of smooth muscu-
lar fibres. About the bases of the secreting tubules the
adenoid tissue is more abundant, and here and there forms
small nodules with ill-defined boundaries. Below, there is
a well-developed muscularis mucosae, consisting of two
distinct layers, an inner transverse and an outer longitu-
dinal layer : the inner is to some extent subdivided into
two laminae, the fibres of one running somewhat ob-
liquely to those of the other; from its inmost surface the
slender bundles above mentioned as running up between
the gastric glands are given off.

CHAPTKR XV. ALIMENTARY CANAL. 177

The submucosa of the stomach is a rather thick layer of
areolar tissue which extends upwards into the folds of the
mucosa which form the rugae. It contains the larger
bloodvessels and lymphatics connected with the vascular
supply of the niucosa, and a well developed plexus of Meiss-
ner situated nearer to the mucosa than to the muscular
coat.

The musculosa is thicker in the stomach thqji in any
other portion of the alimentary canal, the increase in
thickness being chiefly in the inner or transverse coat.
This, in addition to being greatly increased in volume is in
the cardiac portion more or less clearly divided into two
layers lying obliquely to each other and to the longitudi-
nal coat. The plexus of Auerbach is conspicuously seen
between the two coats. On the outer surface is the serosa,
a thin fibrous membrane covered by endothelium, and ad-
hering closely to the longitudinal muscular coat except
along a narrow strip at the attachment of the mesogaster
and the omentum.

As we pass from the stomach to the duodenum the mu-
cosa becomes much thinner, the glands present in the
upper portion of the latter bemg situated in the submu-
cosa; a narrow zone of transition is seen between the
pyloric glands of the stomach and the duodenal glands, or
glands of Brunner. The mucosa, which is thrown into
folds, the valvulae conniventes, is covered with finger-like
or leaf-like projections, the villi, between which are tubu-
lar depressions, the intestinal follicles or crypts of Lie-
berkuhn : the siirface of the villi and the lining of the crypts
are alike covered with simple columnar epithelium con-

178 part; II. HISTOLOGICAL ANATOMV:

taining large numbers of goblet cells, the columnar cells
having vertically striated cajps or borders. Beneath the
epithelium of both crypts and villi is an endothelial base-
ment membrane, sometimes called the subepithelial endo-
thelium of De Bove.

The centre of a villus contains a lymphatic with a
widely opened extremity : around it are scattered bundles
of smooth muscular fibres proceeding from the muscularis
mucosae below : near the surface is a network of capil-
laries formed from the breaking up of a small artery at the
base of the villus and uniting to form a vein near its tip :
the interstices are filled with loose adenoid tissue. Between
the crypts at the base of the villi is a rich network of capil-
laries and Ij'mphatics and imbedded, like those of the villi,
in adenoid tissue, which becomes quite dense about the
bases of the crypts ; here and there well defined nodules
are found.

Immediatel}'^ beneath the layer of adenoid tissue upon
which the crypts abut is situated the muscularis mucosae,
which is not so thick as in the stomach, but presents two
distinct layers, an inner transverse and an outer longitu-
dinal.

The submucosa of the upper portion of the duodenum is
quite thick: in addition to the blood vessels and lymphatics
and the plexus of Meissner, it contains the glands of
Brunner already mentioned. These are compound tubu-
lar glands, whose short alveoli, lined with polyhedral cells
surrounding a distinct lumen, open into slender branching
ducts which pass to the centre of the gland, there to open

CHAPTER XV. ALIMENTARY CANAI,. 179

into the central duct : this, which is lined with short col-
umnar cells, penetrates the mucosa to open between the
villi. The glandular epithelium resembles that of the
pyloric glands, between which and the glands in question
there is a distinct transition at the pylorus : the epithelial
cells contain fine granules similar to those found in the
cells of glands of the serous type, which these glands re-
semble save in the non-essential feature of the larger size
of the lumen.

The transverse layer of the musculosa, as it reaches the
lower end of the stomach, becomes suddenl}^ thickened to
form a muscular ring, the pylorus or gastro-duodenal
valve: passing into the intestine it becomes as quickly
reduced in thickness, forming a la\^er much thinner than
that found in the gastric wall : a similar diminution is
found in the thickness of the longitudinal layer. The mus-
culosa of the small intestine does not otherwise differ
from that of the stomach, like which it is invested by a
serosa formed from the peritoneum.

In the lower portion of the duodenum the glands of
Brunner are wanting, and thesubmucosa becomes greatly
reduced, the canal assuming the structural character com-
mon to the greater portion of the small intestine, whose
division into the regions recognized by the anatomist is
not marked by any histological characters. The only
noteworthy feature in this respect is the specialization
seen in the quantity and disposition of the adenoid tissue
of the mucosa, which attains its highest development in
the lower portion of the small intestine, the ileum.

180 PART II. HISTOLOGICAL ANATOMY.

Throughout the whole of the small intestine adenoid tis.
sue is found in the villi and between the crypts, as in the
upper portions ot the duodenum just described : and as in
that structure, there is a continuous layer thereof between
thecrypts and the muscularis mucosae. This, however, be-
comes much thinner and less dense, while here and there
appear well defined nodules commonly known as solitary
follicles or (by the use of a term equally unfortunate) sol-
itary glands: it should of course be understood that these
bodies have functionally nothing in common with either
glands or follicles as defined on a preceding page: they are
broadly pear-shaped bodies whose bases usually extend
into the submucosa, which is locally thickened in connec-
tion therewith, and whose apices when large project as
rounded eminences upon the surface of the mucosa covered
with a single layer of columnar epithelium, both crypts
and villi being wanting at that point : in structure each is
a dense mass of adenoid tissue in the main uniformly dis-
posed, but with a slight diminution in density toward the
centre of the mass, containing a capillary network and sur-
rounded by numerous lymphatics: the nodules are sur-
rounded by the fibrous tissue of the mucosa, which is here
quite dense, but they have no definite capsule.

The solitary nodules of adenoid tissue give place to a
great extent in the lower portion of the intestine to clus-
ters of similar bodies sometimes called agminated glands,
but better known as Peyer's patches. Where these occur
the mucosa and submucosa are both thicker than in other
portions of the intestine : the nodules are closely crowded
together and in the centre of the mass become confluent :

CHAPTER XV. ALIMENTARY CANAL. 181

the conical apices of most of them project upon the sur-
face of the mucosa: the epithelium of the projections (as
is the case to some extent with that of the solitary nodules
just described) becomes infiltrated with leucocytes as in
the case of that investing the tonsils, often to such an ex-
tent as to be no longer clearly distinguishable, large num-
bers of leucocytes thus constantly entering the intestinal
cavity.

At the margin of the folds which constitute the iko-
colic valve the villi cease abruptly; and as we pass the
margin in question we come upon the wall of the large
intestine, including its greater portion, the colon, with
its anterior prolongation, the caecum (ending in the ver-
miform appendix), and its posterior continuation, the
rectum.

The mucosa of the colon, like that of the rest of the
large intestine, is devoid of villi. It is crowded through-
out its whole extent with tubular follicles resembling the
crj'pts of the small intestine, but larger and showing ordi-
narily a larger proportion of goblet cells. The interstitial
adenoid tissue is scant)', but large solitary nodules occa-
sionallv occur. The muscularis mucosae does not differ
materially from that of the small intestine.

The submucosa is a layer of moderate thickness, con-
taining the larger vessels and the plexus of Meissner. The
musculosa is well developed : the inner or circular layer is
thickened uniformly, while the outer layer is chiefly gath-
ered into three longitudinal bands. The outer surface is
covered for a portion of its extent with a serosa.

182 PART II. HISTOLOGICAL AMATOMY.

The caecum is essentially similar in structure to the
colon. The appendix vermiformis is a rudimentary struc-
ture, variable in size and development: its mucosa con-
tains numerous follicles, like those of the colon, and also
solitary nodules of adenoid tissue: its musculosa con-
sists of two layers, the outer or longitudinal differing
from that of the colon in being of uniform thickness and
thicker than the inner or circular laj^er.

As the alimentary^ canal is continued downward to form
the rectum, its structure undergoes important changes in
each of the fundamental layers, in connection with the
modification which takes place in the function of the ca-
nal: these will as before, be described in connection with
the layers successively.

The mucosa of the rectum, like that of the colon, is^e-
void of villi : it is much thicker than that of the colon, in-
creasing in thickness downwards to the anal opening. In
addition to temporary foldings, chiefly longitudinal, which
exist when the tube is empty, there are present three or
four transverse folds, semilunar in shape, known as the
valves of Houston, and near the terminus several longitu-
dinal folds, the columns of Morgagni ; their relations to
subjacent structures will be mentioned later. The epithe-
lium of the greater portions of the rectum is columnar,
like that of the small and the large intestines : just above
the anus there is in man a transition from simple columnar
to stratified squamous epithelium continuous with that of
the surface of the body : this transition takes place upon

CHAPTER XV. AI.IMENTARY CANAL. 183

the columns of Morgagni higher up than within the de-
pressions which lie between them. Follicles or crypts like
those of the colon abound in the mucosa of the greater
portions of the rectum : the\' disappear at the level of the
commencement of the columns of Morgagni. In addition
to a small amount of diffuse adenoid tissue, occasional
solitary nodules arc found : they are less numerous rela-
tively than in the colon.

The fibrous structure of the mucosa of the rectum is
denser than that of the colon, particularly toward the
lower extremity, where there is a marked increase in the
proportion of elastic fibres and a diminution in the amount
of adenoid tissue, the fibrous layer finally passing without
abrupt transition into the corium of the skin. The mus-
cularis mucosae is gradually increased in extent, and
consists chiefly of longitudinal fibres: at the lower por-
tion these are gathered together in several stout bundles,
which, with the overlying folds of the mucosa, form the
columns of Morgagni above described : these are contin-
ued into tendinous bundles which are inserted into the
skin in the immediate vicinity of the anal opening.

The submucosa of the rectum becomes considerably
thickened toward its lower extremity', in connection with
the increased mobility of the mucosa, and is in addition
reinforced b\' the presence of numerous elastic fibres : it
contains, moreover, a certain amount of adenoid tissue,
in this respect differing markedly from the same la^-er in
other portions of the canal. A distinct plexus of Meissner
is present.

184 PART 11. HISTOLOGICAL ANATOMY,

The musculosa of the rectum undergoes marked in-
crease in thickness from above downward in both the cir-
cular and the longitudinal layer. In the former, there is in
addition to the gradual increase in volume a tendency to
divide into bundles of fibres of gradually increasing size :
at its lowermost limit a number of these bundles are ag-
gregated together to form the internal sphincter muscle
which surrounds the anal opening : distinct thickenings of
the circular layer also underlie the valves of Houston. The
longitudinal layer, as it passes downward, also undergoes
subdivision, the resultant bundles of fibres diverging more
and more from the circular layer, and being roughly divisi-
ble into three strata ; an inner, the bundles of which are
interwoven with those that form the internal sphincter ; a
middle, whose bundles terminate along the boundary of
the external sphincter (an extrinsic muscular structure
composed of striped fibres) ; and an outer stratum whose
bundles unite with the levatores ani and the recto-coccy-
geal muscles.

The extrinsic glands connected with the alimentary canal
are but two in number, the pancreas and the liver, both of
them opening into the duodenal portion of the small intes-
tine, of which they are outgrowths. The pancreas is in
most respects similar in structure to a large salivary gland
of the serous type; its connective tissue framework as well
as its blood and lymph vascular supply being the same,
and the form of the alveoli similar, as well as their arrange-

4,

CHAPTER XV. ALIMKNTARY CANAL. 185

ment in lobules and lobes. The epithelium of the alveoli re-
sembles that of a serous {.^land in its j^ranular appearance,
and in the position of the nucleus; the lumen is also very
small. Certain differences are, however, to be noted. The
ajveoli are as a rule longer and more generally tubular in
form : theepithelial cells are taller and narrow^er, as a rule,
approaching to the columnar form : there have been de-
scribed under the name of centroacinar cells, certain spin-
dle-shaped elements occupying the lumen near the union
of the alveolus with the intermediate tubule; their origin
and function are alike uncertain. The intermediate tubules,
with their characteristic flattened epithelium, are more
extensive than in the salivary glands, replacing in great
measure the intralobular ducts. The pancreas is also
characterized by the presence between the alveoli of pecu-
liar groups or clusters of intertubular cells which form
nodular masses situated in the intralobular connective
tissue or the interlobular septa; each mass having a defi-
nite tuft of capillaries : these bodies whose nature is not
clearly understood, are by some regarded as newly formed
or embryonic alveoli.

The liver is at once the largest and the most highly
modified of all the digestive glands. While it must be re-
garded as formed by the modification of a primarily tubu-
lar structure, the multiplication of the glandular epithe-
lial cells has obliterated the lumen of the tubule in each
instance, the secretion formed by the epithelium being dis-
charged throughtheagency of intercellular channels which
are unlike anything found in any other portion of the
human body. This extreme modification in the arrange-

186 PART n. HISTOLOGICAL ANATOMY.

ment of the glandular epithelium is also accompanied by
an equally great modification in the arrangement of the
blood vessels, the capillary network apparently penetrat-
ing the epithelial masses ; thus forming what at first sight
seems to be an exception to the general rule that epithelia
are devoid of blood vessels. The structure of the liver will
therefore best be understood if we begin by examining the
blood supply and the framework of skeletal tissue with
which it is associated.

The surface of the liver is invested by a thin capsule of
fibrous tissue, surmounted by the peritoneal endothelium.
From this capsule thin lamellae penetrate the surface of
the gland as interlobular septa : in the transverse fissure
it becomes continuous at the porta (which corresponds to
the hilum of an ordinary gland) with an important in-
growth of connective tissue, the so-called capsule of Glis-
son, which forms by far the larger part of the interlobular
tissue, becoming continuous with the ingrowths from the
capsule. The porta is the place of entrance for the nerve-
supply of the liver, and for the large portal vein and the
smaller hepatic artery ; as well as of exit for the bile
duct and for the principal lymphatic trunk : these four
vessels and their ramifications, together with strands of
nerve-fibres, are usually found in close proximity, sur-
rounded by a certain amount of connective tissue, the
whole constituting what is known as a portal canal.
The blood leaves the liver by means of the hepatic veins,
whose branches are formed by the union of the sublobular
veins, the origin of which will be presently described : the
sublobular and hepatic veins are not accompanied in their

CHAPTER XV. ALIMENTARY CANAL. 187

course by branches of other vessels, a character by means
of which they can be readily distinguished from the
branches of the portal vein.

The lobules of the liver, sometimes (but less correctly)
termed the acini or alveoli, arc masses of hepatic cells,
penetrated by capillary networks, polyhedral in form as
the result of pressure, and in man something over a milli-
metre in diameter. Those at the surface of the liver have
the axis vertical or nearly so to that surface: but there is no
such regularity of position in the case of those more
deeply situated. The interlobular septa are in a few mam-
mals (of which the pig is one) quite stout and conspicuous :
in man they are far less developed ; and the boundaries of
the lobules are in consequence not always readily deter-
mined. From certain points on the septa a very scanty
framework of connective tissue traverses the interior of
the lobule, accompanying the capillaries: its presence even
in small quantity is a fact of importance in forming a clear
conception of the essential structure of the liver.

The branches of the portal yein end in veinlets situated
in the septa and hence known as interlobular veins, each
lobule having several surrounding it : these give off capil-
laries (in a manner similar to arteries elsewhere) which
form a network whose meshes converge toward the axis
of the lobule : this is occupied by a single vessel, the cen-
tral or intralobular vein, into which the capillaries
empty. The intralobular vein passes at the base of the
lobule into a sublobular vein, the hepatic veins being as
has been stated, formed by the confluence of the sublobu-
lar veins.

188 PART II. HISTOLOGICAL ANATOMY.

The branches of the hepatic artery accompany those of
the portal vein to the lobules, where they divide into inter-
lobular arterioles situated in the septa, together with the
interlobular veins: like the latter, they terminate in capil-
laries ; these are, however, situated chiefly' in the connec-
tive tissue of the septa and the walls of the veins, a por^
tion of them only penetrating the lobules for a sufficient
distance to form channels of communication with the
capillary network of the lobule, as a means of discharge
of the blood from the arterioles. The portal vein and its
final branches, the interlobular veins, are therefore to be
resfarded as the channels bv means of which the blood is
submitted to the glandular action of the hepatic cells ; the
hepatic artery being probably in large part, if not alto-
orether the channel of nutrition for the skeletal framework
and vascular mechanism of the organ.

The network of capillaries with its accompanying scanty'
connective tissue forming, according to some observers,
perivascular lymph channels, is interwoven with another
and somewhat coarser network, that of the strands of
liver cells or hepatic cords. These are polj'hedral epithelial
cells about twentj^ micra in average diameter, with granu-
lar protoplasm and central spherical nuclei, which perform
the complex secretor}^ function of the liver. The size of
these cords are such that nearly every cell is brought at
some point in contact with a capillary ; the surface of the
cell being in many cases slightly excavated along the line
of contact.

Between the hepatic cells is found a network of minute
passages usually not more than a micron in diameter, the

CHAPTER XV. ALIMENTARY CANAL. 189

so-called bile capillaries or, as they are otherwise termed
the bile canaliculi : these are so situated that they tra-
verse the surface of the cell along a side or an angle oppo-
site that in contact with the blood capillaries, never pass-
ing loetw^een the latter and the cells. At the surface of the
lobule the bile canaliculi communicate with small inter-
lobular bile ducts whose flattened epithelium is continu-
ous with the hepatic cells, and whose basement membrane
is resolved into the scant}* connectiv'e tissue of the lobule.
These unite as they pass to the portal canals to form the
smaller bile ducts situated therein ; the epithelium becom-
ing columnar and the basement membrane stouter. As
the smaller bile ducts come together to form the larger
trunks found in the more central region of the liver the
cells of the columnar epithelium become taller and are
seen to rest upon a distinct membrana propria: while the
fibrous layer beneath is reinforced by smooth muscular
fibres: the largest ducts have in addition srnall mucous
glands in their walls. The structure of the duct leading
to the intestine, and of the gall bladder as well, is essen-
tially the same as that of the largest ducts within the
liver.

If we recall the structure of a gland of the serous type it
will be remembered that the epithelial cells which line the
alveoli are so large as to reduce the lumen to a very slen-
der and often imperceptible channel between their apices:
the bile canaliculi may with propriety be compared to
such channels. The alveoli of ordinary glands may be
either spheroidal or elongated and tubular, the interstices
between them containing a variable amount of connective

190 PART II. HISTOLOGICAL ANATOMY.

tissue which supports a capillary network, whose meshes
conform to the structure and arrangement of the alveoli.
In the lower vertebrates the liver consists of tubular alve-
oli essentially similar to those of other glands. If we con-
ceive of such tubules in the mammalian liver as becoming
elongated and branched, and finally anastomosing, we
shall understand the structure and arrangement of the
hepatic cords. According to this view the anastomoses
formed obliterate the boundaries of the alveoli ; but we
may regard as belonging to one such division all the bile
canaliculi with their surrounding epithelium which lie
nearest to and presumably discharge into one of the inter-
lobular bile ducts : the beginnings of the ducts, with their
flattened epithelium, corresponding in position and struc-
ture with intermediate tubules or intercalary ducts of
other glands : the lobule is thus seen to be in fact as well
as in name a lobule ; that is, a collection of acini or alveoli ;
and the application of either of the latter titles to it is
misleading. The most peculiar features in the structure of
the liver are those connected with the blood supply.

CrtAPTfiR XVI. RESPIRATORY APPARATUS. 191

CHAPTER XVI.
THE RESPIRATORY .APP.VrATUS.

In the description of the alimentary canal which has
just been given, the conspicuous layers, whose modifica-
tions in the various regions have been discussed, are those
most readily demonstrable by the knife of the anatomist:
they are those, moreover, which naturally result from the
double function of the greater part of the canal, the inner
layer, or mucosa, being in substance a glandular layer by
means of which the nutritious portion of the food is
brought into a condition suitable for the accompanying
process of absorption; the outer layer, or musculosa,
being in effect a mechanism by which the food mass is
caused to traverse this glandular surface; while the sub-
mucosa makes possible the necessary movements of the
mucosa and the musculosa on each other. We should,
however, bear in mind the continuity of the submucosa
with the fibrous structure of the mucosa on the one hand,
and on the other with the interstitial connective tissue of
the musculosa, and through it with the fibrosa or serosa:
considering this, and bearing in mind the embryonic origin
of the tissues present, it will be quite clear that another
very natural division of the wall of the alimentary canal,
based on histological rather than anatomical characters,
would recognize two primary la\"ers of tissues (differing

192 PART II. HISTOLOGICAL AN'ATOMY.

vastly in extent), the epithelial layer, hypoblastic in its
origin; and the musculo-skeletal (and vascular) layer de-
rived from the mesoblast, extending from the basement
membrane, just beneath the epithelium w^ithin, to the
fibrosa (or, in the intestine, to the fibrous basis of the
serosa) without: the presence of this fibrous layer as one
of the constituents of the alimentary canal should be
clearh' borne in mind.

It is by an outgrov^'th from the phar3'ngeal region of
the alimentary canal that the respiratory apparatus is
formed, and its structure throughout its whole extent is de-
rived from such modifications of a musculo-skeletal layer
lined with epithelium as are required for the proper per-
formance of the functions of the various regions. These
are in the trachea and bronchial tubes such as will serve to
maintain the patency of these channels for the passage of
the air and in some measure to regulate the same: and in
the airsacs such as will permit the freest interchange of
gases between the air which they contain and the capil-
laries within their walls.

The trachea is plainly seen to be composed of three
well-defined layers, similarly disposed to those which ap-
pear as the chief factors of the alimentary canal. The
inner of these is the mucosa; it differs in some important
respects from the layer of that name in the region just
mentioned. It is lined with w^hat is usually termed a
stratified columnar ciliated epithelium: the layer is but
a few cells deep ; those at the surface are cylindrical or
prismatic, with tapering bases, their free extremities in
most cases beset with numerous fine cilia, whose united

ClIAPTHR XVI. RHSI'IKATOKV APPARATUS. 193

action causes constant currents toward the mouth; here
and there g^ohlet cells occur; beneath the superficial cells,
or intcrsjjersed between their bcises, are elongated and
more or less spindle-shaped cells, which are probably des-
tined to replace them ; still lower are pyriform and spher-
oidal cells, many of which multiply rapidly by cell-divi-
sion, replacing the older superficial cells as occasion re-
quires, resembling thus in function the deepest portion of
the stratified stiuamous epithelium of the oesophagus.

The epithelium of the trachea rests upon a basement
membrane, a homogeneous, elastic layer of considerable
thickness, perforated by occasional fine canals. This is
followed abruptly by a fibrous layer, whose bundles are
irregularly and somewhat loosely disposed and are inter-
mintjled with a considerable number of elastic fibres:
there is a well but not greatly developed network of small
blood vessels, accompanied by nerves and lymphatics, and
a considerable amount of adenoid infiltration. Beneath
the fibrous layer, and continuous therewith, is the elastic
layer, composed of a dense network of elastic fibres, most
of which are longitudinally disposed, which form the inner
boundary of the mucosa : it is best developed in the dorsal
portion of the trachea.

The submucosa. like that of the alimentary canal, is a
layer of areolar tissue serving to unite the mucosa with
the denser layer beyond it. In addition to the larger
blood vessels from which the blood supply of the mucosa
is derived, and their associated lymphatics, it contains
numerous small glands ol the mucous type, whose long
ducts, lined with cuboidal epithelium, traverse the mucosa

194 PART II. HISTOLOGICAL ANATOMY.

to open on its surface. These glands are, as a rule, most
abundant in those portions of the submucosa which are
situated opposite the intervals between the successive
fibrous rings of the trachea. Here and there in the sub-
mucosa an occasional adenoid nodule may be found, fre-
quently associated with a gland or its duct.

No single word will readily indicate the complex struc-
ture of the outer layer of the trachea : if we for conven-
ience make use of the terra fibrosa in referring to it, we
shall designate it by its most constant though not most
conspicuous factor. It is in effect a tube of d^nse fibrous
membrane, rich in elastic fibres, reinforced at regular in-
tervals by the incomplete rings of cartilage which are the
most prominent components of the trachea, and bearing
on the inner su_rface of its dorsal portion the rudiments
of a muscular layer. The rings are composed of hyaline
cartilage w^hich is far less brittle than usual: they are
imbedded in the fibrous layer, which is continuous with
the perichondrium of each, and situated rather toward its
inner than its outer limit. The muscular layer occupies an
elongated rectangular area nearly identical with that
bounded b}^ the lines connecting the dorsal ends of the in-
complete cartilaginous rings, but somewhat more exten-
sive laterally : to the band of muscular tissue thus formed,
the name of the tracheal muscle is given. It consists en-
tirely of smooth muscular fibres; the great majority of
these are arranged in transverse bundles disposed in groups
which are separated by occasional transverse septa of
fibrous tissue ; external to these are a few thin and short
longitudinal bundles forming an imperfectly defined layer:

CHAPTER XVI. RESPIRATORY APPARATUS. 195

the muscular laver is invested outwardly l\v the fibrous
laver which it in some measure replaces, the latter being
here much thinner than in other portions of the trachea.
The bundles of the trans verse la vera re inserted bet ween the
rings upon the fibrous layer; those opposite the rings are
in man inserted on their inner surface; in some of the lower
mammals thev are inserted on the ends and in others on
the outer surface of the rings. The longitudinal bundles
are inserted upon the ends of the rings and upon the fib-
rous layer. It is a noteworthy fact that in the dorsal por-
tions of the trachea some of the mucous glands are situ-
ated external to the muscular layer, their ducts penetrat-
ing it as well as the other layers of tissue beneath the epi-
thelium.

The bronchi, or right and left divisions of the trachea,
while they differ from it anatomically, particularly in the
form of the cartilages, resemble it in all essential respects
as regards their histological structure. As the}- enter the
lungs they branch and subdivide repeatedly, their imme-
diate continuations and their subdivisions, with the ex-
ception of the smallest, being known as the bronchial
tubes, or, as they are sometimes termed, the intrapulmon-
ary bronchi: by the time they are reduced to a diameter
of one millimetre, or thereabouts, they are known from
their size and structure as bronchioles; the smallest of
these are never less than half a millimetre in diameter.

The largest bronchial tubes are essentially like the
extra-pulmonary bronchi in structure : as we pass toward
the bronchioles, their structure undergoes marked though

196 PART 11. HISTOLOGICAL ANATOMY.

gradual modification: that of a tube of medium size may be
described as follows. The epithelium, like that of the
trachea and larger bronchi, is stratified, columnar and
ciliated, and rests on a homogeneous basement membrane.
Beneath this is a fibrous layer containing a moderate
amount of adenoid tissue, and numerous elastic fibres no
longer forming a continuous layer, but gathered into
strands, which form the bases of the folds into which the
inner portion of the mucosa is frequently thrown : exter-
nal to the adeno-fibrous layer is a well-defined muscularis
mucosae, composed of smooth fibres transversely dis-
posed, this layer, which is of variable thickness from
point to point along the tube, is sometimes designated
the bronchial muscle.

External to the muscular layer is the submucosa, com-
posed here, as in the trachea and bronchi, of areolar tis-
sue containing rnucous glands and plexuses. QJLblood and
lymph vessels. The outermost layer, that which we have
for convenience termed the fibrosa, is the one which first
shows signs of reduction: the fibrous membrane is by no
means as dense, nor as clearly defined, either from the sub-
mucosa within or from the adjacent tissues without, as in
the trachea; the cartilaginous rings of the latter are rep-
resented by plates of cartilage of varying size and shape;
and there is nothing present in this laj^er that may be re-
garded as corresponding structurally to the tracheal mus-
cle. Occasional lobules of fat may be seen, and the mucous
glnnds not unfrequently penetrate into this layer, which
thus tends to approach in its structure that of the sub-
mucosa.

CHAPTER XVI. UKSIMKATOKY APPARATUS. 197

As we ])ass toward the smaller bronchial tubes the epi-
thelium becomes gradually reduced to a single layer ot"
columnar ciliated cells resting upon a basement membrane.
The fibrous layer beneath becomes much thin^ner; the rela-
tive amount of adenoid tissue becomes less and less, and
elastic fibres become far less numerous, though they do
not altogether disappear. The muscular layer, on the
other hand, for a time at least, increases in relative thick-
ness, becoming one of the niosc conspicuous features of
small tubes. The submucosa and fibrosa become blended
into one layer ^f loose fibrous tissue rich in lymphatics
and containing blood vessels, the mucous glands djsap-
pearing from the former, together with the cartilaginous
plcites (and the nodules which succeed them) from the lat-
ter.

Within the bronchioles (otherwise designated the term-
inal bronchi) still further reductions of structure take
place: the epkhelium changes from columnar to cubical,
loses its cilia, and later becomes more or less flattened,
forming a single layer of polyhedral granular cells upon
the basement membrane. The latter rests on a thin layer
of fibrous tissue with longitudinal elastic fibres: the muscu-
lar layer is reduced to scattered bundles and later to iso-
lated fibres, without finally disappearing altogether; while
the submucosa and the fibrosa become blended with each
other and with the fibrous layer of the mucosa.

The bronchioles lead into larger pyramidal or irregular
shaped spaces, the infundibula, into each of which open
by wide apertures a large number of the spheroidal or

198 PART II. HISTOLOGICAL ANATOMY.

polyhedral air-sacs or alveoli which are the ultimate
chambers of the lung: each infundibulum with its associ-
ated alveoli making up one of the component lobules
of that organ. The principal change in passing from the
bronchiole to the infundibulum is found in the epithelial
layer: the low granular cells with which the distal extrem-
ity of the bronchiole is lined are found in the infundibulum
in patches which become smaller and less numerous as we
proceed to its farther extremity: between them are found
larger and thinner transparent cells which form a sirnple
squamous epithelium to which the distinctive title of res-
piratory epithelium is applied . The basement membrane,
the longitudinal network of elastic fibres, and the layer
of scattered muscular fibres are continued without essen-
tial change. The fibrous portion of the wall of the bron-
chiole is represented by scattered branched connective tis-
sue corpuscles situated in the interstices between the elas-
tic and muscular fibres.

The alveoli are in the main continuations of the walls of
the infundibulum. The epithelium consists almost entirely
of the large flat cells above mentioned, the smaller granu-
lar cells being scattered sparingly among them either singly
or in groups of two or three: between the cells, and in par-
ticular at the angle where three or four meet, are occasional
stomata, minute openings which communicate with the
lymph spaces below. About the mouth of each alveolus
there is an annular bundle of elastic fibres from which is
given off a network, which, together with a small amount
of fibrous tissue and a few connective tissue corpuscles,
forms the wall of the alveolus and the support of the epi-

CHAPTER XVI. RESPIRATORY APPARATUS, 199

theliuni and capillary network: from the form and dispo-
sition of the alveoli it results that a single layer thus
formed does duty in jj;^reat measure for two adjacent alveoli.
The capillary network contained in the interalveolar
septum thus formed isexceedin«i^ly dense; and itsloops pass
from side to side of the septum in a serpentine course, thus
l)rin<i^ing the blood contained within them as near as pos-
sible to the air in each ot the alveoli.

The lymph spaces in the alveolar walls communicate
with the lymphatics situated in the connective tissue
septa which lie between the lobules. Each lobule, as thus
bounded, is irregularly pyramidal in form, its apex being
situated at the bronchiole, toward which the limiting
septa tend : peripherally, the interlobular septa are con-
tinuous with the denser laj'ers of fibrous tissue which
form the investment of the lobes: the latter, in their turn,
being continuous with the pleura which forms the serous
investment of the surface of the whole lung.

The larynx is a special modification of the proximal end
of the trachea: it differs in its histological structure from
the latter in the following particulars. Its epithelium is
in most portions stratified, columnar and ciljated, as in
the trachea : that of the txue vocal cords and of the epi-
glottis and a portion of the intervening surface is strati-
fied squamous: in that covering the under surface of the
epiglottis numerous taste buds are imbedded. The mucosa
differs from that of the trachea, chiefly in the greater
amount of adenoid tissue, in places resembling the phar-
ynx in this respect : the elastic fibres which run through-
out its extent are greatly increased in number in the vocal

200 PART II. HISTOLOGICAL ANATOMY.

cords, where they constitute the chief part of the mucosa.
The principal cartilages are hyaline, like those of the
trachea: those of Santorini, of Wrisberg and of Luschka
are reticular, as is the epiglottis. The proper muscles of
the larynx are composed of striped fibres, resembling in
this respect the musculosa of the upper portion of the
oesophagus.

The course and arrangement of the blood vessels of the
lung are subjects rather for anatomical than histological
discussion. It should be borne in mind, however, in study-
ing sections of that organ, that the lung has a double
blood supply : the branches of the pulmonary artery ac-
companying the bronchial tubes to their common destina-
tion in the lobules of the lung; while the smaller bronchial
arteries, derived from the aorta, are distributed to the
walls of the air passages themselves and to the surround-
ing structures, as vessels of nutrient supply. A similar re-
lation subsists between the bronchial veins, which empty
into the ascending vena cava, and the branches of the
pulmonary veins.

The lymphatics of the lung are divisible in a^somewhat
similar manner into two groups, those associated with the
bronchial tubes, and those associated with the air sacs.
The former, or bronchial lymphatics, have their origin in
the lymph spaces of the bronchial mucosa, forming a plexus
in the submucosa which empties into larger trunks lead-
ing to the root of the lung. The second group is compound
of two sets, the deep or vascular lymphatics, having
their origin in the connective tissue of most of the lobules
of the lung, and the superficial or, as they are also termed,

CHAPTER XVI. RESPIRATORY APPARATUS. 201

the subpleural lymphatics, which arise in the vicinity of
the lobules near the surface and enter into the plexus which
underlies the pleura: the latter communicates with the
thoracic cavity bv means of occasional stomata. The
deep and superficial plexus alike empty into trunks which
lead to the root of the lung, there to enter, in company
with those from the bronchial lymphatics, into the bron-
chial lymph nodes.

It is a common custom to speak of the lung as essentially
similar in structure to a gland, the alveoli beingcompared
with the similarly named divisions of the latter, theinfun-
dibula and bronchioles to the ductules and intralobular
ducts, and the bronchial tubes to the larger ducts. Com-
parative anatom}-^ shows that the primary condition of
the lung is that of a large sac-like c»utgrowth of the ali-
mentary canal, its unquestionable homologue in the fish-
like vertebrates being the swim-bladder. In some of these
forms it undergoes more or less subdivision b\' foldings of
its inner surface, and assumes something of a respiratory
function; its connection with the alimentary canal remain-
ing simple and entirely membranous: in the lower air-
breathing vertebrates the area of respiratory surface is
increased to a limited extent only by peripheral sacculation,
while a rudimentary trachea and larynx appears: it is
only in the higher reptiles, the birds, and the mammals,
that the lung assumes the compact and spongy structure
due to compound sacculation and associated with the pres-
ence of a well developed system of bronchial tubes. Its
origin is therefore seen to be the reverse of that of a large

202 PART II. HISTOLOGICAL ANATOMY.

gland, which is developed by an increase in the number of
origin alh' small alveoli.

While the respiratory tract is to be regarded as a sub-
divided saccular outgrowth of the alimentarj' canal, we
must not expect to be able to recognize essential similari-
ty of detailed structure in organs differing markedly in
function : a general comparison may, however, aid in
understanding and remembering the character and ar-
rangement of the tissues present. The epithelium is, of
course, continuous throughout. The mucous membrane
of the alimentary canal, with its varying amount of ade-
noid tissue and its subjacent muscular layer, is represented
in the air-passages by the adeno-fibro-elastic layer, to
which in the bronchial tubes amuscularismucosaeis added.
The submucosa of one is continued into that of the other
without essential modification. The outer layer of the
larger air-passages may best be regarded as corresponding
to the fibrosa of the pharynx and oesophagus, reinforced
by the tracheal and bronchial cartilages. The musculosa
of the alimentary canal is represented only by the muscu-
lar area of the trachea with its well developed transverse
and rudimentary longitudinal layer.

CHAPTER XVII. URINARY ORGANS. 203

CHAPTER XVII.
THE URINARY ORGANS.

The urinary organs include the glandular kidneys; their
ducts, the ureters; the bladder, to which the latter lead;
and the urethra, through which the contents of the blad-
der are discharged. The urethra of the female is strictly
a urinary tract, and is throughout its entire extent a por-
tion of the ventral outgrowth of the alimentary canal
from a part of which the bladder is eventually formed : it
will therefore be described in this connection. The pro-
static portion of the male urethra includes all that part of
the tract homologous with the urethra of the female; its
remaining portion is a common channel for the urine and
the seminal fluid, and is formed in connection with struc-
tures accessory to reproduction : the description of its
whole extent will therefore be deferred until it can be
taken up in connection with them in a subsequent chapter.

If a kidney be cut through from the convex surface to the
hilum, either longitudinally or transversely, the surface
of the cut section shows to the unaided eye certain feat-
ures of importance to the study of its histology. The solid
portion is seen to be curved around a central cavity, the
pelvis of the kidney; into this project a number (varying
with the direction of the section) of conical papillae, each
of which is surrounded by a cup-like extension of the pel-

204 PART II. HISTOLOGICAL ANATOMY.

vie cavit}^ termed a calyx. The solid portion itself can be
readily seen to be divided with considerable sharpness into
an outer region, the cortex, forming about one third of
the depth, and an inner, the medulla ; the latter can be
again distinctly subdivided into the boundary layer next
the cortex, and forming about one fourth of the entire
depth, and the papillary portion, v^hich includes the re-
mainder. The papillary portion is distinctly but uni-
formly striated : in the boundary layer radial tracts, termed
medullary rays, similar in appearance to those of the
papillary substance alternate with tracts characterized by
increased transparency.

The entire mass of radiating tracts extending from the
apex of a papilla to the meeting of the cortex and the
boundary layer constitutes what is known as a pyramid
of Malpighi; there are rarely less than ten or more than
twent}^ of them in a single human kidney : in some of the
lower mammals they are numerous, as in man ; in others
there is a single papillary ridge which projects into the pel-
vis along its length, or a single large central papilla: kid-
neys such as the latter are termed umpyramidal. If the
pyramids of Malpighi be conceived of as extended across
the cortex, the kidney would be divided into a correspond-
ing number of parts commonly called lobules : if, how-
ever, we are to compare these glands with others on the
basis of the arrangement of their ducts, these regions will
with greater propriety be termed lobes: in the human kid-
ney they are not structurally separated, but in that of some
mammals, as the otter, each is invested by a capsule of its
own, and is but slightly attached to its fellows : a condi"

CHAPTER XVII. I'KINARV ORGANS. 205

tion characteristic of the human kidney during foetal life.
In all multipyramidal kidneys, like that of man, the corti-
cal portion of each lobe extends around and beyond the
base_oLthe pyramid toward the pelvis of the kidney : these
interpyramidal masses, composed in part of portions of
the cortical substance of the adjacent lobules, and in part
of blood vessels whose origin and course will be presently
described, are known as the columns of Bertin.

The medullary rays seen in the boundary layer are less
readily traceable in the cortex : they are, however, contin-
ued into that layer, growing smaller as they approach the
outer surface, and disappearing altogether before reaching
it. These prolongations of the medullary rays are known
as the pyramids of Ferrein: their bases, as will be readily
understood, rest on the bases of the pyramids of Mal-
pighi, each of which gives rise tg a large number of them.
The intervening portions of the cortex make up what is
known as the labyrinth: -a definite portion thereof, not
marked off bj' any visible separation, is structurally con-
tinuous with each pyramid of Ferrein. Each pyramid with
its associated portion of the labyrinth, and its continua-
tion b}' means of the medullar}' ray to the apex of the
p\'ramid of Malpighi, may, as we shall see, be with pro-
priety regarded as corresponding in extent to a lobule of
other glands. Scattered throughout the labyrinth are
small rounded bodies, the Malpighian corpuscles, barely
visible to the naked eye. Beyond the cortex maybe readily
seen the tough fibrous capsule which invests the whole
gland.

The kidney, like the liver, is characterized by the posses-

206 PART II. HISTOLOGICAL ANATOMY,

sion of a specially modified blood supply, the most impor-
tant features of which are visible to the naked eye or with
a low power of the microscope. If a section be made as
above indicated through a well injected organ the follow-
ing facts may be noted. The renal artery, entering at the
hilum, divides into four or five branches which traverse the
lining of the pelvis, their subdivisions entering the columns
of Bertin. As they pass along the latter they give off twigs
to the cortical substance present, and on reaching the level
of the bases of the pyramids of Malpighi give rise to an
arched plexus through whose meshes the pyramids of Fer-
rein pass to the cortex. From the arcuate arteries of this
region branches are given off which run to the surface of
the kidney, known as the radiate or (from their position
between the cortical areas surrounding the pyramids of
Ferrein) the interlobular arteries: from these are given
off twigs which supply the cortical substance in a manner
presently to be described : their extremities terminate in
the capillary network of the capsule of the kidney. From
the arcuate arteries also arise near the point of origin of the
interlobular branches slender vessels, the arteriae rectae,
which supply the medullary portion, passing directly
toward the apex of the papilla.

There may also be seen in the cortical portion of the kid-
ney, and receiving twigs which proceed from its substance,
radiate or interlobular veins situated in close proximit}^
to the arteries of that name. They arise just beneath the
the capsule by small vessels having a stellate arrangement
(the stellules of Verheyen), and traverse the cortex to enter
into arcuate veins in the main similarly disposed to the

CHAPER XVII. IRINARY ORGANS. 207

arteries of the boundary region ; they resemble the latter
in receiving vena rectae from the medullary portion, and
unite to form trunks which pass by way of the columns of
Bertin to traverse the lining of the pelvis and come together
at or near the hilum to form the renal vein. The vasa
recta pass toward the papillae in groups which alternate
in the boundary layer with the medullary rays, forming
the tracts of greater transparency above mentioned.

The kidney is a compound tubular gland made up of
lobes and lobules which, as We have seen, are not sharph-
defined from each other by fibrous septa. Its component
tubules differ in a marked degree from those of any other
gland in the definiteness of their course; the variation in
size and in the character of the epithelium of the different
regions of each ; and in the peculiar relations which they
sustain to their blood supply. With the exception of the
blood-vessels and their accompanying lymphatics and
nerves, together with the small amount of connective tis-
sue in which these are imbedded, and of a very scanty inter-
stitial tissue, the whole substance of the kidney, both cor-
tical and medullary, is made up of these tubules.

Each uriniferous tubule is made up throughout its
whole extent of a homogeneous basement membrane lined
with a single layei of epithelium. It has its origin in a
Malpighian bod\', the extremity of the tubule being there
expanded to form a thin walled sac whose distal portion
is inverted into the proximal, forming a capsule of Bow-
man: into the cavity of invagination thus formed is thrust
a spheroidal mass of capillaries termed a glomerulus: its

208 PART n. HISTOLOGICAL ANATOMY.

relations will be described later. The capsule is lined
throughout with a sirnple^quamous epithelium ; and with
the accompanying glomerulus makes up the Malpighian
bod\'. The tubule leaves the capsule at a point opposite
the inversion for the glomerulus bv a constricted neck
lined with small cuboidaljoells, expanding at once to
form the proximal convoluted tubule : this is Hned with
cuboidal cells, whose outhnes are irregular and interlock
in such manner as to render it almost impossible to
distinguish the boundaries of adjacent cells when seen
in longitudinal section; the portion qf^thecell next the
b§S£m£Dt membrane is vertically striated, and is, under
the influence of certain reagents, separable into rod-
like processes ; the free surface bears cilia-like processes
projecting into the lumen. The tubule passes by sweep-
ing curves toward the p\'ramid of Ferrein : as it enters it
it turns toward the boundary layer in wavy curves as the
spiral tubule of Schachowa : its structure is essentially
like that of the convoluted tubule, the epithelium being
distinctly "rodded."

Entering the boundary layer, the tubule suddenly be-
comes much smaller and nearly straight in its course
toward the papilla : it is here known as the descending
tubule of Henle: this is the smallest portion of the whole
tubule; its epithelium has the formof plate-like cells, whose
nuclei cause a central thickening which projects into the
conspicuous lumen, making its course apparently irregu-
lar. Shortly after reaching the papillary portion the tubule
bends upward, forming Henle's loop, the diameter in-
creases slightly, and the epithelium assumes the form of

CHAPTER XVn. I'KINAKY ORGAN'S. 209

low poU'hedral cells, greatly reducin<^ the lumen. From
the loop the ascending tubule of Henle passes upward
through the boundary layer without essential modifica-
tion, save a slight increase in diameter, as a straight or
slightly wavy tubule.

The ascending tubule enters the cortex and travels for a
longer or shorter distance in the pyramid of Ferrein, finally
bending abiniptly to enter the labyrinth as an irregular
tubule, whose size and the position of whose lumen varies
greatlv in accordance with irregular variations in the
height of the striated epithelium, the lumen remaining mi-
nute throughout its whole extent : it pursues a zigzagcourse
toward the Malpighian body whence the tubule arose. In
the vicinity of the latter it resumes a nearly uniform dia-
meter and exchanges its angular course for regular curves:
it is now known as the distal convoluted tubule : in size,
form, and structure, it is identical with the proximal. Like
the latter, its course tends toward the pj'ramid of Ferrein;
as it proceeds, it narrows into a junctional tubule, or, as
it is also called, an arched collecting tubule, with low
cuboidal epithelium and a relativeh' large lumen. It passes
toward the axis of the p^'ramid to enter a straight col-
lecting tubule somewhat larger, but otherwise similar in
structure, which may also receive other junctional tubules
from point to point as it passes toward the boundary
layer.

The straight collecting tubule passes along the medul-
lary ray through the boundary layer unchanged in form
or size : after it enters the papillary portion it joins at an
acute angle with similar tubules, the resultant tubules be-

210 PART II. HISTOLOGICAL ANATOMY.

coming larger with each union : the largest (and terminal)
tubes so formed, known as the ducts of Bellini, are lined
with a simple columnar epithelium: their openings, visible
to the naked eye, are scattered over the apices of the pa-
pillae.

The interlobular arteries, as has been previously stated,
pass from the boundary to end in the capillaries of thecor-
tex. Along their course through the cortex they give off
on every side short arterioles which go to the neighbor-
ing Malpighian bodies: these, which are known as the vasa
afiferentia, on reaching these bodies give rise to the sphe-
roidal masses of capillaries called glomeruli, the arteriole
entering the glomerulus at a point opposite the neck of
the capsule : the capillaries unite again to form small ves-
sels, the vasa efferentia, which leave the glomeruli where
the afferent vessels enter.

The efferent vessels, it will be seen, sustain to the glo-
meruli the relation of veins : they do not, however, proceed
to the interlobular veins, but divide shortly after leaving
the glomeruli to give rise to the cortical network of capil-
laries, which is made up throughout the labyrinth of
short irregular meshes interwoven with the convoluted
and irregular tubules, throughout the pyramids of Fer-
rein of long meshes running in the direction of the pyra-
mid. The capillary network is not distinguishable by any
known means into areas corresponding to the Malpighian
bodies, the individual tubules, the lobules, or even the lobes
of the kidney, being apparently continuous throughout
the cortex of the whole kidney.

CHAPER XYII. I RINARV ORGANS. 211

Here and there in the caj)illary network the radicles of
small veins are formed, chiefly in the vicinity of the inter-
lobular veins, to which they lead. The veins in question,
arising from the stellate veins just beneath the capsule,
gather in their course the corticid veinlets and pass to the
arcuate veins of the plexus lying in the boundary. They
thus form the channels of return for the blood which leaves
the interlobular arteries by way of the vasa afferentia,
flowing, as should be noted, through two sets of capil-
laries, those of the glomeruli and those of the cortex, as
well as though the intervening vasa efferentia.

The vasa recta, as has been stated, leave the arteries and
veins of the boundary in close proximity to the interlobu-
lar vessels, in some cases springing as branches from their
bases. They pass in the transparent striae of the boundary
layer to the papilla, the arteries being resolved along the
way into the medullary capillaries, which form a net-
work with greatly elongated meshes running in the direc-
tion of the tubules between which they chiefly lie : in the
tip of the papilla the meshes are much shorter, forming a
denser network about the ducts of Bellini. The capillaries
are gathered up into the radicles of the veins, which lie in
close proximity to the arteries. Occasionally the small
vessels which arise from that portion of the cortical net-
work of capillaries nearest the boundary' la\er, instead
of entering the interlobular veins, pass downward into
the medulla: these, which are known as false vasa recta,
sooner or later divide again into capillaries which enter
into the medullary network. The vasa recta divide the
medulla into regions corresponding to the lobules.

212 PART II. HISTOLOGICAL AXATOMY.

The capsule of the kidne}- is a thin but tough fibrous
membrane, somewhat lamellated in structure, especiall}^
toward the outer surface, and having in its deep portion a
scant}^ plexus of smooth muscular fibres. Atthehilumthe
capsule is continuous with the fibrosa of the ureter. Its sur-
face is invested with a small amount of adventitious areo-
lar tissue which in some cases contains numerous fat
lobules, and by which it is connected ventrally with the
fibrous layer ofthe peritoneum, and dorsally with the fascia
of the adjacent muscles. A plexus of lymphatics has
been described in the capsule whose channels connect with
spaces in the superficial portion of the cortex : it unites at
the hilum with lymphatics which accompany the blood
vessels along and from the medullary rays.

The lining which invests the pelvis of the kidney and its
prolongations, the calyces, may be regarded as the con-
tinuation of the inner coats of the ureter, and may, like
the wall of the latter, be regarded as consisting of an epi-
thelial and a musculo-skeletal layer. The epithelium is of
the transitional type, which is found only upon urinary
surfaces : its peculiarities will be discussed in connection
with the bladder, in whose structure they are most
readily demonstrable. Immediately beneath is a thin but
dense fibrous membrane which, with the epithelium, makes
up the mucosa of the pelvis. This constitutes the sole in-
vestment of the papillae, the two layers ofthe mucosa be-
ing continuous with the epithelium and basement mem-
brane respectively of the uriniferous tubules at the mouths
of the ducts of Bellini. As we pass along the surface of
the calyx, however, the mucosa begins to be separated

CHAPTER XYII. URINARY ORGANS. 213

from the surface of the columns of Berlin by scattered
muscular fibres, which increase in quantity as we approach
the margin of the fold which surrounds the tip of the pa-
pilla : here a well defined muscular ringis formed, which
has been compared to the transverse layer of the muscu-
losa ; it may be regarded as in a certain sense a sphincter
of the papilla. The general surface of the pelvis is lined
by the mucosa and the muscular layer, there being present
between the two an inconspicuous submucosa of areolar
tissue, which also contains a small amount of adenoid
tissue and scattered mucous glands.

The ureter, as has been stated, consists of an epithelial
and a musculo-skeletal layer; the components of the latter
being so distributed as to form with the epithelium a mu-
cosa, a submucosa, a musculosa, and a fibrosa. The mu-
cosa is a continuation of that just described as lining the ;^./tv*i:^
pelvis of the kidney, with which it agrees in structure in
every essential respect : it is relatively greater in quantity,
being thrown, like that of the oesophagus, into longitudi-
nal folds. The submucosa is correspondingly increased in
quantity : as in the bladder, it contains a small amount of
diffuse adenoid tissue, scattered nodules having also been
described, as have occasional mucous glands. The muscu-
losa shows throughout the whole length of the ureter two
well defined layers, an inner longitudinal and an outer
transverse ; and along the lower portion traces of a third
layer, external to the circular, are found, in the form of
scattered longitudinal bundles. Each of the principal
layers contains a comparatively large amount of inter-
stitial connective tissue as compared with the muscular

./^-

,Lm^<^fci/uiM- '• toHMltj^ 1^ J^my\hiyL

214 PART II. HISTOLOGICAL ANATOMY,

layers of the alimentary canal : outwardly this is continu-
ous with a well defined but not very dense fibrosa.

The bladder resembles the ureter in the essential histo-
logical structure of its wall. The epithelium which lines
the mucosa resembles that of the pelvis of the kidney and
of the ureter, as has been stated : the term transitional
generalW applied to.it, has, as it is generally defined, little
meaning; it serves, however, to connote certain character-
istics pertaining to epithelium found only on urinary'
surfaces, and distinguishing it from stratified squamous
epithelium, with which it is often compared. The most
marked pecularity of transitional epithelium is the power
possessed by all of its cells (and not the deeper layers only,
as is the case with stratified squamous epithelium) of
changing and regaining its form in connection with the
stretching or relaxation of the membrane beneath : this
can be most readily seen by comparing sections of the col-
lapsed bladder and of one distended by a hardening fluid:
both conditions will therefore be described.

In the former case the transitional epithelium is seen to
consist of three distinct forms of cells arranged in what
may perhaps be termed as many layers, though, as will be
seen, the boundaries which separate them are not strongly
marked. The surface is invested with a single layer of cells
which are almost spheroidal or cuboidal above (though
usuall}' not as deep as they are wide), the upper surface
being either flattish or slightly convex when the mucosa is
neither stretched nor pressed together, and almost hemi-
spherical when the bladder is strongly contracted: the
lower surface of the superficial cells is sculptured by con-

CHAPTER XVII. IKINAKV ORirANS. 215

cavities which fit closely the surfaces of the cells of the
next layer: two nuclei are sometimes seen in a single cell,
and it is quite possible that the superficial cells still retain
the power of cell-division. The next layer consists of large
pear-shaped cells, their rounded ends fitting into the exca-
vations upon the under surfaces of the superficial cells, and
their smaller tapering extremities extending to the base-
ment membrane beneath. The spaces between the large
ends of the pyriform cells and the membrane are occupied
by the constituents of the third layer, which is one or two
cells deep, and consists of smallerclosely packed spheroidal
or pol3'hedral cells.

If the bladder be distended, the whole epithelial layer
becomes much thinner: the superficial cells become flat-
tened, approaching squamous cells in form; they never,
however, loose their characteristic sculpturing: the pyri-
form cells become greatly shortened : and the spheroidal
cells correspondingly flattened. A knowledge of the ap-
pearance of the elements in both these conditions is
highly important on account of the frequent appearance
of bits of epithelium in morbid urine. It should be remem-
bered also, that these changes doubtless take place regu-
larly with the daily periodic changes in the state of the
bladder. It is well, too, to note the fact that the cells
which compose transitional epithelium are alwa3's closely
in contact, whatever changes of form they may undergo :
and also that this epithelium has a remarkable power of
resisting diffusion into the blood vessels beneath of the
soluble constituents of the urine.

The mucous membrane of the bladder is much thicker

216 PART II. HISTOLOGICAL ANATOMY.

and firmer than that of the ureter, and the transition to
the submucosa is much more abrupt. As in the ureter,
a muscularis mucosae is w^anting, while a small amount
of adenoid tissue is present. The Submucosa contains
numerous elastic fibres, and occasional mucous glands,
particularly toward the base.

The musculosa consists throughout of smooth muscular
fibres whose bundles are in a general way (and particu-
larly at the equator of the bladder) arranged in three
layers, a middle circular and an outer and inner longi-
tudinal : the bundles are, however, quite irregularly dis-
posed, and there is much interstitial connective tissue : it
is, therefore, not always easy to recognize the layers. At
the base of the bladder the circular laj^er is increased in
quantity to form the internal sphincter. The interstitial
connective tissue is continuous externally with a rather
loosely woven fibrosa, which is over a part of the outer
surface of the bladder invested with serous endothelium.

The female urethra continues the wall of the bladder
to the mucous membrane of the vestibule. In its course
the epithelium passes from transitional to stratified squa-
mous, the remainder of the mucosa undergoing no impor-
tant change. The submucosa contains numerous elastic
fibres and near the bladder a number of mucous glands :
its deeper layer is highly vascular. The musculosa con-
sists of an inner circular and an outer longitudinal laj'er
of smooth fibres, and there is no well defined fibrosa.

CHAPTER XVIII. REPRODICTIVE ORGANS. 217

CHAPTER ZVIII.
THE MALE REPRODUCTIVE ORGANS.

The male and female reproductive bodies, or gonads,
are in their orijj^in, mode of development, and primarv
position essentially similar bodies. The reproductive ele-
ments produced in them differ so widely, however, in their
activities as to call for widely differing mechanisms for
their discharge: mechanisms which are nevertheless de-
rived from the modification of intimately allied structures,
whose development is alwa\'s closel}-^ associated with that
of the urinarv apparatus. For this reason the descrip-
tion of the early stages of the latter will be deferred until
the completion of this and the subsequent chapter.

The male gonads, or testes, each with its associated
epididymis, are in the adult human subject suspended
by the spermatic cords in saccular folds of the skin conflu-
ent below the penis and forming the scrotum. The latter
is formed, as will be more fully described later, by the push-
ing down of the abdominal wall on either side of the base
of the penis : its structure therefore conforms to that of'
the body wall, subject, however, to special modifications
in each of its constituent layers.

The skin of the scrotum is thin, corrugated, rich in
brownish pigment and in sebaceous glands, and has scat-

218 PART II. HISTOLOGICAL ANATOMY.

tered over its surface flattened curlinghairs with conspicu-
ous bulbs. The superficial fascia of the groin and adjacent
parts is continued into the scrotum to form a character-
istic tunic, the dartOS, which is best developed in the fore-
part of the scrotum : it is quite highly vascular, and con-
tains numerous smooth muscular fibres ; it is consequently
of a reddish-brown color. The right and left dartos tunics
unite in the mid-plane to form a partition, the septum
scroti. The corrugation of the skin of the scrotum is
caused by the constriction of the muscular fibres of the
dartos.

Immediately internal to the dartos is a denser and firmer
fibrous layer, thin and transparent, known as the sper-
matic fascia: it is derived from the tendon of the external
oblique muscle of the abdominal wall. Within and closeU'
associated with the spermatic fascia is a layer of areolar
tissue which contains numerous bundles of striped muscu-
lar fibres variously disposed, constituting a more or less
continuous muscular layer termed the cremaster, and
formed from an extension of the external oblique muscle.
Still farther within, the fascia transversalis is continued
in the scrotal wall by the infundibuliform fascia, a fib-
rous layer separated from the preceding by loose areolar
tissue and immediately underlying the parietal portion of
the tunica vaginalis, a serous membrane derived from
the peritoneum, which lines the scrotum and is reflected
upon the spermatic cord and testis.

The successive la^^ers characteristic of the scrotal wall
are, therefore, from without inwards, the skin, the dartos
tunic, the spermatic fascia, the cremaster muscle, the in-

CHAPTER XVIII. REPRODUCTIVE ORGANS. 219

fundibuliform fascia, and the tunica vaginalis. Of these
the (lartos forms the basis of the median septum, while
those internal to it invest the cavities which are separated
thereby.

The spermatic cord of either side is composed of the ves-
sels and nerves of the testis together with the duct of dis-
charge, known as the vas deferens : they are imbedded in
areolar tissue and surrounded by the continuations of the
coverings of the testis. The structure of the duct will be
described later.

The visceral portion of the tunica vaginalis invests the
surface of the testis except its posterior border, along
w^hich it is reflected to become continuous with the parie-
tal portion : it is frequently termed the tunica adnata.
Beneath it lies the proper capsule of the testis, the tunica
albuginea, a dense white fibrous layer of considerable
thickness: along the posterior margin this is continued
into the interior for some distance as a wedge-shaped fib-
rous reticulum known as the mediastinum testis, and
also as the corpus Highmori: from the mediastinum ra-
diate stout straight bands of fibrous tissue, the septa or
trabeculae, w^hich unite with the peripheral albuginea,
thus imperfectly dividing the body of the testis into a num-
ber of irregularK' p} ramidal lobules. The inner stratum
of the albuginea is quite vascular, and is sometimes dis-
tinguished as the tunica vasculosa: from it vascular
trunks pass along the septa and form the blood supply of
the lobules.

220 PART II. HISTOLOGICAL ANATOMY.

Each lobule consists of a variable number of seminifer-
ous tubules supported by delicate lamellae of interstitial
tissue continuous with the stout fibrous septa as well as
with the proper membranes of the tubules : within these
lamellae peculiar epithelioid cells are found either singly or
in groups, whose origin and function are still uncertain ;
they are known as the interstitial cells of the testis.

Each seminiferous tubule begins near the periphery of
the testis with an irregularly contorted portion extending
from its rounded extremity nearly to the mediastinum
and known as the convoluted tubule, or the seminifer-
ous tubule in the strict sense : these occasionally branch
near their free extremities, and are said to anastomose in
some instances. As they approach the mediastinum they
become smaller and less irregular, and unite to form the
straight or conducting tubules, which lie in the apices of
the pyramidal lobules. On entering the reticular medias-
tinum the latter anastomose freely to form a network of
tubules of variable size, forming the rete testis, whose
meshes correspond with the spaces of the mediastinum.

The membrana propria of the convoluted tubules con-
sists of several lamellae of endothelioid cells with flattened
oval nuclei. Upon this membrane rests an epithelial layer
several cells deep, from whose inner strata are derived the
male reproductive elements or spermatozoa. The base-
ment membrane of the straight tubules is a continuation
of that of the convoluted : it is lined with a single layer of
cuboidal epithelium. In the rete testis the membrane be-
comes continuous with the reticular framework of the
mediastinum, and can no longer be distinguished: the

CHAPTKR XVIII. REPRODUCTIVE ORGANS. 221

channels are lined with a single layer of flattened epithe-
lialjcells.

The arrano^enient of the epithelial cells of the convoluted
tubules, and the changes which they undergo in the pro-
cess of forming the spermatozoa, which is known as sper-
matogenesis, have been the subject of much controversy
and cannot vet be said to be fully understood. Different
methods have been described with positiveness not only
in difl'erent classes of animals, but in different orders of
the mammals, and even in different members of the same
order. Our knowledge of the facts in the human subject
are, as in most cases, less perfect than of those in the lower
animals. In addition, the same terms have been applied
bv different writers to what are certainly different stages.
The following account is based on that given by Schaefer
in the tenth edition of Quain's Anatomy.

Next to the basement membrane is found a layer of cells
most of which are cubical, clear, and possessed of nuclei
which are in the resting or network phase : here and there
dividing nuclei are seen : these cells ma}' be termed parietal
cells or spermatogonia. Scattered here and there among
them are larger cells which project between the more in-
ternal layers: these are known as sustentacular cells:
w^hen_much elongated and joined to bundles of develop-
ing spermatozoa the}- form the columns of Sertoli.

Within the parietal la\'er is seen a middle layer of some-
what larger spheroidal cells whose nuclei show various
phases of division, the intermediate or spermatogenic
cells. The layer may be one, two or more cells in depth ;

222 PART II. HISTOLOGICAL ANATOMY.

its constituents are probably derived primarily from the
cells of the parietal layer, but increase in number by lat-
eral divisions. They are by some authors termed sper-
matocysts.

The inner layer of distinctly cellular elements consists of
smaller and more numerous cells derived from the inter-
mediate layer, the spermatoblasts (or spermatids of some
writers); these are probably directly transformed into the
spermatozoa. When first formed they compose a layer of
closely packed small granular cells: they subsequently be-
come more and more elongated vertically and collected
into small groups, each of which becomes connected with
one of the sustentacular cells above mentioned to form a
column of Sertoli: later the bundle of spermatozoa derived
from each group of spermatoblasts becomes separated
from its sustentacular cell; the constituent elements are
set free, and accumulate in large numbers in the lumen of
the tubule.

The mature spermatozoon consists of an oval and flat-
tened head, about four and a half micra long, two to three
broad, and one to two thick; a cylindrical rniddle part or
body about six micra long and less than one in diameter;
and a tapering cilia-like tail forty to fifty micra long.
The head consists chiefly if not entirely of the nucleus of
the spermatoblast : the origin of the body and the tail is
not so certain. In the lower animals the form and struct-
ure of spermatozoa vary greatly.

The p3^raraidal lobules and the body of Highmore, with
their investment, the tunica albuginea, compose the whole
of the gonad proper or testis in the strict sense. Closely

CHAPTER XVIII. REPRonrCTIVE ORGANS. 223

associated therewith and often regarded as a portion
thereof, though in reality the beginning of the efferent appa-
ratus, is the epididymis, a tubular mass attached to the
testis in the region of the mediastinum. Associated with
the latter are certain rudimentar\' organs known respec-
tively as the hydatids of Morgagni, lying between the
head of the epididymis and the upper end of the testis,
the vas aberrans, attached to the lower end of the epidid-
ymis, and the organ of Giraldes, found near the base of
the spermatic cord.

The channels of the rete testis anastomose freely through -
out the whole of the mediastinum or body of Highmore.
Those of the upper or anterior portion open into the effer-
ent tubules, also known as vasa efferentia, from twelve
to twenty in number, which penetrate the albuginea and
enter the epididymis, of which they form a part : at first
straight, they soon become coiled in conical masses, the
coni vasculosi, whose bases are turned away from the
testis. These aggregated conical bodies, together with
the upper portions of the canal of the epididymis, into
which the efferent tubules open, form the globus major,
or caput epididymis. Their constituent tuljules are as
large as the convoluted tubules of the testis: they are
lined with a cuboidal or short columnar ciliated epitheli-
um, beneath which is a well defined membrana propria
surrounded b}' a thin transverse layer of smooth mus-
cular fibres.

The canal of the epididymis, exceedingh- flexuous
from the point of its origin in the extremities of the up-
permost vasa efferentia, becomes disposed in numerous

224 PART II. HISTOLOGICAL ANATOMY.

small irregular coils in the middle region and is continued
at the lower portion as a densely convoluted mass, the
globus minor. At its origin it is twice the diameter
of the vasa efferentia: lower it becomes smaller, and is
enlarged again in the globus minor, from which it is con-
tinued as the beginning of the spermatic duct or vas def-
erens. It is lined throughout its course with tall col-
umnar cells (between whose bases smaller rounded cells
occur) : these cells are provided with unusually long cilia
throughout the greater portion of the tube, the cilia dis-
appearing in the lower portion. Beneath the underlying
membrane is a thin circular layer of smooth muscular
fibres continuous with that of the vasa efferentia, which
is surrounded by a thin longitudinal layer. The convolu-
tions of the tube are bound together by interstitial areo-
lar tissue which is here and there replaced by incomplete
fibrous septa [w^hich partially divide the 'epididymis into
irregular lobules.

The hydatids of Morgagni are small saccular bodies
lying between the globus major and the upper end of the
testis. One of these, the stalked hydatid, is provided
with a short peduncle, is usually present, and has a homo-
logue in the female organs of reproduction. The others,
the sessile hydatids, are variable in number and some-
times wanting: they are found in the male only. Both
stalked and sessile hj^datids are lined with cuboidal epi-
thelium upon which cilia have been described.

The vas aberrans is a long narrow blind tube, a diverti-

chaptp:r XVIII. rki»K()I)1'CTivk okcans. JJo

culum of the canal of the epididymis, which arises from
the latter at or near the point where it becomes continu-
ous with the vas deferens. Like the canal, it is exceedingly
tortuous in its course, forming an elongated convoluted
mass which extends upward among the vessels of the
spenut'itic cord. It resembles the vas deferens in struct-
ure. It is almost invariably present, is sometimes branch-
ed, and in some cases more than one such structure oc-
curs.

The organ of Giraldes, also called the paradidymis, is a
small body found on the front of the spermatic cord just
above the globus major. It consists of several discon-
nected irregularly branched tubules lined with columnar
ciliated epithelium. Their coiled masses form irregular
nodules imbedded in the connective tissue of the regions
between the cord and the epididj^mis.

The spermatic duct, or vas deferens, is the continua-
tion of the canal of the epididymis. It is considerably
larger and more complex in structure than the canal,
showing like many of the tubular structures of the bod}',
a definite mucosa, submucosa, and musculosa. The mu-
cosa consists of a stout membrane sometimes throw^n
into Ipngitudinal folds, and bearing a non-ciliated colum-
nar epithelium : the submucosa is composed of areolar tis-
sue somewhat laminated in arrangement : the musculosa
is thick and yellowish in color and comprises an inner cir-
cular and an outer longitudinal layer of smooth muscular
fibres, external to which is a fibrous adventitia. At the

226 PART II. HISTOLOGICAL ANATOMY.

commencement of the duct there is also a longitudinal
layer of muscular fibres internal to the circular layer.

The ampulla, or sacculated enlargement of the duct sit-
uated near its junction with the seminal vesicle, resembles
the rest of the duct in structure, save that the various
coats are somewhat thinner. The tubular diverticula of
thevasa deferentia known as the seminal vesicles are like
the ampullae in structure. The spermatic duct or vas de-
ferens of either side unites with its associated seminal vesi-
cle to form the ejaculatory duct, or common seminal
duct, which completes the passage way from the seminif-
erous tubules of the testis to the urethra : in this region
the walls of the tube are much thinner than in the vas
deferens and the external fibrous adventitia disappears
as the duct enters the substance of the prostate gland.

The male urethra, into which the common seminal ducts
empty within the region surrounded by the prostate gland,
is by virtue of that fact divisible into two distinct regions,
the urinary and the urino-genital : it is the former alone
that corresponds to the female urethra. By its anatomi-
cal relations it is also divided into the prostatic, the mem-
branous, and the penial urethra : the former includes the
first of the two regions above mentioned and a portion of
the other. Histologically the structure differs in the sev-
eral regions chiefly in the character of the epithelium, in
the special structures found in the mucosa of the penial
region, and in the composition of the musculosa.

The prostatic urethra of the male resembles the female
urethra in being essentially a continuation of the wall of

CHAPTER XVIII. REPRODUCTIVE ORGANS. 227

the bladder. It has a mucosa lined with transitional epi-
thelium, which rests upon a membrane rich in elastic fibres.
Beneath is a highly vascular submucosa. and beyond this
a musculosa which consists of an inner longitudinal and
outer circular layer of smooth muscular fibres. In front
of the openings of the common seminal ducts the epjthe-
lium passes by a 'gradual modification from the transi-
tional to a somewhat stratified columnar form. The -^
membranous urethra continues the structure of the an-
terior portion of the prostatic, and in addition receives a
distinct layer of striped muscular fibres from the adjacent
compressor urethrae.

In the penial urethra the epithelium is composed of a
single layer of columnar cells except at the fossa navicu- ^
laris, where it passes into stratified squamous epithelium if
continuous with that of the surface of the glans. Here
and there the mucosa exhibits irregular depressions of
variable size, known as lacunae Morgagni : it also has
connected with it numerous small racemose glands, the
glands of Littre, which are also sparingly found in other
portions of the urethra; they are lined with cuboidal or
low columnar glandular cells.

The penis consists essentially of three masses of what
is commonly called erectile tissue, the right and left cor-
pora cavernosa, and the median inferior corpus spongi-
osum ; the latter being traversed throughout its length by
the penial urethra, and having its distal portion expanded
to form the glans penis : the whole, of course, invested by
the somewhat modified integument. The form and rela-

228 PART II. HISTOLOGICAL ANATOMY.

tions of these bodies are matters for anatomical discus-
sion : we are here concerned with their histological struct-
ure onh'.

What is known as erectile tissue consists primarily
"sirapl}' of a somewhat circumscribed collection of larger
and smaller veins w^hich under certain circumstances may
become distended with blood, thus causing the parts in
which they lie to expand." Its specialization consists
chiefly in the enlargement of the veins to form irregular
sinuses, or cavernae, and their frequent anastomosis ; the
development and modification of the circumscribing skel-
etal tissues ; and accessory modifications of the arteries
of supplv. That the cavernous sinuses are to be regarded
as veins is showm not only b}' the striicture of their walls,
but also by the fact that the blood reaches them chiefly if
not entirely through capillaries. It will be seen from w^hat
has just been said that erectile tissue is not constant in
structure as are adenoid and other compoutjd tissues that
have been previoush^ described. The form assumed in
each locality where it occurs will therefore be briefly
stated.

The corpora cavernosa are surrounded and united by a
stout fibrous envelope, termed, like that of the testis, the
tunica albuginea ; it consists of bundles of white fibres
chieflv disposed in a longitudinal direction and mixed with
numerous elastic fibres: within, the w^hite fibres are chiefly
circularly disposed, surrounding each of the two corpora
to form an individual sheath; these two sheaths are

CHAPTER XVIII. REPRODUCTIVE ORGANS. 229

confluent in the mid-plane through the greater part
of the penis, tbnning a septum which is incomplete by
virtue of the presence of slit-like apertures through which
the erectile tissue becomes continuous from side to side:
these ajiertures are most numerous in the anterior part of
the penis.

From the fibrous sheath numerous stout trabeculae
pass inward to form a reticulum whose meshes are the
cavernous sinuses. They are composed chiefly of white
fibrous tissue, with more or fewer elastic fibres inter-
mingled, and contain in addition numerous bundles of
smooth muscular fibres : their surfaces are lined with the
vascular endothelium of the sinuses. The trabeculae are
stoutest and most numerous near the surface: the sinuses
are correspondingK' largest at the centre of the bod\' ;
thev are also larger near the extremity of the penis than
at the base, in which region their long diameter is as a
rule placed transversely to the penis. The small arteries
which follow the trabeculae in many cases project from
their surfaces in peculiarly curled and coiled loops; they
are hence known as helicine arteries: they are said in
some cases to open directly into the sinuses, but such a
direct communication between an arter\' and a modified
vein is not easily demonstrated under conditions which
exclude the possibility of error.

The corpus spongiosum differs from the corpora caver-
nosa histologically in the thinness and increased elasticity
of its fibrous tunic, which contains so much elastic tissue
as to be yellowish in color ; in the smaller size and greater

230 PART II. HISTOLOGICAL ANATOMY.

uniformity of the trabeculae; and in the lesser amount of
muscular tissue which they contain. The venous sinuses
are smaller and more uniform, forming a spongy mass,
from which the name is derived : their greatest dimensions
are as a rule longitudinally disposed.

In the glans the meshes are quite small and uniform
in size : the erectile tissue passes insensibly into the lower
strata of the integument, with which its surface is invested.
The derma is thin and highly vascular over the surface
of the glans, and the epidermis has the form of a stratified
squamous epithelium devoid of the division into layers
characteristic of the cuticle and resembling in character
that found on the oesophageal mucous membrane. Glands
arewanting, except upon the corona and the cervix, where
modified sebaceous glands, the glands of Tyson, are
abundant. Special nerve terminals, the so-called genital
corpuscles, are present, as are Pacinian bodies.

The skin of the penis is quite thin, highly elastic, and
very movable, and contains but a very little fat : the larger
portion is devoid of hair also: as it passes around the
free margin of the prepuce it changes its structure and the
character of its epithelium, the lining of the prepuce, like
the investment of the glans, having the appearance of a
mucous membrane. At the base it passes on into that of
the pubes, which is quite thick, beset with coarse hairs,
and provided with a dense fatty layer.

The urino-genital tract of the male has associated with
it certain glandular bodies, the prostate gland, which sur-

CHAPTKR XVIII. REPRODUCTIVE ORGANS. 231

rounds the proximal region of the urethra, and the glands
of Cowper, paired organs opening into it near the point
where it enters the corpus spongiosum. Opening into it
ventrally in close proximity to the apertures of the ejacu-
latory ducts, is an interesting rudiment, the sinus pocu-
laris, otherwise known as the uterus masculinus. These
structures will next be described.

The prostate Ts a glandular body which differs from
most organs of the kind in the fact that not only its cap-
suje but also its stroma contains a ver}- considerable
amount of smooth muscular tissue. This muscular tissue /
is in continuity with the musculosa of the urethra and of J
the ejaculatory ducts, and posteriorly with that of the »
bladder. The capsule is divisible into two layers, between
which is found the prostatic venous plexus: from it trabec-
ulae pass inward to form the framework of the gland, con-
' sisting, in addition to the smooth muscular tissue already
J mentioned, of a very small amount of white fibrous tis-
sue and a larger quantity of elastic fibres. The alveoli are )
f tubular, frequently quite elongated and irregular in shape,
I their walls sometimes showing conspicuous folds : the epi-
thelium is columnar and simple save that frequently small
and spheroidal cells are found at the base of the columnar
) cells. The ducts, which are numerous, are lined with col-
umnar epithelium which changes into stratified as it ap-
1 proaches their openings upon the urethra.

Cowper's glands are small bodies of the racemose type,
each consisting of several small lobes. Their capsules and

232 PART II. HISTOLOGICAL ANATOMY

supporting framework resemble those of the prostate to
some extent in the presence in each of a small amount of
smooth muscular fibre : a well defined longitudinal laj'-er
of smooth muscular fibres is also present in the wall of
the principal duct. The acini resemble those of a mucous
salivary gland in form and in the general appearance of
the glandular epithelium : there is a conspicuous lumen,
the cells are pyramidal, and the nuclei are situated near
the base. Nothing resembling the crescents or demilunes
of the mucous glands has been observed. The lobar ducts
are lined with cuboidal epithelium, which passes into col-
umnar in the principal ducts.

The sinus pocularis is the homologue in the male of the
vagina and uterus of the female. It is a diverticulum of
the prostatic urethra having a well-defined muscular wall
and a mucosa containing a number of short tubular
glands which resemble the uterine glands in their form
and structure.

chai'Ti;k XIX. reproductive organs. 23v

CHAPTER XIX.
THE FEMALE REPRODUCTIVE ORGANS.

The female reproductive apparatus consists of the fe-
male gonads, or ovaries, in which the reproductive ele-
ments are formed, the oviducts, or Fallopian tubes, by
which they are conveyed from the ovaries; the uterus, in
which they are received, and in which the fertilized ovum
or oosperm develops into the embryo ; the vagina, by
which the uterus communicates with the exterior, and the
parts composing the vulva, w^hich immediately surrounds
the openingof the vagina. As in the case of the male, there
are also present certain rudimentary bodies, chiefly in the
vicinity of the gonad.

The ovary, like the testis, is an organ in which special-
ized cells, epithelial in their origin, are matured and liber-
ated : the sexual elements differ, however, in an antipodal
manner as regards their size, activity, destination, mode of
transportation thereto, and mode of liberation : there is
a corresponding difference in the structure of the organs
which produce them, the ovary having nothing of that
tubular structure seen in the testis and giving to that
body a close resemblance to a gland.

The framework or stroma of the ovary lacks the abund-
ant white fibrous tissue found in the capsule and trabecu-

234 PART II. HISTOLOGICAL ANATOMY.

lae of the testis : it consists chiefly of a peculiar form of
connective tissue characterized by the presence of elon-
gated nucleated cells which are frequently spindle shaped,
and b}^ the scarcity of true fibrous tissue, either white or
elastic. It contains numerous smooth muscular fibres,
which are most abundant in the deeper portions. Toward
the surface the stroma becomes more dense, forming quite
a well-defined superficial layer, to which the name of the
tunica albuginea has been given ; it lacks the firmness
and definiteness of the layer so designated in the testis.
At the base of the ovary the stroma is especially rich in
blood vessels : the region occupied by them is known as
the zona vasculosa.

The region between the albuginea and the zona vascu-
losa constitutes the parenchyma of the ovary. It is
rather indefinitely divided into a cortical and a medullary
portion by the character of the Graafian follicles contained
in it. It also contains, scattered irregularly through it,
groups of interstitial cells similar to those found in the
testis.

The surface of the ovary is invested with a layer of cells
which are structurally continuous with the serous endo-
thelium of the peritoneum, but which differ therefrom in
form and function. They are cuboidal or low columnar
in^ shape, and constitute the germinal epithelium: they
have no proper basement membrane, but rest directly up-
on the tunica albuginea. Here and there may be seen, es-
pecialh-^ in the embrj^o, certain cells which are larger and
more rounded infoi'm : these are the primitive OVa. These,
during foetal life and possibly in childhood sink into the

CIIAPTKK XIX. RHI'KODrCTIVK DKGANS. 235

Stroma, accompanied by tubular or spheroidal nests of
epithelial cells : it is doubtful whether this ever takes place
in the adult : it is also at present a matter of question
whether the primitive ova increase in number by division
after they have passed into the stroma, or whether all so
situated have come from the epithelium of the surface of
the ovary.

The cortical region of the parenchyma is crowded with
the spheroidal masses of cells formed in the manner just
described, each consisting of one or sometimes two primi-
tive ova surrounded by a layer of epitheHal cells. These
are the primitive Graafian follicles. In those immediately
beneath the albuginea the surrounding layer is usually
but a single cell deep: at first flattened, and hardly dis-
tinguishable from the cells of the adjacent stroma, its cells
soon become cuboidal in form. At the same time the
fibres of the stroma tend to assume a disposition concen-
tric to the follicle, forming the beginning of the theca
folliculi.

As the follicles grow older they tend to sink deeper into
the stroma of the ovary; the cells of the enveloping layer
at the same time proliferating, and the layer becoming
several cells thick. Shortly afterward the ovum leaves its
central position for one nearer one side of the follicle, usu-
uallv that farthest from the surface of the ovary, while
a cleavage takes place in the cellular layer toward the
other side, the space formed becoming infiltrated with a
clear fluid, the liquor folliculi. The follicles now rapidly
increase in size, at the same time sinking into the medul-
lary region, where they are seen as large vesicles filled

236 PART II. HISTOLOGICAL ANATOMY

with fluid, invested by a well-defined theca, and lined by a
layer of small isodiametric cells of irregular form, the
layer being several cells deep : it is now known as the
membrana granulosa. Attached to it at one side of the
follicle is the rounded heap of similar cells which contains
the ovum : it is termed the discus or cumulus prolig-
erus.

The follicles still increasing in size, their outer wall now
tends to approach the surface of the ovary, the fulh' ma-
tured follicle finally projecting somewhat from the surface,
b}^ whose rupture the contained ovum is eventually to be
liberated. The theca is now well defined and consists of
two laj'ers, an inner or vascular, and an outer or fibrous
layer. The cells of the membrana granulosa next the
theca and those of the cumulus next the ovum are dis-
tinctly columnar in form. The most conspicuous as well
as the most important body present is the mature ova-
rian ovum. This is a spheroidal body now much larger than
the primitive ovum from whi&hit was developed, although
small as compared with the ova of many of the lower ver-
tebrates. It is in the human subject about two-tenths of
a millimetre in diameter. It is invested b}-- a thick cover-
ing appearing when seen with microscopes like those used
by the earlier observers to be quite clear : as its optical sec-
tion forms a girdle or zone of considerable breadth about
the ovum it was named by Von Baer the zona pellucida.
Careful examination by modern instruments and meth-
ods demonstrate that it contains innumerable radial
striae: it is therefore now commonly called the zona
striata, or still more accurately the striated membrane.

ClIAPTEK XIX. KKI'UODrCTIVi: ORCANS. 237

Several eminent and accurate observers have described a
delicate membrane, which they call the vitelline mem-
brane, internal to the layer just described : its presence
cannot l^e readily demonstrated with certainty.

Within the envelope just described is the vitellus, or
volk, wrongly so called, since it does not correspond to
the bod v so termed in the eggs of many lower vertebrates:
it is a homogenous protoplasmic mass, semi-fluid in con-
sistencv and highly granular. It contains, usually in an
eccentric position, a large spherical nucleus which was
named by Purkinje the germinal vesicle: the nuclear con-
tents exhibit a coarse network characteristic of the phase
of complete rest : there is usually but a single nucleolus,
which is quite large and rounded, and was called by Wag-
ner the germinal spot.

When the ovum is discharged by the rupture of the fol-
licle u])on the surface of the ovar^^ the follicular cavity is
at first filled with a clot of blood. It is quickly invaded
by growths from the wall of the follicle formed in part of
rapidlv proliferating cells of the membrana granulosa, in
part of folds and procesvses from the theca : it has been as-
serted that the interstitial cells previously mentioned also
enter to a large extent into the ingrowing structure. The
result is the formation (about the shrunken and discolored
clot as a centre) of a mass ofmingled cells and fibres known
as a corpus luteum : this is at first sharply defined by the
presence of the theca, but gradual!}' loses its definiteness,
and becomes continuous with the mass of the ovary, the
peculiar spurious tissue thus formed composing quite a

238 PART II. HISTOLOGICAL ANATOMY-

large part of that organ in age. The corpus luteum formed
concurrently with pregnancy is large and well-defined and
is regarded as characteristic : but corpora lutea equally
large and distinct sometimes (though more rarely) occur
under other conditions.

The oviducts, commonly termed the Fallopian tubes,

. »>
while they vary in form in the several regions distinguish-
ed by the anatomist, are quite uniform in their histologi-
cal structure throughout their whole extent. Continuous
at the isthmus with the uterus, they open at the fimbriated
extremities into the peritoneal cavit}^ : they consequently
present the only instance of direct continuitx^between a
mucous and a serous surface. Like nearly all the larger
tubular structures in the body, the wall is divisible into a
mucosa, a submucosa and a musculosa; to which is added
a serosa derived from their investment by the marginal
fold of the broad ligament.

The mucosa consists of a well developed fibrous mem-
brane moderately rich in elastic fibres, and well supplied
with blood vessels and lymphatics, which supports a layer
of simple columnar ciliated epithelium. An imperfectly
developed muscularis mucosae, consisting of longitudinal
bundles of smooth fibres, is also present. The mucosa
throughout its extent is thrown into longitudinal folds
which in the ampulla and particularly in the infundibulum
are very extensive and have secondary folds upon their
surfaces, giving to the cross section a peculiar arborescent
appearance. As seen in such sections the bases of these
folds often present the appearance of tubular glands ; but

CHAPTER XIX. F<i: PRODUCTIVE ORGANS. 230

true srlands are not present. The inner surface of the fun-
briae is lined with the mucosa, while the outer surface is
covered with the serosa: the two becomingconfluent along
the sides.

The submucosa is a simple layer of areolar tissue of but
slight depth. It is continuous with that of the uterus,
like which it contains small ganglia and scattered multi-
polar cells, forming the rudiments of a plexus. The mus-
culosa consists of an inner circular and an outer longitu-
dinal layer of smooth muscular fibres, the latter being but
imperfectly developed. The serosa consists of a thin fib-
rous membrane supporting the serous endothelium charac-
teristic of the surface of the peritoneum.

Attached to the extremity of the tube or to one of the
fimbriae is frequently found a pedunculated cyst or
stalked hydatid of Margagni. It is the homologue of the
body bearing the same name in the male reproductive ap-
paratus and resembles it in general structure, the cavity
of the sac and also of the pervious portion of the stalk
being lined with cuboidal or columnar epithelium.

Situated in the broad ligament between the ovar\' and
the ampulla of the oviduct is a well-defined mass of ir-
regularly convoluted tubules, known as the parovarium.
It is also called, from its discoverer, the organ of Rosen-
mueller: Waldeyer has proposed for it the name of the
epoophoron. The constituent tubules are lined with low
columnar epithelium : they converge toward each other,
without uniting, at the ends nearest the ovary : the other

240 PART II. HISTOLOGICAL ANATOMY.

extremities diverge somewhat, and terminate in a longi-
■ tudinal tube which runs parallel with the oviduct: in some
of the lower mammals this tube is quite extensive and is
known as the duct of Gartner, a term also applied to it
in the human subject by many. Somewhat nearer to the
uterus than the parovarium a smaller and more irregular
group of rudimentary tubules occurs, similar in structure
to those just described. These have been designated by
Waldeyer the paroophoron. The homologies of these rudi-
ments will be discussed later.

The uterus presents but two distinct regions histologi-
calh%thefundusandbody agreeing in structure and differ-
ing from the cervix. The most characteristic features of
the upper region are found in the mucosa, the stroma of
which is greatly modified, while the muscularis attains a
greater development than does the structure bearing that
name in any^other portion of the body. The surface is inves-
ted with a single layer of columnar ciliated cells directly con-
tinuous with that lining the Fallopian tube. Beneath this
is a very thick mucous membrane containing but a very
small quantity of fibres and composed in large part of
spindle shaped cells similar to those found in the stroma of
the ovary, loosely interwoven: the spongy mass so formed
contains numerous lymph spaces and leucocytes. Im-
bedded in it are great numbers of tubular uterine glands,
wavy or convoluted in their course, not infrequently''
branched, and penetrating to the base of the stroma and
quite frequently between the bundles of fibres of the mus-
cular layer: they are bounded by a delicate basement mem-

CHAPTER XIX. REPRODUCTIVE ORGANS. 24-1

brane which supports columnar cells similar to those lin-
ing the uterine wall : near the blind extremity of the tube
the columnar cells entirely till its cavity: but throughout
the greater part of its extent there is a distinct lumen.

The rnuscularis rnucosae is the chief muscular coat of the
uterine wall; greatly developed at all times, it is enor-
moush' hypertrophied during pregnancy, parth' by the
great increase in number. It consists of bundles of fibres
interwoven with a sparing amount of interstitial connect-
ive tissue, and running in various directions: their dispo-
sition is apparently quite irregular, and cannot be de-
scribed briefly with clearness : it is, moreover, subject to
considerable variations: it can perhaps be best understood
bv regarding it as consisting chiefly ot circularly disposed
bundles which are arranged on the fundus in two sets, one
concentric to the insertion of each of the two oviducts,
and which become gradually combined to form a single
set as the}' approach the lower extremity of the body.

There is associated with the great development of the
rnuscularis mucosae a corresponding reduction of the sub-
mucosa, there being less independent movement of the
mucosa and musculosa in this case than in almost any of
the similar hollow structures. There is, however, a dis-
tinct zone of connective tissue discernible just exterior to
the muscularis mucosae characterized particularly by the
presence of numerous bloodvessels and lymphatics, and
by scattered nervous elements. It is of such slight extent
as to be by some described as wanting.

The musculosa consists of two distinct laj^ers of smooth
fibres both quite thin and varying in their relative devel-

242 PART II. HISTOLOGICAL ANATOMY,

opment in different parts of the organ. The inner or cir-
cular layer is the more uniform in thickness and in the ar-
rangement of its fibres : the fibres of the outer laver are in
the main longitudinally disposed, but are somewhat ir-
regularly arranged upon the fundus, in accordance with
its irregularities of form. This laj-er also gives off bundles
of muscular fibres extending out into the ligaments of the
uterus. The musculosa is invested by a serosa which is a
continuation of the peritoneum.

The cervix differs from the region just described chieflv
in the structure of the mucosa, the stroma of which is
much firmer and richer in fibres, both white and elastic,
the membrane being thrown into characteristic folds, and
in the lower portion beset wnth minute papillae. The up-
per two-thirds is lined with columnar ciliated epithelium
continuous with that of the upper regions : this passes
below into the stratified squamous epithelium which in-
vests the papillated lower third. There are present both
tubular and saccular glands said to be lined in each case
with columnar ciliated epithelium : the saccular glands
contain also goblet cells and secrete the thick mucus found
in the cervix. Here and there spheroidal bodies filled with
a clear yellowish fluid can be seen with the naked eye:
they are probably occluded and enlarged mucous glands,
and are known as ovula Nabothi. The muscularis mu-
cosae is well developed and consists chiefly of circular
bundles : these are accumulated in greater numbers at t2
upper and lower extremities of the cervix to form the
sphincters of the regions in question.

The submucosa is not conspicuous. The musculosa re-

'1/

tM/<

CHAPTER XIX. REPRODUCTIVE ORGANS. 24-3

sembles in structure the same portion of the wall of the
body of the uterus: the imier circular and outer Ion git u-
dinal layers are clearly defined. The region of the cervix
toward the rectum is invested with a peritoneal serosa:
that toward the bladder is separated from that organ b\'
an adventitia of areolar tissue. The portion of the cer-
vix that projects into the vagina to form the OS uteri
agrees in structure on its inner aspect with the cervix : on
its outer it is a continuation of the vaginal wall.

The vagina differs from the uterus in structure in accor-
dance with its differing and double function, it serving at
once as the channel by which the male reproductive ele-
ments are brought into proximity with the female ele-
ments, and as the avenue of discharge for the foetus at its
maturity. Its wall is muscular, dilatable, and highly
elastic, somewhat erectile, and provided with a definite
reinforcement of adenoid tissue ; a feature possessed by no
other portion of the reproductive tract.

The mucosa of the vagina is lined with stratified squa-
mous epithelium, which rests upon a thick mucous mem-
brane. The surface of the latter is beset with minute pa-
pillae which project into the deeper portions of the epithe-
lium but do not produce a noticeable roughness of its sur-
face, the only irregularities observable being those due to
the well-marked folds or rugae. The upper portion is a
dense fibrous layer rich in elastic fibres : below this in the
rugae are networks of large veins supported by fibrous
tissue containing numerous bundles of smooth muscular
fibres which may be regarded as representing the muscu-

244 PART II. HISTOLOGICAL ANATOMY.

laris mucosae, elsewhere absent : a ridge of rudimentary
erectile tissue is thus formed beneath each ruga. Leuco-
cytes abound in the mucosa, and scattered nodules of ade-
noid tissue are found : in the anterior wall near the orifice
there is a \vell-defined adenoid layer. Special nerve termi-
nals, the genital corpuscles of Krause, as found in the
mucosa of the vagina. It is doubtful whether glands of
any sort are present.

The submucosa is quite loose in structure and contains
a venous network whose meshes run chiefly in the direc-
tion of the vagina. Beyond the submucosa is the mus-
culosa, which is not sharply defined, as in most cases, in-
to distinct strata : the inner bundles are in the main circu-
larly disposed, and the outer bundles longitudinally; the
two regions being, however, blended by numerous oblique
bundles. A well-marked fibrosa invests the musculosa : it
is composed largely of elastic tissue and is best developed
on the anterior wall : it also contains an extensive plexus
of large veins intermingled with bundles of smooth mus-
cular fibres and forming a layer of erectile tissue best de-
veloped near the lower extremity.

I^he hymen agrees in its structure with a fold of the
mucosa of the vagina, and can perhaps be regarded as de-
rived therefrom, though its presence in rare instances in
cases of absence of the vagina has caused this mode of
origin to' be questioned by those who regard it as a fold of
the skin of the vestibule.

The vulva includes a number of parts or regions each
characterized by certain histological features worthy of

CHAPTER XIX. REPRODrCTIYE ORGAN'S. 24."

brief mention. The surface of the area known as the ves-
tibule is covered by a mucous membrane continuous with
that of the vagina and of the urethra at their respective
orifices. It is covered with stratified squamous epitheli-
um, contains numerous elastic fibres, and is feebly erectile:
it also contains numerous simple mucous glands. At the
lower limit of the vestibular area are seen on either side of
the vaginal orifice the openings of the ducts of the glands
of Bartholin, small racemose glands of the mucous type
homologous with the glands of Co wper in the male sub-
ject. Beneath the mucous membrane of the vestibule and
somewhat external to the proper limits of the vestibular
surface are paired elongated masses of erectile tissue, the
bulbi vestibuli, whose converging upper extremities are
continuous with smaller plexuses whose vessels are conflu-
ent above with those of the glans clitoridis. The bulbar
regions may be regarded as corresponding to the bilateral
bulbous portions of the corpus spongiosum of the male
subject.

The clitoris consists of two small corpora cavernosa
identical in structure and relations wnth those of the male,
and a small glans of spongy erectile tissue, which is of
course imperforate. Its surface contains numerous geni-
tal corpuscles. Its preputial fold is continuous with the
upper and its fraenum with the lower of the anterior di-
visions of the labia minora. The latter, while resembling
the surface of the vestibule in color and texture, may be
regarded as folds of the skin: the}' contain numerous large
sebaceous glands, but sweat glands are wanting, as are
also hairs : their inner surface contains numerous genital

246 PART II. HISTOLOGICAL ANATOMY,

corpuscles. Fat is wanting in the subcutaneous connec-
tive tissue, but large irregular venous channels are present,
with smooth muscular fibres, composing here as elsewhere
a loose form of erectile tissue. The labia majora are well
defined folds of the skin whose inner surfaces resemble in
appearance the outer surfaces of the labia minora, with
which they are confluent, but differ from them in the pre-
sence of occasional modified sweat glands and minute
hairs, associated with the sebaceous glands common to
both. The thick and rounded margin of the fold affords
a gradual transition from the moist stratified'squamous
epithelium of the mucous type found upon the greater por-
tion of the vulvar surface to the epidermis of the skin,
with which its outer surface agrees in general struct-
ure. In the deeper portions of the integument of the labia
majora is found a layer of tissue similar to that forming
the dartos tunic of the male scrotum, with which the labia
correspond. Like the divisions of that structure they
converge above to be united upon the pubic eminence in
the region known as the mons veneris, characterized, as
in the male, by an abundance of coarse curling hairs, of
numerous enlarged sudoriparous glands, and by a dense
mass of subcutaneous fat.

The mammary glands, while essentially tegumentary
organs, differ so greatly from all other dermal glands as
to require consideration apart from the latter: their func-
tional relations to reproduction render it appropriate to

CIIAI'TEK XIX. KlirKODlCTIVH ORGANS. 24-7

discuss them at this time. I'sualh' but not always rudi-
mentary in the malesuljjcct, they are normally but not in-
variably fully developed in the female. Each mammary
gland so called is in reality an aggregate of fifteen or tvventy
distinct glands, if we regard that term as strictly desig-
nating a secretory body provided with a duct which opens
independently, since each of the ducts opens by a separate
orifice upon the skin of the nipple: but their union into a
single anatomical structure is so intimate that it is more
convenient to designate each of these regions as a lobe ot
a compound racemose gland. The structure of these lobes
and of the whole organ varies greatly in relation to function-
al activity ; but it is characteristic of it at all times that it
possesses an unusuallv large proportion of connective tis-
sue and fat in its composition.

Before the gland has been called into functional activity
the lactiferous ducts are present, as well as the ducts ot
the lobules by whose confluence they are formed : the lat-
ter have at the extremities rudimentary acini, which are,
however, relatively few in number, and consist of masses
of epithelial cells. Acini, lobules, and lobes are alike im-
bedded in an extensive stroma of connective tissue which
forms stout septa not only between the lobes but between
their subdivisions as well ; while a considerable amount of
subcutaneous and interstitial adipose tissue is present.

As pregnancy advances the acini become larger and
more numerous, still consisting, however, of solid masses
of cells. At the time of delivery the central cells undergo
fatt}' degeneration and form the colostrum corpuscles of
the milk of commencing lactation. In the fully active

248 PART II. HISTOLOGICAL ANATOMY.

gland the acini are sgherpidal, comparatirely large, anJ
are lined b}^ a single layer of cells which when at rest are
flattened, but during secretion become cuboidal or colum-
nar in form, their extremities containing one or more large
oil-droplets in each instance: the latter are liberated by the
rupture of the cell substance, a portion of w^hich is prob-
abh^ contributed to the secretion. The basement mem-
brane upon which the epithelium rests consists of an en-
dothelioid layer of connective tissue corpuscles : the intra-
lobular stroma is greatW reduced in proportional quan-
tity. The terminal branches of the ducts have a thin base-
ment membrane like that of the alveoli, which is lined by
a single layer of flattened cells, their appearance resemb-
ling that of the ductules of the salivary glands : the larger
lactiferous ducts have stouter walls, and cuboidal epithe-
lium ; these empty into the still larger channels, the galac-
tophorous ducts, one of w^hich leads, as has been stated,
from each lobe to an independent orifice upon the skin of
the nipple. These large ducts have stout walls of fibrous
and elastic tissue, containing a few smooth muscular
fibres : they are lined with columnar epithelium save in
their outermost portions, where the epithelium becomes
stratified. Each has an enlargement, the ampulla, near
its termination at the nipple, whose structure does not
differ from that of the rest of the duct.

In the intervals between lactation the mammary glands
assume a resting condition in many respects similar to
their primitive state. They always contain, however, a
smaller amount of dense fibrous tissue and a larger
amount of fat, and are consequently much less firm in

CHAPTER XIX. RKPRODUCTIVE ORGANS. 240

texture. At the close ot the reproductive period they be-
gin to undergo a retrograde metamorphosis, the acini and
smaller ducts disappearing, the larger ducts collapsing,
and the shrunken organ consisting chiefly of a mass of
connective tissue and fat.

The nipple is a cvlindrical projection from the surface
of the gland, covered with deeply pigmented skin and
composed of the extremities of the lactiferous ducts, as-
sociated bloodvessels, and smooth muscular fibres ar-
ranged in circular and longitudinal bundles. The der-
mal papillae are rich in nerve terminals: there is no
subcutaneous fat : scattered in the surface are the small
racemose glands of Montgomery : the areola at its base
contains sweat-glands and numerous sebaceous glands.

The intimate relations existing between the urinary
and reproductive systems and the homologies between
the male and female sexual organs cannot be clearly
stated without a description of their formation in the
embr^'o, accompanied by a statement of some at least
of the facts of their comparative anatoni}'. No attempt
will be made at this time to discuss systematically
either the embr\'ologv' or the morphology' of these or-
gans, but such account of each will be alone given as
appears necessar\' to the intelligent comprehension of
the relations and homologies above referred to.

The structure fundamental to the whole of the uro-
genital apparatus is what is known as a nephridium.

250 PART II. HISTOLOGICAL ANATOMY.

In the great majority of the classes of the higher inver-
tebrates the elimination of nitrogenous waste products
takes place through the agency of organs designated
by that term. A nephridium is essentially a tubular
structure opening at one extremity upon the surface of
the bod}' (either directly or indirectly), and at the other
communicating with the bod\^ cavity, or coelom, by a
more or less funnel-shaped extremity termed the ne-
phrostome. Its wall is lined with an epithelium which
is glandular throughout a large portion of its extent :
the funnel-shaped internal opening is commonly and the
larger portion of the rest of the tube frequently cili-
ated, the ciliary movement invariably sweeping toward
the external opening. The tube may be simple and
quite direct in its course, or long, convoluted, and di-
vided into specialized regions. Where a well developed
vascular system is present the more or less coiled ne-
phridial tube is usualh^ provided with a rich net-work
of capillaries. Nephridia may be simple: a single pair
opening right and left on the surface of the body, as in
many mollusks; or a pair being found in each of the
majority of the segments of the body, as in the annelids,
where they were first observed, and designated from
their disposition segmental organs: or they ma}' be
compound, a number of funnel-bearing and ciliated tu-
bules opening symmetrically into lateral tubes which
discharge [the secretion of the tubules upon the surface
of the body or into a posterior cloacal sac, as in the roti-
fers. Without entering into a discussion of the homolo-
gies that may exist between the nephridia of the various

CHAPTER XIX. REPRODUCTIVE ORGANS 251

classes of invertebrates, or between those of either of
them and those of the vertebrates, it may be said that the
urinary apparatus of the hitter conforms, in a general
way at least, to the plan last described.

The gonads, in which the reproductive elements are
formed, are in most classes of Metazoa developed within
the body-cavity, or coelom, into which thev ])rotrude,
and into which the reproductive elements, when fully
mature, are in many species discharged. In some of the
lower forms where this takes place, and in which the re-
productive elements are produced in immense numbers,
the body wall becomes distended by their presence and
finalU' liberates them by rupturing, the parent organism
being thereby destroyed ; a condition comparable to that
of the so-called annual plants, which grow, blossom, ripen a
single crop of seeds, and die. In other cases the ne-
phridia serve as channels for the escape of the reproduc-
tive elements : it will be noted that the small and actively
moving spermatozoa would pass far more readih'
through the nephridial tubules than the large and passive
ova; and it is probabK" true in the case of some species
that the female is destroyed by the rupture of the body
wall after the ripening of the first crop of ova, while the
male elements are liberated through the nephridia, the pa-
rent organism surviving. In most of the higher classes of
animals there are distinct channels of discharge, the gon-
aducts, for the reproductive elements : these are in some
cases clearly formed by the modification of nephridia.

The vertebrate urinary aparatus consists in effect of a
series of nephridial tubules arranged along the dorsal wall

252 PART n. HISTOLOGICAL ANATOMY.

of the body cavity on either side of the mid-plane and
opening into right and left ducts of discharge. The dis-
position, form and relations of the tubules, and the origin
and position of the duct of discharge of the functional kid-
ney undergo important modifications in the various
classes of vertebrates. Taking the group as a whole the
series of tubules may be said to be divided into three reg-
ions, the pronephros or head-kidney, the mesonephros
or primitive kidney, and the metanephros or permanent
kidney of the higher vertebrates. Each of these regions
needs brief consideration.

The pronephros is best developed in the anamnia (cyclos-
tomes, fish-like vertebrates, and amphibia), in all of which
it is present as a well-defined structure in the embr3'o, and
in some of which it is functional in the adult. The tubules
are always few in number, sometimes but one on each
side, open upon the body cavity by distinct nephrostomes,
and discharge into ducts leading to the cloaca and known
as the segmental ducts. Near the opening of the nephro-
stomes there is found on each side a process of the coelo-
mic wall containing a mass of capillaries and constituting
a prominent glomus : the region of the coelom where this
occurs is either partialK' or in some cases wholly con-
stricted off from the rest of the body-cavit}- so as to form
"what is practically an enormous Malpighian bod}':"
thus foreshadowing the arrangement found in the perma-
nent kidncA' of the mammals. The development of the
pronephros is embr\'onic and rudimentarv in the sharks
and their allies among the fish-like vertebrates, and in all
amniota (reptiles, birds, and mammals): in the last named

CHATTER XIX REPRODUCTIVE ORGANS. 253

class it is doubtfully represented in the adult male and fe-
male by rudimentary structures connected with the
surprarenal capsule.

The mesonephros is the functional kidnc}- of all anam-
nia except the sharks and their allies. In this class and in
the various classes of the amniota it is represented by the
embryonic structure termed the Wolffian body. It con-
sists primarily of serially arranged tubules plainh' homol-
ogous with those of the pronephros, although their mode
of development is not identical, being somewhat acceler-
ated ; a point of importance in connection with the origin
of the metanephros. These tubules become variously
coiled and convoluted, and may give rise to branches like
themselves: in many anamnia they open upon the peri-
toneal surface by nephrostomes : the\' have associated
with them glomeruli resembling in their origin the glom-
us just described in connection with the pronephros : in
some cases several tubules open upon a single glomerulus.
The}'^ discharge into a modification of the segmental duct
known as the mesonephric or Wolffian duct: it differs
from the primary segmental duct in undergoing (in some
species) longitudinal cleavage to give rise to a second
canal called the Muellerian duct.

The ripened gonads of many anamnia discharge the re-
productive elements into the body cavity by rupture,
from which the\' escape through openings posteriorly and
ventrally placed and known as abdominal pores. In
other cases abdominal pores are wanting: in the female
the ova pass out through the Mullerian duct or oviduct ;
in the male the testis lies opposite the anterior end of the

254 PART II. HISTOLOGICAL ANATOMY.

primitive kidney, or Wolffian bod}' ; diverticula grow out
from the Wolffian tubules of that region and become con-
nected with the testis, forming channels of escape for the
spermatozoa, which thus reach the Wolffian duct ; the
latter therefore functions both as a ureter and a spermi-
duct.

In the selachians (the sharks and their allies), and in all
amniota a diverticulum is given offfrom the posterior por-
tion of the Wolffian duct. In the former class this be-
comes connected with a mass of serially disposed tubules
arising posterior to the Wolffian body, the metane-
phros, or permanent kidney ; the duct in question being
the true ureter. In the amniota the ureter grows out to-
ward a mass of cells in which true uriniferous tubules
with their associated Malpighian corpuscles are being
formed. It is not certain that this structure is strictly
homologous with the selachian metanephros: it may, per-
haps, be so regarded, the difference in the mode of devel-
opment being due to an intensified acceleration of growth
similar to that already mentioned in the case of the Wolff-
ian body. The latter structure never acts as a kidney in
the higher vertebrates, its sole function being connected
with the transmission of the reproductive elements of the
male : the Wolffian duct becomes the spermiduct or vas
deferens : in the female the whole structure is functionless
and rudimentary.

This digression into the field of comparative anatomy
W'ill not be without value if it aids in making clear the
brief statement of the development and homologies of the
male and female urogenital apparatus of the human sub-

CHAPrEK XIX KKPRODUCTIVE ORGANS. 255

ject based upon the facts nicntloncd therein, whicli is
now to be entered upon.

Early in the development of the embryo there may be
seen ji^rowing downward from the dorsal wall of the body
cavity on eit]ier side a well-defined eminence, the Wolffian
orexcretory ridge: alongitsbasearefound two ducts, the
Wolffian and Mitllcrian. Just niesad of the excretor\' ridge,
and separated from it by so shallow a groove at first as
almost to seem a modification of its mesial surface, is a
second and smaller ridge, the germinal ridge. The latter
becomes the gonad : the epithelium of its surface early be-
comes columnar and shows here and there numerous prim-
ordial ova; the characteristiccellsbcingso termed without
regard to the future sex. The changes of position which
the gonads undergo in each sex will be mentioned later.

The embryonic pronephros is formed at the anterior ex-
tremity of the excretory ridge. The posterior portion, as
it developes backward, becomes the blastema or mass of
cells in which the uriniferous tubules of the kidney are
formed : it is joined by the ureter, whose extremit}' branches
and forms the pelvis and calyx of the kidne\', and possibly
to some extent the collecting tubules: the exact relation
of the two intergrowing structures is not j-et fulh' known.
At first the most posterior, the kidne}^ gradual!}^ advances
in position until it occupies a location anterior to all the
associated structures.

The middle region of the excretory ridge becomes the
Wolffian bod\' : in both sexes it develops as a series of
transverse tubules in relation with the longitudinal Wolff-
ian duct. In the male the tubules of the anterior portion

256 PART II. HISTOLOGICAL ANATOMY.

become connected with the testis, forming the coni vascu-
losi of the epididymis, and possibly the rete testis as welh
those of the posterior portion, corresponding to the strictly
renal portion of the primitive kidney of the anamnia, be-
come rudimentary and form the vasa aberrantia and the
paradidymis, or organ of Giraldes. The Wolffian duct be-
comes the convoluted tubule of the epididymis with its
continuation, the vas deferens, the latter giving off the semi-
nal vesicles as it is transformed into the ejaculatory duct.
In the female the whole structure, being functionless be-
comes rudimentary : the anterior portion of the tubules,
corresponding to the epididymis, becomes the parovarium
or epoophoron ; the posterior portion becomes the paro-
ophoron : the Wolffian duct becomes the rudimentary duct
of Gartner.

The Miillerian duct undergoes corresponding differentia-
tion. In the female, where it is functional, the upper por-
tion terminates in the fimbriated extremity: the middle
portion becomes the Fallopian tube, or oviduct in the
strict sense. The lower portions coalesce on the mid-line,
the upper region of the median structure forming the
uterus and the lower the vagina: a discussion of the mode
of their coalescence would take us again into the domain
of comparative anatomy. In the male the Miillerian duct
is, like the Wolffian duct of the female, altogether function-
less. It early disappears throughout the greater part of
its extent: the upper portion is perhaps represented by the
sessile hydatid of the testis : the coalesced lower portions
form the uterus masculinus, which sometimes showstraces
of a vaginal region.

CHAPTER XIX. REPRODUCTIVK ORCANS. 2o7

In all amniota there is found very early in embryonic
life a vesicular diverticulum of the ventral wall of the in-
testine at a point near its posterior termination: this out-
growth is called the allantois. In the mammals a consid-
erable portion of it is enclosed within the body cavity: the
distal portion of this becomes enlarged to form the urin-
ary bladder; a narrow region corresponding to the true
urethra of both sexes connects it with a proximal enlarge-
ment termed the urogenital sinus: the latter opens in
common with the intestine into a short cloaca formed by
a depression of the ventral surface of the body. The Miil-
lerian and Wolffian ducts open into the sinus: the ureters
at first open into the Wolffian duct, from which they are
derived ; later they open independently into the sinus : as
growth advances they shift their position upward, until
they reach their permanent point of discharge on the sur-
face of the bladder.

The mammalian cloaca is from the first a very shallow
depression : a transverse fold soon separates it into a pos-
terior anal portion, and an anterior region which is the
continuation ofthe urogenital sinus. Just in front of it there
is found the genital eminence, whose posterior surface is
grooved, the margins of the groove forming the genital
folds; in front of its base the skin of the pubis is thrown
into a thick fold (the future mons veneris) which is pro-
longed backward right and left in the genital ridges. In
the male the genital eminence elongates ; corpora caver-
nosa are formed in it : the genital folds coalesce from be-
hind foward, converting the groove on the posterior surface
of the eminence into a canal which is the continuation of

258 PART ir. HISTOLOGICAL ANATOMY.

the narrow and elongated sinus; while the development
of erectile tissue in the walls of the canal gives rise to the
corpus spongiosum : the canal becomes the urethra of the
penis. In the female the eminence remains small and be-
comes the clitoris : the sinus becomes short and broad and
is represented by the vestibule : the genital folds become
the labia minores ; the genital ridges the labia majores.

Glandular diverticula of the sinus are found in both
sexes. The prostate of the male may be regarded as so
derived; there-is no corresponding structure in the female:
the glands of Cowper and the glands of Bartholin are ho-
mologous specializations of urethral structures. The
change of position of the gonads has been already re-
ferred to. Situated at first in the more anterior region of
the abdominal portions of the body cavity, they undergo
an apparent shifting backward in both sexes. The change
is least in the female, where they find a permanent resting
place within the pelvis. In the male they reach the brim
of the pelvis, penetrate the abdominal wall, and push their
way into the genital ridges : these enlarge, become saccu-
lar, and coalesce below the united genital fold to form
the scrotum.

CHAPTER XX. VASCULAK SYSTEM. 259

CHAPTER XX.
THE VASCULAR SYSTEM.

On account of the part played by the smaller arteries,
veins and lymphatics as components of the organs of the
body in which they occur, the structure of such vessels was
described in one of the earlier chapters of this book.
The larger vessels must themselves be regarded as organs
meriting separate consideration equally with the heart,
which is the central organ of the vascular system.

The arteries of the body differ most conspicuously from
the arterioles found in the various organs b}' the relative
increase of the media : but each of the three layers under-
goes both increase and modification. The intima is lined,
as in the smaller vessels, by endothelial cells elongated in
the direction of the tube : but this endothelium rests upon
a subendothelial layer of fibrous tissue with branched
corpuscles that disappears as we pass to the finer subdi-
visions of the vessel : with increase in the size of the artery
it becomes more fully developed. This layer is directly in
contact outwardly with the elastic layer, which is well de-
veloped, especially in the larger arteries, where it has the
form of a fenestrated membrane, the membrana elastica
intimae.

The media consists largeh^ of transversely disposed
smooth muscular fibres, particularly- in the smaller ar-

260 PART II. HISTOLOGICAL ANATOMY.

teries of the limbs: in large arteries, however, there is a dis-
tinct admixture of elastic tissue in the form of a network
of fibres; this is connected with the elastic layer of the in-
tima, and pervades the whole muscular coat: it increases
in proportion with the size of the vessel. The muscular
fibres of the media are quite short, and are very irregular
in form, lacking the definite spindle shape characteristic
of smooth fibres in most places where thej^ occur. In some
of the larger arteries many of the muscular bundles of the
inner part of the media are longitudinally disposed.

The adventitia is the stoutest and most resistant of the
coats of the arteries. It is rich in elastic tissue, especially
toward the media, the larger arteries exhibiting just ex-
ternal to that coat a distinct elastic la^^er, the membrana
elastica externa; this is followed by a region rich in elas-
tic fibres: more outwardl}^ the adventitia consists almost
wholly of closely felted bundles of white fibrous tissue
w^hich at its outer surface passes over into the intersti-
tial areolar tissue found between the vessel in question
and the adjacent organs. In some of the larger arteries
longitudinal bundles of smooth muscular fibres are found.

In connection with the discussion of the respiratory
tract regarded as a diverticulum of the alimentary canal,
it was pointed out that the coats of the latter were desig-
nated by characters readily demonstrable with the dis-
secting knife, but that the proper division based upon his-
tological characters would be into an epithelial and a
musculo-skeletal la3'er. The coats of an artery as above
described are also examples of structures clearly disting-
uishable by anatomical methods : but a consideration of

Cir.VPTF^R XX. VASCl'LAR SYSTIvM. 261

the histology of the wall of an artery will show the cor-
rectness of the description that has been proposed for
it as "composed of muscular and elastic tissue lined
intcrnalh' by endothelium and strengthened externally by
a layer of connective tissue."

The features which have been mentioned as character-
istic of the largest arteries are intensified in the structure
of the aorta. In it the elastic layer of the intima is not so
distinctly membranous, being composed chiefly of fibres
which pass into those of the media, the two coats not be-
ing sharply defined. The media itself not only contains a
ver}' large amount of elastic tissue in proportion to the
muscular tissue present, but is also reinforced b}' a consider-
able quantit\^ of white fibrous tissue, the coat being char-
acterized more especially by its strength and elasticity
than by its contractility. As in sojne of the larger arter-
ies there are both longitudinal and transvere bundles of
muscular fibres. The adventitia is not sharply defined
from the media, and is relatively- thin. The pulmonary
artery agrees in most respects with the aorta in struct-
ure. The larger arteries and veins have, like other or-
gans of the body, their own small vessels of supply ; these
are called the vasa vasorum.

While the heart is quite a complex organ from the ana-
tomical standpoint, its histological structure is compara-
tively simple. It may be regarded as essentially a hollow
mass of muscular fibres of a peculiar sort, having on its
outside an investing skeletal laj-er, and a similar lin-

262 PART II. HISTOLOGICAL ANATOMY,"

ing inwardl\, each of the layers being bounded by an en-
dothelium. The outer la\'er, or epicardium, is the cardiac
portion of the pericardium. It is invested with serous
endothelium which rests upon a membrane whose outer
portion consists chiefl\^ of white fibrous tissue, but which
contains in its deeper layer a considerable quantity of
elastic fibres arranged in a loose and indefinite meshwork.
Beneath the membrane is a stratum of areolar tissue
which is rich in fat cells, and in which run the vessels and
nerves of the wall of the heart ; in the auricles there are
numerous small ganglia connected with the nerves. The
lining membrane, or endocardium, is not unlike the epi-
cardium : its investing endothelium, while continuous
with that lining the arteries and veins, and agreeing in
function with vascular endothelium, is composed of cells
which are not elongated in form, but resemble rather in
outline those found on serous surfaces. The membrane
contains a few muscular fibres, and the subjacent con-
nective tissue but a limited quantity of fat cells. In the
auricles the elastic tissue of the membrane is quite well
developed.

The myocardium or muscular layer of the heart varies
greatlv in thickness, being most developed in the ventricu-
lar wall and least in some portions of the auricles. The
peculiar cardiac muscular elements of which it is com-
posed have already been described : they are arranged in
bundles which form the fibres and lamellae visible to the
naked eye: the disposition of the latter is a subject of an-
atomical rather than histological study. Between the
bundles is a very delicate framework of connective tissue

CHAPTER XX. VASCULAR SYSTEM. 263

which supports the abundant blood and lymph capillar-
ies: patches of adenoid tissue are also of occasional occur-
rence. The peculiar fibres of Purkinje found just be-
neath the endocardium of some mammals are of doubtful
occurrence io man: they are large cellular elements (fre-
quently with two nuclei) whose central portion consists
of clear protoplasm but whose surface has undergone
striation ; they may perhaps be regarded as imperfectly
developed cardiac fibres.

The columnae carneae and the papillary muscles are
alike processes of the myocardium : in them the muscular
fibres are chiefly disposed in a longitudinal direction, and
the endocardial membrane upon their surfaces becomes
almost tendinous in structure; the latter passes over from
the surface of the papillary muscles into the chordae ten-
dineae, whose central strands of fibrous tissue are in the
larger cords reinforced by scattered bundles of muscular
fibres.

The valves of the heart are folds of the endocardium
containing bundles of fibrous tissue and a variable quan-
tity' of elasjtic tissue, the latter being present in greatest
quantity in those places subject to the greatest pressure
produced by the heart's action on the blood : the semi-
lunar valves are farther reinforced by the presence in each
of a nodule composed largely of elasticjtissue, the corpus
arantii.

The large veins differ from the corresponding arteries in
the relative thinness of the media and the thickness of the
adventitia, and in the smaller amount of elastic tissue
throughout the entire structure. The endothelial cells lin-

264* PART 11. HISTOLOGICAL ANATOMY.

ing the intima of the larger veins are but slightly if at all
elono^ated, resembling those of the endocardium in their
polygonal outline: in the smaller tributaries the form char-
acteristic of vascular endothelium is assumed. The media
of the two venae cavae is a continuation of the mj^ocar-
dium of the right auricle, and contains cardiac muscular
elements : that of the larger veins generally contains but a
small quantity of muscular tissue, intermingled with
bundles of white fibres, which replace the elastic tissue
similarly situated in the larger arteries. The media of
some veins, notably those of the nervous axis and^of the
bones, is entirely devoid of muscular tissue. In a few-
cases the muscular bundles situated in the inner part of
the media are longitudinal in their direction. The adven-
titia of the veins consists chiefly of a stout layer of felted
bundles of fibrous tissue, containing a moderate amount
of elastic fibres. In the portal, renal, and some other veins,
the adventitia contains numerous bundles of smooth
muscular fibres longitudinally disposed. The valves of
the veins are folds of the intima, strengthened by bundles
of fibrous tissue arranged parallel to their free borders,
and containing a small amount of elastic tissue: mus-
cular bundles from the media sometimes extend into the
base.

The lymphatic vessels resemble the blood vessels in the
fact that their walls are composed of an inner, a middle,
and an outer coat : each of the three coats is, however,
much thinner than in the case of the blood vessels, the
middle and outer being especially so. It is customary to

CHAPTER XX. VASCULAR SYSTEM. 265

say that they resemble the veins in structure: it is more
accurate to say that in the relative thickness of the coats
and particularly in the proportion of muscular tissue they
approach the arteries : they resemble the veins in the
structure of the intima and in its development into valvu-
lar folds : the adventitia is relatively to the other coats
weaker and simpler than in either arteries or veins. This
is not the case, however, with the most highly specialized
of all the lymphatic vessels, the thoracic duct, whose ad-
ventitia is quite stout and contains longitudinal bundles of
smooth muscular fibres : the intima of the duct is also re-
inforced with a longitudinal network of elastic fibres. In
the smaller lymphatic vessels the adventitia disappears;
the media contains bundles of smooth muscular fibres
obliquely disposed, in addition to the ordinary transverse
bundles.

The development of the heart and of the larger vascular
trunks takes place in such a manner that it is difficult to
discuss it without either taking for granted a knowledge
of or entering into a description of the manner in which
the ground plan of the embryo body is laid down. It is
therefore necessary to defier it until after the subject of
Embryology has been taken up by the student.

Reference mav with convenience at this time be asrain
made to the great serous membranes which line the
principal cavities of the body ; since these latter are best re-

266 PART II. HISTOLOGICAL ANATOMY.

garded as enormous lymph cavities. Their general struct-
ure and particularlj^ that of their characteristic epithe-
lium has been discussed in a previous chapter: it remains
to speak briefly of some of the features characteristic of
each (in so far as they have distinguishing characteristics)
and to describe some allied though different structures
usuall}^ associated with them.

The right and left pleurae, which invest the lungs, line
the thoracic cavities and bound the mediastinum which
separates the latter, while they have the general charac-
ter of serous membranes, vary somewhat in structure in
different localities. They are thickest over the ribs, where
the subserous layer of areolar tissue permits of the ready
removal of the membrane : the stomata of the endothe-
lium are said to occur over the intercostal spaces onlv-. The
membrane becomes thinner and more adherent as it passes
onto the surface of the diaphragm. On the surface of the
lungs it is quite thin and closely attached, the connective
tissue becoming continuous with that of the framework
of the organ: the endothelial cells of the pulmonary
pleura are also taller and more granular. The pericardium,
or heart sac, is in reality but little more than a double
serous membrane: its outer portion being derived from the
pleurae and its inner forming the proper pericardial mem-
brane which is reflected upon the heart as the epicardium :
between the two membranes is a layer of areolar tissue
containing a small quantity of fat and numerous |blood
vessels and lymphatics.

The peritoneum, which lines the cavity of the abdomen,
is the largest and most complicated of the serous mem-

CHAPTER XX. VASCULAR SYSTEM 267

branes. Like the pleura, it is thicker in its parietal than
its visceral portion. The subserous layer of areolar tis-
sue is less developed along the ventral midline and on the
under surface of the diaphragm than elsewhere in the
parietal peritoneum, which is consequent!}' most closely
adherent in these regions : that of the visceral portions is
in every case closely connected with the skeletal structures
of the organs invested. Certain of its folds, termed liga-
ments, from their mechanical relations to the organs with
which they are connected, for example, the spleen, the liver,
and the uterus, have their applied fibrous membranes in
close contact. Others, such as the omenta, have between
the membranes a laj'er of areolar tissue in which a greater
or less amount of fat may be developed. In the me-
sentery the intervening layer contains, in addition to a
variable amount of fat, an extensive system of blood and
Ij'mph vessels: associated with the latter are the numer-
ous nodules of adenoid tissue commonly' called the lacteal
glands : their structure will be described in the following
chapter in connection with other similar bodies. The
tunica vaginalis of the scrotum and testis must be re-
garded as an oft'set from the peritoneum : its structure
and relations have been sufficiently described in a pre-
vious chapter.

The meninges of the brain and spinal cord, and in par-
ticular the pia mater, have been regarded as serous mem-
branes : the}' differ, however, in structure and relations
alike from the ordinary membranes of that name, and will
best be discussed in connection with the nervous axis. The
perilymphatic and endolymphatic surfaces of the internal

268 PART II. HISTOLOGICAL ANATOMY.

ear will be considered in connection with the description of
that organ.

The synovial membranes, which line the capsules of
joints, the sheaths of tendons, etc., may here be consid-
ered, although they differ materially in structure from the
serous membranes wnth which they are often associated
as regards both their organization and also the charac-
teristic fluid which fills the cavities bounded by them.
They are in effect rather dense membranous layers of con-
nective tissue, devoid of any well-defined endothelial in-
vestment : here and there cells and patches of cells may be
seen upon their surfaces, some of which are sufficiently
close together to assume the polygonal outline character-
istic of endothelium; others, how^ever, are distinctly
branched, differing in no essential from ordinary connect-
ive tissue corpuscles : their affinity to endothelium will
perhaps be evident if we recall the definition of it previ-
ousl}^ given to the effect that it is a layer of connective
tissue corpuscles investing a free surface. Fringed vascu-
lar folds occur upons\movial membranes which frequently
bear smaller processes, the synovial villi, in which a cen-
tral strand of fibrous tissue is invested with a layer of
small rounded cells. Articular synovial membranes pass
so gradually into the fibro-cartilage which borders the true
articular cartilage of the joint that it is not possible to
define the boundary between the two : even the cellular
elements gradually losing their processes and presenting
the appearance of cartilage corpuscles.

CHAPTER XXI. THE DUCTLESS BODIES. 269

CHAPTER XXI.

THE DUCTLESS BODIES.

The organs here included under the above title are very
frequently referred to by the name of the ductless glands.
If the term gland be used in its older and looser sense as
a designation for any soft parench3'matous body, then a
distinction between such of those bodies as are provided
with and those devoid of ducts is natural and justifiable.
It is better, however, to use the term in its narrower and
more definite application to bodies composed essentially
of acini or tubules lined with epithelial cells whose funct-
tion is to secrete a specific solution or soluble substance,
ordinarily removed through a definite channel of dis-
charge. If the term gland be thus employed, there is but
one (and a portion of another) of the bodies here consid-
ered to which the expression ductless gland may with any
propriety be applied : as used to designate the others it is
not only erroneous, but tends to give rise to. misleading
conceptions. The expression ductless bodies, while not
without objections, is therefore preferable to the older
title. The name of adenoid bodies has also been pro-
posed, but is apt to lead to the false idea of a community
of structure in all of the bodies in question.

It is quite important to note in this connection that the
caption given to this chapter, unlike any other used hith-

1' ^

\

^5^70 PART n. HISTOLOGICAL ANATOMY. .

erto, does not designate any s\'stem of organs associated
with the performance ofone of the great functions or groups
of functions, or characterized bv anv common structure
or b}' similar anatomical or morphological relations : their
association under one heading is more than anything else
a matter of convenience ; and the order of their consider-
ation largely arbitrary, though not altogether so. The
lymphatic bodies, from their close structural and func-
tional relation to the Ij'mphatic vessels, might with equal
propriety be described in connection with the vascular sys-
tem : the spleen, differing in important respects from the
bodies just named, resembles them in its close relation to
the circulatory system : the thymus has much in common
w^ith them in its structure: to all these bodies (and to these
alone) the term adenoid bodies might with propriety be
applied, owing to the prevalence in them of the tissue of
that name.

The thymus, in addition to the adenoid tissue of which
it is largely composed, contains definite masses of epithe-
lial elements whose derivation will be discussed when
that body is described : the thyroid, is largely composed
of epithelial cells which line acini ; to it more than to any
other of the bodies here considered may the title of duct-
less gland be with propriety applied, as is shown b}^ its
embryonic development. Resembling the thymus and
the thyroid in the origin of some of their elements (and in
little else) are the parathyroids, situated on either side of
each lobe of the thyroid ; and the carotid glands ; they
are closely resembled in structure, though not in origin,
by the coccygeal gland; the term gland is still univer-

CHAI'TKK XXI. THE DUCTLESS BODIES. 271

sally applied to the last two; they have been called, from
their associations, the arterial glands.

The suprarenal capsules, or the adrenals, as they are
sometimes termed, are double structures, consisting in
part of a mass of nervous tissue: they are resembled in
this respect by the pituitary body, which consists in part
of a mass somewhat analogous to the thyroid, and in
part of an atrophied lobe of the brain: the pineal body
is altogether derived from the modification of a portion
of the brain, and might, in common with the pituitary
body, be described in connection with that organ. This brief
enumeration will suffice to show how heterogenous are
the bodies here associated as regards both structure and
anatomical relations: the function of most of them is at
present entirely unknown. An account of the details of
their structure will now be given.

The simplest lymphatic bodies are those rounded masses
of adenoid tissue bounded by a more or less definite fib-
brous layer which have already been mentioned in con-
nection with the description of the alimentary canal. The
lymphatic follicles, as they are commonly called, of the
intestine, whether solitary or clustered (as in the Peyer's
patches of the ileum), and the similar masses whose some-
what confluent aggregations form the tonsils, are exam-
ples of what may perhaps best be called lymphatic nod-
ules. They consist essentiall}^ of spheroidal lumps of ade-
noid tissue somewhat more dense in the outer than the
inner portion, but showing no division into lobes. Thej'
are pervaded by capillary networks, and may be invested

272 PART II. HISTOLOGICAL ANATOMY.

by a large lymph sinus or by a plexus of small lymphatic
vessels. It is not easy to demonstrate among the latter
distinctl}^ efferent or afferent trunks : and from the usual
position of these bodies in mucous membranes, and from
what we now know of the important bacteriophagous
function of leucocytes upon mucous surfaces it may be
questioned whether those formed in the lymphatic nodules
do not find their chief destination there rather than in the
l3''mph vStreara.

The larger and more deeply seated h^mphatic bodies
commonly termed lymphatic glands, together with the
altoofether similar bodies found in the mesentery and called
lacteal glands, for both of which the much better name
of lymphatic nodes has of late years come into use, are
in reality integral parts of the lymphatic and therefore of
the circulatory system. Each is a rounded body showing
a distinct depression, the hilum, on one side, at which
point the blood vessels of the interior enter, and from
which one or more efferent lymphatics leave the organ.
The surface is invested with a stout fibrous^capsule, con-
taining scattered bundles of smooth muscular fibres in the
larger nodes: it has a proper network of blood vessels,
and into it at various points pass afferent lympahtics
which traverse it quite obliquely, sometimes forming
small plexuses or sinuses within it before communica-
ting with the lymph channels within.

The interior is divided into a cortical and a medullary
portion : from the capsule stout trabeculae of fibrous tis-
sue (sometimes containing smooth muscular fibres) pass
inward : they are frequently broad and lamellar in form

CHAPTER XXI. THE DUCTLESS GLANDS. 273

and divide the cortex imperfectly into suljecjual lobules
sometimes called the cortical follicles and compared to
the lymphatic nodules (or so-called follicles) already de-
scribed, from which, however, they differ in important re-
spects: the inner extremities of their trabeculae subdivide,
communicating by their branches with a coarse network
of fibrous tissue which forms the framework of the medul-
larx' portion.

The spaces of the network just mentioned are occupied
b\^ another network of rounded strands of adenoid tis-
sue, the medullary cords, which become continuous at
their extremities with the solid cortical lobules of the
same tissue situated between the trabeculae. It will be
remembered that adenoid tissue ^nsists in effect of reti-
form tissue whose interstices are filled wrth lymphoblasts:
the medullary cords and cortical lobules of adenoid tissue
are in each case smaller than the cavities in which they are
situated, the surrounding spaces containing a coarser ret-
iform tissue (devoid of lymphoblasts and presenting but
little resistance to the passage of fluids) which connects
the cords and lobules with the medullary and trabecular
framework. A network of passages throughout the whole
node, known as the lymph sinuses or lymph channels,
is thus formed, into which the afferent l^'mphatics open
after passing through the capsule, and from which the
efferent lymphatics lead.

The medullary portion of the lymphatic node extends to
the surface at the hilum, the artery of supply entering it
directly : the capillary network of the interior is situated
almost entirely in the deeper portion of the adenoid

,>f

274 PART n. HISTOLOGICAL ANATOMY.

strands and masses. The lymph channels of the medulla
converge at the hilumtororm a plexus, from which a single
efferent trunk may lead, or, in the larger nodes, several
smaller vessels which unite outside the node to form a
single trunk. Pacinian bodies are of frequent occurrence
in the interstitial connective tissue j ust without the ^ap-
sjale at the hiluni.

The relation between the lymphatic nodes and the lymph
stream, as regards the formation of lymphocytes and
their transformation in the blood stream into leucocytes,
has been discussed in a previous chapter. The minute is-
lands of adenoid tissue occasionally found either on or
within the walls of lymphatic vessels, and known as peri-
lymphatic or endolymphatic nodules, may be regarded
as rudimentary organs of the same kind.

The spleen is as closely related to the blood vascular as
the lymphatic nodes to the lymphatic portion of the cir-
culatory system : it may possibly be regarded as derived
from the modification of one of the last named bodies,
though differing from them greatly not only in its vascu-
lar relations, but also in its internal structure. It is inter-
esting to note that in some of the lower mammals num-
erous small accessory spleen-like nodules are norrnally pre-
sent in other regions of the body. Similar bodies are
sometimes found in man in the vicinity of the principal
organ, of which they may be regarded as diverticula.

The spleen is invested by a serosa derived from the peri-
toneum which rests upon a stout capsule of fibrous tissue
which, like that of the lymphatic nodes, contains occa-

CHAPTER XXI. TIIK DUCTLESS GLANDS. 275

sional sraqoth_muscular fibres: it differs from that of the
bodies last mentioned in the greater predominence of elas-
tic fibres, making the organ highly distensible. At the
hilum the capsule is continued into the spleen to form large
trabeculae. which branch and subdivide within, eventually
becoming continuous with the branches of similar though
small trabeculae which pass inward from the capsule at
numerous points: the trabeculae, like the capsule, contain
numerous elastic fibres, and some muscular fibres : the
interior of the organ is pervaded by the large-meshed re-
ticular framework thus produced. Continuous with this
framework is a coarse retiform tissue whose fibrous net-
work is invested with branched corpuscles : in many cases
the fibrous element is quite scanty, and the reticulum con-
sists of little more than branched corpuscles connected
with each other by the tips of their branches, the so-called
reticular cells of the spleen. The intervals between these
cells are filled with blood which contains rather more than
the usual proportion of colorless corpuscles and rather less
of the colored : there are present also numerous unbranch-
ed amoeboid cells somewhat larger than colorless corpus-
cles, the spleen-cells: these, the reticular cells, and the
^asma of the blood itself contains disintegrating colored
corpuscles, and pigment granules derived therefrom : the
whole constitutes the spleen pulp, a reddish brown
mass to which the characteristic color of the organ is due.
The splenic arterj' divides into several branches just be-
fore reaching the organ : these enter at the hilum, follow-
ing the stout trabecular continuations ofthe capsule above
mentioned : within the latter they branch, their branches

^^'"'^" f i J ., r L ,

/luJ.^u^u^^Mf^'^^

276 PART n. HISTOLOGICAL ANATOMY.

in some cases followmg the subdivisions of the trabeculae.
In other cases small branches leave the trabeculae and be-
come divided into brush-like tufts of arterioles: on emerging,
their adyentitia, heretofore continuous with the fibrous
tissue of the framework of the organ, becomes replaced
by a layer of adenoid tissue : here and there this sheath is
suddenh' enlarged to form spheroidal masses which ma}'
be as much as a millimetre in diameter, though usually
less than half as large: thej^are known as the Malpighian
corpuscles of the spleen, and are readily visible to the
naked e^-e as whitish spots in the dark brown pulp. The
adenoid tissue of the corpuscles is permeated b}' capillaries
given off from the arterioles enclosed ; it is quite loose in
the centre but denser at the surface, where it passes over
abruptly into the retiform tissue of the pulp. The cor-
puscles are found surrounding small arteries and appar-
ently strung upon them, or upon their subdivisions, in
which case they look like lateral outgrowths.

fOn leaving the corpuscles the arteries divide into capil-
laries, which, like those arising from the smaller divisions of
the arteries which follow the framework of the spleen more
1 closely, finall}^ open into the spaces of the tissue of the
_y pulp, the endothelial cells of the capillaries gradually be-
1 coming looser, branching, and finally passing over into
I ) the reticular cells ; thus^f|m:din^ the only instance jn the
body where the blood leaves the definite vessels ,prc)ger_to
\\it and circulates in the interstices of the tissues ; a condi-
tion largely characteristic of the circulation of all inver-
tebrates. The veinlets of the pulp originate in the same
way that the arterioles terminate, or rather its converse :

-;Uu^CMf>4^

CHAPTER Xxi. THE DUCTLESS GLANDS. 277

the reticular cells passing over into branched and loosely
disposed endothelial cells which later become closely united
to form the lining of the commencing vessels. The latter
soon enter the trabeculae, where they are gathered into
larger vein&j these anastomose freely within the trabecu-
lae, finalh' uniting to form the few large trunks that leave
the hilum.

The thymus is a bijobed adenoid body situated just be-
neath the sternum in the upper part of the thorax and ex-
tending into the lower part of the neck in the embryo and
the infant: it is gradually reduced to a mere vestige in the
adult. The whole organ is invested by a thin capsule of
fibrous tissue, beneath which it is subdivided into a num-
ber of irregular lobules each but a few millimetres in di-
ameter. The fibrous tissue envelope of each lobule gives
off trabeculae which penetrate the interior in the same
manner as the similar structures in a 13'mphatic node. The
lobule in consequence exhibits a cortical and a medullarv
portion. The cortex is composed of nodules of adenoid
tissue not unlike those found in a tonsil : the medulla is a
mass of adenoid tissue much less dense than that of the
cortex, the transition from the one to the other being so
rapid as to be quite conspicuous in sections. The trabec-
ulae are continuous with the retiform tissue of the cortex
and medulla alike, but there is no distinct raedullar\' frame-
\vork of fibrous tissue and no segregation of the adenoid
tissue in medullary cords: nor are there any lymph chan-
nels in either cortex or medulla.

The most characteristic feature^ of the thymus is the

- — CJUa^iU^' f

278 -- PART II. HISTOLOGICAL ANATOMY.

presence in the medulla of what are known as Hassall's
corpuscles, or, as that histologist termed them, concen-
tric corpuscles. These are peculiar nest-like groups of epi-
thelioid cells which are now" known to be derived from the
breaking up of right and left tubular diverticula from the
cervical hypoblast. Each corpuscle consists of a central
granular mass containing one or more spheroidal cells,
surrounded by two or three layers of concentric flattened
cells : compound corpuscles sometimes occur, two or three
ordinary corpuscles being invested by a common layer of
concentric cells. The adenoid tissue of the organ contains
a rich capillary network, and is the place of origin of nu-
merous large lymphatics.

The thyroid resembles the thymus in being relatively
large in foetal life and infancy : it differs from that organ
in its persistence and evident functional importance in the
adult, as indicated by the grave consequences of its com-
plete extirpation. Like the tubular structures which even-
tually break up into the concentric corpuscles of the thymus,
its characteristic elements are derived from the cervical
hypoblast. It is at first provided with a duct, whose rudi-
ment becomes the foramen caecum of the dorsum of the
tongue: the duct in question in rare instances persists;
in the great majority of cases it aborts, converting the
organ into a true ductless gland.

Unlike most glandular bodies, the thyroid is not pro-
vided with a well-defined capsule. It is invested by a layer
of areolar tissue considerably denser than that connecting
it with adjacent organs but not passing over into a dis-

CHAPTER XXI. THE DUCTLESS GLANDS. 279

tinct fibrous ineinl)ranc. Areolar tissue of variable density
])erva(k's the interior of the organ, forming the support-
ing framework of its structure, the characteristic feature
of which is the jiresence of great numbers of vesicles
united into imperfect lobules. The vesicles are spheroidal,
polyhedral, or sometimes tubular in form, their walls con-
sisting of a single laver of cubqi^al epithelium. The inte-
rior of the vesicle is filled with a glairy yellowish colloid
substance which frequently contains leucocytes and de-
tached epithelial cells. A distinction has been] made by
some observers between the colloid, cells, which are act-
ivelv engaged in secreting the fluid contained in the ves-
icles, and the reserve cells. A definite basement mem-
brane cannot be clearly discerned, the epithelial cells ap-
pearing to rest directly upon the interstitial septa of are-
olar tissue already mentioned : the areolae of the septa
not infrequently contain the colloid secretion of the cells:
elements resembling plasma cells are found in the inter-
stitial tissue. The thyroid is highly vascular, the arteries
being relativel}' quite large, and anastomosing freely : the
vesicles are surrounded by a rich capillar}' network : the
lymphatics are also large and numerous, and the presence
of colloid substance in their interior may sometimes be
detected.

Imbedded in the substance of the thyroid upon both
the lateral and the mesial surfaces of the lobes are small
bodies a few millimeters in diameter to which the name of
parathyroids has been given. They resemble the thyroid
in color ^ind appearance, but differ from it in ^structure,

f';/^/

280 PART n. HISTOLOGICAL ANATOMY.

consisting of solid strands of epithelioid cells apparently
anastomosing, their interspaces being occupied by numer-
ous blood vessels. They have been regarded by some ob-
servers as masses of embryonic thyroid tissue: this view-
is denied by others, who regard their structure as more
nearly approaching that of the carotid glands. There is
usually associated with each a small mass of adenoid tis-
sue containing concentric corpuscles and in other respects
resembling the substance of the thymus.

The carotid glands, situated in the angle between the
branches of the common carotid artery, are small irregu-
larly shaped bodies whose envelope of connective tissue is
continued inward to form a supporting framework im-
bedded in which are nodular masses of epithelioid cells
richly supplied with capillaries. They resemble the para-
thyroids in their origin from the cervical hypoblast, and
both are probably to be regarded as rudiments of larger
and more important organs.

The coccygeal gland is another body of quite similar
structure to those just described, and probably also rudi-
mentary in its character. The epithelioid cells which con-
stitute its distinguishing feature are to some extent dis-
posed in columnar strands as in the parathyroids. Eberth
has described among them nests of cells resembling con-
centric corpuscles. Attempts have been made to show
that the elements of this body are largely nervous in char-
acter, but this view of their nature lacks confirmation.
The mode of development is not known.

CHAPTER XXI. THE DUCTLESS GLANDS. 281

The suprarenal capsules, or, in the lan<2:uage of com-
parative anatomy, the adrenal bodies, since their position
is usually near but not upon the kidneys, as is the case in
man,^ are in some respects the most complex in structure
of all the -ductless bodies, to no other of which are they
nearly allied. Each on section shows to the naked eye a
distinct yellowish cortex, radialh' striated, and a dark
brownish homogeneous medulla, the two being clearly de-
fined from each other. The surface is invested by a thin
but firm fibrous capsule whose deeper portion shows scat-
tered bundles of smooth muscular fibres: from it tough
fibrous septa enter the interior of the organ to form the
cortical framework, which is limited internally b\' a con-
tinuous layer of connective tissue which bounds the medul-
la. The interior of the latter is also pervaded b}' a frame-
work of fibrous tissue.

The cortex is divided by the difference in the disposition
of its septa and in the consequent mode of segregation of
its elements into three distinct zones which pass into each
other without great abruptness : these are the thin zona
glomerulosa just beneath the capsule, the zona fascicu-
lata next within, which forms by far the greater portion
of the cortex, and the zona reticularis, little if at all
thicker than the outer zone, which lies next the medullary
sheath [of connective tissue. The spaces of the fibrous
framework are occupied in the outer zone by rounded no-
dules, in the middle b\' columnar masses, and in the inner
by a network of strands of closely packed polyhedral cells
of moderate size whose protoplasm shows numerous small
oil globules to which the color of the cortex is largeh^ due.

/ A ^ (? c

V. / ,/

282 PART II. HISTOLOGICAL ANATOMY.

The cells of the inner zone are darker in color, frequently
containing brownish pigment : those of the outer in some
animals are occasionalh' columnar in form, being disposed
about an ill-defined lumen in the centre of the nodule.

The medulla contains within the fibrous stroma irreg"
ular cords and masses of cells larger and much more
loosely arranged than those of the cortex : they are de-
void of oil globules and frequentl}^ exhibit branching pro-
cesses. A rich plexus of non-medullated nerve fibres is
present, and connected with great number of ganglion cells
either scattered or clustered in groups of varj'ing size.
Numerous small ganglia are also found upon the nerves
just external to the hilum.

The arteries of supply enter the surface of the capsule by
numerous small branches : wathin, the vessels are distri-
buted to the cortex along its framework, the capillaries
not pervading the cellular masses as in the parathyroids,
carotid glands, and coccygeal gland : thence they pass to
the medulla, which contains a large plexus of veins whose
branches unite into one at the hilum. The cortex is well
supplied with lymphatics which communicate both with
those of the capsule and with those of the medulla.

In the angles of the irregularly pyramidal adrenals of
the human subject the cortex is folded upon itself, the me-
dulla not extending into the fold ; the two layers of the
zona reticularis are, however, separated b^^a continuation
of the connective tissue layer which surrounds the me-
dulla. The distinctness between the cortex and the medulla
is associated with an important difference in their embry-
onic development. The two arise independently of each

CHAPTER XXI. THE DUCTLESS ORGANS. 288

other, and, indeed, remain so throughout life in some of
the fish-like vertebrates: the eortex arises as an outgrowth
from the peritoneum in close proximity to the mesone-
phros: the medulla is derived from an extension of the
adjaeent sympathetic chain of ganglia : from its close con-
nection with which and from its richness in nervous elements
many are inclined to regard the adrenals as essentially
portions of the nervous system.

The pituitary body is also known as the hypophysis x

cerebri. It is a double structure, consisting of an anterior j?,
and a posterior portion : it would be well if the former
term could be restricted to the first of these and the latter ,p(^V.
to the other, since they are essentially different alike in
structure and in origin. The posterior division is in real-
ity a downgrowth of the brain, as the second term im-
plies: it is the rudiment in man and mammals of what is
a distinct and important lobe of the brain in the fish-like
vertebrates. The anterior is an upgrowth from the epi-
blast which lines the oral invagination, and is in sub-
stance an epithelial body: it is also a rudiment of what
was probabh^ in the earlier vertebrates or their inverte-
brate ancestors an important glandular organ; its path- ^^-^■^■
ological relations indicate that it still has a persistent
though as yet unknown function. The contact and cohe-
sion of the two bodies is confined to the mammals : in all
other vertebrates they remain distinct.

The anterior lobe is larger than the posterior, and is of
a darker reddish color. It consists of spheroidal and sim-
ple or branched[_tubularacini ; their closed cavities are lined

28-4 PART n. HISTOLOGICAL ANATOMY.

either by a mass of polyhedral ep^itheliqid cells which fill
the cavitv, or by a layer of true epithelial cells which sur-
round a distinct though sometimes irregular lumen fre-
quently filled with colloid substance similar to that found
in the thyroid : in some of the larger tubules cilia have
been observed upon the cells. Between the vesicles and
tubules is a framework of connective tissue which sup-
ports the numerous blood vessels and lymphatics and is
continuous with the fibrous capsule.

The posterior lobe is at first a hollow diverticulum of
the 'twixt-brain : in man and all mammals its cavity is
nearly or quite obliterated, and it becomes a small solid
mass. The nervous tissues characteristic of the inferior
lobe of the brain of the fish-like vertebrates fail to devel-
ope in the higher forms, their place being taken by an in-
growth of vessels and of bundles of fibrous tissue: within
the meshes of the latter are found numerous peculiar fus-
iform or stellate cells which are frequently pigmented.
Where a vestige of the cavit\^ remains it is lined with col-
umnar ciliated cells similar to those found lining the cavity
of the third ventricle. The relation of this portion of the
pituitary body to the brain is so close that the whole is
frequent^ described in connection wath that organ : but
the larger and apparently more important anterior por-
tion resembles the thj^roid so much both in structure and
in origin as to justif)^ its description among the anom-
olous ductless bodies.

The pineal body, otherwise known as the epiphysis
cerebri has already been spoken of in connection with the

CHAPTER XXI. THE DUCTLESS GLANDS. 285

pituitary body. Like the posterior portion of that organ,
it is stricth' a portion of the brain ; and its structure
should be included in a full account of that organ. Like
its analogue first mentioned, however, it is in mammals
altogether rudimentary and completely devoid of true
nervous tissue: it has therefore no longer any histological
relation with the nervous axis and may as a matter of
convenience be described at this time. It consists essen-
tially of a number of closed spheroidal or tubular acini
lined or in most cases wholh' filled with polyhedral
epithelial cells, and supported by dense interstitial con-
nective tissue. Among the epithelial cells, as well as on
the outer surface of the body, are numerous gritty calcare-
ous particles known as brain-sand: this is also found in
some other portions of the brain. The pineal body in
some of the lower vertebrates contains true nervous
tissue and is connected with a peculiar median sense organ
sometimes called the parietal or pineal e^-e.

Attention was called In the opening paragraphs of this
book to the frequent use of the term Physiological Anat-
omy as a s3'nonym for Histological Anatomy. The dis-
tinction between the two is very well illustrated by the
facts in the case of the bodies described in this chapter.
It was alike the hope and the expectation of the earlier
histologists and physiologists that the investigation of
the elemental structure of each organ of the body would
throw great light upon its functions. This has proved to
be the case in many instances, and physiology has been

286 PART II. HISTOLOGICAL ANATOMY.

under correspondingly great obligations to histology,
which it has been quick to acknowledge: so ready, indeed,
that the student new to the subject is to some extent
liable to loose sight of the important fact that in not a
few instances no such happy result has followed upon
a dvances in our knowledge of minute structure. While,
for example, the diiference in the form and relations of
the cells lining the alveoli of serous and of mucous glands
respectively can be correlated with the difference in the
character of the fluids secreted by them, no one could
have foretold, given the facts of structure, the associated
difference in function ; there is still less evident relation be-
tween structure and function in the case of the cardiac
and pjdoric glands of the stomach ; and none at all that
can as yet be discerned between the form and arrangement
of the hepatic cells and the secretion of bile. What is true
in numerous other instances is eminently so in the case of
the ductless bodies : we have in them examples of quite com-
plex organs whose histological anatomy has in each case
been ver}^ carefully studied; but whose function is in every
instance, save that of those which consist chiefly of
adenoid tissue, almost if not altogether unknown.

CHAI'TKH XXII. THE NERVOUS AXIS. 287

CHAPTER XXII.

THE CBXTK.\L NERVOUS SYSTEM.

The ductless bodies afford us instances of organs exhib-
iting quite complex histological structure, concerning
whose functions little or nothing is known. A case not
exactly the converse of this is presented b}' the nervous
axis; whose ph3'siological anatomy is exceedingly com-
plex, but whose histological structure is far less so. For
example, experiments show that the white matter of either
side of the spinal cord is resolvable into a number of more
or less distinct tracts, none of which presents any single
structural feature or any combination of features b}'^ which
it can be demonstrated : much the same may be said of
the physiological centres that have been experimentally
located in the grey matter of the cord; in no case can
their positions be demonstrated by histological methods.

Failure to distinguish between the provinces of ph3''si-
ological and of histological anatomy has led many writ-
ers upon the latter to include in their description of the
nervous axis many facts (in themselves of the highest im-
portance) which are demonstrable only by methods that
are essentially physiological : to the confusion of the stu-
dent, who is frequently led to expect that his sections will
show him more than our present histological methods at
least make possible; and being disappointed is apt to

288 PART n. HISTOLOGICAL ANATOMY.

doubt the validity of the statements which he has read.
The present chapter will undertake no more than a brief
description of such structural features as can with cer-
taintv be made out by histological methods; in other
words, b^' a study of the form, relations, and groupings
of the structural elements present. The consideration of
the different regions of the brain wnll follow that of the
spinal cord : and both will be preceded by a short account
of the membranes which invest the nervous axis or line
the cavity in which it is contained.

The meninges, or lining and investing membranes con-
nected with the nervous axis, are three in number: of
these the outer, known as the dura mater, or meninx fi-
brosa, lines more or less closely the spinal canal and the
cavity of the skull ; the inner, called the pia mater, or
meninx vasculosa, closeU^ invests the surface of the cord
and the brain; while the third, the arachnoid, or meninx
serosa, situated between the first and the second, is
structurally connected with the latter, the two being de-
rived from the differentiation of a single investing laj'er.
On account of the intimate relation between these two
meninges they are by some anatomists described as one,
under the name of the leptomeninx, the outer membrane
being termed the pachymeninx : a view which is sup-
ported by the comparat ve anatomy of these structures
as well as bj' their embryology ; it is also true, however,
that the two meninges so designated are themselves form-
ed from a single mass of indifferent tissue originally filling
the space between the nervous axis and its case.

CHAPTER XXII. THE NERVOUS AXIS. 289

Tlie dura is a thick and strong mass of white fibrous tis-
sue, rni.xed with a small amount of elastic fibres; the con-
stituent bundles are dis]3osed chiefly in a longitudinal di-
rection in the spinal portion of the membrane, those of the
cranial portion being variously disposed. In the latter re-
gion the dura is closely adherent to thecranial periosteum,
})articularly at the base of the brain; dorsally the two
fibrous layers are less intimatcU' united; it is, indeed, cus-
tomary to speak of the dura as forming the intracranial
periosteum, and as composed of two layers; but the inter-
pretation here indicated is more in accordance with the
facts of structure. The inner surface of the cranial dura is
invested throughout with serous endothelium; a similar
structure has been described upon the outer surface in
places where it is free from the periosteum. The spinal
dura is covered on both sides with a similar investment.
In addition to flattened connective tissue corpuscles, both
the cranial and the spinal dura contain numerous granule
cells. The cranial dura is separated from the cranial
periosteum here and there by the large sinuses which
receive the blood from the veins of the brain, and bv
the accessory or parasinoidal spaces, but is not itself
highly vascular; the same is true of the spinal dura, be-
tween which and the periosteal lining of the spinal canal
is found the internal spinal plexus. Ossification occurs
normally in the principal folds of the cranial dura (the
falx and the tentorium) in some mammals.

The pia consists of two more or less clearly distinguished
laN'crs, an inner or investing layer, closely applied to the
subjacent nervous mass, with whose connective tissue
framework it is directly continuous ; and an outer or vas-
cular stratum, rich in small blood vessels whose branches
enter the nervous tissue beneath. The whole membrane

290 PART II. HISTOLOGICAL ANATOMY.

is made up of interlacing bundles of white fibres, mixed
with occasional elastic fibres; in the spinal pia the prevail-
ing direction of the bundles is longitudinal. The outer
layer of the pia is connected with the arachnoid by
numerous trabeculae o^ connective tissue, which pass
across the subarachnoid space; this is quite extensive
along the spinal cord, and in some places around the
brain ; a nearly continuous series of definitely arranged
strands passes from the pia at the sides of the spinal cord
across to the inner surface of the dura, traversing the
arachnoid; the groups of internally converging bundles
are found between the spinal nerves, with which they
alternate; the whole forms the denticulate ligament of
either side. Upon the free surface of the pin and along the
surfaces of the trabeculae and the ligaments just de-
scribed are found flattened connective tissue corpuscles,
endothelioid in form and position. In the stroma of the
pia plasma and granule cells occur, and in some animals
great numbers of pigment cells.

The arachnoid is composed of loosely interwoven delicate
bundles of fibres, the intervening meshes containingnumer-
ous flattened corpuscles: the outer surface is invested with
a layer of serous endothelium ; it is near to but very spar-
ingly connected with the dura. The inner surface corres-
ponds to the outer surface of the pia, with which it is
united by the loose mesh work of trabeculae above de-
scribed.

The relations of the meninges to each other and to the
adjacent structures are as yet but imperfectly understood,
neither their structure nor their embryonic development
being as yet fully known. Such facts as are known con-
cerning the latter, and such light as is afforded bj-^ com-
parative anatomy, appear to justify the view which

CHAPTER XXII. THE NERVOUS AXIS. 291

regards the subdural space as the primary cleavage of the
mass of connective tissue which in the embryo and in the
lowest vertebrates lies between the nervous axis and the
walls of the cerebrosj)inal canal ; the portion outside this
cleft becoming the dura or pachymeninx, and that inter-
nal to it tlie leptomeninx or arachno-pial membrane; if
this view be correct, the supradural (or so-called "epi-
dural") and the subarachnoid spaces must be regarded
as secondary. As a modification of this view it has been
suggested that the arachnoid is in reality reflected over
the inner surface of the dura, thus forming a continuous
serous membrane which surrounds a serous cavity for
which the name of the arachnoid space (as a substitute
for the more familiar name of subdural space) has been
proposed ; and this interpretation has some facts in its
support.

The spinal cord, surrounded by its meninges and sup-
ported by them, hangs freely in the spinal canal; through-
out a great part of its extent it is nearly cylindrical ; in
the cervical and lumbar enlargements its transverse
diameter is distinctly greater than the dorsoventral. It is
divided into symmetrical halves by the so-called anterior
and posterior fissures; the former of these, the ventral
fissure of comparative anatomy, is a distinct furrow from
a fourth to a third of the diameter of the cord in depth,
containing a fold of the pia which bears an important
relation to the blood supply of the cord : the latter, or
dorsal fissure, while deeper than the former, is not a true
cleft, the right and left masses being separated simph' bv
a stout median septum given off from the inner surface
of the pia. Other septa of a similar nature, but less exten-
sive, are given ofif from the pia at various points along
the sides of the cord, and contribute in some measure to
its subdivision.

292 PART 11. HISTOLOGICAL ANATOMY,

When viewed in cross section the cord appears to the
naked eye to be made up of two differently appearing sub-
stances known from their color as the white and the
gray mitter. The latter is in each half of the cord almost
entirely surrounded by the former, but not uniformly so ;
from its central portion on either side a narrow ridge pro-
jects dorsally (and slightly outward) almost to the sur-
face of the cord ; from its appearance in cross section it
is known as the posterior or dorsal cornu; in a similar
manner the gray matter projects ventrally and outward
to form an anterior or ventral cornu which is much
thicker than the dorsal, but does not approach the sur-
face so nearly; in the thoracic portion of the cord a
slightly projecting ridge extending directly outward be-
tween the dorsal and ventral cornu constitutes the so-
called lateral cornu. The gray matter of the two sides of
the cord is continued inward in the mid-region through-
out the extent of the cord to form the gray commissure,
by which the two halves are connected, and in which runs
the central canal; in consequence of the disposition of the
gray matter of the cord its figure as seen in cross section
is irregularly H-shaped.

The white matter also exhibits certain conspicuous
subdivisions, due in part to the disposition of the gray
matter, in part to the mode of origin of the spinal nerves.
The bundles of fibres which are gathered together at
regular intervals outside the cord to form the posterior or
dorsal roots of the spinal nerves enter the cord close to the
dorsal cornu along a well-defined line on either side, which
lies in a shallow groove. Between the dorsal cornu of
either side and the median septum, which extends inward
to the gray commissure, lies the posterior or dorsal col-
umn of the side in question ; it is limited externally by the
line of entrance of the dorsal roots, between which and

CHAPTKK XXII. THE NERVOUS AXIS. 293

the dorsal midline a well-marked septal process of the pia
suhdivides the column into two important tracts to be
presently described. Between the dorsal and the ventral
cornu of each side is situated the lateral column; anrl be-
tween the ventral cornu and the ventral fissure the an-
terior or ventral column. As has been already stated,
the ventral cornu docs not approach very closelv to the
surface, and the fibre-bundles which enter into the anterior
or ventral roots of the spinal nerves are given off irregu-
larly over a region of considerable width; there is there-
fore no clearlv defined boundary either externally or
internally between the lateral and the ventral columns ;
the whole region from the dorsal cornu to the ventral fis-
sure is therefore sometimes spoken of as the antero-
lateral or lateroventral column. The ventral columns of
the two sides are connected with each other dorsad of the
ventral fissure by a narrow layer of white matter called
the white commissure.

The ajjpearances thus far described can all be readily
seen, most of them with the naked eye; the histological
structure of which they are the expression, in some re-
spects exceedingly simple, is in others of such complexity
that it has hitherto taxed the resources of histological
technique to the utmost. It has long been known that the
gray matter contains numerous multipolar nerve corpuscles
of varying size and of a distinct but not sharply defined
arrangement; their branching processes; non-medullated
nerve fibres; and numbers of the neuroglia cells mentioned
in a previous chapter; that the central canal is lined with
a layer of columnar cells which are sometimes ciliated ;
that the white matter is composed chiefly of medullated
nerve fibres, mingled with which are numerous neuroglia
cells like those of the gray matter: that the pial septa
already mentioned subdivide in the interior of the cord to

294 PART 11. HISTOLOGICAL ANATOMY,

form a framework along which are distributed the inter-
nal vessels of the cord, the general disposition of which
can readih'^ be determined In brief, the cord is known to
consist of multipolar corpuscles and nerve fibres sup-
ported largely by neuroglia cells and provided with a
definite blood-supply. The complexity of the cord depends
not upon the number of kinds of tissue elements present,
no other active organ in the body of equal size and im-
portance containing so few; but upon the arrangement of
the corpuscles and fibres, and particularly upon the rela-
tions existing between them ; and it is only recently that
the latter has been at all satisfactorily determined by
means of technical processes of great efficiency and corres-
ponding delicacy. It will be profitable to consider first
the more conspicuous and therefore longer known feat-
ures of the histological anatomy of the cord, and subse-
quently the minuter details that have more recently been
discovered.

Beginning with the gray matter, the most conspicuous
and in every sense the most important elements are the
multipolar corpuscles scattered throughout it. While
they at first seem to be quite irregularly distributed, the
examination of a number of successive sections from the
same region of the cord, and theircomparison with similar
series from other regions will demonstrate that the corpus-
cles are arranged in groups extending along the length of
thecord throughout the whole or definite portions of its ex-
tent with sufficient regularitv to warrant their design a-
tion as distinct ganglionic columns. Of these the most
readily distinguishable on account alike of its extent and
the size of its constituent corpuscles is the column of the
ventral cornu, or, from the know^n distribution of the
axis-cylinder processes of most of its corpuscles to the ven-
tral roots of the spinal nerves, the motor-corpuscle col-

CHAPTER XXII. THE NERVOUS AXIS. 295

umn; this mav a^aiii be more or less distinctly divided in
some portions of the cord into a mesial tract, situated
adjacent to the ventral fissure ( which probably supplies
motor fibres to the nerves of the dorsal muscles ol the
trunk); a ventrolateral tract, contiguous to the mesial
and extending to the outer side of the cornu, (which
probal)lv supplies fibres to the nerves of the lateral and
ventral muscles of the trunk) ; these two tracts, which are
confluent in the thoracic portion of the cord, are quite con-
st'itit throughout its whole extent; and a third, the. dor-
solateral tract, which is situated, as its name implies, in
the outer side of the cornu and dorsad of the second above
mentioned ; it is present chieflv in the cervical and lumbar
enlargements ( and probablv supplies fiiires to the brachial
and sacral plexuses). Still another group, more centrally
located, can be distinguished in some sections; but it is
much less constant than those above described The cor-
puscles of the ventral cornu are the largest found in the
cord, their diameters ranging in most instances from
sixty-five to one hundred and thirty micra.

At the base of the ventral cornu and near the gray com-
missure is a group of much smaller corpuscles, rarely ex-
ceeding thirty micra in diameter, the medlocentral tract;
it is well developed in the thoracic portions of the cord,
but much less so in the cervical and the lumbar. At the
base of the so-called lateral cornu, also in the thoracic re-
gion, there is a group of corpuscles of similar size, the
mediolateral tract, or, as it is called, the column of the
lateral cornu.

The dorsal cornu exhibits at its juncture with the gray
commissure a well-marked column which extends from the
level of the seventh cervical to that of the third lumbar
nerve ; throughout the thoracic region it is quite conspic-
uous, being roughly cylindrical in form and quite sharply
defined by the arrangement of the adjacent fibres; it was

296 PART II. HISTOLOGICAL ANATOMY.

the first column ofcorpuscles to be clearl}' recognized, and is
known from its discoverer as Clarke's column. Stilling
proposed for it the name of the dorsal ( thoracic j nucleus,
and for similarly placed aggregations situated respectively
at the level of the third and fourth cervical nerves and the
second and third sacral the names of the cervical and the
sacral nucleus. The corpuscles of Clarke's column are
next to those of the anterior cornu in size, being from
thirty to ninety micra in diameter. Small corpuscles,
from twenty to thirty micra in diameter, are scattered
through the rest of the cornu with little definite arrange-
ment; those close to the mesial margin are in some cases
quite distinctly elongated parallel thereto and are known
as marginal corpuscles; near the middle of the cornu the
gray matter is broken up along the mesial side and
to some extent along the lateral side also by bundles
of fibres; the reticular formations thus produced
have associated with them irregularly defined groups of
corpuscles. The posterior cornu is capped dorsally by a
trani*lucent mass, rich in glia cells, known as the gelati-
nous substance of Rolando ; it contains numerous small
rounded elements, about fifteen micra in diameter, gener-
ally regarded as nerve corpuscles, as well as other larger
elements, unquestionably nervous, some of which are mar-
ginal in form and position. Mention may also here be
made of the fact that various observers have described
outlying corpuscles scattered among the fibres of the
white columns, both dorsal and lateroventral.

While the corpuscles are the most conspicuous elements
of the gray matter, they form but a small portion thereof.
By far the larger portion consists of a densely felted mass
of fibres of various kinds ; including small medullated
fibres, nonmedullated fibres, axis-cylinder processes and
their branches, and the dendritic subdivisions of the
protoplasmic processes of the corpuscles. These were

CHAPTER XXII. THE NERVOUS AXIS. 297

formerly suj)posc{l to be continuous .'ind to form the so-
called reticulum of the gray matter: but the evidence of
recent investi^^ations by methods giving results of great
delicacy leads to the belief that no such continuity exists;
what is now generally believed to be the true relation of
the corpuscles and the fibres of the gray matter will be in-
dicated later. To the interlacing fibres and fibrillae
already mentioned are added the numerous delicate fila-
ments proceeding from the glia cells which with the con-
nective tissue elements present make up what is some-
times called the spongy substance of the gray matter.
About the central canal there lies a translucent layer simi-
lar to that which caps the dorsal cornua, the central
gelatinous substance.

The white matter of the cord consists, as has already
been said, chiefly of medullated nerve fibres; these vary
greatly in size, the smallest being but one or two micra
in diameter, while the largest may have a diameter of over
twenty-five micra. The great majority of these fibres are
longitudinally disposed, the most conspicuous exception
to this being found in the white commissure, through
which fibres pass at greatly varying degrees of obliquit}'.
Between the medullated fibres are numerous neuroglia
cells so disposed as to form an interstitial framework be-
tween the subdivisions of the pial septa. Along the sur-
face of the septa the neuroglia cells are gathered in great
numbers, forming a definite investment which is con-
tinuous outwardl}'^ with a well defined layer of consider-
able thickness which lines the inner surface of the pia.

While it is possible to recognize readily that certain
regions of the cord, presently to be indicated, contain
chiefly cither larger or smaller medullated fibres, through
most portions the fibres vary so much and so irregularl}^
in size that no such division into tracts can be made as in

298 PART It. HISTOLOGICAL A.NATOMV.

the gray matter, based on the size and arrangement of the
fibres; the only conspicuous visible feature on which a sub-
division can be based is the presence of the important
secondary septum already mentioned as situated between
the median septum and the dorsal cornu of either side;
this passes obliquely toward the ventral edge of the
median septum, thus dividing the dorsal column into two
well defined portions, long known respectively as the
column of GoU (next to the median septum) and the col-
umn of Burdach (next to the dorsal cornu). The
remainder of the cord has been more or less accurately
subdivided into tracts by other methods; while these
tracts cannot be distinguished by any means now in our
possession in sections of the normal adult cord, some
knowledge of them is necessary in order to understand
the meaning of some of the finer details of structure devel-
oped by recent histological researches in both the white
and gray matter: a brief account of them and of the
methods by which they have been determined will there-
fore be given.

We owe chiefly to Flechsig the discovery that the medul-
lated nerve fibres of the different tracts of the cord attain
their full development at different periods of embryonic
and even of infantile life ; the difference in question being
apparently correlated with similar differences in thecalling
into activity of different powers and faculties of the ner-
vous system : the study of the spinal cords of embryos at
various stages of advancement thus revealing significant
differences in structure not discernible in the cord of the
adult. To Waller we are indebted for pointing out that
an axis-cylinder when severed from the corpuscle of which
it is a process rapidly undergoes degeneration : it follows
from this that in cases of lesion of the cord, either patho-
logical or experimental, not only single fibres but also
tracts composed of fibres of a similar character will un-

CHAPTER XXII. TIIK \ERVOUS AXIS. 209

dergo degeneration in a manner definitely related to the
place of lesion. Without entering into the physiological
significance of the changes involved, it may be said that
degeneration on the side of the lesion toward the brain is
called ascending; and that on the opposite side of the
lesion descending ; and that the same terms are applied
to the tracts in which the changes in question occur. The
method of Flechsig and that based on the Wallerian law
of degeneration give us results which coincide to such an
extent as to determine quite clearly the limit of some of
the tracts about to be mentioned ; the evidence in favor of
the existence of others, while generally regarded as satis-
factory, is by no means as conclusive.

None of the tracts which are thus shown to exist in the
white matter of the spinal cord are more clearly definable
than those which pass from the pyramids of the medulla
oblongata into the ventral and lateral columns of the
cord. The fibres of the pyramids decussating while still in
the medulla, the great majority of them enter a large com-
pact bundle whose cross section is an irregularly triangular
area lying (in the human spinal cord) between the dorsal
cornu, from which it is separated by a thin layer of fibres,
and the lateral surface of the cord, from which it is also
separated by a layer of fibres save in the lumbar region,
where it extends to the surface ; it is known as the lateral
(or the crossed) pyramidal tract. In man the fibres of the
pyramid do not, as a rule, all decussate in the medulla, a
small tract passing down as a thin la\'er on the portion
of the surface of the ventral column which lies within the
ventral fissure on the same side as the pyramid from which
it proceeds; this tract is therefore called the anterior (or
the direct) pyramidal tract ; it does not extend beyond
the middle of the thoracic region of the cord. It is proba-
ble that decussation goes on between the right and left
tracts throughout their whole course, the fibres passing

300 PART II. HISTOLOGICAL ANATOMY.

throuo^h the white commissure, instead of occurring all at
once in the medulla: in some mammals the pyramidal
decussation is complete, and there is in consequence no
direct pyra.nidal tract: this is sometimes the case in the
human subject The existence of both pyramidal tracts
was demonstrated by Tuerck, though his name is usually
associated with the smaller and less constant of the two,
which is CO nmonly designated the column of Tuerck.

Far less clearly defined and less constant in position is
another descending tract, the ventrolateral, or anterior
marginal bundle of Loewenthal: it consists of a thin
layer of fibres situated upon or near the surface of the
ventral and a good portion of the lateral columns : its
fibres proceed fro n the cerebellar cortex ot the same side.

In close contact with the ventrolateral descending
cerebellar tract, the fibres of the two mingling to a greater
or less extent, lies the ventrolateral ascending cerebellar
tract, or anterolateral ascending^ tract of Gowers: like
its companion, it is throughout the larger part of its
course a thin layer of fibres, which is situated between the
tract just mentioned and the surface of the cord: it is
thickest in its most dorsal portion, and is limited in that
direction by the crossed pyramidal tract.

Dorsad of the tract of Gowers and external to the
crossed pyramidal tract is the dorsolateral ascending
cerebellar tract, or the direct cerebellar tract of Flechslg.
It begins in the lower part of the thoracic portion of the
cord (below which level the crossed pyramidal tract comes
to the surface of the cord) and extends from there up-
wards, passing through the restiform body in the medulla
oblongata to reach the middle lobe of the cerebellum. Like
the pyramidal tracts and unlike those just described, the
tract of Flechsig is very clearly defined. Between its dor-
sal border and the line of entrance of the dorsal roots of
the spinal nerves, limited externally by the surface of the
cord, is a narrow tract, the marginal zone of Lissauer.

CHAPTER XXII. THE NERVOUS AXIS. 301

Attempts have been made to farther subdivide the white
matter of the ventral and lateral columns; but the results
on which these attempts arc based arc thus far so conflicting
as to render theconclusions drawn therefrom cpiite doubt-
ful: for the present it is best to include the whole of the
territory enclosing the ventral cornu and perforated by
the fibre-bundles of the ventral roots of the spinal nerves
(excepting, ol course, the tracts already designated) under
the title of the ventro-lateral root zone.

The columns of GoU and ol Burdach, situated in the
dorsal region of the white matter, have already been de-
scribed as limited structurally: they may be otherwise
distinguished, according to their function, as the dorso-
median and the dorso- lateral ascending tracts. Im-
bedded within the latter may be detected a small bundle
of fibres with descending degeneration, known Irom its
outline when seen in cross section as the comma.

Wehaveseen that some of the tracts above described, both
ascending and descending, are in direct relation with the
brain : others are doubtless composed entirely' or in large
measure of fibres that begin and end in the cord itself:
others, and pcirticularly those of the dorsal tracts, are in
direct relation with the spinal nerves; and these latter
organs are in such close connection with the cord as to
merit mention in this connection.

Each spinal nerve possesses, as is well known, a dorsal
and a ventral root. The latter consists of efferent fibres
chiefly if not solely motor in function, which arise from the
axis-cylinder processes of the corpuscles of the ventral cornu,
as has been stated, and pass almost directly out of thecord :
they therefore make no important contributions to its col-
umns. The dorsal root, in addition to its ganglion, which
will be farther discussed in a subsequent paragraph, con-
tains a few fibres probably motor in function : it consists
chiefly of efferent or so called sensory fibres which are some-

302 PART 11. HISTOLOGICAL ANATOMY.

what definitely divided into two groups, a mesial and a lat-
eral, in each bundle; they penetrate the surface of the cord
more or less obliquely and then bifurcate, givmg rise to as-
cending and descending branches, the disposition of whose
terminal and collateral arborizations will be described later.
Recent researches by Cajal indicate that the fibres of the
lateral group, which are slender, have their bifurcation in
the marginal zone of Lissauer and the adjacent part of
the lateral column; their collaterals are few and delicate
and end in the dorsal cornu : the fibres of the mesial group,
which are stouter, reach the columns of Goll and Burdach,
and there bifurcate: their collaterals form by far the larger
portion of those subsequently to be described as derived
from the dorsial column, and in particular those which
form the channels whereby those impulses are transmitted
which are involved in simple reflex movements.

The white matter of the cord, therefore, consists of med-
ullated nerve fibres which may be divided according to
their origin and destination into three groups, the mem-
bers of which are not structurally distinguishable: those
which pass from the cord to end in the brain; those, com-
missural in character, which pass from one portion of the
cord to end in another; and those which pass to the cord
from the brain or from the spinal nerves. From the de-
scription of the nervous elements given in a preceding
chapter it will be evident that each of these fibres consists
essentially of the axis-cylinder process of a corpuscle, and
ends in an arborization, giving off along its course one or
more collaterals. It is probable, but not certain, that
these collaterals have their terminal arborizations in re-
gions of the gray matter of the cord homologous with
that in which the terminal arborization of the fibre itself
is situated.

Leaving out of consideration for the present the fibres

CttAPTEk XXll. THE NERVOUS AXIS. 303

which pass from the cord to the l)rain, it may be said that
the terminal arborizations of fibres, whether collateral or
principal^ which enter the gray matter of the cord are in
close contiguitv or actual contact with the bodies or the
dendritic processes of the corpuscles of the gray matter:
the latter play the part of conductors (and not a nutritive
role merely), and connection is thusestablished asefficiently
ns by the continuity of substance once supposed to exist.
An impulse thus transmitted calls forth the activity of the
corpuscle in question, resulting in a discharge along its
axis-cylinder process which undergoes a similar distribu-
tion.

Regarding the final arborization of a fibre as essentially
similar to thoseof the collaterals, and therefore to beclassi'
fied with them, we shall make the first step toward a con*
ception of the physiological anatomy of the cord by an
enquiry into the distribution of these structures as they
leave the various regions of the white matter. The follow-
ing account thereof, as well as that of the corpuscles of the
gray matter to be subsequently given, is taken almost
wholly from Cajal.

Collaterals of the ventral (anterior) column. These are
larger than those from any other portion of the cord :
springing from the large axis cylinders which compose this
column they pass dorsad in irregular groups to be distrib-
uted within the ventral cornu and particularly about the
motor corpuscles. Some bundles pass to the mid-plane of
the cord and are distributed in the ventral cornu of the
opposite side, constituting the ventral (or anterior) com-
missure of collaterals, situated largely dorsad of the ven-
tral commissure formed of axis cylinders.

Collaterals of the lateral column. These pass inward
to be distributed chiefly in the central region of the gray

304 PART II. HISTOLOGICAL ANATOMY.

matter of the same side: some, however, pass to the mid-
plane, dorsad of the central canal, where thev form a por-
tion of the dorsal (posterior) or gray commissure: they
are divided in it into two bundles, a ventral and a ne-
dian, and are distributed in the central and to some extent
the dorsal region of the opposite side

Collaterals of the dorsal (posterior ) column. The tracts
of Goll and of Burdach,and the marginal zone of Lissauer,
from which these collaterals are chiefly derived, are formed
in great part of the continuations of the fibres of the pos-
terior roots of the spinal nerves. Four groups of collat-
erals may be distinguished.

Sensitivo-motor (or reflexo-motor) collaterals: these
arise not onh' from the continuations of the fibres of the
posterior root but also from the fibres themselves before
their bifurcation; passing across the gray matter, they
terminate in the ventral cornu of the same side.

Dorsal cornu collaterals : these, like the preceding, are
very numerous: they traverse the substance of Rolando
in groups to form immediately ventrad thereof and through-
out the substance of the dorsal cornu a dense network
composed of the intercrossing of their terminal arboriza-
tions.

Clarke's column collaterals: small bundles pass ven-
. trally from the tract of Goll to terminate in the column of
Clarke of the same side, there forming a thick network
about the corpuscles of the column.

Commissural Collaterals: arising chiefly in the tract of
Goll numerous small bundles pass to the most dorsal por-
tion of the gray commissure, which they traverse, to be
distributed in the dorsal cornu of the opposite side: thus
forming the dorsal bundle of the commissure.

Thus, it will be seen, each white column gives off two

CIIAPTKR XXII. THE NKRVOUS AXIS. 305

kinds of coUateriils: those which furnish their terminal
arborizations to the gray matter of the same side, and
those, commissural in chiiracter, which are destined to
ramifv in the gray matter of the opposite side. In either
case their relations are eventually directly with the cor-
])uscles of the gray matter, whose disposition may now
be further considered. E.Kcepting in the substance of Ro-
lando, where some elements of special form ai*e found, the
corpuscles of the dorsal, central, and ventral regions diflfer
but little, save in size. The only important distinction to
be noted in them pertains to the final disposition of their
axis-cylinder processes: on this basis five groups or kinds
of corpuscles may be distinguished : of these the first four
send, as will be seen, their axis-cylinder processes out of
the gray matter into the white, there to become medul
lated : they may be therefore distinguished as corpuscles
with long axis-cylinder processes (corpuscles of the first
class); while the axis-cvlinder processes of the fifth end in
the gray matter not far from their point of origin : they
are therefore corpuscles with short axis-cjdinder processes
(corpuscles of the second class, corpuscles of Golgi). The
five kinds of corpuscles are as follows:

Radical corpuscles, or corpuscles directly related to the
roots of the spinal nerves. These are the motor corpuscles
of the ventral cornua, and comprise the largest corpuscles
of the cord : their axis-cylinder processes are thick and
devoid of collaterals and pass in most cases directly
through the lateroventral column to enter the ventral
roots of the spinal nerves. From a few^ of the corpuscles
the axis-cylinder processes traverse the gray matter to
leave the cord by the dorsal root : they pass through the
spinal ganglia, however, without entering into relation
with their corpuscles, and must be regarded as in all prob-
ability motor in function.

Q'()6 PART n. HISTbLOGICAL ANATOMY?

■ The dendrites of these corpuscles are stout, lotig/aria
ver}' much branched. They ma}' be distinguished as ven-
tro-external, dorsal, and internal (mesial); the latter
branch dichotomously in the Vicinity of the ventral (ante-
rior) commissure ; some of the branches pass the niid-plane
and enter the ventral cornu of the Opposite- side, intercross-
ing with corresponding processes therefrom and forming
the protoplasmic commissure of Caijal. The ventro-ex-
ternal processes terminate in the lateroventral column,
and the posterior in different regions of the ventral cornu.

Commissural Corpuscles: these are smaller than thosie
just described, a:nd provided vC^ith fewer arid shorter den-
drites. Golgi demonstrated their presence in all of the
regions of the gray matter, and that the axis-cylinder pro-
cesses pass to the mid-plane which they crOss ventrally (Iti
the white commissure) to- be continued to the ventrolat^-
eral column of the Opposite side. Cajal has shown that
thfey there under'g'o not a sifi'gle continuation', btit a T-
division: this indicates that'the conitnissural axis-cylinder
process, on reaching the white inatter of the opposite side;
divides into an ascending and a descending fibre of the
column.

Columnar corpuscles : we may thus designate the nutn-
erous medium-sized corpuscles scattered throughout the
whole of the gray matter, of which the axis-cylinder pro-
cesses enter into vertical 'fibres of their own side.' The
greater number of the corpuscles of this kind which occur
in the ventral cornii send their processes to the lateroven-
tral column : those which are sittJated in the dorsal cornu
direct them toward the most dorsal portion of the lateral
column in manycases, though some of the corpuscles found
in the substance of Rolando and the internal portion of
the dorsal cornu send their processes to the dorsal column.

CHAPTKR XXII. THE NHRYODS AXIS; 307

A? rc«;ar(ls tht'columii ol Clarke, two kinds of corpuscles
cati be demonslrated : conimissiiral corpuscles, whose
processes enter the ventral commissure ; and columnar cor-
puscles, whose processes pass to the lateral column to be-
come continuous with the fibres of the cerebellar tract.
This continuation takes place by two methods; by the
formation of a l)end, which furnishes a sinj^le conductor,
ascendiuLj or descending; and by a T-division, which forms
two conductors or vertical branches, one ascending and
the other descending.

Pluricolumnar corpuscles : Elements. are so termed by
Cajal of which the axis-cylinder process is divided while
still in the gray matter into two or more portions which
enter into as many nerve fibres belonging to different col-
umns : thus, for example, an element of this kind may send
one fibre to the ventral column of its own side and an-
other to that of the opposite side; in another the process
may divide into a fibre for the dorsal column and another
for the lateral or ventral, etc.

Van Gehuchten has proposed for the corpuscles here
designated commissural, columnar, and pluricolumnar
the names of heteromeral, tautomeral, and hecateroraeral
corpuscles respectively.

Short process corpuscles: these, which are found chiefly
in the dorsal portion of the gray matter, have slender and
flexuous axis-cylinder processes which speedily end in arbor-
izations situated in proximit\' to Other and adjacent cor-
puscles.

The substance of Rolando merits special mention on ac-
count of the peculiarities of some of the corpuscles con-
tained therein. The latter belong chiefly to the columnar
and short process types, with some pluricolumnar cor-

308 PART II. HISTOLOGICAL ANATOMY.

puscles; whilst commissural corpuscles are not known
with certaint}' to occur. Those characteristic of the re-
gion are of three principal forms disposed in as manv con-
centric zones, passing from without inward, as follows.

The marginal corpuscles are large fu:!iform or flat-
tened elements situated between the substance of Rolando
and the dorsal column of white matter, thus forming a
discontinuous layer. The dendritic processes line the sur-
face of the dorsal column and there ramify: the axis-cylin-
der process, arising sometimes from the border of the cor-
puscles, sometimes from one of its processes, is directed ven-
trally across the substance of Rolando; it then changes its
direction to reach the posterior portion of the lateral col-
umn, with one of the fibres of which it becomes continuous.

The pyriform or fusiform corpuscles are the smallest
elements of the cord: their shape is quite variable, but
forms indicated by the terms above are prevalent. It is
characteristic of them that they are elongated dorsoven-
trally and have great numbers of crooked and intermin-
gled dendrites, the greater portion of which arise from a
ventrally directed stalk which is prolonged almost to the
head of the dorsal cornu. The axis-cylinder process gen-
erally arises from the posterior portion of the corpuscle
and passes either dorsally or laterally to become continu-
ous with one of the fibres of the dorsal column.

The stellate corpuscles are situated nearest to the head
of the dorsal cornu: they unite the substance of Rolando
with that region by means of their abundant spinous den-
drites. The axis-cylinder process is sometimes directed
lengthwise of the cord; it then comports itself as a por-
tion of a short process corpuscle, and appears to end in
the substance of Rolando itself: at other times it is directed
either mesially, to become continuous with a fibre of the
column of Burdach, or laterally, to form a slender fibre of
the marginal zone of Lissauer.

CIIAPTKK XXII. THE NERVOUS AXIS. 309

Bv a CO n():iris :)n of this (Ijscription of the corpuscles as
based upon the destination of their axis-cylinder processes
with that previously j^iven of their distribution in col-
umnar tracts aloni^ the cord, it will be seen that these lat-
ter are in nearlv everv instance composed of corpuscles of
v'lrvinar relatiois. n )t evi.';i the colu n.is of the ventral
cornu consistin'j^ solely of so called motor corpuscles:
Each of these elon^jated clusters may therefore be regarded
as consisting of corpuscles so coordinated in function as to
justify the title of nuclear or ganglionic columns fre-
quently applied to them.

The corpuscles of the spinal ganglia, while they are
situated without the cord, should always be associated
with that structure in any attempt to form a complete
conception of the central nervous mechanism, since their
eflferent axis-cylinder processes enter extensively into rela-
tion with the dorsal columns andcornua. Their spheroidal
or ])vriform contour has been described in a previouschap-
ter, as has also the manner in which two medullated fibres
are derived from the single pole or process borne by the
bodv of the corpuscle. They are unique in the fact that
one of these fibres puts them in communication with den-
dritic or receptive terminals which are far more remote in
most instances than those of any other nervous element.

The central canal of the cord is lined with a columnar
epithelial layer of ependyma cells which are frequently
but not always ciliated. These are in the embryo con-
tinued b\' slender prolongations which reach to the pia,
forming the primary' framework of the cord : such contin-
uations persist in the lower vertebrates, but their presence
in adult birds and mammals is not yet well established.
In connection with the ependyma cells, and possibly de-
rived therefrom, there are found in the adult cord of the

310 PART If. HISTOLOGICAL ANATOMY. ,

higher vertebrates numerous neuroglia cells which are
particularly abundant in the white, matter,, immediatehr
beneath the pi a and along the septa and blood vessels, in
the substance of Rolando, and in the central gelatinous,
substance. . , > , . • , , i

The blood supply of the cord merits special mention. A
s;ingle median ventral spinal artery (the anterior spinal
of human anatomy) runs along the . ventral margin of.
the fold.of the pia which enter§ the ventral fissure: while
a pair of dorsal spinal arteries ;,(or postejioj^ spinals).
are situated just ventrad of the dorsal roots of the
spinal nerves: branches of these arteries ramify. in the ]')ia
tK) form an, extensive plexus. From the, ventral tr,unkv
small, vessels, the central arterioles of Ross, injiumber
several times as many as the iV,erte;brae follow, the .fold of,
the pia into the.median fissure as far as the white commis-,
sure. Here they turn altern^.tely right , and' left to reach:
the central region of the gray matter of, either, side* where,
they breakup into small arterioles and finally into capil;
laries: the central arterioles are distributed, chiefly to the;
gray matter, though some of their divisions penetrate thef
white ma,tter, particularly of the lateral and ventral col-
umns. ' ,

The dorsa,l vessels and the pial plexusgiyeoflT great num,-
bers of peripheral arterioles which follow the dorsal me-
dian septum or the other less prominent septa into the
cord: their terminal divisions^ are .found not only in the
w^hite matter but also in the, outer portion of the grayi
matter and throughout the, dorsal coruua. Each a.irteriole,
whether central or peripheral, has its own proper capil-;
lary area, anastomoses between adjacent arterioles not
being known to occur.

Similarly disposed central and peripheral venules
o^rry the blood from the capillary networks to the irregu-

CHA<»fER XXII. TilK NERtoUS AXIS. 311

Inr plexuses of veins in the ()ia and to tlie ])rinei|)al venous
trunks: these are two in number, a ventral which follows
the ventral artery nmre or less eloselv, and a dorsal,
\\ hieh overlies the niediiin dorsal septum and is not the
compatiion of any artery. ' ' '

^'The brain is the mo'dified and specialized anterior end of
the nervous axis. As we pass from the spinal cord into
the medulla oblonfrata, and thence; throu<;h the region of
the pons, into the crura cerebri, we find that the fibrous
tracts which can be recognized in the white matter of the
coi^d here become subdivided and varibusl}' modified, some
of theni sooii disappearing as such, while others mav be
traced almost to the most anterior portions of the brain;
while new fibrous tracts variouslv related, mav be de-
tected by the method of Waller and particularh'^ bv that
of F'lechsig, The central canal expands here and there to
form the ventricular cavities of the brain ; while its epen^
dymal lining frequently coming in contact with the pial
•investment through the absence of intervening nervous
tissues; is often thrown into A'^ascular folds of greater or
less exrterit. The'gray matter*,' which ih the c6rd sur-
rounds the ceritral canal, now lies chiefly in its floor and
yides, and is penetrated and subdivided by the diversified
fibre tracts already referred to. The ganglionic columns,
■v^'hich in the cord were continuous throughout the whole
or large portions of its structure, are now broken up into
more or less definite nuclear aggregates of corpuscles, ser-
ving a«i centres for specific cranial nerves: some of these
nuclei may perhaps be homologized with portions of some
of the ganglionic columns ; while for others no such rela-
tion is discernible.

In addition to this central prolongation and modifica-
tion of the cord, other structures appear connected there

312 PART II. HISTOLOGICAL ANATOMY.

with Nvliicli. while they must unquestionably be regarded
as developments of the axial region, are of such size and
structural importance as to be properh' regarded as sub-
stantially additions thereto. These, like the cord, and the
basal portions of the brain as well, are composed of more
or less definite fibre tracts and corpuscular areas, related
to each other and to the more axial structures.

Within recent years great progress has been made in the
process of unraveling this complex structure, and it is
safe to say that the general topography of the brain is
known. To attempt to sketch it. however, would take
far more space than the range of the present work con-
templates, and would take us largeh^ into a field where,
while the method has been and must be chiefl\' that of the
histologist, the results belong rather to the domain of the
anatomist. Aledullated fibres, axis-cylinder processes with
their collaterals, and multipolar corpuscles are much the
same in appearance and in relations throughout a large
portion of the brain as in the spinal cord : and we are con-
cerned from a histological standpoint only with those
resions of the brain in which new forms of nervous ele-
m.ents appear or in w-hich some special mode of combina-
tion is demonstrable. The chief of these are the cerebel-
lum, the cerebral hemispheres, and the olfactory bulbs ;
an account of the cerebellum and of the hemispheres will
now be given : that of the olfactory bulb will be deferred
until the sense organ with which it is connected has been
described.

The cerebellar cortex is a superficial la^^er of gray mat-
ter whose well marked folds form the laminae visible on
the surface of the organ to the naked eye: the middle of
each fold is occupied b}' a mass of white fibres in direct
relation to the gray matter. The latter shows to the
naked eye tw^o distinct strata ; an outer and paler known

CHAPTER XXII. THK NERVOUS AXIS. 313

as the molecular layer, and an inner, of a rusty brown
color, called the granular layer. Between these, and par-
tially imbedded in each, is a nearly continuous stratum of
kir;j[e corpuscles, the most characteristic of the cerebellar
cortex, the corpuscles of Purkinje. They are pyriform or
flask-shaped, the large extremity being directed inward:
from the latter an axis-cylinder process is given oft' which
passes through thegranular layer to enter the white mat-
ter as 'ci medullated nerve fibre; during its course through
the granular layer it gives off^ collaterals which in many
cases turn backward to enter the molecular layer.

The outer extremity of the corpuscle is prolonged for a
greater or less distance, but usually soon divides into two
])rincipal branches, which rapidly and repeatedly subdi-
vide to give rise to large numbers of dendritic processes,
many of which are continued to the surface of the cortex:
their surface is beset with .short processes which end
bluntly. The ramification is in every case almost entireh'
confined to a plane transverse to the lamella in which the
corpuscle is situated.

The great majority of the corpuscles of the granular
layer are exceedingly small, with large nuclei and very
scanty surrounding protoplasm : their great numbers,
and their appearance when stained with carmine or other
similar stains led to the name above given for the region
in which the}' occur. For a long time their nervous char-
acter was doubted or denied. The more recent technical
methods have demonstrated it beyond question, and the}'
are now known as granule corpuscles. Their bodies con-
tain a relatively large quantity of rusty brown pigment,
to which the characteristic color of the layer is due, A
few protoplasmic processes are given off b\' each corpus-
cle: these branch sparingly to end in a sm.all number of
dendrites with thickened extremities. From the bod}' of
the corpuscle, or frequently from one of the processes, a

314 PART II. HISTOLOGICAT. ANATOMY.

slender axis-cylinder process is given off, which passes
without collaterals into the molecular layer and there
undergoes a T-division to form two slender tangential
fibres whose course is alwa^'s in the direction of the
lamella in which the}^ occur and therefore at right angles
to the plane of the dendrites of the corpuscles of Purkinje,
with which they are in close contact as the\^ pass. The
tangential fibres have been shown in some of the smaller
vertebrates to run the whole length of the lamellae in
which they are situated.

In addition to the small granule corpuscles there are
present in the inner layer, though in small numbers, other
nervous elements. They are situated near the outer limit
of the layer and are nearly as large as the corpuscles of
Purkinje, which they somewhat resemble in form. Their
outer region sends off numerous protoplasmic processes,
which branch irregularh^ in every direction to form large
numbers of dendrites: these project chiefly into the mole-
cular layer, though many of them lie altogether within
the granular layer. From the inner region there is given off
a slender axis-cylinder process, which branches freelj'-
almost from its origin, the whole giving rise to an exten-
sive arborization-plexus which is situated entirel^within
the granular layer. These corpuscles therefore opb^ng to
Golgi's second type.

The finely dotted appearance to which the molecular
layer owes its name, seen when the lamella is cut trans-
versely, is largely due to the cut ends of the tangential
fibres. There are present in this layer two kinds of cor-
puscles. In the deeper portion are seen numerous corpus-
cles of medium size and irregular form, whose dendritic
processes branch sparingly but extend for some distance
into the surrounding region : the axis-cylinder process
takes a course generally parallel to the surface of the cor-
tex and gives off frequent collaterals ; these pass inward

CIIAPTKK XXII. TIIK NKK VOl'S AXIS. 315

to be ji|)|)lic(l in each instance to the bodv of a cor-
puscle of Puikinje, ujjon whicii, or about the base of the
axis-cylinder process, tliev terminate with little if anv snb-
(bvision: a»nunil)er of such fibrils surround each corjjus-
cle of Purkinje, forming a nest or basket about it: the
cor|)uscles Ironi which they proceed are therefore known
as basket corpuscles.

Throus^hout the molecular laver, but chiefly in the outer
portion thereof, are found numerous stellate COrpuscles,
which though smaller in size, resemble the elements just
described in their general form and in the appearance of
their sparingly brdnched dendrites. The destination of
their axis-cylinder processes is not known

The axis-cylinder processes of the corpuscles of Purkinje
give rise to the only fibres of the central white matter of
the lamella known to be sent inward from the cerebellar
cortex. The fibres which pass out into it have been shown
to terminate in two different methods. Some of them end
in the granular layer, where their extremities branch spar-
ingly, the subdivisions terminating in enlargements in
such a way as to give to the whole somewhat the appear-
ance of a tuft of moss: the}' have therefore been desig-
nated EQQSSy fibres. The others find their way to the cor-
puscles of Purkinje, traverse their surfaces, and subdivide
to follow the branchings of the dendritic processes, thus
forming a terminal arborization which adheres thereto
like a vine to a tree: they have therefore received the name
of climbing fibres.

It will be seen from the above description of the cerebel-
lar cortex that each corpuscle of Purkinje is in relation
with three sets of discharging terminals: the tangential
fibres of the granule corpuscles, the collaterals of the bas-
ket corpuscles, and the arborizations of the climbing
fibres. We are aX present entirel)' ignorant of the func-
tional relations which are based on this structure.

316 PART II. HISTOLOGICAL ANATOMY.

The cerebral cortex has been regarded as composed of
several distinct la^-ers which differ from each other as re-
gards the form and size of the contained corjDuscles. As
our knowledge has increased, the boundaries between
these la\'ers have been found to be less sharply defined
than was at first supposed. Three can with certainty be
distinguished: the outer, or so called molecular laver, the
middle, or p3'ramidal laj-er, and the inner, or polymor-
phous la^er.

The molecular layer, like that of the cerebellar cortex,
owes its characteristic appearance largely to the cut ends
of the collaterals and slender fibres which are denselv in-
terwoven in it and to their terminals. Its outermost por-
tion contains numerous neuroglia cells, which just beneath
the pia form an almost continuous stratum, as in the
spinal cord: this has been described as a distinct layer of
the cortex. vScattered throughout the molecular layer are
numerous nervous elements, the corpuscles of Cajal, that
histologist having first demonstrated their distinguishing
characteristics. Two forms have been described b}^ him,
the fusiform and the stellate. The former are, as their
name implies, spindle shaped, and give off from either end
a polar process which runs parallel to the surface of the
cortex : the two processes cannot be distinguished struc-
turalh'-, and each may take on the character of an axis-
cylinder process : from each collaterals are given off at
right angles or nearly so, which are invariablj'' directed
toward the surface of the cortex. In the second form the
number of similar processes is increased to three or more.
In each case the processes terminate eventually in ramifi-
cations which are turned toward the cortical surface.

Cajal has also described in the molecular la^^er a third
form of corpuscle under the name of polygonal; these
have several protoplasmic processes which end in dendrites

CHAPTER XXII. THP: NERVOUS AXIS. 317

which nuiv extend beyond the molecular layer to enter
that beneath it. The axis-cylinder process may arise
either from the body of the corpuscle or from one of the
processes: its terminal subdivisions are confined to the
molecular layer.

The pyramidal layer is very commonly divided into two
strata, the layer of small pyramids, next the molecular
layer, and the layer of large pyramids, immediately sub-
jacent. The elements characteristic of these two layers
are, however, so nearly alike in everything but size, and
the transition from the one to the other is so gradual in
this respect, that they mav. at least for the present, be
advMutagcouslv considered as one.

The most numerous of the elements peculiar to this layer
are the pyramidal corpuscles: their form is indicated by
their name. The base of the pyramid is turned away from
the surface of the cortex, the apex being directed verti-
cally upward and continued into a long tapering ascend-
ing stem which, even from the corpuscles most deeply
placed, extends nearly or quite to the molecular layer: it
terminates by subdivisions into a tuft of protoplasmic
processes; similar processes are given off" at right angles
along its course; while others are given oft' from the bodv
of the corpuscle, and particularly from the angles of its
base : all these processes are probably to be regarded as
dendritic : their subdivisions are distributed chiefly to the
surrounding substance of the pyramidal layer, save those of
the apical tuft, which are largely situated in the molecular
layer. An axis-cylinder process is given off" from the base
of each pj-ramidal corpuscle, usualU' from a point near the
centre: it is always directed toward the white matter be-
neath the cortex: collaterals are, however, given oft" while
it is still in the gray matter, some of which run horizon-
tally to terminal ramifications within the pyramidal

318 PART 11. HISTOLOGICAL ANATOMY.

layer, while others bend upward at a right angle to termi-
nate in the molecular layer.

Within the lower portion of the pyramidal layer are cdso
found, scattered here and there, certain elements confined
to the cerebral cortex, the carpuscles of Martinotti : they
are also found in the third or polymorphous layer. They
are spindle shaped, roughly pyramidal, or irregular in
form, their distinguishing characteristic being the distribu-
tion of the dendritic processes chiefly outward and down-
ward, and an axis-cylinder process which generally arises
from the uppermost portion of the corpuscle, though some-
times from one of the ascending protoplasmic processes,
and ascends toward the molecular layer in which its rami-
fications are usually situated : in some instances the ter-
minals lie wholly or in part in the uppermost portion of
the pyramidal layer.

The polymorphous layer is so called from the occur-
rence therein of elements which vary greatly in form as well
as in size: they may be ovoid, spindle shaped, pyramidal,
or polygonal. They agree, however, in the fact that their
long axes are as a rule disposed horizontally to the sur-
face of the cortex, and that when a terminal or apical
stalk is present it is never vertically directed as in the
pyramidal layer. Some of the elements present are, as
has just been indicated, ascending corpuscles of Mar-
tinotti: others belong to the second type of Golgi, having
short axis-cylinder processes which break upinto terminal
ramifications in the immediate vicinity of the corpuscle.
Still others give off long axis-cylinder processes which
bend downward to enter the white matter and become
medullated fibres of varying distribution.

The deeper portion of the polymorphous laj'cr contains
chiefly small fusiform corpuscles, which has led to its dis-
tinction en the part of some histologists as a separate

, cnAPTKK xxii. Tin-: nkkvois axis. 319

hiver: tliis is more clearly deHiied in the rej^ioii of the
island of Rjil than elsevvh.'re, where the strataiii in (|ucs-
tion is separated from the rest of the cortex by intervening
white matter, forminf:^ the layer visible to the naked eye,
known as the claustl'um. In most portions of the cortex
the stratum in cjiu'stion is not clearlv definable from the
rest of the ])olymorphous layer.

Brief mention may perhaps l)e made of the composition
of the subjacent white fibrous layer, although the vari-
ous fibres and tracts are not histologically distinguishable,
save to a certain extent by the methods of Waller and of
Flechsig. Fibres formed by the development of medullary
sheaths about axis-cylinder processes which descend from
the corpuscles of the cortical gray matter may pass on
downward as projection fibres to the basal ganglia, the
hindbrain, or the spinal cord itself: other fibres pass as
association fibres to other ])ortions, more or less remote,
of the cortex of the same hemisphere: while others still
pass, chiefly by way of the corpus callosum, as commis-
sural fibres to regions of the cortex of the opposite hemi-
si)here; not unfrequently an axis-cylinder process may be-
come a projection or an association fibre, and one or more
of its collaterals an association or a commissural fibre:
or the converse may occur. Still other fibres pass upward
into the cortex by wa}' of projection, association, or com-
missural tracts to end there, the terminal arborizations
being situated either in the molecular or the upper portion
of the pyramidal layer.

The neuroglia of the brain does not differ from that of
the spinal cord to such an extent as to merit a detailed
description in so brief an account of the organ as is here
given. Much the same mav be said of the ependyma,
which lines the ventricles and passage ways of the brain :

320 PART II. HISTOLOGICAL ANATOMY.

like that of the central canal of the cord, it is composed
chiefly of columnar cells, whose free extremities bear for a
time at least cilia-like processes not known to be vibratile.
Mention should be made of the plexuses of the ventricles,
formed b}^ infoldings of the pia and the ependyma, and
consisting chiefly of a rich network of small bloodvessels
supported by the former and invested b}' the latter. An
outline of the blood supply of the cord was given above:
that of the brain is far too complex and too much a mat-
ter of gross anatomy to be described here. Neither will
any account be attempted in this connection of the devel-
opmental history of the nervous axis, beyond the state-
ment that it is formed entirely from an infolding of the
outer or epiblastic layer.

CHAPTER XXIII. Tlin ORGANS OF SPECIAL SENSE. 321

CHAPTER XXIir.
THE ORGANS OF SPECIAL SENSE.

As was stated in a previous chapter, there are certain
organs in which specially modified receiving terminals are
associated with more or less highly modified forms of epi-
thelium and with other special structures of a skeletal
character to form in each case an apparatus for the recep-
tion of a specific and clearly defined impression ; their
stimulation giving rise to sensations of flavor, odor,
sound, or light, commonly called special, as distinguished
from the more diffused and less clearly definable sensations
of temperature, contact, resistance, etc., received by the
more widely scattered and possibly less specialized termi-
nals described in the chapter referred to.

In each case the special receiving apparatus involved has
associated therewith other special structures, chiefly skele-
tal, whose function it is to render more intense or more
specific the impression received: these associated struc-
tures being, equally with the nervous apparatus in ques-
tion, essential factors of the organ of special sense. As in
the case of the brain, a full description of these structures
belongs rather to the province of anatomy than to that of
histolog}', and would require far more space than can with
propriety be given here: an account of the histological
composition of the essential apparatus will in each case
be given, together with mention of any characteristic
features noteworthy in the tissues of the accessory parts,
some previous knowledge of the anatomy of the organs
in question being presumed.

322 PART II. HISTOLOGICAL ANATOMY.

The immediate organs of taste are the taste-buds, so
called from their spheroidal form, which are situated in
large numbers upon both the outer and the inner sides of
the valle\'S which surround the circum vallate papillae; on
the fungiform papillae; and particularly upon the loose
folds just in front of the anterior pillars of the fauces which
in inan represent the more definiteU' circumscribed foliate
papillae of some of the lower mammals: they are also
found on the soft palate and the epiglottis, and arc scat-
tered here and there over the surface of the tongue. They
are spheroidal bodies, almost completely embedded in the
stratified squamous epithelium of the surface where the\'
occur: the long axis is directed vertically or nearly so to
the surface, and the outer extreinitv tapers slightly, on
which account their form is sometimes described as flask
shaped. The mass consists of a number of elongated epi-
thelial cells, of which some are spindle shaped, or flattened,
and are known as sustentacular cells : a layer of these
completed covers the outer surface, their grouping recall-
ing somewhat the surface segmentation of acantelope:
others are scattered irregularly throughout the interior.
Between them lie other cells whose bodies, except just
around the large nucleus, are slender and almost filamen-
tous; these are the so called gustatory cells. The outer
extremity of each ends in a ciliary process, the taste-hair,
which projects with its fellows through a small circular
opening in the squamous epithelium known as the gus-
tatory pore: its inner extremity is slender, often bifur-
cated, and frequently more extensively branched ; its sub-
divisions, which are sometimes varicose, reach to the base
of the taste-bud.

Numerous attempts have been made to demonstrate a
structural relation between theelements just described and
the nerve fibres which pass to the taste-buds from the sub-
divisions of the glossopharyngeal nerve, but thus far

CH.VPTKK XXIII. TlIK ORCANS OF SPKCIAI, SENSE. 323

without success. This ucrvc, like the dorsal root of a
spinal nerve, bears a j^anglion near its point of union with
the nervous axis: examination by the chromate and silver
method shows that tiie axis cylinders of the nerve fibres
end in the taste-buds by branching ainoni^f the cells in the
interior, forniin*,^ what are known asintrabulbar ramifica-
tions: the honiolosi^y existing between the glossopharvn-
jjeal and the spinal nerves would indicate that these are to
be regarded as dendritic processes at the ends of long af-
ferent fibres, similar to the "free endings" in the epidermis
described in the chapter on the nervous tissues. Accord-
ing to this view it is questionable whether the so called
gustatory cells are in reality nervous in character, and
some have regarded them and the sustentacular cells as
alike modified epithelial elements whose form and arrange-
ment favors the stimulation of the nerve terminals. There
is, however, a close relationship between the senses of
taste and of smell ; and the structure of the receiving ap-
paratus of the latter suggests an explanation of the struc-
ture of the taste-buds in which the gustatory cell would
form the first member in a series of nervous elements.
Reference will be made to this after the organ of smell has
been described: the statements already made, however,
represent the present extent of our knowledge of the facts
in the case.

Mention should be made here of the fact that some of
the medulla ted nerve fibres of the subdivisions of the
glossopharyngeal nerve going to the taste-buds terminate
bv free endings in the stratified epithelium immediately
surrounding those structures, forming what are termed
peribulbar ramifications : these are generally regarded as
fibres of general and not of special sensibilitv. The glands
of a serous type, known as the glands of Ebner, which
are closely associated with the taste-buds, have been de-

324 PART ir. HISTOLOGICAL ANATOMY.

scribed in connection with the tongue. They are to be
distinctly regarded as accessor}^ to the apparatus of taste
perception, the fluid secreted by them aiding in the solu-
tion of substances whose flavor is to be perceived by the
taste-buds.

The sense of taste resembles most forms of general bodih-
sensation in requiring the actual contact of the object per-
ceived: the remaining special senses resemble the thermal
sense in being capable of givingknowledge of objects at a
distance. Of these the first and as regards its receiving
mechanism the simplest is that of smell. The accessory-
muscular and skeletal structures which make up the facial
region known as the nose require no special description
from a histological standpoint. The air passages which
they enclose are lined with a mucous membrane known as
the pituitary or the Schneiderian membrane : the vesti-
bule, into which the nostril opens on either side, is lined
with stratified squamous epithelium continuous at the
margin of the nostril with the epidermis, of which it is a
modification : the remainder is divided into two portions,
the lower or respiratory and the upper or olfactory : the
former is lined with stratified ciliated columnar epithelium,
similar to that of other respiratory^ passages, beneath
which is a highly vascular membrane which contains a
considerable amount of adenoid tissue here and there gath-
ered into distinct nodules, and numerous racemose glands,
some of which are mucous and others are serous in char-
acter: large numbers of goblet cells are also distributed
throughout the epithelium.

The olfactory region of the nasal mucosa can be distin-
guished with the naked eye b}' means of its well marked
pigmentation, it being of a yellow color in man and some

CHAPTER XXIII, THE ORGANS OF SPECIAL SENSE. 325

of the lower mnmmals, nnd of a yellowish brown in
others. The fibrous layer is more highly vascular than in
the respiratory region, but contains less adenoid tissue: it
contains numerous glands, the glands of Bowman, which
diflTer from the racemose glands above mentioned in being
tubular, rarely branched, and but slightly bent or convo-
luted: the distal extremity is frequently the largest, the
tube tapering toward the duct, which is always slender,
and opens either upon the mucous surface or occasionally
into a small ciliated crypt: the epithelium of the glands
of Bowman is of the serous type, but the tubules resem-
ble those of mucous glands in having a conspicuous lumen:
ordinary racemose glands are also occasionally found in
the olfactory region.

The epithelial layer of the olfactor}' region is composed
chiefly of two kinds of elements. The first comprises the
non-ciliated columnar supporting cells: these are chiefly
prismatic in form throughout the greater portion of their
extent, but with tapering inner extremities, and with oval
nuclei situated at an approximateK' uniform distance from
the surface: other more deeply situated epithelial cells are
])yramidal in form, their bases resting on the fibrous layer;
they ma\' perhaps be regarded as immature supporting
cells. Interspersed among the columnar epithelial cells are
large mambers of slender elements of the second kind,
whose outer extremities terminate in tufts of hair-like pro-
cesses, the olfactory hairs, which project above thegeneral
surface: the middle portion is suddenly thickened to con-
tain large spheroidal nuclei : these are the olfactory cells:
their slender varicose inner portions are now known to be
continuous with the medullated fibres of the nerves of
smell. They must therefore be regarded as nervous ele-
ments, the thickened middle portion which contains the
nucleus constituting the bod\' of the corpuscle, and the
peripheral portion a greatly reduced dendritic region con'

326 Part ii. histological anatomy.

sivSting of a single protoplasmic process; while the proxi-
mal portion passes over into an axis-cylinder process
which shortly becomes a non-meclnllated nerve fibre.

The olfactory nerve-fibres can bo follov^^ed through the
cribriform plate to their passage into the surface of the
olfactory bulbs, whose structure may now be considered :
as has been stated in a previous chapter, the^^ constitute
a distinct region of the brain; but their histological struc-
ture is so intimately associated with their relation to the
sense of smell as to make their description appropriate in
this connection. Each olfactory bulb is a rounded mass
at the anterior extremity of the longer or shorter olfac-
tory tract (or olfactory nerve, improperly so called): the
whole is an outgrowth from the hemisphere and originally
contains a cavity, the olfactory ventricle, which is a di-
verticulum of the lateral ventricle: in many mammals this
cavity persists throughout life ; in some it persists in the
bulb, that of the tract being obliterated ; in man and the
Primates generally it disappears altogether in the adult.

In passing across a section of the olfactory bulb from
the surface in close proximity to the cribriform plate to the
ependymal lining of the olfactory ventricle a more or less
distinct stratification may be observed: the number of
layers distinguished by difterent observers varies accord-
ing to the degree of subdivision recognized: Cajal desig-
nates five, distinguished by histological characteristics de-
monstrable with the aid of the silver chromate method, in
addition to the ependymal la\'er, which is wanting in
those forms in which the bulb is solid.

The first of these is the superficial layer of nerve
fibres: this is a thin stratum of slender non-medullated
fibres arranged in a felted mass: it is composed exclu-
sively of the constituents of the bundles which pass
through the perforations of the cribriform plate, whose
origin, as we have seen, is in the axis-cylinder processes of

CllAPTHK XXIll. Tin: ORGANS OF SPFXIAI. SKNSK. 327

ihc olfactory cells. The fibres leave the layer inwardly
cither siniilv or in stnall *;roups to enter the second stra-
lurn. or layer of olfactory glomeruli: the bodies whose
presence distin«i:uishcs this layer have lon<; been known to
histolosiists.as spheroidal masses present inlarji^e nnnibers
near the surface of the bulb: it is only recently that their
structure has been at all understood : they are composed
in])arL hv the dense tufts of varicose fibrils which form the
terminal arborizations of the olfactory fibres entering
them from the superficial layer; in part by the similarly
tufted dendritic ramifications of the extremities of pro-
cesses derived from corpuscles situated in a deeper layer
to be presently described: they are, therefore, the places
where ingoing impulses are transmitted from the first to
the second of a series of nervous elements.

The third stratum is termed by Cajal the molecular
layer: as is the case with other structures similarly desig-
nated in various parts of the nervous axis, the finely
punctate appearance which characterizes it when seen in
section is due to the cut ends of numerous fibres, and of
fibre-like processes from the corpuscles of the layer next
adjacent. Cajal describes in addition, as i)cculiar to this
layer, certain elongated or fusiform corpuscles whose
peripheral extremities are continued by slender processes
which run to the glomeruli and there terminate in small
dendritic tufts which are subsidiary to those of the cor
puscles of the layer next within: their proximal extremi-
ties give rise to axis-cylinder processes which run to the
innermost layer and there bend strongly to pass toward
the olfactory tracts, which they enter as medullated
fibres.

The layer of mitral corpuscles is the fourth of the suc-
cessive zones : it consists of large nervous elements chiefly
disposed in a sitigle stratum, whose general form is indi-
cated by their title. The base of each corpuscle is directed

328 PART rr. histological anatomv.

toward the outer surface of the bulb: it gives off, in mam-
mals usually from a point near its centre,a stout descend-
ing process which traverses the molecular la^^er to form
in one of the glomeruli the important dendritic ramifica-
tion already described as one of the essential constituents
of each of those bodies. From the margin of the base are
given off stout protoplasmic processes which diverge
greatly, their ramifications interlacing to form a layer in
which the corpuscles lie: their finer subdivisions extend
obliquely into the molecular layer. The inwardly directed
apex of the corpuscle gives rise to a stout axis-cylinder
process which penetrates the layer next within and there
bends abruptly to run backward in that layer, giving off
collaterals whose terminal ramifications are in the mole-
cular layer, and eventually to become the axis-cylinder of
a medullated fibre of the olfactory tract.

The form and relations of the mitral corpuscles are sub-
ject in different vertebrates to variations in detail that are
of such importance, as bearing upon their functions, as to
merit description here. Tn mammals generally each mitral
corpuscle bears but a single descending process: this may,
however, divide and send branches to more than one glom-
erulus. In birds each mitral corpuscle gives off several
descending processes to as many glomeruli : in either of
these two ways a single corpuscle is put in relation with
a number of the bipolar nervous elements of the olfactory
mucosa. In some mammals, however, the glomeruli are
relatively large, and each receives the dendritic ramifica-
tions of several descending processes from as many mitral
corpuscles: in such cases a single bipolar element may
transmit a stimulus to several mitral corpuscles: this lat-
ter condition obtains in the olfactory bulbs of mammals
possessed of a high degree of olfactory sensibility.

Within the layer of mitral corpuscles is found the granu-
lar layer or deep layer of fibres: both of these terms being

ciiMTKu xxiii Tin: ouc.ANS OF siMXiAL shnsp:. 829

ai);)lic(l to the tilth stratum as here defined. , Omitting
tro 11 eoiisideration tlie ependymal liniiiij; of the eavity of
the bulb, by some regarded as belonging to this layer, but
which does not differ in any essential respect from the
ependvma which everywhere lines the cavities of the ner-
vous axis, the stratum consists almost exclusively of the
two kinds of nervous elements indicated by the titles
above given. The principal constituents are the nervoUS
fibres, which represent the axis-cylinder processes already
described as entering this layer from the subjacent corpus-
cles: as these pass along the length of the bulb they give
off numerous collaterals, of which some run horizontallv
to end among the adjacent granules; others descend ver-
tically to terminate by interlacing ramifications among
the lateral protoplasmic processes of the mitral corpus-
cles.

The name of granule corpuscles is used to designate
the abundant cellular elements, probably nervous in char-
acter, which are grouped in numerous clusters through-
out the laver, the fi.bres above mentioned running in inter-
lacing bundles among these clusters. They vary more or
less in form, but are usually provided at their inner ex-
tremities with several short, slender, rapidly branching
processes: the outer extremity bears a single stouter
process which runs to the inner surface of the molecular
layer, there to ramify among the lateral processes of the
mitral corpuscles. Other corpuscular elements, which are
more doubtfully nervous in function, have been described
in this layer.

The medullated fibres of this layer pass through the ol-
factory tract to enter the hemisphere and there to be dis-
tributed to their destinations in the cortex. In addition,
Cajal describes in the tract eflferent fibres which pass into
the deep fibre layer of the bulb to end in arborizations
which are situated chiefly among the central processes of

33U PART II. HISTOLOGICAL ANATOMY.

the granule corpuscles. In those mammals in which the
cavity of the bulb is ol)literated in the adult, the ependy-
mal lining is replaced b\^ a gelatinous mass containing
numerous neuroglia cells. The dorsal portion of the
bulb is in mammals generalU' far simpler in structure
than the ventral, the latter, by virtue of its position, being
brought into far more intimate relations with the olfac-
tory mucosa.

At the close of the description of the structure of the
taste-buds reference u^as made to the close relationship
of the senses of taste and of smell, and a possible resem-
blance in structure in the two organs was intimated.
That resemblance, if it exists, is chiefly between the gus-
tatory and the olfactory cells : the former are strikingly
like the latter as regards their general form, and particu-
larly as regards the central nucleated portion and the
peripheral process. As we pass inward, however, the
likeness is less evident : the olfactory cell is plainly a ner-
vous element, being continued by an axis-cylinder process
which ends by arborizations in one of the glomeruli of the
olfactory bulb: if the gustatory cell is a nervous element,
its inner portion must be regarded as a corresponding
arborization-region, very greatly reduced, and probably
discharging the impulses which it transmits upon the
intrabulbar ramifications of the fibres of the glossopharyn-
geal nerve. Such an arrangement, if it exists, is without
parallel as far as known ; but would find its nearest repre-
sentation in the olfactory apparatus. If, on the other
hand, the gustatory cells are epithelial, and not nervous,
the structure of the taste-bud approaches most nearly to
that of a tactile or pressure-organ, a form of sensation
having little relation to the sense of taste, which resem-
bles that of smell (and no other sense) in that it enables
us to take cognizance of stimuli that must be regarded as

CHAPTER XXIII. 1 HI. ORGANS OF SPECIAL SENSK. 3^1

essentially chemical. It should be noted that true gusta-
tory sensations are also received on portions of the
tongue in which taste-buds have not been found, and that
these organs have been descriljed upon surfaces other
than that of the tongue which are certainly not gusta-
tory. We are here undoubtedly confronted with a prob-
lem whose solution depends upon discoveries yet to be
made.

The apparatus of sight is far more complex in structure
than that of smell, alike in its essential portions and in
those which are accessory thereto. The former include
the capsule, fibrous in man and in the mammals generalh'
throughout the larger portion of its wall (though ]iartl\-
cartilaginous or bony in some vertebrates) ; the apparatus
of refraction, with its mechanisms of adjustment; and
the structures directly involved in the reception of light
stimuli and their conversion into nervous impulses. The
accessory parts are the protecting eyelids; the investing
membrane common to them and to the eyeball ; and the
glands whose secretions maintain the proper condition of
this membrane. It will be convenient to proceed, in des-
cription, from the more external accessory parts to the
more deeply seated and more complex essential structures:
in each case considering anatomical characters only in so
far as necessary for the elucidation of the histology of the
parts in question.

The eyelids are essentially muscular folds of the skin,
modified chiefly upon their inner surfaces. The outer sur-
face resembles the skin of adjacent portions of the face in

332 PART II. HISTOLOGICAL ANATOMY.

the presence of diminutive hairs, accompanied by small
sebaceous glands, and by occasional sweat glands : it is
thrown into small irregular folds. The underlying corium
is loose in texture: it also differs from that of the rest of
the skin of the face in the presence of considerable numbers
of branched pigment corpuscles. At the free margin ol the
lid the surface curves inward ; the corium becomes more
dense; and the hair follicles are suddenly enkirged for the
development of the long, stout, and recurved cilia, or eye-
lashes. Sebaceous glands open into the follicles of the
cilia, as do also some of the ducts cf modiiied sweat
glands known as the glands of Moll, others opening
freely at the surface.

As the integument approaches the inner surface of the
lid it bends almost at a right angle at the palpebral bor-
der, and at the same time becomes modified in "structure
to form the palpebral conjunctiva, which lines the sur-
face of the lid in contact with the eyeball. Beneath the in-
tegument is situated the orbicularis muscle, composed of
striated fibres whose bundles run in a general way par-
allel to the palpebral border: the group of bundles situ-
ated just within the border fold, and separated from the
mass of the orbicularis by the follicles of the cilia, is dis-
tinguished as the ciliary or marginal muscle, or as the
muscle of Riolan. Just interior to the orbicularis mus-
cle lies the palpebral fascia, a layer of fibrous tissue which
separates the tegumental from the conjunctival portion
of the lid: in the upper lid it is blended with the tendon of
the levator muscle.

The palpebral conjunctiva consists of a layer of strati-
fied columnar epithelium containing scattered goblet cells,
and resting upon a definite basement membrane: and a
dense mass of fibrous tissue, the tarsus, or the tarsal carti-
lage (erroneously so-called, as it is entirelv devoid of carti-
lage corpuscles). The stratified columnar epithelium of the

CHAI'TKK XXIII. THE ORGANS OF SPKCIAL SENSE. 333

conjinictiva passes tjradually at the palpebral border into
the stratified squamous epithelium ot the integument :
the tarsus may be regarded as the continuation of the
denser portion of the tegumentary corium. Upon the
inner surface of the lid there can be seen with the naked
eye. a number ol vertical rows o( apparently granular
masses, of a yellowish color. These are the tarsal or
Meibomian glands. com])ound structures of the sebace-
ous type which are imbedded in the tarsus: each consists of
a straight or somewhat curved conducting tube or duct
lined with cuboidal epithelium, into the sides of which
o])en numerous sebaceous saccules resembling in everv es-
sential those found in connection with the hair follicles;
the ducts open by minute orifices upon the margin of the
lid. their mouths being lined for a short distance with
stratified squamous e])ithelium.

Along the proximal margin of the tarsus, and partly
imbedded therein are scattered branched tubular glands of
the serous type, the accessory tear glands: they discharge
their secretion upon the adjacent conjunctival surface.
The conjunctival surface of this vicinity is frequentlv
thrown into folds, chiefly involving the epithelium, whose
appearance in cross section has led to their being de-
scribed as glands. The connective tissue between the base-
ment membrane of the conjunctiva and the tarsal plate
contains diffuse adenoid tissue which is occasionallvgath-
ered into nodules in the human subject: in some of the
lower mammals these nodules are quite numerous and
well-defined.

Bevond the base of the eyelids the conjunctiva passes
over upon the eyeball at the fornix conjunctivae. Goblet
cells are more numerous here than upon the palpebral sur-
face; the fibrous portion contains a number of distinct
adenoid nodules, and a few mucous glands : inwardlv.it
passes over into a loose la\er of subconjunctival areolar

334 PART II. HISTOLOGICAL ANATOMY.

tissue which permits of a considerable amount of motion.
The continuation of the conjunctiva upon the e\'eball will
be best described in connection with that structure. The
plica semilunaris, a vertical curved fold at the inner
angle of the e3'-e, representing the third e\^elid of many
lower vertebrates, is a mere fold of conjunctiva; it con-
tains internall}' in some mammals, and sometimes in man,
a thin slip of h3'aline cartilage: as well as a rudimentary
racemose gland regarded as representing the Harderian
gland generalh'^ present in the eyes of those vertebrates
which have a functional third eyelid. The adjacent ca-
runcle is a rounded fatty mass, with an investment agree-
ing with the integument in structure and containing mi-
nute hairs and modified sweat glands. The conjunctiva,
w^hich is highH' sensitive, is richly supplied with the nerve
terminals already described as end-bulbs.

The lachrymal gland, situated in the supero-lateral
portion of the orbit, consists of two somewhat distinct
portions, sometimes described as the superior and inferior
lachrymal glands. The whole mass consists of an aggre-
gation of compound racemose glands which open by in-
dependent ducts upon the conjunctival surface in the region
of the superior fornix. The acini, which may be either
simple or branched, are lined by granular cells with large
spherical nuclei, agreeing in this respect with the alveoli
of serous glands ; from which the}' differ, however, b}^ the
presence in each of a distinct and sometimes a large
lumen. They open into ductules lined by flattened or low
columnar cells: these lead into ducts whose epithelium is
distinctly columnar, and in which a second layer of small
cells has been described as situated near the basement
membrane.

The secretion of the lachrjanal gland, after washing the
surface of the eyeball, is carried away by the lachrymal

C!I.M'TI-:K XXIII. TIIK ORC/ANS OF SI'KCIAL SENSE. 335

canals, whlcli open on the ])alj)chral borders near their
inner extremities. Each canal is lined with stratified squa-
inons epilheliuin, which rests upon a tihrous layer rich in
clastic fibres: external to this is a layer of striated mus-
cular fibres which are <2^cnerally dis|)osed lon<;itudinally.
The canals discharge into the lachrymal sac, which is
continued to the nasal cavity by the nasal or lachrymal
duct. The sac and the duct are both comj^osed of ahi elas-
tic fibrous layer containing considerable adenoid tissue,
and lined bv a mucous membrane which is invested by
columnar ejiithelium resembling that of the nasal cavity.

The visual capsule consists of two distinct strata ; the
outer, or skeletal, which is known throughout the greater
]K)rtion of the eyeball as the sclerotic, but is transformed
in front to form the transparent cornea; and the inner, or
musculo-vascular, which is composed of the posterior
choroid, and the anterior iris: the essential nervous struc-
ture of the eye lies immediately interior to the choroid.
The cornea is a part of the refracting apparatus of the
eve, and the iris a portion of the regulatory mechanism :
but each ma\' be conveniently described in connection with
the stratum of which it is a portion.

The whitish sclerotic is a dense fibrous layer resembling
somewhat a greatly thickened membrane; the interlacing
fibre bundles are arranged chiefly in antero-posterior and
in transverse directions : elastic fibres are sparingly pres-
ent: the fixed corpuscles are flattened, and lie in definite
lacunae of irregular form. The inner kiyer is rich in brown-
ish pigment and is known as the lamina fusca: between
it and the outer layer of the choroid are extensive lymjih
spaces lined with endothelium and traversed by blood-
vessels and strands of connective tissue. The sclerotic is
nearh' twice as thick in its posterior as in its anterior por-
tion : where the optic nerve enters the eye the sclerotic be-

336 PART II. HISTOLOGICAL ANATOMY.

comes continuous with thesheath of that cylindrical bodv:
the circular area enclosed is suddenly thinned, and is
pierced by a number of small openings; it is therefore des-
ignated the lamina cribrosa. Over the larger portion of
the sclerotic its outer surface is invested by a thin layer of
connective tissue loosely uniting it to the capsule of Tenon,
a membranous sac lined with endothelium and enclosing
the space of Tenon by means of which the e^'eball is sep-
arated from the fat masses lining the orbit. In front the
sclerotic, is invested, as far as the scleral sulcus bv which
it is separated from the cornea, by the scleral conjunctiva:
this consists chiefly of stratified squamous epithelium
resting upon a thin fibrous membrane which is connected
to the sclerotic by a scanty layer of looser connective tis-
sue: it contains numerous end-bulbs, and, as is well known,
is extremely sensitive.

The cornea is readily distinguishable from the sclerotic,
not only by its transparency, but also by its greater con-
vexity. Its outer surface is covered by a layer of stratified
squamous epithelium continuous with that ol the scleral
conjunctiva: the layer is several cells deep, the outer ele-
ments being strongly flattened, but retaining their nuclei,
and the inner being digitated in a manner similar to the
prickle-cells of the epidermis. The deepest cells rest on a
thin, dense, homogeneous layer of closely felted fibres, the
membrane of Bowman, or external limiting membrane,
which possibly represents the fibrous portion of the con-
junctiva. Beneath this membrane, and closel}^ connected
with it is the substantia propria of the cornea, a mass of
the corneal tissue described in detail in a previous chapter:
at its margin it passes over into the substance of the scle-
rotic, of which it is presumably a modification. Internally
this mass is invested with a thin homogeneous elastic in-
ternal limiting membrane, otherwise known as the
membrane of Descemet: its inner surface is covered

CHAPTER XXIII. THK OKOANS OF SPECIAL SENSK. 337

with a layer of endoLhcliuni and bounds the anterior
chamber of the eye. Around its margin the meml)rane of
Deseeniet is continued by a number of processes to form
the pectinate ligament by which the cornea is attached
to the iris.

The choroid is nearly coextensive with the sclerotic. It
consists of a vascular layer of fibrous tissue which is ex-
ceedingly rich in large pigment corpuscles, imparting to it
a color which varies from brown to black: in the human
eye it is dark brown in color. Its outermost portion, con-
sisting entirely of pigmented connective tissue, forms a
layer distinguished as the lamina suprachoroidea : it
lies immediate!}' within the lamina fusca of the sclerotic,
from which it is largely separated, as has been already
stated, by extensive lymph spaces lined with endothelium.
The body of the choroid is rich in bloodvessels, which are
disposed in tw^o strata; an outer containing the arteries
and veins, wdiich are arranged in a characteristic manner,
and an inner, the capillary tunic, or tunic of Ruysch: the
fibrous structure between the two is a layer of connective
tissue rich in elastic fibres which in some mammals is so
well developed as to form a distinct layer which is visible
through the capillary tunic and the retina, and is known
as the tapetum. Within the capillary tunic is a thin trans-
jjarent layer, the membrane of Bruch, or vitreous mem-
brane.

The anterior portion of the choroid is modified by the
foldings of its inner surface known as the ciliary processes,
and by the thickening due to the presence of the layer of
bundles of smooth muscular fibres termed the ciliary
muscle: the whole region, including a marginal zone, the
ciliary ring, in which the capillary tunic is less well de-
veloped than in the choroid generally, is sometimes desig-
nated the ciliary body. The ciliary processes, upwards
of seventy in number in the human eye, are meridion-

33H PART II. HISTOLOGICAL ANATOMY.

ally disposed folds which begin just anterior to the cili-
ary ritig and rise gradually to the height of a half of a
millimetre or so, to terminate abruptly at the margin of
the iris: like the rest of the choroid, they are quite vascu-
lar, the vessels being imbedded in a pigmented stroma of
connective tissue, and are limited internally by the vitreous
membrane. Upon their surfaces are pouch-like depressions
lined by the epithelial layer, presently to be described,
with which the vitreous membrane is invested in this re-
gion; these have been called ciliary glands : their gland-
ular function is, however, doubtful.

The ciliary muscle is b_v some regarded as a portion of
the choroid, by others as interposed between it and the
sclerotic. It is composed of bundles of smooth muscular
fibres (of striated fibres in birds), most of which arise
from the pectinate ligament at the region where the scle-
rotic, the cornea, and the iris come together: of these the
greater number run meridionally to be inserted into the
choroid, and are therefore sometimes regarded as forming
a distinct muscle, the tensor choroideae: the remainder,
known as radial bundles, run obliqueh' to those just de-
scribed, assuming a direction which tends toward the cen-
tre of the eye, and being inserted in the ciliary processes.
Other bundles, internally situated, are arranged in a more
or less definite circular tract, known as the ring-muscle
of Mueller- The ciliary muscle as a whole is triangular in
cross section : it is thickest in hypermetropic eyes, due
largely to an increase in the size of the ring-muscle.

The stroma of the choroid is continued forward from be-
yond the ciliary body to form the principal portion of the
iris: the latterishighly vascular, but not so much so as the
rest of the musculo-vascular layer: it is also somewhat dif-
ferent in texture, approaching more nearh'tothe vStructure
of retiform tissue. It contains numerous pigment cor-
puscles, of different colors in different persons, on whose

CIIAPTHK XXIII. Tin: ORGANS OF SIM:CIAI- SKXSK. 331)

presence the color of the iris depends: i\n exception to this
occurs in cases where the iris is blue: here pigment cor-
puscles are wanting in the stroma, the color depending
entirely on the appearance of the post-iridal pigment (to
be described-later) as seen through the body of the iris.

The anterior surface of the stroma is somewhat con-
densed, and leaves a layer of endothelial corpuscles con-
tinuous at the irido-corneal angle with those w'hich invest
the membrane of Descemet. The posterior surface is
formed by a homogenous layer continuous with the vitre-
ous membrane of the choroid and the ciliary body : against
it lies the layer of pigment corpuscles above referred to.
Imbedded in the stroma of the iris near the pupillary mar-
gin is an angular layer of smooth muscular fibres, the
sphincter papillae: near the posterior surface are radi-
iiting bundles forming a thin la3'er which is not continu-
ous, known as the dilator pupillae: its existence is ques-
tioned by some histologists, the demonstration of the
scattered bundles of smooth muscular fibres, as distin-
guished from the adjacent bloodvessels and bundles of
connective tissue, being quite difficult.

The region lying between the outer margin of the
iris and the sclero-corneal sulcus is one of great import-
ance; the iris, the choroid, the ciliarx' muscle, the cornea
and the sclera all coming together in this vicinit\'. Just
external to the irido- corneal angle, and among the
fasciculi which make up the pectinate ligament, lies a loose
network of trabeculae of white and elastic fibres whose
interstitial cavities, imperfectl}^ lined with endothelial cells,
are known as the spaces of Fontana: they communicate
freeK' with the anterior chamber of the eye and contain
the same fluid. External to these and fairly within the
sclerotic portion of the region is situated an annular
space, irregularly flattened and in places subdivided : it
is called the canal of Schlemm : whether it is a lymphatic

340 PART II. HISTOLOGICAL ANATOMY.

or a venous channel, and whether or not it communicates
with the spaces of Fontana are still matters of dispute.

The parts concerned with the processes of refraction by
means of which distinct images of things seen are formed
on the sensitive surface within the eye are the cornea, a
description of which has already been given; the aqueous
humor, a water}^ fluid in which leucocytes are occasionalh^
found, but containing no other tissue elements, which fills
the space between the cornea and the capsule of the
lens: the crystalline lens, with its capsule, by means of
which it is suspended at right angles to the eye immedi-
ately behind the iris ; and the vitreous humor, which fills
the cavity posterior to the lens. The regulatory mechan-
isms are the iris, which, by modif\ang the size of the pupil-
lary aperture, governs the amount of light which passes
through the lens and indirectly (to some extent) the sharp-
ness of the image formed by it ; and the ciliary muscle,
whose action modifies the convexity of the lens and thus
affects its definition : an account of these having already
been given, there remain for description the lens and the
vitreous humor, with the capsules by which they are
surrounded.

The crystalline lens is composed of an epithelial layer
and a fibrous mass whose components are modified epi-
thelial corpuscles. The epithelial layer consists of cu-
boidal elements which form nearly the whole of the ante-
rior surface: as they approach the equator they become
columnar, and as thej^ reach that region become greatly
elongated and assume the shape of the long hexagonal
])rismatic fibres of which the greater part of the lens is
composed : at the equator these retain their nuclei for
some time after birth, but those of the greater portion of
the mass are quickly lost.

The fibres run from the anterior to the posterior por-

CHAPTER XXIII. IHK ORGANS OF SPECIAL SKXSE. 341

tion of the mass, l)elii<.^ so disposed that their extremities
eome to<2^ether in eaeh region along radifiting sutural
pL'ines ])rimarily three in number; the sutures of the an-
terior region alternate with those of the posterior region
in position; as a consequence, the fibres pass obliquely
from one region to another: the degree of obliquity is in-
creased by the fact that the length of the fibres is such
that those which arise nearest the centre of the anterior
region terminate posteriorly nearest the equator; and
vice versa. As a consequence of this arrangement of the
fibres, stellate figures are formed w^hich can be easil}' seen
in the artificially hardened lens, and traces of which have
been discovered in the living e\^e by the aid of the ophthal-
moscope. Hardening in alcohol reveals a tendency to
lamination, particularly in the outer portion, the laminae
peeling ofifin triangular patches which separate along the
sutural planes.

The capsule of the lens is a transparent elastic sac
which completely encloses that body. It is apparenth'
homogenous in structure in the adult, but is regarded b}-
some as composed of two laminae: an inner, cuticular in
character, formed b_v the activity of the epithelial elements
within while still embryonic : and an outer, composed of
fibrous tissue. It is thicker in front than behind, in rela-
tion with the greater change of curvature which takes
place in the more highly convex posterior surface in visual
adjustments. The support of the capsule and lens in place
will be described later.

The vitreous humor, or vitreous body (as it also called),
is a semi-fluid mass of extreme tranparenc}' derived from
the modification of a quantity' of gelatinous tissue b\' the
infiltration of lymph to such an extent that over ninety-
eight per cent, of its substance is water. Slender trans-
parent fibres are scattered through the mass, and occa-
sional corpuscles are found in it : these are generally of ex-

342 PART ir. HISTOLOGICAL ANATOMY. '

qeedingly irregular shape, and are often extensively vacuo-
lated ; they are probably modified leucocytes. Under cer-
tain methods of hardening the vitreous body can be seen
to present evidences of a laminar structure; whether
this is real, or the result of the treatment employed, is not
3^et certain.

The vitreous body is invested by a thin transparent
capsule, the hyaloid membrane, which is structureless
throughout the greater portion of its extent. Opposite
the optic nerve it is reflected forward to line the slender
hyaloid, canal which perforates the vitreous body, termi-
nating in front at a point opposite the centre of the pos-
terior surface of the crystalline lens: here the hyaloid
membrane is reflected outward from the anterior extremi-
ty of the canal to line the patellary fossa, on the anterior
surface of the vitreous body, which receives the convexity
of the lens and its capsule, the latter being in contact
with the hyaloid membrane.

At the margin of the patellary fossa the hyaloid mem
brane lining it becomes continuous with that investing
the outer surface of the vitreous body: here the membrane
is distinctly fibrous, the fibres in some cases penetrating
the gelatinous mass within. It gives off from its outer
surface a fibrous layer which closely invests the ciliary
processes as the zone of Zinn, or zonula ciliaris : its free
portion extends beyond the ciliary processes to be inserted
upon the capsule of the lens at its equator, thus forming
the suspensory ligament of the lens above mentioned.
The mode of insertion of the suspensory ligament is such
as to leave a narrow circular space between the two
layers of the hyaloid, known as the canal of Pettit.

The structures directly involved in the reception of light
stimuli and their conversion into nervous impulses are
contained in the retina, a highly complex organ formed by

CHAPTKR XXIII. THE OK(iANS OF SPECIAL SENSE. 34-3

the modification of a direct outgrowth from the brain.
This outgrowtli, at first vcsicuhir in form, is afterwards
doubled upon itself in such a manner as to form a sphe-
roidal cup composed of two layers and situated chiefl\'
between the vitreous humor and the choroid, its basal
stalk-like portion beconiinfr the optic nerve. The fibres of
the optic nerve are continued over the inner surface ot the
retina, in a manner to be presently described, as far as the
outer or posterior extremities of the ciliary processes; the
sinuous line which marks the limit of their distribution
being known as the ora serrata. In front of this line the
surface of the ciliary processes is invested by a much sim-
pler ciliary portion of the retina, which is in turn con-
tinued into the iridal portion, or uvea, which lines the in-
ner surface of the iris: the structure of these outlying por-
tions will be best understood after a description of that
of the principal portion of the retina.

When examined by the ordinary methods of hardening
and staining the retina shows in transvervse section a num-
ber of distinct strata or layers, tolerabl}^ uniform in
structure throughout the greater portion of its extent.
These are usually stated as eight in number: here, how-
ever, as in the other sense organs, our views have in re-
cent 3'ears been greatly modified by the results of the
chromate and silver method. An account of the eight
layers will first be given, and the relation of their compo-
nents as now understood subsequently considered.

Beginning at the inner or anterior surface of the retina,
there may be discerned next to the hyaloid membrane
which invests the vitreous bod}'^ a delicate layer which is
apparently (and onK' apparently) continuous : it has been
designated the internal limiting membrane: it is in
reality a mosaic formed b}^ the thin expanded ends of sup-
porting structural elements, the fibres of Mueller, which

344 PART II. HISTOLOGICAL ANATOMY.

pass verticalh' toward (but not to) the outer surface of
the retina.

Next to the internal limiting membrane, so called, is
seen the first definite stratum of the retina, the layer of
nerve fibres. These, which are non-medullated in most
cases, are arranged in small bundles which form a plexi-
form meshwork over the inner surface, radiating from the
optic nerve, or, more exactly, converging to it from all
portions of the retina. This layer, like nearly all the
others, is quite transparent, and is itself insensitive to
light.

The layer of nerve fibres is succeeded outwardly by the
ganglionic layer, a stratum of relatively large multipolar
nerve corpuscles, either spheroidal or pyriform in shape,
whose axis-cylinder processes are continuous with fibres
of the preceding layer, and whose other processes ramifj-
in the la3^er next beyond. The ganglionic layer varies in
thickness in different portions of the retina, being in some
places two or three corpuscles deep, but over the greater
portion of its surface consisting of a single layer of cor-
puscles: toward the ora serrata these become separated
from each other by considerable intervals.

Immediately beyond the ganglionic layer is situated the
inner molecular layer: this, which is in most portions
of the retina the thickest of the visible strata, is appar-
ently composed of a granular mass, which is in reality the
expression in section of the cut extremities of the rami-
fying processes of the corpuscular elements of the layers
next adjacent.

External to the inner molecular layer is seen the inner
nuclear layer, composed chiefly of closely aggregated
bipolar and multipolar nerve corpuscles, the former pre-
dominating. The corpuscles var^^ greatl}^ in size, but are,
on the average, decidedly smaller than those of the gan-
glionic layer: the disposition of their processes will be de-

CIIVrTIvK XXIII. Tin: ORC.ANS OF SPIXIAL SKXSE. 34-0

scribed later. Xiicleatcd cMilari^ciiK'nts of the sustentacu-
lar elements, or fibres of Mueller, are also found in this
Layer to some extent.

The inner nuclear layer is followed by the outer molecu-
lar layer, a <;ranular stratum closely resembling in its
appearance the inner molecular layer, but differing from it
gi'catlv in extent, being usually the thinnest of the various
la vers of the retina.

Bevond the outer molecular layer, again, is seen a layer
of cor j)uscular elements, the so called outer nuclear layer,
which as ordinarily seen resembles the inner nuclear layer
as closely as do the two molecular layers : nearly all of the
constituent elements are distinctly bipolar, and, as will
presently be shown, their relations are quite different from
those of the elements of the inner of the two apparently
similar layers. The corpuscles of this layer are, further-
more, closely connected with the elements of the succeed-
ing layer in a manner which differentiates them sharply
from any other retinal elements.

The outer surface of the outer nuclear layer is sharply
defined, the sustentacular tissue of the retina here termin-
ating soabruptly as to lead to the description of a definite
external limiting membrane: recent researches have
shown that the application of the term membrane in this
connection is even less justifiable than in the case of the
inner boundary of the retina.

External to the outer nuclear layer, and apparently
resting upon the so-called external limiting membrane, is
the bacillary layer, or layer of rods and cones. It consists
exclusively of the two kinds of elements designated by the
latter of these titles. Those termed rods are b\^ far the
most numerous in nearly all portions of the retina : each
consists of an inner or basal portion, somewhat thicker
in the middle than at the extremities, and an outer or
terminal portion, slightl}' longer than the basal in man

34-6 PART II. HISTOLOGICAL ANATOMY.

and most mammals, which is of nearly uniform diameter
throughout its entire length. The outer segment of each
rodistransversely striated, and can be resolved into a num-
ber of thin disks by the aid of certain reagents : the outer
portion of the basal segment is longitudinally striated.
The cones resemble the rods in consisting of two segments:
the inner of these is much stouter than the basal segments
of the adjacent rods, and is thicker at the base than at the
outer extremity : like the corresponding regions of the
rods, the outer portion of the basal segment is longitudi-
nally striated. Upon its free extremity is seated the outer
segment, which is shorter than that of the rods, and
tapers to a point : it shows transverse striations.

The layer of rods and cones was for a long time re-
garded as the limiting stratum of the retina, the adjacent
layer of pigment cells being associated with the choroid,
with which it is in close contact. With advancing know-
ledge of the embryological development of the eye, how-
ever, it has become evident that it must be regarded as
retinal in nature. It consists of a single stratum ol pris-
matic cells, hexagonal in form and so heavily loaded with
jjigment as to hide the large central nucleus: the outer
surface of the cell, in contact with the choroid, is smooth:
the inner is prolonged by numerous slender processes
which extend between the rods and cones of the adjacent
layer to a distance which varies in relation with the in-
tensity of the stimulus acting upon the retina.

The layers of the retina, as above described, are now
known to be the expression as seen under the more familiar
methods of preparation of a system of nervous elements ar-
ranged in a manner not unlike that which is found in the
other organs of special sense: their disposition may be
briefly described as follows, disregarding for the present
the layer of pigment cells just mentioned.

The rods and the cones of the bacillary layer are in real-

CHAPTER XXIII. THE OnOANS OF SPECIAL SENSE. 34-7

itv structurally continuous with the elements of the outer
nuclear laver in such a manner that they may with propri-
ety be rci^arded as their peripheral prolongations, or as
their greatly morlified dendritic portions. Each rod is
continued within the external limiting membrane by a
slender filament of greater or less extent which terminates
at the outer pole of one of the spindle sha])ed corpuscles ot
the nuclear layer: the body of the corpuscle is transversely
striated in a characteristic manner: from its inner pole it
gives oft' a fine varicose filament, the homologue of the
axis-cylinder process, which extends to the outer molecu-
lar layer and there terminates in a small knob-like expan-
sion, representing a greatly modified terminal arborization
the rod-corpuscles are situated at various levels in the outer
nuclear layer from the outer to the inner surface, their
peripheral and central filamentous prolongations varying
in length in a corresponding manner.

The base of each cone is continued beneath the external
limiting membrane by a stout strand of protoplasm
which passes almost immediately into the nucleated cor-
puscle, the latter being situated just within the membrane:
the corpuscle is continued inward b\' a stout smooth
fibre which passes directly across the outer nuclear layer
to end within the surface of the outer molecular layer by
a disk-like expansion from whose margin slender filaments
are given off, forming a rudimentary arborization.

The rod and cone elements, including both the bacillary
and the nuclear portions, as far as the outer molecular
layer, may be regarded as forming the first of the three
groups of nervous elements proper to the visual appa-
ratus: they are frequently distinguished as the neuro-
epithelial layer, or the layer of visual cells.

The outer molecular layer, like the glomeruli of the ol-
factory bulb, may be regarded as chiefly made up of the
interlacement of arborizations and dendrites, the central

348 PART II. HISTOLOGICAL ANATOMY.

terminals of the rod and cone elements here coming into
relation with the peripheral terminals of the corpuscles of
the inner nuclear la^'-er; or, as it is now frequently desig-
nated, the layer of bipolar corpuscles.

Each of these corpuscles is prolonged peripherally b\' a
filament which terminates at the outer molecular layer b\'
a group of dendritic processes: Cajal has shown that
those of some of them pass the outer portions of that
layer to form close tufts about the knob-like terminals of
the rod -elements; the central processes (or axis-cylinder pro-
cesses) of the same corpuscles passing to the innermost
portion of the inner molecular layer: for these he has pro-
posed the name of rod-bipolars. The others he has shown
to ramifx' extensivel}^ in the inner portion of the outer
molecular layer, in relation with the terminals of the
cone-elements; while their central processes terminate in
arborizations w'hich are situated at various levels in the
stratified inner molecular laver: these he calls cone-
bipolars.

The same investigator has demonstrated in the outer-
most portion of the layer of bipolar corpuscles elements
varying" in size, whose dendrites ramify in the outer mole-
cular layer and whose axis-cylinder processes run for
longer or shorter distances horizontally to end in arbor-
izations distributed in the same layer ; for which reason
he calls them the horizontal corpuscles of the retina. In
the innermost portion of the same layer of corpuscles he
has described pear-shaped amacrine corpuscles of vary-
ing size, whose processes branch and ramify in the inner
molecular layer at various levels corresponding to those
indicated in connection with the terminals of the cone-
bipolars. The nature and functions of the horizontal and
amacrine corpuscles of this layer may perhaps stiil be re-
garded as matters of question ; the bipolar corpuscles

CHAPTKK XXIII. THK OKCAXS OF SriaiAL SENSE. 349

clearly rorin ihc second members of tiie series ol nervous
elements involved in visual sensation.

The inner molecular layer resembles the outer (and, in-
deed, all the so-called molecular layers of the nervous sys-
tem) in consistins^ chieHy of an interlacement of central
and peripheral terminal filaments. It shows, as has been
indicated, evidence of stratification, due to the termina-
tion, at more or less definite levels, of the central pro-
cesses of the cone-bipolars, and the associated horizontal
distribution of their arborizations ; in relation not only
with the terminals of the processes of the amacrine cells,
but also with those of the dendrites of the corpuscles of
the layer next within.

The layer of ganglion corpuscles, the third and in-
nermost members of the visual series, is composed of ele-
ments which vary much in size: according to Cajal, the
smallest corpuscles send their dendrites into the inner-
most stratum of the inner molecular layer: the largest to
the outermost stratum : and those of intermediate size in
like manner to the intervening strata ; the arborizations
of the terminals from the rod-bipolars being distributed
in all cases in the innermost stratum. From each of these
corpuscles an axis-cylinder process is given off which
eventually becomes one of the fibres of the optic nerve, its
terminal arborizations being situated in the brain. Cajal
has also described in the optic nerve fibres which come
from the brain and enter the retina, terminatingby arbor-
izations within the layer of bipolar corpuscles: he regards
them as conve^-ing centrifugal impulses.

The layer of bipolar corpuscles and that of ganglionic
corpuscles, taken together, have been designated the cere-
bral layer, as distinguished from the neuro-epithelial
layer or layer of visual cells. The cerebral and neuro-
epithelial layers, taken together, are formed from the an-
terior (and principal) lamina of the collapsed optic vesi-

350 PART II. HISTOLOGICAL ANATOMY.

cle, the layer of pigment cells alone representing the pos-
terior lamina.

At the macula lutea the retinal layers are notably
thickened, the layer of ganglionic coi'puscles in particular
becoming several cells deep. Passing towards the centre
of the macula, the layers become rapidl}^ thinned to form
the fovea centralis, in which cone-elements only are pre-
sent in the neuro-epithelial layer, and cone-bipolars in the
cerebral layer, the central processes of the latter passing
obliquely outward to enter the inner granular layer at
the margin of the fovea. Where the optic nerve pierces
the retina the retinal structures are of course wanting.

The characteristic retinal layers disappear at the ora
serrata, the layer of visual cells first becoming absent.
Over the ciliary portion of the retina the posterior lamina
of the optic vesicle is represented by a layer of pigment
cells as elsewhere: the anterior lamina bj' a layer of col-
umnar cells l\nng between the pigment layer and the hya-
loid membrane: they have large oval nuclei near their outer
extremities. In the iridal portion of the retina both lam-
inae are represented by layers of pigment cells. The
masses of retinal pigment cells between the ciliary pro-
cesses form the so-called ciliary glands alread}'' mentioned.

The apparatus of hearing, like that of sight, comprises
a receiving and a transmitting mechanism in addition to
the structure which contains the special terminal organs
involved. The receiving neuro-epithelium is in some respects
more complex than in any of the other organs of special
sense: the accessory mechanisms are far simpler than
those of the apparatus of sight.

CIIAPTHU XXIII. Tin: ORGANS OF SIM;CIAI. SKXSK. 351

The pinna cotiijists essentially of a sheet of yellow fibro-
cartilage covered by integument: funncl-shiiped or vari-
ously modified in mammals generidly, it is in man crum-
pled and comparatively rudimentary, but retains its char-
acteristic structure. In the lobe of the ear the cartilage
is rejjUiced by a mass of fjit. The skin u]Jon the outer or
convex surface does not in man differ materialK'from that
of the adjacent portion of the head : that of the inner sur-
face is thin and but slightly mobile upon the subjacent
cartilage, and is devoid of sweat glands. The hairs of the
integument of the pinna are in man very small, with rela-
tivelv large sebaceous rrlands. The small intrinsic muscles
which pass from certain folds of the cartilage to others,
beneath the integument, are composed of slender striated
fibres.

The external auditory meatus, in part cartilaginous
and in part bony, is lined by a closely adhering tegumen-
tary layer, continuous with the skin upon the inner side
of the pinna, which grows thinner and simpler in struct-
ure as it passes inward. The portion of the skin which
invests the surface of the outer or cartilaginous portion of
the tube contains fine hairs which, like those of the pinna,
iire accompanied by well developed sebaceous glands : the
fibrous tissue subjacent to the corium contains in addi-
tion numerous convoluted tubular ceruminous glands
which greatly resemble sweat glands in form and struct-
ure but are larger and more closely aggregated : they are
farther characterized by their brownish color and the
highly refracting fatty particles seen in their secretion.
The lining of the deeper bony portion of the meatus is
devoid alike of hairs and of glands.

The membrana tympani, which separates the external
meatus from the middle ear, is composed of a fibrous lay-
er invested outwardly by a continuation of the integ-

352 PART II. HISTOLOGICAL ANATOMY.

ument from the bony wall of the meatus, and inwardly
bv the mucous membrane which lines the whole of the
tympanic cavity. The fibrous layer consists chiefly of ra-
diating bundles diverging chiefly from the point of attach-
ment of the malleus: there are in addition circularly dis-
posed bundles of fibres, chiefl\^ near the margin of the
membrane, which form a so-called inner layer. The tegu-
mentarv la\'er resembles that of which it is a continua-
tion: the mucous la\'er consists of a thin membrane rich
in elastic fibres which bears a single layer of cuboidal epi-
thelium whose component cells are devoid of cilia.

The auditory ossicles possess to some extent the char-
acter of dense bone, their thicker portions showing dis-
tinct though small Haversian systems of lamellae: defi-
nite marrow^ cavities exist in the interior of the principal
masses of the malleus and incus. The articular surfaces
are in each case invested with hyaline cartilage. The
muscles connected with the ossicles are composed of stri-
ated fibres. Both bones and muscles are invested wnth
the mucous membrane which lines the tympanic cavity.

The Eustachian tube, which connects the tympanic
cavit}^ with the pharynx, has a bony wall in its posterior
portion; in the anterior portion the wall is in part com-
posed of h\^aline cartilage, sparingly reinforced by bund-
les of white fibres, and in part of dense fibrous membrane.
It is lined throughout its extent by a mucosa which is a
continuation of that of the phar\'nx, and is in turn con-
tinued by the lining of the tympanic cavity: like that of
the phar3'nx, the mucosa is invested wath a layer of strat-
ified columnar ciliated epithelium. In the cartilaginous
])ortion of the tube there is a submucosa which contains
numerous mucous glands and a considerable, cpiantity of
diffuse adenoid tissue; in this respect, again, recalling the
structure of the ph^-rynx. In the bony posterior portion

CHAPTER XXIII. THE ORGANS OF SPKCMAL SENSE. 853

the mucosa is devoid of ;2;laii(ls and adheres more elosely
to the wall of the tube.

The cavity of the tympanum, like that of the mastoid
cells leadiuij out from it. may be re<^arded as an expan-
sion of that of the Eustachian tube. It is lined with a
mucous membrane which has already been frequentlv re-
ferred to : between the membrane and the bony wall of
the cavity is a submucosa consisting of interlaced fibrous
bundles among which are seen numerous spheroidal bodies
in manv wavs resem])ling Pacinian corjjuscles.

The disposition of this fibrous network, the irregulari-
ties of the bony surfaces involved, and the structures pre-
sent in the tympanic cavity cooperate to throw the mu-
cosa into cons]3lcuous folds; their disposition is a matter
for the anatomist rather than for the histologist. The
epithelium of the tympanic cavity is columnar ciliated
over the greater ])ortion of the surface; that of the mas-
toid cells is devoid of cilia. The existence of distinct
glands in the mucosa of the tympanum is a matter of
question.

The inner ear is in the strict sense the organ of hearing,
the middle and outer regions being merely accessory
thereto. It consists of the membranous labyrinth, in
which the specially modified neuro-epithelial structures in-
volved in audition are situated, enclosed in the cavity
within the periotic mass known as the bony labyrinth.
The membranous labyrinth is formed by the ingrowth of
the integument of the side of the head : this is at first a
simple saccular or flask-shaped cavity lined with epithe-
lium derived from the ectoderm, and communicating with
the surface by a small aperture. Later, this aperture is
obliterated and the sac is divided into two principal re-
gions, the utricle and the saccule, which are in the adult
onlvindirectlv connected. The three semicircular canals

354 PART II. HISTOLOGICAL ANATOMY.

are developments of the wall of the utricle and together
with it form the labyrinth in the limited sense in which the
term was formerly used. The cochlea is an extension of
the saccule.

The bon}' walls of the cavity which contains the utricle
and the semicircular canals are lined with a thin perios-
teum invested by flattened connective tissue corpuscles
which form an endothelium. The membranous structures
enclosed within are adherent to the periosteum along one
side of each canal and upon a portion of the surface of the
utricle: throughout the rest of their extent they are sep-
arated therefrom by a space filled with perihmph which is
traversed by frequent trabeculae of fibrous tissue, the free
surface being similarly invested. The membranous wall
consists of a layer of connective tissue containing numer-
ous elastic fibres, within which is a dense clear tunica
propria, whose surface throughout the canals shows nu-
merous low papillary eminences; lining the tunica is a
layer of polygonal pavement-epithelium cells.

In the ampulla of each of the semicircular canals the
tunica propria is much thickened along a projecting ridge,
the transverse septum, upon whose summit is situated a
crista acustica, or ampullar area of auditory neuro-epi-
thelium. In passing from the general surface of the am-
pulla upon the sides of the septum the pavement epithe-
lium becomes first cuboidal and then columnar in form,
the columnar cells being surmounted by a distinct cutic-
ular layer. Within the crista the epithelium consists of
cells of two sorts: fibre cells, whose elongated bodies ex-
tend through the whole epithelium, their bases being
larger than their free extremities, and their nuclei being
variously situated within the basal half; and hair-cells,
cylindrical elements which are situated in the outer half
of the epithelium only, their nuclei being situated near

CIIAPTICK XXIII. TFIi: ORC.AXS OF SPKCIAL SKNSE. 'A3~}

their rounded inner extremities, find their free ends bear-
ui'fl, Ion;; taperin.L!^ lilainonts, the auditory hairs.

Branches of the auditorv nerve are distributed to each
crista: as the fibres enter the epithelium they loose the
medullarv sheath, and quickly divide into fibrils which
ramify extensively in the vicinity of the hair-cells, their
free extremities being in every ease in direct contact with
these ej)ithelial elements: the relation between the nerve
terminals and the epithelial cells must be regarded as sim-
ilar to and as specializations of that elsewhere described
in connection with the free endings of nerve fibre in the
epidermis.

In material hardened for section cutting the surface of
each crista is found to be covered by a dome-shaped mass
of a clear colorless substance of unknown composition
and origin, in which the auditory hairs are imbedded : to
this mass the term cupula is applied. Under suitable
reagents the auditory hairs can be made to break up into
numerous fine cilia-like filaments, indicating that the hairs
are compound structures.

The surface of the utricle bears a large patch of neuro-
epithelium, the macula acustica or macula cribrosa, as it
is sometimes termed, essentially similar in structure and
in the mode of nerve supply to one of the cristae of the
ampullae. There is not such a marked thickening of the
subjacent tunica propria, and the auditory hairs are not
as long as those of the ampullar organs: the surface of
the macula is invested b\' a soft gelatinous mass in which
are imbedded numbers of crystals of calcium carbonate
known as otoliths. A macula in every way similar to
that of the utricle is found in the saccule.

The cochlea, a development of a portion of the saccule,
with which it is directly connected in the lower verte-
brates and in the embryo, is in the mammal in great
measure constricted off from that region in the adult, be-

856 PART II. HISTOLOGICAL ANATOMY.

ing connected with it only by a slender tubular passage,
the canalis reuniens. It should be regarded, however, as
a tubular diverticulum of that division of the primary
auditorv vesicle, differing from the rest of the membran-
ous labyrinth in its spirally coiled form, its mode of at-
tachment, and particularly in the complexity of its neuro-
epithelium, which here attains a degree of specialization
found in no other organ

Regarding, for convenience, the position of the whole
structure as so far shifted from that which it occupies in
the living body as to bring the base of the spiral into a
horizontal plane, the apex pointing upward, the cochlear
tube mav be said to be adherent outwardly for about one
third of its surface to the bony wall of thecontainingcav-
itv; and to be connected inwardly with the central bony
spiral lamina b\' two flat membranes, the lower of which,
the basilar membrane, is nearly horizontal in this posi-
tion, while the upper, the membrane of Reissner, slopes
at an angle of about forty-five degrees: the tube is there-
fore approximately triangular in cross section.

The periosteum of the outer wall is much thickened
alone: the area of adhesion of the cochlear tube to form
the spiral ligament, the greatest elevation being at the
point of attachment of the basilar membrane, where a
fibrous ridge is found known as the crista basilaris; a
short distance above this a second ridge is seen, the vas-
cular prominence, containing one or more conspicuous
bloodvessels : the somewhat concave surface between this
and the ridge to which the membrane of Reissner is at-
tached, known as the stria vascularis, exhibits a histo-
logical structure without parallel in the entire body. It
contains a rich network of capillaries, imbedded in ele-
ments apparentl}' epithelial in character, and commonly
so described: the superficial cells (which entirely overlie
the capillaries) are certainly in continuity with the epithe-

CHAPTKK XXIII. THK ORGANS Ol- SI'IXIAI. SKNSK. Sf)?

Hiim linin«T the rest of the tube; those situated between
the capillMi'ies may very possibly l)e epithelioid connect-
ive tissue corpuscles not unlike those known to occur
elsewhere.

The membrane of Reissner is an exceedingly delicate
sheet of connective tissue invested on the side toward the
scala vestibuli with a layer of endothelium some of whose
cells are pigmented : the inner side is lined, like the greater
portion of the membranous labyrinth, by a j^avement
epithelium composed of polyhedral cells: the three com-
ponent layers are of nearly equal thickness. The inner
and lower edge of the membrane is united to the middle
or inner portion of the limbus, a peculiar thickening of
the periosteum of the upper surface of the bon\' spiral
lamina.

The portion of the limbus situated outwardly from the
attachment of the membrane of Reissner terminates ab-
ruptly b}' a border excavated by the spiral groove, which
is bounded by an upper and a lower lip. The upper sur-
face of the upper lip is ridged and grooved and its margin
developed into numerous tongue-like processes, the audi-
tory teeth: the surface of the ridges and the teeth is in-
vested by polyhedral pavement epithelium : that of the
grooves is columnar: this is continued over the spiral
groove by a layer of cuboidal cells continuous at the
lower lip of the groove with the epithelial structures upon
the upper surface of the basilar membrane. The lower
lip extends to the margin of the bony spiral lamina.

The basilar membrane extends from this margin to the
basilar crest of the spiral ligament. Its middlelayer consists
of a sheet of homogeneous ground substance containing
scattered nuclei, and having embedded in it an immense
number of straight stout fibres running radially from the
spiral lamina to the basilar crest. The surface toward
the scala tympani is covered by a layer of connective tis-

358 PART II. HISTOLOGICAL ANATOMY.

sue whose elements do not take on a definite endothelial
form, but are largely spindle-shaped fibres disposed at
right angles to the fibres of the middle layer. The inner
surface is invested with epithelium continuous with that
lining the rest of the tube: that of the outer half of the
membrane, or zona pectinata, is but slightly modified :
that of the inner half, or zona tecta, is greatly modified
to form the characteristic structure of the cochlea.

This structure, the organ of Corti, is a neuro-epithelium
not unlike those found in the cristae and maculae acnsticae;
being composed, like those bodies, of hair cells and sup-
porting cells: its greater complexity is due chiefly to the
form and arrangement of the latter elements. The central
feature is a series of arches formed by the convergence
above of an inner and an outer rod of Corti, the outer
rods being longer and more slanting than the inner: the
triangular space beneath them, which runs the whole
length of the cochlea, beingknown as the tunnel of Corti.
Each rod consists of a broad basal portion, or foot, a
slender shaft, and an enlarged head, that of the inner rod
having a concave surface upon its outer side into Vt^hich is
fitted a corresponding convexity upon the inner aspect of
the head of the outer rod : both the inner and the outer
rods bear outwardly directed flattened phalangeal pro-
cesses, those of the inner rods overlapping the inner por-
tions of the processes of the outer rods. Both inner and
outer rods are invested with a layer of protoplasm which
is accumulated at the base on the side toward the tunnel
in a mass containing an oval nucleus. The inner rods are
narrower and more numerous than the outer.

On the inner side of the upper extremities of the inner
rods is situated a row of inner hair cells, cylindrical in
form, and, like those of the auditory structures, only
extending through the upper half of the layer: the upper
extremity of each Ijears a number of hair-like processes :

CHAPTER XXIII. TIIH ORGANS OF SPECIAL SKNPE. 359

the lower extremity is routuled and contains a spherical
nucleus. Internal to the inner hair cells are columnar sup-
portin«; cells which pass gradually over into the epithe-
lium of the spiral groove.

On the outer side of the heads of the outer rods are rows,
three or four in number, of outer hair cells, essentially
like those of the inner row. Between them are the ujiper
extremities of the cells of Deiters, elements somewhat re-
sembling the rods of Corti : each has a spindle-shaped
basal portion containing a spheroidal nucleus, and a slen-
der rigid upper portion which terminates in an outward-
ly directed phalanx. The phalangeal processes of the rods
of Corti and the phalanges of the cells of Deiters are united
by their angles to form a reticular membrane through
whose apertures the extremities of the outer hair-cells
project. Between the outer rods, the hair-cells, and the
cells of Deiters are intervals, the spaces of Nuel, which
communicate with each other and with the tunnel of
Corti, the whole being filled with a semifluid substance.

The organ of Corti may be said to be limited by the
hair cells and the cells of Deiters : the latter pass over out-
wardly into tall columnar elements, the cells of Hensen,
whose nuclei are situated in their large upper extremities.
These pass rather abruptl}' into the shorter columnar
cells of Claudius, between which and theepitheliumof the
outer wall of the tube a gradual transition is seen along
the surface of the zona pectinata.

The bundles of nerve fibres distributed along the cochlea
pass along the under surface of the bony spiral lamina
to its margin from the spiral ganglion of the mod-
iolus. Here they penetrate the basilar membrane : the
fibres loose their medullar}' sheath and are distributed to
the epithelium in a manner quite similar to that described
in the account of the cristae, some of the fibres travers-
ing the tunnel of Corti to reach the vicinity of the outer

360 PART II. HISTOLOGICAL ANATOMY.

hair cells. From the margin of the upper lipofthelimbus
a cuticular fold, the membrana tectoria, extends out as
far as the outer cells. It probably rests upon the organ
of Corti durinof life.

A comparison of the es.sential structures of the various
organs of special sense shows that they agree in being
modified epithelia containing more or less specialized ter-
minals of nervous elements. These epithelia are in [each
instance derived from the ectoderm : in the case of the re-
tina indirectl}^ the organ in question being formed as a
diverticulum of the nervous axis, which is itself formed
from an infolding of the ectoderm : in the case of the other
sense organs the derivation from the ectoderm is direct.
The nerve terminals of the organs of taste and of hearing
resemble each other in consisting of fibrils ramifying be-
tween the specialized epithelial cells, though these senses
are not at all related as regards the character of the stim-
uli to which they respond. Similarly, the terminals of the
organ of smell and of sight are somewhat alike, greath'
as these senses differ. Farther investigation may explain
these apparent resemblances and differences and exhibit a
still deeper unity of structure in the mechanisms of special
sense.

INDEX.

361

INDEX.

Pack

Achromatic spindle 120

Achromatin 117

Adenoid nodules 180, 183, 271

Adenoid tissue, 40, 80, 178, 183, 196

Adipose tissue 39

Adrencils 281

Adventitia 92, 94, 260, 264

Agminated glands 180

Allantois 257

Alveoli, pulmonary 188

Amacrine corpuscles 102, 111

Amitotic division 119

Amoeboid motion 33, 80

Ampulla 226

Ampullae, auditory 354

Aorta 261

Apolar c(irpuscles 110

Appendix vermilormis 182

Arborizations 109, 111

Areas of Colinheim 88

Areolae 39, 55, 66

Areolar tissue 38

Arteries 91, 92, 259

Articular corpuscles 105

Auditory meatus 551

ossicles 362

teeth 347

Aucrbach, plexus of. 173, 174

Axilemma 108

Axis-cylinder 101, 105

Axis-cylinder process 102, 110

Bacillary Layer 345

Hellini, ducts of 210

Bertin, columns of 205

Bile capillaries 189

Bipolar corpuscles 102, 109

Bladder, urinary 214,257

Blastoderm 128

Page

Blood 14, 73

corpuscles 74

platelets 74, 82

Bone corpuscles 51

nutrition of. 57

structure of. 32, 50, 53

Brain 311

Bronchi 195

Bronchial muscle 196

Bronchioles 197

Brunner, glands of. 177

Caecum 182

Calyces of kidney 204, 212

Canal of Schlemm 339

Canaliculi 49, 51

Capillaries 91 , 93

Capsule of Bowman 207

of Glisson 186

of lens 341

of Tenon 336

Cardiac fibres 85

Carotid glands 280

Cartilage 25, 29, 30

calcified 27

cellular 26

elastic 27

hyaline 26

ossification in 60

reticular 27

transformation of. 64. 65

vascularization of. 63

Cartilage bone 61

Caruncle.... 334

Cell-division 2.S, 119

Cells 14, 115

of Claudius 359

colloid 278

of Deiters 359

362

INDEX.

Cells of Hensen 359

tactile 103, 104

Cellular tissue 38

Cement um 164, 166

Central canal of cord 292, 309

Centres of ossification 61

Centroacinar cells 185

Cerebellar cortvx 312

Cerebral cortex 316

Ceruminous glands 144

Cervix uteri 242

Chondrin ^9

Chondroclasts 56

Chondrogen 29

Chordae tcndinea 263

Choroid 337

Chromatin 117

Chromoplasm 117

Chromosomes 120

Ciliary processes 337

Circumferential lamellae 52

Clitoris 245,258

Cloaca 257

Coelom 250

Coccygeal gland 280

Cochlea 355

Cohnheim, areas of. 88

Collagen 29

Collaterals Ill

of cord 303

Colloid cells ■ 278

Columnae carnae 263

Columns of Bertin 205

of Burdach 298

Clarke's 296

of Goll 298

of Morgani 182

of Sertoli 221

of spinal cord 292

of Tuerck 300.

Coni vasciilosi 223

Conjunctiva 332

Connective tissues 24, 32, 38

Contractile tissues S3

Cord, spermatic 217

spinal 291

Corium 138, 155

Cornea 336

Corneal tissue 32, 48

Corpora cavernosa 228

Corpus Arantii 263

Highmori 219, 223

luteum 237

spongiosum 229

Corpuscles 14, 23

accessory 36

adventitious 36

amacrine 102, 111

apolar 110

articular 105

basket 315

bipolar 102, 109

blood , 73

bone 51

of Cajal 316.

colored . 74

colorless 74, 77

concentric 278

. corneal 49

fixed 31, 36

genital 105, 230. 244

of Gran dry 103

, of Hassali 278

Malpighian 205. 276

. of Martinotti 318

of Melssner 104

— —migratory 33, 36

mitral 327

multipolar 102, 110

of muscle 87

nerve 102, 111

nerve-fibre 101

of Purkinje 313

pyramidal 317

red 74

of retina 348

of spinal cord 305

tactile 104

unipolar.... 102. 110

of Vater 105

wliite 74

Cowpcr, glands of 231

INDEX.

363

Crcmaster 218

Crescents of Ciianuzzi 158

Crista acustica 354

Crtista i>etrosa IfiG

Crypts of Lielirrkuhn 177

Crystalline letis 340

Cumulus proligerus 236

Cupula 355

Cytology 15

Daktos 218

DeBove. endothelium of. 178

Deiters, process of. 102

Demilunes of Heidenhain 159

Dendrites 102, 109, 110

Dense bone 52, 69

f«)rmation of. 64

Dental papilia 167

Dentinal tubules 165

Dentine ..32, 48. 164

Dcutoplasm 116

Discharging terminals 106-

Discus proligerus 236

Disks, tactile 104

Division, amitotic 119

karyokinetic 119

Dobie's line 88

Dorsal coniua 292

Duct of Gartner 240

Ductless bodies 269

Duodenum 177

Dura 289

E.\R 350

Ebncr, glands of. 170

Ectodei-m 128

Ectoplasm 116

Elastic cartilage 27

fibres 34

membrane 41

tissue 43

Elastin 35

Elementary particles 82

Enamel 164, 166

germ 167

End-bulbs of Krausc 104

Endings, free nerve 103

Endocardium 262

Endochondral bone 64, 67

Endomysium 90

Endoneurium 112

Endoplasm HG

Endosteum 59

Endothelium 20, 44

of DeBove 178

End-plates, motor 106

Entoderm 128

Eosinophile cells..... 37

leucocytes 79

Ependyma 309

Epiblast 128

Epicardium 262

Epidermis ,. 135

Epididymis 217, 223

Epineurium 112

Epiphysis cerebri 284

Epithelium 17

ciliated.... 19

columnar 8l

germinal 234

— — glandular IS

pavement 18

polyhcdrai 18

respiratory 198

simple 19

spheroidal .^. 19

squamous 18

stratified 19

transitional 19, 214

Epoophoron 239

Erectile tissue 228

Erectorcs fili 15o

Erythroblasts .'i5, 81

Erythrocytes 74

Eyelids 331

Factors of strictire 14

Fallopian tubes 238

Fasciculi, muscular 89

Fat cells .... 37

364

INDEX.

Fenestrated membrane 42

Ferrein, pyramids of. 205

Fertilized ovum 86

Fibres 34

elastic 34

gray 101. 107

medullated 101

of Mueller 344

non-meduUated 101

of Purkinjc 563

of Remak lOl

smooth muscular 84

striated 87

of Tomes 165

white 34, 101, 107

Fibrillae, muscular 88

• primitive 107

Fibrin 73

Fibro-cartilage 27

Fibrosa 173, 194, 196, 216, 244

Fibrous membrane 41

tissues 31, 32, 42, 45

Fields of Cohnheim 88

Fixed corpuscles 31, 36

Follicles 157

intestinal 177

Foveolae 56

Free nerve endings 103

Funiculus 112

Ganglia 113

Ganglionic columns 294

Gartner, duct of. 240

Gelatin 29

Gelatinous fibres 101

tissue 32

Genital corpuscles 105, 230, 244

eminence 257

ridges 257

Germinal epithelium 234

ridge 255

spot 237

vesicle 237

Gianuzzi, crescents of. 159

Giant cells 55

Giraldes, organ of 223

Glands 157

of Bartholin 245

of Brunner 177

cardiac 175

carotid 280

ceruminous 144, 351

coccygeal 280

of owper 231

of Ebner 170

gastric 175

lachrymal 334

of Littre 227

mammarj' 246

Meibomian 145, 333

- of Moll 332

of mouth 160

mucous 158

peptic 175

prostate 231

pyloric 175

salivary 161

sebaceous 144

serous 158

of skin 142

sudoriparous 152

of Tyson 230

unicellular 156

uterine 240

Glandular epithelium 18

Glans penis 230

Glia-cells 46,113

Glisson, capsule of. 186

Glomeruli, renal 207

olfactorj' 327

Goblet cells 19. 156

Golgi, organs of. 103

Gonads 217

Graafian follicles 234

Grandry, corpuscles of. 104

Granule cells 37

Gray commissure 292

fibres 101, 107

matter of cord 292

Gums 154

Haematoblasts 82

INDEX.

365

Haemoglobin 76

Hairs U6

Hassall's corpuscles 27H

Haversian canals 52

spaces ". 53

systems .- 52. 166

Head kidney 252

Heart 261

Heidenhain, demilunes of. 159

Henle's layer l+H

loop 208

sheatli 113

He|)atic cords 188

Horny layer 135

Houston, valves of. 182

Howship's lacunae 56

Huxley's layer 148

Hyaline cartilage 26

Hyaloid menibrane 342

Hyaloplasm 1 15

Hydatids of Morgagni 223. 224

stalked 239

Hymen 244

Hypoblast 128

Hypophysis cerebri 283

I.NCREMENTAL LINKS OF SALTER, 165

Infundibula, pulmonary 197

Intima 92, 259, 264

Interglobular spaces 165

Interlobular cells of pancreas.. 185

Internal sphincter 184

Internodes 101

Interstitial lamellae 52

Intestinal follicles 177

Intramembranous ossification, 60, 61

Involuntarv muscular tissue 84

Jelly of Wharto.n

33

Karvokinksis 119

Kidneys 203

bloodsupply of : 205

permanent 252

primitive 252

Krausc, end-bulbs of 104

membrane of 88

Laijia majora 246. 257

minora 245. 257

Lachrymal glands 334

Lacteal glands 267

Lacunae, of bone 49, 50

Howship's 56

of Morgagni 227

Lamellae, circumferential 52

interstitial. 52

Ivameilated tissues 47

Lamina cribrosa 335

fusca 335

- - suprachoroidea 337

Large intestine 181

Larynx 199

Lateral cornu .- 292

Leucocytes 33, 36, 74. 77

Lieberkuhn, crypts of. 177

Lines of Schreger 165

Lingual papillae 169

Littre, glands of. 227

Liver 185

Lymph 96

capillaries 92, 96

vessels 92,97, 264

Lymphatic glands 80, 272

Lymphobiasts 81

Lymphocytes 80

Lymphoid tissue 80

Macula acistica 355

Malpighian corpuscles 205, 276

Malpighi, lacunae of 227

pyramids of 204

Mammary glands 246

Marrow 40, 53, 54. 81

Matrix 23

Media 92, 259. 264

Mediastinum testis 219

Medullated fibres 101. Iu6

Medullary sheath lOl

Meibomian glands 144

Meissner, corpuscles of. 104

366

INDEX.

Meissner, plexus of 172, 174, 178, 183

Melanin 37

Membrana granulosa 236

propria 41

tectoria 360

Membrane, basilar 356

bone 61

of Bowman 336

of Descemtt 336

of Krause 88

Nasm\'tirs 166

periodontal 166

serous 98

tympanic 351

Membranous labyrinth 353

Meninges 267, 288

Menisci, tactile 104

Merkel, tactile cells of 103

Mesenchyma 130

Mesentery 267

Mesoblast 128

Mesoderm 128

Mesogaster 177

Mesonepliros 252, 254

Metanephros 252

Microcytes 75

Migratory corpuscles 33, 36

Mitosis 119

Mons veneris 246

Morgagni, columns of. 182

hydatids of 223. 224

Motor end-plate 106

Mucosa, 154, 155. 172, 175, 177,

181, 182, 192, 215,238,240,243

Mucous glands 158

layer of skin 135

membranes 153

tissue 32

Muellerian duct 253

Multipolar corpuscles 102, 110

Muscle 89

bronchial 196

corpuscles H7

papillary 263

of Kiolan 332

tracheal 194

Muscular tissues 84

cardiac 36

smooth 84

striated 37

Muscularis mucosa. ...lt>6, 172, 174,

176, 178, 183, 241.
Musculosa, 172, 174. 177, llsi, 184,

216. 239, 241, 244.

Myelin 101

Myeloplaxes 55

Myocardium 262

Nails 15 1

Nasmyth's membrane 166

Nephridium 249

Nephrostome 250

Nerve corpuscles 102, 111

fibres 101, 109

fibre corpuscles 101

terminals 102

Nerves 112

Nervous tissues 99

Neurilemma 101, 109

Neuro-epithelium 222, 325, 347,

354, 308, 360.

Neuroglia 46, 113

Neurokeratin 108

Neuroplasm 107

Nodes of Ranvier 101

Nodules, adenoid IHO, 183

Non-meduUated fibres 101, 105

Nuclear division 118

Nucleoli - 117

Nucleus 16, 110

Odontoblasts 164

Oesophagus 174

Olfactory bulbs 325

cells 325

glomeruli 327

hairs 325

Omentum 177, 267

Oosperm 128

Organ 14, 127

of Corti 358

of Ciraldcs 223

INDEX.

367

Orjjf.'iii of (iolgi 103

of Roseuniucllcr 2.'}9

Os uteri 24-:{

Osier's jj;ra miles 82

Ossein 51

Osseous tissue 50

Ossification 59

centres of (il

in cartila^'c 60. 63

in membrane 60, 61

Osteo!)lasts 5+. 59

Osteoclasts 56

Osteogenetic fibres 62

- layer 54,59

tissue 66

Otoliths 355

Ova, primitive 234-

primordial 255

Ovary 233

Oviducts 238, 253

Ovula Nabotlii 242

Ovum 128, 236

I'aciman noDiKs 105, 230

Palate 154

Pancreas 184

Pannicuius adiposus 140

Pa])i!lae, foliate 322

of kidney 203

of Iin<,nial 169

Papillary muscles 263

Parablast 130

Paradidymis 225

Paraplasm 116

Paratliyroids 279

I'arietal cells 176

Paroophoron 240

Parovarium 239

Pavement epithelium 18

Pelvis of kidney 212

Penis 227, 257

Peptic cells 176

Pericardium 266

Perichondral bone 64. 69

Perichondrium 30

Periodontal membrane 166

Perimysium 90

Perineurium 112

Periosteum 53, 54. 66, 68

Peritoneum 266

Perivascular lymphatics.. 98

Permanent marrow 67

Peyer's patches 180

Pflue^er, tubes of. 161

Pharynx 173

Pia 289

Pigment cells 37

Pineal body 284

Pituitary body 283

Plain muscular tissue 84

Plasma of blood... 73

cells 37

Platelets, blood 74, 82

Pleurae 266

Plexus of Auerbach 173, 174, 177

of Meissner..l72, 174, 178, 183

Plica semilunaris 333

Poles of nerve corpuscles 102

Polyhedral epithelium 18

Portal canals 186

Prickle cells 19

Primary areolae 65, 66

marrow 67

vascular invasion 67

F'rimitivc fibrillae 107

ova 234

sheath 101

Process, axis cylinder 102

of Deiters 102

Pronephros 252

Prostate gland 231

Protoplasm 15, 115

Protoplasmic processes 102, 110

Pulmonary artery 261

Pupil, muscles of 339

Pylorus 1 79

Pyramids of Ferrein 205

of Malpighi 204

Pyriform corpuscles 101, 109

368

INDEX.

Ranvier, nodes of 101

Receiving terminals 102

Rectum 182

Red corpuscles 74

marrow 55

Remak, fibres of 100

Rete mucosum 135

testis 220

Reticular cartilage 27

Reticulum 1 15, 121

Retiform tissue 4-0

Retina 344

Retinal pigment 346

Retzius. stripes of. 106

Ring-muscle 338

Root-sheatli of hair 147

Saliva 163

Salivary glands 161

Salter, incremental lines of. 165

Sarcolemma 87

Sarcoplasm 88

Sarcous elements 88

Schachowa, spiral tubule of 208

Schmidt, medullary segments of 108

Schreger, lines of. 165

Schwann, white substance of.... 101

Sclerotic 335

Scrotum 217, 258

Sebaceous glands 144

Secondar3' areolae 65, 66

Segmental ducts 252

organs 250

Segmentation 128

Seminal vesicles 226

Seminiferous tubules 220

Serosa 173, 177, 181, 239

Serous endothelium 20, 98

glands 158, 159

membranes 99, 265

Sertoli, columns of 221

Sharpey's fibres 53

Sheath, Henle's 113

Simple epithelium 19

Sinus pocularis 231, 232

Skeletal tissues 45

Skin 135, 141, 217, 230, 331, 351

Small intestine 179

Small vessels 91

Smooth muscular fibres 84

Sole-plate 106

Solitary follicles 170

Spaces of Fontana 339

interglobular 165

Spermatic cord 217

Spermatozoa 220, 222

Spermiduct 254

Spheroidal epithelium 18

Spinal cord 291

ganglia 309

nerves 301

Spleen 274

Spirem • 119

Spongioplasm 115

Spongy bone- 52

Squamous epithelium 18

Stalked hydatid 239

Stellate corpuscles 113

Stellules of Verhuyen 206

Stigmata 94

Stomach 175

Stomata 98, 198

Stratified epithelium 19

Stratum adiposum 140

corneum 137

epitrichium 137

granulosum 136

lucidum 137, 152

Malpighii 136, 152

papillare 139

reticulare 139

squamosum I'SH

Stria vascularis 356

Striated muscular fibres 87

Striped muscular fibres ... 87

Stripes of Retzius 166

Subcutanea 140

Submucosa 154, 156, 172, 174

177, 178, 181, 183, 193, 196, 216
239, 241, 244.

INDEX.

369

Substance of Rolando Ii95. 307

Sudoriparous slj^nds 142

Su|)rarciial i.'a|)Siiles....j 2Ml

Svmpatlictic system 110

Synovial nienihrancs 268

Tactilk cklls 103, lO-l-

cor|)usclcs 104-

disks 104

hairs 150

Taste-buds 322

Teeth 16+

Tendon cells 43

tissue 42

Terminals, discharf^in^ 106

nerve 102

receiving 10 i

Testis 217, 219

Theca folliculi 235

Thoracic duct 265

Thymus 277

Thyroid 27S

Tissue 13

Tomes, fibres of. 165

Tongue 168

Tonsils 171, 173

Trabeculae of spongy bone 53

Trachea 192

Tracheal muscle 194

Tracts of cord 299

Transitional epithelium 19

Tubes of Ptlueger 161

Tunica albuginea 219, 229, 234

Tympanum 353

I'mbilical cord 33

Uniceilular glands 156

Unipolar coipuscles 102, 110

Ureter 213

Urethra, female 216

male 226

Urinary bladder 214, 257

Uriniferous tubules 207

Urogenital sinus 257

U'tcrine glands 240

Uterus 240

masculinus 231

Vagina 243

Valves ot heart 263

of Houston 182

of veins 264

Vas aberrans 223, 224

deferens 219, 224, 225

Vasa cfferentia 223

vasorum 261

Vascular endothelium .20, 90

Vascularization of cartilage 63

Vatcr, corpuscies of. 105

Veins 91, 94, 263

I Ventral cornu 292

I Verhuyen, stellules of. 206

Vermiform appendix 182

Vessels 91

Vestibule 245

Villi 177

Vitelline membrane 237

Vitellus 237

Vitreous body 33,341

Volkmann's canals 52

Vulva 244

Wharto.n's jelly 33

White commissure 293

fibres 34, lOl, lo7

fibrocartilage 28

fibrous tissue 42

matter of cord 292

substance of Schwann 101

Wolffian body ... 253

duct 253

Yellow fibres 34

fibro-cartilage 27

marrow 56

Zona pellccida 236

striata 236

vasculosa 234

Zone of Zinn 343

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