A Course in Normal Histology 
By Professor Rudolf Krause, Berlin. Translation by Philip J. R. Schmahl, M.D., 
New York. A Course in Normal Histology. A Guide for Practical Instruction in 
Histology and Microscopic Anatomy. With 30 Illustrations in the Text and 208 
Colored Pictures, Arranged on 98 Plates, After the Original Drawings by the Author. 
Part I, 96 pages, Cloth, 75 Cents. Part II, 416 pages, Cloth, $5.50. 
CONTENTS 
Part I.-The Microscope-Rules for the Use of the Microscope-General Micro- 
technique Methods-Methods of Preservation-Preparation of Sections-Staining 
Methods-Impregnation Methods-Methods of Injection-Mounting and Finishing 
-Mensuration and Drawing. 
Part II.-I. The Cell-II. The Tissues of the Animal Body-1. Epithelium- 
2. Connective Tissue-3. Cartilage-4. Bone-5. Muscle-6. Nerve Tissue-7. Blood 
-III. Organs-7. Circulatory Organs-2. Glands-3. Organs of Digestion-4. Or- 
gans of Respiration-5. Urinary and Genital Organs-6. Organs of Motion- 
7. Organs of the Nervous System-8. Organs and Sense-9. The Skin-Index. 
ABSTRACTS FROM REVIEWS 
"The illustrations are excellent and well 
chosen; the descriptions are clear and concise. 
. . -Johns Hopkins Hosp. Bulletin. 
"This is the most comprehensive work on 
histology, and one every one should possess. 
. . ."-Wisconsin Med. Recorder. 
"An abundance of text-books exists on the 
subject of histology, some of them having been 
standards for many years, but no more beauti- 
fully illustrated volume than this has appeared 
at so low a price. . . -Medical Record 
(N. y.) . 
"This is a collection of two hundred and 
eight superb histological plates of tissues and 
organs with their brilliant stains, all presented 
with a care and degree of perfection seldom 
equalled in works available to the student."- 
New York Med. JI. 
"The publications of the Rebman Company 
excel in respect of illustration, and this one 
is no exception. A student who owned this 
handsome volume might well consider himself 
fortunate."-Canadian Med. Assoc. JI. 
"This is the finest series of colored plates, 
both microscopic and gross, as yet presented for 
study. The Rebman Company in the last ten 
years has given a great boost to medical science 
in the United States by their masterly array 
of French, German and Italian authorities, 
lavishly illustrated and indispensable to anat- 
omy, pathology and differential diagnosis. The 
device of their trademark on every volume, 
"Age quod agis" (Do what thou doest), should 
be an inspiration to the student as it has been 
to the publishers in putting before us those 
fundamental works such as the present treat- 
ise. They have stability and permanent value. 
• • ."-Indianapolis Med. JI. 
"We might cast our superlatives not only at 
Krause, but also at Schmahl (the translator), 
at Rebman (the publisher) and at the art 
printers. We hope that our American schools 
will adopt this work for practical histology 
and leave the theory to the lecturer. . . - 
Archives of Diagnosis. 
"Krause's teachings are beginning to have 
effect in the English-speaking countries, and 
doubtless by the translation of his book more 
rapid strides will be made in bringing his- 
tology into its own. . . -Maryland Med. JI. 
"Rebman Company is giving us an excellent 
series of scientific text-books translated from 
European texts, of which this is one of the 
best examples. It is fitting that a teaching 
book on Histology should excel in its illustra- 
tions, since it is only through the visual faculty 
that histology can be properly imparted and 
acquired. And this book does excel in its 
illustrations, containing as it does some of the 
finest pictorial representations of tissue-struc- 
tures that it has ever been our pleasure to 
see."-Clinical Medicine. 
"The work is written from the modern 
standpoint and is especially to be commended 
for its practical character."-Interstate Med. JI. 
"To teachers and students alike it will prove 
acceptable as a proper guide in Histology."- 
Dominion Med. Monthly. 
"This work-as would be expected at the 
hands of such a man-covers the ground 
clearly and tersely, not only as to text, but 
by arrangement and type. . . ."-JI. of Oph- 
thalmology, Otology and Laryngology. 
"To one even slightly familiar with histology 
as a subject, the work has hardly an equal as 
a reference book."-New Orleans Med. and 
Surg. JI. 
"The plates are most excellent and leave 
nothing to be desired."-Canadian Practitioner 
and Review. PUBLISHER'S ANNOUNCEMENT 
The Plates and Figures quoted in the pages of "A Textbook of Histology" 
refer to the Plates and Figures contained in R. Krause's "A Course in Normal 
Histology," and are cited by the author for the purposes of illuminating his 
lectures. 
We offer this work to teachers and students alike as a guide in Histology. 
Professor Krause strikes a keynote in his preface to the book. The study of 
Histology should go hand in hand with that of Anatomy. The one cannot 
be separated from the other nowadays without serious injury to the student. 
The reason why we publish this work in two parts is apparent. 
The First Part is simply a guide to the technique of Microscopy and may 
be used by students of medicine as well as by those who pursue subjects of 
science foreign to medicine. 
The Second Part deals exclusively with Histology and therefore appeals 
chiefly to the medical man. 
The author's aim is to place at the disposal of the student a book of 
reference which is both practical and theoretical, as well as to furnish the 
teacher with a textbook that will serve as a detailed, clear, concise and method- 
ical guide through the course of Microscopy and Histology. 
So far as the student of medicine is concerned he will in the first part find 
much valuable information appertaining to the microtechnique with which the 
student of pathology must needs be familiar. 
The subject of microtechnique is in most all of the English written books 
on this topic treated but briefly. Krause's work not only acquaints the student 
in a thorough fashion with the theory and manipulation of the microscope, but 
introduces him also to all the methods employed in preparing a specimen 
for microscopic examination from start to finish. 
Professor Krause justly attaches great importance to the matter of draw- 
ing the specimens from the microscope. The illustrations in the second part 
are true reproductions of the colored drawings made by the author himself, 
and are intended not only to illuminate the text matter, but also to stimulate 
the student to practical efforts in reproducing on paper his own microscopical 
findings. (See preceding page.) 
F. J. Rebman, Pres. 
REBMAN COMPANY. 
New York, 141, 143, 145 West 36th Street. A TEXTBOOK 
OF 
HISTOLOGY 
BY 
RUDOLF KRAUSE 
A. O. PROFESSOR OF ANATOMY AT THE UNIVERSITY OF BERLIN 
TRANSLATED FROM AN ORIGINAL MANUSCRIPT AND PRINTED 
ONLY IN THE ENGLISH LANGUAGE 
WITH THIRTY-SIX ILLUSTRATIONS IN THE TEXT, 
THREE OF WHICH ARE COLORED 
THE REFERENCES TO ILLUSTRATIONS GIVEN IN THE TEXT RELATE TO 
THE COLORED ILLUSTRATIONS CONTAINED AND PUBLISHED 
IN DR. RUDOLF KRAUSE'S " A COURSE IN 
NORMAL HISTOLOGY " 
NJEW YjO R K 
REBMAN Cq|m P A N Y 
141, 143 AND 145 WEST 36th STREET Copyright, 1915, by 
R E B M A N COMP A N Y 
New York 
All Rights reserved 
PRINTED IN AMERICA INTRODUCTION 
The bodies of all of the higher animals, including man, are composed of 
a large number of single organs, with a distinct function allotted to each 
in the general economy. To give a simple example: The kidney is the organ 
of the human body to which is entrusted the duty of excreting the end products 
formed elsewhere by metabolism. The ureter, bladder, and urethra are supple- 
mentary organs attached to it to provide means for the final elimination of 
its product from the body, and we call any such interrelated group a 
system of organs. 
In order to fulfill its own special duty each organ has its own peculiar 
structure, which is fixed and unchangeable under normal conditions; i.e., it 
is constructed of certain tissues arranged in a definite manner. The same 
tissues are met with in almost every organ, but their arrangement in each 
is quite typical. 
Organs are composed of epithelial, supporting, muscular and nervous tis- 
sues. These are not the primary components of the human body, but each 
is made up of minute, elementary particles, called cells, with an intermediate 
substance derived from these cells in many cases. Cells are therefore the 
real elements of the human body, and every description of its minute structure 
must begin with and be based upon them. Hence the investigation of all of 
the material composing the microscopic anatomy of the human body falls 
naturally into three large sections: 
I. The cell. 
II. The tissues. 
III. The structure of the individual organs. 
III TABLE OF CONTENTS 
Introduction iii 
PAGE 
Publishers' Announcement iv 
A. THE CELL 1 
Form and Size of Cells 1 
Construction of the Cell 2 
The Cell Body 2 
The Nucleus 4 
The Central Bodies 6 
The Chondriosomes 7 
The Reticular Apparatus 7 
The Biological Properties of the Individual Constituents of the Cell . . 7 
The Manifestations of Life in the Cell 8 
The Metabolism of the Cell 8 
Reaction of the Cell to External Stimuli 10 
The Propagation of the Cell 11 
Indirect Segmentation 12 
Direct Segmentation 15 
B. THE TISSUES 16 
Epithelial Tissue 17 
Simple Epithelium 20 
Stratified Epithelium 21 
Stratiform Epithelium 23 
Glandular Epithelium and Glands 24 
The True Glands 25 
Closed and Solid Glands 29 
Muscular Tissue 32 
Smooth Muscular Tissue ,32 
Striated Muscular Tissue 33 
The Cardiac Muscle Fiber 38 
Nervous Tissue 39 
Nerve Cells 41 
The Nerve Fiber 45 
Glia Tissue 47 
The Development of Nervous Tissue 48 
The Supporting Tissue 49 
The Chordal Tissue 50 
The Vesicular Supporting Tissue 50 
The Gelatinous Tissue 51 
V VI 
B. THE TISSUES-Continued 
Muscular Tissue 
The Vitreous Tissue * * 
The Reticular Tissue 51 
The Neuroglia Tissue 52 
The Connective Tissue 52 
Cartilaginous Tissue 57 
Hyaline Cartilage 58 
Fibrocartilage 60 
Elastic Cartilage 61 
Osseous Tissue 61 
PAGE 
C. MICROSCOPIC ANATOMY OF ORGANS 66 
I. THE VASCULAR SYSTEM 66 
1. The Blood 66 
The Red Blood Corpuscles 67 
The White Blood Corpuscles 68 
The Blood Platelets 70 
The Haemokonia 71 
Physiological Importance and Replacement of the Corpuscular 
Elements of the Blood in Adults 71 
2. The Blood Vessels 72 
The Capillaries 73 
The Arteries 73 
The Veins 75 
The Vessels and Nerves of the Blood Vessels 76 
3. The Heart 76 
4. The Lymph 79 
5. The Lymphatics 79 
6. The Lymphoid Organs 80 
7. The Spleen 82 
II. THE DIGESTIVE ORGANS 84 
1. The Lips 84 
2. The Teeth 86 
a. The Dentine 88 
b. The Enamel 89 
c. The Cement 89 
d. The Dental Pulp 90 
e. The Periodontal Membrane 90 
f. The Gums 90 
3. The Tongue 91 
4. The Palate 94 
5. The Salivary Glands 95 
The Submaxillary Gland 96 
The Sublingual Gland 98 
The Parotid Gland 98 VII 
C. MICROSCOPIC ANATOMY OF ORGANS-Continued 
II. THE DIGESTIVE ORGANS PAGE 
6. The Pharynx 99 
7. The CEsophagus 100 
8. The Stomach 101 
9. The Intestine 105 
The Liver Ill 
The Gall Bladder 115 
The Pancreas 116 
The Peritoneum 119 
III. THE RESPIRATORY ORGANS 121 
1. The Nasal Cavity 121 
2. The Larynx 123 
3. The Trachea and the Extrapulmonary Bronchi .... 125 
4. The Lungs and the Intrapulmonary Bronchi 126 
5. The Pleura 129 
6. The Diaphragm 130 
7. The Thyroid Gland 131 
8. The Parathyroids 132 
9- The Thymus 133 
IV. THE URINARY ORGANS 136 
1. The Kidney 136 
2. The Pelvis of the Kidney and the Ureter 144 
3. The Urinary Bladder 145 
4. The Urethra 146 
a. The Male Urethra 147 
b. The Female Urethra 148 
5. The Suprarenal Capsules 149 
6. The Carotid Gland 151 
7. The Glomus Coccygeum 151 
V. THE MALE SEXUAL ORGANS 152 
1. The Testicles 152 
2. The Excretory Passages of the Testicles 156 
a. The Epididymis 156 
b. The Vas Deferens 156 
c. The Seminal Vesicles 157 
d. The Ejaculatory Ducts 157 
e. The Prostate 157 
f. Cowper's Glands 159 
g. The Semen 159 
h. The Penis 162 
VI. THE FEMALE GENITAL ORGANS 165 
1. The Ovary 165 
2. The Fallopian Tube 170 
3. The Uterus 171 VIII 
C. MICROSCOPIC ANATOMY OF ORGANS-Continued 
VI. THE FEMALE GENITAL ORGANS PAGE 
4. The Vagina 174 
5. The External Female Genital Organs 176 
a. The Vestibule of the Vagina 176 
b. The Clitoris 176 
c. The Labia Minora 175 
d. The Labia Maj ora 177 
VII. THE ORGANS OF MOVEMENT 178 
1. The Muscles 178 
2. The Tendons 180 
3. The Bones 181 
4. The Joints 183 
5. The Synarthroses 184 
VIII. THE ORGANS OF THE NERVOUS SYSTEM 185 
1. The Spinal Cord 185 
a. The Gray Matter ' 188 
b. The White Matter 190 
c. The Anterior Roots 192 
d. The Posterior Roots 192 
e. The Neuroglia of the Spinal Cord and the Central Canal . .193 
f. The Blood Vessels of the Spinal Cord 194 
g. The Lymphatics of the Spinal Cord 195 
2. The Spinal Ganglia 195 
3. The Brain 197 
a. The Nuclei of the Afterbrain and Midbrain 197 
b. The Cortex of the Cerebellum 201 
c. The Nuclei of the Cerebellum 203 
d. The Nuclei of the Midbrain and Interbrain 203 
e. The Hypophysis 205 
f. The Epiphysis 206 
g. The Cerebral Cortex 207 
h. The Olfactory Bulb 209 
4. The Membranes of the Central Nervous System .... 210 
5. The Cerebral Ganglia 211 
6. The Sympathetic Ganglia 211 
7. The Peripheral Nerves 212 
IX. THE ORGANS OF SENSE 214 
1. The Eye 214 
a. The Retina 214 
b. The Optic Nerve 220 
c. The Choroid 221 
d. The Ciliary Body 222 
e. The Iris 224 
f. The Lens 225 
g. The Cornea 227 IX 
C. MICROSCOPIC ANATOMY OF ORGANS-Continued 
IX. THE ORGANS OF SENSE page 
h. The Sclera 229 
i. The Vitreous 230 
k. The Zonula Ciliaris 230 
1. The Blood Vessels of the Eyeball 231 
m. The Lymphatics of the Eyeball 232 
n. The Nerves of the Eyeball 233 
o. The Eyelids and the Conjunctiva 233 
p. The Lacrimal Glands 236 
q. The Lacrimal Passages 237 
r. The Extrinsic Ocular Muscles 237 
2. The Ear 238 
a. The Internal Ear 238 
b. The Middle Ear 245 
c. The External Ear 247 
3. The Olfactory Organ 249 
4. The Organ of Taste 250 
X. THE SKIN 252 
The Hair 255 
The Nails 257 
The Sweat Glands 258 
The Sebaceous Glands 259 
The Blood Vessels of the Skin 260 
The Lymphatics of the Skin 260 
The Nerves of the Skin 260 
The Mammary Gland 262 LIST OF ILLUSTRATIONS 
FIG. PAGE 
1-Structure of a Cell 2 
2-Indirect Segmentation of the Nucleus (Stage of the Spire?7i) .... 12 
3-Indirect Segmentation of the Nucleus (Stage of the Formation of the Pole- 
field') 12 
4-Indirect Segmentation of the Nucleus (Stage of the Monaster) . ... 13 
5-Indirect Segmentation of the Nucleus (Barrel Stage) 14 
6-Indirect Segmentation of the Nucleus (Stage of the Daughter Spirem) . . 14 
7-Ciliated Cell 17 
8-Simple Flat Epithelium (Schematic) 20 
9-Simple Cuboidal Epithelium with Intercellular Spaces and Cement Wedges 
(Schematic) 20 
10-Simple Cylindrical Epithelium with Intercellular Spaces and Cement 
Wedges (Schematic) 21 
11-Stratified Flat Epithelium with Intercellular Spaces and Intercellular 
Bridges (Schematic) 22 
12-Transitional Epithelium (Schematic) 22 
13-Stratified Cylindrical Epithelium (Schematic) 23 
14--Stratiform Epithelium (Schematic) 23 
15-Sensory Epithelium of the Membranous Labyrinth of the Ear (Schematic) 24 
16-Simple Tubular Gland (Schematic) 25 
17--Sweat Gland (Schematic) 26 
18-Simple Branched Tubular Gland (Schematic) 26 
19-Compound Branched Tubular Gland (Schematic) 26 
20-Simple Alveolar Gland (Schematic) 27 
21-Simple Branched Alveolar Gland (Schematic) 27 
22-Compound Branched Alveolar Gland (Schematic) 27 
23-Simple Alveotubular Gland (Schematic) 28 
24-Branched Alveotubular Gland (Schematic) 28 
25-Compound Branched Alveotubular Gland (Schematic) 29 
XI XII 
FIG. PAGE 
26-Transverse Striation of a Muscular Fibril 35 
27-Schematic Representation of a Motor Neuron 40 
28-Motor Nerve Cell of the Spinal Cord {Semischematic) 43 
29-Schematic Picture of the Structure of a Lymphatic Gland 81 
30-Schematic Picture of the Structure of the Spleen {In 3 Colors') ... 82 
31-Structure of the Liver {In 3 Colors') 112 
32-Renal Canaliculi {After Peter) 138 
33-The Divisions of the Blood Vessels in the Human Kidney {Schematic) {In 
3 Colors) 144 
34-Human Spermatozoon {After Maves) 160 
35-Shapes and Sections of the Spinal Cord at Various Levels 187 
36-The Columns of the White Matter of the Spinal Cord at the Level of the 
Fifth Cervical Nerves 190 A 
THE CELL 
We understand by a cell the final, elementary particle of any living 
organism which is able to live independently under certain circumstances. 
A large class of very low organisms, the protozoa, are nothing else than 
living single cells, and we learn from embryology that all other animals, even 
the most highly organized, come from a single cell, the ovum, or from the 
union of the ovum with the seminal cell. The union of the ovum with the 
seminal cell is the beginning of the fecundated egg, which finally produces the 
complete organism by continuous segmentation. Therefore we must remember, 
when we speak of the cell as the element, or the elementary organism of the 
human body, that this name is not given it because of the simplicity of its 
structure, it has, on the contrary, a very complicated structure which is as 
yet only partially understood. It is not an elementary particle in a chemical 
sense, it is only a biological unit. None of its parts can exist continuously 
without the others; the union of all is necessary to the life of an elementary 
particle. 
Form and Size of Cells 
The fundamental form of an animal cell is that of a sphere or of an ellip- 
soid, but this undergoes numberless modifications, chiefly due to the limitations 
of space. The cells that take part in the formation of tissue lie close together 
and adapt their shapes to the spaces in which they are confined, and thus 
become cylindrical, cuboidal, flat, spindle-shaped, or polyhedral. Another 
important factor in determining the shape of the cell is the function of the 
organ of which it forms a part; thus muscle cells become spindle-shaped, or 
threadlike, and nerve cells throw out numerous processes, with which they 
seek to join others like themselves, conformably to their functions. Finally 
many cells change their shapes in response to external or internal stimuli, and 
it happens frequently that a local movement of the cell may be brought about 
by such an active change of form. 
Cells vary in size as well as shape. The great majority of those of the 
human body are too small to be seen with the naked eye, but the muscle cells, 
nerve cells, and ova are exceptions. The human ovum has a diameter of 
200 p, and can therefore be seen without artificial aid. The ova of many ani- 
mals are incomparably larger; this is particularly true of birds, in which the 
yolk is an enormously enlarged cell. 
1 2 
Construction of the Cell 
The classical morphological definition of a cell given first by Max Schultze 
in 1861 is: A bit of protoplasm with a nucleus within it. Even now, after the 
lapse of more than fifty years, this definition has changed very little. The 
cell body and the nucleus are the most important constituents. The cell 
cannot last continuously without a nucleus, and free nuclei, without enveloping 
cell bodies, are just as little to be met with. 
In the course of these fifty years we have learned, however, to recognize 
that there arc other essential constituents pertaining to the cell, including the 
a, protoplasm, consisting of 
hyaloplasm and granoplasm; b, 
paraplasm; c, mitochondria; d, 
ectoplasm; e, archoplasmic rays; 
f, central bodies; g, nuclear 
framework (linin) ; h, achro- 
matic nuclear membrane; i, 
nuclear chromatin; k, chromatic 
nuclear membrane; Z, nucleoli; 
m, nuclear juice. 
Fig. 1.-Structure of a Cell. 
membrane formations, the central bodies, the reticular apparatus, 
and the chondriosomes, or mitochondria. We will discuss each of these 
constituents in turn, beginning with the cell body. 
The Cell Body 
The body of the living cell is of a gelatinous or semisolid consistence, and 
is originally colorless, transparent, and weakly refractive. When it appears 
to be colored it is because of the presence of special pigment, or of fatty 
substances. 
The main part of the cell body is composed of protoplasm. This is 
not chemically uniform, but is made up of a mixture of nucleoalbumins, pro- 
teids, globulins, and traces of albumins. Its composition probably varies in 
different kinds of cells. It is always alkaline, insoluble in water, and coagu- 
lable by the precipitants of albumin-heat, alcohol, mineral acids, and the 
salts of the heavy metals. 3 
In many living cells the body appears to be homogeneous and structure- 
less, but in others a structure can be discerned more or less clearly. If we 
kill a cell in a suitable manner the structure of its body stands out more 
distinctly, and we can recognize that it varies in different cells. 
Very often the cell body presents the picture of an alveolar structure (Pl. 1, 
Figs. 1 and 2), i.e., the protoplasm forms partitions that pass through the 
body from the periphery to the nucleus and unite to form more or less regular 
alveoli. The individual alveoli may be completely separate from one another, 
or they may communicate, as they do in many cases. Granules appear to be 
deposited at the points where the walls of the alveoli meet, and, by using the 
highest magnifying power, we find that such granules are to be seen everywhere 
in the walls of the alveoli. These are the microsomes of the protoplasm, or 
the plasmosomes, and we speak of this framework and the granules together 
as the granoplasm, to distinguish it from the structureless part of the pro- 
toplasm, which we call the hyaloplasm. Sometimes nothing can be seen of 
an alveolar structure in the cell body, but the lattei' contains threads, or 
fibrils, running through the hyaloplasm, and then we say that the cell body 
has a fibrillary structure (Pl. 2, Fig. 6). These protoplasmic threads may 
constitute quite definite structures, as in the muscles, nerves, and many epi- 
thelial cells, and are then connected closely with the function of the cell, inas- 
much as the body has assumed a fixed, stable formation fitted for the perform- 
ance of its function, but in many other cases a fibrillary structure of the proto- 
plasm is not of so great consequence. In such cases the fibrils are not arranged 
so typically, but run more irregularly, interlace, cross one another, or form 
true networks in the cell body. We call these threadlike constituents embedded 
in the hyaloplasm the cytomitome. Wherever the formation of a cytomitome 
has taken place we always find with it in the hyaloplasm larger or smaller 
microsomes, and it is quite possible that the fibrils of the cytomitome, which 
often present swellings that make them look like strings of pearls, are pro- 
duced by the lining up one after another of numberless minute microsomes. 
In that case the cytomitome is simply a certain phase of the granoplasm. 
Very often the greater part of the cell body appears to be granular, and 
then we speak of a granular protoplasm. Such an appearance may be due to 
the presence of many large microsomes in an otherwise hyaline protoplasm, 
but it may also be simulated in a cell body of alveolar structure when the alveoli 
are filled with some substance that makes itself prominent. 
In addition to protoplasm the cell body very often contains another sub- 
stance, known as the paraplasm, or the deutoplasm, which fills the inter- 
spaces left open by the protoplasm between the alveoli. This may be fluid 
or solid. In the majority of cases it is formed of materials that owe their 
presence to the activity of the protoplasm itself, or have been taken up from 
outside into the cell body. Glycogen is found very extensively as a deuto- 
plasmatic constituent of cells in the form of fine granules, or of coarse flakes 
in the retiform spaces of the protoplasm (Pl. 3, Fig. 7). Fat enjoys an 
equally wide distribution, appearing in minute particles which, when in great 
numbers, often fill the cell body, and may blend into large drops (Pl. 3, 4 
Fig. 9). Other substances met with are mucus and its rudiments, lipoids, 
enzymes, and the vitelli of embryonal cells. 
Externally the protoplasm becomes condensed and excludes the cell body 
from the outer world as by a membrane, but this so-called cell membrane is 
nothing else than protoplasm, and must not be confused with the membrane of 
the plant cell, which consists of cellulose and differs both chemically and 
morphologically from the cell protoplasm. It is better therefore not to call 
it a membrane, but to speak of it as an ectoplasm, or a crusta. 
The Nucleus 
There is only one exception to the rule that a nucleus is to be found in 
every cell-it cannot be seen in the erythrocytes, the red blood corpuscles of 
man and the mammals. These cells are nucleated at first, but the nucleus 
gradually becomes invisible as they take up the coloring matter of the blood, 
the haemoglobin. It has not yet been determined with certainty whether 
the nucleus becomes extruded from the cell, dies, or is simply hidden. 
The great majority of cells contain only one nucleus apiece, yet there are 
varieties in which two nuclei are to be seen very often, like those of the liver. 
Polynucleated cells are comparatively rare in man under normal conditions, 
though they are much more common in the lower animals. The human red 
bone marrow contains polynucleated cells, which have also been called giant 
cells on account of their size (Pl. 2, Fig. 6) ; and the osteoclasts, which, as 
we shall see later, play an important part in the development of the medullary 
cavities of the bones, are likewise polynucleated. Such cells appear far more 
often in pathological conditions, such as rapidly growing tumors, like car- 
cinomata, or tuberculous or syphilitic neoplasms, when more than a hundred 
nuclei may be found in a cell. 
The size of the nucleus is usually proportionate to that of the cell body, 
so that we may say as a general rule that the larger the body the larger the 
nucleus, but there are many exceptions to this rule. In certain blood cells, 
the lymphocytes, the body is so small as to form only a narrow ring 
about the nucleus. The size of the nucleus also fluctuates considerably during 
the life of many cells, as in the ovum. In the glandular cells the nucleus may 
be greatly reduced in size and deformed by the pressure of the deutoplasmatic 
substances produced or stored away in the cell body. 
The shape of the nucleus is even more variable than its number or size, 
and is very often influenced greatly by that of the cell body. Its fundamental 
form is spherical. If the cell body elongates, the nucleus does likewise and 
changes from a sphere first into an ellipsoid, and finally into a cylindrical or 
spindle shape. Very often spherical nuclei are to be seen that seem to be 
indented at a certain place (Pl. 2, Fig. 4). The indentation may be so deep 
as to break through into the center of the nucleus and to convert the sphere 
into a ring. Annular nuclei may be of uniform thickness everywhere, or they 
may have nodules in places that cause them to resemble rosaries, or rings of 
pearls. Nuclei that are shaped like horseshoes, or are tabulated, are not 
uncommon (Pl. 2, Fig. 5). The neutrophilic leucocytes and the eosinophilic 5 
blood cells are particularly rich in such dissimilar forms of nuclei. In the 
bone marrow and the spleen of many mammals, such as rabbits, hedgehogs, and 
moles, we find cells that contain nuclei which have the form of spherical, fenes- 
trated shells, and in which an intranuclear and an extranuclear cell body can 
be distinguished, connected by numerous bridges that break through the spher- 
ical shell (Pl. 2, Fig. 6). In cells that possess an active power of movement, 
like the lymphocytes of the blood, the form of the nucleus changes with the 
movement. That the nucleus has no fixed, unchangeable form may be seen 
very beautifully while watching the circulation in the frog, when the shape of 
the cell body and of the nucleus of the blood corpuscle will be observed to 
change greatly as it passes through narrow places, or enters lateral twigs. 
The position of the nucleus within the cell varies a great deal. It is often 
central in the spherical nerve cell, while in the likewise spherical ovum it is 
almost always excentric. In cylindrical cells it lies sometimes in the middle, 
sometimes near the base, sometimes near the surface. Its location may be 
changed passively by the pressure of the adjacent cells, or by that of the 
deutoplasmatic contents of the cell itself, but many nuclei seem to possess a 
powei* of active movement. Distinct amoeboid movements may be seen in the 
nucleus of the human ovum, the so-called germinal vesicle. 
Turning now to the structure of the nucleus, if we stain a section of the 
human liver with haematoxylin the nuclei of the liver cells stand out as circular 
bodies bounded externally, toward the cell body, by a blue, irregular outline, 
and containing numerous more or less minute particles stained blue. This 
substance, thus brought into prominence by the ordinary stain of the nucleus, 
bears the name of chromatin. It is so distributed as to form an incomplete 
membrane about the nucleus, the chromatic membrane, and to appear 
within it in larger or smaller masses. If we would study the properties of 
this body more closely we must employ a more delicate method of staining. 
The best way is to treat the preparation with a mixture containing an acid 
and a basic coloring matter (Pl. 1, Figs. 1 and 2). The chromatin selects the 
basic color and becomes stained accordingly; we therefore say that chromatin 
is basophilic. The basophilia of chromatin depends on its content of nu- 
cleinic acid, an albuminoid rich in phosphorus, which forms the characteristic 
product of chemical decomposition, the so-called nuclein base. Only a very 
minute portion of this is free in the chromatin, the most of it is combined 
with other albumins to form the so-called nuclein. The amount of chromatin 
in the nucleus varies a great deal; often its particles can scarcely be seen 
(Pl. 1, Fig. 3), while in other cases they almost fill its entire interior (Pl. 1, 
Fig. 2). 
If the chromatin contains only a little nucleinic acid it may cease to be 
basophilic and stain with the acid instead of the basic dye. We then speak 
of oxyphilic chromatin, which is found in the germinal vesicle of the ovum 
and in the nucleus of the nerve cell. 
The nucleus contains in addition to chromatin a corpuscle, usually spher- 
ical, which is stained with haematoxylin; this we call the nucleolus. The 
nucleolus varies much in size and is quite large in some nuclei, as in the ger- 
minal vesicle of the ovum and in the nucleus of the nerve cell. It is by no 6 
means rare for several to be present. Its position is inconstant; it may be 
central or excentric, but it is never in the chromatic membrane. When the 
preparation is stained with haematoxylin the nucleolus has the same color and 
intensity as chromatin, but when a mixture of acid and basic dyes is employed 
it behaves differently from chromatin in that it selects the acid stain, and thus 
proves itself to be oxyphilic (Pl. 1, Fig. 1). It is composed of pyronin, which 
differs from nuclein in containing little phosphorus. 
The name chromatic substance is used to cover both the chromatin 
and the nucleolus, because they have a great affinity for coloring matters and 
may be stained very easily. 
The nucleus contains in addition to the chromatic substance achromatic 
substances, which are more difficult to demonstrate. They are oxyphilic, i.e., 
they always select the acid dyes from a mixture, and are therefore similar 
in behavior to the cell protoplasm and the nucleolus. Two of these may be 
differentiated, the amphipyrenin and the linin. The former creates a 
membrane between the nucleus and the cell body, the achromatic membrane, 
while the linin forms a framework throughout the interior of the nucleus, the 
achromatic framework. The achromatic nuclear membrane is internal 
to the chromatin granules of the chromatic membrane, and the particles of 
chromatin have the same relation to the achromatic framework. The nucleolus, 
or nucleoli, is usually situated at the points of junction of this linin frame- 
work. All the meshes of this network are filled with the nuclear fluid, which 
is likewise achromatic and oxyphilic. A final constituent of the same nature 
is lanthanin, which appears in the form of little granules. 
The Central Bodies 
The central bodies, which are usually just visible with the microscope, 
were discovered by Ed. v. Beneden in 1876, and have been determined to be 
constant constituents of cells by very modern researches. They appear as 
two or more sharply defined granules usually lying close together in the cell 
body, never in the nucleus (Pl. 2, Figs. 4 and 5). As regards their position, 
all that can be said is that they have two places of predilection-the center 
and just beneath the free surface of the cell. The central position is charac- 
teristic of the colorless blood cells and of the cells of the smooth muscles, 
while the superficial is peculiar to the various epithelial cells (Pl. 8, Fig. 25). 
They are generally two in number, and are frequently joined by a narrow 
bridge, a centrodesmose. Three central bodies occur in the nerve cells, the 
epithelial cells of the epididymis, and the cells of the lacrimal gland. Cells that 
deviate from the ordinary type in size vary with regard to the number of 
central bodies they may contain; as many as a hundred have been counted in 
the giant cells of the bone marrow (Pl. 2, Fig. 6). 
The central bodies are oxyphilic, like protoplasm, yet many observations 
go to show that they are not of the same chemical composition. 
They often form the center of a system of protoplasmic threads that radiate 
outward and are lost in the network of the cell protoplasm after a longer or 
shorter course (Pl. 2, Figs. 4 and 5). This radiating portion of the cell 7 
protoplasm is called the archoplasm. The central bodies may also be sur- 
rounded by and lie in the center of a bright areola, through which the archo- 
plasmic rays pass. At the margin of the areola each ray has a little thicken- 
ing, a minute nodule, so that the central bodies seem to be within a sphere, 
which is called a centrosphere, while the entire structure is denominated a 
centrosome. Typical centrospheres are to be seen in the white blood cells 
and in the pigment cells, but chiefly in the ovum before and after impregnation, 
and in the segmentation cells, or blastomeres, of the impregnated ovum. In 
the majority of cases the central bodies have no centrosphere, but simply lie 
in a rather bright areola of protoplasm. 
The Chondriosomes 
The Chondriosomes, or mitochondria, are granules found in the protoplasm 
that stain differently from the microsomes. They were found first in ova and 
spermatozoa, but have now been demonstrated in almost all kinds of cells, and 
may be seen with beautiful distinctness in the cells of the embryonal body. 
As a rule they unite to form threads which run in wavy courses through the 
cell body (Pl. 3, Fig. 9, bf), or accumulate near the nucleus so as to form a 
sometimes spherical, sometimes crescentic body, known as the accessory 
nucleus. 
The Reticular Apparatus 
The final constituent of the cell is the reticular apparatus (Apparato 
reticolare), which was discovered by Golgi in the nerve cells, and has since 
been found in many others. In the most perfect cases it forms a network 
in the cell body about the nucleus, but in other varieties of cells it lies near 
the nucleus and envelopes the central bodies. 
The Biological Properties of the Individual Constituents 
of the Cell 
All important vital processes take place witinn the cell body, and particu- 
larly in its chief component, the protoplasm. We shall speak of this in an- 
other place. The question now arises of what importance the nucleus is to the 
cell. That it is of very great importance is proven by the fact that cells 
cannot continue to live without nuclei. This fact has been demonstrated 
experimentally many times; the nuclei have been removed from large cells, 
after which the cells live for a while, but soon die. The cell also loses with 
its nucleus its most important vital properties, and seems to retain only one, 
the power of movement. Infusoria deprived of their nuclei move just the same 
as normal ones, and the ciliated cells of the higher mammals continue to make 
regular strokes with their cilia after their nuclei have been removed. The 
nucleus has been thought by some to be the chief place:of oxidation of the 
cell, but it seems to take no greater part in oxidation processes than the cell 
body. An important part in secretion has been ascribed to it, but without 8 
cogent proof. Hence, although the activity of the nucleus is important to 
the life of the cell, it is apparently much less so than that of the cell body, 
while at the present time it is thought to be the most important, though per- 
haps not the sole bearer of the hereditary substances, which we have to seek 
in its chromatin. 
The central bodies likewise are extremely important, as they play a deter- 
mining part in the segmentation of the cell, as we shall see later. 
The main activity of the mitochondria appears in embryonal life, when 
they are employed in the development of important differentiation products 
of the cell, but in postembryonal life the duty devolves upon them to produce 
certain constituents of secretion, as in the glandular cells. 
Nothing positive is yet known concerning the physiological attributes of 
the reticular apparatus. 
The Manifestations of Life in the Cell 
Inasmuch as every organism is composed of cells, every manifestation of 
life must have to do with, originate from, and come to pass in these cells, and 
we may therefore say that every vital process, even when exhibited in so com- 
plicated an organism as the human body, consists of a great number of single 
processes, each of which takes place in a certain cell. 
All of the manifestations of life that we are able to observe in animal bodies 
may be traced back to three elementary activities of the animal cells, thus: 
1. The ability to admit into itself suitable foreign substances, to disinte- 
grate them, to utilize them in its own construction, and to evacuate the prod- 
ucts of elaboration. We call the phenomena that result from the exercise of 
this power metabolism. 
2. The ability to receive energy from without in the form of stimuli and 
to transform it into heat, light, electricity, or mechanical energy, i.e., to trans- 
form it into change of form and movement. 
3. The ability to propagate. 
We will now discuss in some detail these three fundamental activities of the 
animal cell. 
The Metabolism of the Cell 
To repeat a comparison that has been made a great deal, each animal cell is 
a little chemical laboratory; it takes from its surroundings oxygen, water, and 
organic substances, supplies its own needs from them, and either eliminates 
the products of chemical decomposition, or forms from them simply con- 
structed materials that are of service to the entire organism. The chemical 
processes that take place are extremely complicated. The fundamental differ- 
ence between the cells of plants and of animals, and the importance of those 
of the former to those of the latter, are shown in the metabolism. The plant 
cell takes up simple inorganic substances in solution and transforms them 
by a wonderful process of synthesis into organic substances, carbohydrates, 
fat, and albumin. The carbohydrate, with its relatively simple molecule, is 
produced first, and from it is formed albumin with its highly complex molecule. 9 
These organic bodies elaborated by the plant cell serve as nutriment to the 
animal cell, which has absolutely no other resource, as it cannot live on inor- 
ganic substances. The metabolism of the plant cell is a process of synthesis, 
that of the animal cell one of analysis, for it breaks up the complex bodies 
it admits into simpler ones. 
Oxygen alone of the gases is of importance to the animal cell, while the 
plant cell may need carbon dioxide also. All animal cells without exception 
require oxygen, and cannot live without it. The taking up of oxygen by the 
cell we call respiration. 
The more highly differentiated are the cells in an organism the more sensi- 
tive they are to lack of oxygen. While the cells in a frog may live for hours 
without it, a few minutes of total deprivation suffice to kill human cells. 
It was formerly thought that the processes of oxidation in the cell took 
place almost exclusively in the nucleus, and that this was the respiratory 
organ of the cell, but modern research has proved this idea to be erroneous, 
and that the nucleus and the cell body share equally in these processes. Cells 
without nuclei behave exactly the same as regards oxygen as those with nuclei. 
During the admission of oxygen highly organized bodies come into being in 
the cell, and are broken up again immediately with the production of carbon 
dioxide. This carbon dioxide is the end product of the gaseous changes in 
the animal cell and is thrown off by it as excrement, while it is assimilated, 
by green plants under the influence of light, and is synthetized into a combina- 
tion forming a more highly complex molecule, that of carbohydrate. 
The principal fluid taken up by the animal cell is water. Life cannot 
last long where there is no water, for no organism can protect itself for any 
length of time against evaporation, and it is imperative that the evaporated 
water be replaced. Everything that the cell takes up and utilizes must be 
furnished it in a fluid form, so Nature has supplied the human body with a 
complete system of glands, the duty of which is to furnish secretions that 
render fluid the nutritive material introduced into the body, the greater part 
of which, including the albuminoids, the fat of meat, and the starch of cereals, 
is certainly insoluble in water. 
Diffusion processes play a very considerable part in the entrance of water 
into the cell, but the living protoplasm is able to detain, in opposition to the 
current, the bodies dissolved in the fluid, and to select from among them the 
substances that it will permit to enter, and to reject those it does not want. 
The elimination of the fluid products of metabolism is accomplished through 
an active contraction of the cell body. A diminution in the size of the latter 
may always be observed when this takes place. It may be that the nucleus 
also takes an active part in the process, for it has not only been observed to 
change its shape, but a protrusion of its constituents has been described as 
occurring in many processes of secretion. 
The statement made above, that the animal cell can admit substances only 
when they are in solution, needs limitation in many respects. In the lowest, 
single-celled animal organisms, the protozoa, the cell, i.e., in these cases the 
organism itself, can admit solid bodies, render them fluid, and then assimilate 
them. With the aid of the microscope we can follow this process in these cells 10 
in all its details. Either every part of the surface of the cell is fitted to 
receive the nutrient body, so that the latter is wrapped about by the cell body, 
and removed into it, or the cell has at a certain place a sort of mouth sur- 
rounded by cilia, through which the nutrient substance is passed into the cell 
body. 
In the higher animals only a very few kinds of cells have this power of 
admitting corpuscular elements, and of these the lymphocytes of the blood 
are the most important. These cells have the power to move independently, 
and to press into all clefts in the tissue, so that there is hardly a place in the 
human body that is beyond their reach (Pl. 1, Fig. 3, wz). As they pass along 
they seize upon all of the fragments of cells and foreign substances they meet, 
and render them innocuous to the organism, thus taking an important part in 
the protection of the body against an invasion of foreign bodies. 
Reaction of the Cell to External Stimuli 
The animal cell possesses to a high degree the ability to respond to any 
change in the external factors that affect it. We call such changes stimuli. 
The greater the intensity of the stimulus the sharper is the response. When 
the intensity is extreme it may kill the cell. Stimuli are divided into chemical, 
mechanical, thermic, photic and electrical. 
The commonest manner in which a cell responds to stimuli is by way of 
movement, i.e., by a change in the form of the entire cell, or of some of its 
parts. The phenomena that may be observed are the amoeboid movement, 
contraction, the movement of certain parts within the cell, and the 
movement of special accessory organs, such as cilia or flagella. 
Amoeboid movement can be studied best in the amoebas, a family of 
the microscopic rhizopods, in the early stages of the development of the lower 
metazoa, and particularly in the lymphocytes of the blood of all the higher 
animals. At a certain place in the cell, which is globular when at rest, one 
or more processes protrude, and are either drawn in again or are followed 
by the body so that a change of location takes place. The movement is ex- 
tremely slow (Pl. 1, Fig. 3, wz). 
While the amoeboid movement takes place in the protoplasm of the cell 
body, which is no further differentiated, contraction always occurs in cer- 
tain differentiated formations of the cell body that usually run parallel through 
the cell, the contractile fibrils, and therefore differs from the former in always 
being in only one direction. These contractile fibers are met with in the higher 
animals only in quite definite tissue cells, those of the muscles, a description 
of which will come later. 
Movements within the cell without change of its form are met with 
throughout the vegetable kingdom as protoplasmic currents, but in the higher 
animals they are to be seen only in the pigment cells. Pigment is usually 
found in cells in the shape of little, variously colored granules or rods, which 
either have been formed there as products of metabolism or are derivatives 
of the coloring matter of either the blood or the bile. They are distinguished 
by their content of iron and sulphur. Pigment occurs in all possible cells, but 11 
plays its most important part in the cells of the connective tissue and of the 
epithelia. It determines the color of the skin, which is changeable in many 
animals. These changes of color are effected very often by photic causes, by 
exposure to or deprivation of sunlight. Guided by such stimuli the pigment 
eithei' flows toward the periphery of the cells, spreads itself over the entire 
cell body, or flows back and agglomerates into a small, compact lump. We 
find something quite similar in the human retina. 
The final form of movement is that which is exhibited by means of specially 
constructed accessory organs, more or less long and numerous threads that 
project from the free surface of the cell, known as cilia or flagella. While 
flagella serve as organs of locomotion to the protozoa and the young of many 
metazoa, moving the animal about in the water by their strokes, ciliated cells 
occur in the higher animals only in the linings of cavities, where their duty 
is to set in motion any fluid contained therein, and any foreign bodies that 
may happen to enter, by strokes made unceasingly in only one direction. The 
details concerning ciliated cells and the ciliary movement will be given in the 
description of the ciliated epithelium. 
But the cell may respond to external stimuli not only through the produc- 
tion of mechanical energy; it may produce light, as is often seen in pelagic 
animals, or heat, or electrical force, but the latter is generally very slight 
and attains a considerable strength only in the so-called electric fishes, in 
which it is produced in special organs. 
The Propagation of the Cell 
The final, and perhaps the most important, property of the cell to be 
described here is its power to reproduce. Just as every cell is able to assimilate 
nutrient bodies brought to it from without and to utilize them in its growth, 
so is it able to produce new individuals of the same kind. Growth and repro- 
duction are originally similar processes. If the supply of food and assimilation 
is in the long run greater than its need, the cell multiplies itself. 
Cell increase is greatest in childhood; later, when the body has reached its 
full development, it gradually declines, but does not cease altogether until the 
death of the individual. All cells, with few exceptions, have a limited length 
of life, so they are constantly dying and being replaced. 
The replacement is always secured through segmentation of the remaining 
cells, a process that involves both the cell body and the nucleus. While the 
division of the former takes place in the same way almost everywhere, that of 
the latter may be accomplished in one of two quite different ways: the indirect 
division of the nucleus, or mitosis, and the direct division of the 
nucleus, or amitosis. A feature common to both processes is that the 
nucleus is always the first to divide, and then, when its division is quite, or 
practically complete, the division of the cell body takes place. In indirect 
segmentation each of the daughter cells obtains an almost mathematically 
exact half of the mass of chromatin in the nucleus, but this is not the case in 
direct segmentation. Indirect segmentation plays by all means the greater 
part in the life of the cell, so we will describe it first. 12 
Indirect Segmentation 
As stated above, the most important constituent of the nucleus, its chrom- 
atin, is divided as exactly as possible into halves in indirect segmentation. 
First, the chromatin that is scattered about in the nucleus in larger or smaller 
particles collects together to form a long thread, which breaks into a certain 
number of pieces of equal length, and then each 
piece splits longitudinally into equal parts. Each 
daughter nucleus receives one-half of each of these 
pieces. In addition the central bodies separate, so 
that each daughter cell contains one of them. 
The process of indirect segmentation is com- 
monly divided into three successive stages: the 
prophase, the metaphase, and the anaphase. 
In the prophase the chromatin scattered 
about in the nucleus collects together, ranging its 
granules close together so as to form a dense chro- 
matic network (Pl. 4, Figs. 11 and 12). The 
threads of this network give way in numerous places 
and the substance composing them collects in other 
places, until finally a single long thread of chro- 
matin is produced, which runs through the nucleus 
in numberless convolutions (Pl. 4, Fig. 13). Its surface is at first irregular 
and rough, but gradually becomes smooth, and the nucleus enters the stage of 
the spirem, or ball of smooth thread, which very soon breaks down into a num- 
ber of pieces of equal length, the chromosomes. 
The number of chromosomes is constant for 
each genus of animals, and varies from two and four 
in the intestinal worm of the horse (Pl. 4, Fig. 15), 
to over a hundred in many anthropods. The human 
cell, like that of many other mammals, contains 
twenty-four chromosomes. They stain with basic 
dyes more intensely-that is, they are more baso- 
philic-than the chromatin of the resting nucleus; 
this is to be ascribed to a higher content of free 
nucleinic acid. 
Very soon after the formation of the chromo- 
somes changes take place in their shapes and in 
their positions in the nucleus. They bend into more 
regular loops, with the convexity of each loop 
facing in one direction, the two free ends in the 
opposite. 
During this stage, or earlier in many cases, the nuclear membrane gives 
way and permits the chromosomes to lie free in the cell body. The nucleolus 
disappears; what becomes of it no one yet knows with certainty. 
The central bodies play an extremely important part and undergo a con- 
siderable change in the prophase. At this time they lie in the so-called polar 
field, i.e., at the place in the cell toward which the convexities of the chromo- 
Fig. 2.-Indirect Segmenta- 
tion of the Nucleus (stage 
of the spirem). 
Fig. 3.-Indirect Segmenta- 
tion of the Nucleus (stage 
of the formation of the 
pole field). 13 
somes are directed. Even in cells that show no traces of archoplasm when at 
rest, a system of rays develops about each, radiating in all directions. Part 
of these archoplasmic rays are lost in the indifferent cell protoplasm, part 
attach themselves to the chromosomes that now lie free in the cell. This 
picture soon changes as the central bodies, which have been lying close together, 
now separate, one moving toward one pole of the nucleus, the other toward 
the opposite. Each takes with it its archoplasmic radiation, which causes 
each to resemble a sun gliding along what was formerly the nuclear membrane. 
They are also connected by archoplasmic rays which stretch out between them. 
Therefore, when the central bodies have arrived at the poles of the nucleus, 
we can distinguish three kinds of archoplasmic fibers: first, those that radiate 
from the central bodies and are lost in the cell protoplasm, the polar spindle 
fibers; second, those that join the central bodies to the chromosomes, the 
traction, or mantle fibers; third, those that pass from one central body 
to the other, the central spindle fibers. 
As soon as the central bodies have reached the poles of the nucleus the 
chromosomes change their places until they are arranged in one plane with 
the convexities of all the loops directed inward, the free ends outward. Seen 
from the side this extraordinarily characteristic stage of mitosis gives the 
picture of a plate occupying the equator of the cell, the equatorial plate 
(Pl. 4, Fig. 14), while when seen from above it often presents a remarkably 
regular star, the mother star, or monaster (Pl. 4, Fig. 15). The central 
spindle fibers extend from one central body to the other directly through the 
equatorial plate; the mantle fibers pass, like the rods of two open umbrellas 
tilted toward each other, from each central body to the ends of the chromo- 
somes. This ends the prophase and the mitosis enters the metaphase. 
Although the exact division of the entire quantity of chromatin is essen- 
tially facilitated in the prophase by the formation 
of chromosomes, i.e., by the collection of the parti- 
cles of this substance scattered about in the nucleus 
and its separation into a certain number of equal 
portions, yet the division is made complete in the 
metaphase by the splitting of each chromosome 
longitudinally into two halves, each mother chromo- 
some thus becoming two daughter chromosomes of 
equal size. This does not always take place in 
exactly the same stage of mitosis; it may be con- 
summated during the prophase before the chromo- 
somes form the monaster, and then the daughter 
chromosomes lie parallel to each other for a much 
longer time, but usually the splitting occurs in the 
beginning of the metaphase, when the stage dur- 
ing which the daughter chromosomes lie near to- 
gether is short and cannot always be easily observed (Pl. 5, Fig. 16). 
Now the daughter chromosomes separate, one moving with its convexity 
in front toward each central body (Pl. 5, Fig. 17). It is not quite correct 
to say that the chromosomes move, for they can scarcely have an active motil- 
Fig. 4.-Indirect Segmenta- 
tion of the Nucleus (stage 
of the monaster). 14 
ity; it is more probable that the central bodies form fixed points from which 
contractile spindle fibers, particularly the mantle fibers, extend to and are 
inserted into the ends of the chromosomes, and that the movements of the 
latter are due to the contraction of these spindle fibers. Some investigators 
doubt this, and believe that the central bodies are 
kinetic centers which exert an influence over the 
chromosomes comparable with that exerted by a 
magnet over bits of iron, in which case the spindle 
fibers may be the visible phenomenon produced by 
this force, like the lines of magnetic power shown 
with iron filings. 
As stated above, the daughter chromosomes 
move with their convexities forward. This brings 
the convexities of all toward the central bodies, 
their free ends toward the equator of the cell, and 
results in a figure not unlike a barrel. We speak 
of this as the barrel stage of the mitosis. Ap- 
parently it lasts quite a while, for it may be ob- 
served very easily and often. The three portions 
of the spindle fibers can be differentiated very dis- 
tinctly. The polar fibers extend from the central bodies to the periphery of the 
cell, the mantle fibers, which are very short, to the chromosomes, while the cen- 
tral spindle fibers run approximately parallel between the free ends of the 
daughter chromosomes. They naturally cross the 
equator of the cell, where each has a little nodule. 
These nodules, taken together, form a plate situ- 
ated in the equator, the cell plate. The barrel 
stage now passes over into that of the daughter 
stars, in which the daughter chromosomes are 
arranged in stellate figures. 
In the anaphase the division of the cell body 
is completed and the two daughter stars are 
changed into two resting nuclei. The division 
of the cell body takes place at the equator of 
the cell. A circular furrow appears and cuts 
deeper and deeper into the cell body. Shortly be- 
fore the division is complete a little heap of gran- 
ules is formed by the compression of the bundle 
of spindle fibers and the consequent crushing of 
the cell plate, from which the central spindle 
fibers radiate in each daughter cell to the 
chromosomes. 
The transformation of the daughter stars into two resting nuclei is 
quite similar to a reversal of the prophase. The chromosomes turn their 
convexities toward the central bodies again and unite to form a thread that 
is at first smooth, later irregular (Pl. 5, Fig. 18). Threads of chromatin 
grow out from the irregularities of the thread and join to form a network. 
Fig. 5.-Indirect Segmenta- 
tion of the Nucleus (bar- 
rel stage). 
Fig. 6.-Indirect Segmenta- 
tion of the Nucleus (stage 
of the daughter spirem). 15 
The chromatin also forms a chromatic membrane at the periphery of the 
nucleus, inside of which it breaks into little particles which fit into the linin 
framework that now appears (Pl. 5, Fig. 19). The nucleolus reappears to- 
gether with the chromatic membrane, while the central bodies and the spindles 
likewise pass through retrograde changes. The mantle fibers disappear com- 
pletely, as does the central spindle, the granular plate of which is demonstrable 
for a while as a little corpuscle between the two daughter cells. Finally the 
central bodies part and each seeks its characteristic situation in its own cell. 
Then we have two resting daughter cells, each of which is in every way like 
the mother cell, through the segmentation of which they originated. 
Indirect segmentation is to be met with wherever tissue growth is taking 
place or dead cells have to be replaced, and is a sign that the cells are vigor- 
ous. This is not always true of direct segmentation, which is often to b^ seen 
where living cells have been destroyed. 
Direct Segmentation 
An accurate halving of the chromatin is not contemplated in direct seg- 
mentation, which takes place either with the formation of a constriction, or 
by a process of budding, with subsequent fragmentation of the nucleus. 
In the former case the nucleolus divides, the two daughter nucleoli separate, 
while at the same time the nuclear membrane presents a more or less regular, 
circular, constricting furrow. The two halves of the nucleus separate, the 
connecting pedicle becomes longer and thinner until it breaks, leaving two 
separate nuclei, each accompanied by a nucleolus. In budding a little lump 
appears on the surface of the nucleus, enlarges more and more, and cuts itself 
off from the mothei' nucleus after it has enclosed a nucleolus. Budding very 
often occurs at several places on the surface of the nucleus, so that the latter 
is finally broken up into a large number of daughter nuclei, it is broken into 
fragments. The buds may remain for a long time connected with the mother 
nucleus by threadlike bridges, so the pictures produced may be remarkably 
multiform. 
In many cases the central bodies also appear to take an active part in the 
amitosis, in that they interpose themselves between the two halves of the 
nucleus and accelerate the process (Pl. 2, Fig. 5). 
In mitosis the division of the cell body almost always follows or accom- 
panies that of the nucleus, but in amitosis no such division of the cell body 
takes place, as a rule, so this process leads to the formation of polynucleated 
cells. 
Amitotic segmentation is to be observed chiefly in the colorless blood cells, 
but it may also be seen in the epithelium of the bladder, and in the cells of 
the liver. 
Finally, it should be mentioned that transition forms between mitotic and 
amitotic segmentation have been described in which the constriction through 
the nucleus takes place after the chromatin within it has collected into a regular 
form, but without the development of chromosomes. B 
THE TISSUES 
Now that we have studied cells, which are the elementary constituents of 
the animal body, we will proceed to the next higher unit, the tissues that are 
formed from them. We understand by a tissue a complex of cells 
which present the same individual characteristics. During the first 
stages of development all of the cells of the embryonal body are remarkably 
alike, though perhaps not perfectly the same, but very soon they separate into 
groups that have common individual characteristics. They assume definite 
arrangements, and form special characteristic structures within their cell bodies, 
or develop special intermediate substances between themselves. The tissues 
thus started naturally have to perform the various duties of the entire organ- 
ism, so to each is assigned distinctly defined functions. 
Tissues are commonly divided into four distinct groups: 
1. Epithelial; 
2. Muscular; 
3. Nervous; 
4. Connective. 
The cells themselves play the essential parts in the epithelial, muscular, and 
nervous tissues, in which only an insignificant intercellular substance is devel- 
oped, but the condition is quite the reverse in connective tissue, in which the 
development of the cells is subordinated to that of the intercellular substance. 
With regard to the derivation of the various tissues from the three primi- 
tive organs of the embryo, all three-ectoblast, mesoblast, and entoblast-give 
rise to epithelial tissue. Muscular tissue comes mainly from the mesoblast, 
and only to a slight degree from the ectoblast. Nervous tissue comes exclu- 
sively from the ectoblast. Connective tissue arises from the mesoblast, the 
entoblast, and probably also from the ectoblast. 
16 EPITHELIAL TISSUE 
Epithelial tissue consists almost wholly of cells placed together in rows, 
which differ both in their mode of arrangement and in their special construction. 
As its name indicates, it covers .the other tissues, separating them from the 
outei' world and lining all the cavities of the body. Thus it forms an extremely 
important protection. In addition to this function it has another which is 
perhaps equally essential, to provide for the secretions and excretions of 
the body, so the processes of metabolism that have been de- 
scribed play a particularly great part in their cells. To attain 
this end the epithelium of the surface of the body, and more 
especially that lining its cavities, is invaginated to form 
glands. A third function of the epithelial tissue is to receive 
stimuli that fall upon the body. 
Each epithelium has a free surface, which looks outward, 
or, in a cavity, inward, and a base with which it rests upon the 
subjacent tissue. It is usually separated from the latter by a 
thin, structureless membrane called the basal membrane. 
Special formations that serve various purposes may develop 
on the surfaces of the epithelial cells, chief among which are 
the cilia. 
Cilia are fine filaments that project from the free surfaces 
of these cells and maintain during life a constant movement in 
a fixed direction. Although they are always rather short in 
man, they attain quite a length in the lower vertebrates and in 
the invertebrates, where they are often considerably longer 
than the cell itself (Pl. 9, Figs. 28 and 29). In the lower ani- 
mals a single cilium may project from a cell, or several may 
coalesce to form a sort of w7hiplash, as in a flagellated cell 
(Pl. 10, Fig. 30), but in man we always find many cilia of 
equal length standing close together. The free surface of the 
ciliated cell is provided with a homogeneous band of proto- 
plasm through which each cilium passes to the outermost periphery of the cell, 
where it terminates in a little nodule, the so-called basal corpuscle. In many 
cases the cilia continue in the cell body as fine, converging threads to the base 
of the cell, thus forming the root of the cilia. 
Ciliated epithelium is found throughout the entire respiratory tract, as far 
as the smallest bronchi, the tympanic cavity, the auditory canal, the central 
canal of the spinal cord, the uterus, and the efferent ducts of the epididymis. 
The construction of the so-called hair cells, found in the crista? and 
macula? acustica? of the membranous labyrinth, is very similar to that of the 
ciliated cells in principle, but basal corpuscles are absent and, as the hairs have 
no movement of their own during life, we must consider these corpuscles to be 
the kinetic centers of ciliary movement. 
The so-called cuticular band appears as a thin layer on the free surface 
Fig. 7. 
Ciliated Cell. 
17 18 
of epithelium beyond the crusta. It may seem to be homogeneous and without 
structure, as in the ciliated cells, or it may have a fine, transverse striation, 
caused by the presence of many short rods, processes of the cell protoplasm, 
when it is called a rod band. Such a rod band is met with in the epithelial 
cells of the intestine (Pl. 9, Fig. 27; Pl. 8, Fig. 26), and is probably connected 
with the absorption of certain nutritious substances by the intestinal epithelium. 
This is indicated by the fact that it is met with in different conditions of 
development during different phases of absorption. Similar formations occur 
on the epithelial cells of certain sections of the uriniferous tubules, where the 
rods are coarser and can be seen more distinctly than in the intestinal epithelium. 
Such a formation is called a bristle band (PL 6, Fig. 21). The relations 
of these rods to the activity of the cell are more evident here, for the bristle 
band is present only on the secreting cells and is absent on the inactive ones. 
Still longer processes are met with on the epithelial cells of certain sections 
of the epididymis, where they closely resemble cilia, except foi' the absence of 
basal corpuscles. 
The bodies of the epithelial cells also undergo considerable modifications 
in form and structure to accord with the particular duties they have to perform. 
Wherever the main duty of the epithelium is to act as a protective covering, 
as over the outer surface of the body, the superficial portion of the cell body 
is transformed into keratin, and the cell becomes horny. The hair and nails 
consist essentially of such cornified cells. A calcareus epithelium, the 
enamel, covers the crowns of the teeth and gives them their great hardness and 
power of resistance. 
Another function of epithelial tissue, aside from the glandular epithelium 
which will be described later, is that of absorption and secretion. It is 
mainly through the latter power that epithelial cells bring about very striking 
and important changes in the body. The chief secretory product of these 
cells is mucus, which, in the epithelium of the intestinal canal and of the 
respiratory tract, is elaborated from the protoplasm within the cell by the 
mucinogen granules. These can be distinguished from the granules of the 
cell protoplasm by their higher refractive power, and particularly by the fact 
that they are not oxyphilic, but are basophilic, and therefore stain with basic 
instead of acid dyes. During the further course of the formation of mucus these 
mucinogen granules increase in size and number, while the protoplasm between 
them changes its staining reaction and becomes basophilic. The granules develop 
chiefly in the distal portion of the cell, near the free surface. This portion 
becomes considerably distended by the great accumulation of mucinogen gran- 
ules, bulges, and contrasts sharply with the proximal, more slender part of the 
cell, which, as a whole, now has the shape of a goblet. Such cells, filled with 
mucus, are called goblet cells (PL 8, Fig. 26; PL 9, Fig. 29). The slender 
proximal portion contains unaltered protoplasm and the nucleus, which is 
usually more or less deformed by pressure. The surface of the cell ruptures 
when the process within it reaches its acme, probably because the contents 
are caused to swell by the admission of water, and the mucus, i.e., the mucinogen 
granules plus the intergranular substance, pours forth as a homogeneous or 
a filamentous mass into the interior of the cavity lined by the epithelium. The 19 
empty and collapsed cell refills with oxyphilic protoplasm from its proximal 
portion, after which it differs in no way from the adjacent epithelial cells, 
in one after another of which the same process takes place. 
The secretory activity of the epithelium may also result in the destruction 
of the cells that take part; they are compressed by their vigorous neighbors 
and cast off. The process of casting off is still more marked in the true pro- 
tective epithelia, such as that of the cornea, or of the mouth. The cells thus 
cast off must be replaced of course, and this replacement is accomplished by 
indirect segmentation of the remaining cells (Pl. 10, Fig. 31). 
Epithelial cells are divided according to their shapes into flat, cuboidal, 
cylindrical, and polymorphous. A flat cell is one in which the breadth 
greatly exceeds the height; a cuboidal, one in which the breadth and height are 
approximately equal; a cylindrical, one in which the height greatly exceeds 
the breadth. A flat cell therefore has about the shape of a tile, such as are 
frequently used for floors; a cuboidal cell that of a dice, or often that of a 
prism in which the transverse is about equal to the vertical diameter; the 
cylindrical cell that of a prism which has been much lengthened along its 
longitudinal axis. The bodies of the polymorphous cells are irregularly poly- 
hedral, and are often provided with numerous short offshoots. Sometimes they 
look like open umbrellas, whence they are called umbrella cells. 
As regards the way in which epithelial cells are connected, it has already 
been said that they lie close together in the epithelium. Their smooth surfaces 
may be pressed together so very closely as to leave no visible apertures, and 
special methods of preparation are necessary to enable us to demonstrate an 
intermediate substance, the cement, or intercellular substance. This 
is present particularly wherever flat cells unite in a single layer to form an 
epithelium, and is very easily stained black with silver salts. Such a silvered 
epithelium shows on its surface a network of fine lines which mark the inter- 
cellular substance. The meshes of the network are the bodies of the epithelial 
cells (Pl. 6, Fig. 20). 
In other cases the interspaces between the individual cells are rather large 
and form intercellular spaces and clefts, which are filled during life with 
tissue fluid. These clefts are closed at the free surface by edges of firm inter- 
cellular substance interposed between the tops of the cells. These so-called 
cement edges can also be easily stained, and then the surface may present 
a picture very like that given by the intercellular substance of flat epithelium 
(Pl. 8, Fig. 25). 
Another interesting condition is to be found frequently. Such intercellular 
clefts may contain narrow bridges of protoplasm that unite the cell bodies. 
These are called cell bridges, or intercellular bridges. When these are 
present we can no longer speak logically of individual cells, for the entire 
epithelium forms a single large mass of protoplasm, a synctium, in which the 
nuclei lie scattered about. Such cell bridges are to be found in many epithelia, 
but not in all, as has frequently been claimed. The intercellular clefts are of 
importance to the epithelium in that they enable nutrient fluid to enter between 
the cells (Pl. 11, Fig. 32). 
Cell bridges are formed of ordinary cell protoplasm, but fibrous differentia- 20 
tions of the cell body, the so-called epithelial fibers, may pass through them. 
These fibers develop chiefly in the superficial layers of the cell protoplasm 
and, in their courses, pass through not only one, but quite a number of cell 
bodies. They have a considerable importance in dense epithelia in that they 
increase the powei' to resist external forces. 
We come now to the mutual position of the cells from which the common 
classification of the different kinds of epithelium as simple, stratified, and 
stratiform, is derived. 
Simple Epithelium 
Simple epithelia, i.e., those formed of single layers of cells, may be flat, 
cuboidal, or cylindrical, in accordance with the type of the cells of which they 
are composed. 
Simple flat epithelium consists of thin, tilelike cell bodies, the lateral 
surfaces of which are in apposition one with another. The cell plates are often 
so thin that the nuclei produce elevations 
on the free surface. The cells do not have 
straight lateral margins, as a rule, but ir- 
regular ones that fit one into another, while 
between them lies an intercellular substance 
which forms a network of irregular, fine 
trabecula? (Pl. 6, Fig. 20). Cilia are never found on simple flat epithelium, but 
epithelial fibers may be seen in the cell bodies running parallel to the free sur- 
face only. Flat epithelium is distributed very widely throughout the human 
body; it lines all of the serous cavities, like the pleura, the pericardium and 
the peritoneum, and also the entire vascular system, the heart, arteries, veins, 
capillaries, and lymphatics. In both of these places it is frequently known as 
endothelium. Other flat epithelia arc the respiratory epithelium of the al- 
veoli of the lungs, the indifferent parts of the epithelium of the labyrinth, and 
the endothelium of the cornea. In the last may be found cell bridges and epithe- 
lial fibers. 
Simple cuboidal epithelium is glandular epithelium zar The 
expression cuboidal must not be taken too literally, for, aside from the fact 
that the cells are almost always bounded by more 
than four sides and therefore resemble prisms 
rather than dice, they may often assume the form 
of truncated cones or pyramids with their bases 
resting on the basal membrane. As we shall see 
later, this is the case particularly in the glands, 
where many cells are grouped into a relatively small space (Pl. 3, Fig. 8). 
Cuticular formations, cilia, and bristles are to be found on simple cuboidal 
epithelium (Pl. 6, Fig. 21). Intercellular clefts and cement edges are 
quite generally present. The nucleus lies either near the center of the cell, 
or toward its proximal end. Such cuboidal epithelium is found in most glands, 
as has been already mentioned, where it is secretory. It has a respiratory 
function in the bronchioles, and is provided with cilia in the minute bronchi 
Fig. 8.-Simple Flat Epithelium 
(schematic). 
Fig. 9.-Simple Cuboidal Epithe- 
lium with Intercellular Spaces 
and Cement Wedges (schematic). 21 
and in the tympanic cavity. The epithelium of the choroid plexus of the brain 
is also simple cuboidal and has cilia during embryonal life. 
Simple cylindrical epithelium consists of a single layer of elongated 
cells which have a more or less regular prismatic form. As a rule, the cells 
are not of the same transverse diameter throughout their length, but become 
more slender toward their proximal ends, which lie upon theii' basal membrane 
(Pl. 8, Fig. 26; Pl. 9, Fig. 27). Intercellular spaces closed in with cement 
almost always exist between the cells. Cell bridges also may be observed. In 
the lower epithelium the nucleus usually lies near the base of the cell, in the 
higller nearer the center. Goblet cells are often interposed between the true 
cylindrical ones (Pl. 8, Fig. 26). The free surface often has a cuticular 
band, more rarely cilia (Pl. 9, Fig. 28). This is the characteristic epithelium 
of the gastrointestinal canal, which it lines from the cardiac orifice of the 
stomach to the anus. In the stomach all of the 
cells are filled with mucus, in the intestine the 
ordinary cylindrical cells have many goblet cells 
between them. The former have on the free 
surface a cuticulai' band composed of short rods, 
and are preeminently absorptive. Simple cylin- 
drical epithelium also lines the excretory ducts 
of the large intestinal glands, the liver and the 
pancreas, the bile duct, the cystic duct, the gall 
bladder, the hepatic duct, and the pancreatic 
duct. At the entrances of these excretory ducts 
into the glands the cylindrical cells become shorter 
and at last cuboidal, or, in the case of the pan- 
creas, flat. Finally we meet with a simple cylindrical epithelium in the excretory 
passages of the sexual glands; in males in the vas deferens, the seminal vesicles, 
the ejaculatory duct, and in many parts of the urethra, in females in the 
Fallopian tubes, and in the uterus, where part of the cells have cilia. 
Fig. 10.-Simple Cylindrical 
Epithelium with Intercellular 
Spaces and Cement Wedges 
(schematic). 
Stratified Epithelium 
A stratified epithelium consists of two or more layers of cells, none of which 
extend through its entire thickness, i.e., none of them has one end at the free 
surface and the other at the basal membrane. The number of cells placed one 
over another is very variable; in the simplest cases it is two, but in very thick 
epithelia it may be twenty or more. True stratified epithelia are never ciliated. 
They are classified according to the cells of which they are composed into 
stratified flat epithelium, transitional epithelium, and stratified cylindrical 
epithelium. 
Stratified flat epithelium always shows an unmistakable regularity in 
its construction. The deepest, proximal layer consists of cylindrical cells of 
greater or less height, on which rest distally one or more layers of polymorphous 
cells, and the epithelium is completed at its free surface by one or more layers 
of quite flat cells (Pl. 10, Fig. 31). Between the cells are very distinct inter- 
cellular spaces, through which pass cell bridges (Pl. 11, Fig. 32). As the 22 
isolated cells, when torn apart, seem to be provided with numerous short pro- 
cesses they have been termed prickle cells, or crenated cells. Epithelial 
fibers are particularly prominent in stratified flat epithelia, in which they run 
vertically through its entire thickness from base to surface, and then bend over 
to form arches. The flat cells in the most superficial layer are usually horny. 
Stratified flat epithelium is always a protective covering without secretory 
functions, and as such it covers the entire surface of the body under the name of 
epidermis. Stratified flat epithelium also covers the anterior surface of the 
Fig. 11.-Stratified Flat Epithelium with Intercellular Spaces and Intercellular 
Bridges (schematic). 
cornea, is present at the entrances and exits of the digestive, respiratory, and 
genitourinary organs, and lines the mouth, fauces, vestibule of the nose, the 
navicular fossa of the penis, the female urethra, the vagina, and the portio 
vaginalis of the uterus. We also find it in the external auditory canal, on the 
vocal cords, and on the posterior surface of the epiglottis. 
Transitional epithelium is very like the preceding, but the strict regu- 
larity in its construction is wanting. The proximal layer of cells is not so 
typically cylindrical, and the most distal layers are not so markedly flat. Its 
Fig. 12.-Transitional Epithelium (schematic). 
free surface is usually covered by a layer of polymorphous cells, which we 
call cover cells, or umbrella cells, from their form (Ph 11, Fig. 33). 
Cornification, intercellular spaces, and epithelial fibers never occur in this 
form of epithelium, which is found only in the urinary passages, the pelvis of 
the kidney, the bladder, and the prostatic portion of the male urethra. 
Stratified cylindrical epithelium is of still rarer occurrence. It is 
always composed of only a few layers of cells, the most proximal of which are 
small and cuboidal, with a row of conical cells resting their bases upon them. 23 
The apices of these conical cells point toward, but do not reach the free surface, 
while between them lie the true cylindrical cells with their proximal ends pointed, 
and their distal ends bounding the free surface. The middle layer may be 
wanting, and then the cylindrical cells lie directly on the cuboidal. Stratified 
Fig. 13.-Stratified Cylindrical Epithelium (schematic). 
cylindrical epithelium is never ciliated in man, in whom it is found lining the 
submaxillary and parotid ducts, the membranous and cavernous portions of 
the male urethra, and the palpebral conjunctiva. 
Stratiform Epithelium 
We call an epithelium stratiform when all of the cells in it either start 
from the basal membrane and only a part of them reach the free surface, or 
start from the free surface and only a part reach the basal membrane, while the 
rest end within the epithelium. An epithelium like this cannot be said to be 
stratified, although it may easily be mistaken to be such, particularly when the 
nuclei lie in rows one above another. In stratiform epithelium we have to do 
with only two varieties of cells, one of which is long, cylindrical, and reaches 
Fig. 14.-Stratiform Epithelium (schematic). 
from the surface to the basal membrane, while the other is crowded in between 
these at either the superficial or deep surface. 
Such a stratiform epithelium is the ciliated, found in the respiratory 
organs from the vestibule of the nose to the smaller bronchi, where it gives place 
to a simple cuboidal epithelium (Pl. 9, Fig. 29; Pl. 11, Fig. 35). The ciliated 
cells pass through its entire thickness and taper very much proximally, while 
between their tapering ends lie conical cells with their broad bases on the basal 24 
membrane and their apices lost between the ciliated cells. Goblet cells m the 
same situation as the ciliated cells are often met with in this epithelium. Strati- 
form ciliated epithelium is also found in the Eustachian tube, in the epididymis, 
and in the spermatic ducts. 
The sensory epithelium of the membranous labyrinth is stratiform, but 
the conditions present are the reverse of those in stratiform ciliated epithelium. 
Long, slender cells, with their nuclei in their proximal thirds, pass through the 
entire thickness of the epithelium, and rest with broad bases on the basal 
membrane. Between these slender, so-called filiform 
Dells, which serve as supports, lie broader, prismatic 
cells that reach from the free surface only to about 
die middle of the epithelium, and carry on their distal 
ends shorter or longer hairs, the auditory hairs. The 
nucleus of each of these hair cells lies in its proximal 
portion. Thus the nuclei of the epithelial cells form 
several rows, those of the auditory cells the most distal 
pne, those of the filiform one or more proximal. 
It should be mentioned in conclusion that there are 
i few kinds of epithelium that do not conform to this 
classification. Among these in man is the sper- 
matozoa producing epithelium of the seminife- 
rous tubules, which is composed of several superim- 
posed layers of roundish cells, the seminal cells, with 
the long Sertoli's cells between them extending from 
the basal membrane to the lumen of the canal. 
The retina is a sensory epithelium that, very like 
the auditory, contains supporting cells, known as 
Mueller's fibers, which pass through its entire thick- 
ness, but instead of forming a single layer, as in the 
eai; the sensory cells are in several layers, one over another, and are provided 
with many long offshoots. 
Fig. 15.-Sensory Epi- 
thelium of the Mem- 
branous Labyrinth of 
the Ear (schematic). 
Glandular Epithelium and Glands 
Although epithelial cells in many places have a secretory function, yet most 
of the secretion and excretion in the human body is performed by special 
epithelial formations known as glands, and not by the superficial epithelium. 
We mean by a gland a proliferation of the epithelial tissue that takes 
place, as a rule, below the level of the epithelium, the essential ob- 
ject of which is to obtain an enlargement of the secreting surface. 
In the great majority of cases these proliferations are hollow, and are there- 
fore invaginations of the superficial epithelium. A glandular cavity, that 
varies a good deal in shape, is formed and is lined with glandular cells, 
which empty their secretory products into it. The primary opening of this 
cavity usually remains patent, so that the secretion is poured through it over 
the surface of the epithelium. The system of cavities in a gland may be very 
extensive, and different portions may assume different functions; in such cases 25 
the parts distant from the surface are devoted mainly to secretion, while those 
in immediate proximity to it serve simply to carry the secretion away, and the 
gland becomes thus divided into a secretory and an excretory portion. 
But the open connection between the glandular cavity and the surface of 
the epithelium does not remain patent in all cases. It may become obliterated, 
and then we have a closed gland beneath the surface, the secretion of which 
is carried away by the lymph and blood channels. 
These true glands with glandular cavities contrast with others in which 
epithelial cords proliferate deeply, have no cavities from the first, and do not 
become canalized later. Such are known as solid glands. The vascular sys- 
tem must care for their secretions, as well as for those of the closed glands, so 
both varieties are included under the name haematopoietic glands. 
We will now familiarize ourselves more closely with the details of the true, 
open glands. 
The True Glands 
Every true gland originates in a depression of the superficial epithelium in 
one of two principal ways. In one form the invagination, with its peculiar 
glandular epithelium, is of approximately the same transverse diameter every- 
where, so that in its simplest type it resembles the finger of a glove; in the 
other it forms a little hollow ball. The former we call a tubular, the latter 
an alveolar gland. These two simple, fundamental types are complicated in 
many ways. The gland may ramify, or many glands may unite to form a single 
large one, and therefore we have simple, branched, and compound glands. 
The free ends of tubular glands may also dilate and give rise to a mixed variety 
known as alveotubular glands. We will now study each of these varieties. 
The simple tubular gland is a short, straight, or slightly curved, blind 
tube that passes from the surface of the epithelium down 
into the subjacent tissue. At its mouth the superficial 
epithelium changes gradually into glandular. Lieber- 
kuehn's intestinal crypts are examples of such very simply 
constructed glands (Pl. 53, Fig. 126). 
Convoluted glands are distinguished from the fore- 
going by the considerably greater length of the glandular 
tube, and the fact that the latter has numerous windings 
at its blind end, which form a convolution of a glomerule. 
In consequence of its great length the tube is divided into 
a convoluted, secretory portion, and a straight excretory 
duct. Such are the sweat glands of the skin (Pl. 93, Figs. 
197 and 198). 
In the simple branched tubular gland the tubu- 
lar invagination soon divides into two or more branches 
that lie close together, penetrate deeply and have blind 
ends. The simple initial portion usually remains lined with the superficial 
epithelium, and narrows just in front of the place of division, forming what 
is known as the neck of the gland. The branched parts, or body of the 
Fig. 16. - Simple 
Tubular Gland 
(schematic). 26 
gland, follow straight, or slightly tortuous courses, widen a little at their 
blind ends to form the base of the gland, and arc lined with secretory epithe- 
Fig. 17.-Sweat Gland 
(schematic). 
Fig. 18.-Simple Branched Tubular 
Gland (schematic). 
Hum. The fundus glands of the stomach and the glands of the uterus are of 
this type (Pl. 50, Fig. 119). 
Compound branched tubular glands are formed by the union of 
many glands of the kind just described, which empty their secretions into one 
Fig. 19.-Compound Branched Tubular Gland (schematic). 
or more common outlets. In such glands as these the glandular tubes may 
reach extraordinary lengths, while their different sections may assume various 27 
secretory, and, undei' certain circumstances, absorptive functions. Such are 
the serous glands of the tongue, the lacrimal glands, the kidneys and the 
testicles (Pl. 58, Fig. 138; Pl. 61, Fig. 146). The liver also is one, but it 
Fig. 20.-Simple Alveolar Gland 
(schematic). 
Fig. 21.-Simple Branched Alveolar 
Gland (schematic). 
differs from the others in that the branching canals unite to form a network 
within it. 
The simple alveolar gland is a little vesicle lined with secretory cells 
Fig. 22.-Compound Branched Alveolar Gland (schematic). 
beneath the epithelium. It may be spherical or oval in shape. It communi- 
cates with the free surface by a narrow neck, where the secretory epithelium 28 
becomes very thin and changes into the superficial. The only simple alveolar 
glands are the little sebaceous glands of the skin. 
When two or more such simple alveoli empty into a com- 
mon duct we have a simple branched alveolar gland. 
The efferent duct may be short with all the alveoli grouped 
at its end, or it may be long, with the alveoli attached along 
its sides. The large sebaceous glands of the skin are of the 
first type (Pl. 94, Fig. 201), the Meibomian glands of the 
lids of the second (Pl. 90, Fig. 193). 
Many branched alveolar glands may unite, in the same 
way as the tubular, and then we have a compound 
branched alveolar gland. The excretory part of 
these glands, which are all large, is greatly developed and 
is divided sharply from the secretory part, which is con- 
fined to the alveoli. The alveoli empty into the primary 
efferent ducts, several of which join to form a secondary 
duct, and so on until finally we come to the principal excre- 
tory duct. The alveoli may communicate with the primary 
duct by short wide openings, or by long, narrow channels, 
when they seem to hang by slender pedicles and resemble 
grapes to such an extent that these are also called acinous 
glands. The mamma is an example of the former variety of compound 
branched alveolar gland (Pl. 98, Fig. 
208), while the pancreas is a typical 
acinous gland (Pl. 55, Fig. 132). 
Therefore, the most important 
part, the secretory portion, of a tu- 
bular gland has the form of a tube, 
that of an alveolar gland the form of 
a globule. The alveotubular is a 
mixed variety, the origin of which is 
thought to be an alveolar dilatation 
of the blind end of the secretory 
tubule. 
The simple alveotubular 
gland needs no further description 
of its structure. The secretory epithe- 
lium adjoins the superficial and ex- 
tends throughout the tubule and the 
alveolus. The tube is usually con- 
stricted like a neck at the junction 
of the superficial and the glandular 
epithelium. The pylorus glands of 
the stomach are of the type of the 
simple, and frequently of the 
branched alveotubular gland (Pl. 50, Fig. 121). 
A short excretory duct is formed in the branched alveotubular gland. 
Fig. 23. 
Simple Alveotubu- 
lar Gland 
(schematic). 
Fig. 24.-Branched Alveotubular gland 
(schematic). 29 
The secretory tubules branch not only end in alveoli, but are also studded with 
them. Littre's glands of the male urethra exemplify this variety. 
Many of the glands already mentioned unite to form a compound 
branched alveotubular gland. Here we usually find a long excretory 
duct, lined with indifferent epithelium, which presents many branches. The 
indifferent is replaced by secretory epithelium in branches of a certain caliber.. 
At the end of each tubule are groups of spherical or oval alveoli, usually 
Fig. 25.-Compound Branched Alveotubular Gland. The indifferent parts of the 
epithelium are bright, the secretory parts are rather darker (schematic). 
separated from the tubule by a short, indifferent, intermediate portion. This 
structure is common to all mucous glands, the large glands of the mouth (Pl. 48, 
Figs. 115 and 116), Brunner's glands of the duodenum, and the prostate. 
The lungs also may be regarded as a compound branched alveotubulai' gland 
(Pl. 58, Fig. 137). 
Closed and Solid Glands 
The only closed gland in the human body is the thyroid, in which we have 
roundish or oblong glandular alveoli lined with secretory epithelium, the so- 30 
called follicles. The secretion is thrown out into the cavity of the alveolus, 
and probably is emptied through apertures in the epithelium into the surround- 
ing lymph spaces, when the follicle collapses (Pl. 40, Fig. 100). 
There is no cavity to contain secretion in the solid glands. The secretory 
epithelium forms trabecula with cell lying close to cell, so that quite a number 
appear to be grouped in a section of a trabecula. These epithelial trabeculae 
may run in various directions in different cases, and have a varying arrange- 
ment in the gland, but, as a rule, they unite to form a network. In the meshes 
of this network lie the blood vessels, with which these glands are always 
abundantly supplied, that dispose of the secretion. Here belong the parathyroid 
glands, the suprarenal capsules, and the hypophysis (Pl. 41, Figs. 101 and 102). 
Now that we have learned to recognize and to classify glands in this manner 
from their outer forms, it is necessary to go rather more minutely into their 
finer construction and their functions. We shall considei' only the epithelium, 
for the description of the less important accessory structures, such as the 
membrana propria, connective tissue, muscular tissue, etc., appertains to the 
special microscopic anatomy of the organ concerned. 
The duty of secretory glandular cells is either to elaborate in their own 
bodies certain substances from the materials brought to them by the blood, and 
to empty such products into their cavities, or to extract these substances already 
prepared from the blood and then simply to excrete them. Thus urea and uric 
acid, the two most important constituents of the urine, are not elaborated by 
the secretory cells of the kidney, but are products of the disintegration of 
albuminoids in other cells of the body, and are simply excreted by the renal 
epithelium. But the secretions of most glands are elaborated in their glandular 
cells, where the process of formation may be followed under the microscope. 
The preliminary steps of secretion frequently appear in the glandular cells as 
granules, and it is now evident that the formation of the latter starts from 
the microsomes lying in the protoplasm. These living microsomes take up 
matter from the material brought to them and elaborate it in a certain way 
within their own bodies. Consequently they increase in size, change their loca- 
tions, and finally appear as dead granules of secretion in the meshes of the 
protoplasm. 
In addition to this taking up and elaboration of albuminoid substances, 
the absorption of water by the glandular cell is an important factor. This 
process runs parallel with the other, but we do not know as yet how it takes 
place, or whether it is through the mediation of special organs, the filamentous 
structures of the proximal part of the glandular cell, as has been claimed. 
The question of the involvement of the nucleus in the process of secretion 
is the subject of much discussion. As has been mentioned already, certain 
alterations may be perceived in the nucleus during the various phases of the 
process. While the nucleus lies near the center of the resting cell, which con- 
tains no secretion, it moves gradually toward the base as the cell fills with the 
primordial particles of secretion, until it comes into such close apposition that 
it can be discerned only with difficulty. Coincidently with this change of loca- 
tion a diminution of its volume usually takes place. The chromatin becomes 
compressed into a smaller space, so that the nucleus of a cell full of secretion 31 
stains more strongly and looks more compact than that of a cell not thus 
filled. This reduction of volume may be regular or irregular; in the former 
case the nucleus preserves its spherical shape, in the latter it becomes greatly 
deformed and presents dentations and projections, as may be seen very dis- 
tinctly in the nucleus of the mucous cell. 
These visible changes make it probable that the nucleus takes an active 
part in the process of secretion, and that it furnishes secretory material, but 
nothing is positively known concerning this. It has been claimed that in 
certain glandular cells the nucleolus escapes from the nucleus during secretion 
and disappears in the cell body. 
The secretion is emptied from the free surface of the glandular cell into 
the glandular cavity either directly, or through minute diverticula of the latter, 
which are called secretory capillaries. These are of two kinds, the inter- 
cellular and the intracellular. Intercellular secretory capillaries are 
minute tubules which follow a straight or slightly curved course between any 
two cells and have blind ends in front of the membrana propria. They may also 
be forked (Pl. 48, Fig. 115). Such intercellular secretory capillaries are 
found in all of the salivary glands, the liver, the pancreas, the pylorus glands, 
the lacrimal gland, etc. The intracellular secretory capillaries extend 
from the lumen of the gland into the bodies of the glandular cells, or enter the 
latter as extensions or lateral branches of the intercellular variety. Within 
the cell bodies they either end in cul de sacs, or they form an intracellular' 
system of canals by branching and anastomosis (Pl. 50, Fig. 120); we find 
examples of the former in the cells of the liver, of the latter in certain cells of 
the fundus glands of the stomach. 
Although glandular cells, like all others, have only a limited length of life, 
and we therefore meet with dead ones from time to time, yet they are not 
destroyed by the process of secretion itself. There is only one known exception 
to this rule; in the sebaceous glands the glandular cells perish, the nuclei and 
cell bodies break down and are cast off as constituents of the secretion. MUSCULAR TISSUE 
Every appreciable movement of the human body, or of any of its parts, 
depends on the existence of a special, muscular tissue, the greatest part of 
which by far is derived from the mesoblast. The tissue is composed of specific 
muscle cells, which shorten by contraction in response to stimuli and are able 
to do so because of the presence of certain differentiation products of their 
bodies, the contractile fibrils, protoplasmic threads of varying thickness 
that run approximately parallel through the length of the cell body. The 
result of this arrangement is that the shortening brought about by the con- 
traction of all the fibrils in a cell always takes place in only one direction. 
The contractile fibrils develop from the protoplasm of the embryonal muscle 
cell and the formations known to us as mitochondria. The greater part of the 
cell protoplasm is used for this purpose, the balance, known as sarcoplasm, 
lies in the perfect muscle cell as indifferent protoplasm and envelops the fibrils. 
We distinguish two kinds of muscles in the human body, the involuntary 
or organic, and the voluntary or animal; the former independent of the 
will, the latter subject to it. These two kinds of muscle differ essentially in 
the construction of their contractile fibrils. They are homogeneous through- 
out their entire length in involuntary muscles, which are also called smooth 
muscles because of this fact. In the voluntary muscles, on the contrary, 
they are composed of substances of unequal refractive power, which give the 
cells a transversely striated appearance. Voluntary muscles consequently are 
also called striated muscles. 
The muscle of the heart occupies an intermediate position between 
these two varieties, as it is not subservient to the will, and yet its cells contain 
transversely striated fibrils. 
Smooth muscle fibrils are found throughout the animal kingdom, but stri- 
ated fibrils are absent in the lowest animals. They gradually appear as the 
scale of life rises, and reach their highest development in the arthropods and 
vertebrates. 
Smooth Muscular Tissue 
Smooth muscular tissue is found chiefly in the interior of the human body 
in the form of continuous membranes in the walls of the hollow digestive, respi- 
ratory, and genitourinary organs, where it takes charge of the onward move- 
ment of the chyme, the expulsion of foreign bodies, or of epithelial secretions 
that act as such, the evacuation of the urine, and the ejection of the genital 
products. It forms a very important constituent of the vascular system, and 
is also found in the skin and within the eye. 
Smooth muscle cells are long, thin, spindle-shaped, or frequently ribbon- 
32 33 
shaped, thickest in the middle and tapering into points at each end (Pl. 23, 
Fig. 59). Their length varies fiom 40 to 500 their greatest transverse 
diameter from 5 to 10 [X. In a cross section the cells usually appear to be 
roundish, polygonal with rounded corners, or oval (Pl. 23, Fig. 60). 
The body of the smooth muscle cell usually has a longitudinal striation, 
though this is often very indistinct, due to the presence of the contractile 
fibrils, which are divided into the internal and the limiting. The latter 
form a single layer in the periphery of the cell and may be distinguished by 
their somewhat greater thickness from the internal fibrils, which fill the interior 
of the cell, where they may either be very compact or lie more loosely. All 
of these fibrils run parallel through the length of the cell, but whether they 
become more and more slender as the ends of the cell taper into points, or 
the outer fibrils terminate sooner than the inner ones, is not yet certain. 
Between the fibrils lies unchanged protoplasm, the sarcoplasm, contain- 
ing varying quantities of fine granules. The sarcoplasm also forms a cortical 
layer over the outer fibrils, known as the sarcolemma, in which larger 
granules are to be found (Pl. 22, Fig. 57). 
Each smooth muscle cell contains a nucleus, which is always situated 
near its center where it produces a marked swelling. It is usually cylindrical, 
or drawn out still more like a rod, and frequently does not occupy exactly 
the longitudinal axis of the cell, but seems to be somewhat hollowed toward the 
latter. In the depression thus formed lie two .little central corpuscles. 
Both the cell body and the nucleus undergo great changes of shape during 
contraction. The contractile fibrils become much thickened at certain places, 
called contraction nodes, where the individual fibrils draw close together 
and seem to blend. Dark lines are formed in the stained specimen which run 
transversely or obliquely over the-cell. 
Smooth muscular tissue is formed from these smooth fibers lying close to- 
gether. No such protoplasmic cell bridges are to be found as are often to be 
seen in epithelial tissue, but the fibers are held together by connective tissue 
with a small quantity of cement. The cells may be assembled to form bundles 
which may either lie parallel and unite to form muscular coats, or may inter- 
lace and decussate (Pl. 22, Figs. 57 and 58). 
Striated Muscular Tissue 
Striated muscular tissue is perhaps the most important of all the tissues 
in the construction of the human body. The muscles composed of it form 
over 40 per cent, of the body weight. They comprise all of the motor muscles 
of the trunk, head and limbs, the so-called skeletal muscles, those of the upper 
part of the alimentary and respiratory tract, i.e., of the mouth, tongue, phar- 
ynx, larynx, and the upper two thirds of the oesophagus, and those of the 
orbit, the tympanum, the external genital organs, and the anus. 
The cells that compose this tissue, the striated muscle cells, differ 
essentially from those of the smooth muscles that have just been described; 
they are much larger, arc of a different shape, have striated instead of smooth 
fibrils, and contain many nuclei instead of a single one. 34 
The form of a striated muscle cell is that of a very long prism, the edges of 
which are more or less rounded off. Consequently, it presents on cross section 
the picture of a polygon with rounded corners, and on longitudinal section a 
band so long that it is very seldom seen throughout its entire length (Pl. 25, 
Figs. 63, 64 and 65). For this reason it is commonly spoken of as a striated 
muscle fiber, but we must always bear in mind that it is only a single cell, 
no matter how extreme its length may be. 
The diameter of the striated muscle fiber is about the same throughout its 
course, except that each end tapers to a blunt point, and is considerably 
greater on the average than that of the smooth muscle, cell. Although striated 
muscle fibers are to be found that are only 10 thick, or even thinner, the 
great bulk of them range in thickness up to 100 |X, but this maximum is never 
exceeded. The thickness depends in the first place on the age of the individual, 
as it increases from birth until growth is complete, and in the second place on 
the size of the muscle and the degree to which it has been developed. Large 
muscles and robust persons have largei' fibers than small muscles and weakly 
individuals. The length also is very variable, as it depends on the length of the 
muscle to which it belongs. We know that they run the entire length of small 
muscles, but it is difficult to determine whether this is the case in long muscles 
or not, though fibers over 10 cm. long have been isolated from them. 
Irregular forms of muscular fibers are not uncommon. Thus we find in 
the tongue and in the ocular muscles fibers that split into many small branches, 
branched muscle fibers. We meet with such branched fibers in the muscle 
spindle also, but here the branches reunite and give rise to a muscular net- 
work (Pl. 68, Fig. 160). 
As in the smooth muscle cells, the contractile fibrils form the most essen- 
tial constituents of the striated muscle fiber, and in addition we have in each 
the sarcoplasm, the sarcolemma, and the nuclei. Central bodies have not been 
found. 
. The contractile fibrils occupy the entire length of the body of the 
muscle fiber, and are commonly packed very closely together. We suppose, for 
positive proof can hardly be obtained, that each contractile fibril runs the 
length of the fiber to which it belongs. The fibrils are round threads 2 p. thick, 
and, if we isolate them by means of suitable chemical agents, they present 
structural bands of light and dark segments that alternate regularly, the 
brighter bands being the broader. If we move the micrometer screw during the 
observation the picture changes, the light bands becoming dark, the dark ones 
light. This phenomenon shows that we have to deal with two substances of 
different refractive power. With the aid of the polariscopic microscope the 
broad bands between the crossed Nicol prisms attain their maximal brightness 
and darkness four times during a rotation of 360°, and are therefore doubly 
refractive, or anisotropic, while the narrow bands are equally refractive 
in all directions, are singly refractive, or isotropic. Hence in every fibril 
broad anisotropic bands, marked as Q, alternate regularly with narrow isotropic 
bands marked as I. Each anisotropic band is further divided into two halves 
by a narrow isotropic stripe (h), and likewise each isotropic band by a narrow 
anisotropic stripe (Z). Starting with Z we have the following regular order 35 
of rotation: Z, I, Q, h, Q, I, Z, I, Q, h, Q, I, Z, etc. We designate any segment 
of a fibril lying between two Zs as a muscular compartment, i.e., the indi- 
vidual muscular compartments are separated by the narrow anisotropic Z 
stripes, or intermediate discs. 
When the fibril contracts the picture changes and becomes simpler. The 
contraction passes like a wave over the fibril, which becomes shorter and 
thicker. The thickening affects the anisotropic 
segments, while the isotropic on the contrary be- 
come smaller, so that the former blend, and we 
have the broad, doubly refracting bands forming 
a simpler alternation with the narrow singly re- 
fracting ones. But the refractive conditions 
also have changed; the two substances have ap- 
proximated in their refractive powers and con- 
sequently are no longer to be so sharply dis- 
tinguished from each other. 
While the contractile fibrils lie parallel in the 
muscle fiber they are arranged at the same time 
so that the corresponding bands all lie at the 
same level and run across the entire fiber without 
interruption. We may therefore speak of them 
as discs placed one after another in the fiber 
(Pl. 27, Fig. 70). 
We may break down the fiber into discs by 
means of certain chemical agents that cause the 
individual parts to swell, or to shrivel. The 
fiber then falls apart into separate pieces, like 
a roll of coins, but these pieces never correspond 
to a single muscular compartment. The inter- 
mediate discs that separate the compartments 
are extremely resistant, and the separation takes 
place either in the middle disc (h), or within the 
isotropic disc I. 
We will now consider more minutely the arrangement of the contractile 
fibrils within the muscle fibers. On cross section the fiber exhibits more or 
less distinctly a network with fairly fine, irregularly triangular, quadrangular, 
or polygonal meshes, which blends at the periphery with an outline surrounding 
the fiber. This picture we call Cohnheim's fields (Pls. 24-27). We learn 
from corresponding longitudinal sections that Cohnheim's fields are the cross 
sections of the contractile substance, which is placed in the form of long prisms 
in the muscle fiber. These prisms are the muscle columns, and are composed 
of the contractile fibrils. The diameter of Cohnheim's fields, or of the muscle 
columns, may be said to be about 5 [X, differing in different muscles, but approx- 
imately the same in any one. That of the fibril, on the contrary, appears to 
be fairly invariable, so a muscle column is composed of fewer contractile 
fibrils in one muscle than in another. 
Another important constituent of the striated muscle fiber is the sar- 
Fig. 36.-Transverse Striation 
of a Muscular Fibril. A, 
during rest; B, during con- 
traction. 36 
coplasm. This appears in the living or surviving muscle fiber as a homo- 
geneous, or very indistinctly granular mass, which, while fresh, stains strongly 
with haematoxylin. The contractile substance remains almost wholly unstained, 
so the sarcoplasm can be brought out in this way very plainly. It is this that 
makes Cohnheim's fields prominent in the cross section of a muscle fiber, as it 
separates the muscle columns from one another (Pls. 25-26). Cohnheim's 
fields are the cross sections of the muscle columns; the network that separates 
them and forms longitudinal partitions that unite laterally to create long tubes 
in the muscle fiber, is the sarcoplasm. 
It is extremely difficult to preserve the sarcoplasm. Most of our preserva- 
tives cause it to swell and destroy it, while they save the muscle columns. Foi' 
this reason the picture presented by the cross section of a preserved muscle 
fiber differs materially from that of a fresh one. The very regular division into 
fields in the latter is replaced by one which is very irregular, because the swelling 
of the sarcoplasm produces large, communicating spaces, between which lie 
muscle columns blended irregularly into fields of contractile substance. Hence 
the cross section of the preserved fiber does not present the true Cohnheim's fields. 
The septa of sarcoplasm are not so thin and delicate, or so regularly ar- 
ranged and joined together in all muscle fibers as would appear from the above 
description. In some places they are thicker and more irregular, especially 
where three meet and bring together a larger mass of sarcoplasm (Pl. 25, Fig. 
65). As such fibers are usually more slender, and have a smaller cross section 
than others, the amount of sarcoplasm in them may be equal, or even greater 
than that of the contractile substance. Such fibers are said to be rich in 
sarcoplasm, or cloudy. They are found in man in muscles that are con- 
stantly active and make very quick movements, such as those of the eye. The 
fibers of the skeletal muscles have the structure previously described, and are 
said to be poor in sarcoplasm, clear muscle fibers. 
Sarcoplasm is divided into hyaloplasm and granoplasm, the same as any 
other protoplasm. It also contains granular formations which, taken together, 
forms the interstitial substance of the muscle fiber. Generally speaking 
we may say that muscle fibers rich in sarcoplasm appear to be richer in inter- 
stitial granules than those that are poor in sarcoplasm. The granules lie chiefly 
at the points of junction of the network of Cohnheim's fields, and therefore run 
in parallel courses through the fiber (Pl. 26, Fig. 68). They are most numerous 
at the periphery. We learn from longitudinal sections that these granules 
are deposited at the level of either the isotropic disc I, or of the anisotropic disc 
Q, and we call them accordingly I or Q granules. 
Various substances take part in the chemical composition of the interstitial 
granules. The first of these to be mentioned is fat, which occurs in very 
variable quantities in the muscle fibers, the content depending to a great degree 
on the nutritive condition of the individual. There are some interstitial granules 
that consist wholly of fat, and others in which a nucleus of fat is enveloped in 
material that is soluble in water. This may be seen very beautifully when we 
treat a fresh section of muscle with distilled water; the enveloping substance is 
dissolved and the previously invisible fat granules come into view (PL 24, Fig. 
64). Another important component of the interstitial granules is glycogen. 37 
Some granules consist wholly of this substance, and it may be that it forms the 
soluble envelopes which have just been described. Lecithin also takes part in 
the composition of the interstitial granules. 
The sarcoplasmic septa unite at the periphery of the muscle fiber to form 
an outer covering called the sarcolemma (Pls. 24-26). Thus the conditions 
here are the same as in every other cell of the body, for the sarcolemma is in 
fact a specific membrane. If we isolate fresh muscle fibers and place them in 
distilled water, the water will destroy both the contractile substance and the 
sarcoplasm, leaving the sarcolemma perceptible, at least in places, as an empty 
tube. This is not contradictory of what has been said above. We must con- 
sider the sarcolemma as intermediate between the crusta and the pellicle of the 
cell; like the one it passes over immediately into the sarcoplasm, while it has 
the firm consistence and greater power of resistance of the other. 
The last constituent of the muscle fiber remaining to be discussed is the 
nucleus. Each muscle fiber originates from a cylindrical cell of the myotome 
of the embryonal body, which contains a single nucleus, but as it increases in 
length indirect segmentation takes place continuously in the nucleus so that 
finally the fiber when completed is a polynucleated cell. The number of nuclei 
varies; the thin fibers contain more on an average than the thick, but the 
number depends greatly on the length of the fiber, very long ones having 
hundreds. 
In man the nuclei almost always lie peripherally, just beneath the sar- 
colemma, and have their inner sides covered by a small quantity of sarcoplasm, 
but in certain muscle fibers, as in those rich in sarcoplasm, they are most fre- 
quently removed from the sarcolemma and lie embedded in sarcoplasm between 
the muscle columns. This is likewise the condition in the so-called red muscles. 
The very marked redness of certain muscles is attributable in the first place 
to the richness of their vascular supply, in the second place to the amount of 
haemoglobin, or of its derivative myohsematin, dissolved in the sarcoplasm of 
their fibers. The peripheral position of the nuclei is characteristic of the 
muscles of man and of animals; in all other vertebrates they are distributed 
uniformly in both the internal and peripheral layers of the fibers (Pl. 26, Figs. 
67 and 68). 
The nuclei of the muscle fiber are ellipsoidal in shape, frequently have an 
irregular, uneven surface, are rich in chromatin, and usually contain several 
nucleoli. 
As is well known, the muscles of a grown man may be enlarged considerably 
by good nutrition and suitable exercise. This enlargement is due primarily to 
a stoutening of each individual muscle fiber, not by an increase in the size of the 
individual fibrils, but by a multiplication of their number through longitudinal 
division. The nuclei arrange themselves in several longitudinal rows, between 
which the division takes place. 
In the branched muscle fibers, found in the tongue and in the muscle spindles, 
the conditions are quite similar to those in the simple fibers. The fibrils pass 
from the thicker trunk into the more slender branch without splitting or 
branching themselves, and so follow uninterrupted courses throughout the entire 
fiber. 38 
The Cardiac Muscle Fiber 
The branched striated muscle fibers lead us to those of the muscle of the 
heart. Although it is very rarely that striated muscle fibers in general split, 
branch, anastomose, and form networks, this behavior is a characteristic of 
those of the cardiac muscle, in which all of the fibers branch dichotomously 
as a rule in every plane. By the junction of these branches the musculature 
of the heart forms a very extensive muscular network that has exactly the 
same components as the fibers of the skeletal muscles, contractile fibrils, sar- 
coplasm with interstitial granules, sarcolemma, and nuclei (Pl. 28, Fig. 71). 
The fibrils course without interruption through the fibers, and it is im- 
probable that they split or branch. They are transversely striated through- 
out their entire length and show the same alternation of isotropic and anisotropic 
substances as those of the fibers of the skeletal muscles, from which they differ 
in only one essential point. The striation appears to be broken at certain 
places and the fibrils to be homogeneous for a little way, as though a short rod 
had been interposed in the course of the fibril. These places are spoken of as 
interpolated sections. The interpolated sections of the adjoining fibrils 
may lie together so as to form a plate across the fiber, or they may be at 
slightly different levels so as to create a picture that resembles a flight of steps, 
whence they are called cement striae, or stairs (Ph 28, Fig. 71). The 
stairs may pass through the entire thickness of the fiber, or through only a part 
of it; in the first case all of the fibrils are broken, in the latter only a part 
of them are interrupted. 
There has been much discussion concerning the nature of these stairs. 
Some hold them to be contraction phenomena of the fibrils which take place 
during the death of the fiber, and to be to a certain degree artefacts. Others 
believe them to be preexisting formations of functional importance, either a 
sort of intervening tendons, or of accretion lines from which the growth 
of the fiber proceeds. Finally they have been taken to be limiting cell lines, 
and have been called cement lines, as they are composed of a special cement 
substance. By treating the cardiac muscle with strong liquor potassae the fibers 
can be broken into segments that have these stairs at their ends and contain a 
variable number of nuclei. According to this theory the cardiac muscle is 
composed of roughly prismatic cells that send out short, likewise prismatic 
processes in all directions, which join the separate cells and so produce the 
apparently fibrous network. 
The fibers of the human cardiac muscle also differ materially from those 
of the skeletal muscles in the arrangement of the contractile fibrils. Instead 
of forming regular, prismatic muscle columns, the fibrils unite into irregular 
leaves, which converge from the periphery toward the axis of the fiber. The 
individual leaves may split and join together. The axis of the fiber is quite 
free from fibrils (Pl. 28, Fig. 73, c). 
The interspaces between the leaves of fibrils are filled with sarcoplasm, 
which forms similar, though decidedly narrower leaves (Pl. 28, Fig. 73, a). 
The finer points of structure are the same as those of the skeletal muscle fibers. 
Fresh hyaline protoplasm contains many interstitial granules that have the 39 
same composition as in those fibers (Pl. 28, Fig. 73, b), and are commonly 
very fine. The isotropic and anisotropic substances of the contractile fibrils 
are also disposed in the same typical manner. 
The leaves of sarcoplasm start at the periphery from a sarcolemma 
(Pl. 28, Fig. 73, a). 
The nuclei always lie about the axis, surrounded by a small quantity 
of sarcoplasm. The latter is usually so little that the leaves of fibrils seem to 
reach the membrane of the nucleus, and the leaves of sarcoplasm to start directly 
from the latter. The light areolae that appear to surround the nuclei in the 
fixed specimen are artefacts caused by the shrinkage of the leaves of fibrils and 
the swelling of the sarcoplasm, just as they are formed in the skeletal muscle 
fibers (Pl. 28, Fig. 73, a and b). 
Just as the same structure is not present in all true striated muscle fibers, 
so there are marked differences in fibers from different parts of the heart. The 
structure above described is common to the great mass of the musculature of the 
auricles, ventricles, and papillae, but we find other conditions in the cardiac mus- 
cle system that joins the auricles and ventricles, starting from the valvular sinus 
coronarii of the right ventricle and extending to the ventricular septum, a system 
of great functional importance and known as the bundle of His. In man the 
fibers here are thicker and much richer in sarcoplasm than those of the ordinary' 
cardiac muscle, the fibrils also are thicker and do not lie so closely together, 
and the interstitial granules are more numerous and more dense. In hoofed 
animals and other large mammals these fibers can be seen even more plainly, first, 
because of their thickness; second, because the fibrils form a peripheral mantle, 
as in the embryonal muscle fiber, while the interior contains simply sarcoplasm 
and nuclei. These fibers are named after their discoverer Purkinje's fibers. 
Nervous Tissue 
Although the power to conduct stimuli is common, to a certain degree, to 
the protoplasm of every living cell, yet it is possessed xar by the 
nervous tissue. Every external stimulus that falls upon our bodies is re- 
ceived by either this tissue itself, or by special perceptive cells, and transmitted 
to the central nervous system where it is elaborated. Special fibrils are dif- 
ferentiated from the protoplasm of the cells that make up nervous tissue, just 
as in the muscle cells; we call these neurofibrils, and we ascribe to them the 
special conduction of stimuli. 
The great majority of nerve cells lie in the central nervous system, the 
brain and spinal cord, or in peripheral masses of varying size called 
ganglia. Isolated nerve cells are met with much more rarely. 
Muscle cells have a certain uniformity of shape and appearance, but this 
is entirely lacking in nerve cells, as might be expected from the multiform 
duties they are called upon to perform. In order to conduct stimulus from the 
periphery to a central organ, elaborate it, and transmit it to a muscle fiber, 
they have to develop many long offshoots that connect all of these places. 
These offshoots are of two kinds, the neurites and the dendrites. The 
former are usually single, while more than one of the latter commonly emanate 40 
from the cell body. The neurite may pass through the central organ as a 
single unbranched process for a long distance, or it may run from the organ 
to the periphery. It usually has a sheath, and this combination of a neurite 
Fig. 27.-Schematic Representation 
of a Motor Neuron. 
a, neurite; b, dendrite; c, nucleus 
of the nerve cell; d, tigroid sub- 
stance; e, neurofibrils radiating from 
the cell body into the neurites; f, 
collaterals; g, axis cylinder of the 
nerve fiber which leaves the central 
nervous system at h; i, cylindro- 
conical medullary segments; k, Ran- 
vier's constrictions with cement discs; 
I, Schwann's sheath with Schwann's 
nuclei; m, muscular fiber in which the 
axis cylinder ends at n in an antlered 
formation. 
with its sheath is what we call a nerve fiber. Dendrites likewise may become 
nerve fibers, so that in certain cases two fibers may develop from one nerve cell, 
but, as a rule, the dendrites do not attain such great length as the neurites, and 
do not leave the central organ. 41 
It is evident from the above that, although these extremely long nerve 
fibers are integrant parts of the cells, they acquire a certain independence, 
enough so that we may consider nervous tissue to be composed of nerve cells 
and nerve fibers. The nerve cell with the dendrites and the nerve fiber 
coming from it form the nerve unit, the neuron. 
In addition to these specific cells and fibers, nervous tissue contains other 
elements that have the same origin, but play the more subordinate part of filling 
and supporting material. These are the branched glia cells, from the bodies 
of which glia fibers extend in a manner similar to that seen in connective sub- 
stances, which we shall study later. Glia tissue, formed of glia cells and glia 
fibers, we therefore regard as a supplement to nervous tissue. 
Nerve Cells 
Nerve cells vary a great deal in shape, but may be said to have three funda- 
mental forms, spherical, spindle-shaped, and polyhedral, between which every 
possible transition may be found. The shape of the cell is influenced very 
materially by the number of its processes, and we distinguish accordingly as they 
have one, two, or more unipolar, bipolar, and multipolar cells. 
Unipolar cells occur preeminently in the spinal ganglia and in those of 
the cranial nerves, the Gasserian, the geniculate, the superior and inferior of 
the pneumogastric nerve, the petrosal, and the jugular, where they often have 
the shape of a pear, or of an egg. A fairly thick process comes from the cell, 
curls closely about its body in numerous convolutions, and then starts off in 
a straight course (Pl. 29, Fig. 74; Pl. 83, Fig. 182). At a greater or less 
distance from the cell it splits into two branches, one of which goes to the 
periphery as a sensory nerve fiber, while the other enters the spinal cord as a 
fiber of its posterior root. We may call the former a dendrite, the latter a 
neurite. In embryonal life the cell has two processes instead of one, but as 
development proceeds the two draw near together and finally blend at the 
place where they leave the cell. In the lowest vertebrates, the fishes, the cells 
maintain this embryonal condition throughout life. 
Bipolar cells occur in the vestibular and spiral ganglia of the auditory 
nerve. The generally roundish cell body is pointed at each end, from which it 
sends processes, one to the sensory epithelium of the ear, the other to the 
brain as a fiber of the auditory nerve. Here also we may call the former a 
dendrite, the latter a neurite (Pl. 29, Fig. 75). 
The great majority of nerve cells are multipolar. They vary extremely 
in shape, but may be said to belong to two types, one of which is denominated 
the motor, the other the sensory, with every possible transition between them. 
The motor cell has a large, polyhedral cell body from which processes 
radiate in all directions (Pl. 29, Fig. 76; Pl. 79, Fig. 176, vwz; Pl. 81, Fig. 
178, vwz). These processes usually start from the cell body with a fairly 
broad base, taper gradually, continuously divide dichotomously, attain a very 
considerable length, and terminate in quite fine points. These are the dendrites. 
In comparison with them the origin of the neurite, which is always single, is very 
difficult of detection. The neurite arises from a conical elevation of the cell 42 
body as a very delicate, slender thread, which first assumes a considerable 
caliber at a little distance from the cell, and then remains of the same size for 
the rest of its course. Together with its sheath it becomes a nerve fiber, which 
is never the case with the dendrites of the motor cell. 
Very rarely a neurite behaves differently, and suddenly splits up into a 
large number of very fine fibrils soon after it leaves the cell. Such cells are 
said to be of Golgi's type, while those that behave in the ordinary manner 
are said to be of Deiter's type. 
The sensory cell is characterized by an elongated body, which may be 
of a perfect spindle shape in typical cases (Pl. 79, Fig. 176, coz, hhz; Pl. 81, 
Fig. 178, coz). The dendrites come from the two poles of the cell body, and may 
be extremely long, but are never as stout at their origin as those of the motor 
cell. The neurite behaves similarly to that of the motor cell, coming from either 
the cell body itself, or from one of the dendrites. As said above, many transi- 
tional forms exist between these two types, and we often find it difficult to 
determine from its shape alone, in an individual case, whether a cell belongs to 
the one or the other. 
Another peculiarity of the neurites of both motor and sensory cells is 
the possession of collaterals. By this term we understand fine, lateral twigs 
that usually leave the neurite at a right angle and split up into terminal filaments 
after a relatively short course. They end within the central organ, or a 
ganglion, and during its course through the central organ a neurite may give 
off a very large number of such collaterals. 
Neurites terminate in many ways. Motor neurites end in all of the muscle 
fibers in the form of motor end plates (Pl. 70, Fig. 162). Other neurites split 
up at their ends and form fiber baskets, or clawlike end organs, with which 
they embrace other nerve cells, while still others branch and twine as it were 
with their twigs about the bodies and dendrites of other nerve cells. 
The structure of the nerve cell also offers much of interest. As stated 
at the beginning of this chapter, the neurofibrils form their most essential 
component. Like contractile fibrils they are differentiations of the cell pro- 
toplasm. They are absent from the nerve cells in the early stages of embryonal 
life, but gradually develop in the cell bodies and extend out into the processes 
of the latter. They resemble contractile fibrils in being smooth threads, but 
differ from them in always being uniformly isotropic throughout their entire 
length. They have slight refractive power and are therefore comparatively 
difficult to detect when not stained. They are considerably more slender than 
contractile fibrils on an average, yet stout ones are found together with others 
that are very slender, and many observations go to show that their bulk changes 
with the functional condition of the cell. 
The neurofibril is also a much more labile structure than the contractile 
fibril, and undergoes marked changes very soon after death, becoming first 
like a string of pearls, and later breaking down into clear granules. Most 
observers describe these fibrils as solid, but some believe them to be hollow tubes 
that contain fluid. 
Another essential point of difference between the neurofibrils and contractile 
fibrils is that the former branch and form networks. 43 
Neurofibrils are oxyphilic, like contractile fibrils, but are very difficult 
to stain by our ordinary methods. 
The manner in which the neurofibrils are arranged in the body of the 
nerve cell is very inconstant. They are never distributed quite uniformly, 
but radiate into it in bundles from the neurites and dendrites. The bundles 
of fibrils either simply traverse the body of the cell from the neurite to the 
dendrites, or from one dendrite to another, or they form a network within the 
body from which other bundles of fibrils are collected and sent off (Pl. 30, 
Fig. 28.-Motor Nerve Cell of the Spinal Cord (semischematic). 
a, dendrites; b, neurite; c, neuroplasm; d, tigroid substance; e, neurofibrils; f, pigment. 
Fig. 77). These fibrillary nets lie either in the superficial parts of the cell 
body, or pressed close to the nucleus. Both sorts may be found in the same cell, 
and are then joined by bands of fibrils. 
In the neurite the neurofibrils at first lie very close together, but farther 
along they loosen up somewhat and form the axis cylinder of the nerve fiber, 
which will be described later. In the dendrites they are packed less closely. 
Wherever a dendrite branches the fibrils usually form little nets. 
The neurofibrils lie embedded in neuroplasm, which corresponds to the 
sarcoplasm of the muscle cells. It is not always easy to demonstrate this 44 
substance, but it often shows an exquisite vacuolized structure, in which the 
vacuoles are very fine, and have the neurofibrils running in their walls. 
The nerve cell contains a third component which is more striking to the 
eye in a properly killed cell, and can be stained much more easily than either of 
the others, the tigroid substance. The bodies of living or surviving nerve 
cells show an almost homogeneous appearance, with at most, at least in many 
cases, indications of the neurofibrils to be seen within it. Yet if fixing reagents 
that precipitate albumin are used to preserve and stain it, the picture changes 
at once and the cell becomes mottled. This is due to the appearance within the 
cell body of flakelike deposits, which are known under the names of tigroid 
substance, and Nissl's clods, or granules (Pl. 31, Fig. 78). 
These bodies vary a great deal in appearance. In the large motor cells 
they form irregular, angular, or roundish bodies of conspicuous size, which 
often exceed in dimensions the large nucleolus of the nerve cell. Smaller cells 
frequently contain very few large flakes, while in the cells of the spinal ganglia 
they are so small and so closely packed as to make the cell look dusty. Their 
situation also varies; they may form a circle about the nucleus, a perinuclear 
ring, or they may lie wholly in the periphery and form a marginal areola. 
The form and arrangement of the clods are fairly constant for the different 
varieties of nerve cells and are therefore of great help in distinguishing the 
latter apart. Clods are to be found not only in the true cell body, but also 
for a distance in the dendrites, where they have a more elongated, spindle shape. 
The only parts of the cell that are free from them are the neurite and its cone 
of origin (Pl. 31, Fig. 78). 
When we examine these clods with the highest powers of the microscope 
we find them to be composed of fine granules, and it can be perceived that 
they do not form a deutoplasmatic constituent of the walls of the alveolae of the 
neuroplasm, but rather a sort of microsome. 
These clods of Nissl have so great an affinity for basic dyes that they have 
been pronounced basophilic bodies, but this is only when the conditions described 
in the chapter on "The Cell" have not been observed correctly, for they take 
the acid rather than the basic component of a mixture of coloring matters and 
are therefore oxyphilic. Their chemical reactions are as follows: They are dis- 
solved completely by alkalies, like soda and liquor potass®; they are partially 
dissolved by the gastric juice and trypsin; they are precipitated by reagents 
that precipitate albumin. These reactions indicate that the tigroid contains 
a complex body, composed of nuclein and a substance resembling globulin. 
Among the other structural parts of the body of the nerve cell is the 
reticular apparatus, which is similar to that found in other cells of the 
body. It forms a relatively wide meshed network of medium sized threads 
about the nucleus, which may, perhaps, be a system of canals filled with a 
substance that possesses to a high degree the power to reduce osmium terroxide. 
Central bodies do not appear to be present in the adult nerve cell, but 
they are to be found in the embryo in pairs close to the nucleus. 
Another substance very often met with in the body of the human nerve cell 
is a lipoid, or fatty pigment. Usually its granules are very small, of a 
yellowish or brownish color, and may lie massed together, or be distributed 45 
pretty uniformly. The amount of pigment increases as the person grows 
older. These pigment holding cells impart a stain, which is macroscopically 
visible, to many places in the central nervous system, such as the substantia 
nigra, and the locus coeruleus. 
The single nucleus of the nerve cell is distinguished by its size. It is 
almost always globular, and rarely exhibits slight irregular dimples, or indenta- 
tions (Pl. 31, Fig. 78), but in spindle cells it is commonly ellipsoidal in shape. 
As a rule its size bears a certain proportion to that of the cell, but there are 
exceptions in which it is enclosed in a very thin cell body, as in the little granular 
cells of the cerebellar cortex. 
The nucleus of the nerve cell is pool' in chromatin in comparison with those 
of other cells, so that it appears clear and vesicular in the stained specimen. 
The chromatin forms a chromatic membrane, and also a few particles situated 
for the most part about the nucleolus. In addition radiating, oxyphilic cords 
are to be seen running through the nucleus; these are called sometimes oxyphilic 
chromatin, sometimes linin. 
But the most prominent part of the nucleus of the nerve cell is the nucle- 
olus, which attains a size met with elsewhere only in the ovum, and is situated 
more or less exactly in its center. Its staining reaction is not constant; as a 
rule it is purely oxyphilic, like all other nucleoli, but in many cases it shows 
a certain affinity for basic dyes. This indicates that it is composed of two 
different substances, pyrenin and nuclein, or, in the opinion of some authors, 
pyrenin and plastin. Small granules frequently appear in the nucleolus, and 
are known as nucleoluli, or corpusculi intranucleolares. 
Exceptions to this description are to be met with. Thus the nuclei of the 
little granular cells of the cerebellar cortex are rich in basophilic chromatin, 
and this is generally true of all nerve cells in their early stages of development. 
In the course of development the nucleus becomes poorer in chromatin, and the 
latter gradually loses its capacity to stain with basic dyes. 
The Nerve Fiber 
Every nerve fiber originates as an offshoot of a cell situated in either the 
central nervous system, or a peripheral ganglion. This projection of the cell 
forms the most important constituent of the fiber, and we designate it as an 
axis cylinder. It forms the only constituent at the place where the fiber starts 
from the cell, but very soon it becomes surrounded by two structures, the 
medullary sheath and the sheath of Schwann. It would be more correct 
to say that the axis cylinder may become enwrapped by these sheaths, foi' it 
may remain entirely naked, may have but one of the two sheaths, or may have 
both; consequently we may distinguish four different kinds of nerve fibers: 
1. Those that consist of naked axis cylinders ; 
2. Those in which the axis cylinder is covered by a medullary 
sheath; 
3. Those in which the axis cylinder is covered by a sheath of 
Schwann ; 
4. Those that consist of an axis cylinder, a medullary sheath, 
and a sheath of Schwann. 46 
The only nerve fibers that consist of naked axis cylinders throughout are 
found in the olfactory nerve, but the axis cylinders of all fibers are naked 
for a little way at their origins, as has been mentioned already, and become 
so again at their ends, after they have lost their sheaths. These naked initial 
and end pieces may be pretty long under certain circumstances, as in the intra- 
oculai* portion of the optic nerve, and in the sensory posterior root fibers in 
their courses within the gray substance of the spinal cord, but in most cases 
they are quite short. 
Fibers that consist of an axis cylinder and a sheath of Schwann are charac- 
teristic of sympathetic nervous system, and are usually known as non- 
medullated, or Remak's fibers. They are to be found in the ganglia, 
principal trunk and nerves of the sympathetic system, and also in large numbers 
in other peripheral nerves. 
The fibers of all cranial and spinal nerves are composed of axis cylinders 
encased peripherally in medullary sheaths and sheaths of Schwann. 
The axis cylinder is therefore the most essential portion of the nerve 
fibei' and is never absent. It is a round cord, varying in diameter from a frac- 
tion of a micromillimeter to ten micromillimeters (Pl. 71, Fig. 165), that ex- 
tends without interruption from the cell in which it originates to the end of 
the fiber. Its structure also shows it to be a true cell process, foi' it is com- 
posed of the two fundamental parts of the body of the nerve cell, neuroplasm, 
called here axoplasm, and neurofibrils. The axoplasm is a semisolid mass 
of alveolate construction, which is very difficult to demonstrate. The neuro- 
fibrils run parallel to one another in the alveolate walls (Pl. 31, Fig. 79), and 
naturally have the same properties as those of the nerve cell, of which they are 
direct continuations. Just as in the more slender initial portion that has been 
described, they are compressed closely at many places in the fiber and perhaps 
fuse into thicker fibrils. 
The medullary sheath forms a cylindrical mantle about the axis cylinder 
which is not continuous, but is broken into segments, one placed close behind 
another. The length of these medullary segments depends on the thickness 
of the fiber; the thicker the fiber, the longer ai^ the segments. When the fiber 
is very thick they may be nearly a millimeter long. The thickness of the 
medullary sheath varies, but is always less than that of the axis cylinder, and 
may be so extremely thin as not to exceed 5 The fiber appears to be con- 
stricted between the segments because of the absence of the sheath at these 
points, forming what are known as Ranvier's nodes or rings, one of which 
intervenes between every two segments (Pl. 31, Fig. 79; Pl. 83, Fig. 182; 
Pl. 84, Fig. 184, mlrf). 
Each medullary segment consists of two chemically and physically different 
substances, myelin and neurokeratin, which taken together form the 
myelin or nerve medulla. Myelin is a very refractive body which forms 
lumpy masses in water, a property that is also characteristic of lecithin, which 
forms its chief constituent together with protagon and fat. Neurokeratin on 
the contrary has only a slight refractive power and resembles keratin chemically. 
The nerve medulla has the characteristic property, in common with fat, of 
strongly reducing osmic acid and of holding chronhcematoxylin very firmly. 47 
It exists in the medullary sheath in a semifluid form together with a system 
of firm, supporting fibers, which form a superficial, wide meshed network 
that extends from section to section like a funnel down to the axis cylinder in 
such a way that each medullary segment in turn consists of quite a number 
of cylindrical pieces that overlap one another somewhat at their ends. These 
are called the cylindo-conical segments (Pl. 31, Fig. 79). 
The ends of every two medullary segments are separated from each other at 
Ranvier's constrictions by a plate of cement substance that surrounds the axis 
cylinder, the intermediate disc. Like all cement substances it reduces 
silver salts, so that if a fresh medullated nerve fiber is treated with a solution 
of silver nitrate a transverse trabeculum appears at this place, and is often 
crossed by a longitudinal one, forming what is known as Ranvier's cross. 
The sheath of Schwann, or the neurilemma, is nucleated, unlike the 
medullary sheath, and is to be regarded as the matrix of the latter, as the nerve 
medulla is formed from it, or from the cells of which it is composed. On the 
fresh nerve fiber it appears as a quite pale outline external to the medullary 
sheath and broken like it by Ranvier's nodes, while firmly adherent to the 
intermediate discs. In general it is structureless, but a granular protoplasm 
can be seen in many places, which is massed somewhat about the nucleus. Each 
nucleus in the human sheath always corresponds to a medullary segment, so 
that we may also speak of the nucleated segments of the sheath of Schwann 
(Pl. 31, Fig. 79; Pl. 71, Fig. 165). 
Glia Tissue 
Although nerve and glia tissue have exactly the same origin they develop 
in quite different directions. While the one develops into a typical stimulus, 
conducting and elaborating tissue, the other assumes rather the type of con- 
nective substance, as shown by the fact that its cells compose a fibrous sub- 
stance, although lacking in its independence. Glia tissue consists of glia cells 
and glia fibers. 
Glia tissue is also known as neuroglia. It is found outside of the central 
nervous system only in the optic nerve. It serves as a supporting and filling 
substance and is perhaps of value in the isolation of nerve conduction. 
Glia cells present two modifications,-the ependymal cells and the 
astrocytes. The ependymal cells form the epithelial covering of the system 
of cavities found in the central nervous system, lining the central canal of the 
spinal cord and the ventricles of the brain, yet undergo a sweeping reduction 
in adults. In young persons they form a single layer of cylindrical or cuboidal 
■cells, the free surfaces of which look into the ventricles, or the central canal, 
and are provided with cilia. At their opposite ends they taper into processes 
that extend for longer or shorter distances into the brain or spinal cord (Pl. 80, 
Fig. 177, ep). 
The astrocytes, on the contrary, are to be found everywhere in the central 
nervous system scattered about amongst the true nerve cells and forming 
masses of glia in many places. Their special relations will be discussed later in 
the description of the central nervous system (Pl. 80, Fig. 177, glz). 48 
The astrocytes are, as their name implies, for the most part branched, 
stellate cells with small bodies and many processes. The latter either may 
radiate from the body in all directions, or may have a more polar arrange- 
ment when the cells are more spindle-shaped. In man the astrocytes usually 
contain several nuclei, which vary a great deal in appearance, and are often 
lobulated, hour-glass shaped, or perforated. They differ from the nuclei of 
the nerve cells by being smaller, and by appearing darker than those clear, 
vesicular nuclei when ordinary staining methods are employed, because they 
are always fairly rich in basophilic chromatin. 
Glia fibers are uniform threads of greater oi' less thickness and often of 
extraordinary length. They originate in the bodies of the astrocytes and, 
when fully developed, run through these bodies and their offshoots in such a way 
that they may be said to be intracellular for at least a large part of their course. 
Probably this is not true of all glia fibers; it is more likely that some of them 
may dissociate themselves completely from their cells of origin and become 
independent. The substance of the glia fibers is not the same as that of the 
protoplasm of the astrocytes, of which the fibers are differentiation products. 
These glia fibers envelop the central nerve cells with networks, wind every- 
where between the central nerve fibers, for a sort of external limiting membrane 
on the outer surface of the central organ, and develop similar membranes that 
separate all of the vessels of the central nervous system from the nerve tissue. 
The Development of Nervous Tissue 
It may be well to glance briefly over the histogenesis of nervous tissue in 
order that we may understand better its construction. 
The entire nervous tissue of the human body originates from an inva- 
gination of the ectoderm, the neural canal, the wall of which consists, at its 
end, of several layers of ovoid and cylindrical cells. These are at first absolutely 
undifferentiated embryonal cells, but very soon they begin to develop in two 
different directions and to form neuroblasts and spongioblasts. The 
neuroblasts may be identified at an early stage by the fact that neurofibrils 
with a netlike arrangement develop in their bodies, advance peripherally in the 
direction of the principal axis of the cell, and form the axis cylinder process 
or neurite. The dendrites develop later from the body of the neuroblast, which 
thereupon becomes a nerve cell. 
The spongioblasts are the mother cells of the glia tissue. They are cylindri- 
cal, originally reach from the lumen of the neural canal to the so-called marginal 
film, and likewise give off many processes. Part of them gradually draw back 
from the inner surface of the neural canal and come to lie with their cell 
bodies between the neuroblasts, while the remainder form an epithelium that 
lines the neural canal, thus giving origin to the astrocytes and the ependymal 
cells. 
All of the nerve cells that compose the peripheral ganglia come from cells 
that have migrated from, or have separated themselves from the neural canal. 
In all of these cases the neuroblasts are escorted by indifferent cells, which 
accompany nerve cells everywhere and are to be found in all ganglia. 49 
The peripheral nerves grow in the way already described from neuroblasts 
situated either centrally or peripherally. Up to a certain stage in its develop- 
ment a peripheral nerve consists wholly of neurofibrils and axoplasm; then the 
indifferent cells just mentioned accumulate along it and enclose it with a sheath 
of Schwann, which is therefore the primary sheath of the nerve. This may 
remain its only coat, or nerve medulla may develop in its cells and gradually 
enwrap the axis cylinder with a medullary mantle. In the central organ, where 
the sheath of Schwann is not found about the nerve fibers, the function of form- 
ing the medulla is taken over by the glia cells, which are, to a certain degree, 
the functional equivalents of the cells of Schwann. 
The Supporting Tissues 
The final class is the supporting tissue, which differs in many ways from 
the other three. This originates from the mesenchyma, i.e., from cells that 
leave the inner, middle and perhaps also the outer germinal layers and interpose 
themselves between the primary blastoderms. Only one supporting tissue 
develops directly from the entoderm, the chorda dgrsalis, and this is of only 
transitory importance in man. 
The tissues that have been heretofore described have active functions in 
the way of secretion, absorption, conduction, etc., but supporting tissue plays 
a passive part, giving, when fully developed, support and stability to the body. 
It not only forms the skeleton, but it is also a constituent of almost every organ 
in the human body, which it enwraps in special envelopes, separates one from 
another, and often enters and subdivides into smaller portions. 
Supporting tissue also displays its passive character in that, at least in 
most cases, its essential part is situated not in the cells, but in an intercellular 
substance that is secreted by them and, if present at all in other tissues, is 
so to a very subordinate degree. This intercellular substance may or may not 
have a definite form; it may be soft and like mucus, firm and flexible, or hard 
and rigid, so it is best to differentiate this tissue in accordance with the nature 
of the intercellular substance present with it. 
We may first divide supporting substances into two large groups, those 
with little or no intercellular substance, and those with such substance. 
1. Supporting tissue with little intercellular substance. A sup- 
porting function can be exerted by such a tissue as this only by the cells them- 
selves when they are placed close together and a certain power of resistance is 
imparted to them by the development of fluid within them, and the formation 
of a firm, membranous crusta or pellicle. This class includes the chordal 
tissue and the vesicular supporting tissue. 
2. Supporting tissue with intercellular substance. This large 
group, which embraces the great bulk of the supporting tissues, needs to be 
subdivided according to the nature and formation of the intercellular substance. 
These subdivisions are: 
a. One in which the intercellular substance is soft, of mucous or fluid con- 
sistence, and may contain fibrils. This includes the gelatinous and the 
vitreous tissues. 50 
b. One in which the intercellular substance consists wholly of fibrils that 
are not intercellular for the most part, but remain enclosed in the cell bodies. 
The reticulated or adenoid supporting tissue belongs in this category, 
and we might add the neuroglia tissue which has already been described. 
c. One in which the intercellular substance consists of fibrils that are free 
from the cells to a much greater extent. No other material is added in either 
this variety, or in any of the preceding. Here belongs the true connective 
tissue with its subgroups to be described later. 
d. One in which the intercellular substance consists of fibrils which are 
consolidated by a secretion from the cells which is of firm consistence and hyaline 
appearance. This embraces cartilaginous tissue. 
e. One in which the intercellular substance consists of fibrils that are 
impregnated with lime salts and unite to form lamellae which are joined together 
by a cement. This tissue possesses firmness and the power of resistance to an 
extraordinary degree, and furnishes in the osseous tissue the most perfect 
type of support. 
The Chordal Tissue 
The support of the embryonal human body in the early stages of its develop- 
ment is furnished by a cord situated between the neural and the visceral tubes 
in the axis of the body and known as the chorda dorsalis, or notochord. 
This will receive only a brief mention here as it is of quite subordinate impor- 
tance in the adult body. Its tissue consists of chordal cells, which are 
vesicular, vary in size from medium to large, and have shapes that depend 
materially on the closeness with which they are packed together. A fluid is 
formed within the cells by a liquefaction of the protoplasm, and the turgor 
thus produced imparts a certain degree of firmness to the tissue. Externally 
a very dense ectoplasm encloses the cell as with a membrane, and may contain 
fibrillary formations, the so-called tonofibrils. The chordal cells are covered 
by a layer of small protoplasmic cells, the chordal epithelium, which is 
their matrix, and then follows the chordal sheath that envelops the entire 
chord. In proportion as the vertebral column approaches development the 
notochord disappears, so that at birth only some slight traces of its tissue 
remain in the so-called gelatinous nucleus of the intervertebral discs. 
The Vesicular Supporting Tissue 
The principal characteristic of vesicular supporting tissue is the presence 
of large vesicular cells that contain fluid under sufficient pressure to keep them 
tense. Externally each cell is enclosed in a sort of membrane that resembles a 
crusta and, like it, is immediately connected with the protoplasm of the cell 
body. As a rule, the cells lie together in large masses with true connective tissue 
fibrils between them (Pl. 15, Fig. 44). 
Although this tissue is of frequent occurrence in the lower animals, it is 
found in man in only a few such places as the little sesamoid bones in the 
tendon of the peroneus longus. 51 
The Gelatinous Tissue 
This forms in man and the higher mammals in the forerunner of fibrillary 
connective tissue, and is to be found in its pure form only in young embryos, 
as it early passes through transition stages into connective tissue by the develop- 
ment of fibrils. It is preserved longest in the Wharton jelly of the umbilical 
cord. Gelatinous tissue is composed of medium sized cells that are not aggre- 
gated very densely and are provided with numerous processes which anastomose 
and form a protoplasmic network having more or less large meshes. These 
meshes are filled with jelly, a secretory product of the cells, which therefore 
represents here the intercellular or basic substance of supporting tissue. This 
jelly is of semisolid consistence and is probably a mixture of true mucin, which 
is insoluble in water and can be precipitated by acetic acid, and mucoids, which 
are soluble in water and cannot be precipitated in that way. 
The Vitreous Tissue 
Although the greatly branched cell is an important constituent of the gela- 
tinous tissue, the vitreous contains either none at all, or only a few unimportant 
ones in places. The place of the cellular network is taken by a maze of extremely 
delicate fibrils that branch, anastomose, and form meshes, some close, some 
wide, according to their location. These meshes contain a fluid known as the 
vitreous humor, which contains mucoids, but no mucin. Unlike the gelatinous 
tissue the vitreous is not exclusively of mesenchymatous origin, for cells from the 
ectoderm, particularly those of the inner layer of the retina and of the lens, take 
part in its formation. They send out very minute processes and form the 
matrix of the vitreous tissue, and later have added to them wandering cells 
from the mesenchyma. 
The Reticular Tissue 
Reticular shows an unmistakable likeness to gelatinous tissue, of which it 
is simply a development in that it is formed fundamentally of branched anasto- 
mosing cells, but there is no gelatinous basic substance filling the meshes of the 
network. The characteristic feature of this tissue is the differentiation of fibrils 
in the cells, which are not confined to the cell bodies, but extend into the pro- 
cesses and so form a second fibrillary network within the protoplasmic net. 
In time the fibrils become strong fibers with many branches, but they always 
maintain their intracellular position, though often they are quite superficial 
and are covered by only a very thin layer of protoplasm (Pl. 15, Fig. 43). 
This is preeminently the form of the supporting tissue of the lymphatic 
organs. When lymph cells accumulate in its meshes it becomes adenoid or 
lymphoid tissue. It is found in the lymphatic glands, the tonsils, the follicles 
of the intestinal wall, the thymus, the spleen, and it also forms the tunica 
propria of the stomach and the intestines. 52 
The Neuroglia Tissue 
Though this has been dealt with under the description of nerve tissue it 
should be mentioned in this place. Like the vitreous it is of ectodermal origin, 
and consists of cells and intracellular fibers like the reticular tissue, but differs 
from the latter in that its cells do not form a network, and its cell processes 
run out freely. Neuroglia fibers also have a greater degree of independence 
than those of the reticular tissue, as they are in places intercellular in their 
courses. 
The Connective Tissue 
Connective tissue, also known as fibrillary connective tissue, is the most 
widespread of all the supporting substances in the human body. It comes from 
gelatinous tissue and is therefore of mesenchymatous origin. It is characterized 
by the development of an intercellular substance composed of fibers which casts 
the cells far into the background. We need to consider it minutely because 
of its importance, and will first study more closely its two components, its cells 
and its fibers. 
Although the cells are greatly overshadowed by the fibrous intercellular 
substance, the part they play is very important and they show a great diversity 
in form and construction. They are all derivatives of those branched, mesenchy- 
matous cells with which we became acquainted in gelatinous tissue, with the addi- 
tion of numerous blood cells that, at a later period, wander out from the vessels 
and entci' the gaps and chinks to be found everywhere between the connective 
tissue fibers. Mesenchymatous cells have a great capacity for transformation. 
They may produce fibrous intermediate substance and thus pose as fibroblasts, 
the true connective tissue cells, or they may undertake other functions and, 
without producing any intermediate substance, may sever their connections 
with their neighbors, draw in their processes, cause their bodies to assume a 
spherical form, and then, like certain colorless blood cells, make amceboid move- 
ments and become known as wandering cells. Still, all of the wrandering cells 
of the connective tissue are not mesenchymatous, many of them are lymphocytes 
that have migrated from the blood vessels. The two varieties possess the same 
characteristics and cannot be distinguished apart later. It is also uncertain 
but what fibroblasts of the perfected connective tissue may in later life be trans- 
formed likewise into wandering cells. These wandering cells undergo many 
changes; they may become stationary again and form clasmatocytes, or they 
may become plasma cells, or mast cells, through special changes in their 
bodies. As these changes may all occur in a cell that has wandered into, or has 
originated at a place, we must look upon both the connective tissue cells and the 
colorless cells of the blood as ancestors of these various forms. We will now 
study rather more closely the individual kinds of cells met with in the fully 
developed connective tissue. 
The characteristics of the mesenchymatous cells remain fairly well preserved 
in the fibroblasts, often called the fixed connective tissue cells (Pl. 12, 
Fig. 36, fbl), which are large and stellate, have numerous processes, and have 
shapes that depend materially on their locations. By means of their processes 53 
they are closely attached to the fibers of connective tissue and often accompany 
them for considerable distances. The processes of neighboring cells frequently 
anastomose, but, as a rule, without forming a closed protoplasmic network. The 
arrangement of the cells, like their shapes, is dependent to a great degree on 
their location; they are to be found scattered about, grouped in irregular 
masses, and in regular formations. 
An important attribute that fibroblasts have in common with many other 
kinds of cells is the power to appropriate fat. The fat of the human body 
is a variable mixture of free fatty acids, stearic, palmitic, and oleic, and neutral 
fats, i.e., the propenyl ethers of these acids. It is introduced into the organism 
with the food, broken up in the intestine, absorbed by the cells of the intestinal 
epithelium, recombined, and carried to all of the organs by way of the circula- 
tion of the blood and lymph. In the fibroblast it makes its first appearance in 
the form of very minute, strongly refractive particles that blend very soon 
into large drops, which may enlarge so much by accretion as to reduce the cell 
protoplasm into a narrow marginal zone about the nucleus. This we call a 
fat cell (Pl. 14, Fig. 42). 
Under the microscope these drops of fat become evident through their great 
refractive power; each acts as a convex lens. Another characteristic is the 
deep black stain imparted to them by osmic acid in consequence of their 
deoxidation power. They may also be stained red by such dyes as sudan and 
scarlet. 
Pigment is very often met with in the body of the fibroblast, where it 
appears in the form of little yellow, brown, or black granules that are embedded 
in the protoplasm and may be agglomerated into large masses. Fibroblasts 
that are thus pigmented are called pigment cells (Pl. 14, Fig. 41). They 
can be recognized from the facts that their bodies are much branched, and that 
they are highly capable of moving about so as to be able to carry the pigment 
from one place to another, as, for example, to the epithelium. Whether the 
pigment comes from the blood, or is elaborated in the cell bodies themselves, re- 
mains an open question. It is also uncertain whether the pigment cells, which 
play an important part in the lower animals under the name of chromatophores, 
are really mesenchymatous elements, or may not be wandering cells from the 
ectoderm. 
The wandering cells are derived from the mesenchyma, as before stated, 
in that the bodies of the mesenchymatous cells become rounded and mobile. 
They multiply greatly and wander everywhere throughout the connective tissue. 
In later embryonal life this transformation of fibroblasts is greatly over- 
shadowed by the immigration of large numbers of lymphocytes from the blood 
vessels, which also become wandering cells. Wandering cells vary a great deal 
in form and size, but are always smaller than the fibroblasts. This is also 
true of their nuclei, which are always very rich in chromatin and are often 
irregular in shape. The cell bodies exhibit no peculiarities aside from the 
changes in shape brought about by the amoeboid movements. The wandering 
cells are to be met with in all parts of the connective tissue and elsewhere as well, 
for they frequently pass out of this tissue into the epithelium that covers it, 
and into the cavities of the body (Pl. 1, Fig. 3; Pl. 8, Fig. 26; Pl. 9, Fig. 27; 54 
Pl. 11, Fig. 35). The most important attribute they possess is the capacity to 
surround and incorporate foreign substances through their power of amoeboid 
movement. They thus dispose not only of dead constituents of the normal 
body, but also of elements that have penetrated from without, foreign bodies, 
bacteria, and the like. In doing this they play an extremely important part in 
the economy of the body by rendering harmful substances innocuous and have 
consequently received the name of phagocytes. 
The clasmatocytes are similar in certain ways to the fibroblasts; they are 
of the same size and branch in a like manner (Pl. 12, Fig. 36, cl). The nuclei 
are always rather smaller, the cell bodies are basophilic and always contain very 
variable granules capable of being stained, scattered irregularly through them. 
Modern researches seem to show that clasmatocytes are nothing else than rest- 
ing wandering cells. 
Plasma cells are more rounded and are to be found but sparsely in the 
connective tissue. The nuclei are small and have near them distinct central 
bodies within a clear areola. The protoplasm is more strongly basophilic than 
that of the clasmatocytes. We must regard them as special modifications of 
wandering cells. 
A very characteristic form of the connective tissue cells has been named 
by Ehrlich mast cells, and he believes that they are produced from fibroblasts 
by fattening (Pl. 12, Fig. 36, mz). They are roundish, or rather elongated 
cells without processes, which are very numerous in the subcutaneous connective 
tissue and are characterized by the presence of many large granules within 
their cell bodies. While these granules stain with basic dyes they do not take 
the color of the dye inself, but rather that of its free color base (Pl. 50, Figs. 
119 and 121, mz). For example, thionin is the base of thionin hydrochlorate; 
the color of the dye is blue, that of its base is red, and when mast cells are 
stained with this dye the nuclei become blue while the granules become red. The 
granules of the mast cell share this attribute, which is called metachromasia, 
with many other constituents of tissue, for instance mucus. Mast cells are to 
be found only in the connective tissue, never in the blood. They multiply by 
mitosis and originate perhaps from the clasmatocytes. 
A final constant constituent of connective tissue is found in the eosinophile 
leucocytes, which are also to be found normally in the circulating blood 
(Pl. 12, Fig. 36, coz). They are of medium size and contain in their bodies 
many fine granules which usually surround the nucleus closely, are oxyphilic, 
and are stained with peculiar intensity by eosin. The form of the nucleus varies ; 
it may be dumbbell-shaped, lobulated or annular. Whether all of these cells 
have migrated from the blood vessels, or whether some at least are derived from 
the wandering cells in the connective tissue, still remains an open question. 
The intercellular substance of the connective tissue consists of fibers 
which arise in the superficial layers of the cells, but latei' become free and then 
show ability to grow independently. They are divided into two varieties, the 
collagenous and the elastic fibers, in accordance with their chemical, physi- 
cal, and morphological characteristics. 
The collagenous fibers, also called simply connective tissue fibers, 
generally resemble bands, but sometimes round cords, that vary in length and 55 
thickness (Pl. 12, Fig. 36, coif). An indistinct longitudinal striation, that may 
often be noticed, is the imprint of the fact that they are composed of very 
minute fibrils bound together with a very small quantity of cement. The 
cement may be dissolved away by treating the fiber with lime water, or baryta 
water, so as to allow the rather swollen fibrils to become free. The fibers unite to 
form bundles of greater or less thickness, and the individual bundles may branch 
and join together to form networks, but this is not always the case. 
As regards their physical and chemical properties the collagenous 
fibers are colorless, elastic, have only a slight luster, and are doubly refractive. 
They swell in aqueous dilutions of alkalies and acids, especially of acetic acid, 
into an almost homogeneous, glassy mass. In the first stage of this swelling 
bundles are to be seen that appeal' to be bulging in places, while between these 
bulges the swelling of the bundle is impeded by encircling cell processes. These 
fibers become completely dissolved in water heated in 120°C. and form a gelat- 
inous mass on cooling. The dissolution takes place more easily if the fibers 
are first caused to swell by the addition of dilute acetic acid, and then heated. 
Chemically the fibers consist of collegen, and albuminoid that is changed 
by boiling into glue, or gelatin, to which they owe their name. Another im- 
portant peculiarity of these fibers is that after they have been swollen by 
treatment with an acid they become dissolved by the gastric or the pancreatic 
juice with the formation of gelatose. As regards staining they are oxyphilic. 
The elastic fibers vary much in caliber; sometimes they are immeasurably 
slender, but in many cases they have a considerable thickness (Pl. 12, Fig. 36, 
elf; Figs. 37 and 38). They possess quite a number of marked characteristics 
that enable us to differentiate them easily from the fibers just described. They 
can never be made out to be composed of fibrils. They incline strongly to 
branch and anastomose, and they may form networks so extensive as to look 
like fenestrated membranes through a broadening of the fibers with a cor- 
responding diminution in the size of the meshes. As implied by their name they 
are much more elastic than the collagenous fibers. When cut across they retract 
with a characteristic rolling in of their free ends. Wherever they are collected 
in large numbers they present a yellowish white color that distinguishes them 
clearly. Elastic fibers have a much greater luster than the collagenous, in con- 
sequence of their higher power of refraction, but they are less strongly aniso- 
tropic. They also exhibit a much greater power of resistance to physical and 
chemical agents; they do not swell, but simply become dissolved in concentrated 
mineral acids, yield no gelatin on boiling, and are more resistant to the gastric 
and pancreatic juices. They consist of elastin, an albuminoid closely related 
to collagen, but containing less sulphur, which shows affinity to both acid and 
basic dyes. In microscopical preparations the elastic fibers, when present in 
small numbers, are not easy to distinguish from the mass of collagenous, but they 
can be brought out at once, when the tissue is fresh, by the addition of a drop 
of dilute acetic acid, as this causes the collagenous fibers to swell and leaves the 
elastic fibers sharply differentiated. 
Mention should be made here of a peculiar kind of fibers found in only a 
few places in the human body, e.g., in the parenchyma of the liver, the lattice 
fibers. These are for the most part very delicate and tortuous, for exquisite 56 
networks, and are to be distinguished from the elastic fibers through their 
chemical and staining reactions. Chemically they resemble collagenous fibers, 
into which they change in many places, but they must differ as they are much 
more unyielding and can be demonstrated only by the aid of the gold and silver 
impregnation method. 
Now that we have learned to recognize the various elements of which con- 
nective tissue is composed, we will turn our attention to the various forms in 
which this tissue appears in the human body, which are, in accordance with the 
arrangement or character of the intercellular substance, and the peculiarities of 
the fibroblasts, loose connective tissue, tight connective tissue, elastic 
tissue, pigmented connective tissue, and fat tissue. 
1. Loose connective tissue is distributed very widely throughout the 
human body. It forms the supporting framework of each of the parenchyma- 
tous organs, frequently subdivides these into lobes and lobules, joins the skin to 
the musculature, separates the muscles, forms the sheaths of the muscular fas- 
ciculi, and joins many organs together. The characteristic feature of this 
tissue is that its bundles of fibers have no regular arrangement, but cross and 
interlace in all directions. Wide interspaces exist between the bundles, which 
are filled with lymph, afford abundant opportunity to the wandering cells 
for their movements, and are in open communication with the system of lym- 
phatic vessels of the body. 
2. Tight connective tissue originates from the preceding under the 
influence of such mechanical forces as tension and pressure, which force the 
bundles of fibers tightly together and into regular courses to form cords or 
plates. It is found in the tendons, fasciae, aponeuroses, and ligaments, as well 
as in the sclera and cornea. The interspaces filled with lymph, that exist in loose 
connective tissue, are naturally done away with either entirely or for the most 
part, leaving only room between the bundles for the cells. The latter also 
lack the profusion of form that typifies those of loose connective tissue. They 
are chiefly fibroblasts arranged characteristically in columns or rows, one cell 
directly adjoining another. Thus in tendons the connective tissue fibers form 
bundles that run longitudinally with the cells lying between them in longitudinal 
rows. Each of these cells sends out winglike processes that penetrate between 
the tendinous bundles and surround them with an incomplete protoplasmic 
tunic. The fibroblasts, the so-called tendon corpuscles, thus come to present 
a stellate picture in cross sections of tendons, while in longitudinal sections cell 
bodies are to be seen irregularly following each other (Pl. 13). Loose connec- 
tive tissue is moreover to be found associated with the tight in such organs as are 
composed of the latter. 
When the amount of elastic fibers is considerably in excess of that of the 
connective tissue fibers we speak of an elastic tissue. This is usually met with 
in the form of coherent membranes, networks, or felted fibers, as in the walls of 
the blood vessels, and in the respiratory organs, but it may also be found in 
larger masses, as in the ligamentum nuchae, which is only rudimentary in man. 
When fully developed it is composed of thick elastic fibers which run parallel 
to one another, are sheathed with cells in the same way as tendon fibers, and 
are separated from one another by collagenous connective tissue (Pl. 12, 57 
Fig. 88). It can readily be distinguished macroscopically from either loose or 
tight connective tissue by its pronounced luster and its somewhat yellowish 
color, which has caused it to be called sometimes yellow connective tissue. 
Pigmented connective tissue, or pigment tissue, is found in the 
human body only in the middle coat of the eye and in the iris. It is characterized 
by the fact that the fibroblasts have taken up pigment and become pigment 
cells, while between them lie networks of connective tissue and elastic fibers 
(Pl. 87, Fig. 188, chp). 
Fat tissue occurs in the form of little lobules joined together into larger 
masses by loose connective tissue (Pl. 93, Fig. 197, sbc). Each lobule is com- 
posed of fat cells, i.e., of fibroblasts the bodies of which have become distended 
with fat. Connective tissue fibers appear, though very sparsely, between the 
individual fat cells. In well nourished, strong persons this tissue attains a high 
degree of development in the panniculus adiposus of the skin. It likewise 
forms a fatty capsule in the subperitoneal tissue about the kidney, and ap- 
pears in great masses in the mesentery and omentum. When fat tissue is 
caused to retrograde by a suitable diet the nature of the change depends on the 
age of the individual; in young persons the fat cells change back into ordinary 
fibroblasts while their fatty contents atrophy, and the transformation is thus 
made into loose connective tissue, but in old persons the cells atrophy and a 
mucous fluid accumulates in them. 
Cartilaginous Tissue 
The intercellular substance of cartilage consists of the two constituents 
which form that of connective tissue, collagenous and elastic fibers, with the 
addition of an amorphous basic substance outside of these fibers, that is 
excreted by the cells and may cover up completely all of the formed constituents. 
This it is that stamps cartilage with its own physical properties and gives it a 
certain firmness combined with flexibility which renders it a true supporting 
substance. In order that we may understand the condition better it seems 
advisable that we review briefly the development of cartilage. 
Cartilaginous tissue comes from the same mesenchymatous cells as connec- 
tive tissue. At the places where it begins to appear these cells first lose their 
characteristic stellate form and become little roundish cells, lying close together, 
which are known as chondroblasts. Each of these embryonic cells gives off 
collagenous fibrils, but at the same time changes the peripheral layers of its 
body into a crusta, or sort of membrane, which remains throughout in con- 
tinuous connection with the cell body, has the same staining reaction, and is 
therefore oxyphilic. The cells are thus moved somewhat apart from one another, 
and the cartilage enters upon the prochondral stage. We have here con- 
ditions similar to those presented by the vesicular supporting tissue, which may 
therefore be considered as intermediate between connective and cartilaginous 
tissues. Then comes the characteristic excretion from the bodies of the cartilage 
cells of a fluid containing chondroitin-sulphuric acid, which gives this 
substance the marked basophilic property that causes it to contrast with cell 
protoplasm. This substance naturally first permeates the crusta of the cell so 58 
that it may be differentiated sharply from the cell body by staining. At the 
same time the crusta and the body separate morphologically, and a pellicle or 
cell capsule comes from the former. With this the cartilage enters its proto- 
chondral stage. The further development can be readily understood; a 
secretion conies unceasingly from the cells, appears between them as a basic 
substance and so surrounds and envelops the collagenous fibrils that they escape 
ordinary observation. The oxyphilic collagenous fibrils spread farther apart, 
and the protochondral passes into the chondral stage. 
We have therefore to differentiate in fully developed cartilaginous tissue 
the cartilage cells and the basic substance, the latter of which envelops the 
fibrils. Ordinarily these fibrils cannot be detected readily in the basic sub- 
stance, which alone is to be seen and gives the tissue the uniform, hyaline appear- 
ance of hyaline cartilage, but they may be more or less prominent, and then 
we speak of a Rbrocartilage. In both varieties the fibrils are collagenous. 
When elastic fibers appear in the basic substance they exhibit a vary marked 
tendency to the formation of networks, and we have elastic cartilage, or 
reticular cartilage 
Hyaline Cartilage 
Hyaline cartilage is found in man chiefly during a temporary stage in the 
development of the body framework, in which it exists alone for a considerable 
period during embryonal life as the primordial skeleton. Although it at- 
tains a firmness that is not to be underestimated, it is unequal to bear the 
burden of a human body, and is replaced by the far stronger osseous tissue 
during the second half of fetal life, or in part after birth. The lower we descend 
among the vertebrates the greater is the part we find this cartilage to play in 
the final skeleton, and in the lowest, the cyclostomata, the selachii, and many 
ganodei, it does not go on to the development of bone anywhere.. In human 
adults hyaline cartilage is to be found chiefly as a covering to the ends of bones 
that come together to form joints, symphyses, or synchondroses, in places 
where tendons run in bony grooves, or some similar condition exists. It also 
forms the framework of the nose, larynx, trachea, and bronchi, but it should be 
noted that ossification take's place later in life in many of these places. 
As regards the physical aspect of hyaline cartilage, it has a peculiar, unique 
consistence, is firm, clastic, flexible, easily cut, is bluish white in color, and is 
feebly lustrous. In thin layers it is translucent and doubly refractive. This 
latter property depends on the content of collagenous fibrils in its basic sub- 
stance. 
The cartilage cells lie in cavities, called cartilage cavities, two or more 
often together (Pl. 16, Figs. 45 and 46). The cells are of medium size, rich 
in protoplasm, and vary a great deal in shape on account of the limited quarters 
furnished by the cartilage cavities, in which they lie pressed together so closely 
as to flatten each other. They fill the cavities completely, and yet are quite 
sensitive to mechanical and chemical influences, which cause them to shrink back 
from their capsules and to shrivel. The same result can be produced by passing 
an electric current through living cartilage, the cells retract and do not return 
to their former shape. 59 
The cell body is usually vacuolized (Pl. 16, Fig. 46). Many plasmosomes, 
some smaller, some larger, lie in the walls of the vacuoles, with near them thread- 
like rows of granules, the mitochondria. The contents of the vacuoles consists 
chiefly of glycogen, which is quite a characteristic constituent of cartilage 
cells. It appears in them in the form of large or small granules during fetal 
life, even after the basic substance has been formed, and is demonstrable in them 
until old age. The cartilage cell also contains minute particles of fat, that 
may blend into large drops, which appear before the glycogen and gradually 
disappear at the age of puberty. 
Each cartilage cell contains a moderately large nucleus, which is usually 
spherical, though it may be ellipsoidal in the peripheral parts where the cells 
are larger (Pl. 16, Fig. 45). In young, growing cartilage the nuclei may often 
be seen in mitosis, and cells with two nuclei are frequently to be observed. In 
the latter cases the segmentation of the cell body did not follow that of the 
nucleus immediately (Pl. 16, Fig. 46, b). 
The basic substance of hyaline cartilage appears in the fresh section 
to be uniform, dull, and without structure, except that immediately about 
the cells are to be found the cartilage capsules, which become evident as 
narrow edges through their somewhat stronger refractive power. More of its 
structure is revealed when the cartilage is fixed in a suitable manner. Then it 
can be seen that the entire basic substance is divided into two portions, the cell 
territories, and the framework. The cell territories are islands of basic 
substance that enclose two or more cartilage cells with their capsules, and are 
separated from one another by a net or framework composed of the rest of 
the basic substance. Still more details may be brought out within the cell 
territories if we stain with a suitably chosen mixture of an acid and a basic dye 
(Pl. 17, Fig. 47). Each cartilage cell is enclosed in a strongly basophilic 
cartilage capsule, those of neighboring cells frequently blending. About 
the cartilage capsule, enclosing two or more encapsulated cells, lies a rather 
broader ring which is much less basophilic ; this we call the inner cell 
areola. Several inner cell areolae are finally embraced by an outer cell 
areola, which is not basophilic, but oxyphilic. The outer cell areola together 
with its contents constitutes a cell territory. The framework may be dis- 
tinguished from the outer cell areola by the fact that it is much less oxyphilic, 
oi' is perhaps very feebly basophilic. 
The entire basic substance of hyaline cartilage is permeated by collage- 
nous fibrils. These can scarcely be seen in fresh tissue, and only under very 
favorable circumstances, as in the superficial layers of a joint cartilage. If we 
first treat the tissue with gastric or pancreatic juice they can be seen very well 
with the polariscopic microscope. They run in parallel courses about the car- 
tilage cells and form a dense felt of minute fibers in the framework. The 
differences of stain mentioned above to be seen in different parts of the basic 
substance depend on the preponderance in any place of the oxyphilic collagenous 
fibrils, or of the basophilic basic substance. When the former predominates, 
as in the outer cell areola, the tissue appears to be oxyphilic, while if the latter 
has the ascendancy the collagen is obscured and basophilia prevails, as in the 
capsule and the inner cell areola. 60 
Next to collagen the most important constituent of the cartilaginous basic 
substance is chondroitin-sulphuric acid, a monobasic nitrogenous acid, 
which is combined with an albuminoid and gives the substance its basophilic 
reaction. It is secreted by the cartilage cells, a fact that explains why the 
parts of the basic substance lying nearest to the cells exhibits the strongest 
basophilia. When cartilage is boiled the collagen contained in it yields a 
gelatinous substance known as chondrin, which differs from gelatin in its con- 
tent of the compound of chondroitic acid with albuminoid, and of the mineral 
salts of the same acid. 
A deposit of lime salts almost always takes place in old age in the basic 
substance of cartilage, which renders the elastic, flexible tissue rigid and brittle. 
These salts appear first about the cartilage cells in the form of very minute 
particles, which soon unite to form large, calcareous masses. Cartilage can also 
ossify in latei- life, i.e., its tissue may be destroyed and replaced by that of bone. 
Calcification and ossification of cartilage are therefore two fundamentally dif- 
ferent processes. 
Its tissue changes at the periphery of the cartilage, its basic substance con- 
stantly decreasing and its cells drawing closer together, and passes over into a 
compact, firmly built connective tissue permeated with elastic fibers, the peri- 
chondrium, which separates it from the surrounding loose connective tissue 
(Pl. 16, Fig. 45). The perichondrium is rich in cells, and its fibroblasts may 
become cartilage cells by cutting themselves off with basic substance. Hence 
the appositional growth of cartilage comes from the perichondrium. 
Growth also takes place within the substance of cartilage itself by segmentation 
of the cartilage cells and the secretion of basic substance; this is called its 
interstitial growth. 
Fibrocartilage 
By fibrocartilage, or connective tissue cartilage, we mean a car- 
tilage in which the collagenous fibers can be distinctly seen without special 
preparation. Such cartilages are not too abundant in the human body. They 
are to be found chiefly in the intermediate discs and the margins of the sockets 
of the various joints, the fibrocartilagines intercalares, and the labia glencidalia. 
Examples are the articular cartilages of the sternoclavicular and knee joints, 
the outer, laminar parts of the intervertebral discs, and the interpubic fibro- 
cartilages. 
The picture presented by fibrocartilage under the microscope is extremely 
variable, because of the transition forms that may be exhibited, on the one 
hand into tight connective tissue, on the other into hyaline cartilage. In the 
former case lines of cartilage cells are to be found between the densely packed, 
parallel collagenous fibers, each provided with a distinct cartilage capsule, 
while scarcely anything more is to be seen of the basic substance. The appear- 
ance of the hyaline basic substance becomes more distinct as the cartilage cells 
become more pronounced; it may be seen as cartilaginous islands or bands within 
the tight connective tissue, and finally preponderates in bulk to such an extent 
that the collagenous fibers are completely hidden and the typical picture of 61 
hyaline cartilage is developed (Pl. 17, Fig. 48). Elastic fibers are to be found 
here and there among the collagenous, but they are in no way prominent. 
When the basic substance of cartilage is not present, or simply forms the 
cartilage capsules, this tissue has been called chondroid connective tissue. 
Elastic Cartilage 
Collagenous fibers are strongly in evidence in fibrocartilage, but in elastic, 
or reticulated cartilage the structural picture is dominated by networks of 
elastic fibers. Like fibrocartilage this variety is found in only a few places 
in the human body. It forms the framework of the auricle, external auditory 
canal, and Eustachian tube, and takes part in the structure of the larynx, where 
it forms the epiglottic, Santorinian, and part of the arytenoid cartilages, par- 
ticularly of the vocal processes of the latter. 
The cells of elastic cartilage differ in no way from those of hyaline, and 
its basic substance is of the same composition, but elastic fibers are added, 
which can be readily seen without staining, impart a rather yellow color to the 
cartilage, make it less translucent, and increase its power of resistance. They 
are sometimes extremely delicate, sometimes coarser, and may attain quite a 
considerable thickness (Pl. 16, Fig. 49). They give off many branches which 
anastomose and form networks that permeate the entire basic substance of the 
cartilage and hold the cartilage cells in their larger meshes. At the periphery 
of the cartilage these networks pass over without interruption into the elastic 
fibers of the perichondrium. The peripheral fibers are the thinnest and run 
for the most part parallel to the surface of the cartilage, but farther in they 
become thicker and placed more vertically to the surface, while in the center they 
again become thin and form at this place an extremely dense, fine-meshed felt. 
Osseous Tissue 
The supporting substance of the human body attains its highest development 
and its greatest functional capacity in bone, a tissue that appears very late in 
the course of evolution. No bone is developed in the invertebrates, or in the 
lovzest vertebrates, the amphioxus and the cyclostoma. Ossification appears 
first in the connective tissue of the skin of the selachians, where it forms a protec- 
tive exoskeleton, which reaches its highest development among the reptiles, in 
turtles and crocodiles. Later, bony tissue develops in the cartilaginous, as in 
the skin, for the purpose of forming an osseous endoskeleton, which first 
appears in the ganoids and is wholly absent in the selachians. Bone may there- 
fore originate directly from connective tissue, or in connection with cartilage. 
We meet with something similar in man. The great majority of his bones, 
those of the vertebrae, the limbs, the base of the skull, and of the tongue, are 
cartilaginous at first and are called primary, while those of the sides and vault 
of the skull, and those of the face originate directly from connective tissue and 
are said to be secondary bones. 
The formation of bony tissue proceeds from cells called osteoblasts. 
Opinions are still divided with regard to the origin of these cells; the majority 62 
of histologists consider them to be fibroblasts, i.e., connective tissue cells, but 
some think them to be cells that have migrated from the ectoderm. In the 
simplest formation of bone these stellate cells are deposited thickly upon the 
bundles of collagenous fibers and are bound there by a cement containing lime, 
causing the collagenous fibers to calcify. The cells then secrete more fibrils and 
cement about themselves and thus come to lie in a substance composed of col- 
lagenous fibrils enveloped in a material containing lime, the basic substance 
of bone. Now a second layer of osteoblasts begins, through the activity 
of which another layer of basic substance is formed, and so on. The result 
of this is that the completed bone tissue has an exquisite lamellated structure. 
Between the lamellae of basic substance lie the bone cells, the former osteoblasts. 
This is the simple course of ossification in the secondary bones, those 
that originate directly from the connective tissue, but the conditions are 
much more complicated in the primary bones. In these bones is deposited layer 
on layer in a quite similar manner on the surface of the cartilage from the 
richly cellular perichondrium, forming the perichondral ossification, but 
osteoblasts also penetrate with the blood vessels into the interior of the car- 
tilage, where they destroy the tissue and hollow it out to form the primary 
medullary cavity, from which ossification proceeds to transform what re- 
mains of the cartilaginous tissue, endochondral ossification. 
We need to study this last process rather more closely. At one or more 
points, called ossification, or calcification points, lime salts are excreted 
by the cartilage cells into the surrounding basic substance. Toward these 
points the richly cellular perichondrium proliferates, and there the cartilage is 
destroyed, the cartilage cells die, and a hollow cavity, filled with a very cellular 
periosteal tissue and blood vessels, is formed at the site of the calcification 
point, the primary medullary cavity. As this cavity enlarges with the 
advancing destruction of the cartilage, trabecula? are preserved and remain 
joined to the principal mass, the directing trabeculae of the ossification* 
Upon these the osteoblasts lodge and begin their bone forming activity, so 
that each bony trabecula that originates from the directing trabeculae contains 
within itself at first a cartilaginous one, which later ossifies. This formation 
of bone with coincident destruction of cartilage extends gradually from the 
medullary cavity toward the periphery, until finally the endochondral and 
perichondral ossifications meet. Endochondral ossification is productive of ai 
network of anastomosing bony trabeculae, which form spongy bone sub- 
stance, while the periosteal ossification gives us compact bone substance. 
The blood vessels play an important part; on the one hand, they destroy car- 
tilage in their advance from the medullary space; on the other, they become 
grown about by newly formed bone in the perichondral ossification, and so 
come to lie in bony grooves that later become tubes, together with the osteo- 
blasts that accompany them. Thus it happens that compact bone substance 
is traversed by a system of tubes, the Haversian canals, which lodge the 
nutrient blood vessels. The final form of the medullary cavity is determined 
later by the partial destruction of the remodeled bone substance by certain 
large, polynucleated cells, the osteoclasts, the genesis of which has not yet 
been learned with certainty. Some believe them to be metamorphosed fibro- 63 
blasts, others think that they come from the epithelium of the blood vessels, 
and still others hold them to be cartilage cells that have been set free by the 
destruction of the cartilage. 
Following this review of the development of bone tissue, we have to inquire 
more minutely into its construction, and to become somewhat acquainted with 
its physical properties. As stated above, it exceed^ cartilage in firmness and 
hardness, and it cannot be cut, when fully developed, because of the lime salts 
it contains, but when the latter have been removed by treatment with dilute 
mineral acids we find it to resemble cartilage. On the other hand, if the 
organic substance is destroyed by heat, it becomes very brittle and friable. 
Thin layers of bone tissue are quite translucent and have a high degree of 
anisotropia due to the quantity of collagenous fibrils that they contain. 
Wherever bone tissue is present in considerable quantity its basic sub- 
stance has a markedly lamellar structure, due to the deposit of osteoblasts 
in layers on the collagenous fibrils in its development (Pl. 20, Fig. 52). Each 
lamella is composed of collagenous fibrils, bound together with cement. 
If a thin section of bone is carefully heated red hot so as to destroy the former, 
the cement alone will be left, and may be seen to be permeated by a great many 
densely packed tubes, which are the spaces in which the fibrils lay. The fibrils 
are grouped into little bundles in the lamella?, those in each lamella running 
about parallel, but those in different lamellae running in various directions, so 
that the fibrils in one may be perpendicular to those in another, and in a cross 
section through several lamellae we may see in one points formed by the trans- 
versely cut fibrils, and in another, often adjoining, lines where they have 
been cut longitudinally. Punctate and striate lamellae may alternate. If we 
examine such bone lamellae with the polariscopic microscope we will find that 
in a certain position of the analyzer the striate lamellae appear to be dark, 
the punctate light, and that when the ocular has been rotated 90° both appear 
to be of the same brightness. This is known as pleochroism. 
In addition to the fibrils that form a constituent of the lamellae, we find in 
the superficial layers of bone tissue bundles of thicker collagenous fibrils, which 
are often very thick, intermixed with elastic fibers. These do not belong to 
the individual lamellae, but pass through them vertically or obliquely from the 
periosteum. They are known as Sharpey's fibers, and appear much more in 
embryonal than in fully developed bones. 
The cement unites the collagenous fibrils into bundles, fills the spaces 
between the latter in the lamella, and finally binds the lamellae together. It 
holds the lime salts. 
In speaking of the arrangement of the lamella of the basic substance, it 
should first be understood that when bone tissue exists simply as an extremely 
thin plate, no lamellae are formed, and that the fibrils run only in one direction. 
As sOon as the plate becomes somewhat thicker lamellae appear with their 
surfaces parallel, but the arrangement becomes much more complicated wher- 
ever the mass of tissue is considerable, as in the compact bone found in the 
diaphyses of the long, and the cortices of the flat, bones. 
In a cross section through the diaphysis of a human humerus we can 
distinguish three different systems of lamellae (Pl. 18, Fig. 50). First we 64 
have lamellae that run parallel to the outer and inner surfaces of the bone; 
these are the basic lamellae, which we may subdivide into the outer and 
the inner. They form only a comparatively small part of the entire bone, 
and are far surpassed in bulk by another system situated between them, which 
we call the special or Haversian lamellae. These give the bone its 
peculiar structural imprint, and appear in the section as more or less regular 
concentric circles about a hole. Substantially each Haversian system of 
lamella? consists of a hollow cylinder with very thick walls, which are in turn 
composed of several cylinders of basic substance, one within another. The 
hollow cylinder encloses the Haversian canal, which frequently does not 
lie exactly in its axis. In a longitudinal section of bone (Pl. 19, Fig. 51) the 
Haversian canals may be seen to run nearly parallel to one another, to unite 
by means of oblique branches, and so to form a system of canals throughout 
the bone substance for the accommodation of the nutrient vessels. 
Each system of Haversian lamellae consists of an indefinite number of 
single lamellae; the systems lie at various distances apart, and are sharply 
differentiated from the surrounding bone tissue. This latter, which occupies 
the entire space between the systems of Haversian lamellae and the outer and 
inner basic lamellae, presents what are called the interstitial or interposed 
lamellae, some of which run parallel to the outer basic lamellae, while the 
rest follow no typical course. 
The systems of Haversian lamellae are united with the interstitial lamellae 
that surround them by a cement that gives rise to the outlines which sharply 
delimit the systems in the cross section, the cement lines (Pl. 20, Fig. 52). 
The cement lines are always irregular, with many bulges jutting outward, 
and not only form frames to the Haversian lamellae, but also divide the inter- 
stitial into separate systems. 
The basic substance of bone differs in its chemical composition from 
every other tissue of the human body in the great amount of its mineral con- 
stituents, which comprise from 30 to 40% of its mass. The chief of these 
is calcium phosphate, followed at a long distance by calcium carbonate, mag- 
nesium phosphate, calcium fluoride, calcium chloride, and ferrum oxide. Of 
the organic constituents, which amount to 20 to 25%, the most important is 
ossein, a collagen mixed with mucoid and albumoid. 
The bone cells represent the original osteoblasts, and lie in little cavities 
of the basic substance, the bone cavities or lacunae (Pl. 21, Figs. 53 and 
54). These lacunae are shaped like pumpkin seeds, and are always intra- 
lamellar, a row of them to be found between every two lamellae. In each row 
they lie apart at a certain distance and send out numerous branching tubules 
in all directions, the bone canaliculi, which run through the lamellae and 
anastomose not only with neighboring canaliculi, but often with others situ- 
ated at a distance, and thus form a system of canals that permeates the 
entire basic substance and permits the circulation of the nutrient fluid. The 
canaliculi of the innermost row of lacunae in each Haversian system open 
directly into the Haversian canal. 
Each lacuna and each canaliculus is surrounded by a thin layer of basic 
substance, called the limiting sheath, which may be isolated by treating 65 
thin discs of bone for several hours with concentrated hydrochloric acid. 
Bodies shaped like pumpkin seeds and provided with many processes may then 
be seen, the so-called bone corpuscles, which conceal within themselves the 
bone cells. These limiting sheaths are composed of a specially modified, re- 
sistant layer of basic substance that contains no collagenous fibrils (Pl. 21, 
Fig. 55). 
In the lacunae lie flat cells provided with many processes that extend into 
the canaliculi. These are the bone cells. How far their processes go, and 
whether they anastomose with those of the neighboring cells, are questions 
that are not easily answered (Pl. 21, Fig. 56). 
Dentine is a variety of bone tissue which will be described in the discus- 
sion of the structure of the teeth. c 
MICROSCOPIC ANATOMY OF ORGANS 
I. THE VASCULAR SYSTEM 
1. THE BLOOD 
The blood, which carries nutriment and oxygen to all parts of the human 
body, consists of a fluid known as the blood plasma, and of corpuscular 
elements suspended in it, the blood corpuscles. We have to deal here 
chiefly with the latter, which are very closely related to the cellular elements 
of the connective tissue, as must be evident from what has been said in the 
preceding chapter and is particularly prominent in the part on embryology, 
as both come from the mesoderm. 
The first signs of the formation of the blood are met with in the wall of 
the vitelline sac in the form of the so-called blood islands. These are 
masses of cells that have wandered out from the mesoderm, and have spread 
out between it and the vitellus like a network. The peripheral cells unite 
very soon, to form a continuous epithelium, while those situated more internally 
remain isolated and float about in a fluid secreted within the blood islands. 
The epithelial tube separates its contents from the neighboring mesoderm and 
vitellus, and forms the most primitive part of the blood vessel that thus 
originates. The cells that remain isolated are the primitive blood cor- 
puscles, or the primitive mesamoeboids. 
While the formation of blood vessels proceeds subsequently in like manner 
in the embryonal body, important changes take place in these primitive mesa-i 
moeboids. First they multiply by indirect segmentation many times repeated, 
then a not inconsiderable portion of them wander out of the primitive vessels 
into the surrounding mesoderm, and above all they exhibit an advancing differ- 
entiation in two principal directions. Part of them elaborate a coloring matter 
that contains iron, the haemoglobin, envelop themselves with a membrane 
and become the red blood corpuscles, or erythrocytes. In doing this the 
corpuscles pass through various stages of transformation, manifest at first by 
a diminution in size and a greater staining power of the nucleus. The primitive 
mesamoeboid enters the ichthyoid stage as a spherical cell containing haemo- 
globin and a small nucleus, and in this stage it undergoes a lively multiplication 
by indirect segmentation. By the further diminution in size of the nucleus 
and massing of its chromatin, and by the enveloping of the cell in a mem- 
brane, it enters the sauroid stage, in which multiplication no longer takes 
place. The red blood cell then attains its final form by casting forth its; 
nucleus, that has become pycnotic, and by flattening its body, so that finally, 
instead of a spherical, nucleated cell, we have a non-nucleated, flat, round disc, 
the young erythrocyte. 
66 67 
A certain percentage of the primitive mesamoeboids that do not elaborate 
haemoglobin within their bodies continue as they are for a long time, and are 
transformed into erythrocytes later in embryonal life, but the rest remain 
throughout life destitute of haemoglobin and containing nuclei. These form 
the white blood corpuscles, or leucocytes, which are to be found in the 
blood of the adult bearing a certain percentage to the red blood corpuscles. 
While all of the erythrocytes present the same morphological picture in adults, 
the white blood corpuscles exhibit quite a variety of forms, but these are only 
developments, and finally degenerations of the young form. 
The manufacture of blood in the embryo takes place not only in the blood 
vessels of the vitelline sac and of the embryonal body, but also at special 
chosen places, which are chiefly the liver and the bone marrow for the red 
blood corpuscles, the lymphoid organs, such as the lymphatic glands, the 
tonsils, the lymphatic follicles, and the spleen, for the white cells. 
The Red Blood Corpuscles 
The red blood corpuscles, or erythrocytes, form by far the greatest 
part of the corpuscular elements of the blood. A cubic millimetei' of human 
blood contains between four and five millions of erythrocytes, and the ratio of 
their number to that of the leucocytes is about 500: 1. 
The erythrocytes are round discs that have a diameter of 7 to 7.5 [*. 
They are considerably larger in the embryo, and under abnormal conditions 
such large erythrocytes may be found in the blood of adults, when they are 
called megalocytes. The thickness of an erythrocyte is greatest at its margin, 
where it is about 2 [x, and least at its center, so that it resembles a biconcave 
lens with a rounded margin. 
When spread in a layer that is not too thin the erythrocytes, in conse- 
quence of a sort of capillary attraction, place themselves with their broad 
sides toward each other, like the separate pieces of a roll of coins standing 
more or less perfectly on their edges (Pl. 32, Fig. 80). 
The color of the erythrocyte is a light yellow, depending on its content 
of the coloring matters of the blood, hasmoglobin and oxyhaemoglobin, the 
former of which is being produced constantly by reduction of the latter. Both 
are proteids easily soluble in water, insoluble in alcohol characterized by the 
iron they contain, and may be obtained in crystalline form very easily from 
many animals, but only with difficulty from man, blood, or haemoglobin 
crystals. Haemoglobin and oxyhaemoglobin have quite characteristic absorp- 
tion spectra, and are found exclusively in the erythrocytes in firm combination 
with their substance. If distilled water is added to blood it becomes stained 
because it dissolves the coloring matter out of the corpuscles, while the blood 
becomes lake colored. The corpuscles themselves undergo an extensive change; 
they become less plainly visible and shadowy, on account of the loss of their 
coloring matter, and become much swollen. Dilute salt solution acts in a 
manner similar to distilled water on the erythrocytes, which are hypoisotonic 
to the blood serum. Stronger solutions of salt, which are hyperisotonic, 68 
cause the erythrocytes to shrivel, to become indented, and to take the form 
of thorn apples or of morgensterns. 
The erythrocytes are markedly oxyphilic; they stain with acid dyes, and 
owe this property to the coloring matters of the blood. 
Very little is positively known concerning the minute structure of the 
erythrocytes. The fact that a nucleus is never present in adults has already 
been mentioned. Even with the greatest enlargement no structural details can 
be perceived in its body, yet for various reasons we have to suppose that it is 
enveloped in a membrane, which encloses a framework, or stroma, containing 
the blood coloring matters. 
The White Blood Corpuscles 
The white blood corpuscles, or leucocytes, differ in various ways 
from the erythrocytes, but chiefly in the absence of haemoglobin and the 
presence of a nucleus. They are much fewer in number in the blood, but 
definite figures can hardly be given as their number fluctuates considerably, 
according to the place in the body, the age, and the physiological condition 
of the individual. The younger a person is the greater is the number of 
his leucocytes. A cubic millimeter of the blood of a new born child contains 
from 12,000 to 13,000. Blood taken from peripheral vascular areas con- 
tains more than that from central areas, and more leucocytes are to be found 
in the blood after a full meal than during hunger. The minimum number 
may be said to be 5,000, the maximum 10,000, the average 7,500 to the cubic 
millimeter of blood. 
The leucocytes can always be recognized easily in the circulating blood 
from the fact that they roll singly along the wall of the blood vessel, while 
the erythrocytes occupy the axis of the vessel in a continuous current. Fre- 
quently the leucocytes remain hanging on the wall of the vessel, which indi- 
cates that these bodies have a certain stickiness. The erythrocytes are ex- 
tremely plastic, as is shown by the fact that they very often are caused to 
undergo in the vessels many changes of form by the blood current that revert 
immediately, but this is not the case with the leucocytes, which are more re- 
sistant to such mechanical influences. On the other hand, they have, to a high 
degree, the power to change their own shapes, and to migrate from the vessels 
into the connective tissue, where we have learned to know them as wandering cells. 
Leucocytes in a state of rest are more or less spherical cells, more rarely 
ovoid, that vary extremely in size. The smallest are about as large as erythro- 
cytes, the largest attain a diameter as great as 20 a (Pl. 31, Fig. 80). 
The leucocytes of the human blood show essential differences in size, in 
the shape of the nucleus, and above all in the structure of the cell body. The 
latter may elaborate within itself fine or coarse granules, or granulations, 
which give the leucocytes a very characteristic appearance. The granula- 
tions are characterized by their form and their color reaction, and play a 
very important part in the classification of leucocytes. Various views are held 
as to their nature; some authors consider them to be secretory products of the 
cell body, others take them to be foreign bodies that have been seized by the 69 
phagocytic leucocyte and are being worked up in its body, and still others 
believe them to be constant cell organs comparable to the microsomes of other 
cells, which take an important part in assimilation. 
The following varieties of leucocytes may be differentiated in normal human 
blood: 
1. Lymphocytes. These are spherical cells about as large as erythro- 
cytes, 7 to 9 [x, with spherical or slightly indented nuclei that occupy the greater 
part of the cells and are surrounded only by a more or less narrow ledge of 
protoplasm. The nucleus exhibits a distinct framework, presents one or two 
nucleoli, and has a strong basic affinity. The same affinity is possessed by the 
cell body, in which distinct chondriosomes have recently been demonstrated. 
The body of the lymphocytes contains no true granulations (Pl. 32, Fig. 81). 
Lymphocytes appear in the blood of children that are quite large and 
attain a diameter of even 15 P-, but differ in no way as regards their structure 
from those of adults. These are called lymphoblasts. 
Lymphocytes have amoeboid properties and are able to pass through the 
walls of the vessels into the tissues, but this property is possessed to a consid- 
erably higher degree by the granular leucocytes. 
A cubic millimeter of blood contains in children 7000 to 8000 lymphocytes. 
After the age of ten years the number decreases considerably, so that in the 
adult it is only 1500 to 2000. Hence in childhood the lymphocytes amount to 
from 40 to 60%, and in adults to only 20 to 25% of the total number of 
leucocytes. 
2. Transition forms. These, which have a diameter of from 12 to 20 p, 
are the largest cells of the human blood. The nucleus may vary a great deal 
in shape; it may be roundish, oval, have a slight indentation, or have greater 
indentations that give it a lobulated appearance, or it may be shaped like a 
horseshoe. The chromatin takes only a weak stain, so the nucleus appears light. 
In the cell body are to be seen delicate, but distinct granulations, that are neu- 
trophilic, concerning the nature of which there is still a lively discussion (Pl. 
32, Fig. 81). 
The transition forms amount to only 6 or 8% of the total numbei' of 
leucocytes. 
3. Neutrophilic leucocytes form the great bulk of the colorless blood 
cells. They are of medium size, having a diameter of from 9 to 12 p. The nucleus 
exhibits the most extensive polymorphism; usually it is broken into several lit- 
tle pieces that are joined together by slender bridges, which are frequently 
arched. These bridges often are so extremely delicate as to easily escape obser- 
vation, and then the cells may seem to be polynucleated. The nucleus is rich in 
chromatin and stains strongly with basic colors. The protoplasm of the cell 
body is oxyphilic and contains many fine neutrophilic granules (Pl. 32, Figs. 
81 and 82). 
Four to five thousand are to be found in a cubic millimeter of blood, and 
they form 65 to 70% of all of the white blood corpuscles. 
4. Eosinophilic leucocytes. These are somewhat larger than the neu- 
trophiles, as they have a diameter of 12 to 15 p. The nucleus is similar to that 
of the preceding, but on the whole is rather coarser, and its chromatin is more 70 
weakly basophilic. But their distinguishing characteristic is furnished by the 
eosinophilic granules, which are coarse and regularly round in comparison with 
the granules of the neutrophiles. They refract light powerfully, have a peculiar 
yellowish color, and are strongly oxyphilic. They contain an albuminoid with 
much iron, which is in no way identical with haemoglobin. In addition to this 
albuminoid, the eosinophilic granule contains a lipoid, a fatlike body, that gives 
it its great luster (Pl. 32, Fig. 81). 
The blood of adults contains 100 to 200 eosinophilic leucocytes to the cubic 
millimeter; they therefore constitute only 2 or 3% of the entire quantity of 
white blood cells. 
5. Mast cells. These cells, with which we became acquainted in the study 
of connective tissue, have a diameter of 8 to 10 The nucleus has numerous 
indentations, is tabulated, is weakly basophilic, and appears very bright. Gran- 
ules are embedded in the oxyphilic protoplasm, which are as large as, or larger 
than those of the eosinophilic leucocytes. They refract light weakly and are 
very soluble in water. In contrast to the eosinophilic granules they are de- 
cidedly basophilic and exhibit the phenomenon of metachromasia that has been 
described. They always contain glycogen, and may hold lipoids, like the 
eosinophilic granules. 
Although the mast cells of the blood and of the connective tissue are very 
similar, there are weighty reasons which lead us to believe that the two have 
nothing to do with each other (Pl. 12, Fig. 36). 
The mast cells form only a minute constituent of the normal blood; only 
about forty are to be found in a cubic millimeter, so they comprise not over 
0.5% of the entire number of leucocytes. 
Two theories exist today concerning the genetic relations of these varieties 
of leucocytes, one called the polyphyletic, the other the monophyletic. 
According to the former all granular leucocytes are well differentiated forms 
of cells that develop from one or more mother cells, or myeloblasts, situated 
in the bone marrow, while the blood cells that are not granular, the lympho- 
cytes, differentiate themselves from the lymphoblasts formed in the lym- 
phatic glands. This gives us two different primitive forms for all leucocytes, 
the myeloblasts and the lymphoblasts, coming from different places of origin. 
According to the monophyletic theory there is, on the contrary, only one 
primitive form from which all leucocytes become differentiated. Once the cell 
has started in a certain direction it may multiply by segmentation, but it can- 
not be transformed into another kind of cell. 
The Blood Platelets 
Human blood contains, in addition to the erythrocytes and leucocytes, a 
third morphological element, the blood platelet. These are considerably 
more numerous in the blood than the leucocytes, as a cubic millimeter contains 
about 250,000, so they are present in the ratio of sixteen or twenty erythro- 
cytes to one blood platelet. 
In ordinary preparations of blood they usually form little groups (Pl. 32, 
Fig. 81). Each platelet is a spherical corpuscle about 3 p. in diameter. In 71 
its center is a granular substance which is considered by many writers to be a 
rudimentary nucleus. The substance of the corpuscle shows a slight affinity 
for basic dyes, while the granules are more strongly basophilic. They never 
contain haemoglobin. 
When special precautions have been taken the blood platelets exhibit every- 
where the form of little rods, and they are of this form in the circulating 
blood. 
Whether, as has been claimed, the blood platelets play an important part 
in the coagulation of the blood cannot yet be told with certainty. The coagu- 
lation of the blood, i.e., the excretion of firm fibrin from the blood plasma, 
depends on the presence of a ferment, the fibrin ferment, which is present in 
the blood and, according to the views of many authors, in the blood platelets 
as thrombogen. Outside of the vessels kinase is provided by the leucocytes, 
which changes the thrombogen into profibrin ferment, that furnishes with 
the lime salts of the blood the fibrin ferment. 
The Haemokonia 
There remains to be mentioned a final morphological element of the blood 
which has been discovered very recently by means of the ultramicroscope, the 
haemokonia, or blood dust. These are always in very lively dancing motion, 
and probably are not all of the same nature. In many cases they are granules 
of fat, the number of which seems to be increased enormously after a full meal. 
Physiological Importance and Replacement of the Corpuscular 
Elements of the Blood in Adults 
The erythrocytes carry oxygen to all parts of the body. It is for this 
purpose that they are provided with haemoglobin, which unites with oxygen 
in the lung to form oxyhaemoglobin, and gives it up in the capillaries to the 
tissues and their cells. The transportation of carbonic acid produced in the 
tissue cells is also cared for in part by the erythrocytes, with the haemo- 
globin of which the gas may unite, but the bulk of the latter is carried in the 
venous blood combined with the constituents of the blood plasma. 
The erythrocytes are being used up constantly during life. They die and 
are broken to pieces chiefly in the spleen, bone marrow, and lymphatic glands, 
where their remains are seized upon by special leucocytes, the macrophagi, 
and transformd into pigment, which is the matrix of the biliary coloring 
on the bone marrow. 
The replacement of the dead erythrocytes devolves exclusively on the bone 
marrow, where new ones originate from the so-called normoblasts, i.e., cells 
from 6 to 12 [X in diameter, that contain nuclei and haemoglobin, but lose the 
former through pycnosis. For more concerning this subject, see the chapter 
on the bone marrow. 
The leucocytes also have important duties in the economy of the body, 
the most important of which depends fundamentally on their power of amoe- 
boid movements, that enables them not only to change their places, but 72 
also to seize upon foreign bodies, in other words to act as phagocytes. The 
neutrophilic leucocytes and the large mononucleated, nongranular transition 
forms are the principal ones that undertake the duties of phagocytosis, and are 
known as microphagi and macrophagi. They are able to seize upon and 
render harmless not only the products of disintegration in the body itself, but 
also foreign elements, like bacteria, parasites, and particles of carbon. Sub- 
stances of a fermentative nature also are formed in the bodies of the leuco- 
cytes and perform various duties, such as the breaking up of fat, the di- 
gestion of albumin, oxidation, and reduction. These activities are 
probably associated with the granulations. 
Like the erythrocytes the leucocytes are constantly dying in the body and 
their remains being disposed of by similar cells. Replacement takes place in 
several places. Lymphoblasts are continuously formed by mitotic segmen- 
tation of the cells present in the lymphatic glands and follicles, are transformed 
into lymphocytes, and are carried by the lymph current into the blood. Leuco- 
cytes also develop in the bone marrow, where the erythrocytes are formed. 
They come from cells that are identical with the lymphoblasts of the lymphatic 
glands and are known as myeloblasts; these diminish in size and first become 
myelocytes, which, with the elaboration of specific granulations and a char- 
acteristic transformation of the nucleus, provide the various kinds of granular 
leucocytes. The bone marrow cannot be said not to furnish lymphocytes, al- 
though it is chiefly concerned in the production of granular leucocytes, neither 
can the lymphatic glands be held to purvey lymphocytes exclusively, for granu- 
lar leucocytes originate in them, though in comparatively small numbers. An- 
other place where leucocytes develop is the spleen, in which both granular and 
nongranular white blood corpuscles are produced. 
Up to the most recent times a great difference of opinion prevailed concern- 
ing the origin of the blood platelets, but now it may be regarded as certain that 
they also come from the bone marrow, where they are produced by the constric- 
tion of large cells, which we shall learn to know as giant cells in the description 
of that organ. 
2. THE BLOOD VESSELS 
The blood vessels develop with their contents, the blood, from the mesoderm 
as a common matrix, as we have seen in the preceding chapter. The most 
peripheral cells of the blood islands lie close together and unite to form an 
epithelium, and thus give rise to a much branched space in the wall of the vitelline 
sac that is closed on all sides, the primitive blood vessel. The separate 
blood islands unite, but at the same time send out shoots, by segmentation of the 
epithelial cells, which are at first solid but soon become hollow through separa- 
tion of their cells. These shoots anastomose, so that finally the entire wall of 
the vitelline sac is permeated by a dense network of young blood vessels. From 
the vitelline sac the vessels pass over into the embryonal body, though blood ves- 
sels are also formed independently in this. 
At first the entire vascular system of the embryo consists of such primitive 
vessels arranged in a plexus, then individual trunks become further dififerentia- 73 
ted, others disappear, and the final condition gradually develops. During 
this time the vessel wall undergoes typical changes. The vascular epithe- 
lium, which originally consisted of only a single layer of cells, is later 
strengthened by the surrounding mesenchymatous cells that envelop it like a 
sheath, and the primitive becomes the secondary blood vessel. These are 
the stellate cells, the processes of which anastomose. Wherever large trunks 
are to be formed the mesenchymatous sheath gradually becomes stronger, its 
cells lodge more densely and become transformed, part into smooth muscle 
cells, part into collagenous and elastic fibers, until the vessel is enwrapped 
externally to the epithelium by a sheath consisting of smooth muscle fibers, 
connective tissue, and elastic tissue, which we call the tunica media, while 
the epithelium forms the tunica intima. Outside of the media then appears 
a second sheath composed principally of connective tissue, the tunica adven- 
titia. The intima has about the same structure in all vessels, but the media 
and adventitia not only differ quite materially in the arteries and veins, but in 
both present typical variations according to the size of the vessels and its posi- 
tion in the body. 
The Capillaries 
The capillaries have the original construction and plexus like arrangement 
of the secondary blood vessels of the embryo completely preserved; i.e., they 
consist simply of an epithelial tube, the tunica intima, enveloped by a cellular 
adventitial sheath (Pl. 33, Figs. 83 and 84). 
The cells that constitute the intima of the capillaries are very thin, elongated, 
irregular plates, so arranged that their long diameters lie in the longitudinal 
direction of the vessel (Pl. 33, Fig. 84). The usually oval nucleus bulges the 
thin cell body into the lumen. The individual cells are joined together very 
firmly by a cement which is stained black by silver salts. Wherever several cells 
come together this cement is frequently massed into a larger quantity. 
The adventitial sheath consists of stellate cells, the processes of which anasto- 
mose and interweave about the epithelial tube. They are contractile, and are 
able to so compress the tube by their contraction as to cause the lumen to 
completely disappear. 
The Arteries 
In contrast with the capillaries the three layers of the walls of the arteries 
can always be demonstrated, with the exception only of the transition places 
from arteries into capillaries, the so-called precapillary arteries, the walls 
of which are like those of the capillaries but for the presence of a network of 
fine elastic fibrils between the epithelium and the adventitial sheath. 
The intima of the arteries consists, as in the capillaries, exclusively of the 
epithelial tube, which presents about the same picture in all and is composed 
of long, irregular cells. The epithelial cells are shorter in the largest arteries 
than in the medium sized and small. Their long axes, as well as those of their 
ovoid nuclei, lie longitudinally with the direction of the vessel (Pl. 84, Figs. 85 
and 86). 74 
Much greater differences may be found in the media, the most important 
component of which is formed of smooth muscle fibers, which in general 
have a circular course, so that their longitudinal axes are perpendicular to 
the longitudinal axis of the vessel. As the nuclei of these cells are likewise 
fairly long we now have two kinds of nuclei in the arterial wall, those that 
are placed longitudinally, the epithelial nuclei, and those that are placed 
transversely, the muscular nuclei. In the smallest arteries the muscle fibers of 
the media are at first pretty separate, surrounding the epithelial tube like 
rings, but very soon they unite to form a continuous layer, and then are placed 
in several layers, one upon another. Generally speaking we may say that the 
thickness of the musculature increases with the calibei' of the vessel, yet we can 
distinguish among the larger arteries those that are strong and those that are 
weaker in muscular tissue. Among the former are the femoral and radial ar- 
teries, among the latter is the carotid. But the musculature gradually recedes 
into the background in the largest arteries, so that the aorta and the pulmonary 
artery are destitute of it at the places where they originate. 
Elastic tissue plays an important part in the media. It appears, as we 
have seen, in the precapillary arteries, and increases continuously with the 
calibei' of the vessel until finally it completely displaces the muscular tissue 
(Pl. 35, Figs. 87 and 88). The elastic tissue is found in the media in the form 
of nets which are mainly arranged concentrically, so that they are to be seen 
in the cross section of the vessel usually as very tortuous, circular lines. The 
elastic network finally becomes changed to an elastic membrane, as its meshes 
become smaller until their only remaining traces are numerous apertures, such 
as we meet with chiefly in the large arteries. These elastic networks and 
membranes lie everywhere between the muscular fibers, enveloping and separating 
them. The two alternate in the large arteries, a layer of muscular fibers with 
an elastic lamella (Pl. 35, Fig. 88). 
One elastic lamella, which we call the elastica interna, is particularly 
strong in all arteries, even the smallest (Pl. 35, Figs. 87 and 88). It is very 
tortuous and lies just beneath the epithelium in small arteries. In the medium 
sized vessels it is separated from the epithelium by a longitudinally striated, 
delicate layer containing elastic fibrils, within which scattered muscular fibers 
may be detected in the aorta. The musculai' fibers therefore occur between 
the interna afid the elastica interna. Formerly not only the epithelium, but also 
this layer and the elastic interna were included in the term intima. 
Besides the circular network of elastic fibers we meet with in the media 
radiating elastic fibers, but these are always absent in the cerebral arteries 
and in the aorta. They may run through the entire thickness of the media 
in either straight or curved courses, may furcate, and usually pass over into the 
elastic interna. Many authors believe these fibers to be the dilatators of the 
arteries. 
In addition to the elastic tissue we find in the media between the muscle 
fibers a collagenous tissue, connective tissue cells and fibers. 
The greatest part of the adventitia of the arteries consists of collagenous 
connective tissue, the bundles of which run chiefly in a longitudinal direction, 
but also take a circular course in the inner portion. Between these bundles 75 
are nets of elastic fibers, which are often very coarse, and are placed most 
densely at the margin of the media, where they form a sort of elastic limiting 
membrane, called the elastica externa (Pl. 35, Fig. 88). The elastic net- 
works are developed most strongly in the adventitia of the medium sized arteries 
and gradually disappear in the largest. The adventitia also contains mus- 
cular tissue in the form of little bundles of smooth fibers running longitu- 
dinally, which never form coherent masses, but are always scattered about. 
They appear first in the medium sized arteries and are absent in the largest. 
The adventitia of all arteries blends without any sharp demarcation with the 
connective tissue surrounding the vessels. 
The Veins 
The well-known macroscopical differences between the wall of a vein and 
that of an artery are that the former is the thinner in vessels of the same size, 
and that it does not increase in strength in the same proportion to its caliber. 
Valves also are developed in veins, though not in all, which prevent a reflux 
of the blood current. The thinner condition of the venous wall is essentially due 
to a weaker development of the media, while the adventitia is usually thicker 
than that of the artery, but a sharp delimitation of the two is very rarely 
possible. 
The epithelium does not differ from that of the artery. 
The media is weakly developed, is entirely absent in many veins, like the 
vena cava superioris, and even where it is strongest it is only a weak layer of 
circular, smooth muscle fibers associated with bundles that run longitudinally. 
Between the muscle fibers are networks of elastic fibers, but these by no means 
attain the magnitude of those in the wall of the artery, and never form true 
elastic membranes (Pl. 35, Fig. 89; Pl. 36, Fig. 90). An elastic interna 
can usually be made out, but it is never anything more than a fine fibrous net- 
work. In the majority of the larger veins it rests directly on the epithelium, 
but in those of medium size a longitudinally striated connective tissue may 
be interposed, and in the veins of the lower extremity, especially in the vena 
dorsalis penis, bundles of smooth muscle fibers run obliquely or longitudinally 
internal to the elastica interna. 
The media passes over without any demarcation into the adventitia, 
which forms the lion's share of the entire wall of the vein, and always contains 
a considerable quantity of muscular tissue, consisting exclusively of bundles 
of smooth muscle fibers running longitudinally. They usually lie scattered 
about, separated by richly developed collagenous connective tissue, but may 
unite to form a continuous longitudinal muscular layer (Pl. 36, Fig. 90). 
The adventitia is extremely rich in connective tissue, the bundles of which 
run in all directions, some circularly, some obliquely, some longitudinally, cross 
and interweave together. Elastic tissue is scanty in the adventitia of the veins 
and is found chiefly in the form of delicate networks about the muscle bundles. 
The valves are duplicatures of the wall of the vein, in the construction of. 
which the intima and media take part, but not the adventitia (Pl. 35, Fig. 
89). The epithelium on the peripheral, convex side is the same as that of the 76 
intima of the vein, but that on the central, concave surface, the one toward 
the heart, is composed of irregular polygonal cells. The elastica interna ex- 
tends from the wall of the vein in undiminished strength over the peripheral 
surface of the valve, yet ends at its margin, is wholly absent beneath the epithe- 
lium of the central surface, and reappears gradually at the base of the valve. 
The musculature of the media shows at the base of the valve a not inconsiderable 
thickening, and extends for a very little way into the valve itself, yet the mus- 
cular bundles disappear very soon so that by far the greater part of the valve is 
free from muscular tissue and is composed exclusively of bundles of connective 
tissue, part of which take a circular, part a radiating course, with some scanty 
elastic fibers between them. 
The Vessels and Nerves of the Blood Vessels 
The walls of the capillaries and of the small arteries and veins obtain their 
nutrition from the blood coursing through the vessels, but those of the larger 
arteries and veins have their nutriment supplied by special vessels known as 
vasa vasorum, which come from neighboring blood vessels, run in the adven- 
titia, and penetrate with their capillaries into the media (Pl. 35, Fig. 88). 
All blood vessels are supplied by nonmedullated fibers from the 
sympathetic nerve, which weave about the capillaries in wide meshes and 
form in the walls of arteries and veins three plexuses that are connected together. 
First comes a plexus fundamentalis in the adventitia; then a plexus 
perimuscularis at the border between the adventitia and the media; and 
finally a plexus intramuscularis in the media. The fibrils coming from the 
last attach themselves with little end bulbs to the muscular fibers. The vessel 
wall also contains medullated sensory fibers, which likewise form a plexus 
in the adventitia, from which extend delicate fibers that become nonmedullated 
and spread out in the border toward the media in end organs like antlers. 
3. THE HEART 
The development of the heart follows the same course in principle as that 
of the blood vessels. It starts from two masses of cells paired on each side of 
the middle line in the most causal portion of the rudimentary head between the 
mesoderm and the entoderm. Each of these are grouped like epithelium and 
form little tubes, from which grow the primitive vessels, just as in the blood 
islands. When the foregut closes itself these two heart rudiments gradually 
approach until they finally blend and furnish the single heart tube. This last 
consists exclusively of the epithelial tube, but soon the cells of the visceral wall 
of the coelom protrude from the heart rudiment into pleuropericardial cav- 
ity, the so-called cardiogenous plate, apply themselves closely to the epi- 
thelial tube and differentiate themselves into striated muscle cells. In time a 
powerful muscular mantle is formed about the rudiment of the heart, which is 
now passing through its well-known process of metamorphosis. Outside of this 
lies the epithelium of the pericardial cavity, so that now we can distinguish 
three different layers of the heart: the innermost the epithelium, or endo- 77 
cardium, derived from the vessel cells; outside of this the musculature or 
myocardium, derived from the wall of the coelom and developed to very 
different degrees of powei' in the various parts of the heart; and outside of 
this an epithelial layer which represents the epithelium of the coelom itself, 
the epicardium (Pl. 36, Fig. 91). 
The endocardium lines the entire inner surface of the heart and has when 
fresh either a reddish, or an almost pure white appearance, as in the left auricle 
and on the valves, with every possible shade between these two extremes. Its 
most important constituent is the cardiac epithelium, but in addition we 
include in the endocardium the tissue on which the epithelium rests and which 
join it to the myocardium. 
The epithelium of the heart is directly continuous with that of the vessels 
entering or leaving, and differs in no way from it. Next to the epithelium is a 
layer of connective tissue which is developed to different degrees in different 
parts of the heart, and in which elastic and smooth muscle fibers evolve mainly in 
postembryonal life. The elastic fibers form networks and increase in thickness 
from within outward. The smooth muscle fibers ran in the endocardium as 
scattered bundles, or unite into denser bands, as in the wall of the septum in the 
left ventricle. Externally to the elastic network lies a layer of connective tissue 
which is commonly known as the subendocardial tissue. 
The myocardium forms by far the greatest part of the substance of the 
heart and consists mainly of the cardiac muscle fibers already described, 
which unite to form a meshwork, or wickerwork, as we have seen (Pl. 28, Figs. 
71 and 72). The cardiac muscles arise from a connective tissue supporting ap- 
paratus that may be called the skeleton of the heart, which includes the 
annuli fibrosi of the atrioventricular valves and of the mouths of the arteries, 
the septum membranaceum, and the trigona fibrosa. It is the province 
of descriptive anatomy to depict these formations and the arrangement of the 
muscle of the auricle and ventricle, so we shall deal only with their minute 
structure. 
The skeleton of the heart, which may be cartilaginous or even bony in 
the lower animals, is composed in man of a dense connective tissue that is per- 
meated with fat cells. A vesicular supporting tissue also appears between 
the masses of connective tissue in the trigona. 
The muscle fibers of the heart unite first into small bundles, which in 
turn form either thicker fasciculi or single layers. They are separated and 
encased by loose connective tissue in which numerous elastic fibers are to be 
found, that can scarcely be perceived in the newborn, but always become more 
powerful in the course of life in response to the functional demands made upon 
the muscles (Pl. 36, Fig. 91). The connective tissue also enters into the pri- 
mary bundle and surrounds each muscle fiber with a delicate envelope. 
The musculature of the ventricles is independent of that of the auricles, 
and the two are united only by a system of muscular fibers known as His's 
bundle, or the atrioventricular bundle. This can be distinguished ma- 
croscopically from the rest of the musculature of the heart, from which it is 
separated by a strong connective tissue sheath. The muscular fibers them.- 
selves are richer in sarcoplasm and poorer in fibrils than those of the rest of 78 
the muscles, and also contain less glycogen. At their commencement in 
Tavara's nodes the fibers lose their identity and pass over into the muscu- 
lature of the auricle, while in the musculature of the ventricle they lose them- 
selves in Purkinje's threads, which are much less prominent in man than in 
many mammals, but are characterized by their greater abundance of sarcoplasm 
and their marginal fibrils. 
The valves of the heart consist of a connective tissue foundation covered 
by the endocardium on both sides (Pl. 37, Fig. 92). This lamina fibrosa of 
the valve is poor in elastic fibers, while the valvular endocardium is permeated 
very strongly with elastic networks. The atrioventicular valves in the 
embryo are almost entirely muscular, but in the adult the muscular tissue, though 
present, is much reduced in quantity. The semilunar valves contain no 
muscular elements, but consist wholly, with the exception of the epithelial coatj 
of connective tissue, the fibers of which run in various directions with finer or 
coarser elastic fibers between them. The noduli of the margin of the valve are 
produced by a deposit of fat in the connective tissue. 
The epicardium forms the outer covering of the heart and passes over 
on to the large vessels in the pericardium. Toward the pericardial cavity 
it is covered by a single layer of low polygonal cells, the epicardial epithe- 
lium, which rests on a layer of connective tissue that contains many elastic 
fibers and passes directly into the adventitia of the large vessels at the places 
where they are given off. The epicardium is joined to the myocardium by a 
connective tissue that is developed to different degrees in different places, the 
subepicardial tissue, which is continuous with the connective tissue of the 
myocardium. A development of fat takes place in this, especially at certain 
places of predilection, such as the sulcus coronarius and the sulci interven- 
triculares. 
The pericardium has a structure quite like that of the epicardium, and 
like it presents toward the pericardial cavity a single layer of epithelium 
consisting of rather higher cells. Beneath the epithelium lies a connective 
tissue permeated with elastic networks, which in turn is joined to the neighbor- 
ing organs by a sub pericar dial connective tissue containing fat. 
The heart is supplied with blood through the two coronary arteries, 
which were formerly thought to be terminal arteries, but it is known now that 
both they and their branches anastomose freely. The arteries and their larger 
branches run first in the subpericardial connective tissue, then penetrate into 
the myocardium, lying here in the connective tissue septa, and finally break up 
into capillaries which enmesh the muscle fibers. The veins that carry away 
the blood generally accompany the arteries and are without valves, except where 
they empty into the coronary sinus. 
The lymphatics of the myocardium surround the cardiac muscle fibers 
as cleft-shaped hollow spaces that gather together into little vessels which run 
between the muscle bundles and empty into the larger lymph channels that 
accompany the blood vessels. The endocardium and epicardium likewise con- 
tain lymphatics which form extensive networks in the subendocardial and subepi- 
cardial tissue. Little lymphatic glands may also be found in the course of the 
lymphatics in the subepicardial tissue. 
The nerves of the heart come from the cardiac branch of the vagus 79 
and the cardiac nerves of the sympathetic, which unite to form the car- 
diac plexus. From this comes the coronary plexus, which accompanies both 
of the coronary arteries. From this coronary plexus, which lies beneath the 
epicardium, the nerves enter the wall of the heart together with the blood vessels, 
and form an extensive network in the epicardium, myocardium and endocardium, 
which enmeshes the muscle fibers in the myocardium. Fibers extend out from 
this plexus to end on the muscle fibers with little end bulbs. We also find 
sensory endings in the heart in the form of terminal tufts with many twigs in 
the connective tissue of the myocardium, endocardium and epicardium. 
The plexuses of cardiac nerves are plentifully provided with ganglia (Pl- 
37, Fig. 92). They are to be found in the coronary plexus, and also in the 
extensions of the latter into the subepicardial connective tissue of the chambers. 
Each ganglion contains a varying number of capsulated nerve cells of the sym- 
pathetic type, part of the neurites of which end in the neighboring cells, while 
part leave the ganglia to end in more distant ganglion cells. Medullated 
nerves also enter- the ganglion, perforate the cell capsules, and end in the gan- 
glion cells. 
4. THE LYMPH 
The lymph is a fluid that coagulates like blood spontaneously outside of its 
vessels. It comes from various sources, the tissues, the serous cavities, and the 
chyle of the intestines. Like the blood it consists of a plasma with corpus- 
cular elements suspended in it. The latter are lymphocytes, which the 
current of lymph is carrying to the blood, where we have already become 
acquainted with them. Together with them we find, though much more sparsely, 
transition forms and neutrophilic leucocytes. 
The lymph plasma also contains minute particles of fat brought to it in the 
chyle from the intestines, and is the fat absorbed for nutrition. When the quan- 
tity of fat is small, as during fasting, the lymph is almost as clear as water, but 
when the quantity is great, as after a full meal of fatty food, it is milky in 
appearance. 
5. THE LYMPHATICS 
The blood vessels form a completely closed system of cavities, but the same 
cannot be said of the lymphatics, for theii' beginnings, or roots, frequently are 
clefts in the connective tissue of such organs as the skin. Here belong also the 
serous canaliculi of the cornea. The lymphatics at their beginnings also form 
sheathlike spaces about the blood vessels, as may be seen very beautifully about 
those in the Haversian canals of bone, and those of the iris. The secretory 
parts of many glands are surrounded in a similar way by such lymph sheaths. 
The roots of the lacteals of the intestines are closed cavities with which we shall 
become better acquainted in the study of the intestinal villi. From these roots 
the lymph flows into very narrow channels, the lymphatic capillaries, 
which form an extensive, large meshed plexus. The lymphatic capillaries are 
quite like those of the blood in their structure, with the exception that the 
lining epithelium is always composed of larger cells. They have no valves. 
Small lymphatics are formed by the confluence of the lymphatic capil- 80 
laries, and have a connective tissue envelope added to the epithelium. Very soon 
smooth muscular fibers appear in it, and then its structure is quite similar to 
that of a small vein. We can distinguish an intima, a media, and an ad- 
ventitia, but these are very imperfectly separated. Beneath the epithelium is 
a thin layer of connective tissue permeated with network of elastic fibers. The 
media is poorly developed and contains muscular fibers that run in a circular 
course, together with some scanty elastic fibers. The adventitia consists mainly 
of bundles of connective tissue running longitudinally, with smooth muscle fibers, 
likewise running longitudinally, in the larger lymphatics and the thoracic 
duct, but never in such thickness as in the large veins. 
The lymphatics, like the veins, are provided with valves to prevent a re- 
flux of the lymph stream, but they are more delicate and are situated much more 
closely together. 
The larger lymphatics are supplied by special little blood vessels and 
nerves. Nonmedullated nerve fibers surround them with a very dense plexus 
lying in the adventitia, from which delicate fibers extend into the media and 
end on the muscle fibers. 
6. THE LYMPHOID ORGANS 
Organs are inserted in numerous places in the lymphatic system where they 
must be passed by the lymph stream, which provide for the lymph its corpus- 
cular elements and also serve as a sort of filter to arrest any foreign bodies that 
may have entered the lymph stream. 
The simplest of these lymphoid organs is the lymphatic follicle, or soli- 
tary follicle, roundish or ovoid bodies enclosed in a more or less distinct con- 
nective tissue capsule. The interior of the latter is filled with reticulated 
tissue, the meshes of which often show a distinct concentric arrangement, par- 
ticularly in the periphery, and are filled with lymphocytes. The lymphocytes 
are most scanty in the periphery of this adenoid tissue just beneath the capsule, 
so as to leave a narrow interstice known as the lymph sinus. In the center 
of the node appears the so-called germinal center, where the lymph cells be- 
come larger, the nuclei brighter, and many mitoses are always to be found; in 
other words, where the replacement of the cells continually removed from the 
lymph stream passing through the follicle takes place. The lymphatic enters on 
one side of the lymph sinus and passes out of the other, so that we may look 
upon the sinus as a dilatation of the lymphatic. 
Such solitary follicles are found in numerous places in the human body, 
especially in the various mucous membranes. 
When several solitary follicles unite they form an aggregated follicle, 
such as we find in the mucous membrane of the digestive tract, as follicular 
glands in the tongue, as tonsils in the fauces, and as Peyer's patches in the 
intestines. 
The lymphatic glands, or lymphatic nodules, present a further de- 
velopment of the purpose of the follicles. Like them they are interpolated in the 
current of lymph and chyle, and, as a result of their labyrinthine structure, the 
lymph is kept longer in the gland, is filtered better, and is given more oppor- 
tunity to become loaded with lymphocytes (Pl. 37, Fig. 93). 81 
Each lymphatic gland is encased in a capsule that consists mainly of reticu- 
lated tissue in which elastic fibers are to be found. This capsule sends processes, 
septa and trabecula, from place to place within the org;an, that soon unite 
so as to form a layer of little cham- 
bers open toward the center. Toward 
the interior of the gland these cham- 
bers run out into tubes which anas- 
tomose and form a network. Thus 
both the peripheral chambers and 
the central system of tubes are sepa- 
rated by the trabecula sent out from 
the capsule. The trabecula consist 
of reticulated tissue, like that of the 
capsule, and are strengthened by the 
blood vessels which run in them. 
The lymphatic gland is common- 
ly shaped more or less like a bean, 
so that we may distinguish a convex 
and a concave surface. On the latter 
the capsule appears to be drawn in, 
and here it penetrates into the inte- 
rior of the organ so that the periph- 
eral layer of chambers is absent. At 
this place, which is known as the 
hilus of the gland, the blood ves- 
sels enter the organ and then course 
within the trabecula. 
The system of hollow spaces 
formed by the trabecula and bounded 
externally by the capsule, is filled by 
the parenchyma of the lymphatic 
gland. This consists peripherally of more or less roundish masses, the cortical 
follicles, which taper off toward the interior of the gland into the so-called 
medullary fasciculi, or lymphoid cords, which unite and form a frame- 
work, the medulla of the lymphatic gland, that is separated from the capsule 
by the cortical follicles and comes in contact with it only at the hilus. 
The parenchyma is nothing else than adenoid tissue, i.e., reticulated tissue 
infiltrated with lymphocytes. The processes of its cells and its collagenous 
fibers join the capsule on the one hand, the trabecula on the other, but the most 
peripheral portions, those parts directly adjoining the capsule and the 
trabecula, are not infiltrated with lymphocytes, so that both the cortical collides 
and the lymphoid cords are separated from the capsule or trabecula by larger 
or smaller interspaces through which pass the fibers of the reticulum. These 
spaces form the lymph sinus, of which there are two parts, the superficial and 
the deep. The structure of the cortical follicles is like that of the solitary 
follicles. Each has a germinal center, where lymphoblasts are formed by 
segmentation of the cells present (Pl. 38, Fig. 94). 
The paths of the lymph stream through the lymph sinus are now clearly 
Fig. 29.-Schematic Picture of the Structure 
of a Lymphatic Gland. 
1, capsule; 2, trabeculum; 3, hilum; 4, cor- 
tical follicles; 5, medullary cords; 6, superficial 
lymph sinus; 7, deep lymph sinus; 8, germina- 
tion center of the cortical follicles; 9, vasa 
afferentia; 10, vas efferens. 82 
marked out. The entering lymphatics, vasa efferentia, usually are several 
little trunks that approach the convex surface of the gland, perforate the cap- 
sule, and pour their lymph into the superficial lymph sinus situated between the 
capsule and the cortical follicles. From here the lymph courses toward the 
center until it reaches the deep sinus about the lymphoid cords. From this 
labyrinth of fissures it gathers finally in the neighborhood of the hilus, where 
the sinuses come together, and form a little vessel, the vas efferens, which 
leaves the gland at the hilus. The lymph stream in passing through the gland 
thus has to pass through not only a very long, but a very narrow channel, so 
that it is furnished the most abundant opportunity to load itself with lympho- 
cytes, and at the same time to get rid of the foreign bodies it has been carrying. 
7. THE SPLEEN 
The structure of the spleen coincides with that of the lymphatic glands in 
so many points that we may consider it as a lymphatic gland interpolated into 
the circulation of the blood. It is like them of purely mesodermal origin, and it 
appears first as a conical thickening of the dorsal mesogastrium. This rudi- 
ment remains undifferentiated for a long time, until an outer capsule ai 1 an 
inner framework is developed with the entrance into it of the blood vessels. Then 
follows a marked growth of the organ, which forms an important center for the 
development of the red blood corpuscles during a certain period of emb yonal 
life. 
Like the lymphatic glands the spleen is enveloped in a capsule wk ^h sends 
numerous trabecula into the interior of the organ (Pl. 38, Fig. 95). The 
capsule is adherent to the peritoneum and consists of a dense connect *e tissue 
permeated with elastic networks, and also containing smooth muscle fibers. The 
spleen likewise has a hilus, where the capsule enters the organ along with the 
blood vessels. 
The capsule sends processes, called trabeculae, from its entire inner sur- 
face into the interior of the organ where they unite in an irregular manner. 
The trabeculae have the same structure as the capsule and carry the blood 
vessels in the same way as those of the lymphatic glands. 
The splenic pulp corresponds to the parenchyma of the lymphatic glands. 
It is an adenoid tissue traversed by numerous blood vessels, and fills the entire 
space within the capsule and the framework. But while we meet with lympho- 
blasts and lymphocytes almost exclusively in the parenchyma of the lymphatic 
glands, we find here other elements, entirely aside from the cells of the reticulum. 
First, there are red blood corpuscles, both normal and dying, and their 
remains. Then come lymphocytes and macrophages, with their bodies 
stuffed with these remnants. In the spleen of embryos and of children we also 
find the nucleated rudiments of the red blood corpuscles and giant cells, such 
as we find in adults only in the red bone marrow. 
The erythrocytes present give the splenic pulp a dark red appearance in 
which punctate white portions stand out in relief. This is the adenoid tissue 
of the spleen, which has no erythrocytes and presents a quite typical and in- 
tricate arrangement, intimately associated with the course of the blood vessels. 
We will therefore turn our attention first and briefly to these. Fig. 30.-Schematic Picture of the Structure of the Spleen. 
1, capsule; 2, trabeculum; 3, hilum; 4, pulp; 5, artery; 6, 
vein; 7, trabecular artery; 8, trabecular vein; 9, Malpighian 
corpuscle; 10, penicillate artery; 11, capsular artery; 12, 
arterial capillaries that open into the pulp; 12*, arterial capil- 
laries that open into a splenic sinus; 13, splenic sinus; 14, 
pulp vein. 83 
The arteries of the spleen enter the organ at the hilus together with the 
veins, ramify, and always course in the trabecula, from which they do not 
emerge until their diameter has diminished to about 200 Each twig is soon 
surrounded, usually at a place of branching, with a sheath of adenoid tissue, 
which appears as a little, roundish nodule about 0.5 mm in diameter, that is 
macroscopically visible and is known as a Malpighian corpuscle (Pl. 39, 
Fig. 97). The arteries coming from the corpuscles soon decrease greatly in 
caliber and break up into quite a number of minute twigs known as the peni- 
cillate arteries or penicilli. Each penicillate artery is invested after a 
prolonged course with a theca of fibers running longitudinally. The thecal 
artery is very short and is marked by the fact that its epithelial cells project 
strongly into the lumen. Very soon it loses the theca and becomes an arterial 
capillary which opens into the splenic sinuses. The sinuses form a mesh of 
medium sized blood spaces throughout the entire pulp of the spleen and give off 
the veins of the pulp, which assemble into the veins of the trabecula that ac- 
company the corresponding arteries. The splenic sinus is lined with a quite 
peculiar epithelium consisting of long, so-called rod cells, the nuclei of which 
project far into the lumen, while from their cell bodies come ledgelike projec- 
tions that run longitudinally, and others that take a circular course and inter- 
lock with the former in grooves (Pl. 38, Fig. 96). Another conception is that 
these ledges are fibers that have become independent of the rod cells, so that 
longitudinal and circular fibers have to be distinguished. Spaces are left be- 
tween the cells or the fibers through which the sinus communicates with the pulp. 
The space between the sinuses of the spleen is filled with the adenoid, loose 
tissue of the pulp. This contains a system of cavities into which a part of the 
arterial capillaries open, and between which and the sinus many communica- 
tions exist, so that two ways are open for the blood current to take through 
the spleen, one directly through the arterial capillaries into the sinus, the other 
more troublesome and affording more resistance, from these capillaries through 
the pulp into the sinus. In both cases the veins of the pulp and of the trabecula 
care for the outflow. This arrangement makes the spleen an important factor 
in the regulation of the circulation of the blood in the abdominal cavity. 
In addition to this the spleen is beyond doubt an important center for the 
development of lymphocytes, known also as splenocytes, in extrauterine life. 
These originate in the adenoid tissue and reach the sinus, part by diapedesis, 
part through the direct communications. In intrauterine life the spleen is also 
an important center for the development of erythrocytes, but this function 
ceases after birth, and then the spleen serves as a place for the annihilation of 
erythrocytes, which break down in the pulp and are seized by the macrophages. 
The haemoglobin thus set free is carried by the blood current to the liver and 
there transformed into biliary pigment. 
The spleen is extremely well supplied with nerves. By far the majority 
of these are nonmedullated fibers, yet medullated ones are to be found amongst 
them. They accompany the blood vessels, enter the pulp with them, and form 
dense plexuses about the arteries, the veins and the sinus. Sensory nerve end- 
ings may also be made out in the spleen, especially in the trabecula, which ex- 
plain how swelling of the spleen may cause severe pain. The nonmedullated 
fibers are exclusively for regulating the dilatation and contraction of the vessels. II. THE DIGESTIVE ORGANS 
1. THE LIPS 
The lips bound the oral fissure, which is the entrance to the digestive tract. 
They are covered externally by the skin of the face, internally, toward the 
vestibulum oris, by the mucous membrane of the mouth. These coverings shade 
into each other in the oral fissure, in which the transition portion is brought 
out in strong relief to the skin by its red color. We may thus distinguish in a 
cross section of a lip a cutaneous, and a mucous membranous portion, 
separated by striated muscles that serve to move it, the muscular stratum 
(Pl. 44, Fig. 108). 
The skin of the lip is like that of the rest of the body, which will be de- 
scribed in a later chapter, with certain characteristics of its own. The epithe- 
lium, or epidermis, is a stratified flat epithelium consisting of a few layers of 
cells, the outermost of which are horny. The epidermis rests on a connective 
tissue corium, which, contrary to the rule elsewhere on the surface of the body, 
contains only a very slight development of papillae. In it lie many sweat glands, 
hair bulbs, and sebaceous glands, which are particularly evident in males. 
Internally to the corium is the loose connective tissue of the subcutis, the sub- 
cutaneous tissue. 
These conditions change at the place where the skin passes over into the 
transition portion, in that the hair bulbs and sweat glands disappear, while 
the sebaceous glands remain, at least in a large percentage of the cases. At 
the same time the thickness of the epithelium gradually increases through multi- 
plication of the layers of cells, and the cornification of the outer layers de- 
creases, so that the epithelium becomes more transparent and allows the con- 
tents of the subjacent, extremely well developed blood capillaries to show 
through. The layer of connective tissue beneath the epithelium, which we must 
now call the tunica propria, corresponding to the corium of the skin, shows 
a distinct development of papillae, though they are still quite inferior. 
The transition into the true mucous membrane of the mouth takes 
place at a greater or less distance in different individuals, and is characterized 
by a still thicker epithelium, atrophy of the sebaceous glands, entire disappear- 
ance of cornification, and greater prominence of the papilke. 
It may be well at this point to elucidate briefly what is meant by a mucous 
membrane. We understand by a mucous membrane the lining of a cavity 
that communicates with the surface of the body, which, as its name implies, 
secretes a fluid mucus in many, though not in all cases. This secretion keeps the 
membrane moist and gives it its typical softness and lubricity. The mucous 
membrane is covered by an epithelium differing in structure in different cases, 
which rests on a tunica propria, consisting either of collagenous connective 
84 85 
tissue, or of reticulated tissue, and forms crests or papillce that project into 
the epithelium. Between the epithelium and the propria there is often interposed 
a membrana propria, or basal membrane. Next to the propria com- 
monly follows a tunica submucosa, which unites the mucous membrane 
proper with the musculature, or with its bony or cartilaginous substratum, and 
consists of loose connective tissue. In the submucosa, or in the propria, lie 
glands that open on the surface of the mucous membrane. When these glands 
are large and massive they often are removed far from the vicinity of the 
mucous membrane and are then united with it by long excretory ducts. 
To return now to our subject, the mucous membrane of the lips, and 
of the mouth as well, is quite generally covered by a stratified flat epithelium in 
which there are many layers of cells. It never shows cornification except on the 
tongue. The propria is a purely connective tissue formation in the mouth and 
is composed of interwoven and decussating connective tissue fibers. It forms 
papillae that often, especially near the transition portion, project deeply into 
the epithelium, but without causing the latter to protrude in adults. In new- 
born infants, on the contrary, the epithelium itself forms such papillae, or villi, 
which naturally have a papilla of the propria as a basis over which they form 
an epithelial covering. 
In the connective tissue submucosa of the lip, that now follows, lie numer- 
ous glands, the glandulae labiales, corresponding to the glandulae buccales 
of the mucous membrane of the cheek. Both are compound branched alveo- 
tubular glands. Their fairly wide excretory ducts usually pass obliquely 
through the propria to the epithelium and open in the depression between two 
papillae. In the submucosa the excretory duct enters the true gland, which lies 
here in the form of a large, distinctly lobulatcd mass, divides into many inter- 
lobular branches, from which a twig enters each little lobule, where it breaks up 
into the intermediate portions that lead to the terminal chambers, some of 
which are serous, some mucous. These glands bear a close resemblance 
to the sublingual gland, which may be referred to for further details. 
The muscular stratum interposed between the skin and the mucous mem- 
brane consists of the fibers of the orbicularis oris. They follow a circular 
course about the oral fissure and therefore appear cut across in a cross section 
of the lip. They are striated muscular fibers of medium thickness gathered 
into larger and smaller bundles. The stratum bends like a hook at the place of 
transition and ends about where the cutaneous portion begins. In addition to 
those that unite to form bundles, we always meet with numerous isolated fibers 
in the subcutaneous tissue, and between them fibers either alone or in bundles 
that have been hit longitudinally, which therefore pass obliquely through the 
lip from without inward. They also appear everywhere between the bundles 
of the orbicularis oris, wind between the glands, and reach the propria of 
the mucous membrane. These are known in their entirety as the compressor 
labii. 
The supply of blood vessels varies in different parts of the lip. It is 
least in the cutaneous portion, most abundant in the transition and mucous mem- 
brane portions. The superior labial or coronary artery and vein of the two 
sides anastomose and form a corona vascularis internal to the muscular stratum 86 
in the submucosa near the inner angle of the lip, from which twigs pass to the 
two portions and form wide meshed plexuses in the subcutis and submucosa. 
Still finer twigs extend from these to the corium and the propria. Each papilla 
of the corium or propria contains a minute arterial twig, which breaks up there 
into capillaries that reunite in one or more little veins. 
The lymphatics form a cutaneous and a submucous network that com- 
municate in the transition portion. Numerous little trunks assemble from these 
networks and lead to the submaxillary, superior cervical, and submental lym- 
phatic glands. 
The lips are supplied by both motor and sensory nerves, through the facial 
and the trigeminal. The former ends on the muscle fibers, the latter partly 
free in the epithelium of the skin and mucous membrane, partly in special end 
organs in the subcutis of the type of Krause's and Vater-Pacini's corpuscles. 
The structure of the cheeks is so familiar to that of the lips .that no further 
description is necessary. 
2. THE TEETH 
The teeth are hard organs that are phylogenetically nothing else than 
ossified cutaneous papilla?, such as we find very extensively in the skin of the 
lower vertebrates. Ontogenetically they develop from two different parts of the 
mucous membrane of the mouth, the epithelium and the connective tissue of 
the propria. 
The first rudiment of a tooth appears in a thickening of the epithelium which 
extends deeply along the margin of the embryonal jaw in the form of a crest, 
the tooth band. Corresponding superficially to it is a furrow along the 
mucous membrane of the jaw, the dental groove. The epithelium for the 
formation of each tooth dips in deeply in the form of a cone that is rather club- 
shaped at its end. A proliferation of connective tissue grows about the base 
of this club, separates it from the neighboring connective tissue, and thus con- 
verts the epithelial cone into the dental sac, the connection of which with the 
tooth band is still preserved. The peripheral parts of the dental sac then pro- 
liferate more deeply into the tissues, while the epithelium covers the connective 
tissue about the base as a cap on a head, so now we are able to distinguish 
two parts of the dental sac,-the epithelial covering, or enamel organ, 
and a mass of connective tissue with a broad base in the interior, the 
dental papilla, which is directly continuous with the subjacent connective 
tissue. 
There are ten such dental sacs in each of the upper and lower jaws, one for 
each milk tooth. While the dental sac hangs at first like a pear on the epithe- 
lial stalk, the latter gradually moves down later to the inner side of the sac and 
develops there a second clublike thickening, the rudiment of the permanent 
tooth. As soon as this is formed the stalk separates from the sac and dies, 
together with the tooth band. 
About the beginning of the fourth month of the human embryo important 
changes take place in the further enlargement of the dental sac, now lying in 87 
the deep part of the mucous membrane covering the alveolar margin, part of 
which goes to form the enamel organ, part the dental papilla. 
While the enamel organ consists at first of the same kind of epithelial cells, 
a differentiation now becomes noticeable. The peripheral cells join closely to- 
gether into a single layer of epithelium, while the central ones assume on the 
contrary the character of mesenchymatous cells, move apart, become stellate* 
anastomose with their processes, and secrete a gelatinous substance between 
themselves. Thus we have two parts of the enamel organ, the enamel epithe- 
lium, and the enamel pulp enclosed by the former. In that portion of the 
enamel epithelium that lies directly on the surface of the papilla the cells are 
long and cylindrical, while those that cover the enamel pulp above are always 
lower and flatter. We call the former the enamel cells, and their entirety 
the enamel membrane, or the inner enamel epithelium, while the latter 
is known as the outer enamel epithelium. 
Changes take place at the same time in the dental papilla. The connective 
tissue cells that lie just beneath the enamel cells separate themselves from the 
others of the papilla, elongate into long, cylindrical elements placed together 
like epithelium, and bound on all sides the substance of the papilla toward the 
enamel membrane. These are known as the odontoblasts. Thus we have 
now in the dental sac two epithelial caps, one over the other, the upper of which, 
the sac supposed to be standing upright, originates from the epithelium of the 
mouth and therefore from the ectoderm, the lower from the connective tissue 
of the mucous membrane of the mouth and therefore from the mesoderm. 
From these two epithelia comes the hard substance of the tooth, which makes 
its first appearance about the end of the fifth month of embryonal life. The 
odontoblasts secrete the greater part of this, the dentine, the enamel membrane, 
and the smaller part of the enamel. The dentine is secreted by the odonto- 
blasts in the form of a mass that is at first structureless and is deposited be- 
tween them and the enamel membrane. The odontoblasts grow with processes 
in this predentine. Each cell sends out a process that is frequently branched, 
which grows with the increase of the predentine, passing through its entire 
thickness. Collagenous fibers very soon appear in this predentine, running 
parallel to the surface of the dental rudiment, and then calcification begins and 
advances from within outward, changing the predentine into dentine. The den- 
tine forms a constantly thickening mantle about the dental papilla, which as- 
sumes the shape of the complete tooth, so that finally the papilla comes to lie 
in a little cavity in the tooth, the pulp cavity, and is joined to the subjacent 
connective tissue by the narrow root canal, which passes through the lower, 
projecting segment of the dentine, the dental root. In the young tooth what 
was the papilla forms the contents of the pulp cavity, the pulp, the most super- 
ficial cells of which are odontoblasts. 
The development of the enamel sets in somewhat later than that of the 
dentine. First each enamel cell excretes from its surface a cuticula, from 
which a process extends toward the dentine, Tomes's process; it calcifies and 
is separated from its neighbor by a richly developed cement. The calcifica- 
tion advances with a gradual reduction of the enamel cells to the outer surface, 
and changes each enamel cell into a long, prismatic, calcified formation, the 88 
enamel prism, which stands about perpendicularly to the surface of the 
dentine. The young enamel prisms are separated from one another by an 
abundant cement, which gradually disappears as the prisms continue to grow. 
The entire enamel membrane does not take part in this formation of enamel, 
only the portion surrounding the upper segment of the dental rudiment, which 
later becomes the crown of the tooth. Below the inner and outer enamel 
epithelia begin to draw together at an early period with atrophy of the enamel 
pulp, and the farther the development of the enamel advances the more does this 
approximation progress upward, until finally the entire enamel pulp is oblit- 
erated and even the remains of the enamel epithelium are destroyed. Of the cells 
of the enamel membrane the cuticula alone is preserved, which after birth covers 
the young enamel as a continuous layer, the cuticula dentis. 
A third tissue is added to the dentine and enamel in the first month of life, 
the cement, which is a bony substance secreted by the connective tissue about 
the root of the tooth. 
We have therefore to distinguish in the complete tooth:-a, The dentine, 
which forms the main part of the entire tooth and encloses the pulp cavity 
and the root canal, but never appears on the surface; b, the enamel which 
covers the dentine like a cap, is thickest above, gradually becomes thinner until 
it ceases at the neck of the tooth, and therefore is present only over the free 
crown of the tooth projecting from the gums where it is covered by the cuti- 
cula dentis; c, the cement, which begins where the enamel ceases, covers the 
entire root with a thin layer, and extends to the beginning of the root canal, 
the foramen apicis; d, the pulp, which fills the cavity of the tooth. To these 
may be added e, the periodontal membrane, and f, the gums. We will 
briefly discuss each of these in turn. 
a. The Dentine 
We can differentiate a dentine of the crown and a dentine of the root. Its 
structure can easily be understood from what has been said concerning the 
embryology of the teeth. The basal substance consists of collagenous fibrils 
which run parallel to the inner surface of the tooth, but cross in all possible 
directions. They form numerous layers one over another and are not calcified 
themselves but are held together firmly by a calcareous cement. Within this 
basic substance uncalcified places are to be found in the crown dentine near the 
border between the enamel and the dentine in the form of irregular cavities, the 
interglobular spaces, into which the calcareous basal substance projects 
with roundish knobs, the dentine globules. The interglobular spaces in the 
crown dentine are quite large and form a continuous layer parallel to the border 
of the dentine enamel; in the dentine of the root they are much smaller, but 
have the same position and arrangement, and here form Tomes's granular 
layer (Pl. 43, Figs. 104 and 105). 
The basal substance of dentine is permeated by dental tubules, or 
canaliculi, which radiate from the pulp cavity to the margin of the enamel 
or cement in such a way that they run almost exactly transversely in the root 
and always obliquely upward. They are so dense as to give the dentine a char- 89 
acteristic striated appearance. Where they open into the pulp cavity they 
have a diameter of 4 or 5 p., which gradually diminishes as they give off numer- 
ous fine lateral twigs through which both neighboring and distant canaliculi 
form many anastomoses. They divide dichotomously sooner or later, and have 
blind ends at the enamel dentine margin, oi' form arching connections. Part of 
them pass this border and end in the enamel with a little swelling. The inter- 
globular spaces are permeated by the canaliculi, which frequently end in the 
dentine of the root in Tomes's granular layer (Pl. 43, Figs. 104 and 105). 
The basal substance forms particularly resistant sheaths about the canaliculi 
and theii- twigs, just as in bone, the so-called Neumann's sheaths. 
b. The Enamel 
The enamel is the hardest organic substance of all animal tissues and con- 
tains the least water. It is composed of enamel prisms bound together with 
cement. The prisms are not .of uniform thickness, but become thicker from 
the dentine to the outer surface (Pl. 43, Fig. 106). In the cross section they 
are pretty regularly pentagonal or hexagonal, and are indistinctly striated in 
the longitudinal section. The substance of which they are composed is doubly 
refractive. As the enamel prisms pass through the entire thickness of the 
enamel in screwlike windings they are never met with throughout their entire 
length in a section, but only in short transverse or oblique segments. In the 
longitudinal section of a tooth dark lines appear in the enamel which are known 
as the striae of Retzius (Pl- 42, Fig. 103; Pl. 43, Fig. 104). In the neighbor- 
hood of the neck they run obliquely to the surface of the dentine, but the 
farther up we go in the tooth the more acute becomes the angle formed, until 
they are almost parallel just over the top of the dentine. Schweger's striae, 
which appear as horizontal parallel lines on the surface of the enamel, do not 
change in this way; they are to be seen as transverse bands in the section. 
The surface of the enamel is covered by the cuticula dentis, or enamel 
cuticle. This is, as we have already seen, the original cuticle of the enamel 
cells, is extremely resistant, calcified, structureless, and attains a thickness of 
only 1 or 2 p.. 
c. The Cement 
All parts of the outer surface of the tooth that have no enamel, that is, the 
neck and the root, are covered by the cement, which usually extends a little 
way over the enamel (Pl. 42, Fig. 103). Its thickness increases from above 
downward and is thicker on the permanent than on the milk teeth. It originates 
from the connective tissue about the dental sac in the way we learned in the 
study of bone substance, and therefore consists of a fibrillary, stratified basal 
substance, in which are placed bone cells with numerous offshoots enclosing 
bone cavities (Pl. 43, Fig. 105). The thinner the layer of cement, the more 
sparse are the bone cavities, which are almost entirely wanting in the region of 
the neck. Haversian canals may also be met with in old, thick cement. 
Sharpey's fibers, with which we became acquainted in the study of bone, may 
be seen very distinctly in the cement passing vertically through the lamellae of 90 
the basal substance that run parallel to the surface. They appear in the sec- 
tion of the macerated tooth as little hollow tubes. 
d. The Dental Pulp 
The pulp fills the pulp cavity and the root canal or canals, and con- 
sists of connective tissue richly supplied with vessels and nerves (Pl. 44, Fig. 
107). It contains numerous fine, decussating connective tissue fibers with very 
many stellate connective tissue cells between them. Toward its surface the 
tissue becomes richer in cells, denser, and finally consists next the dentine of a 
coat of long cylindrical cells, the odontoblasts. These cells send out several 
fine processes into the pulp, where they are lost, and each cell also sends a long 
fiber to the dentine, the dentine fiber, oi' Tomes's fiber, which enters a 
canaliculus and extends in it a long way. 
The blood vessels that enter the pulp through the root canal course 
through its middle and send branches to the periphery, where a dense capillary 
network is formed which reaches into the layer of odontoblasts. 
The pulp is also supplied with lymphatics, that gather into several little 
trunks in the root canal and leave it to enter the maxillary or deep cervical 
glands. 
The nerves accompany the blood vessels, branch and form a wide meshed 
plexus within the pulp from which fibers extend, after losing their medulla, to 
form a second plexus within the layer of odontoblasts. Minute fibrils from this 
penetrate the dentine with the dentine fibers and form there a third plexus, 
which again sends fibers to the border between the dentine and the enamel, or 
the cement, where they end with little clublike thickenings. No nerves exist in 
the enamel or the cement. 
e. The Periodontal Membrane 
By the periodontal membrane, or the periodontium, we understand 
the tissue that joins the root of the tooth to the alveolus and its neck to the 
gums. It consists mainly of connective tissue fibers, part of which extend 
transversely, part obliquely between the parts mentioned. They are inserted 
at one end into the cement of the root, at the other into the bony substance of 
the alveolus, where they appear as Sharpey's fibers. The intermediate space 
is filled with a loose connective tissue containing blood vessels and nerves. 
f. The Gums 
We call that part of the mucous membrane of the mouth the gums which 
covers the alveolar processes of the jaws, clothes the neck of each tooth that 
protrudes from the alveolus, and forms a fold over the septa interalveolaria be- 
tween every two teeth. The gums are firmly united to the periosteum of the 
alveolar process by its subcutaneous tissue, and are also connected with the 
neck of the tooth through the periodontal membrane. Its structure is in general 
that of the mucous membrane of the mouth already described, from which it 91 
differs in having high papillae of the propria, and in the total absence of glands. 
It is richly supplied with blood vessels and sensory nerves, and possesses a very 
extensive lymphatic network within the propria. 
3. THE TONGUE 
The tongue is substantially a muscular organ that develops in the floor 
of the mouth from four separate rudiments, two of which are paired and two are 
not, that appertain to the ventral ends of the first three branchial arches. These 
rudiments unite, the two paired ones forming the body, the others the root of the 
tongue, which is phylogenetically the older portion. The musculature, which 
for the most part belongs to the postbranchial myotome, then invades this con- 
nective tissue and epithelial rudiment from behind. We have therefore to con- 
sider first the mucous membrane with the submucosa, and then the mus- 
culature. 
The mucous membrane of the tongue resembles on the whole that of 
the lips and cheeks, yet displays characteristic differences. In the anterior 
portion of the front part of the tongue the papillary development attains a 
magnitude found nowhere else in the mouth. The part lying behind the sulcus 
terminalis, the after part of the tongue, has on the contrary no papillae. We 
meet with high papillae in the propria of the mucous membrane of the lips and 
especially of the gums that are always covered so smoothly with epithelium as 
to scarcely appear superficially, but in the mucous membrane of the tongue the 
papillae are so high as to project above the surface as pointed or fungus shaped 
elevations. These papillae may be divided according to their shape into filiform, 
fungiform, circumvallate, and foliate. 
The filiform papillae are spread uniformly over the entire front part of 
the tongue, rise with broad bases out of the propria, and either simply run to 
a point, or form a sort of socle or base from which the slender, secondary 
acuminate papilla projects (Pl. 45, Fig. 109). Such a socle may support as 
many as twenty secondary papillae. They stand very close together in the 
mucous membrane, have a quite pale or grayish color, and give the surface of 
the tongue its peculiar velvety appearance. 
The fungiform papillae are usually shorter. The slender body is thick- 
ened above so that the papilla looks like a club, or a mushroom (Pl. 45, Fig. 
109). Here also secondary papillae project from a papillary base, yet the 
epithelium passes smoothly over them and fills more or less completely the inter- 
vening depressions, so that the secondary papillae can scarcely be seen ex- 
ternally. The fungiform papillae differ from the filiform not only in their shape, 
but also in their color; they are always bright red, and are scattered over the 
back of the front part of the tongue, more numerous in the anterior than in the 
posterior parts. 
The circumvallate papillae are found only in small numbers, seven to 
twelve, at the line of demarcation between the front and back parts of the 
tongue, arranged in the form of an obtuse V open forward, the apex of which 
lies in the foramen caecum. They are inverted, broad, truncated cones, the 
smaller parts of which are sunken deeply into the mucous membrane so that 92 
the bases, 1 to 2 mm broad, look upward and are placed in the plane of the 
surface of the mucous membrane (Pl. 45, Fig. 110). The latter forms about 
the papillae a low wall which is separated from it by a deeply cut, circular 
groove. The surface of the circumvallate papillae is studded with low secondary 
papillae which do not project externally. 
The foliate papillae, which are rudimentary in man, are found in the back 
part of the margin of the tongue as several low folds that run obliquely to the 
surface of the tongue and are separated by slight furrows. Each fold is studded 
with numerous secondary papillae. On the back part of the tongue the place of 
the papillae is taken by the lingual follicles, roundish or elongated, flat ele- 
vations of the mucous membrane that may be as much as 4 mm in diameter. 
They have in the middle a little opening which leads into a short, blind canal, 
usually running obliquely to the surface. 
The epithelium of the mucous membrane of the tongue is of the flat 
variety and is composed of many layers. It covers all of the papillae, dips into 
the sulci about the circumvallate papillae, and lines the cavities of the lingual 
glands. Its most superficial layers are cornified, where they pass over the 
filiform papillae. The outermost horny cells give these papillae the gray color 
that contrasts with the coloi' of the fungiform papillae which are not horny. 
These horny cells, which are constantly being cast off and appear in large masses 
in the saliva, form very large, flat elements that often take the form of little 
cones stuck into one another. 
Another characteristic of the epithelium of the tongue is the appearance 
of peculiar budlike formations, which are the end organs of the glossopharyn- 
geal nerve and are known as taste buds (Pl. 45, Fig. 110; Pl. 92, Fig. 197). 
Although these are found in all of the fungiform papillae of fetuses and infants, 
they are confined in adults mainly to the epithelium of the lateral surface of the 
circumvallate papillae and the opposite surface of the surrounding sulcus. They 
also occur in the epithelium of the foliate papillae. Their structure will be de- 
scribed when we come to speak of the organ of taste. 
The propria of the mucous membrane of the tongue consists of a dense 
connective tissue containing many cells. It forms the main part of the papillae. 
In the region of the lingual follicles it undergoes an important alteration in 
that lymph follicles are embedded in it, and the connective tissue changes 
into reticulated tissue, the meshes of which seem to be stuffed with lymphocytes. 
Quite a number of such follicles lie in each of the lingual follicles, where they 
lie closely together and surround the cavity of the sac with a complete lymphoid 
mantle just beneath the epithelium. Lymphocytes wander continually from the 
follicles through the epithelium into the cavity of the lingual follicle, where they 
seem to be destroyed. 
The submucosa of the tongue joins the mucous membrane and the mus- 
culature, and is continuous with the connective tissue of the latter. On the 
dorsum of the tongue the connective tissue is greatly developed and has received 
the name of fascia linguae. The lingual glands lie in the submucosa, fre- 
quently reaching inward to between the muscular bundles. They are distributed 
very unevenly over the after part and the posterior portion of the fore part 
of the tongue, leaving the anterior three quarters of the latter free, except for 93 
a small area in the middle line of the apex, where they form near the under 
surface the Blandin-Nuhn's, or anterior lingual glands. 
The great bulk of the lingual glands, particularly all of the posterior ones, 
are purely mucous, and belong to the compound branched alveotubular type. 
The excretory ducts open either on the surface of the mucous membrane, or 
in the cavities of the lingual follicles, are lined with cylindrical epithelium, 
traverse the propria, and reach the lobes of the gland situated in the submucosa. 
A branch of the efferent duct enters each lobule, breaks up into many twigs, and 
terminates in the end chambers lined by high cuboidal mucous cells. 
Serous glands, or Ebner's glands, are found only beneath the circum- 
vallate and foliate papillae, where taste buds are present (Pl. 45, Fig. 110). 
They are compound branched tubular glands. Their excretory ducts, likewise 
lined with cylindrical epithelium, open in the floor of the groove surrounding 
the papillae, branch in theii' course through the submucosa, and change into the 
end tubules in the gland. The end tubules are lined with a single layer of 
cuboidal or conical cells, between which many secretory capillaries are to be 
found. The cells elaborate a secretion rich in albumin, the initial stages of 
which distend their bodies with fine granules. 
The Blandin-Nuhn's is a mixed gland, the structure of which is just 
the same as that of the sublingual gland, which will be described later. 
The musculature of the tongue consists of striated muscle fibers that 
appertain to the outer and inner muscles of the tongue (PL 45, Figs. 109 and 
110). Among the former are the hyoglossus, the genioglossus, and the stylo- 
glossus ; among the latter are the longitudinal, transverse, and vertical lingual 
muscles. These muscles are so arranged that in a cross section of the tongue 
part of the bundles are seen running from above downward, part from right to 
left, and part cut transversely. They are imperfectly separated by the median 
septum lingua?, composed of connective tissue, into right and left halves, and 
they end in the fascia lingua?. 
Part of the arteries of the tongue extend to the muscles, part enter the 
submucosa and thence supply the mucous membrane. They form a rich ar- 
terial plexus from which twigs extend on the one hand to break up into long 
capillary meshes about the muscle fibers, on the other into the propria where 
they form capillary loops in the papilla? just beneath the epithelium. The 
glands also are enmeshed by a well developed capillary plexus. The veins 
formed by the confluence of the capillaries follow the same course as the 
arteries. 
Networks of lymphatics are found both in the propria and its papillae, 
and in the submucosa, the efferent trunks of which lead, some to the cervical, 
some to the lingual lymphatic glands. 
The nerve supply of the tongue is very complicated. The hypoglossus 
furnishes all of the muscles with motor twigs which terminate in end plates on 
the striated muscle fibers. The nerve of taste is the glossopharyngeal, the 
lingual branches of which penetrate to the submucosa of the back part of the 
tongue and the region of the circumvallate and foliate papillae, where they form 
a large plexus from which nerves extend to the mucous membrane. Part of the 
fibers end free in the epithelium, part form another plexus about the taste buds 94 
from which twigs enter each. The glossopharyngeus also sends out gustatory 
fibers by way of the minor superficial petrosal nerve, the otic ganglion, and the 
chorda tympani, which reach the anterior part of the tongue in company with 
the sensory fibers of the lingual branch of the trigeminus. The sensory fibers in 
turn form a plexus in the submucosa from which fibers rise into the papillae, 
where some terminate in the epithelium, and others enter special end corpuscles 
within the papillae, part of which are of the Vater-Pacini, part of the Krause 
and Ruffin type of construction. The glossopharyngeus is the secretory nerve 
of the lingual glands. 
Small ganglia occur in the plexus formed by the sensory nerves of the 
tongue, consisting of a few cells that belong to the sympathetic type. 
4. THE PALATE 
The hard palate, that separates the mouth from the nasal cavity, is cov- 
ered by a mucous membrane the characteristic feature of which is that it is 
very firmly and tightly joined to the periosteum, especially in its anterior por- 
tion. It differs in no way from that of the lip. Palatine glands are to be 
found in the submucosa; they are most abundant in the posterior part, become 
more and more scanty as we pass forward, and are completely absent in front 
of the bicuspids. Like the glands of the posterior part of the tongue they are 
purely mucous, and their excretory ducts empty on the surface of the mucous 
membrane through numerous openings that are visible to the naked eye. 
The soft palate, which is a continuation of the preceding, is a membrano- 
musculai' fold that incompletely separates the cavity of the mouth from that 
of the pharynx. In front it belongs to the mouth, behind to the pharynx, and 
this dual nature is stamped in the structure of its mucous membrane. 
The mucous membrane of its anterior surface is the same as that 
of the hard palate, like it consisting of a stratified flat epithelium and a propria 
having low papillae. The papillae become higher and denser the nearer we ap- 
proach the free margin. The submucosa contains quite a layer of mucous 
glands, a direct continuation of the glands of the hard palate, as well as 
numerous fat cells, and is permeated toward the propria with dense elastic 
networks. 
The mucous membrane of its posterior surface is, on the contrary, 
a continuation of that of the posterior nares, and like the latter is covered 
with stratified cylindrical epithelium provided with cilia. The transition be- 
tween these two territories is not at the free margin, but on the posterior sur- 
face, so a part of the latter is covered with stratified flat epithelium. A propria 
with numerous high papillas comes next to the epithelium. In the submucosa 
lie glands, which are not purely mucous, but are mixed and are of the type 
of those of the lips. 
A strong muscular layer is interposed between the mucous membranes 
of the anterior and posterior surfaces; this serves to move the soft palate and 
consists of striated fibers. Many collections of glands enter between the muscle 
bundles from the anterioi' surface. 95 
The blood vessels and lymphatics resemble those of the lips and cheeks. 
The lymphatics empty into the deep cervical glands. 
The nerves of the palate are partly motor, partly sensory. Those that 
supply the muscles come from the trigeminus and from the pharyngeal plexus, 
the sensory nerves from the sphenopalatine ganglion. 
The tonsils are embedded in the sinuses tonsillares between the pillars of 
the fauces on each side. These are nothing else than greatly enlarged follicular 
glands, such as are seen on the posterior part of the tongue, and may be 
imagined to originate from the packing together closely of from twelve to fifteen 
such follicles. Correspondingly the tonsil presents on its surface oval openings, 
each of which leads into a follicular cavity, called the tonsillar sinus, that is 
not simple as in the follicles of the tongue, but presents many prongs (Pl. 46, 
Fig. 111). Otherwise the structure is just the same as that of the lingual 
follicles. Mucous glands are to be found in the submucosa which for the most 
part open in the floor of the follicle cavity. The tonsil is separated from the 
surrounding mucous membrane by a tight connective tissue capsule furnished 
by the submucosa. Each follicle is supplied by it with a separate sheath that 
reaches to just beneath the surface. 
The lymphatics form in the tonsil a very dense plexus between the in- 
dividual follicles, with branches leading into each, and empty into the deep 
cervical glands. 
5. THE SALIVARY GLANDS 
The large salivary glands of the mouth, the submaxillary, sublingual, 
and parotid, develop like the small glands of the mucous membrane from 
eversions of the epithelium. A solid cone of the latter grows down into the 
tissue and, after it has reached a certain length, sends out lateral branches from 
place to place, which in turn send out more sprouts, until they finally end in 
terminal buds. This glandular trunk is at first solid, but becomes hollow later 
by a separation of its epithelial cells, and then through a differentiation of 
these, becomes divided into an excretory and a secretory system. At a very 
early period a capsule is formed about the gland rudiment from the surround- 
ing connective tissue, that is at first purely cellular. It forms septa that sep- 
arate the terminal regions of the excretory ducts and divides the entire rudi- 
ment into numerous lobes. The connective tissue cells also lie about the excre- 
tory ducts and end pieces of the gland rudiment and surround them with a 
basketlike envelope. 
The secretion of the large salivary glands, together with that of the 
glands of the mucous membrane, forms the saliva, a viscid, frothy fluid as clear 
as water, of neutral or slightly alkaline reaction, made up of 9.4% organic, 
0.6% inorganic matter, and 99% water. Among the former constituents are 
the salivary mucin, an albuminoid, ferments, oxydase, and sulphocyanate of 
potassium. The most important of the ferments is ptyalin, which changes 
starch into dextrin and sugar, and thus transforms a large part of the food 
into a soluble form in which it can be absorbed. The fluid also contains morpho- 
logical elements, the salivary corpuscles, round, rather swollen cells, in 96 
which a nucleus cannot always be demonstrated. They contain numerous 
granules that exhibit Brown's molecular movement very beautifully, a condition 
that indicates at once that we have to deal here with dead cells. Formerly 
it was quite a general belief that these were lymphocytes which had been formed 
in the lymphoid organs of the mouth, the tonsils and the follicular glands of 
the tongue, as well as in similar organs in the pharynx, and had reached the 
mouth through the epithelium, but we know now that the greater part if not all 
of the salivary corpuscles are not lymphocytes, but neutrophile leucocytes 
which wander into the salivary glands and also into the mouth through the 
epithelium, where they reach the saliva, swell up and die. They furnish the 
saliva its oxydase, or enzymes. 
We will pass now to the consideration of the individual glands. 
The Submaxillary Gland 
The submaxillary, the largest of the human salivary glands, is a mixed 
gland, i.e., its secretion is a fluid containing mucus and albumin, which is liquid 
and only slightly viscid. 
Its excretory duct, which opens in the mouth close to the frenulum 
linguas on the caruncula sublingualis, is a fairly thin walled duct lined with a 
double layer of cylindrical epithelium and encased in a sheath of connective 
tissue containing many elastic fibers. 
The body of the gland is enveloped by a connective tissue capsule, and is 
divided by numerous septa into a large number of larger tertiary, and smaller 
secondary and primary lobes and lobules. 
Within the gland the excretory duct breaks up into numerous branches 
which first run between the tertiary lobes and then, constantly dividing and 
becoming smaller, between the secondary lobules. Here the double layer of 
cylindrical epithelium becomes single, the connective tissue sheath gradually dis- 
appears, and the lumen contracts (Pl. 47, Fig. 114 ; Pl. 48, Fig. 116). When the 
little excretory ducts finally arrive in the septa that separate the primary 
lobules they divide again and an important change takes place in each section. 
The lumen dilates again somewhat and at the same time the cells become higher 
and cylindrical. While the cells in the little excretory ducts are almost cuboidal 
and are distinctly separated by boundary lines, these cells are higher and very 
imperfectly defined. The nucleus lies close to the lumen, and the basal segment 
of the cell appears to have radiating striae. This striation is due to the fact 
that the cell protoplasm here forms long, radiating meshes with numerous 
microsomes embedded in the walls. We call this formation, which we meet with 
in other glandular cells as well, Heidenhain's rods. Near the nucleus this 
rod structure ceases and the entire cell body is distended by similar granules 
that lie so close as to completely cover the protoplasm. This part of the 
glandular cavity is known as the salivary tubules. They belong not to the 
excretory, but to the secretory portion of the glandular substance. 
The salivary tubules, which thus lie between the primary lobules, are com- 
paratively short and soon break up, with considerable diminution in size, into 
the intermediate pieces tha.t enter the primary lobules and on which the 97 
terminal chambers hang like berries on a common stem (Pl. 47, Fig. 114). 
These are either spherical or rather elongated, saclike, and together compose 
a primary glandular lobule. In the intermediate piece the cylindrical epithelium 
of the salivary tubule changes into a lower, cuboidal, indifferent variety, which 
gives place in the terminal chambers to the true secretory epithelium. 
The latter varies in the different terminal chambers. The spherical ones 
are lined with cuboidal, or better conical, cells and enclose a very narrow cavity 
from which numerous secretory capillaries extend between the glandular 
cells (Pl. 48, Fig. 115). Roundish nuclei are to be seen in the cells, sometimes 
near the center, sometimes close to the base, and the cell bodies contain more 
or less numerous granules. When the gland is at rest they lie uniformly thick 
throughout the entire cell, but during its activity they become discharged into 
its lumen, where they very soon become fluid and are lost to sight. The granules 
in the cell gradually decrease, advancing from the base toward the lumen, and, 
when the activity is maintained long enough, a time comes when the cells are 
completely empty of granules. Then follows their replacement from the micro- 
somes of the base. This proceeds from threadlike formations near the nucleus 
known as basal filaments, which are beyond doubt of a mitochondric nature. 
Usually the secretory activity of the gland is less intense than this, so that the 
discharge and new formation of granules keep pace with each other. The 
granules consist of a substance like albumin, certainly contain the oxydase of 
the saliva, and probably also the ptyalin, or its primary stages. 
The elongated, saclike, terminal chambers present essentially different pic- 
tures. In them the low cells of the intermediate pieces are replaced by high 
cuboidal or cylindrical cells which exhibit all of the well known properties of 
mucous cells. At the blind ends of these mucous sacs are groups of albuminous 
cells arranged like hoods, which terminate the sacs and are known as Gian- 
nuzzi's crescents. The lumen of the saclike terminal chambers is consid- 
erably wider than that of the spherical. From its end pass out several secretory 
capillaries to penetrate between the crescent cells (Pl. 48, Fig. 115). 
The majority of the terminal chambers composing the primary lobules of 
the gland in man are spherical, albuminous alveoli; the mucous ones are greatly 
overshadowed and there are many lobules in which they are entirely absent. The 
human submaxillary is doubtless a mixed, yet it is a preponderatingly 
serous gland. 
Each terminal chamber is surrounded by a structureless membrana propria, 
but between it and the epithelium is interposed a layer of branched, stellate cells 
that anastomose with one another, the so-called basket cells. These may also 
be demonstrated on the salivary tubules, but disappear on the excretory ducts. 
The blood vessels enter the gland with the submaxillary duct and on the 
whole follow the same course as its branches and twigs. When they arrive at 
the primary lobules they break up into capillaries which wind about the ter- 
minal chambers just beneath the membrana propria. 
The lymphatics accompany the blood vessels, twisting about them, enter 
the primary lobules, and end in spaces that surround the terminal chambers 
with the capillaries that are woven about them. 
The nerves of the submaxillary gland come from the submaxillary gan- 98 
glion, composed of sympathetic ganglion cells, which receives sensory fibers from 
the trigeminus through the lingual nerve, secretory intermediate fibers through 
the lingual nerve from the chorda tympani, and sympathetic fibers from the 
external maxillary plexus. The fibers that enter the gland from it are for the 
most part medullated, with a few that are nonmedullated. They accompany 
the blood vessels and lymphatics and are furnished with numerous little sympa- 
thetic ganglia. Within the primary lobules the nerve fibers form medullated 
and nonmedullated plexuses about the terminal chambers, from which pro- 
ceed only nonmedullated fibers that pass through the membrana propria and 
apply themselves with little swellings to the secretory cells. The nerves behave 
in quite a similar manner toward the salivary tubules. 
The Sublingual Gland 
After we have become intimately acquainted with the structure of the sub- 
maxillary, that of the other salivary glands may be dealt with more briefly. 
The sublingual gland consists of a large and several smaller parts which 
likewise have a major and several minor sublingual ducts. Like the sub- 
maxillary it is a mixed gland of compound branched alveotubular structure, 
and the principal differences between the two are that the parenchyma of the 
sublingual consists mainly of mucous terminal chambers with Gianuzzi's cres- 
cents, and that the cells of its salivary tubules have no rod structure (Pl. 46, 
Fig. 112). Consequently the saliva secreted by the sublingual is much more 
viscid, more alkaline, and less albuminous than that from the submaxillary. It 
contains only small quantities of ferments and oxydase. 
The relations of the vessels and nerves are the same as in the submaxillary 
gland. 
The Parotid Gland 
The parotid, unlike the sublingual, is a purely serous gland. Its 
parotid duct, as well as the rest of its efferent system, is exactly the same 
as that of the submaxillary. The smallest excretory ducts change into salivary 
tubules with rod epithelium, and the terminal chambers are all spherical, al- 
buminous alveoli (Pl. 47, Fig. 113). 
The saliva from the parotid is consequently free from mucin, liquid, neutral, 
or only slightly alkaline, and abounds in albuminous ferments and oxydase. 
The lobulation of the gland by the densely developed interlobular con- 
nective tissue is very prominent. 
The nerves of the parotid come from the glossopharyngeus and reach the 
gland by way of the tympanic nerve, the tympanic plexus, the minor super- 
ficial petrosal, the otic ganglion, the auriculotemporal and its parotid branches, 
and are associated with sympathetic twigs from the plexuses about the vessels. 
Small ganglia are also to be found in the interlobular connective tissue. The 
endings of the nerves are the same as in the submaxillary gland. 99 
6. THE PHARYNX 
Conformably to its double function as the entrance for food and for air, 
the pharynx has a mucous membrane clothed with epithelium that differs in 
different places. The nasal part, or naso-pharynx, i.e., the part that is 
closed below by the velum during the act of swallowing and into which the 
posterior nares open, is lined with the stratiform ciliated epithelium of 
the latter, while the oral and laryngeal parts have a stratified flat epithe- 
lium. 
The propria is well developed everywhere; it contains many high papillae 
in the oral and laryngeal parts, and is characterized by the great networks 
of elastic fibers within it, that separate it from the subjacent muscles, into 
which it sends interfascicular septa. Lymph follicles also occur in the 
propria of the pharyngeal mucous membrane, which coalesce in two places into 
tonsillar formations, the pharyngeal tonsil in the middle of the roof of the 
pharynx and the tube tonsil at the mouth of the Eustachian tube. Both of 
these resemble in their minute structure the faucial tonsil. 
A submucosa is developed only in the laryngeal part of the pharyngeal 
mucous membrane, so the pharyngeal glands lie for the most part between 
the musculai' bundles beneath the mucous membrane. These glands are found 
in all parts of the pharynx and the great majority of them are purely mucous, 
mixed glands occur only in the nasal part. 
The muscular coat of the pharynx is composed of the constrictors and 
levators, which contain only striated muscle fibers. It is covered externally 
by a connective tissue fascia rich in elastic fibers, which is known as the fibrous 
membrane, or the tunica adventitia of the pharynx. 
The pharynx receives its blood chiefly from the ascending pharyngeal 
artery, the branches of which supply the muscles, glands and mucous membrane. 
The veins as well as the lymphatics form a superficial, mucous membranous, 
and a deep, muscular plexus. The lymphatics flow into the retropharyngeal 
and the deep cervical glands. 
The nerves of the pharynx come in part from the glossopharyngeal, in 
part from the vagus, and in part from the superior cervical ganglion, and 
form a plexus provided with a ganglion on the outer surface of the constrictor 
medius, whence fibers pass to the muscles (vagus) and the mucous membrane. 
An organ that should be mentioned here occurs in the roof of the pharynx 
at the mouth of the canalis craniopharyngeus and is known as the Hypophysis 
of the roof of the pharynx. It consists of a connective tissue capsule, 
which encloses a parenchyma composed of epithelial cords and tubes lined with 
epithelium, the cells of which are similar to those of the anterior lobe of the 
hypophysis cerebri. The parenchyma is abundantly supplied with blood vessels. 100 
7. THE (ESOPHAGUS 
After the complete closure of the entodermal intestinal tube its uppermost, 
blind segment, which joins the ectodermal oral diverticulum by bursting through 
the pharyngeal membrane, consists of an epithelial tube surrounded by a thin 
layer of mesenchymatous cells. The primarily fairly wide lumen later becomes 
narrow, and is lined by a double layer of cuboidal epithelium. In consequence 
of a lively cell division the epithelium comes to be formed of several layers and 
cilia appear on the layer facing the lumen. The cylindrical ciliated cells unite 
into groups, between which lie areas covered with cuboidal cells without cilia. 
Then the entire superficial layer of cells is cast off and we have an epithelium 
composed of several layers of cells, the uppermost of which flatten more and 
more. During this time the entire section has greatly enlarged and the tube 
lies in folds running longitudinally. The development of glands begins in the 
fourth month by the invagination of the epithelium in the cranial and caudal 
parts in the form of branched tubes. Similar invaginations are formed later 
in all of the other parts of the mucous membrane. The mesenchymatous cells 
enveloping the epithelial tube differentiate in two directions: one to provide a 
connective tissue propria and submucosa, the other to form myofibrils within 
themselves and to surround the lower two thirds of the tube with a mantle of 
smooth muscle fibers. Striated muscle fibers are formed about the upper third, 
but at a considerably later period. 
The mucous membrane of the oesophagus, which lies in several longi- 
tudinal folds, is lined by an extensively stratified epithelium resembling that, 
of the mouth, but nowhere horny (Pl. 49, Fig. 117). Islands not rarely occui* 
within this flat epithelium of simple mucous cylindrical epithelium, such as 
those we shall soon become acquainted with in the stomach. These places are 
called islands of the gastric mucous membrane. 
The epithelium rests on a connective tissue propria, which forms high 
papilla?, and is separated from the subjacent submucosa by a layer of smooth 
muscle fibers (Pl. 49, Fig. 118). This we meet with here for the first time, and 
it will accompany us through the entire digestive tract; we call it the mus- 
cularis mucosae. It appears in the uppermost part of the oesophagus in 
connection with the elastic membrane mentioned under the pharyngeal mucous 
membrane, as separate bundles of muscular fibers which soon come together 
and form a continuous layer. 
Glands also appear in the oesophagus for the first time within the propria, 
for hitherto they have been met with only in the submucosa, or in the sub- 
jacent muscles. These, known as cardiac oesophageal glands, are con- 
stant only in the lowermost segment of the oesophagus, where they surround 
the orifice into the stomach. They belong to the branched tubular or alveo- 
tubular type. They resemble perfectly the fundus and pyloric glands of the 
stomach, to which we may refer for a description. They are less constant, but 
are to be found in the upper part of the oesophagus, particularly at the level 
of the lower margin of the cricoid cartilage, where they are spoken of as the 
Upper cardiac oesophageal glands. These glands may be associated 101 
again with the above mentioned cylindrical epithelium, which appears in the 
form of depressed dimples. We have then in the oesophagus true gastric mucous 
membrane islands which can be seen plainly with the naked eye. 
Lymph follicles also appear in many places in the propria, as in the 
mouth and pharynx. 
The submucosa, which is strongly reinforced by elastic networks, con- 
tains purely mucous, compound branched alveotubular glands, that may be 
considered a continuation of those of the mouth and pharynx. They lie in little 
groups which are fairly thick in the upper part of the oesophagus and gradually 
diminish in number downward. The excretory duct is usually dilated some- 
what like an ampulla, passes obliquely downward through the propria and opens 
at the top of a papilla. 
The muscularis of the oesophagus presents an inner, circular, and an 
outer, longitudinal layer. In the upper third it consists exclusively of striated 
fibers united into bundles. At the beginning of the middle third smooth fibers 
appeal' among the striated, rapidly increase in number, and soon displace the 
latter entirely, so that at the middle the muscularis is composed wholly of 
smooth fibers. The connective tissue separating the muscular bundles blends 
internally with the submucosa, and externally forms a strong fibrous mem- 
brane, much reinforced by elastic fibers, which envelopes the entire organ and 
connects it with neighboring organs. 
The blood vessels and lymphatics are quite like those in the pharynx; 
the latter empty into the deep cervical, bronchial, posterior mediastinal, and 
cardiac glands. 
The oesophageal branches of the vagus nerve form a plexus on both 
the anterior and the posterior surfaces of the organ, from which fibers pierce 
the longitudinal musculature and form a plexus between it and the circular, 
which contains many nonmedullated fibers and ganglia. It is analogous to1 
Auerbach's plexus, which we shall meet with in the intestine. A second plexus, 
composed exclusively of nonmedullated fibers and ganglia, lies in the sub- 
mucosa. The intermuscular plexus furnishes the motor fibers for the muscularis, 
the submucous the motor fibers for the muscularis mucosae, the sensory for the 
epithelium and the secretory for the glands. 
8. THE STOMACH 
The stomach, which appears as a spindle shaped, laterally flattened dila- 
tation of the primitive intestinal tube, is enveloped by mesenchymatous cells 
and lined by a double layer of epithelium, the inner of which is composed of 
cuboidal, the outer of cylindrical cells, and is separated from the mesenchyma 
by a distinct basal membrane. The surface is smooth at first, but changes soon 
appear that are due to an unequal growth of the epithelium and lead to the 
formation first of folds in the mucous membrane, and then of depressions in 
its surface, the primitive gastric dimples. The floor of each dimple is 
formed by a single layer of cylindrical cells, while between them the epithelium 
remains of two layers. Granules very soon appear in certain ones of these 
cylindrical cells in the floor of the dimple that stain vividly with acid dyes. 102 
These are the parietal cells. While the primitive gastric dimples are still 
wholly epithelial, hollow branching tubes begin to press forward in the sur- 
rounding mesenchyma, while at the same time the part of the mesenchyma set 
apart now for the propria drives forward the epithelium toward the lumen, so 
that it loses its double layer and is reduced to a single layer of cylindrical cells. 
Thus we have now a cylindrical superficial epithelium that sinks in like a pit, 
from the outer end of which a gastric gland extends into the propria. Dif- 
ferences between the fundus and the pylorus are evident at an early period, in 
that the dimples in the latter are much deeper than in the former, but particu- 
larly in the fact that delomorphous cells do not develop in the pylorus, where 
the epithelium lining the primitive dimples and later the glands consists of un- 
differentiated cells. It is not until very late, probably about the time of birth 
that a differentiation takes place in these cells within the glands both of the 
fundus and of the pylorus, when very fine granules develop in them and they 
become what are known as mother cells. Then we have only mother cells in 
the pylorus glands, and both these and the delomorphous cells in the fundus 
glands. Yet for a long time after birth a great part of the cells remain un- 
differentiated, multiply by indirect segmentation, and furnish the material for 
new glandular and superficial cells. At the same time the cylindrical epithe- 
lium of the surface begins to become mucous. 
The first trace of muscular elements in the wall of the stomach is found 
in embryos in the second half of the second month of pregnancy, in the form 
of circular smooth muscle fibers in the mesenchyma surrounding the epithelium, 
which soon forms a complete circular musculature. The longitudinal muscles, 
which lie outside of these, appear much later. The muscularis mucosas can be 
distinctly perceived in the fourth month. 
The gastric mucous membrane of adults increases constantly in thickness 
from the cardiac orifice to the pylorus. Its free surface lies in folds, that 
form meshes running longitudinally, and presents many little depressions, the 
gastric dimples, that may be seen with the naked eye. Toward the pylorus 
the dimples become continually wider and deeper, while the parts of the mucous 
membrane between them project into the lumen like villi. 
The simple cylindrical epithelium lines the entire inner surface of 
the stomach, including the dimples; its cells secrete the gastric mucus (PL 
50, Fig. 119). These are long cells that taper only a little toward the propria, 
project their rounded free surfaces into the lumen, and have distinct cement 
edges between their free ends. They somewhat resemble goblet cells in con- 
struction, in that they present a deep, protoplasmic portion enclosing a nucleus, 
and a superficial, mucous portion. In the latter are numerous granules, pre- 
cursors of mucus, which swell more and more toward the lumen. This mucous 
portion of the cell contains two little central bodies. But the mucous cells of 
the gastric epithelium differ from goblet cells very characteristically in the fact 
that their mucous contents are not basophilic, because albuminoids are mixed 
in considerable quantities with the gastric mucus. 
The propria of the gastric mucous membrane consists principally of 
reticulated tissue, yet true connective tissue with very fine collagenous fibers 
is also to be found in it. Toward the surface the reticulated tissue becomes 103 
thickened and forms beneath the epithelium a sheetlike layer of anastomosing 
cells, which also embraces the glands of the propria on all sides. In the meshes 
of the reticulated tissue lie many lymphocytes, yet true follicles do not occur 
normally in the stomach. Mast cells are also met with very often in the propria. 
The propria is separated from the underlying submucosa by a well developed 
muscularis mucosae, the smooth muscle fibers of which form several inter- 
secting layers (Pl. 49, Fig. 118), from which slender little bundles constantly 
separate and mount at an oblique or a right angle up into the propria to lose 
themselves in the ridges about the gastric dimples (Pl. 50, Fig. 119). 
The propria is also the seat of the gastric glands, the secretion of which 
is the gastric juice. The latter is a specific, thin, watery fluid, the secretion 
of which is excited by the mere sight of food, but much more by its introduc- 
tion. It contains, in addition to water, inorganic constituents and mucus, 
mainly ferments and hydrochloric acid. Three ferments have been dis- 
covered in it: pepsin, which dissolves albumin, rennin, which coagulates milk, 
and steapsin, which decomposes fats. Free hydrochloric acid is present in 
the gastric juice and changes the propepsin excreted by the glands into pepsin. 
The gastric glands are found in the propria of the entire mucous membrane 
from the cardiac zone to the pylorus, and present two forms that differ in many 
respects, the fundus and the pyloric glands. 
The fundus glands occupy by far the greater part of the gastric mucous 
membrane, that of the entire body and the fundus ventriculi. They are of the 
branched tubular variety, each opening in a gastric dimple with a short narrow 
portion, the neck of the gland. Just back of its mouth the gland breaks up 
into two or more branches which pass through the propria in parallel, often 
slightly winding courses, and end with a quite small swelling just in front of 
the muscularis mucosas (Pl. 50, Fig. 119). Each of these tubes may be sub- 
divided into a body and a fundus. These glands are aggregated very closely, 
as many as 100 have been counted to the square millimeter, so that the propria 
forms only thin septa between them. 
The lumen is widest in the neck as a continuation of the gastric dimple, 
contracts in the body to a narrow fissure, and broadens again into a slight dila- 
tation at the fundus. The cells that line the tube are of two kinds. Some are 
generally medium sized cuboidal cells that lie along the entire length of the tube 
and increase in size with hunger. These we know as chief cells. They re- 
ceive their characteristic imprint from the fairly coarse granules they contain, 
which stain with basic dyes, the same as the chromatin of the nucleus. The 
general opinion is that these granules, which are difficult to preserve, are the 
initial stages of pepsin, the propepsin, so that of the two principal constituents 
of the gastric juice the pepsin is secreted by the chief cells (Pl. 50, Fig. 120). 
The second kind of cells found in the epithelium of the fundus glands is the 
parietal. They differ from the chief cells in being considerably larger, and 
of a spherical or pyriform shape. They are not distributed uniformly in the 
epithelium like the others, but are aggregated much more closely in the upper 
part of the gland than in the lower, and may be entirely absent in its fundus 
(Pl. 50, Fig. 119). Exceptionally a parietal cell wanders into the epithelium 
of the gastric dimple. They press in as it were from without between the chief 104 
cells, but frequently do not reach the lumen of the tube, and then are connected 
with it by a narrow pedicle. Another and more important difference is shown 
by their staining, as, unlike the chief cells, they have a strong affinity for acid 
dyes. The cell body contains quite minute, dustlike granulations that stain 
rather darker than the protoplasm (Pl. 50, Fig. 120). The secretion of hydro- 
chloric acid is ascribed to the parietal cells, chiefly because the acid appears 
only in the parts of the mucous membrane where parietal cells arc to be found. 
Up to the present time the acid itself has not been demonstrated in the cells. 
Minute lateral canaliculi branch from the lumen of the fundus gland, each 
one of which enters a parietal cell and here forms a capillary basket in the 
peripheral layer of cells, the branches of which are very minute during hunger, 
but are thick and coarse during digestion. These basket capillaries are 
beyond doubt the tubules through which the secretion of the parietal cells es- 
capes (Pl. 50, Fig. 120). 
The pyloric glands are found only in the pyloric region, where they are 
not aggregated so closely as the fundus glands, but are separated by large 
trabeculae of the propria. They are of the branched alveotubular variety. The 
neck develops from the gastric dimple, which here is usually very deep, and 
from this come several branching, very tortuous tubules that pass through the 
propria and end in a fundus that is distinctly thickened like a club (Pl. 50, 
Fig. 121). The course and arrangement of the glands is such that a section 
through the mucous membrane of the fundus differs materially from one through 
that of the pylorus; in the one densely packed tubes are to be seen cut longi- 
tudinally, running parallel through the propria with little propria tissue be- 
tween them, in the other we find sections of tubules almost all cut transversely 
or obliquely, united into little complexes and separated from their neighbors 
by large masses of propria. 
The glandular epithelium presents equally great differences, for the pyloric 
glands are lined with only one kind of cells. Although these differ slightly 
from the chief cells of the fundus glands, they resemble them in all essential 
particulars so that we may unhesitatingly pronounce them the same. In addi- 
tion, the secretion from the pyloric part contains pepsin and no hydrochloric 
acid, and many transition forms between the two kinds of glands are to be 
found at the margin of the pyloric portion toward the corpus ventriculi. 
A third variety of glands is the cardiac, which usually extend only a few 
millimeters from the oesophagus into the stomach, and give place very soon to 
the fundus glands. The cardiac glands are situated within the propria and 
are quite like the pyloric in their structure. Compound branched alveotubular 
glands are also found that extend into the submucosa and resemble Brunner's 
glands of the duodenum, which will be described later. 
Outside of the muscularis mucosae in the wall of the stomach is the sub- 
mucosa, a very well developed connective tissue containing fat, which is con- 
tinuous with the connective tissue of the muscularis. 
The muscularis consists mainly of circular, smooth muscle fibers that 
form a continuous layer. It attains its greatest thickness in the pylorus. Ex- 
ternally to this circular muscular layer there appears in some parts of the 
stomach a layer of muscular fibers running longitudinally, that form a contin- 105 
uation of the outer muscular coat of the oesophagus, and is most marked on 
the two curvatures (Pl. 49, Fig. 118). Finally we find on the inner side of 
the circular muscles fibers that run obliquely. They are best developed at the 
cardiac region, where they form a continuation of the circular muscles of the 
oesophagus, and arch from here over the sides of the body of the stomach to 
be lost on its greater curvature. 
Outside of the muscularis lies the peritoneum, which envelops the 
stomach on all sides. 
The arteries of the stomach come from the coeliac, course first in the 
subserosa, and send their branches through the muscularis into the submucosa, 
in which they form a large plexus. From this twigs rise into the propria and 
form about the glands capillary plexuses with elongated meshes that are par- 
ticularly dense in the region of the glandual neck. The veins assemble just 
beneath the epithelium, run down perpendicularly between the glands, and 
form first a plexus on the muscularis mucosa?, from which twigs lead to a sub- 
mucous plexus. The muscularis also is supplied by this submucous arterial 
and venous plexus. 
The wall of the stomach is richly provided with lymphatics. Their roots 
form the perivascular spaces that lie about the superficial capillaries and veins, 
and open into independent little trunks between the glands which form a sub- 
glandular plexus outside of the muscularis mucosa?. This in turn empties into 
a submucous plexus, which also receives the lymphatics of the muscularis, and 
from which efferent vessels pass to accompany the arteries and to empty into 
the gastric and pancreatico-splenic lymphatic glands. 
The nerves of the stomach come from the vagus and the sympathetic, and 
form an anterior, a posterior, and a superior gastric plexus. They accom- 
pany the vessels into the wall of the stomach, where they form an intramuscular 
plexus between the circular and the longitudinal muscles, and then a submucous 
plexus. Both of these are well provided with ganglia and they anastomose 
with each other. The intramuscular plexus furnishes the motor end fibers that 
terminate on the muscle fibers with little end plates. From the submucous plexus 
come numerous fibers that surround the glands with an extremely fine meshed 
plexus. They also enter between the epithelial cells and terminate on them with 
little swellings. 
9. THE INTESTINE 
In the early stages of development the intestine passes straight through 
the embryonal body, and, like the stomach at this time, is lined with a double 
layer of cuboidal epithelium and surrounded by an envelope of mesenchymatous 
cells. As soon as convolutions begin to appear in the intestinal tube the epithe- 
lium proliferates strongly and becomes stratified in its upper portion. Vacuoles 
appear in the epithelium and the proliferation becomes so great in the duodenum 
that the lumen becomes choked in many places. The differentiation of smooth 
muscle fibers in the mesenchyma surrounding the epithelial tube begins in em- 
bryos 10 to 12 mm long. These take a circular course about the tube and 
develop into a continuous layer of circular muscles. The epithelium placed be- 106 
tween it and the mesenchyma now proliferates toward the latter and gives rise 
to longitudinally running folds. In the more caudal sections on the contrary 
the epithelium advances toward the mesenchyma with the formation of pro- 
trusions, the so-called diverticula, which later are drawn in again into the 
lumen. In the still more caudal sections of the intestine, that later become the 
colon, there is neither a marked formation of folds nor great epithelial pro- 
liferation. The epithelium here likewise at first consists of a double layer, but 
this changes early to a single layer of cylindrical cells. 
By the time the embryos are 20 mm long a proliferation of the mesenchyma 
takes place in the small intestine, which advances in the form of fingerlike pro- 
cesses toward the epithelium, creating protrusions in the mucous membrane of 
the intestinal cavity, which is now open again everywhere; we know these as 
villi. Beginning in the most cranial portion of the intestine their development 
constantly proceeds toward the caudal end, so that in embryos 50 to 60 mm 
long the inner surface of the entire small intestine from the stomach to the 
colon is covered with villi, the number and development of which steadily de- 
creases from the cranial to the caudal end. 
Between the villi the epithelium runs down forming simple, straight tubes. 
These are Lieberkuchn's glands. In addition we find in the cranial sections 
the formation of special protrusions that are developed in connection with the 
pyloric glands of the stomach and are branched like them. We call these 
Brunner's glands. 
Signs of differentiation begin to appear in the mesenchyma between the 
circular muscles and the epithelium in embryos 100 mm long. The smooth 
muscle fibers of the muscularis mucosae appear in it and divide the propria from 
the submucosa. The former changes into reticulated tissue, in which lymphoid 
follicles develop in many places, while the submucosa forms collagenous fibrils 
and becomes loose connective tissue. Shortly before the advent of the mus- 
cularis mucosae longitudinal smooth muscle fibers appear outside of the circular 
muscles, which gradually come to form a longitudinal muscular layer that is 
not continuous throughout the entire length of the intestine. 
We have thus in the intestinal wall of adults most internally the mucous 
membrane, consisting of epithelium and propria, and separated from 
the submucosa by the muscularis mucosce. Then comes the muscula- 
ture of the intestine, composed of inner, circular, and outer longitudinal mus- 
cles. Finally the intestine is invested by the peritoneum throughout its en- 
tire length. 
Although there is no doubt that a certain amount of absorption takes place 
in the upper part of the digestive tract, in the mouth, pharynx, oesophagus, 
and stomach, yet the intestine is the organ in which absorption reaches its 
acme. In order to provide the most extensive possible absorptive surface in 
the least space its mucous membrane is in the first place thrown into folds, 
and in the second is provided with the numerous protrusions we have just been 
studying, the villi. 
The folds of the intestinal mucous membrane are the Kerkring's 
folds, or valves, or the valvulae conniventes, crescentic elevations running 
vertically to the longitudinal axis of the gut, that reach their greatest height 107 
opposite the insertion of the mesentery and gradually grow less toward the 
latter (Pl. 51, Fig. 123). They begin about 5 mm from the pylorus, reach 
their greatest development in the jejunum, and disappear entirely in the lower 
part of the ileum. The plicae semilunares take the place of Kerkring's 
folds in the colon (Pl. 52, Fig. 124). 
The villi furnish a far greater increase of surface than the folds. Like 
them they are elevations of the mucous membrane, the form of which varies in 
different parts of the intestine. Above they are broad and leaf-shaped, below 
their breadth gradually decreases until they finally become finger-shaped. They 
also decrease in number from above downward and disappear entirely in the 
lower part of the ileum. None are to be found in the colon or rectum. The 
number present in the entire intestine has been calculated to be over ten millions. 
Experimental researches on animals have shown that the number and shape of 
the villi depend to a great degree on the kind of food, whether vegetable or 
animal. Between the bases of the villi lie the mouths of Lieberkuehn's glands. 
As regards the structure of a villus, we can distinguish a body and an in- 
vesting epithelium (Pl. 52, Fig. 125). The body of the villus is an ele- 
vation of the propria of the intestinal mucous membrane, and consists like it 
of reticulated tissue. On its surface the reticulum is thickened into a sort of 
membrane from which meshes extend essentially in a radiating direction to the 
axis of the villus. The latter is occupied by a lymph space, the central chyle 
vessel, or lacteal, into the epithelial lining of which the cells of the reticulum 
continue. 
In the meshes of the reticulum one always meets with a large number of 
leucocytes, transition forms, and especially macrophages that come from 
the latter. Mast cells and eosinophile cells are less numerous. 
Besides blood vessels, the course of which will be described later, the body 
of the villus contains smooth muscle fibers. They branch out of the mus- 
cularis mucosae, just as in the propria of the stomach, several little bundles to 
each villus, in the body of which they form by anastomosis a very wide meshed 
network, and are inserted with somewhat thickened ends into the limiting layer 
described above. A very fine homogeneous basal membrane is interposed 
between this limiting layer belonging to the reticulum and the epithelium. The 
simple, cylindrical villous epithelium forms at the same time the superficial 
epithelium of the mucous membrane throughout the greater part of the small 
intestine, as the villi are so extremely dense. It is composed of long, prismatic 
cells, polygonal on section, that taper as they go down and rest with their small 
ends on the basal membrane (Pl. 8, Fig. 26; Pl. 52, Fig. 125). Distinct inter- 
cellular spaces remain between the cells, especially in the deeper parts, and are 
closed at the surface with cement edges. Intercellular bridges also may be 
observed between the individual cells. Each cell contains an ovoid nucleus sit- 
uated in its basal third, and nucleoli can always be plainly seen. The cell body 
consists of a hyaline or finely granular protoplasm, vacuolized in the portions 
adjacent to the lumen, in which very distinct fibrils appear. These pass through 
the cell body in a spiral course, surround the nucleus, and become invisible in 
the basal parts. Just beneath the free surface lies a double central body, and 
we find mitochrondria here as well as in the opposite segment of the cell. 108 
On the free surface the cells are covered by a rod band, such as has been 
described already in the general part of this work. 
Goblet cells are to be found between these typical cylindrical cells. For 
the structure of these, also reference must be made to the general part. They 
are more abundant at the bases than at the apices of the villi, in the epithelium 
of which may be found every transition between typical cylindrical and ripe 
goblet cells. 
Little more remains to be said of the superficial epithelium. In the lower 
part of the ileum, where there are no villi, it presents the same characteristics 
as on those structures, except that the goblet cells become more numerous the 
nearer we approach the rectum. At the anus the intestinal changes quite ab- 
ruptly into stratified epithelium which differs from that of the skin in that the 
uppermost layer of cells is cuboidal or cylindrical. This transition epithelium 
soon gives way to the epidermis. 
The propria of the intestine is, as we have seen, reticulated tissue, forms 
the main part of the bodies of the villi, and surrounds the glands, or crypts 
of Lieberkuehn, which are sunk in between the villi. These crypts are simple, 
straight or slightly bent, short tubes that penetrate only to the muscularis 
mucosae (Pl. 53, Fig. 126). They are found in the entire intestine from the 
duodenum to the rectum, but are much farther apart in the lower sections than 
in the upper, and they increase considerably in length from above downward. 
The lumen is comparatively wide and is lined with an epithelium that differs in 
only a few points from that of the surface. First, we meet with here a new 
kind of cell, the Panethian, which are cylindrical, or more often pyriform in 
shape, lie in the base of the gland, and are filled with fine granules that are 
strongly acidophilic. Probably the secretion is indebted to these cells for one 
of the intestinal ferments. Then we find quite small, very darkly stained cells, 
the so-called peg cells, which are collapsed Panethian cells that have lost their 
secretion (Pl. 53, Fig. 126). The main part of the epithelial cells of Lieber- 
kuehn's crypts consist of superficial epithelium and goblet cells, the latter pre- 
dominating considerably in the colon and rectum. Indirect segmentation is 
seen with extreme frequency in the epithelial cells, which probably provides for 
the replacement of the goblet cells that are destroyed by degrees in producing 
the secretion. 
In the propria of the intestine are also found lymph follicles as little 
nodules lying between the glands of Lieberkuehn. They are not confined to the 
propria in their growth, but break through the muscularis mucosas and pene- 
trate into the submucosa (Pl. 52, Fig. 124). In the ileum large adenoid masses 
unite to form conglobate follicles, which are known as Peyer's patches. 
Here the masses reaching to the musculature are covered by a thin mucous 
membrane, with only scattered villi and Lieberkuehn's glands between the 
separate follicles, while on the top of the follicle the adenoid masses cov- 
ered only by epithelium protrude into the lumen of the gut. Yet the 
adenoid tissue attains its greatest extent in the vermiform appendix, 
where the entire propria is diffusely infiltrated with lymphocytes, and we also 
find many solitary follicles, so that in this section of the intestine Lieberkuehn's 
glands suffer a great reduction and are met with only quite sporadically. 109 
The muscularis mucosae is well developed throughout the intestine. 
As the outer limit of the propria it passes just outside of Lieberkuehn's glands 
and enters into the folds, both in the small intestine and in the colon. It 
consists for the most part of two thin layers of muscular fibers-an inner, 
in which the fibers are circular, and an outer, in which they run longitudinally. 
Its relations to the musculature of the villi have been described above. 
The submucosa does not attain any considerable development in the in- 
testine. It is thickest in the upper duodenum, becomes slighter lower down, 
and increases again in thickness in the colon. It consists of loose connective 
tissue with elastic fibers, and contains glands only in the duodenum. These, 
known as Brunner's glands, are most abundant in the upper part of the 
duodenum, while they occur only here and there in the descending portion 
and are quite absent in the lower part. The large Brunner's glands alone are 
submucous, the small ones remain in the propria, like Lieberkuehn's (Pl. 51, 
Fig. 122). They are compound branched alveotubular glands, similar to the 
mucous glands of the mouth, but from which they differ in their epithelial 
lining. The mouth of the quite short excretory duct is found either between 
the glands of Lieberkuehn, or in the fundus of one of them. After the duct 
has broken through the muscularis mucosae it breaks up immediately into 
quite a number of branches, which branch again and end with short alveolar 
dilatations. The lining of the excretory duct is a simple cylindrical epithe- 
lium, but deeper in it assumes the peculiarities of the epithelium of the pyloric 
glands, as may be shown by many transitions between the two kinds. Brunner's 
glands also contain cells that are quite similar to the Panethian cells of Lieber- 
kuehn's crypts. 
The muscularis of the intestine consists, like that of the stomach, of 
smooth muscle fibers in two layers, an inner circular, and an outer longitudinal. 
Both layers are well developed on the duodenum, yet the inner is three or four 
times as thick as the outer. Both diminish in thickness in the jejunum and 
ileum, but the circular musculature remains the stronger, while in the colon the 
longitudinal finally becomes incomplete and is confined to three thick longi- 
tudinal bands, the taeniae. In the large intestine the haustra are formed, 
saclike pouches in the wall that are separated by circular furrows. The mus- 
cularis of these haustra consists exclusively of circular muscles, while they are 
actually produced by the longitudinal muscles, which act like short ligaments. 
The circular furrows, the plicae semilunares, thus contain all of the layers of 
the intestinal wall and are not folds of the mucous membrane like Kerkring's 
folds of the small intestine. The muscularis increases again in the rectum and 
there attains its greatest thickness. The strong layer of longitudinal fibers 
passes partly into the levator, partly into the rectovesical musculature, while 
the circular layer becomes much thickened at the anus and forms the sphincter 
ani externus. 
The musculature of the intestine contains much connective tissue with many 
elastic fibers, which extends inward into the submucosa, and outward into the 
subserosa of the peritoneal covering. 
The arteries, coming from the mesenteric and the hemorrhoidal, are sit- 
uated on the whole the same as those of the stomach (Pl. 53, Fig. 127). We 110 
have first a large arterial plexus in the submucosa from which branches extend 
on the one hand to the muscularis, on the other to the propria, and form a 
second plexus on the muscularis mucosae. From the latter develop the capil- 
laries that enmesh Lieberkuehn's glands closely, as well as twigs that enter the 
follicles, while two or three little arteries pass from it into each villus and form 
a capillary network in the outer layers of its body, from which the blood is 
collected into veins. These villous veins, together with the vessels that originate 
in the glandular and follicular capillaries, penetrate into the submucosa, where 
they form a venous plexus corresponding to the arterial. From this little 
trunks pass through the musculature and form with the veins of the latter a 
second plexus in the subserosa, from which larger trunks pass into the mesentery. 
The roots of the intestinal lymphatics are found as perivascular sheaths 
about the intestinal glands, but they occur mainly in the villi, where they 
form the central chyle space (Pl. 52, Fig. 125). Each villus contains at least 
one such chyle space, and the large leaflike villi of the duodenum each contain 
two, which are united by transverse anastomoses. They are lined with flat 
epithelium and end oi' begin blind in the upper third of the villus. The chyle 
space tapers downward and appears at- the base of the villus as a lymphatic 
or chyle vessel, takes up the perivascular lymph spaces about Lieberkuehn's 
glands and the lymphatic networks about the intestinal follicles, and passes 
through the muscularis mucosae into the submucosa, in which a wide meshed 
lymphatic plexus originates that presents large varicosities. A second plexus 
lies between the circular and the longitudinal muscles, joined on one side with 
the submucous plexus, on the other with a third plexus lying in the subserosa, 
into which larger trunks from the submucous plexus also open directly. 
The chyle vessels, or lacteals, assemble finally from the subserous plexus and 
enter the mesentery with the blood vessels. Those of the duodenum empty 
into the superior and inferior pancreatic, those of the jejunum and ileum 
into the mesenteric, those of the caecum and vermiform appendix into the 
precaecal, retrocaecal and ileocaecal, those of the colon into the mesocolic, those 
of the rectum into the hemorrhoidal and anorectal lymphatic glands. 
The nerves of the intestinal canal come from the great nervous plexuses 
of the abdomen, the coeliac, mesenteric, aortic, and hypogastric, which belong 
partly to the vagus, partly to the sympathetic. They form in the wall of the 
intestine three plexuses that correspond to those of the lymphatics, first a 
subserous, then a myenteric, commonly known as Auerbach's plexus, and 
finally a submucous, commonly called Meissner's plexus. All three of 
these are united by numerous anastomoses. Auerbach's plexus is of coarser 
frame and wider meshed than Meissner's, as well as considerably richer in 
sympathetic fibers. The ganglia lie at the meeting points of the plexus and 
are composed of multipolar sympathetic cells. All of the fibers of the plexus 
are nonmcdullated. Branches extend from Auerbach's plexus into the mus- 
culature and end on the muscle fibers. From Meissner's plexus fibers extend 
to the glands, the villi, and the entire superficial epithelium. They form fine 
networks that enwrap the glands and pass through the bodies of the villi. 
Sympathetic cells are interpolated in all of these networks. 
The function of the intestine is in the first place to complete the digestion 111 
of the food introduced that has been begun in the upper part of the tract, 
and then to absorb the products of this process and to supply them to the 
organism. It has therefore both a digestive and an absorptive function. 
The former is aided by the secretions of the two large intestinal glands, the 
liver and the pancreas, but the glands of the intestinal wall itself, Lieber- 
kuehn's and Brunner's, provide a watery, light, alkaline fluid, known as the 
intestinal juice. This contains in addition to such inorganic substances as 
sodium carbonate, chloride and phosphates, four ferments, three of which 
decompose sugar, invertin, lactase, and maltase, as well as one that 
decomposes albumin, erepsin, and also a body known as enterokinase, 
which changes the protrypsin of the pancreatic juice into trypsin. The secre- 
tion of these important ferments we must describe with the intestinal glands. 
The amount of them in the intestinal juice constantly decreases from above 
downward and is nonexistent in the colon, which is not surprising, as the glands 
of Lieberkuehn situated here contain goblet cells almost exclusively. The ques- 
tion now arises whether the epithelium of the villi, as well as of the glands, 
takes part in the secretion, and from the observations that have been made it 
must be answered in the affirmative, at least so far as the erepsin and the 
enterokinase are concerned. 
But the principal function of the epithelium of the villi is to absorb. Water 
may be absorbed throughout the entire intestine, and the same is true of 
dissolved carbohydrates; both enter the blood capillaries through the intes- 
tinal epithelium. Concerning the absorption of albuminoids decomposed by 
the action of the gastric and intestinal juices, we know only that their products 
of decomposition enter directly into the blood. Whether, as often claimed, 
the intestinal leucocytes play an essential part is not yet positively proven. 
We are best informed concerning the absorption of fat, which is easily demon- 
strable with the microscope. The fat of the food is completely decomposed 
in the intestine, and the fatty acids thus set free are rendered soluble by the 
bile, and in this form they are taken up by the epithelium of the villi through 
the rod band. Within the cells the fatty acids are again combined into neutral 
fats and in this form the fat is conducted through the body of the villus in 
the central chyle space, and from there into the lacteals. The muscles of the 
villi play a part in its transportation that is not to be underestimated; when 
they contract the villus must become shorter, the body and with it the central 
chyle space must be compressed, and the contents of the latter be emptied into 
the lacteal, in which the valves which appear in the submucous plexus prevent 
a reflux. 
The Liver 
The liver originates, like all intestinal glands, as a protrusion of the ento- 
dermal intestinal tube, and is demonstrable in embryos 2 and 3 mm long as a 
diverticulum directed ventrally just beneath the spindle-shaped stomach. A 
lively proliferation of the epithelium now takes place in the ventral wall that 
takes the form of solid cell trabecula which unite into a network. While this 
trabecular formation constantly extends farther and farther in the ventral 
direction a cell mass grows out from the caudal wall of the primary liver sac, 112 
the rudiment of the gall bladder. The primary liver sac increases in length, 
and we have in embryos 9 and 10 mm long a hollow duct leading out from the 
intestine, lined with several layers of cylindrical cells, the ductus choledochus, 
which splits into two solid cellular cords, the short, caudal one of which ends 
with a distinct, clublike thickening, the rudiment of the ductus cysticus, while 
the longer, cranial one divides repeatedly and passes over into the true liver, 
now evident as a considerable organ. At this time it consists of cellular 
trabecula? joined together into networks, each composed of several cells in 
the section. 
Even in its first stages the young rudiment of the liver is closely related 
to the omphalomesenteric blood vessel that grows into it. Twigs shoot out from 
this vessel between the trabeculae and thus give rise to a system of blood spaces 
in the meshes formed by them. These completely fill the meshes and stick close 
to the epithelial trabeculae. As brought out in the chapter on "Blood," the 
embryonal liver is intimately connected with the formation of blood. An 
enormous accumulation of primitive mesamoeboids very soon appears in the 
diverticula of the hepatic blood spaces, which undergo great proliferation. 
These are the foci for the development of blood in the embryonal liver, and 
they disappear completely at the time of birth. We find in these foci ery- 
throcytes in all stages of development, as well as numerous giant cells. 
The trabeculae of the liver are solid, without a lumen from the beginning. 
The lumen appears first in embryos about 10 mm long, and originates simul- 
taneously in many places of the liver rudiment by the separation of the con- 
tact surfaces of the cells. 
Gradually the lumina thus produced coalesce, and we thus obtain a system 
of anastomosing canaliculi that are lined with liver cells, and always have 
from three to five cells about the lumen. But this condition undergoes a change 
in the course of latei' embryonal life through the growth of the liver tubule, 
as we must now call it. A displacement of the cells takes place so that the 
lumen of the tubule is bordered by only two liver cells with their' broad sides 
touching. 
Simultaneously with the canalization of the trabecula? of the liver the cystic 
and hepatic ducts and the gall bladder become hollowed out. In the former 
the cells simply separate, but in the gall bladder large vacuoles are formed 
which gradually blend, so that finally a cavity is formed, which is lined at 
first with several layers of epithelium, later with two, and finally with only one. 
As is evident from this embryological study the liver is a compound 
branched tubular gland with many deviations from the general type, the most 
important of which are that its tubules unite to form a network, that neigh- 
boring tubules lie together, and that a lumen develops between their cells, so 
that a glandular cell does not belong to only one tubule, as in all other glands, 
but to several at the same time. (See Fig. 31.) 
The liver has a double function: to produce its peculiar secretion, like 
every other gland, which is carried by the excretory ducts into the intestine, 
and to elaborate in its cells substances that pass directly into the blood and 
are carried by it to the organism. The secretion of the liver is the bile, a 
yellow brown, sticky, viscid fluid of intensely bitter taste and alkaline reaction. Fig. 31.-Structure of the Liver. 
1, interlobular biliary duct; 2. intermediate piece; 3, liver 
cell trabecula, one of which divides at 3*, and two come to- 
gether at 3** with the lumen of a biliary capillary between 
them; 4, biliary capillary; 5, interlobular vein sending its 
capillaries, that embrace the liver cell trabecula, into the 
lobules; 6, central vein, into which these capillaries assemble; 
7, sublobular vein; 8, twig of the hepatic artery which branches 
in the interlobular connective tissue. 113 
It contains considerable quantities of solid constituents, the most important 
of which are the biliary salts and the biliary pigments. The former are 
sodium salts soluble in water of two organic acids, glycocholic and taurocholic, 
which in turn are important products of the decomposition of the albumin of 
the food. The biliary pigments, bilirubin and biliverdin, are likewise the 
alkaline salts of two acids, and the latter is a product of the oxidation of the 
former. Both come from the blood and are derivatives of haemoglobin. The 
chief use of the bile when emptied into the intestine is to dissolve fatty acids 
and to activate the fat-decomposing ferment of the pancreatic juice. 
The products of the liver cells borne away by the blood are glycogen and 
urea. Glycogen is a polysaccharide and is doubtless a reserve substance 
formed preponderatingly in the liver, though also in other places, from the 
sugar brought by the blood, and perhaps also from fat and albumin, and 
stored away to be given up to the blood at the proper time. Urea, also one 
of the most important products of the decomposition of albumin, is formed 
not exclusively in the liver but in many other organs as well. 
Like every other large gland of the digestive tract, the liver is surrounded, 
by a capsule, Glisson's capsule, external to w7hich lies the peritoneum ovei' 
the greater part of its surface. Glisson's' capsule consists of connective tissue 
containing a few elastic fibers, and sends septa into the parenchyma of the 
liver. It also enters the organ with the blood vessels and biliary passages at 
the porta of the liver, and finally divides the parenchyma into small prismatic 
areas 1 to 3 mm in diameter, which are called hepatic lobules. Although 
these lobules are quite distinctly visible macroscopically in consequence of the 
difference in the vascularity in their peripheries and centers, yet their delim- 
itation by the connective tissue forming the interlobular septa is very imperfect, 
so much so that it is difficult to distinguish the lobules apart microscopically 
(Pl. 53, Fig. 128). Great numbers of fibers extend from the interlobular 
connective tissue into the lobules and surround the cellular trabeculag with a 
close-meshed network that is difficult to demonstrate by ordinary histological 
methods, and can be made visible only by a special procedure. These inter- 
lobular fibers, which also occur in other glands, are known as the lattice 
fibers. 
We will begin the description of the structure of the liver with the system 
of efferent ducts, taking the same course that we have followed in the con- 
sideration of other glands. The hepatic duct, the excretory duct of the 
liver, is lined with a simple, cylindrical epithelium full of goblet cells, next to 
which comes a connective tissue propria, with circular and longitudinal smooth 
muscular fibres outside of this. Entering the porta of the liver the duct 
divides first into coarse and then into finer branches, which run with the 
branches of the hepatic artery and portal vein in the connective tissue, and 
have the same structure as the hepatic duct except that the enveloping mus- 
cular fibers are absent and that the epithelium contains no goblet cells. The 
cylindrical cells present very distinct cement edges. The very minute inter- 
lobular ducts form wide-meshed plexuses about the blood vessels, from 
which numerous twigs extend to the periphery of the lobule and then turn 
toward the parenchyma, at the same time splitting up. This last part of the^ 114 
excretory duct system, which may be called the intermediate piece, as in 
the salivary glands, has a very narrow lumen and is lined with low cuboidal 
cells (Pl. 54, Fig. 129). Following along we see how the much larger liver 
cells suddenly take the place of the cuboidal in the duct, and the intermediate 
piece is changed into the liver cell trabecula, or liver tubule, its 
lumen into the biliary capillary. While the lumen of the intermediate 
piece is surrounded by four or five epithelial cells, the biliary capillary is 
bounded by only two liver cells. The liver cell trabeculae, which are the secre- 
tory tubules of the liver, pass through the lobules from the periphery to the 
axis, where they bend about and mingle with their neighbors. On the way 
they divide repeatedly, the branches anastomosing and forming radiating 
meshes in the lobule. The conditions then become very complicated, because 
neighboring trabeculae lie together and a biliary capillary runs again in the 
middle of their touching surfaces, so that the same cell may be utilized in the 
wall of two or more biliary capillaries. In this the liver is unique and differs 
from all other glands, as in them each cell always appertains to only one 
secretory space. (See Fig. 31.) 
The biliary capillaries thus form a network of minute canaliculi which 
passes through the entire hepatic lobule and lies within the liver cell trabeculae. 
Their diameter scarcely exceeds 2 p. Whether they have blind ends4or, more 
correctly, blind beginnings, in man is hard to say. 
The liver cell, the secretory cell of the hepatic tubules, has a polyhedral 
or prismatic form (Pl. 54, Fig. 130). It contains one, or very often two, 
spherical nuclei. Mitotic and amitotic figures are not infrequently met with 
in them. The cell body consists of a reticular or vacuolized protoplasm, the 
appearance of which varies a great deal according to the functional condition 
(PL 1, Fig. 1; Pl. 2, Fig. 7). After a full meal the cell body is occupied 
by coarse, irregular masses of a substance which is very slightly soluble in 
water, glycogen, while in hunger the glycogen is given up to the blood and 
the cell is full of little vacuoles that still contain minute granules of the sub- 
stance. The cell also contains fat in larger or smaller drops, according to 
the state of the nutrition. The secretion of the liver cells, the bile, cannot 
be demonstrated in them under normal conditions, but in animals, and also 
in man under pathological conditions, indications have been found that the 
nucleus plays a great part in the secretion of the bile, with which the frequent 
occurrence of a double nucleus is in harmony. In such cases crystals of a 
substance resembling haemoglobin have been found in the nucleus. The forma.- 
tion known as Kupffer's vacuoles probably serve to expel the bile from the 
cells. These are little, spherical vacuoles situated in the protoplasm of the 
cell near the biliary capillaries, which communicate with the latter through 
extremely minute tubules. 
The liver is also distinguished from all other glands by the nature of its 
blood supply, which is double, as in the lungs, in that the entering blood 
stream is divided into a functional and a nutritive portion. The functional 
blood, i. e., that which carries to the liver cells the material for secretion, 
streams through the portal vein, while the hepatic artery conducts only the 
blood for the nutrition of the liver tissue. All of the blood is carried 115 
away through the hepatic veins. The portal vein enters the substance of the 
liver at the porta in company with the hepatic artery and the hepatic duct. 
From its branches finally arise the interlobular veins, which run in the inter- 
lobular connective tissue enveloping each lobule with a dense vascular plexus. 
Numberless capillaries extend from this over the entire periphery of the lobule 
into its substance and form in it a close meshed vasculai' plexus that weaves 
about all sides of the liver cell trabeculae, almost completely filling the meshes 
formed by these. It follows at once from the above description of the rela- 
tions between the biliary capillaries and the liver cells that the biliary and 
blood capillaries never come in contact, but are always separated by the sub- 
stance of the liver cells. Just as the liver cell trabeculae radiate toward the 
axis of the lobule, so do the blood capillaries, which finally empty into the 
central vein occupying the axial space. That is, the central vein does not oc- 
cupy the entire axial space, but, like the fruit stalk of a raspberry, it passes 
through only about two thirds of the entire length of the lobule, in order to 
receive from all sides the blood capillaries of the lobule. The central veins 
open into the sublobular veins, which run in the interlobular tissue, but are 
separated from the biliary ducts and the interlobular veins. The larger veins 
formed by their union likewise always remain isolated, and the hepatic veins 
finally open far from the porta into the inferior vena cava. The hepatic 
artery plays a very humble part as compared with the portal vein. It accom- 
panies the latter closely in all its branching and ends with capillaries in the 
interlobular tissue, the blood from which is carried away in the sublobular 
veins (see Fig. 31) (Pl. 54, Fig. 130). 
The lymphatics of the liver arise in unwalled spaces about the intra- 
lobular blood vessels. Although the existence of such spaces has been much 
contested of late, their presence is indicated by the fact that after ligation 
of the biliary duct the bile very soon appears in the thoracic duct. These 
unwallcd intralobular spaces open on the surface of the lobules into the inter- 
lobular lymphatics, which either accompany the portal vein to the porta and 
empty into the hepatic glands, or wind about the veins of the liver and then, 
accompanying the vena cava, open into the anterior mediastinal glands. Be- 
sides these deep lymphatics, the liver has a system of superficial ones, some of 
which go to the glands just mentioned, others to the aortic, cardiac, superior 
pancreatic, and sternal glands. 
The nerves of the liver come from the hepatic branches of the vagus and 
from the coeliac plexus. The great majority of them are nonmedullated. They 
enter with the vessels at the porta and accompany them in their course. In 
the interlobular connective tissue they form plexuses studded sparsely with 
nerve cells, from which twigs enter the lobules and twine about the cell tra- 
beculae. Knob-shaped endings on the liver cells have been described. It has not 
yet been established experimentally that the nervous system exerts an influence 
on the secretion of bile. 
The Gall Bladder 
The gall bladder, the development of which has already been explained, 
forms a muscular sac which receives the bile after it has been secreted in the 116 
liver and gives it up to the intestine at the propel' time. Its mucous mem- 
brane forms numerous folds and is lined by a single, high, cylindrical epi- 
thelium, the cells of which have a cuticular border on the free surface and 
contain numerous minute droplets that stain like mucus. These droplets leave 
the cells through minute canaliculi that pass through the cuticular border. 
It is questionable whether they are composed of a true mucin, or a nucleo- 
albumin. Next outward from the mucous membrane comes a connective tissue 
propria, richly provided with elastic fibers, and then a muscularis, con- 
sisting of a strong inner layer of circular muscles and a thin outei* layer of 
longitudinal ones. The peritoneum covering the outer surface of the gall blad- 
der is joined to the muscularis by a connective tissue subserosa. 
The nerves of the gall bladder form a dense plexus of nonmedullated 
fibers between the two muscular coats that is furnished with many ganglia. 
It sends its fibers chiefly to the muscles. 
The structure of the ductus cysticus and choledochus correspond in 
the main to that of the gall bladder, and differ only in the presence of glands, 
which may continue moreover on the neck of the latter. They are rather 
sparse in the cystic duct and attain their greatest development in the common 
bile duct, or choledochus. Their epithelial cells have the same structure as 
those of the gall bladder. , 
The Pancreas 
The pancreas appears in the human embryo quite a short time after 
the development of the primary liver sac in the form of two diverticula, one 
on the cranial, the other on the caudal side of the hepatic protrusion. The 
former is the dorsal, the latter the ventral rudiment. Each is at first like 
the rudimentary liver, a rather compressed, saclike, lateral diverticulum of 
the intestinal wall, which in its further growth draws longitudinally into the 
mesogastrium and then communicates with the lumen of the intestine through 
a hollow pedicle. Finer twigs sprout out now from the gradually lengthening 
rudiment, which have at their ends clubbed thickenings that soon become 
hollow, the young alveoli. Through the rotation of the axis of the intestinal 
tube and the gradual elongation of the ductus choledochus, these two rudi- 
ments approximate, grow together, and finally become completely united in 
embryos about 20 mm long. The two efferent ducts from the primitive sac- 
like rudiments form many anastomoses with each other, the dorsal usually 
undergoes involution, and the ventral becomes the main excretory duct of the 
gland. While the entire glandular parenchyma in embryos about 50 mm long 
consists of little spherical alveoli that are connected with the smaller excretory 
ducts by slendei* tubes, there subsequently appear in many places, and first 
in the youngest parts of the body of the gland, solid cellular sprouts, which 
may not become hollow even later, but lie in the parenchyma between the 
alveoli as independent, growing masses of cells constricted off from the epithe- 
lium of the duct. These are the formations known as Langerhans' islands, 
which later develop in all parts of the gland. 
The pancreas is of the compound branched alveotubular type, and greatly 
resembles the serous salivary glands of the mouth in its plan of structure. 117 
It differs in man from all other glands in the possession of masses of cells that 
are not canalized, Langerhans' islands, and of the centroacinar cells 
to be described later. 
The secretion from the pancreas, the pancreatic juice, is a colorless, 
viscid, alkaline fluid that becomes gelatinous in the cold and is curdled by heat. 
It contains five ferments as characteristic constituents: amylopsin, which 
changes starch into sugar; steapsin, which decomposes fat; trypsin, which 
digests albuminous bodies; rennin, which coagulates milk; and a ferment 
that breaks up glucose. As was brought out in the discussion of the intestinal 
juice, trypsin is not contained as such in the pancreatic juice, but in a primor- 
dial form as protrypsin, which is changed into trypsin by the enterokinase of 
the intestinal juice. 
The pancreas is exquisitely lobed. Its connective tissue capsule pene- 
trates into the interior of the organ in the form of septa that subdivide the 
parenchyma into successive lobules ever becoming smaller, just as in the 
salivary glands. Numerous fibers of the kind described in the liver as lattice 
enter the lobules from the interlobular connective tissue and surround the 
alveoli with their plexuses. 
The principal excretory duct of the pancreas, Wirsung's duct, is lined 
with a mucous membrane having a coat of simple cylindrical epithe- 
lium. Next to this is a connective tissue propria containing more or less 
numerous mucous glands. External to the propria is the muscularis, 
consisting of an inner, strong, circular coat, and an outer, longitudinal one 
that is developed only in places. At its beginning it lies in close relation to 
the ductus choledochus and opens with it on the plica longitudinalis duodeni. 
The duct is nowhere free, but is surrounded by glandular substance through- 
out its entire course. It receives many coarse, lateral branches, passes through 
the whole length of the gland, gradually becoming smaller, and ends in its 
terminal twigs in the cauda. The finer the ducts become the lower becomes 
the lining epithelium. The musculature disappears in the coarser branches. 
From the most minute interlobular ducts branches pass into the lobules 
and become intermediate pieces on which the end pieces hang (Pl. 55, 
Fig. 132). Some of the latter are purely alveolar, some are rather elongated 
tubules. In no part of the excretory duct is there a secretory epithelium, 
such as we see in the salivary tubes of many of the glands of the mouth; the 
epithelium everywhere is indifferent. 
In the salivary glands the conical secretory cells of the end piece directly 
adjoin the lower ones of the intermediate piece, but in the pancreas the con- 
ditions are somewhat different. The intermediate piece cells slip over the 
secretory ones so that we have both kinds in the end pieces, the conical or 
cuboidal secretory cells and the flat ones of the intermediate pieces directly 
bordering the lumen, which we know as centroacinar cells (Pl. 55, Fig. 
132). The latter do not form a continuous coat, but lie apart, so that the 
lumen is directly bounded in many places by the secretory cells. The pancreas 
is the only gland in the human body that contains centroacinar cells. 
The secretory cell of the end piece is conical or cuboidal, according to 
the form of the latter, and contains a roundish nucleus with nucleoli. The cell 118 
body varies much in appearance with the functional condition of the gland. 
During hunger, when the cell is filled with secretion, a broad, dark inner zone 
distended with granules, and a narrow outer zone containing the nucleus, may 
be seen. These granules contain the abovementioned fennents of the pan- 
creatic juice, or their preliminary stages, and lie in vacuoles of the proto- 
plasm, which thicken toward the base of the inner zone about the nucleus. 
When the gland begins to secrete during digestion, particularly in consequence 
of the passage of the acid gastric juice from the stomach into the intestine, 
the cells evacuate their secretory granules, their inner zone becomes smaller, 
the denser protoplasm of the outer zone moves after it, and the nucleus mi- 
grates to the centei' of the cell. Fibrous differentiations of the protoplasm 
appear in the outer zone, similar to those found in the serous cells of the sali- 
vary glands of the mouth. We call these basal filaments. Their purpose 
is to reform the secretion; the filaments break it up into granules and these 
are transformed into secretion. The further the formation of secretion ad- 
vances the larger the inner zone becomes again, so much the more these fila- 
ments are used up, and their remains may lie about the nucleus of the cell 
full of secretion in the form of a cap, the so-called accessory nucleus. 
Without doubt we have to do here with mitochondrial formations in these 
basal filaments. 
Between the cells of the end pieces are numerous secretory capillaries, 
like those of the serous glands of the mouth, while the end pieces themselves 
are interwoven about by numerous fine fibers of connective tissue, similar to 
the lattice fibers of the hepatic lobules. 
Langerhans' islands, already mentioned in the embryological introduc- 
tion, form the second main constituent of the parenchyma of the pancreas. 
They form in the adult spherical or ovoid masses of cells placed in the interior 
of the lobules, and are therefore surrounded on all sides by the secretory 
end pieces and are fairly well delimited from them (Pl. 55, Fig. 131). Their 
average diameter is 150 p, but this fluctuates like their number, which, accord- 
ing to modern researches, is between 170,000 and 200,000. They increase 
in number from the caput to the cauda and form at least 8% of the entire 
parenchyma of the gland. 
The islands are wrapped about by fine connective tissue fibers, but so very 
imperfectly that they cannot be said to have a capsule. The parenchyma 
consists of roundish island cells with relatively large nuclei. The cell body 
is usually homogeneous and clear, the result of which is that the islands stand 
out in relief from the glandular parenchyma when properly stained (Pl. 55, 
Figs. 131, 132). Under certain circumstances fine granules are observed in 
the cells, and this has been interpreted as a sign that they have a secretory 
function. The cells form either a quite irregular mass, or one in which anas- 
tomosing cords can be perceived. The abundant supply of the islands with 
blood capillaries is striking. 
The islands originate like the rest of the glandular epithelium from the 
epithelium of the young efferent duct, as brought out in the embryological 
introductory remarks, and the question arises whether they remain connected 
with the rest of the glandular epithelium or not. Views are divided in regard 119 
to this. Some investigators take the stand that the islands are continuously 
connected with the secretory end pieces, that cells are constantly used up in the 
activity of the islands and replaced by transformation of end pieces into 
island cells, so that the proportion between the parenchyma of the gland 
and that of the islands remains undisturbed under normal conditions. Others, 
on the contrary, maintain that the tissue of the islands is absolutely free and 
independent. 
With regard to the physiological properties of Langerhans' islands, path- 
ological research has lately shown that they exhibit more or less serious 
changes in all cases of diabetes mellitus. From this, and from the results of 
experimental research, we must conclude that the island tissue is a gland 
with internal secretion. It provides a secretion that is given off directly 
into the blood vessels, and exerts a specific influence over the metabolism of 
sugar in the body. Hence, two secretory processes run parallel in the pan- 
creas as in the liver, an outer and an inner; in the liver both issue from the 
same cells, in the pancreas from different ones. 
The arteries, coming from the hepatic, splenic, and superior mesenteric, 
follow in general the course of the excretory duct, enter the lobules and form 
capillary baskets about the end pieces. The arteries of the islands first form 
a plexus about each, from which numerous tortuous capillaries pass through 
the island. The veins behave like the arteries. The vessels of the islands 
seem to have a certain independence of the others. 
The lymphatics arise in spaces that sheathe the end pieces within the 
lobules of the gland, pass first between the lobules into vessels with walls, 
which twine about the blood vessels and empty into the pancreatico-duodenal, 
splenic, aortic, cardiac, and hepatic glands. 
The nerves of the pancreas are for the most part sympathetic, and come 
from the hepatic, splenic, and superior mesenteric plexuses. They form an 
extensive plexus in the interlobular connective tissue, provided with very many 
ganglia, from which fibers enter the lobules and twine about the end pieces. 
Within the lobules we meet with large sympathetic cells having long processes. 
The nerves end between the cells of the end pieces. Many fibers also enter 
Langerhans' islands. 
The Peritoneum 
The entire peritoneum is covered by an epithelium which represents the 
most superficial layer of mesenchymatous cells of the former wall of the 
coelom, and is composed of a single layer of large, low cells. Each cell has 
one flat, roundish nucleus, more rarely two, that bulges the free surface some- 
what. The bodies of the adjoining cells are united by numerous protoplasmic 
bridges, so that the entire peritoneal epithelium forms a sort of synctium, 
in which the limits of the cell territories remain firmly fixed by the following 
arrangement. Each cell has on its free surface a homogeneous, structureless, 
cuticular band; these bands are not confluent like the cell bodies, but are 
separated on all sides and luted together by a special cement. Thus a net- 
work of tortuous cement lines is formed in the superficial layer of the peri- 120 
toneal epithelium which reduces silver salts and can easily be brought into 
view by treatment with them (Pl. 6, Fig. 20). In many places, though not 
everywhere, fine short hairs project from this cuticular band which usually 
are stuck together and must be regarded as a sort of brush band. Besides the 
large celled epithelium, there is a small celled one, but it is confined 
to the lower surface of the diaphragm, where it covers the places between 
the tendinous bundles. It differs from the other both in the smaller size of 
its cells and in having larger quantities of cement between the cell plates. 
These cells possess a certain contractility, so that the interstices between 
them, through which absorption takes place from the abdominal cavity into 
the lymphatics, may be enlarged. These heaps of cement were formerly 
described as stomata, but no open communication seems to exist between the 
abdominal cavity and the lymphatics. 
The epithelial cells of the peritoneal cavity possess, as former mesenchy- 
matous cells, a great ability to wander. They may break loose from their 
connections, become free in the abdominal cavity, and here develop phago- 
cytic properties to a high degree. They may perhaps develop typical 
granulations in their bodies and become true granular leucocytes. 
The peritoneal epithelium rests on a structureless, thin basal membrane, 
which is followed by the propria that consists of a superficial layer composed 
mainly of elastic fibers, and a deep one of connective tissue which blends with 
the loose connective tissue of the subserosa. The elastic fibers form true net- 
works, while those of connective tissue interlace in all directions. The sub- 
serous connective tissue unites the parietal peritoneum with the subjacent 
abdominal musculature, the visceral peritoneum with the muscularis of the 
intestines, uterus, bladder, etc., and with the connective tissue coats of the 
glandular organs in the abdomen and pelvis. 
The mesentery and omentum are simply duplicatures of the peritoneum 
in which the subserosa of both layers blends into a median lamella bearing 
the vessels and containing much fat. The fat cells cluster into masses that 
accompany the blood vessels over broad areas. We meet with many plasma, 
mast, and wandering cells in the connective tissue in addition to fibroblasts 
and fat cells. While the omentum is a perfect membrane, covered with epithe- 
lium on both sides, in the newborn the cellular coat gradually becomes im- 
perfect in later life until finally apertures are formed here and there. In 
adults the omentum is cribriform in appearance; the beams between the aper- 
tures consist of connective tissue, and are incompletely clothed with epi- 
thelium. 
The vessels of the peritoneum come from those that supply the abdom- 
inal organs and muscles, run for long distances in the subserosa, and form a 
vascular plexus in the propria. The peritoneal nerves, part of which are 
vasomotor, part sensory, have a like origin, and form plexuses in the subserosa 
and propria. Part of the medullated sensory fibers have free ends, part ter- 
minate in special end corpuscles of the Vater-Pacini type, which unite into 
groups, especially in the peritoneum of the anterior abdominal wall, situated 
just beneath the basal membrane. They come into account in the perception 
of the sensation of pain. III. THE RESPIRATORY ORGANS 
1. THE NASAL CAVITY 
The nasal cavity in man has a double function, inasmuch as it is the 
olfactory organ, and also serves to warm, moisten and filter the entering cur- 
rent of air. 
Embryologically it develops from two parts, one of which is the paired 
olfactory fossa, or primary nasal cavity. This is lined with stratified 
cylindrical epithelium and opens at its posterior end with the primitive pos- 
terior nares into the mouth, from which it is separated by the primitive palate. 
In this primary nasal cavity the turbinates develop, and also Jacobson's organ, 
which latei' undergoes involution. Then a substantial part of the mouth 
unites with the primitive nasal cavity to form the definite nasal cavity 
through the formation of the secondary palate joining the primitive posterior 
nares toward the cauda. 
Conformably to the double function of the nasal cavity we find two areas 
of the lining mucous membrane that differ in structure, the respiratory 
and the olfactory regions. We will deal with the former alone at this 
place and defer the latter until we come to speak of the organs of sense. 
The olfactory region occupies only a very small part of the nasal mucous 
membrane, being confined to the superior turbinate and the corresponding part 
of the septum, while all the rest of the cavity belongs to the respiratory region. 
The outer entrance of the nasal cavity is the vestibule. Here the skin 
of the alae and of the upper lip passes into the nose and forms its lining. At 
first it retains completely the character of the skin, i.e., it is covered by a 
stratified flat epithelium, the most superficial cells of which are horny. 
The epithelium rests on a connective tissue corium that forms numerous 
papillae, the height of which increases at first, after which they gradually 
spread out and disappear. The corium in the vestibule is richly provided 
with networks of elastic fibers and contains hairs, sweat glands, and sebaceous 
glands. These characteristic formations of the skin disappear about the 
middle of the vestibule, together with the horny layer, so that in the posterior 
and upper parts of the vestibule we have a stratified flat epithelium resting 
on a connective tissue propria, in which mixed, seromucous glands, similar 
in structure to those of the lip, take the place of the sweat glands. 
The transition of the skin of the vestibule into the respiratory mucous 
membrane of the nose takes place in the lateral wall about at the limen nasi, 
on the septum in the region of the tubercle, on the floor still farther back, 
though these conditions vary in different persons. The transition is not so 
sudden as it is at the junction of the oesophagus and stomach, but the stratified 
flat epithelium gradually assumes the character of epithelium of the respira- 
tory mucous membrane. That is to say, it becomes a stratiform ciliated 
cylindrical epithelium, the height of which ranges from 15 to 40 p (Pl. 
11, Fig. 35; Pl. 56, Fig. 133). The entire thickness of the epithelium is 
121 122 
passed through by the ciliated cell, which rests with a slender foot on the 
floor of the epithelium, and thickens gradually as it approaches the free surface. 
The intermediate spaces between their foot ends are filled with conical cells 
that rest with broad bases on the epithelial floor and taper upward, but never 
reach the free surface with their thin, finely attenuated points. Between the 
ciliated cells are many goblet cells, which like them descend to the floor 
of the epithelium. The ciliated cells present all the characteristics that have 
already been described; their cilia attain a length of about 6 pi and stroke 
toward the posterior nares. Distinct cement edges can be demonstrated 
between the upper ends of the ciliated and goblet cells. 
The epithelium rests on a thin basal membrane that cannot be demon- 
strated in all parts of the nasal mucous membrane. Wherever it appears 
well developed it is cribriform, and processes of the connective tissue cells of 
the propria pass through the apertures it presents into the epithelium. 
The propria of the nasal mucous membrane forms no papillae and varies 
much in thickness, reaching its maximum on the interior turbinate. Its super- 
ficial parts consist of loose connective tissue, but the deeper we go the more 
abundant do we find networks of elastic fibers. Numerous glands are situ- 
ated in the propria. First we find simple crypts, short diverticula of the 
epithelium into the propria which are lined substantially with goblet cells,, 
but the mucous membrane also contains many branched alveotubular nasal 
glands, which resemble in their structure the glands of the lips and are 
therefore of a mixed nature. In addition to these, little lymph follicles are 
to be found in the propria. 
The propria of the nasal mucous membrane is extremely rich in blood 
Vessels, which mainly come from the ethmoidal and nasal arteries. The 
arteries in the deeper layers send their branches through the entire propria 
and break up into a capillary plexus beneath the epithelium and surrounding 
the glands. Thence the blood flows in a superficial venous plexus, and from this 
into a deeper one. These venous plexuses may assume the character of large 
masses of erectile tissue in certain parts of the nasal cavity in post-em- 
bryonal life, particularly on the inferior turbinate and on the medium surface 
of the middle turbinate. Here strong bundles of smooth muscle fibers, chiefly 
circular, are embedded in the venous walls, and this muscularity is surrounded 
by a connective tissue adventitia containing elastic fibers. By this means 
great masses of blood are accumulated in the mucous membrane of the tur- 
binates for the purpose of warming the inspired air. 
The lymphatics form a wide-meshed plexus in the mucous membrane of 
the respiratory part, collect into little trunks, part of which pass forward 
to the mandibular glands, part backward to the pharynx and to the deep 
cervical glands. The lymph passages of the nasal mucous membrane are con- 
nected with the arachnoidal space of the brain. 
The nerves of the respiratory nasal mucous membrane come from the 
sphenopalatine ganglion, and are in part sensory fibers from the trigeminus, 
in part fibers from the sympathetic. They form a plexus in the propria and 
some of them end free between the epithelial cells of the mucous membrane, 
some of them in the musculature of the vessels. 123 
The mucous membrane of the accessory sinuses resembles, on the 
whole, that of the nasal cavity, except that it is thinner throughout and 
contains fewer glands and no erectile tissue. 
2. THE LARYNX 
The lower respiratory passage appears first in embryos 2 to 3 mm long 
as a groovelike recess in the ventral wall of the epithelial intestinal tube just 
beneath the last pharyngeal pouch. This groove dilates at its caudal end into 
an unpaired rudiment of the lungs. The margins of the groove come closef 
and closer together from the sides until finally separation takes place and we 
have on the ventral side of the foregut a second epithelial tube that opens 
cranially into the pharyngeal cavity and has the rudimentary lung at its 
caudal end. Like the intestinal tube it consists of a double layer of epithelium, 
on the superficial cells of which cilia gradually appear. The cranial end of this 
respiratory tube, which subsequently grows considerably in length, forms with 
the adjoining parts of the pharyngeal rudiment the larynx. In embryos 7 to 
8 mm long appear two sagittal elevations and one frontal across these, the 
arytenoid and epiglottic tubercles, the latter of which is directly contiguous 
to the rudiment of the tongue. In the mesenchyma about these epithelial 
rudiments a dense stratification of rounded mesenchymatous cells takes place 
in embryos 10 to 15 mm long, whence originate chondroblasts that produce 
cartilage. The advanced growth of the arytenoid tubercules leads to a median 
agglutination, so that now the pharynx communicates with the trachea only 
through a narrow transverse slit ventrally from the epiglottic rudiment. Cau- 
dally from this the epithelium puts out two lateral buds that are at first 
solid, but soon become hollow, connect with the lumen of the larynx, and form 
the ventriculum laryngis. The caudal one is marked by the absence of cilia 
on its epithelium and becomes the vocal cords, while the cranial portion fur- 
nishes the false vocal cords. The rudiment of the thyroid cartilage is paired; 
the two halves unite with another, unpaired, median rudiment into a single 
cartilage, which at first is in continuous connection with the great cornu of the 
hyoid bone, but this is severed about the end of the third month by the trans- 
formation of a piece of cartilage into connective tissue. The cricoid cartilage 
appears before the thyroid as a sort of tracheal ring. Simultaneously with 
the cartilages appear the rudiments of the laryngeal muscles, the striated 
fibers of which differentiate strongly in the elongated mesenchymatous cells, 
The glands of the laryngeal mucous membrane originate from the fourth 
embryonal month on as protrusions of the epithelium. 
The larynx is lined by a mucous membrane, which is of a bright red 
color during life. The only exception to this is the covering of the vocal 
cords, which stand out distinctly because of their gray color. The mucous 
membrane has first an epithelium 50 to 60 p thick, which has the same 
character as that of the respiratory part of the nose, i.e., it is a stratiform 
ciliated epithelium in which many goblet cells occur. But exceptions 
to this rule are furnished in various parts of the larynx. From above the 
stratified flat epithelium of the pharynx enters the opening of the larynx 124 
and covers the entire oval surface of the epiglottis as well as a strip about 
10 mm broad of its laryngeal surface. From this point the flat epithelium 
forms a marginal zone about the entrance to the larynx that varies a great 
deal in different individuals, and may send tonguelike strips in an irregular 
way into its interior. The vocal cords alone are quite constant in having 
their free edges projecting into the larynx covered with stratified flat epithe- 
lium. On the other hand, islands of respiratory epithelium are found on the 
oval surface of the epiglottis. 
Taste buds occur in varying numbers in the epithelium of the laryngeal 
mucous membrane, but only in the stratified flat epithelium. They are most 
common on the laryngeal surface of the epiglottis and on the plicae aryepi- 
glottica?. They are never met with on the vocal cords. 
The epithelium rests everywhere on a homogeneous basal membrane, 
which is first developed in postembryonal life. Like that of the nasal mucous 
membrane it is full of fine pores. 
The propria forms papillae wherever stratified flat epithelium is present, 
except on the vocal cords, where ridges running lengthwise take their place. 
It is very firm and compact on the laryngeal surface of the epiglottis, on the 
vocal cords, and in the lower part of the larynx, less firm in the false vocal 
cords, and quite loose on the oral surface of the epiglottis, in the plicae aryepi- 
glotticae, and on the posterior surface of the larynx. It is composed of con- 
nective tissue which forms very many networks and contains strongly developed 
elastic fibers, especially in its deeper layers. These form elastic membranes 
lying immediately upon the perichondrium and are thickened in places into 
bandlike masses, the most important of which have been named the false and 
the true vocal cords. The latter consists of a network of thick elastic fibers, 
the longitudinally running meshes of which are so narrow that the fibers run 
almost parallel. On this fibrous mass rests the epithelium, separated from it 
only by a thin layer of connective tissue and the basal membrane (Pl. 56, 
Fig. 134). 
Lymph follicles appear in the propria in the region of the ventriculus 
laryngis. The propria is also the seat of the laryngeal glands, which pene- 
trate more or less deeply into the cartilage in many places, especially on the 
epiglottis. They are absent only on the vocal cords, and are most numerous 
on the false vocal cords and in the mucous membrane of the ventricle, where 
their secretion serves to keep the vocal cords moist. In form and histological 
structure they resemble the glands of the respiratory nasal mucous membrane. 
A connective tissue submucosa is developed only in the lower segments 
of the larynx; in the middle and upper1 portions the clastic tissue of the pro- 
pria passes directly into the perichondrium, or the interfascicular connective 
tissue of the laryngeal muscles, which is likewise full of elastic fibers. 
The muscles of the larynx are composed of bundles of striated fibers. In 
the vocal fold the longitudinal bundles of the vocal muscle and of the thyreo- 
arytaenoidcus externus lie directly on the vocal cords. 
The cartilages of the larynx are hyaline, with the exception of the epi- 
glottis, the cartilagines corniculatae and cuneiformis, and the vocal process of 
the arytenoid, which are elastic. They are enclosed by a perichondrium which is 125 
rich in elastic fibers and frequently undergoes an asbestic change in the chemical 
constitution of its basic substance, the collagenous fibrils. Soon after puberty 
ossification of the cartilage takes place, earlier and more completely in males 
than in females. 
The arteries of the laryngeal mucous membrane, which come from the 
laryngeal and thyroid, form a wide meshed plexus in the deepest layers of the 
propria, and then a closer one in the middle layers. From the latter minute 
twigs extend and break up into capillaries beneath the basal membrane and in 
the papillae of the propria. They assemble into veins which accompany the 
arteries. 
The lymphatics form a plexus in the upper, and another in the deeper 
layers of the mucous membrane, and are developed most abundantly in the 
ventricle, least so on the vocal cords. The vocal cords divide the lymphatics 
of the larynx into two independent districts, those of the upper flowing into the 
deep cervical glands, those of the lower into the prelaryngeal, pretracheal, and 
paratracheal. 
Motor nerves supply the muscles of the larynx, sensory and vasomotor 
the mucous membrane. They come from the laryngeal nerves, with which run 
many sympathetic fibers, and form a plexus in the propria provided with little 
ganglia and isolated nerve cells. Part of the fibers coming from this plexus 
end free in terminal arborizations between the cells of the epithelium, while 
part form tufts in the connective tissue from which branched fibrillae extend to 
end in terminal plates. They also twine about the glands and vessels. 
3. THE TRACHEA AND THE EXTRAPULMONARY 
BRONCHI 
The structure of the mucous membrane of the trachea and of the extra- 
pulmonary bronchi deviates only in non-essentials from that of the respiratory 
mucous membrane of the nose (Pl. 57, Fig. 135). The epithelium is strati- 
form, ciliated, and cylindrical, increases only a little in height in the trachea, 
and contains many goblet cells. Islands of stratified flat epithelium are found 
in it and are particularly common in the pars membranacea tracheae. 
The propria, which is separated from the epithelium by an always distinct 
basal membrane, is very rich in elastic fibers that form a thick layer just 
beneath the latter, with the fibers running lengthwise and anastomosing freely. 
Many lymphocytes are met with between the elastic fibers. Under certain 
circumstances they may accumulate about the efferent ducts of the glands into 
follicular formations. A second elastic network lies beneath the propria, blends 
with the perichondrium on the cartilage, and elsewhere separates the propria 
from the submucosa. 
The propria is abundantly supplied with large glands, exactly like the 
laryngeal glands in structure. They are most numerous in the interstices of 
the cartilage and in the pars membranacea. 
A submucosa can be said to exist only in the membranous portion, where 
it consists of loose connective tissue with numerous elastic fibers, contains many 
fat cells, and joins the propria with the subjacent musculature. 126 
Thc musculature consists of smooth muscle fibers that in their entirety 
form a band on the dorsal side of the membranous portion. The fibers run 
transversely and are inserted by means of short tendons into the free ends of 
the cartilaginous rings. By their contraction they draw the latter together 
and thus produce a not inconsiderable narrowing of the lumen of the trachea. 
To the dorsal side of these lie isolated bands of longitudinal fibers, part of 
which cross over to the musculature of the oesophagus. 
The cartilaginous rings of the trachea and extrapulmonary bronchi are 
composed of hyaline cartilage and are covered by a perichondrium that is un- 
commonly rich in elastic fibers and is in continuous connection with the an- 
nular ligaments which join the ends of each imperfect ring. In later life an 
asbestic change and ossification appears in the cartilages of the trachea, but 
this comes later than in those of the larynx and is mainly confined to the upper 
half of the trachea. 
The arteries for the upper half of the trachea come from the inferior 
thyroid, for its lower part from the internal mammary, and for the extra- 
pulmonary bronchi from the aorta itself. They form little annular arches out- 
side of the annular ligaments, which send branches to the mucous membrane, 
where they form an arterial plexus in the deeper layers of the propria. From 
this twigs are sent to the capillary plexus enveloping the glands and lying be- 
neath the basal membrane. The course of the veins corresponds to that of the 
arteries. 
The lymphatics form a very dense network in the propria, assemble into 
trunks that pass through the annular ligaments, and reach the paratracheal 
and deep cervical glands. 
The Nerves, which come from the inferior laryngeal and directly from 
the vagus, and are associated with numerous sympathetic fibers from the pul- 
monary plexus, are quite similar in their behavior to those of the larynx. They 
form three plexuses in the propria of the mucous membrane, in which numerous 
ganglia are embedded. 
4. THE LUNGS AND THE INTRAPULMONARY BRONCHI 
In embryos 2 to 3 mm long the rudiment of the lung forms a little sac- 
like dilatation at the caudal end of the just separated respiratory tube. This 
divides very soon into two branches, which in turn distend at their ends. Of 
these two principal bronchi the left divides into two, the right into three 
branches. The entire bronchial tree gradually develops by continuous dichoto- 
mous budding and increase in length, with terminal buds that are saclike and 
dilate into infundibula. Finally alveoli develop in the walls of the latter and 
of the respiratory bronchioles that lead to them. 
The epithelial rudiment of the lung, which thus comes from the entoderm, 
is enclosed in its growth by a mass of mesenchyma that forms its outer covering. 
During the fourth month of pregnancy this proliferates strongly about the 
terminal branching and divides it off into little districts of parenchyma about 
0.25 mm in diameter, the pulmonary lobules. In these constantly growing 
lobules the amount of connective tissue decreases as the branching of the epithe- 127 
lial tube and the formation of alveoli increase. The individual lobules are de- 
fined very sharply from one another by the interlobular septa of connective 
tissue. After birth this interlobular tissue becomes greatly reduced, so that 
the boundary lines between the lobules become more and more effaced. The re- 
sult of this is that several neighboring lobules blend, and the definitive pul- 
monary lobule is a conglomerate of several embryonal lobules. 
The epithelium of the bronchi acquires cilia progressively from above down- 
ward, gradually becomes stratiform, and finally simple, ciliated cylindrical 
in the smallest bronchi. The young alveoli are lined with a single layer of 
cuboidal cells, and while the number of the latter constantly becomes greater 
their diameter remains about the same during the last months of embryonal 
life. At the time of birth a great distention takes place, while the cells thin out 
considerably and assume the character of respiratory epithelium. The mesen- 
chymatous envelope of the bronchi likewise develops from above downward. 
First a propria develops in which successive glands appear. Cartilaginous 
nuclei originate in the bronchial wall and lead to the formation of cartilaginous 
plates down into the smallest bronchi. The clastic tissue appears relatively 
late and extends not only to the bronchi and bronchioles, but also to the walls 
of the terminal chambers and alveoles, which it surrounds with fine networks. 
The function of the lungs is to remove from the venous blood, brought to 
it from the right side of the heart through the pulmonary artery, the carbonic 
acid that is constantly formed in all of the cells of the body by the process 
of combustion, and at the same time to load it with oxygen from the air for 
the purpose of further oxidation. Air that has been warmed and moistened in 
the upper respiratory passages is sucked into the alveoli by the inspiratory 
movements of the thorax and diaphragm, here gives up a part of its oxygen to 
the blood, receiving in exchange carbonic acid and water vapor, and is pressed 
out by expiration. The entrance of oxygen into the blood, and the passage 
of carbonic acid into the air, take place through the cells of the respiratory 
epithelium of the alveoli in accordance with the laws of absorption. 
The lungs are constructed on the principle of the compound branched alveo- 
tubular glands. The main bronchi give off lateral bronchi that branch 
at first monopodically, later dichotomously, and gradually diminish in caliber. 
The bronchioles pass from the smallest bronchi each into a lobule where it 
splits up into the intralobular bronchioles, from which by further sub- 
division come the respiratory bronchioles. The alveoli in these are few, 
but in the terminal twigs they become much more numerous, so that finally they 
lie in contact with each other. These terminal twigs of the respiratory bron- 
chioles, thickly studded with alveoli, we call the alveolar passages. The 
alveolar passage dilates at its end into the infundibulum, which is likewise 
studded with alveoli (Pl. 57, Fig. 136). 
The lobulated structure of the lung is very evident on its surface. It ap- 
pears to be divided into polygonal fields 5 to 10 mm in diameter, which are 
the bases of the conical, peripheral lobules. Toward the interior the lobules 
become irregularly polyhedral. They are separated by a scanty connective 
tissue connected with that accompanying the bronchi and the vessels. It con- 
tinually diminishes after birth, as has been remarked already (Pl. 57, Fig. 136). 128 
Like every other gland the lung, with its efferent ducts, the bronchi and 
bronchioli, is lined by a mucous membrane, which lies in many longitudinal 
folds in the bronchi. Its epithelium shows no noteworthy differences from 
that of the extrapulmonary bronchi, up to the entrance of the bronchiolus into 
the pulmonary lobule, but within these it increases in height and loses its basal 
cells, so that we have a simple ciliated cylindrical epithelium in which the goblet 
cells soon disappear. The cylindrical cells then gradually become lower and 
lose their cilia, so that in the respiratory bronchioles we have a low cuboidal 
epithelium, which gradually changes into the respiratory epithelium that 
lines the alveoli and also part of the respiratory bronchioles. This consists of 
large, extremely thin, irregularly bounded cells luted together by a cement. 
The nuclei all lie in the meshes of the network of blood capillaries, while the 
parts that lie on the capillaries themselves are without nuclei. The nucleated 
portion of the cell consists of a granular protoplasm, while the other part ap- 
pears to be perfectly homogeneous. 
The basal membrane beneath the epithelium of the bronchi becomes thin- 
ner and more delicate from above downward, and can scarcely be detected in 
the bronchioles. 
The propria forms in the bronchi folds of the mucous membrane, in keep- 
ing with the longitudinal ridges, that consist essentially of elastic fibrous net- 
works. Toward the outside these networks are sparser with loose connective 
tissue lying between them. The propria is pretty strongly infiltrated with 
lymphocytes. The deeper we go the thinner the propria becomes, but it always 
has fairly strong elastic fibers, and finally it passes over into the basal mem- 
brane of the alveoli. The basal membranes of adjoining alveoli blend and form 
the alveolar septa. This basal membrane is homogeneous, structureless, and 
has running in it numerous elastic fibers joined into networks, together with 
a few connective tissue fibers. The elastic fibers surround the alveoli like 
baskets, and form a circle about the base of each. The basal membrane also 
encompasses the blood capillaries accompanied by minute elastic fibers and is 
clothed on its inner side by the respiratory epithelium (Pl. 58, Fig. 137). 
We see the musculature in the trachea as a continuous layer stretched 
between the ends of the imperfect cartilaginous rings. With the disintegration 
of the latter the muscular fibers advance farther inward and form a wicker- 
work in the propria, the meshes of which constantly grow larger as we go 
downward, but are maintained to the respiratory bronchioles, and sometimes 
circular fasciculi may be demonstrated in the alveolar passages outside of the 
structureless membrane. 
The glands of the bronchi become steadily smaller and scarcer downward, 
but otherwise differ from the tracheal glands only in their relation to the mus- 
culature ; their excretory ducts pass through the latter and their glandular 
bodies are placed outside of it between the fragments of cartilage. They dis- 
appear entirely in the walls of the intralobular bronchioles. 
The cartilaginous rings of the larger bronchi become, through partial 
fusion, irregular plates, the processes of one projecting into the interstices 
of another, so that in the cross section the lumen of the bronchus is surrounded 
by several pieces of cartilage, partially overlapping or at a distance apart. 129 
Then follows a progressive splitting up of these cartilaginous plates, which 
become constantly more scanty and finally disappear at the entrance of the 
bronchi into the lobules. Therefore the intralobular bronchi have neither 
glands nor cartilage in their walls. 
The blood supply of the lungs is double, like that of the liver, the pul- 
monary artery taking the place of the portal vein, the bronchial arteries that 
of the hepatic. The pulmonary artery brings to the lungs the functional blood 
from the right side of the heart, i.e., venous blood which is to be arterialized 
in the alveoli of the lungs. Its branches accompany the bronchi and their 
branches, yet without coming into close relations, and finally enter the lobules 
with them. Here they weave a dense capillary network about the respiratory 
bronchioles and the pulmonary alveoli, embedded in the basal membrane of the 
latter. The arterialized blood is now gathered from this network into little 
veins that appear on the entire periphery of the lobules, unite into larger trunks, 
and then, following the course of the bronchi and of the pulmonary artery, 
unite to form the pulmonary vein. The bronchial arteries, on the contrary, 
bring arterial blood rich in oxygen to the tissues of the lungs directly from the 
aorta. They also follow the course of the bronchi, but continually send out 
twigs into the bronchial walls, supplying them and the surrounding connective 
tissue with blood. In the former they form two plexuses, one in the muscularis, 
and one beneath the epithelium. They do not enter the lobules, but run in the 
interlobular tissue to the surface of the lung, where they form another capillary 
plexus in the pleura. The bronchial veins follow a course quite similar to that 
of the arteries, but they do not carry away all of the nutritive blood of the 
lung, for part of the small veins empty into the pulmonary veins. 
The lymphatics of the lungs are divided into the superficial and the deep. 
The superficial ones form a network over the surface of the entire lobe be- 
neath the pleura, and the efferent vessels from this lead to the pulmonary hilum 
and thence to the bronchopulmonary glands. The deep lymphatics follow the 
course of the bronchi and blood vessels, and form a perilobular network about 
each lobule, but do not appear to enter them. They also empty into the pul- 
monary glands. 
The nerves enter the lung from the pulmonary plexus and come partly 
from the vagus, partly from the sympathetic. They accompany the branches 
of the pulmonary artery and the bronchi and have many ganglia. In the pro- 
pria of the bronchi they form a plexus from which fibers extend to the epithe- 
lium, the glands and the muscles, enter the lobules with the bronchioles and 
weave about the alveoli. Knoblike endings on the cells of the respiratory 
epithelium have also been described. 
5. THE PLEURA 
The visceral pleura, covering the surface of the lungs and penetrating 
deeply into its incisures, has essentially the same structure as the parietal which 
lines the thoracic cavity, and consists of a loose connective tissue in which 
are embedded many elastic fibers. It has no vessels and is covered next the 
pleural cavity by a simple flat epithelium quite like that of the peritoneum. 130 
Between the epithelium and the connective tissue is a thin, structureless basal 
membrane. The pleura is joined to the subjacent tissues by a subpleural 
connective tissue in which lie the blood vessels and lymphatics. Its nerves 
come from various sources, the vagus, phrenic, sympathetic, and intercostals, 
and form a plexus provided with ganglion cells. 
6. THE DIAPHRAGM 
The rudiment of the diaphragm is composed not of one, but of several com- 
ponents, the transverse septum, mesentery, Wolffian folds, and body wall. At 
first it consists purely of connective tissue, but in the second embryonal month 
muscular fibers enter and soon appropriate it, so that in the third month the 
diaphragm is completely permeated by them. Later the muscular fibers undergo, 
involution in the central part, leaving a central tendinous part surrounded 
by a muscular part. 
The structures present in the muscular part are the pleura, muscula- 
ture and peritoneum. Concerning the structure of the first and last noth- 
ing remains to be added to what has been said. The muscular fibers are 
striated and of a caliber varying in thickness from 5 |J to 20 p. Segmenta- 
tion may be observed very often in these fibers, just as with the muscular fibers 
of the tongue. Neighboring fibers anastomose and form networks. 
In the tendinous part these muscular fibers become tendinous and 
are collected into bundles that take radiating or arched courses that are rather 
complicated. These tendinous fibers form two layers, one on the peritoneal, 
the other on the thoracic side, which contain strong elastic networks. In 
the center the two layers come pretty close together, but peripherally they are 
separated by connective tissue permeated by fat cells and interwoven with 
elastic networks. The peritoneum and pleura are the same as in the mus- 
cular part. 
The arteries of the diaphragm come partly from the internal mammary, 
partly from the aorta, and form in the subserous and subpleural connective 
tissue large plexuses, from which twigs pass on the one hand to the muscle or 
tendon bundles, and weave capillary plexuses about them, on the other into the 
peritoneum or pleura and form subepithelial capillary plexuses. The veins 
follow the same course as the arteries and empty into the vena cava inferior. 
The lymphatics form a subpleural and a subperitoneal plexus which have 
many connections and, like the blood vessels, send out twigs to weave networks 
about the tendon bundles and muscular fibers. The vessels assembled from these 
networks turn toward the pleura and run in numerous trunks to the sternal, 
and anterior and posterior mediastinal lymphatic glands. 
The innervation of the diaphragm is supplied by the phrenic nerve, which 
is mainly composed of thick and thin medullated fibers, but also contains many 
that are nonmedullated. Within the diaphragm its branches form two plexuses, 
a subpleural and a subperitoneal, in which fibers from the solar plexus and 
from the vagus also occur. These two plexuses have many connections. Some 
of the fibers from the plexus supply the muscular fibers with motor end plates, 
others terminate in the connective tissue between the muscular and tendinous 131 
bundles cither free in antlered end formations, or in club-shaped end corpuscles. 
Finally a delicate end plexus is formed beneath the epithelium of the pleura 
and peritoneum. 
7. THE THYROID GLAND 
We will discuss the thyroid gland in connection with the digestive and 
respiratory organs, because it is related to them embryologically and originates 
from the entoderm of the upper part of the foregut. The first signs of the 
thyroid appear very early, in embryos 2 to 3 mm long, in the form of an 
epithelial cone that grows out of the ventral wall of the cavity of the mouth 
between the deep parts of the three primary rudiments of the tongue which have 
been mentioned. It lies at the upper end of the respiratory tube, which is 
splitting up at this time, and forms two lateral lobes that grow rapidly, while 
the pedicle joining it to the base of the tongue, the ductus thyreoglossus, be- 
comes very slender and finally disappears, except for a blind sac at its cranial 
end, the foramen caecum. This unpaired, median rudiment associates itself with 
a paired lateral proliferation of epithelium which comes on each side from the 
fifth pharyngeal pouch. The rudiment of the thyroid is quite solid at first, 
but begins to become hollow in embryos 6 mm long, and sends out shoots that 
are at first solid, later hollow, which become cut off by the surrounding con- 
nective tissue until they lie close together as little closed sacs lined with 
epithelium. 
The thyroid is a closed gland, i.e., a gland with a cavity, but no excre- 
tory duct, and consists of numerous, densely packed vesicles lined with epithe- 
lium, the so-called thyroid follicles. These are commonly ovoid in shape, 
but may be elongated, and are studded with secondary protrusions (Pl. 40, 
Fig. 100). They scarcely exceed 0.1 mm in diameter and decrease in size 
with advancing age. The epithelium lining the follicles consists of a single 
layer of cells, the height of which varies. Usually they are about as tall as they 
are broad, but in some places they are cylindrical, in others quite flat. Each 
cell contains a spherical nucleus and a filamentous protoplasm in which mito- 
chondria can be demonstrated. From the latter proceeds the formation of the 
secretion, which appears first in the form of minute, very acidophilic granules. 
As the secretion augments in the cell the granules blend and the entire cell body 
stains intensively. Such are called colloid cells to distinguish them from the 
former, which are known as chief cells. In addition we find in the zone ad- 
joining the lumen minute granules of a fatty substance. 
The secretion, the colloid, is emptied by the epithelial cells into the lumen 
of the follicle and fills it completely with a rather granular, very acidophilic 
substance. Its chief constituent is thyroglobulin, which may be decomposed 
by various agents into an albumin and iodothyrin. The content of iodine 
is characteristic of colloid. The contents of the vesicles also include the re- 
mains of cast off and broken down epithelial cells. The question how the colloid 
is emptied out of the closed vesicles is answered in various ways by different 
authors. The only thing certain is that it is to be found in the lymph passages 
about the follicles. Some think it gets here by osmosis, others think that it 132 
passes through passages and clefts between the cells. Another theory is that 
the pressure of the ever increasing contents destroys the epithelium lining the 
follicle at a certain place, that the follicle then ruptures and allows the colloid 
to escape into the lymph spaces. 
The colloid is carried by the lymphatics into the blood stream and is borne 
by it to the various organs of the body, where it has in some a stimulative, in 
others an inhibitive effect, according to the nature of a so-called hormon. 
When the secretion of the thyroid colloid is stopped, either by disease or by 
the extirpation of the organ, the result is arrest of growth, decrease of in- 
telligence, hypoplasia of the genital organs, diminution of the amount of 
haemoglobin in the blood, general emaciation, and finally death. 
The thyroid is enclosed in a connective tissue capsule, which sends septa 
into the interioi' of the organ and divides it into lobes and lobules that become 
smaller and smaller, just as in the salivary glands. From the interlobular septa 
lattice fibers penetrate between the follicles and form basketlike capsules about 
them. No membrana propria has been demonstrated about the follicles. 
The thyroid arteries are characterized by a relatively large caliber. They 
branch and anastomose freely in the interlobular tissue, enter the lobules and 
form about each follicle a large capillary basket placed just beneath the epithe- 
lium. The veins form several plexuses in the gland and conduct the blood into 
the thyroid veins. 
The lymphatics likewise form networks about the follicles closely adjoining 
the epithelium. The above mentioned intercellular passages are supposed to 
open into these perifollicular networks. The lymphatics of the thyroid lead 
into the pretracheal, prelaryngeal, and deep cervical glands. 
The nerves of the thyroid are for the most part sympathetic in nature 
and enter the gland with the blood vessels. They weave meshes about the 
follicles and end with little nodules on the epithelial cells. It has not yet been 
demonstrated with certainty that the nerves exert any influence over the se- 
cretion. 
8. THE PARATHYROIDS 
The parathyroids, or parathyroid glands, belong to the class of the 
so-called branchiogenous organs, i.e., they develop from the epithelium of the 
pharyngeal pouches and clefts. They originate in the third and fourth of the 
five pharyngeal pouches that develop in man, in epithelial thickenings directed 
dorsocranially. When the third and fourth pharyngeal pouches arc cut off the 
parathyroids come into close relations to the thyroids, which are maintained 
thoughout life, and then appear as two little corpuscles lying in the furrow be- 
tween the thyroid and the oesophagus, one at the upper, the othci- at the lower 
end of the lateral lobe of the thyroid. 
Each parathyroid is enclosed in a connective tissue capsule, which sends 
into its interior septa that unite into a network. Within the meshes thus 
formed lies the parenchyma, in which the cells lie in reticulate columns, or 
in irregulai' heaps, or are grouped to form true follicles, each with a lumen. 
The last constantly grow more frequent with advancing age and differ in noth- 133 
ing from the follicles of the thyroid. Two different forms of cells may be 
recognized as composing the epithelial columns and heaps, the chief cells and 
the oxyphilic cells. The former are usually the smaller and contain in their 
bodies larger masses of glycogen, the latter contain fine, oxyphilic granula- 
tions togethei' with particles of fat. The follicles secrete colloid, which is also 
found in the peripheral parts of the corpuscle, frequently in clefts between 
the cells. The colloid of the parathyroid appears to contain less iodine than 
that of the thyroid. 
The removal of the parathyroids leads to an increased muscular excita- 
bility, to the production of muscular spasms that are associated with serious 
trophic lesions in various organs. 
The parathyroids are very richly supplied with blood vessels. The cap- 
illaries weave plexuses about the heaps and trabecula of cells, and have the 
character of sinusoids. 
The nerves are similar to those of the thyroid. 
9. THE THYMUS 
The thymus also is a branchiogenous organ. It originates from epithelial 
thickenings that first extend in a ventral direction from the blind ends of the 
third and fourth pharyngeal pouches in embryos 5 to 6 mm long, and therefore 
is considerably later in development than the thyroid. The upper of the two 
rudiments grows toward the cauda, cuts itself off from the pharyngeal epithe- 
lium, and unites with its mate on the opposite side of the median line into an 
unpaired organ in which the paired rudiment can be recognized only in its 
cranial portion. The rudiment from the fourth pharyngeal pouch either under- 
goes involution, or develops into an accessory thymus. The thymus consists 
at first of densely packed epithelial cells. In embryos 50 to 60 mm long a 
loosening takes place in the mass of cells, which become stellate and anasto- 
mose. In the center of the lobules now formed by the entrance of connective 
tissue, the reticulum thus created is denser than in the periphery, and the cells 
are also larger, so that a medullary and a cortical substance can be distin- 
guished. Then begins the most important stage in the histogenesis of the thy- 
mus ; lymphocytes wander into the reticulum from outside and transform the 
originally epithelial rudiment into a lymphoid organ. 
In its highest development, i.e., at the time of puberty, the structure of the 
organ is as follows: Each of the two lobes has a thin connective tissue capsule, 
which enters into the organ and divides it into secondary lobules about 10 mm 
in diameter, and these into smaller primary lobules (Pl. 39, Fig. 98). In other 
glands the lobules are completely separated by the connective tissue, but this 
is not the case in the thymus, where the parenchyma of all of the lobules is con- 
tinuously connected, the connective tissue on each lobule leaving a sort of hilus 
through which a pedicle of parenchyma passes and unites with its neighbors, 
so that the final result is a thick parenchymatous cord forming the axis of each 
lobe. 
The parenchyma of the thymus consists of the reticulum and of the 
thymus cells. In each lobule a clear, neutral medullary, and a dark periph- 134 
eral cortical substance may be recognized. We will turn first to the 
reticulum, which involves the whole lobule and naturally enters also into the 
parenchymatous cord. As has been stated already it is composed of stellate 
epithelial cells which are larger and richer in protoplasm in the medulla, smaller 
in the cortex. This reticulum resembles that of the lymphatic glands, although 
it is of entirely different origin, and the similarity is increased by the fact that 
fibers appear in the cells, enter the cell processes and behave quite like the 
intercellular fibers of reticulated tissue. In addition true collagenous fibers 
enter the parenchyma with the blood vessels, which naturally have nothing to 
do with the cells of the reticulum. 
Within the reticulum lie the thymus cells, found in both the medulla and 
the cortex, but much more abundantly in the latter. The medulla also con- 
tains special structural elements, Hassall's corpuscles. The majority of the 
cells, the so-called little thymus cells, are lymphocytes that have wandered 
in and have suffered only immaterial changes in their structure. They multiply 
within the parenchyma of the thymus and wander out from it as in other lym- 
phoid organs. Eosinophile leucocytes also occur with them. Hassall's 
corpuscles, which are to be observed first in embryos 60 to 70 mm long, 
originate from the medullary cells and are larger or smaller, ovoid or spherical 
cellular masses having a distinctly laminated structure (Pl. 40, Fig. 99). One 
cell is always placed like a dish about the other cells, so that the central ones 
are the oldest. As enlargement advances all of the signs of degeneration ap- 
pear in the central cells, their nuclei become pycnotic and disorganized, while 
the cell bodies undergo hyaline degeneration. The blending of neighboring 
corpuscles finally create formations that are visible to the naked eye. They 
attain their greatest development coincidently with that of the entire organ. 
Many leucocytes are also to be found together with the epithelial cells. 
The thymus increases in size with fair rapidity after birth and reaches the 
acme of its development with the onset of puberty. Then signs of involution 
appear. Involution proceeds rapidly at first, then more slowly, so that only 
traces of the parenchyma remain by the sixtieth year. Its first indication is a 
poverty of the parenchyma in lymphocytes, as the multiplication of the latter 
in the organ comes to a standstill. Then the cells of the reticulum break down 
here and there, and are replaced by an increase of the connective tissue con- 
taining many fat cells. 
With regard to the physiology of the thymus, modern researches agree 
that the organ has relations to the general growth of the body and to the meta- 
bolism of lime. Animals from which the thymus has been removed exhibit a 
marked inhibition of growth and an insufficient calcification of newly formed 
bone substance. Changes take place in their bones that constitute the clinical 
picture known in man as rickets. The thymus also appears to exert a powerful 
influence over the development of the testicles and ovaries. Premature puberty 
appears in animals on which a thymodectomy has been performed. Removal 
of the thymus seems to result in a poverty in the blood of leucocytes, especially 
of the transition forms. 
The arteries of the thymus come from the internal mammary, the innomi- 
nate, and the inferior thyroid. Their branches run between the lobules into a 135 
plexus about each lobule, from which capillaries pass through the cortex and 
medulla to assemble into veins that follow the course of the arteries. 
The lymphatics form plexuses about each primary lobule from which 
twigs radiate through the cortical layer in order to form a second plexus on 
the surface of the medullary substance. The latter is in turn permeated by 
a network of lymphatic capillaries that weave about Hassall's corpuscles. The 
larger lymphatics accompany the blood vessels and empty into the anterior 
mediastinal and deep cervical glands. Whether lymphocytes leave the thymus 
by way of the lymphatics has not yet been determined with certainty. 
Nerves from the vagus and sympathetic reach the thymus in company with 
the blood vessels, but they are very scanty and nothing of the particulars of 
their relations within the gland is yet known. IV. THE URINARY ORGANS 
1. THE KIDNEY 
The conditions presented by the embryology of the kidney are more com- 
plicated than those found elsewhere, because transitory urinary organs appear 
during embryonal life, and then disappear to give place to the permanent ones. 
The development of the urinary organs starts from the primitive segment 
columns, those portions of the mesoderm that connect the articulated primi- 
tive segment with the nonarticulatcd remainder. Little canals develop toward 
the cranial end of the columns, in embryos 2 to 3 mm long, that constitute, 
when taken together, the pronephros, or head kidney. These open on the 
one hand into the cavity of the body, while on the other they unite to form a 
longitudinal, cellular cord which grows caudally, remains for a long time con- 
nected with the epidermis and opens into the cloaca. This is the Wolffian 
duct. 
The primitive segment columns that lie to the caudal side of the pro- 
nephros coalesce and then break down, with the exception of the most caudal 
segment, into a number of roundish masses of cells lying one behind another, 
which become hollow, like so many vesicles. While the pronephros undergoes 
involution as far as the Wolffian duct, this row of vesicles develops into a long, 
voluminous organ, the Wolffian body. With regard to the special process 
of involution we shall only say that a canaliculus comes from each vesicle and 
is attached at one end to the Wolffian duct, while it terminates at the other in 
a vesicular enlargement, the glomerulus of the Wolffian body, which is in- 
vaginated like the finger of a glove by a twig from the aorta. 
Neither the pronephros nor the Wolffian body produce any secretion. The 
latter persists for a long time, and begins to retrogress in its cranial and caudal 
portions in the third month of pregnancy, while its middle portion takes part 
in the formation of the genital organs, as we shall see later. 
The development of the definitive kidney sets in only a little later than 
that of the Wolffian body, and proceeds from the most caudal part of the primi- 
tive segment columns not broken down into balls or vesicles, and from the 
caudal portion of the Wolffian duct. A hollow shoot proliferates from the 
latter toward the former, shoving before it the tissue that covers it like a cap 
on the cranial side. Thus we have in embryos 5 or 6 mm long a hollow duct 
running dorsocranially from the place of opening of the Wolffian duct into the 
cloaca, and pouched at its blind end, on which rests a mass of cells, the metane- 
phrogenous tissue. The pouched end now grows, while the column con- 
necting it with the Wolffian duct constantly becomes longer and trough-shaped, 
so that the metanephrogenous tissue comes to lie in the trough, which is now 
directed dorsally. The hollow connecting column develops into the ureter, 
136 137 
the trough into the pelvis of the kidney and the excretory part of the 
renal parenchyma, the metanephrogenous tissue into the secretory portion of 
the latter and the renal connective tissue. The development of the excretory 
part of the parenchyma takes place in such a way that one terminal and two 
central tubes grow out from the trough-shaped renal pelvis, ramify, and form 
the primary collecting tubes. Thereby the metanephrogenous tissue is split 
up at first into four, and then into as many complexes as there are twigs, i.e., 
each branch and each twig has a cap of this tissue on its blind end. Each cap 
divides into two layers, an outer, which gives rise to the connective tissue, and 
an inner which is transformed into an epithelial vesicle. Each vesicle changes 
into a tortuous tube having one end connected with the blind end of its collect- 
ing tube twig, and the other spread out like a spoon. Blood vessels arise in 
the hollow of the spoon that are connected with vessels entering here from the 
aorta, and become completely enveloped by the spoonlike end of the canaliculus, 
so that finally they are enclosed on all sides by a double layer of epithelium. 
The two layers of epithelium blend at the place where the vessels enter, while 
at the opposite pole of the corpuscle the canaliculi proceed out of the external 
layer. Thus we have, omitting the pelvis, to distinguish three parts of the renal 
parenchyma; the first is excretory, comprises the efferent and collecting tubes, 
and develops from the Wolffian duct; the second is secretory and conics from 
the metanephrogenous tissue; while the third, having the same origin, repre- 
sents the end of the glandular portion, the Malpighian corpuscles. Hence, 
while in all other true glands the parenchyma develops through continuous 
proliferation or protrusion of the epithelium from one place, the mouth of the 
excretory duct, in the kidney the excretory and secretory segments of the duct 
system develop separately. We shall meet with a similar condition in the 
closely related sexual glands. 
The human kidney is a compound branched tubular gland. The excretory 
duct of each, the ureter, is dilated at its attachment to the pelvis of the 
kidney, which is prolonged into two or more large cups, the calices majores. 
Each large calix divides again into two or three small ones, the calices 
minores, which embrace the renal papillae, one or several blended together. 
Each of the seven to twenty papillae has a circular furrow within which the 
epithelium of the calix blends with that coming from the papilla, the latter 
being again continuous with that of the papillary ducts. These papillary 
ducts open in widely varying numbers at the top of each papilla; the small ones 
presenting ten or twelve, the large, coalesced ones as many as a hundred open- 
ings. Very soon after their entrance into the medullary substance of the kidney 
these papillary ducts divide dichotomously into the third order of collecting 
tubules, these in like manner into the second order, from which the collecting 
tubules of the first order are given off as lateral twigs. The collecting tubules 
of the second order are the longest; they begin at the end of the inner third of 
the parenchyma of the kidney and give off their first terminal branches at the 
beginning of the outer third. Those of the first order branch at right or acute 
angles and pass over into intermediate pieces that run in two short zigzag 
courses. Each of these turns again toward the center and passes into Henle's 
loop, which may be divided into a distal and a proximal limb. The former 138 
runs about parallel to the collecting tubes toward the hilum of the kidney, 
passing through a more or less large extent of renal medulla, suddenly bends 
about and becomes the proximal limb, which 
runs back close to and parallel with the distal 
to the renal cortex, where it passes into the 
convoluted tubule, in which the canali- 
culus forms several very tortuous loops lying 
close together, and finally ends in Bow- 
man's capsule, which lies within the con- 
volution. 
The urine, excreted by this complicated 
apparatus, is a yellowish, neutral or slightly 
acid, aromatic fluid with an average specific 
gravity of 1,017. It contains about 4% 
of solid constituents, of which two thirds are 
organic. The principal organic substance is 
urea, which originates everywhere in the 
body, partly as the direct, partly as the in- 
direct decomposition product of the meta- 
bolism of albumin, and is excreted only 
through the kidney. Another important one 
is uric acid, which is present in the urine in 
combination with alkalies and is produced by 
the recomposition of nucleinic acid. Of the 
other organic constituents hippuric acid is 
of interest to us, because we can say with cer- 
tainty that it is formed within the kidney it- 
self from glycocoll and benzoic acid. Com- 
mon salt and phosphoric acid may be men- 
tioned as among the inorganic constituents. 
Although the most of, and the most im- 
portant, constituents of the urine are brought 
to the kidney by the blood, yet this organ is 
not simply a filter, but it functionates the 
same as other glands. The excretion of most 
of the water and salt probably takes place in 
Bowman's capsule, that of the organic con- 
stituents in the cells of the convolution and 
in the distal limb of Henle's loop. Water is 
probably absorbed from the proximal limb of 
the latter on the contrary so as to inspissate the urine, while the collecting tubes 
and papillary ducts serve exclusively to conduct away the urine. 
We shall now discuss the individual sections of this complicated system, be- 
ginning with the papillary ducts, and consider the calices and ureter later. 
The papillary ducts, which open on the surface of the papilla in the 
area cribrosa, are very short, about 0.2 to 0.3 mm wide, and are lined by a 
single layer of high cylindrical cells. Each cell projects with a round top into 
Fig. 32. Fig. 32.-Renal Canaliculi (after Peter). 
A, with long loop; B, with short loop. 1, papillary duct; 
2, collecting tubule of the third order; 3, collecting tubule of 
the second order; 4, collecting tubule of the first order; 5, 
intermediate piece; 6, distal limb of Henle's loop; 7, proximal 
limb of Henle's loop; 8, radiating piece; 9, convoluted tubule; 
10, Bowman's capsule. 139 
the lumen and contains a spherical or ovoid central nucleus (Pl. 59, Fig. 
141). The cell body appears very clear and bright about the nucleus, darker 
in the periphery where the protoplasm contains threadlike mitochondria. Just 
beneath the free surface is a double central body. The epithelium rests directly 
on the strongly developed connective tissue of the renal papilla, as there is no 
membrana propria. The individual papillary ducts are fairly far apart. 
The collecting tubules are exactly the same in their histological struc- 
ture as the papillary ducts, except that the epithelium constantly becomes lower 
as the caliber decreases, and that a distinct, structureless membrana propria 
gradually appears. Naturally they come closer together as they approach the 
cortex, so as to be separated only by a scanty amount of connective tissue 
(Pl. 59, Fig. 140). In the minutest tubules, which are about 15 or 20 [X in 
diameter, the epithelium is cuboidal and rests on a well-developed membrana 
propria. Each cell contains a double central body just beneath the free sur- 
face, that sends a fine thread to the nucleus and a little flagellum into the lumen 
of the canal. 
The primary collecting tubule passes over into the intermediate piece 
in which the lining cells gradually become lower, and Haidenhain's rod struc- 
ture becomes more and more perceptible at their bases. As regards the central 
bodies the cells resemble exactly those of the collecting tubules. The inter- 
mediate piece has two parts that are almost vertical to each other, as we have 
seen above (Fig. 32). They are often studded with pouches and continue into 
the distal limb of Henle's loop that radiates inward, is slightly tortuous at first 
and then becomes straight. 
The two limbs of Henle's loop, the distal or ascending, and the 
proximal or descending, lie close together and parallel, but we have also to 
differentiate two sections of the loop that differ in caliber and in their epithe- 
lial lining, one dark, thick, and lined with cuboidal cells, the other bright, 
thin, and lined with quite flat cells (Pl. 59, Fig. 140). The transition from 
one to the other of these sections never takes place in man in the vertex of the 
loop, but in either the distal or the proximal limb. In the long loops, i.e., 
those in which Bowman's capsules are situated in the central part of the cortex, 
it occurs in the distal or ascending limb. While in the short ones, or those 
having their Bowman's capsules in the peripheral parts of the cortex, it takes 
place in the proximal or descending limb (see Fig. 32). In every case a more 
or less long transition piece follows the intermediate piece and then the thick, 
dark section of the loop; this occupies only about three fourths of the distal 
limb of the long loops and suddenly passes into the thin section, which bends 
about farther toward the papilla and returns toward the cortex as the proximal 
limb. In the short loops we have likewise first a transition piece, then the thick, 
dark section, which bends about the vertex of the loop into the proximal limb, 
in which the transition into the thin bright section takes place. The latter is 
quite short and very soon passes into the straight initial portion of the con- 
voluted tubule. 
The epithelium of the thick, dark section consists of cuboidal cells with 
irregular outlines. The cells have many crestlike processes that fit into each 
other so as to produce an extremely complicated picture. The cell body con- 140 
tains Heidenhain's rods and is given a dark appearance by numerous granules. 
The relations of the central bodies are the same as in the intermediate piece. 
Toward the latter the cells gradually clear up, become lower, and assume the 
characteristics of the intermediate piece cells. 
The clear, thin section of Henle's loop has a diameter only about half as 
great as that of the other. As the cells that line it are very flat the lumen 
seems relatively large, although its actual size is no greater. The lining cells 
are large, flat, and provided with numerous short processes, like many pigment 
cells. Here also the processes of one cell fit into the interstices of another. 
The cells surround the entire lumen, so that we usually find in cross section 
only one nucleus bulging into the canal. The protoplasm is very clear, slightly 
granular, and encloses a double central body with a short flagellum extending 
into the lumen. 
The transition of the proximal limb of the loop into the convoluted 
tubule takes place rather suddenly with a considerable increase of caliber. 
First the so-called end piece, or radiating piece, of the convoluted tubule 
runs toward the cortex until it reaches the level of a Bowman's capsule, where 
it forms several coils, which lie in part farther out than the capsule, then turn 
toward the latter and pass into it with a necklike constriction (Pl. 59, Fig. 
139). The entire length of the canal in the convolution averages 14 mm. At 
the transition from the loop into the end piece the cells increase considerably 
in height and lose at the same time their crestlike processes. The cell body 
is still clear at first, and the protoplasm only slightly granular, but the granules 
soon become denser, the cell body darker, Heidenhain's rods appear in the basal 
zone, and the bristle fringe on the surface of the cell. The convoluted tubules 
are from 35 to 45 p thick and are lined with cuboidal cells of about the same 
height and breadth, which have irregular, sinuate outlines, like those of the 
dark section of the loop. The large spherical or ovoid nucleus lies in the 
center of the cell. The cell body has a cloudy, granular protoplasm, and ex- 
hibits Heidenhain's rods very distinctly in its basal part. The roundish tops 
of the cells, in which the protoplasm is clearer, protrude into the lumen. The 
free surface of the cell is covered by a bristle band, beneath which is a double 
central body. Between the nucleus and the bristle band is a double central 
body. Externally the canaliculi are bounded by a distinct, structureless mem- 
brana propria (Pl. 6, Fig. 21). 
The last section of the renal system of canals is Bowman's capsule, 
into which the initial piece of the convoluted tubule passes with a necklike 
constriction. This may be compared to a double walled goblet, of which the 
initial piece of the convoluted tubule forms the stem and then swells out to form 
its outer or parietal wall. The parietal passes over into the inner, visceral wall 
at the mouth of the goblet, which is very small. The cavity of the goblet is 
completely filled by loops of vessels, the glomerulus, which cover the inner 
wall everywhere so densely as to form a sort of epithelium. The cleft-shaped 
secretory space, which opens into the convoluted tubule, lies between the visceral 
and the parietal layers of the capsule. The entire Bowman's capsule, together 
with the glomerulus contained within it, is known as the Malpighian cor- 
puscle of the kidney (Pl. 59, Fig. 139). This corpuscle has a diameter of 141 
120 to 130 |x, and is enveloped by a structureless membrana propria, which is 
a direct continuation of the membrane of the canaliculi. The cells of the ini- 
tial piece of the convoluted tubule become lower at the neck, lose their bristle 
bands and rod striations, and are continuous with the quite flat cells of the 
parietal wall of the capsule. They are very large and their limits can be de- 
termined only with great difficulty. The ovoid nuclei protrude into the lumen. 
At the place where the blood vessels enter, i.e., about opposite the place of 
entrance of the urinary tubules, the parietal bends over into the visceral layer. 
The vascular loops of the glomerulus are covered by flat cells, the limits of 
which cannot be made out, as a synctium. 
The water of the urine is filtered from the vascular loops of the glomerulus 
through the visceral layer of the capsule into the space between this and the 
parietal layer, and runs from this space into the convoluted tubules where the 
principal part of the solid constituents are excreted. Material changes take 
place in the cells of the convoluted tubules during this secretory activity. 
During rest the ceils load themselves with secretory material and protrude with 
clear tops into the lumen so that the latter becomes narrow and stellate. The 
bristle band is indistinct, while Heidenhain's rods on the contrary are very dis- 
tinct. When the cell throws out its secretion the bristle band becomes much 
more evident, the clear top is lost, and the lumen becomes larger and rounder. 
The rods break down into granules so that the cells have a more uniform gran- 
ular appearance. Modern researches have shown that we must consider Heiden- 
hain's rods to be mitochondrial formations through the breaking down of which 
secretion is provided. 
It still remains for us to study the topographical relations of the in- 
dividual segments of the renal tubule. We may distinguish macroscopi- 
cally in the kidney a central medulla, and a peripheral cortex, the former 
of which is whitish, the latter of a rather brownish color. The medulla presents 
the renal pyramids, the bases of which merge in the cortex while their apices 
lie in the papillae (Pl. 58, Fig. 138). Between the bases of the pyramids the 
cortex penetrates centrally for a certain distance in the form of the renal 
columns, that separate the pyramids. The portion of the latter between the 
columns, the limiting layer of the medulla, appears striated radially be- 
cause here the cortex enters the medulla for a distance in the form of thin 
striae. The same relations are present within the renal columns, so that the 
entire limiting layer is composed of radiating, alternate light and dark striae. 
The light stripes arc the irradiations of the medulla into the cortex and are 
called medullary rays, the dark ones come from the cortex and form the 
renal labyrinth. Large blood vessels run in the medullary rays to the cor- 
tex and bound the areas of the parenchyma known as renal lobules. Lach 
lobule has the form of a cone with its base in the cortex, its apex projecting 
into the medulla, with a medullary ray forming its axis, but not reaching its 
base, and surrounded by a mantle of renal labyrinth. 
If we study now the course of the system of canals in the kidney with refer- 
ence to this arrangement of the longitudinal section, we find it to be as follows: 
The papillary ducts and collecting tubules together form the medulla itself. 
The collecting tubules of the second order enter the medullary rays and here 142 
give off at an angle the collecting tubules of the first order, which enter either 
the renal labyrinth, or the cortex proper, and there change into intermediate 
pieces. The loops arising from the latter run again toward the center in the 
same medullary ray with the collecting tubes of the second order for a greater 
or less distance into the medulla, bend about and return in the same medullary 
ray to the labyrinth or the cortex, where they change into the convoluted tu- 
bules and finally end in the capsules. Thus the Malpighian corpuscles lie in 
the renal labyrinth and the cortex, yet in such a way that the outermost layer 
of the latter contains none. The convoluted tubules have the same position, 
but extend into the outermost layers of the cortex and form the chief con- 
stituent of the latter. Henle's loops run in the medullary rays and in the 
medulla, the intermediate pieces in the labyrinth and in the cortex. The col- 
lecting tubules of the first order enter the labyrinth and cortex from the medul- 
lary rays, those of the second lie first in the medullary rays and then enter the 
medulla, in which the remaining collecting tubes and the papillary ducts natur- 
ally are situated. 
The stroma of the kidney is very poor in true, gelatin furnishing con- 
nective tissue, which is found only in company with the larger vessels. Between 
the canaliculi and about the capsule there is a dense meshwork of minute fibrils 
that are similar to the lattice fibers of the liver and are thought by many 
authors to consist of reticulated tissue. The network gradually increases in 
amount in the medulla and attains a fair degree of development in the papilla? 
about the papillary ducts (Pl. 59, Fig. 141). On the outer surface of the 
kidney it is continuous with the renal capsule, yet the connecting threads 
are so slender that the capsule can be separated from the parenchyma more 
easily than that of any other organ. The renal capsule is a very dense, firm 
membrane composed of bundles of connective tissue that cross and interlace 
in all directions. 
The arteries of the kidney, coming from the renal, are two main trunks, 
a ventral and a dorsal, which divide repeatedly and advance their branches 
peripherally as the interlobar arteries between the pyramids parallel to the 
surface. At the border between the medulla and cortex they break up into the 
interlobular arteries, which run toward the surface of the kidney within the 
labyrinth, thus between the lobules, in slightly tortuous courses, then usually 
perforate the capsule and terminate in the fatty capsule. On the way the 
interlobular arteries give off numerous lateral branches, each of which enters a 
Malpighian corpuscle as a vas afferens. In the corpuscle it divides into 
three or four branches, each of which breaks up into capillaries, so that the 
glomerulus produced consists of the same number of lobes. The capillaries 
reunite in the same way into three or four little trunks and these into a vas 
efferens, which leaves the Malpighian corpuscle close to the vas afferens (Pl. 
60, Fig. 143). The vas efferens has the same structure as the vas afferens, 
except that its lumen is rather smaller, and must be regarded as an artery, 
while the glomerulus is considered to be a wonderful arterial network. 
Very soon after its exit the vas efferens breaks down into true capillaries which 
form plexuses about all of the urinary tubules in the cortex and labyrinth. 
These capillaries then go secondarily to the tubules of the medullary rays 143 
which the vasa efferentia do not directly enter. Moreover the vasa efferentia 
entei* the glomeruli in the most central part of the renal labyrinth, as well as 
the medulla, where they form long capillary plexuses about the collecting tu- 
bules and Henle's loops. These vasa efferentia are also known as arteriolae 
rectae spuriae. The rest of the medulla is supplied by the arteriolae rectae 
verae, which are given off at right angles from the interlobular arteries, radiate 
into the medulla in long tufts, and form long capillary plexuses about the 
straight urinary tubules. Thus the medulla is supplied partly with blood that 
has passed through the glomeruli, partly with blood that has not. 
The venous blood of the cortex flows part toward the capsule, part 
toward the medulla. The capillaries that weave about the canaliculi in the 
outer zone of the cortex assemble into little venous trunks that turn toward 
the surface of the kidney, the venulae rectae corticales, and empty at right 
angles into little veins running parallel to the surface, the venae stellatae 
(see Fig. 33). Inasmuch as all of these veins in a little district of the cortex 
run in a radiating manner toward the center of the district, and there unite, 
stellate figures are produced on the surface of the kidney, when the veins are 
artificially or naturally distended, that join together, the stellulae Ver- 
heynii. An interlobular vein flows from the center of this venous star, 
follows the course of the interlobular artery and empties into an arciform vein. 
The arciform veins run along the margin of the medulla and form a venous 
plexus from which the interlobar veins lead away, taking the course of the 
interlobar arteries. The arciform veins also receive the venae corticales 
profundae, which conduct the venous blood from the capillaries about the 
tubules in the inner zone of the cortex, as well as the venulae rectae, which 
bring all of the venous blood of the medulla, with the exception of that from 
the apices of the papillas, which flows away through the veins of the renal pelvis. 
The lymphatics of the kidney form a system of enclosed tubes which 
generally follow and twine about the blood vessels. In the cortex the lymphatic 
capillaries weave about the urinary tubules, surround each Malpighian cor- 
puscle with a delicate plexus, and enter the glomerulus with the vas afferens. 
The lymph escapes in two directions: one into the capsule, where there is an 
extensive lymphatic plexus that anastomoses with the lymphatics of the fatty 
capsule, the other along larger trunks that accompany the blood vessels to the 
hilum of the kidney and pass thence to the aortic glands. 
The nerves of the kidney come from the sympathetic aortic plexus about 
the renal artery, which receives a branch from the minor splanchnic, and also 
contains fibers from the vagus. Then enter the parenchyma with the arteries, 
weaves about the urinary tubules of the medulla and cortex, and end with arbor- 
escent twigs between their epithelial cells. They also enter the Malpighian cor- 
puscles and are distributed partly to the vascular loops, partly to the epithelial 
cells of Bowman's capsule. Part of the fibers that enter the kidney are vaso- 
motor, part sensory, part secretory. Until quite recently it was supposed that 
the kidney had no true secretory nerves, but their presence has been demon- 
strated experimentally of late. They doubtless come from the vagus. 144 
2. THE PELVIS OF THE KIDNEY AND THE URETER 
The surface of the renal papillae, large and small calices, pelvis, and ureter 
is covered by a so-called transitional epithelium, i.e., by a stratified epithe- 
lium the cells of which do not materially decrease in height and are irregular 
in shape. The most superficial layer is composed of the so-called cover cells, 
which are large, polygonal, and flat, send taglike processes between the deeper 
layers, stain more strongly than these, and have a band of condensed proto- 
plasm on theii' free surfaces. The simple cylindrical epithelium of the papillary 
ducts directly adjoins it in the area cribrosa. The epithelium is rather thin on 
the papillae, but increases in thickness considerably in the calices and pelvis, 
where it is composed of as many as ten layers of cells and has a maximum thick- 
ness of 70 p. 
The epithelium rests directly on the connective tissue propria, which is 
permeated by nets of elastic fibers. On the papillae the propria is very thin 
and is continuous with the papillary stroma, later it assumes a considerable 
thickness and juts into the epithelium in the form of longitudinal and circular 
ridges. In the ureter it forms longitudinal folds covered with epithelium and 
projecting into the lumen (Pl. 60, Fig. 144). Delicate septa that anastomose 
with one another penetrate still farther from the ridges of propria in the calices 
and pelvis into the epithelium and reach the covering layer, imperfectly divid- 
ing the epithelium into roundish or polygonal districts. The mucous membrane 
contains no glands. 
Externally to the propria lies the muscularis, which cannot be sharply 
differentiated from it. It consists of smooth muscle fibers, begins at the place 
of origin of the little calix as a circular muscular layer about the papilla, the 
sphincter papillae, then forms an irregular plexus on the calices, and a 
second circular muscle at the mouth of the calix into the renal pelvis. It be- 
comes stronger in the pelvis and forms a continuous circular muscular layer, 
outside of which lie irregular, longitudinal muscular bands. In the ureter it 
increases considerably and may be divided into three layers: a strong inner 
layer of separate longitudinal bundles, a thick circular layer, and some scattered 
longitudinal bundles outside of this (Pl. 60, Fig. 144). The nearer we ap- 
proach the bladder the more powerful does the musculature become, until finally 
it almost completely displaces the connective tissue of the propria. Near the 
mouth of the uretei' strong external longitudinal muscles appear, which, to- 
gether with the connective tissue adventitia, form the ureteral sheath. Ar- 
riving at the wall of the bladder the ureteral musculature maintains its indepen- 
dence and ends in the propria. 
Outside of the muscularis of the calices, pelvis and ureter there is a connec- 
tive tissue adventitia, which attains its greatest magnitude in the lower part 
of the latter through the reinforcement of the muscle just described. 
The arteries of the calix, pelvis, and upper part of the ureter come from 
the renal, those of the lower part of the ureter from the internal spermatic, 
median hemorrhoidal, and inferior vesical. The smallei* branches run in the 
propria and develop on one side a subepithelial, on the other an intramuscular Fig. 33.-The Divisions of the Blood Vessels in the Human 
Kidney (.schematic). 
1, interlobar artery; 2, interlobular artery, the end piece of 
which, 2', enters the capsule; 3, vas afferens; 4, glomerules; 
5, vas efferens; 6, capillaries of the convoluted urinary canali- 
culi; 7, capillaries of the medullary rays; 8, arteriola recta 
spuria; 9, arteriola recta vera; 10, capillaries of the medullary 
substance; 11, venula recta corticalis; 12, venae stellatae that 
unite 12' to form a stellula Verhezini; 13, vena interlobularis; 
14, vena arciformis; 15, vena corticalis profunda; 16, venula 
recta. 145 
capillary plexus. Capillaries from the formei' frequently enter the epithelium 
with the connective tissue septa and project in it like buds. The veins form a 
plexus in the propria and another in the adventitia. 
The lymphatics form plexuses in the propria and muscularis, and empty 
into the aortic and hypogastric glands. 
The nerves of the renal pelvis and of the upper part of the ureter come 
from the renal plexus, those of the lower half of the ureter from the spermatic 
and hypogastric plexus. They form a plexus in the adventitia and another in 
the muscularis, both provided with ganglia. Twigs from the muscular plexus 
enter the mucous membrane, but nothing is known in regard to their endings. 
3. THE URINARY BLADDER 
The bladder originates in the fifth week of embryonal life from the blind 
end of the intestinal tube, the entodermal cloaca, through the growing out 
of the septum urorectale from the lateral walls of the latter. It divides 
the entodermal cloaca into a posterior segment communicating with the in- 
testine, the rectum, and an anterior, including the urachus and connected with 
the Wolffian duct, the sinus urogenitalis. The latter breaks externally 
into the urogenital fossa in embryos about 16 mm long. After the ureters 
have grown out from the Wolffian ducts the common opening of the two widens 
into the urogenital sinus, which grows constantly in length, takes up this dilated 
portion, and removes the opening of the ureter cranially from that of the 
Wolffian duct, which now turns again to the cauda. The segment of the uro- 
genital sinus lying between the two contracts at the beginning of the third 
embryonal month, and thus separates the cranial portion with the urachus and 
the openings of the ureters to form the bladder. The epithelium of the bladder 
therefore has a double origin: part of it comes from the entodermal terminal 
portion of the intestine, part from the mesodermal epithelium of the Wolffian 
ducts and the ureters. It is flat before the growing out of the septum uro- 
rectale in the region of the ventral wall of the cloaca, later becomes cylindrical 
after separation from the rectum, and then becomes stratified in two or three 
layers. The surrounding mesenchyma condenses, the cells elongate, and smooth 
muscular fibrils begin to develop in embryos 20 to 25 mm long. 
The structure of the bladder, which serves as a collecting reservoir for the 
urine flowing out of the ureters, conforms closely to that of the ureter. We 
may differentiate in its wall a mucous membrane, a submucosa, a muscularis, 
and an adventitia. In addition a large part of its outer surface is covered by 
peritoneum. 
When the bladder is empty its mucous membrane lies in prominent folds, 
except at the trigonum, but when it is full they are quite low. The mucous 
membrane is covered by only from two to four layers of epithelium, the 
deepest layer of which is composed of cylindrical cells, while the most super- 
ficial resemble the covering epithelium of the renal pelvis (Pl. 11, Figs. 33 and 
34). The form of the cells as well as the thickness of the epithelium depend 
to a high degree on the condition of the organ as regards fullness. When the 
bladder is full the epithelium is low and the cells are broad, but when it is empty 146 
the cells become narrower and higher and overlap one another somewhat, so 
that a multistratous condition may be simulated. 
The epithelium rests directly on the connective tissue propria and pro- 
jects into it solid cones of cells, short crypts, and true branched glands. 
These are found almost wholly in the fundus of the bladder near the mouths 
of the ureters, and are lined with a single layer of cylindrical cells. The pro- 
pria contains networks of elastic fibers that increase in strength toward the 
outside, and passes without a sharp line of demarcation into the submucosa 
together with an increase of the elastic tissue. Little lymph follicles may 
be found here. 
The muscularis is composed of smooth muscle fibers and consists of three 
interlacing layers, an inner and an outer longitudinal, and a middle circular 
(Pl. 61, Fig. 145). The first is strongest over the fundus of the bladder, the 
circular muscles are well developed everywhere and form a muscular swelling 
about the opening of the urethra, the sphincter vesicee. The outer mus- 
cular layer extends from the neck of the bladder chiefly over the anterior and 
posterior surfaces. Between the muscle bundles lies abundant connective tissue 
reinforced by networks of elastic fibers. 
The outer surface of the muscularis is covered by a thin connective tissue 
adventitia, which blends posteriorly with the subserous tissue of the peri- 
toneum and anteriorly with the connective tissue of the pelvis. 
The arteries of the bladder come from the hypogastric and the median 
hemorrhoidal. Theii- twigs form first a perivesical plexus, from which branches 
penetrate the bladder wall and form a submucous plexus, and from this comes 
the capillary plexus of the mucous membrane and of the muscularis. The 
veins form a submucous, an intramuscular, and a perivesical plexus; from the 
last the blood passes into the great vesical plexus. 
The lymphatics form one plexus in the propria and another in the mus- 
cularis. They lead to the vesical, hypogastric, and iliac glands. 
The bladder contains both motor and sensory nerve fibers. Most of 
them come from the sympathetic hypogastric plexus, a minority from the 
spinal sacral plexus. Within the bladder wall they form plexuses richly studded 
with ganglia and single nerve cells, from which fibers extend to the muscles and 
the epithelium. Free arborescent terminations and end corpuscles of the Krause 
and Vater-Pacini type are also to be seen. 
4. THE URETHRA 
When the ureter has separated from the Wolffian duct and turned cranially 
with the bladder, that is now becoming roomy, the portion between it and the 
mouth of the Wolffian duct contracts, separates the bladder from the urogenital 
sinus and forms the primary urethra, the epithelium of which therefore has 
a double origin like that of the bladder. In the ventral part it comes from the 
Wolffian duct and is therefore mesodermal, while in the dorsal part it is 
that of the cloaca and consequently entodermal. The further development of 
the urethra differs in the two sexes and follows that of the sexual organs. In 
females the urogenital sinus attached to the primary urethra broadens greatly 147 
in a sagittal direction and moves more and more toward the cauda, so that 
finally the primary urethra, which shows only a little longitudinal growth, 
comes to lie with its mouth at the cranial end. In males, on the contrary, a union 
takes place between the urethra, the urogenital sinus and the urogenital fossa 
along the lower side of the penis, which is of ectodermal origin. This groove 
is closed in by the growing forward of the genital folds and a tube is formed 
that leads from the bladder to the tip of the penis, the male urethra, which, 
as we shall see presently, serves also as the excretory duct for the male germinal 
glands, and therefore functionates as the duct for both the urinary and genital 
organs. The lining epithelium comes mainly from the entoderm, but there are 
portions that come from the mesoderm, and the terminal section is from the 
ectoderm. Rudiments of glands appear in the mucous membrane of the male 
urethra in the third embryonal month that appertain to the sexual apparatus 
and will be treated of under that subject, but there also appear, in both the 
male and the female urethra, little glands that are properly their own, diver- 
ticula of the epithelium. 
a. The Male Urethra 
The mucous membrane of the urethra lying in the penis lies in many 
longitudinal folds that are obliterated when the tube is dilated during the 
evacuation of the urine, and possesses a variable number of recesses that have 
been called lacuncB (Pl. 64, Fig. 151). We distinguish three parts of the male 
urethra: the prostatic, the membranous, and the cavernous, the first occupying 
about two tenths, the second one tenth, and the third seven tenths of its entire 
length. 
The epithelium covering the mucous membrane varies in these different 
parts. It is a simple continuation of the vesical epithelium at the bladder end 
of the urethra, and is transitional over the entire prostatic portion. The inner- 
most layer of cells gradually becomes higher, and this results in a stratified 
cylindrical epithelium in the membranous portion. Inwardly we have a row of 
high cylindrical cells, outwardly one to three layers of low cuboidal cells, and 
this forms the entire mucous membrane covering of the membranous and 
cavernous portions of the urethra. In many places, particularly in the lacunas, 
the cuboidal cells are wanting, and we have a simple instead of a stratified cylin- 
drical epithelium. Islands of stratified flat epithelium that vary in size may 
also be met with. The stratified cylindrical is replaced by a stratified flat 
epithelium at the outer end of the urethra, which lines the fossa navicularis 
and the external orifice and is continuous with that of the glans penis. 
The epithelium rests directly on the propria, as there is no membrana 
propria. The propria consists of longitudinal and circular bundles of con- 
nective tissue and contains many cellular elements in its subepithelial parts. 
Lymphocytes also are to be seen and may accumulate into little follicles. The 
propria contains many elastic fibers, forms the longitudinal folds that have 
been mentioned as obliterated during the evacuation of the urine, and has many 
papillfe in the fossa navicularis. 
The above mentioned diverticula of the mucous membrane, the lacunae, 
enter the propria, which is also the seat of numerous glands, known as Lit- 148 
tre's. These are of the branched alveotubular type, often open into the 
lacunae, and usually extend into the submucosa (PL 64, Fig. 151). They are 
situated mainly in the upper and lateral, less often in the lower wall of the 
urethra, and are lined with cylindrical or high conical cells with vacuoles in 
their protoplasm containing very oxyphilic granules. Groups of such cells 
also appear in the floor of the lacunae. 
The submucosa merges without any distinct border line with the propria, 
and blends with the muscularis, which is developed only in the posterior half 
of the urethra. The musculature consists of bundles of smooth muscle fibers 
that form an inner, longitudinal, and an outer, circular coat. 
The blood vessels and nerves of the male urethra will be described together 
with those of the penis. 
b. The Female Urethra 
The female urethra is only about 3 cm long, and its relations are much 
simpler than those of the male. It likewise has a mucous membrane lying 
in longitudinal folds. The epithelium presents variations that are not imma- 
terial in different individuals. The transitional epithelium of the bladder 
always enters the urethra, but soon changes to a stratified flat variety that 
continues over the external orifice and the vestibule of the vagina. In other 
cases a stratified cylindrical epithelium like that of the greater part of the 
male urethra is interposed between the transitional and the stratified flat, but 
sooner or later it gives place to the latter. 
The propria is of connective tissue abundantly supplied with elastic fibers, 
and always has numerous papillae that project into the epithelium. As in 
males, it contains crypts of the mucous membrane and Littre's glands. 
Two large complexes of glands are formed in the anterior part of the urethral 
mucous membrane which empty through special excretory ducts, the ductus 
paraurethrales, into the vestibule of the vagina near the mouth of the urethra. 
Structurally they resemble the Littre's glands of the male. 
The propria of the female urethra possesses a rich venous plexus, the 
walls of which have no muscular fibers, that as a whole forms an erectile 
tissue. The corresponding arteries lie considerably farther out. Numerous 
bundles of smooth muscle fibers run between the veins, part of them circular, 
part longitudinal, which unite externally to form a strong layer bf longitudinal 
muscles. Still farther outward are circular bundles of striated fibers that form 
near the bladder a closed ring, later an incomplete one open below. 
The propria of the female urethra also contains a very extensive lymphatic 
plexus, especially in its deeper layers, which is connected with the lymphatics 
of the fundus of the bladder. 
The nerves come from the hypogastric plexus of the sympathetic, and 
from the pudendal. They form plexuses in the propria studded with little 
ganglia, and terminate partly free in the epithelium, partly on the muscle 
fibers, and partly in special end corpuscles. 149 
5. THE SUPRARENAL CAPSULES 
The only thing there is to justify a discussion of the suprarenal capsules 
in connection with the urinary organs is that they are related by derivation 
and topographically, for as regards function they have nothing in common. 
The suprarenal capsules originate from two quite different rudiments, the 
interrenal organ and the suprarenal, or adrenal organ. The former 
appears in the fourth week of development in the form of budlike proliferations 
of the epithelium of the coelom, which unites into a median cellular mass on 
each side of the cranial end of the Wolffian body. The suprarenal organ 
originates in the second embryonal month by a migration of cells from the 
rudiment of the sympathetic to the dorsal side of the preceding, that form 
little complexes within which the cells differentiate in two ways. Some become 
true nerve cells with numerous processes, the others remain without processes 
and elaborate within themselves a substance characterized by its reaction to 
the salts of chromic acid; in other words, become chromaffin cells. In the 
lower vertebrates the interrenal and the suprarenal organs remain separate 
throughout life, or are connected only superficially, but in the higher verte- 
brates and man the cells of the suprarenal organ enter the substance of the 
interrenal and become enveloped by it, so that we have an epithelial cortex 
of the suprarenal capsule derived from the epithelium of the coelom, and a 
medulla derived from the nervous system, hence from the ectoderm, 
the latter remaining in constant connection with the sympathetic nervous 
system. 
The suprarenal capsules may be grouped best with the parathyroids as 
regards structure and function. Like them, they are solid glands, with no 
openings between the cords and trabecula of their parenchyma, and naturally 
no excretory duct. The secretion is taken away by the blood. 
The organ is covered by a thick capsule of connective tissue, containing 
many fat cells, that sends septa into the parenchyma. Both the capsule and 
the septa contain, in addition to bundles of connective tissue, many elastic 
fibers joined together into networks. Throughout the greater part of the 
cortex these connective tissue septa form tubes lying close together, that radi- 
ate and gradually grow smaller, to unite into networks in the neighborhood 
of the medulla. As soon as they arrive at the medulla the septa break up 
into numerous bundles of fibers that enter the substance and are there lost. 
The glandular substance is interposed in this connective tissue stroma in 
the form of solid cellular cords, which vary in course and arrangement 
in different parts of the cortex, so that three zones may be distinguished within 
the latter, the zonae glomerulosa, fasciculata, and reticularis. The 
cellular cords begin in the zona glomerulosa in the form of spherical balls, or 
bend about in short loops; then they take a radiating, slightly tortuous course 
in the zona fasciculata, which is by far the greatest of the three, to join and 
form a network within the less massive zona reticularis (Pl. 41, Fig. 101). 
The cells that compose these cords exhibit essentially the same structural 
relations in all three zones. They are polyhedral and rather elongated in the 
zona glomerulosa. They lie closely together, so that from two to four cells 150 
may be seen in a cross-section of a trabeculum, and are largest in the zone 
fasciculata. Each cell contains a spherical nucleus and a vacuolized cell body; 
the vacuoles are smallest in the zona reticularis, largest in the zona fasciculata. 
The vacuoles contain coarse granules of a fatlike lipoid, developed from 
mitochondria in the cell protoplasm. A reticular apparatus has also been 
demonstrated very recently in these cells. A brown pigment is to be found 
in addition to the lipoid in the cells of the zona reticularis. 
The medulla is not always sharply defined from the cortex, like which it 
consists of cellular cords that unite to form a network, but the meshes are 
larger and the cords thicker than in the cortex. The cells also are considerably 
larger, are polyhedral, have nuclei that are usually eccentric, and hold very 
fine granules in the meshes of their vacuolized protoplasm. They contain 
what is called the chromaffin substance, because of its great affinity for 
the salts of chromic acid, by which it is stained intensely brown, but it retains 
this property only a few hours after death (Pl. 41, Fig. 101). This sub- 
stance, known also as adrenalin, is probably attached to the granules in a 
fluid condition and is given up by them to the blood of the surrounding veins. 
This adrenalin, which has also been produced synthetically since its con- 
stitution as a relative of pyrocatechin was discovered, acts quite specifically 
on the sympathetic nervous system and the organs innervated by it, as 
is clearly shown by the considerable increase of blood pressure that follows 
an injection of this substance. Animals from which the suprarenal capsules 
have been extirpated soon die with symptoms of general loss of strength. 
The blood of such animals proves to be very toxic, and the supposition is 
that it contains poisons which under normal conditions are rendered harmless 
by the suprarenal capsules. It seems probable from comparative experiments 
that this vital, antitoxic action proceeds from the cortex and a lipoid pro- 
duced by it, so that we have to ascribe different functions to the two genetically 
different parts of the suprarenal capsule, the cortex and the medulla. 
Some of the arteries of the suprarenal capsule come directly from the 
aorta, some from the inferior phrenic and the renal. They pass through 
the capsule into the cortex, where they follow the course of the connective 
tissue septa, weave their capillaries about the cellular cords, and form a 
wide-meshed capillary plexus in the zona reticularis. Wide sinusoids develop 
from this in the medulla that fill the retiform spaces and assemble into the 
central vein. Another set of arteries, the perforating, take a radiating course 
in the septa through the cortex into the medulla, and there break up into 
capillaries that twine about the cellular cords of the medulla and then open 
into the sinusoids. 
The lymphatics form three plexuses that are connected-a subscapular, 
a cortical and a medullary. They run in the connective tissue septa, and send 
out fine lateral twigs which embrace the cells of the cortex and medulla with 
capillary plexuses. The outflow of the superficial vessels is through little 
trunks that perforate the capsule, while the lymphatics of the medulla leave the 
organ with the central vein at the hilum. Part of the lymphatics of the 
suprarenal capsule empty into the para-aortic glands, part into the posterior 
mediastinal. 151 
The nerves of the suprarenal capsule arise from the coeliac ganglion and 
the phrenic and renal plexuses. The splanchnic must be considered to be 
its secretory nerve. Like the lymphatics, the nerves form a capsular, a cor- 
tical and a medullary plexus, the last of which is best developed. Numerous 
sympathetic ganglia, composed of large and small multipolar cells, are to be 
found within the medullary plexus. The fibers that branch from this plexus 
pass to the chromaffin cells and terminate on them with basketlike endings. 
6. THE CAROTID GLAND 
A little nodule situated at the bifurcation of the common carotid artery, 
and known as the carotid gland, is connected genetically with the adrenal 
system. It belongs to the group of paraganglia derived from the sympa- 
thetic nervous system, and has its analogues in the medullary substance of 
the suprarenal capsule, in Zuckerkandl's organ lying at the bifurcation of 
the abdominal aorta, and in other accumulations of chromaffin substance that 
we find scattered about in the sympathetic ganglia. 
The carotid gland, which is also called the paraganglion intercaroti- 
cum, is an accumulation of polyhedral cells, a greater part of which correspond 
in their structure and in their microchemical reactions to the chromaffin cells 
of the medulla of the suprarenal capsule. Many vascular twigs and nerve 
fibers enter the nodule, which is enclosed in a connective tissue capsule, and 
typical sympathetic nerve cells may be demonstrated in it. 
7. THE GLOMUS COCCYGEUM 
The glomus coccygeum should be mentioned by way of appendix. This 
was formerly considered to be one of the paraganglia, but modern researches 
have shown that it has nothing to do with the adrenal system, and that it 
proceeds from a proliferation of the wall of the median sacral 
artery, in which the smooth muscle cells lose their typical appearance. The 
little corpuscle is abundantly supplied with vessels, which are covered as with 
a mantle by these proliferated media cells. V. THE MALE SEXUAL ORGANS 
1. THE TESTICLES 
Both the testicle and the ovary, the male and female germinal glands, 
originate from the metamorphosis of a common undifferentiated sexual 
rudiment, which we shall describe briefly. 
The ventral surface of the Wolffian body becomes covered by the epithelium 
of the coelom at the end of the first month of pregnancy, when it has developed 
its essential parts, which projects into the abdominal cavity as a high cylin- 
drical epithelium and forms a longitudinal genital ridge. Large, roundish 
cells appear very soon in its middle portion along with slender cylindrical 
ones that wander in from outside; these are the germinal cells. These 
cells, which may be recognized easily from their structure, have been traced 
backward in their development, and have been found to originate through 
segmentation of cells that separate themselves from the remaining somatic 
cells with the segmentation of the ovum, later emerge from various places in 
the embryonal body, and all migrate into the genital ridge. The middle 
portion of the genital ridge consequently increases considerably in thickness 
and becomes detached from the retrograding cranial and caudal portions as 
the germinal epithelium. A cellular mass exists in the mesenchyma be- 
tween the germinal epithelium and the Wolffian body, which expands in the 
form of a network of cellular cords and is known as the reteblast. This 
probably arises also from the epithelium of the coelom. 
Coincidently with the development of the germinal epithelium a second 
canal, Mueller's duct, appears near the excretory duct of the Wolffian 
body through a groove-shaped depression and a later separation of the coelom 
epithelium from the surface of the Wolffian body. Its development advances 
constantly from the cranial to the caudal end of the latter, so that it finally 
opens into the sinus urogenitalis together with the Wolffian duct. 
While the germinal epithelium proliferates into the much thickened mesen- 
chyma in the form of germinal cords, the rudiment detaches itself still 
more from the Wolffian body and forms the undifferentiated genital 
rudiment, composed of the germinal cords, the reteblast, and the middle 
part of the Wolffian body; the caudal and cranial portions of the last and 
of the genital ridge retrograde and become the bands of the germinal gland. 
The excretory ducts are the Wolffian and Mueller's, which lie close together 
caudally on the genital cord and open into the sinus urogenitalis, which at this 
time is beginning to separate from the cloaca. 
The testicle develops from this undifferentiated germinal trunk by the 
germinal cords that separate the mesenchyma pushing to the surface, and the 
dissolution of their union with the germinal epithelium, which is reduced to a 
single layer of cells. We have then the free surfaces of the germinal trunks 
152 153 
covered by a simple cuboidal epithelium, beneath which the mesenchyma forms 
a strong layer, the so-called albuginea; this sends septa, the septula testis, 
inward to the surface of the Wolffian body, with the cellular germinal cords 
situated between them. The germinal cords grow longitudinally in the spaces 
between the septa, their sections toward the Wolffian body remaining straight 
and forming the tubuli recti, while their peripheral portions become very 
tortuous and give off likewise tortuous lateral sprouts. These are called the 
tubuli contorti, but they do not become open until shortly before birth. 
The tubuli recti converge inward and then unite with the network of 
the reteblast, which becomes the rete testis after its canalization. 
The efferent passages of the epididymis are now to be added to the 
parts of the parenchyma of the testicle that come from the epithelium of the 
coelom. These originate from the Wolffian body. The epithelium of the 
capsule of the latter begins to proliferate in the region of the germinal trunk, 
and advances toward the rete testis to unite with it. While the glomeruli 
enclosed by the capsule of the Wolffian body pass away, the newly formed 
canals continue in its canaliculi and open as such into the Wolffian duct, 
which now becomes the spermatic duct. 
The testicles maintain their high position until the third month, when they 
begin to sink, taking with them their blood vessels and their peritoneal cover- 
ing, pass through the inguinal canal, and finally come to lie in a cutaneous 
pouch called the scrotum. The details of this change of position will not 
be discussed here. 
The testicle is covered externally by a dense connective tissue capsule, the 
tunica albuginea (Pl. 61, Fig. 146). It consists of numerous lamellae of 
connective tissue, one over another, that enclose richly developed clastic net- 
works. This albuginea is thicker above and behind, and finally passes over 
into a powerful mass of connective tissue known as the corpus Highmori, 
or mediastinum testis. The albuginea is covered externally by the visceral 
layer of the tunica vaginalis propria, which is firmly adherent to it, is 
composed of a subserosa and an epithelium, like the peritoneum of which it 
is, in fact, a part, and is continuous above with the parietal layer, from which 
it is separated on its anterior, lower, and lateral surfaces by a fine interstice. 
The albuginea sends out numerous partitions from its inner surface, the 
septula testis, which converge toward the upper, posterior pole of the organ 
and there pass over into the mediastinum testis. The parenchyma of the 
testicle is divided by them into very many conical lobules. From the apex 
of each lobule comes a seminal duct, at first straight and known as the tubulus 
rectus, but peripherally cast into coils and called the tubulus contortus, 
which divides several times in succession, the individual branches again thrown 
into coils, and finally loops about the neighboring tubules, so that each lobule 
of the testicle is a dense convolution of tubules which gradually coalesce and 
empty into a tubulus rectus. The diameter of the tubule constantly decreases, 
from 200 to 20 p.. Although the tubulus rectus is quite short, the length of 
a tubulus contortus is estimated as between 70 and 80 cm. 
Each tubulus contortus has a dense sheath consisting of several lamelke 
of connective tissue with flat connective tissue cells and elastic fibers between 154 
them. On the inner side of this sheath is the epithelium, which has two com- 
ponents. As we saw when studying the embryology, the germinal cords that 
form the matrix of the tubuli contorti are composed of two kinds of cells, the 
immigrated germinal, and those of the epithelium of the coelom. The former 
now preponderate and lie in the child's testicle in several superimposed layers; 
we call these the primitive spermatic cells, or the archispermatocytes. 
They are roundish cells lying together rather loosely, each of which contains 
a nucleus and a distinct double central body surrounded by a thick, fila- 
mentous envelope, the idiosome. The cells from the epithelium of the coelom 
give way before the primitive spermatic cells, and are to be seen here and 
there as narrow, long, cylindrical formations, each of which rests upon the 
envelope with a broad base enclosing a nucleus, and reaches up between the 
primitive spermatic cells to the lumen of the canal. These are called foot 
cells, or Sertoli's cells. The semen is produced by the primitive spermatic 
cells, the foot cells playing only a subordinate part (Pl. 62, Fig. 147). 
The formation of semen, or spermatogenesis, begins in man from 
the fourteenth to the sixteenth year, and continues without interruption, un- 
der normal conditions, until the onset of old age. Several successive stages 
may be recognized in the process, which is distributed uniformly over the 
entire length of each tubule in such a way that each successive segment is 
always found in a different stage from the preceding, so that the entire process 
may be followed in one tubule. The same is true of domesticated animals,, 
but in wild animals a short period of semen formation, known as heat, is 
followed by a period of complete rest, during which the epithelium of the 
tubuli contorti assumes the characteristics of that of childhood. 
We shall now review very briefly these stages of spermatogenesis (Pl. 62» 
Fig. 147). At the beginning of puberty mitotic segmentations appear in the 
primitive spermatic cells that lead to the formation of a new generation, called 
spermatogonia, or spermatic mother cells. They differ from the pre- 
ceding only in being somewhat smaller and assemble into a continuous layer 
just beneath the membrane of the tubule. During the second stage a lively 
process of segmentation takes place in these spermatogonia. The daughter 
cells increase considerably in extent, form one or two rows situated more 
internally, and are known as spermatocytes. Each spermatocyte divides 
into two cells, the prespermatids, and each of these immediately divides 
again into two cells, the spermatids, so that four equivalent spermatids 
come from each spermatocyte. The prespermatids differ from their mother 
cells only in being smaller, but the still smaller spermatids differ from them 
in an important characteristic; they have only half as many chromosomes, 
because in the final segmentation the chromosomes do not split longitudinally, 
but twelve of the twenty-four in the mother star enter one, and the remaining 
twelve the other spermatid. This is called a reducing segmentation, and 
is found to play a similar part in the development of ova. Hence the number 
of chromosomes remains constant in fecundation, which consists of a fusing 
of the chromatin of the ovum and of the semen. 
The young spermatids, each with only a half number of chromosomes, lie 
in several rows directly adjoining the lumen of the canal, and are transformed 155 
into spermatozoa. In this process, to be described presently, the cells 
gradually become pointed and lie with their points in the central ends of Ser- 
toli's cells, which about this time are provided with lobate processes and 
probably supply them with nutrition. 
The individual tubuli contorti of the testicular lobules are separated by 
a sparse amount of connective tissue containing the vessels and nerves. We 
also find special cells in masses or cords, that are known as Leydig's cells, 
or the intermediate cells of the testicle, and together form the inter- 
stitial gland of the testicle (Pl. 62, Fig. 147). They are spherical, or 
made polygonal by pressure, 15 to 20 p. in diameter, with spherical nuclei and 
double central bodies. The protoplasm of the cell body is strongly vacuolized, 
with granules of a lipoid substance in the vacuoles, and acidophilic granula- 
tions and pigment granules in the protoplasm itself. These intermediate cells 
are of mesenchymatous origin and are developed more abundantly in the 
embryo than in the adult. Experimental researches, and still more the study 
of abnormal conditions, have made it probable that a substance is produced 
by these cells which enters the blood and causes the development in the body 
of the secondary characteristics of the male. Whether they also take part in 
the nutrition of the developing spermatozoa has not yet been determined 
with certainty. 
The tubuli recti are, as we have seen, the short, straight ends of the 
tubuli contorti. They are lined with a simple cylindrical epithelium, which we 
have to consider to be a continuation of Sertoli's cells. At the transition 
of the straight seminal tubules into the rete testis the cells become quite 
low and the connective tissue wall is lost, so that the tubules composing the 
network look like clefts in the connective tissue of the mediastinum. 
The arteries of the testicle come from the internal spermatic, enter the 
mediastinum, and break up here into branches that follow the course of the 
septula and send twigs into the lobules, the capillaries of which run in the 
interstitial tissue and weave about the tubuli contorti, but are always sepa- 
rated from their walls by connective tissue. The veins follow the course 
of the arteries and empty into the pampiniform plexus. 
The lymphatics of the parenchyma of the testicle are imperfectly known, 
but they seem in general to follow the course of the blood vessels, and like 
them to form fine capillary plexuses in the interstitial tissue. They appear 
in the mediastinum and form a plexus in the spermatic cord, the efferent ves- 
sels of which lead to the paraaortic and preaortic lymphatic gland. 
The nerves of the testicle come first from the spermatic plexus about 
the internal spermatic artery, and second from the deferential plexus about 
the vas deferens. By far the majority of them are nonmcdullated fibers that 
follow the course of the vessels into the mediastinum and form plexuses in 
the septa and in the albuginea. They also enter the lobules and weave about 
the convoluted tubules, forming a plexus the terminal twigs of which penetrate 
into the deeper layers of the epithelium. 156 
2. THE EXCRETORY PASSAGES OF THE TESTICLE 
a. The Epididymis 
The first segment of the excretory passage of the testicle is formed by 
the epididymis, which comes from the tubules of the Wolffian body, and is 
composed of the ductuli efferentes and the ductus epididymidis. The ductuli 
efferentes are encased by a striated membrana propria resting internally 
on the epithelium. It presents on its surface numerous pitlike depressions 
that do not bulge outward over the greater part of the membrana propria, 
but in rare cases lead into true branched glandular ducts (Pl. 62, Fig. 148). 
The epithelial cells are long and cylindrical between the pits, and decrease in 
height as they pass down to the floor of the pit, where they become low cylin- 
drical, or cuboidal. Very distinct cement wedges are always to be found be- 
tween the cell bodies. The nucleus lies in the center of the cell oi' near its free 
end. Numerous acidophilic and pigment granules are to be found in the 
vacuolized cell protoplasm. The free surfaces of many, but not of all the cells 
have tufts of cilia that stroke during life toward the ductus epididymidis. 
These have basal corpuscles, and a double central body beneath the ciliated 
margin. The secretory granules mentioned above also occur in these ciliated 
cells; when a cell is full of them the ciliated border is cast off and the secretion 
escapes, after which the cilia are replaced. 
The structure of the ductus epididymidis, formed by the junction of 
the ductuli efferentes, is materially different. It likewise has a striated mem- 
brana propria, outside of which are several layers of circular, smooth muscle 
fibers (Pl. 62, Fig. 148). The epithelium has perfectly smooth surface and 
is much higher than in the ductuli efferentes. It is composed of two rows of 
cylindrical cells 40 to 50 [X long, with more spherical cells between their bases. 
Each cylindrical cell has a nucleus situated about in its center, and numerous 
secretory granules in its cell body. From its free end extends a tuft of threads 
20 to 30 [X long, that stain intensively, but the only thing we know of their 
nature is that they are not cilia. 
b. The Vas Deferens 
The mucous membrane of the vas deferens lies at first in longitudinal 
folds, but farther down oblique folds appear between these, and finally quite a 
system of communicating folds is formed (Pl. 63, Fig. 149). It is lined with an 
epithelium that differs only a little from that of the ductus epididymidis. 
The cylindrical cells are still higher and gradually lose their granulai' contents. 
The pigment granules persist, multiply, and give the surface of the mucous mem- 
brane a brownish appearance. In the vicinity of the ampulla the tufts of 
threads projecting from the free surface of the cells pass away, and we have 
only a double row of pigmented epithelium, which may lose its basal cells and 
become simple cylindrical. 
The epithelium rests upon a connective tissue propria that is separated 
from it by a striated basal membrane. The propria contains numerous 
elastic fibers and forms the basis of the folds. It passes over into a well 157 
developed submucosa of connective tissue, that leads to the muscularis. 
The last is the most essential part of the wall of the vas deferens, which is 
over 1 mm thick and is composed of three layers of smooth muscle fibers, the 
outer and the inner longitudinal, the middle circular. 
Externally the wall is covered by an adventitia composed of bundles of 
connective tissue and elastic fibers. 
c. The Seminal Vesicles 
The seminal vesicles have essentially the same structure as the vas deferens, 
of which they are simply diverticula. The tendency of the mucous mem- 
brane in the lower parts of the vas deferens to lie in anastomosing folds be- 
comes much more marked and results in the formation of numerous larger and 
smaller fossae, the walls of which are in turn covered with smaller anasto- 
mosing folds. 
The epithelium is simple cylindrical, as in the ampulla of the vas deferens, 
and its cells contain, in addition to a finely granular brown pigment, many 
strongly acidophilic secretory granules. 
The propria, submucosa, and adventitia are the same as in the vas 
deferens. The musculature has the same layers, but is on the whole more 
weakly developed. 
The arteries of the seminal vesicles come from the inferior vesical and 
median hemorrhoidal, and form an intramuscular and a submucous plexus, as 
do also the veins. The latter also form an extensive plexus in the adventitia, 
which is connected with the neighboring venous plexuses. 
The lymphatics are composed like the veins and flow into the hypogas- 
tric glands. 
The nerves come from the hypogastric plexus and form a plexus studded 
with many ganglia in the adventitia, from which fibers extend to the muscula- 
ture and the mucous membrane. 
d. The Ejaculatory Ducts 
The mucous membrane forms anastomosing folds also in the relatively 
short ejaculatory ducts. The epithelium is partly simple cylindrical on the 
tops of the folds, partly stratified cylindrical in the fossae, and otherwise 
presents the characteristics of that of the vas deferens. In the vicinity of its 
mouth it gives place to the transitional epithelium of the prostatic portion of 
the urethra. 
The propria does not differ materially from that of the vas deferens. The 
muscularis also is composed of the same three layers. At the entrance into 
the prostate the two outer layers are split apart by the veins that are strongly 
developed here, and the combination forms a sort of cavernous body. 
e. The Prostate 
The prostate appears in the third month of embryonal life, growing from 
the dorsal and ventral walls of the prostatic portion of the urethra in the form 
of solid and branching cellular cords that later become canalized. While 
the dorsal part retrogrades, the ventral rudiment proliferates into the sur- 158 
rounding mesenchyma, which becomes differentiated into smooth muscle cells. 
Each cellular cord is a branched individual gland, and the whole of them, 
fifteen to thirty in number, later become united into a common glandular mass 
by the developing smooth muscular tissue. The muscle is not developed in 
females and the retrograde glandular tubes open as the ductus paraurethrales 
in the vestibule of the vagina. 
In adults the prostate is a glandular mass of the form and size of a chestnut, 
through which pass the urethra and the ejaculatory ducts. It consists of a 
variable number, at least twenty, separate branched alveolar glands; 
the excretory ducts of some of these coalesce shortly before they empty, but the 
most of them remain open, each with its own duct, into the urethra on each side 
of the colliculus seminalis. 
The epithelium of the excretory ducts is that of the prostatic portion of 
the urethra near their mouths, but this transitional variety gives place very 
soon to a simple cylindrical. The secretory cells of the alveoli are like- 
wise covered with cylindrical epithelium (Pl. 63, Fig. 150). The height of the 
cell depends on the secretory conditions. The nucleus lies either at the base 
of the cell, or toward the center. The basal part of the protoplasm appears 
dense and contains mitochondria, its central part is vacuolized and contains in 
the vacuoles little drops of lecithin and granules of secretion that stain strongly. 
The latter are also to be met with in the lumen of the alveoli after they have 
been cast off by the cells. The lumen of the gland almost always contains 
spherical concrements that may have as great a diameter as 1 mm, and exhibit 
a concentric stratification. These prostatic concretions are produced by 
the deposit of layers from the secretion. 
There seems to be no membrana propria. A connective tissue provided 
with numerous elastic fibers adjoins the epithelium on the outer side, and is 
permeated more or less by bundles of smooth muscle fibers. This muscu- 
lature is so abundant in many cases as to cast even the glandular tissue into the 
shade, while in others it is more weakly developed. An involution takes place 
in old age, after the cessation of the sexual function, of the glandular sub- 
stance and musculature at the cost of the proliferating connective tissue. 
The prostate also contains the so-called utriculus prostaticus. This is 
a blind sac about 10 mm long, though in rare cases its length may be several 
centimeters, which lies embedded in the substance of the prostate and opens on 
the colliculus seminalis between the two ejaculatory ducts. It is originally 
lined with stratified flat epithelium, but later this is replaced by a stratified 
cylindrical epithelium. A scanty muscularis adjoins the connective 
tissue propria of the mucous membrane. 
Embryologically this is a rudimentary masculine vagina, for it originates 
from the caudal end of the coalesced Mueller's ducts, which, as we shall soon 
learn, gives rise to the female vagina. 
The arteries of the prostate, which are only moderately developed, come 
from the inferior vesical and the median hemorrhoidal, branch in the stroma, 
and form capillary plexuses about the glandular alveoli. The veins into which 
these assemble open into the vesico-prostatic plexus. 
The lymphatics form a plexus about the alveoli that is connected with 159 
another beneath the capsule. The efferent vessels lead from the latter to the 
iliac, hypogastric, vesical, and hemorrhoidal lymphatic glands. 
The nerves of the prostate come from the hypogastric plexus, and contain 
both mcdullated and nonmcdullated fibers. In the stroma they form a plexus 
studded with numerous ganglia, from which fibers extend to the muscles and to 
the glandular epithelium. The medullated fibers terminate in special encapsu- 
lated end corpuscles lying in the stroma and in the capsule. 
f. Cowper's Glands 
These are compound, branched, alveotubular glands, the alveoli of 
which communicate, situated in the trigonum urogenitale close to the bulbus 
corporis cavernosas urethrae. The excretory duct of each gland, about 30 
to 40 mm long and opening in the cavernous part of the urethra is lined by a 
stratified cylindrical epithelium, which may be simple cuboidal in the 
branches. These branches have many lacunal dilatations and lead either 
directly, or through the medium of further tertiary passages, into the ter- 
minal chambers, which anastomose together. The last are lined with a 
simple epithelium, the cells of which are dark and cuboidal, or clear and 
cylindrical according to the secretory condition. The nucleus lies in the 
basal part, the protoplasm is reticulate. Secretory capillaries lie between the 
cells, and a distinctly perceptible network is formed by the cement wedges. 
Whether these are mucous cells, as claimed by many, has not yet been deter- 
mined with certainty. 
The epithelium rests everywhere on a well developed membrana propria. 
Between the alveoli is a connective tissue stroma containing numerous elastic 
fibers and bundles of smooth and of striated muscle fibers, that forms a cap- 
sule about the gland, the muscular fibers of which join the musculature of the 
trigonum. 
The arteries come from the bulbourethral. The lymphatics empty into 
the hypogastric glands. The secretory nerve is the pudendal. 
g. The Semen 
The semen, or sperm, is the milky, viscid, slightly alkaline fluid, with a 
peculiar odor, that is ejected from the ejaculatory ducts. The solid constitu- 
ents amount to about 10%, and over 80% of these are of organic nature, 
chiefly nucleoproteids, albumin, albumose, and mucin. If semen is allowed to 
dry sperm crystals form in it which contain spermin as a base in combination 
with phosphoric acid. When potassic iodiodate is added to the sperm we obtain 
little dark brown crystals that resemble those of haematin; this is known as 
Floren's reaction. 
Microscopically the semen contains spermatozoa as its principal con- 
stituent, together with cells more or less changed by death, coming from the 
excretory passages, wandering cells that have passed through the epithelium, 
and larger or smaller spherical concrements of the prostate. 
The semen, the quantity of which may amount to 5 or 6 ccm at an ejacula- 
tion, is composed of the secretions from the testicle, the excretory passes, and 
the glands associated with the latter. The testicle furnishes the spermatozoa, 160 
the epididymis a very albuminous secretion that serves as a medium for their 
nutrition and suspension. The seminal vesicles have a double physiological 
function: to serve as reservoirs for the semen, and to furnish a secretion to 
render the fluid thinner and so to favor the active movements of the sperma- 
tozoa. The secretion of the prostate, which is materially 
thinner than the semen, also reduces the consistency of 
the latter and probably furnishes the spermin. Lit- 
tle is known positively concerning the secretion of Cow- 
per's glands, which is believed by many authors to be 
mucous. 
The spermatozoa, the most important constituent 
of the semen, remain to be described. The number pres- 
ent in an ejaculation varies a great deal, but may be given 
as 6000 to the cubic millimeter on the average. They 
are threadlike formations 50 to 60 |j long that maintain 
a lively, serpentine movement in the seminal fluid, in which 
they rotate about their longitudinal axes and are able to 
overcome a pretty strong resisting current. Their mo- 
tility lasts for days in a test tube, is stimulated by dilute 
alkalies and neutral salts, and is inhibited by acids. The 
rapidity of the movements is given as 15 to 20 p in a 
second under normal conditions. 
Each spermatozoon is divided into &'head, a con- 
necting piece, and a tail. The head is about 4 or 5 p 
long, 2 or 3 pi broad, and only 0.05 pi thick in the posterior 
section (Fig. 34). It has the form of an oval disc 
edged in front like a chisel. It is strongly refractive to 
light and has on its surface a line convex forward that 
separates a larger anterior part from a smaller poste- 
rior. The former is continuous with the chisel-like edge, 
that is also known as the perforatorium, with which 
the spermatozoon cuts into the ovum and enters into its 
substance.. Special pointed and hooklike perforatoria are 
frequently found in animals. The staining reaction shows 
that the head consists mainly of chromatin, but no fur- 
ther structural details can be made out (Pl. 62, Fig. 147). 
The anterior part is covered by the cephalic cap, a 
special, thin, nonchromatic substance, because of which 
the posterior part always seems to stain more intensively 
with basic dyes than the anterior. 
The connecting piece is a cylinder 5 or 6 pi long, 
having an oval cross section and an even diameter of 
about 1 pi. Its most anterior part, directly adjoining the head, is known 
as the neck. It contains axis filaments, consisting of fine longitudinal 
fibrils, which do not reach the head, but end at the peripheral part of the 
neck in several little nodules, the anterior part of the posterior cen- 
trosome. Similar nodules are situated at the junction of the neck and 
Fig. 34. Fig. 34.-Human Spermatozoon (after Maves). 
1, head; 2, connecting piece; 3, tail; 4, anterior part of the 
head; 5, posterior part of the head; 6, axial thread; 7, anterior 
centrosome; 8, anterior part of the posterior centrosome; 9, 
involucrum; 10, spiral thread; 11, mitochondrial envelope; 12, 
posterior part of the posterior centrosome; 13, endpiece of 
the tail. 161 
head, and form the anterior centrosome. The anterior centrosome 
and the anterior part of the posterior centrosome are united by a homo- 
geneous intermediate piece, the massa intermedia. The axis filaments are 
covered in the region of the intermediate piece by an extremely thin, homo- 
geneous membrane, the involucrum, which also extends over the tail. Then 
follows externally the contractile spiral thread, which rises in the anterior 
part of the posterior centrosome, wraps about the intermediate piece in close 
spirals, and is inserted into the circular posterior part of the posterior 
centrosome. The spiral thread is embedded in the homogeneous substantia 
intermedia, and is surrounded by the mitochondrial sheath, which contains, 
the mitochondria and extends forward to the anterior centrosome. 
The tail is subdivided into a main -part and am end part. The former is 
40 to 50 p long and comprises about seven tenths of the entire length of the 
spermatozoon, while the end piece is of about the same length as the connecting 
piece. The entire length of the tail is passed through by axis filaments that 
gradually become more slender and a^e finally drawn out into a very fine point, 
and is covered by the involucrum. The latter presents a fairly marked thick- 
ening behind the posterior part of the posterior centrosome, tapers gradually 
with the axis filaments and ends with the main piece, leaving the fine, short end 
piece uncovered. 
Aberrant forms of spermatozoa are not uncommon in human semen, such 
as those in which two connecting pieces and two tails extend from one normal 
head, the tails either completely separate or united posteriorly. Double headed 
spermatozoa also are comparatively common. 
The development of the spermatozoa deserves a brief consideration. In the 
description of the epithelium of the convoluted tubules of the testicle we learned 
that the spermatids are their formative cells. These are spherical, with 
rathei* eccentrically placed nuclei and double central bodies just beneath the 
surface. During the metamorphosis the spermatids mass in the central, ends 
of Sertoli's cells and maintain this position throughout the entire process (Pl. 
62, Fig. 147). First the nucleus takes a more eccentric position and moves 
entirely into the end of the cell directed toward the wall of the tubule. It loses 
its spherical shape and becomes ovoid, its chromatin becoming condensed at the 
same time and finally formed into a homogeneous mass. Simultaneously the 
entire cell elongates, becoming at first ovoid and later conical. The apex of 
the cone is directed toward the wall of the tubule and is completely filled by the 
nucleus, while the base projects into the lumen of the canal. The two central 
bodies lie at first far from the nucleus at the periphery of the cell, one behind 
the other, so that we may distinguish them as a proximal and a distal with 
regard to the nucleus. A fine thread grows out of the latter, perforates the 
surface of the cell and protrudes free into the lumen of the canal. The proxi- 
mal central body grows out into a transverse rod, and now both central bodies 
begin to move toward the nucleus. Having arrived, the posterior central 
body adheres to the posterior pole of the nucleus, while the distal lies farther 
back and sends its caudal filaments through the entire substance of the 
cell into the lumen of the canal. This distal central body now breaks up into 
a proximal head and a distal ring, the latter of which moves toward the posterioi* 162 
pole of the cell along an axis filament. The young spermatozoon has therefore 
in this stage the shape of a truncated cone, the axis of which is two or three 
times as long as its base. Its anterior end is completely filled by the nucleus, 
which is now an oval disc. At its posterior pole is the proximal central body 
firmly attached to it, and at a little distance the proximal part of the distal 
central body, from which extend the axis filaments in the axis of the cell, passing 
through the distal ring-shaped portion of the distal central body at the distal 
pole of the cell, and out of the cell as a fine, long thread. Now the mito- 
chondria, which have been distributed uniformly throughout the cell, assemble 
about the axis filaments, part of them forming the spiral thread, the rest 
lying about it as a mitochondrial covering. The remainder of the cell 
protoplasm breaks down and the axis filament becomes surrounded by an in- 
volucrum that probably originates through secretion. As soon as the meta- 
morphosis of the spermatozoon is complete the latter leaves Sertoli's cell and 
enters the lumen of the tubule. It is still incapable of movement at first, and 
probably obtains its motility in the epididymis through the action of the secre- 
tion poured out there. 
h. The Penis 
The development of the external male genital organs proceeds quite like that 
of the internal ones from an undifferentiated stage, which we find in human 
embryos 20 to 30 mm long. On the ventral side of the anal depression lies the 
outer opening of the urogenital sinus, which runs along the ventral surface of 
the genital eminence. The latter runs caudalward in the roundish glans, 
its lateral surfaces formed by two folds of skin fused on the ventral surface, 
the genital folds. It is enclosed at its base by two likewise ventrally fused 
genital prominences. 
The genital eminence now elongates in males and forms the penis. The 
urogenital sinus along its ventral surface is closed in by a caudally advancing 
adhesion of the urogenital folds, so that the urogenital tube, the male ure- 
thra, is enclosed in the ventral part of the penis and opens with its external 
orifice at the anterior end of the glans. The genital prominences grow ven- 
trally at the root of the penis and unite in the middle line to form two little 
folds of skin united medially, the scrotum, into each of which a testicle 
descends later. 
A circular line of epithelium proliferates from the posterior circumference 
of the glans backward and deeply into the tissue, so as to form a collarlike 
fold of skin about its base, the rudiment of the prepuce. The ring is in- 
complete ventrally, as at this time the urogenital sinus is still open in the region 
of the glans. When, after closure of the sinus, the prepuce grows forward to 
cover the glans it remains attached ventrally to the latter by a firm, bandlike 
connection known as the frenulum. 
A thickening of the mesenchyma takes place at a very early time in the 
rudimentary penis, through which at first two cylindrical bodies originate that 
lie in apposition in the middle line and along the dorsal side of the urethra, 
the corpora cavernosa penis. The corpus cavernosum glandis de- 
velops in a similar manner in the glans at a later period, and the corpus 163 
cavernosum urethrae about the urethra. Cleft-shaped hollow spaces then 
appear in these at first densely cellular bodies and communicate with each other. 
As the cells bordering them assume the characteristics of epithelium and the 
entire rudiment gains access to the venous system the bodies that were at first 
solid become erectile tissue. The mesenchymatous cells between the vascular 
spaces become transformed in great part into smooth muscle cells. 
The penis is covered by a direct continuation of the skin of the neighboring 
parts. It has long hairs at the root of the penis, only short ones along its 
shaft. The deep layers of the epidermis contain pigment, the corium forms 
many high papillae and is plentifully supplied with elastic fibrous networks 
that blend externally into a strong elastic stratum. The corium contains seba- 
ceous and sweat glands. Anteriorly the skin is prolonged into a simple dupli- 
cature called the prepuce or foreskin, the outer layer of which is exactly 
the same as the skin of the penis, except that it contains no hairs, while the 
inner layer differs in having less cornification, a smaller amount of pigment, 
and no sebaceous or sweat glands. The inner layer passes over upon the glans 
in the sulcus coronarius. The epithelium at the external orifice is continuous 
with the stratified flat epithelium of the fossa navicularis. 
Next internally to the skin is the tunica dartos, a layer of smooth mus- 
cular fibers, chiefly circular, which is connected with the layer of the same name 
in the scrotum and is lost anteriorly between the two layers of the foreskin. 
Then comes the subcutaneous tissue, a well developed connective tissue, 
poor in fat, in which run the vessels and nerves. This continues forward into 
the foreskin, where it unites the outer and inner layers. The strongly developed 
elastic fibers thicken internally into a powerful elastic stratum known as the 
fascia penis. The tendons of the bulbocavernosus and ischiocavernosus mus- 
cles radiate into its posterior part, and it ends anteriorly at the coronary sinus. 
Beneath the fascia penis lie the corpora cavernosa, joined to it by the 
subfascial connective tissue. Each corpus is surrounded by a dense con- 
nective tissue albuginea, and the two unite in the middle line to form the 
septum penis (Pl- 64, Fig. 151). This is thickest behind and becomes in- 
creasingly thinner and more incomplete in front, so that here the two corpora 
cavernosa frequently anastomose. Septa extend from the albuginea into the 
interior of each corpus cavernosum and form by branching and anastomosis a 
system of communicating hollow spaces, which in general are smaller in 
the center of the organ, forming the central plexus, larger in the periphery, 
where they form the coarse cortical plexus, and smallest in the extreme periph- 
ery, creating the peripheral cortical plexus. They are lined with simple flat 
epithelium which resembles that of the vessels and lies directly on the septa. 
The latter consist of connective tissue reinforced with elastic, and contain 
bundles of smooth muscular fibers arranged quite irregularly. Most of 
the muscular bundles are contained in the septa of the corpus cavernosum 
penis, which are nevertheless thinner than those of the corpus cavernosum ure- 
thrae, for although the latter are poorer in muscular tissue they have their 
connective tissue more strongly developed. 
The arteries of the penis are from the external and internal pudendal. The 
former supply the skin of the penis, the latter divides into the urethral, the bulbi 164 
urethras, the profunda penis, and the dorsalis penis. The first two enter the 
corpus cavernosum urethras, the third the corpus cavernosum penis. The 
dorsal artery runs in the subfascial connective tissue to the glans and gives off 
numerous branches to the corpora cavernosa. After these have passed through 
the albuginea a part enter the above mentioned peripheral cortical plexus just 
beneath the latter, the spaces of which must be regarded as capillaries which 
open into the central plexus. The intermediate coarse cortical plexus is on 
the contrary supplied directly by other arterial twigs. Both the central and the 
coarse peripheral plexuses are of venous nature and are openly connected. A 
third part of the arteries are nutrient, run in the septa, and open into the central 
plexus. Finally little arteries, the arteriae helicinae, come from the profunda, 
project their incurved ends into the spaces of the plexuses and open into them. 
All of the arteries of the corpora cavernosa have a powerfully developed 
circular musculature, and we also find in many of them cushionlike thicken- 
ings beneath the epithelium, which protrude into the lumen over wide areas 
and consist chiefly of fine, circular elastic fibers. Similar cushions occur also in 
the subfascial veins. 
The efferent veins, venae emissariae, develop from the spaces of the 
plexuses in such a way that those coming from the central must pass through 
the lacunae of the cortex. They assemble into the venae profundae penis and 
the unpaired vena dorsalis penis. 
The lymphatics of the penis are superficial and deep. The superficial 
form plexuses in the entire cutis and assemble into subcutaneous longitudinal 
trunks that empty into the inguinal glands. Those of the glans and of the 
corpora cavernosa belong to the deep; they assemble into one dorsal subfascial 
trunk that also empties into the inguinal lymphatic glands. 
The nerves of the skin of the penis come from the pudendal and the 
perineal. They form a large plexus in the cutis from which fibers ascend, part 
to penetrate between the cells of the epidermis, especially in the glans, while 
part enter special encapsulated end corpuscles situated in the papillae of the 
corium. The nerves of the corpora cavernosa come from the sympathetic plexus 
cavernosus penis, which is a continuation of the plexus prostaticus, and anasto- 
moses frequently with the dorsal nerve of the penis, which is a branch of the 
pudendal. Together with this plexus are cerebrospinal fibers that reach the 
penis from the lumbar plexus and are known as the nervi erigentes. They may 
receive a reflex stimulation from the skin of the penis that excites an erection. 
Nothing definite is known concerning the way in which they end in the erectile 
tissue. VI. THE FEMALE GENITAL ORGANS 
1. THE OVARY 
The development of the ovary, the female germinal gland, from the un- 
differentiated germinal rudiment is materially different from that of the 
male germinal gland. The most important differences are that in the female 
there is no development of a typically glandular structure with a secretory and 
excretory system of ducts, and that consequently the reteblasts of the germinal 
rudiment and of the genital part of the Wolffian body are never able to play an 
important part, but portions disappear entirely, leaving insignificant traces 
of the remainder. 
The first female characteristic of the genital rudiment is that the germinal 
epithelium sets up a lively proliferation and forms a thick cellular layer, the 
cortex of the germinal rudiment, which closes off the medullary sub- 
stance from the mesenchymatous stroma enclosing the reteblast by a more 
or less distinct primary albuginea. While the reteblast gradually disap- 
pears the connective tissue of the medulla enters the cortex of germinal cells 
and breaks down its cellular mass into complexes ever becoming smaller, the so- 
called ovarian vesicles. This condition of proliferation of the germinal 
epithelium and subdivision of the material thus produced by connective tissue 
entering from the medulla is maintained until the third year of life. The ovar- 
ian vesicles consist of germinal cells or oogonia, and undifferentiated 
epithelial cells, derivatives of the epithelium of the coelom. The cells pro- 
duced by the division of the oogonia are the oocytes, equivalent to the sperma- 
tocytes of the testicle. While the latter very soon produce the prespermatids, 
and these the spermatids by reducing segmentation, the oocytes have a very 
long life. They become separated in the subdivision of the ovarian vesicle by 
the connective tissue and lie isolated in the latter surrounded by some un- 
differentiated epithelial cell. In the lattei' part of embryonal life these cells 
lie like an epithelium about each oocyte, the entire formation is enclosed in a 
connective tissue envelope and receives the name of primary follicle. The 
formation of these primary follicles extends from the primary albuginea toward 
the surface, so that about the third year of life the entire cellular material of 
the cortex has been transformed into primary follicles. When this condition is 
complete the primary albuginea disappears and there is developed in its place 
over the surface of the cortex, outside of the primary follicles, the definitive 
albuginea, a strong connective tissue, outside of which may be found the re- 
mains of the germinal epithelium as a single layer of cuboidal cells. 
The reteblast never plays a great part in the ovary. It forms a network 
and unites with the genital cords that enter from the epithelium of the 
Wolffian body. The reteblast and genital cords, together with the masses of 
165 166 
mesenchyma situated here and the blood vessels that enter, form for a time the 
rete ovarii, but after the primary albuginea has disappeared the most of this 
primary medullary substance is taken up by the growing follicles to re- 
turn after these are destroyed and to be transformed into the secondary 
medullary substance. 
A part of the tubules of the Wolffian body with a piece of the Wolf- 
fian duct is preserved and lies on the completed ovary as the epoophoron. 
Each primary follicle therefore consists of an oocyte surrounded by a 
mantle of epithelial cell, the follicular epithelium, and a connective tissue 
capsule, the theca folliculi. It begins to enlarge, grow, and mature in the 
last part of embryonal life, but the ovum does not attain its full maturity in 
the ovary, the final stage of the ripening process takes place in the efferent 
passages. Inasmuch as the separate stages of this process, with the excep- 
tion of the final one, may be observed in the perfect ovary of the sexually mature 
woman, they will be described later. The greater part of the follicles that 
mature in the early years of life are destroyed. The discharge of the first 
mature ova from the ovaries marks the onset of puberty. 
The ovary is covered externally by the tunica albuginea, an envelope 
composed of several layers of connective tissue fibers, the existence of which has 
recently been denied. Externally the albuginea is covered by a homogeneous, 
elastic membrane, which in turn is covered externally by a single layer of 
cuboidal, so-called germinal epithelium. 
The parenchyma of the ovary is divided into a cortex, or zona paren- 
chymatosa, and a medulla, or zona vasculosa (Pl. 64, Fig. 152). The 
two blend without a sharp line of demarcation and are developed through a 
stroma consisting of connective tissue fibers, in which follicles are embedded. 
These fibers are joined into bundles in the outermost cortex, are straight, run 
parallel to the surface, and interweave a great deal; in the deeper parts of the 
cortex they are more isolated and tortuous, run perpendicularly or obliquely 
to the surface and form a stroma that embraces the follicles. A very few 
smooth muscle fibers and cells are to be met with between the fibers and 
bundles of connective tissue, the cells most abundantly between the follicles. As 
we shall see later they arise from the cells of the walls of the follicle and are 
developed most strongly in the ovary of the child, to later gradually disap- 
pear. They contain granules of a yellow fatty body, lutein, and as a whole 
form the interstitial ovarian gland. 
The fibers of connective tissue are thicker in the zona vasculosa, or 
medulla, than in the cortex, and interlace quite irregularly. The cortex is 
characterized by the follicles it contains, the medulla by its blood vessels,, which 
enter at the hilum where this zone reaches the surface of the organ. The 
medulla is poorer in cells than the cortex. It is not unusual to meet with a 
plexus of cellular tubes at the hilum, that is nothing else than the remains of 
the rete ovarii. 
The follicles lie enclosed in the connective tissue stroma of the cortex in 
such a way that the youngest stages are farthest out, the oldest farthest in (Pl. 
65, Fig. 153). But as the follicle increases enormously in its ripening the 
most mature ones finally reach through the entire cortex. The great majority 167 
of these follicles are destroyed in the course of the sexual life and only a small 
minority, about 0.6%, come to expulsion and full maturity. 
The smallest follicles, the primary, are found in the outermost cortex be- 
neath the above mentioned layer of connective tissue. They are very numer- 
ous and densely packed in the newborn, but their number diminishes as age in- 
creases. The follicle has a diameter of 60 to 70 p, and consists of an oocyte 
and a single layer of flat follicular cells. The ovum, or oocyte, may have 
as great a diameter as 65 p. A clear, reticulated protoplasm surrounds the 
nucleus, which averages 30 p in diameter, and is commonly known as the 
germinal vesicle. It has a well developed basophilic framework of chro- 
matin and a large nucleolus, or germinal spot. 
The further development of the follicle that begins in the last part of 
embryonal life, but makes a special advance at the time of puberty, takes place 
first in the ovum, second in the follicular epithelium, third in the immediate 
vicinity, and leads to the formation of a vesicle 1.5 cm in diameter, filled with 
fluid, the Graafian follicle. We shall next discuss these changes more in 
detail. 
The ovum increases considerably in size during the growth of the follicle, 
from a diameter of 60 p to one of 200 p. Therefore it increases three or 
four times in bulk, while the germinal vesicle grows at the most only one and a 
half times, so that the cell body has a considerably greater ratio to the nucleus 
in the mature than in the immature ovum. Minute yolk globules are de- 
posited in this cell body, beginning from the nucleus in the meshes of the proto- 
plasm, which finally occupy the entire body of the ovum, with the exception of 
a narrow, marginal zone. Then the germinal vesicle, which has been lying 
in the center, takes an eccentric position and its chromatin steadily loses its 
basophilia until it becomes oxyphilic. Finally the ovum is surrounded by a 
homogeneous capsule, the zona pellucida, in the formation of which the 
substance of the ovum and the follicular epithelium are probably uniformly 
involved. 
The follicular epithelium, which consists at first of a single layer of 
flat cells, has a lively growth. The cell becomes cuboidal, then cylindrical, and 
then begins to divide, a process that it repeats many times, so as to form about 
the ovum a coat composed of many layers of cells, the follicular epithelium, or 
the membrana granulosa. After the follicle has attained the size of 200 
to 300 p hollow spaces appear in the membrana granulosa, starting in the 
portion of the membrane that lies next to the surface of the ovary by the 
vacuolization and the rendering fluid of the contents of its cells. The cavity 
has at first the shape of a cap with its convex side outward, its concave side in- 
ward and turned toward the ovum. By gradual enlargement a large cavity 
finally develops which is lined by several layers of granulosa epithelium, and into 
which projects at a certain place an epithelial elevation, the cumulus ovi- 
gerus, enclosing the ovum. 
The changes in the vicinity of the follicle consist of the development of a 
capsule about the follicle from the surrounding ovarian stroma, the theca 
folliculi. 
Such a complete follicle has a size of from 0.5 to 1.5 cm, and is en- 168 
wrapped by the theca folliculi, of which we may distinguish two parts, the 
theca externa and theca interna. The former is composed of many in- 
terlacing connective tissue fibers, the latter of large spindle-shaped, or more 
roundish cells packed closely together. They contain minute drops of fat as 
well as granules of a yellow lipoid, lutein, and are known as the thecal lutein 
cells. Internally the theca interna is bounded by a structureless vitreous 
membrane, the lamina vitrea. 
The follicle is lined by several superimposed layers of follicular epithe- 
lium that rises in the part looking toward the medulla into the cumulus ovi- 
gerus protruding into the cavity of the follicle. The deepest layers of cells, 
those next to the lamina vitrea, as well as those directly surrounding the ovum, 
are cylindrical; all the rest are polyhedral. The cells anastomose by means 
of fine protoplasmic processes, so that the entire follicular epithelium is a 
synctium. 
The cavity of the follicle contains the liquor folliculi, of which from 
1 to 4% of the constituents are solid, including albumin and pseudomucin. 
As a rule the ovum lies alone in the cumulus ovigerus; it is seldom that 
we find several ova in the human follicle, as we do in that of the rabbit. It has 
a diameter of about 200 y, and is encased in the zona pellucida, which aver- 
ages 12 y in thickness, and in which a fine, radiating striation may be perceived. 
The body of the ovum has a very fine alveolar structure and contains in the 
alveoli the very minute yolk globules, that occupy the entire body, with the 
exception of a narrow, peripheral zone. The eccentrically placed germinal 
vesicle has a diametei' of about 25 y and contains the germinal spot, 
about 5 y in diameter, and a framework of chromatin, both of which are 
oxyphilic. 
As soon as the follicle has enlarged enough to reach the albuginea a degener- 
ation takes place in the neighboring parts of its wall and the internal pres- 
sure increases at the same time until a rupture takes place into the abdominal 
cavity. The ovum, surrounded by a crown of follicle cells, the corona 
radiata, is seized by the fimbriae and guided into the Fallopian tube, where it 
passes through its final process of maturation, and where fructification takes 
place. This ripening process has not yet been observed in the human ovum, 
but consists of the expulsion of directing bodies, which become two in 
number through two successive segmentations, the last of which is a reducing 
division, as separate little corpuscles and die, so that an essential difference 
is furnished herein from spermatogenesis. In the latter each spermatocyte 
provides four spermatozoa, while in oogenesis only one ovum capable of fructi- 
fication originates from each oocyte. 
The cycle of ovulation, the expulsion of the ovum, does not always seem 
to proceed in the same way. In very many cases ovulation takes place at regu-. 
lar intervals, so that an ovum is expelled about every twenty-eight days, 
probably about twelve days before the onset of menstruation, but in other 
cases ovulation takes place irregularly, by fits and starts, although the menstru- 
ation may follow its regular cycle. Ovulation comes to an end with the cessa- 
tion of menstruation, the menopause, about the forty-fifth year of age. 
The store of ova or follicles is now exhausted, and the senile ovary is practi- 169 
cally a body of connective tissue that no longer has follicles, contains only scanty 
traces of yellow bodies, and whose vessels degenerate more and more. 
Soon after the ovum has left the cavity of the follicle the walls of the latter 
collapse and from it develops the corpus luteum. If pregnancy ensues a 
large corpus luteum Verum results that lasts several months, otherwise a 
small corpus luteum falsum is formed that soon disappears. At the height 
of its development the corpus luteum consists of lutein cells, of connective 
tissue fibers that twist about these cells, and of blood vessels from the theca that 
run between them. The luteum cells are 50 p or less in diameter, each with a 
large spherical nucleus and a vacuolized protoplasm that contains granules of 
lutein in the vacuoles. Views differ as to their origin, but the theory most 
widely accepted is that they originate through proliferation of the granulosa 
cells. In the center of the body there is at first a cavity filled with follicular 
Huid and extravasated blood, but this is soon replaced by connective tissue. 
As soon as the height of the development of the corpus luteum is passed the 
lutein cells degenerate and break down, the connective tissue shrinks, and the 
blood vessels collapse. The entire body loses its yellow color, sinks together 
and is now known as the corpus albicans. 
Probably the lutein cells of the corpus luteum and the cells of the interstitial 
germinal gland are equivalent. They produce a substance that enters the 
blood and is perhaps of importance in the implantation of the ovum in the 
uterine mucous membrane. 
The great majority of the follicles formed in the ovary do not attain full 
■development, but pass away prematurely. While this may be their fate at any 
stage, it is chiefly so in middle life, when the follicles have well developed thecas, 
but are without cavities. This is called atresia of the follicles. Degener- 
ative processes play a part, seize first upon the ovum, then upon the follicular 
epithelium, and destroy both, while the fibrous and the cellular constituents 
of the theca proliferate. The cells of the proliferated theca interna then 
form what we have learned to know as the interstitial ovarian germinal gland 
■enveloped in connective tissue fibers. They resemble in all essentials the lutein 
■cells of the corpus luteum. 
The arteries, several large branches of the uterine and ovarian, enter at the 
hilum, take a very tortuous course, branching continually, to the margin of 
the medulla, where they form a dense vascular plexus, from which arteries branch 
into the cortex to form fine meshed capillary plexuses about the follicles and in 
their thecas. The veins form an extensive plexus in the medulla, but parti- 
cularly in the hilum, that gives the tissue a spongy appearance. 
The lymphatics weave about the follicles in like manner as the blood ves- 
sels and form two plexuses, one superficial, beneath the albuginea, and one 
deep, at the border between the medulla and the cortex. Little trunks pass from 
this into the medulla to anastomose again in the hilum into a plexus. The 
efferent vessels empty into the aortic lymphatic glands. 
The nerves, that come from the spermatic and renal, enter at the hilum 
with the blood vessels, form a wide plexus in the medulla, from which fibers 
pass into the cortex and twine about the follicles. The majority are non- 
medullatcd, the minority medullated. 170 
From the rudimentary organ near the ovary we have the epoophoron, the 
residue of the genital portion of the Wolffian body. Its canaliculi have a 
cuboidal or ciliated epithelium which rests on a connective tissue propria. A 
circular layer of smooth muscle fibers is often added externally. 
The paroophoron, lying in the broad ligament between the branches of 
the internal spermatic artery, is a residue of the caudal portion of the Wolf- 
fian body, and in well marked cases it may be seen how this is composed of the 
capsule of the Wolffian body and convoluted tubules with cuboidal epithelium. 
Both of these pass away in later life and become evsts with irregular masses of 
epithelium. 
The pedunculated hydatids also are derived from the Wolffian body, 
are connected with the epoophoron in many cases, and have a cuboidal, or a 
cylindrical epithelium. 
2. THE FALLOPIAN TUBE 
In males the Wolffian duct carries away the germinal products, but in 
females this duty devolves upon Mueller's duct, the development of which 
has been described already. It runs at first to the lateral side of the Wolffian 
duct, crosses it farther toward the cauda, and comes to lie close to its partner 
of the opposite side, each flanked laterally by the Wolffian duct. The most 
caudal portions of all four are included in the genital cord. A fusion of the 
two Mueller's ducts takes place in both sexes, but it extends considerably farther 
cranially in the female. The fused portion forms the uterovaginal canal, 
the free cranial portion on each side the oviduct, or Fallopian tube. The 
epithelium of the latter consists at first of several rows of cylindrical epithe- 
lium, latei' becomes simple cylindrical, and gains a ciliated margin toward the 
end of pregnancy. At the beginning of the fourth embryonal month the sur- 
rounding mesenchyma forms several longitudinal folds covered with epithelium 
that protrude into the lumen of the oviduct. Secondary folds grow out from 
these primary ones and straighten the lumen very much. About the same time 
that these folds are formed smooth muscle fibers appear in the mesenchymatous 
wall of the oviduct, which are at first circular in arrangement and have longi- 
tudinal muscular bands added later. The formation of folds in the mucous 
membrane extends not only to the entire length of the oviduct, but extends to 
the open cranial end, from which the folds grow out in the form of several 
fingers, the fimbriae. They lie close to the young ovary, at first on its lateral, 
later on its medial and dorsal sides. 
The mucous membrane of the complete oviduct is thus characterized 
by its marked folds, which constantly increase in height from the ostium 
uterinum through the isthmus to the ampulla, where they are very high, pro- 
trude into the axis of the canal, and have numerous secondary folds that fill the 
lumen completely (Pl. 65, Fig. 154). At the ostium abdominale the mucous 
membrane continues over the fimbriaa, covered by a moderately high, 15 to 
20 |j, simple cylindrical epithelium. Two sorts of cells may be distinguished, 
one broad and clear, the other narrow and dark, each of the former surrounded 
by an areola of the latter (Pl. 65, Fig. 155). The broad clear cylindrical 171 
cells are provided with cilia that have distinct basal corpuscles at the ends 
implanted in the surface of the cell. The ciliary movement is toward the ostium 
uterinum. The nucleus lies in the center of the cell. Secretion granules 
may be seen in the cell bodies, that vary in development and probably consist 
of mucigen. Part of the narrow cells are filled with such granules, part are 
not, but they have no cilia. Comparative studies of the tubal epithelium of 
various animals have shown that they issue from the ciliated cells, the ciliary 
margins of which have been destroyed by the pressure of the secretion. The 
cell then collapses and exhibits in this condition only a double central body. 
Later the ciliary margin is reformed and the cell begins anew the elaboration of 
secretory material. 
The epithelium rests directly on the richly cellular propria, there being 
no basal membrane. The propria forms the basis of the primary and second- 
ary folds and is covered by a muscularis mucosae, the fibers of which run 
longitudinally. The submucosa is poorer in cells than the propria, contains 
elastic fibers, and in it run the larger vessels and nerves. The propria and 
submucosa are permeated uniformly by elastic fibrous networks, which are bet- 
ter developed in the adjoining muscularis. The muscularis is strongest in 
the isthmus and is composed of an inner circular, and an outer longitudinal 
layer of smooth muscle fibers. 
Externally the tube is covered by peritoneum, the subserosa of which 
contains very strong elastic fibrous networks and smooth muscle fibers. The 
serous passes over into cylindrical epithelium on the fimbriae. 
The arteries come from the uterine, branch in the subserosa, pass through 
the muscularis, and form capillary plexuses first in the latter, then in the sub- 
mucosa and propria. The veins form three plexuses in the wall of the tube, 
one submucous, one intramuscular, and one subserous. The last is best de- 
veloped on the lower border of the tube and opens into the uterine, internal 
spermatic, and epigastric veins. 
The lymphatics form three similar plexuses and assemble into several 
little trunks that unite with those of the ovary. 
Part of the nerves, which enter with the vessels, are branches of those of 
the uterus, part of those of the ovary, form plexuses like those of the vessels, 
and end in the epithelium and on the muscles. Ganglia are present in the 
plexuses. 
The Fallopian tube conducts the ovum from the ovary to the uterus, its 
place of implantation. The fimbriae near the ovary probably produce a strong 
current by means of the cilia of the epithelium, which guides the ovum when it 
comes forth from the ovary into the ostium abdominale. Within the tube the 
final ripening and the fructification take place. 
3. THE UTERUS 
As stated above, the uterus originates through the fusion of the caudal 
sections of both Mueller's ducts into the genital cord, or, to put it more 
accurately, from the uterovaginal canal, a tube lined with cylindrical epithe- 
lium and encased in mesenchymatous tissue, which divides cranially into the 172 
two Fallopian tubes, and opens caudally into the urogenital sinus in com- 
mon with the urethra. It lies to the dorsal side of the latter and of the bladder. 
Changes appear in this uterovaginal canal in the fourth month of pregnancy, 
such that the caudal half is closed by a proliferation of its epithelium and is 
bent simultaneously at an obtuse angle to the cranial portion. Soon after- 
ward a differentiation takes place in the cranial portion of smooth muscle 
fibers in the mesenchymatous envelope, and from this develops a muscular 
organ with a simple lumen like a canal lined with cylindrical epithelium, the 
uterus. Its demarcation from the vaginal portion by an epithelial prolifera- 
tion takes place at the end of the fifth month in such a way that starting some- 
what caudally from the beginning of the occlusion the epithelium advances 
cranially in the form of a cuff that surrounds the caudal end of the uterus. 
When the central parts of the epithelial vagina degenerate to form a lumen, this 
formation extends into the cuff so that the caudal end of the uterus protrudes 
foi* a distance into the vagina as the portio vaginalis uteri. A residue of 
the epithelial degeneration is a stratified flat epithelium that lines all parts of 
the canal, covers the outer surface of the portio, and passes over into cylindri- 
cal epithelium at the entrance to the uterus, the os uteri. 
The uterus of the sexually mature female is a very strong muscular organ, 
the wall of which presents from within outward a mucous membrane, or 
endometrium, a muscularis or myometrium, and a peritoneal coat 
or perimetrium (Pl- 66, Fig. 156). The mucous membrane of the uterus is 
continuous with that of the Fallopian tubes at their mouths, and with the 
vaginal portion at the os uteri externum. It is about 1 mm thick, without 
folds in the body, but with a system of folds in the cervix on the rectovesical 
wall, the arbor vitae, a longitudinal ridge with several lateral ones branching 
from it at an angle. The longitudinal fold marks the place where the two 
Mueller's ducts became fused. 
The mucous membrane is covered by a single layer of ciliated cylin- 
drical cells that are from 20 to 30 y long. Each cell has an ovoid nucleus, 
situated at its base, and a quite feebly basophilic protoplasm. The relatively 
short cilia are implanted in the free surfaces of the cells, each with a basal 
nodule. The epithelial cells are rather higher and the cilia somewhat longer in 
the cervix, otherwise they are exactly like those in the corpus. The child's 
uterus has a simple cylindrical epithelium without cilia, and the latter disappear 
again after the menopause, after which the cells decrease much in height (Pl. 
66, Fig. 157). 
The epithelium rests on a homogeneous membrane propria. 
The powerfully developed propria is of adenoid nature. It is a network 
of anastomosing stellate and spindle-shaped cells which is densest beneath the 
epithelium. The collagenous fibers are extremely fine and are mixed with 
elastic fibers. Lymphocytes lie in the meshes of the reticulum. 
The propria contains the uterine glands, which also penetrate into the 
muscularis. They are simple or slightly branched invaginations of the uterine 
mucous membrane, which under normal conditions are from 100 to 200 y apart 
and from 1 to 2 mm long (Pl. 66, Fig. 156). Their epithelium resembles that 
of the surface, but gives a much more distinct mucous reaction, although the 173 
granules of mucigen cannot be demonstrated in the cells themselves. Secretion 
is always to be found in the lumen of the gland in the form of threadlike 
masses, which are carried by the current produced by the cilia into the cavity 
of the uterus. The cervical glands in the cervix are alveotubular and of 
greater extent (Pl. 67, Fig. 158). The epithelium consists of ciliated cylindri- 
cal cells, mixed with true goblet cells. While the cervical glands appear in the 
fifth month of pregnancy, the uterine ones develop about the time of birth, or 
during childhood. 
The muscularis forms the main part of the wall of the uterus and con- 
sists of bundles of smooth muscle cells about 50 p long, surrounded by an en- 
velope of elastic and collagenous fibers. The following layers are commonly 
distinguished from within outward. The stratum submucosum, a thin, 
slight layer of longitudinally running muscular bundles; the stratum Vas- 
culare, the most powerful layer, in which the bundles take for the most part a 
circular course, but are split up into many groups by others that run longi- 
tudinally, and containing many large blood vessels, as implied by its name; the 
stratum supravasculare, which is not very powerful and contains an equal 
number of bundles that follow a longitudinal and a circular course; and the 
stratum subserosum, the outermost layer, which contains only longitudinal 
bundles. As already stated, much elastic and collagenous tissue is found be- 
tween the muscular bundles and fibers. The elastic fibers form a stratum that 
separates the muscularis from the subserosa and pass from this into the sub-* 
serous and supravascular strata. The vessels that radiate into the stratum 
vasculare from the adventitia are numerous, while they are very few in the 
stratum submucosum. 
The serosa rests immediately upon the above mentioned elastic stratum. 
During the period of sexual activity the wall of the uterus exhibits regularly 
recurring transformations that are called menstrual. Menstruation begins 
with puberty, in our latitude about the age of fourteen or fifteen, recurs every 
twenty-eight days and ceases about the forty-fifth year in the so-called climac- 
teric, or menopause. When pregnancy supervenes menstruation is arrested 
until after parturition and the period of lactation. The external manifestation 
of menstruation is an outflow of blood mixed with uterine mucus from the 
vagina. The entire process may be divided into four stages which correspond 
to the changes that take place in the uterine mucous membrane, an interval 
of fourteen days, a premenstrual period of seven days, a true menstrual 
period lasting three days, and a postmenstrual period of four days. 
The condition of the uterine mucous membrane described here as normal is 
found only in the postmenstrual period. On the last day of this period and 
during the interval there is a considerable thickening of the mucous membrane 
associated with a marked growth of the uterine glands. Very many karyokinetic 
figures may be seen in the nuclei both of its cells and of those of the propria. 
The glands, which have become corkscrew-shaped, now begin to secrete and the 
process enters the premenstrual period, in which the glandular cells enlarge 
greatly, become high cylindrical and project far into the lumen of the gland. 
The glandular tubes may be thrown into little folds, giving the glands a sawlike, 
or bushlike form. The cells in the propria become much swollen, rounded, and 174 
gathered into larger complexes. Just before the onset of the menstrual period 
the process of secretion reaches its acme and signs of degeneration begin to ap- 
pear, first in the nuclei, and then in the glandular cells. The blood vessels are 
much dilated, the wall is loosened up, great numbers of leucocytes entei' the 
stroma, and finally there comes a gush of blood and an expulsion of the loosened 
up mucous membrane, wherewith the process enters the menstrual period. The 
separation takes place within the deep layers of the mucous membrane, leaving 
the basal layer next to the muscularis. The shreds of mucous membrane are 
either extruded with the blood by the contraction of the muscles of the uterus, 
or break down in the uterine cavity and are removed by phagocytosis. Re- 
generation sets in immediately from the preserved portions of the glands with a 
lively multiplication of cells. The remains of the mucous membrane thus be- 
comes covered by a new epithelium and grows again very rapidly to its former 
thickness. 
The processes that take place in this menstrual cycle render the mucous 
membrane of the uterus fitted for the reception of an impregnated ovum. If 
none such is present the proliferated mucous membrane is cast off, but if an 
ovum has become impregnated in the Fallopian tube it nests in the proliferated 
mucous membrane and the latter becomes the decidua graviditatis. 
The arteries of the uterus come from the uterine and the ovarian. They 
enter the lateral surfaces, pass through the external musculai' layers, and form 
an extensive plexus in the stratum vasculare, from which part of the twigs go 
to the muscles, part to the mucous membrane, where they form a dense capillary 
plexus about the glands, reaching to the epithelium. The veins that arise there- 
from are relatively small, anastomose much with one another, thus forming sev- 
eral plexuses in the uterine wall, and empty into the uterine and uterovaginal 
veins. 
The lymphatics are not yet known with perfect certainty. It is a ques- 
tion whether any provided with walls exist in the uterine mucous membrane, but 
such are certainly present in the musculature and the subserosa. The efferent 
vessels, divided into several little trunks, pass out of the cervix and the corpus 
to open into the iliac lymphatic glands. 
The nerves form an extensive plexus provided with numerous ganglia on 
the lateral surfaces of the uterus. They are mainly sympathetic, associated 
with a few medullated fibers, and come from the hypogastric plexus and the 
sacral nerves. Within the walls of the uterus they form a large plexus, the 
branches of which go to the muscles and vessels, to end between the epithelia of 
the glands and of the surface. Ganglion cells do not appear to be present in 
this plexus. Contractions take place in the uterus after destruction of the 
spinal cord and are influenced only a little by the central organ. 
4. THE VAGINA 
The embryology of the vagina was discussed under the preceding subject. 
After the occlusion of its lumen by proliferation of epithelium the epithelial 
mass constantly increases in circumference, its caudal end alone remaining thin 
and narrow, so that after the restoration of its lumen the vagina is relatively 175 
wide and opens outward through a narrow aperture surrounded by a fold of 
mucous membrane, the hymen. Originally both the vagina and the urethra 
open at an acute angle into the urogenital sinus, but toward the end of the 
fourth embryonal month the now relatively short sinus increases much in breadth 
sagittally and becomes the vestibule of the vagina, while the openings of the 
urethra and vagina separate more and more, the latter turning toward the 
anus, until both open separately into the vestibulum vaginas. 
The vagina is lined by a mucous membrane that lies in numerous trans- 
verse folds or rugas (Pl. 67, Fig. 158). They unite on the rectal and vesical 
walls, like the system of folds in the cervix, the longitudinal ones, the columnas 
rugarum, marking the original construction from two adjoining tubes. It is 
covered by a much stratified flat epithelium, about 200 p thick, in which the 
beginnings of cornification may frequently be seen. This epithelium continues 
in the vault of the vagina over the portio vaginalis, covering it as far as the 
external os where it changes rather suddenly into the cylindrical epithelium of 
the uterine mucous membrane. At the entrance to the vagina it is continuous 
with the similarly constructed epithelium of the vaginal vestibule. 
The propria of the vagina juts out with numerous high, slender papilbn 
into the epithelium, passing through as much as three fourths of its thickness. 
Its connective tissue is very dense, finely fibrous, and abundantly supplied with 
elastic fibers. Follicular accumulations of lymphocytes are formed in many, 
places. Glands are totally wanting. The propria forms the basis of the sys- 
tem of folds and of the hymen, which is nothing else than a special high fold of 
mucous membrane that forms its outer limit. 
The adjacent submucosa is materially looser than the propria, and joins 
the latter to the muscularis, the smooth fibers of which are a continuation 
of the longitudinal fibers of the stratum vasculare. To the inner side of the 
thick longitudinal stratum we find some scanty circular fibers. Numerous elas- 
tic fibers run between the bundles. 
Externally to the muscularis comes the connective tissue adventitia, which 
unites the vagina with the paravaginal tissues. 
The arteries come from the vaginal and the median hemorrhoidal. They 
run first in the adventitia, thence send branches into the musculature and mucous 
membrane, where they form a capillary plexus that stands out particularly well 
in the papillae of the propria. The veins form a plexus which is always denser 
in the deeper parts, and empties into the uterovaginal and vesical plexuses. 
The lymphatics form a plexus like that of the veins. They assemble into 
several little trunks, the lower of which go to the inguinal, the middle and upper 
to the hypogastric and iliac lymphatic glands. 
The nerve fibers coming from the uterovaginal plexus and the sacral 
nerves form a plexus studded with ganglia in the wall of the vagina. Their 
twigs end free between the epithelial cells in the mucous membrane, and on the 
muscular fibers and the vessels. 176 
5. THE EXTERNAL FEMALE GENITAL ORGANS 
The external female genital organs develop from the undifferentiated 
rudiment, as we have already seen. The genital eminence, which becomes 
the penis in males, attains no such dimensions in females, but has in general 
the same structure and is known as the clitoris. The genital folds extending 
out from it, which in man serve to close in the urethra, grow in woman into two 
folds of skin that border the vestibule of the vagina, the labia minora, and 
form above the frenulum clitoridis and the praeputium clitoridis. The 
genital prominences, which in man go to form the- scrotum, enlarge in 
woman into the labia majora. 
a. The Vestibule of the Vagina 
The vestibule of the vagina, situated between the labia minora, is the sagit- 
tally broadened urogenital sinus and contains in its wall a pair of erectile bodies, 
the bulbus vestibuli, which corresponds to the corpus cavernosum urethrae of 
the man. It is supplied with blood by the external pudendal artery and con- 
sists essentially of a dense venous plexus. 
The glandulae vestibulares open into the vestibule. These are two 
glandules vestibulares majores, or Bartholin's glands, and several 
glandules Vestibulares minores. The formei' lie on each side just beneath 
the posterior end of the bulbus vestibuli, and are analogous to, as well as con- 
structed the same as Cowper's glands in the male. The excretory duct is lined 
with stratified flat epithelium and opens on each side into the lower half of the 
vestibule at the base of the labia minora. 
The glanduhe vestibulares minores are little, simple or branched tubes found 
between the introitus vaginae and the orificium externum urethrae and about the 
hymen, and are lined with cuboidal mucous cells. 
b. The Clitoris 
The clitoris corresponds embryologically to the penis, with the exception 
of its urethral portion, and like it has two corpora cavernosa, a glans, and 
a prepuce formed from the labia minora. The structure of all of these 
scarcely differs from that of the corresponding parts in the male. The arteries 
come from the pudendal; the venous blood collects into the vena dorsalis cli- 
toridis. The skin of the clitoris is very richly supplied with sensory nerve 
fibers, part of which end free in the epithelium, part in capsulated corpuscles. 
c. The Labia Minora 
The labia minora resemble the skin in structure. They consist of connective 
tissue richly provided with elastic, and containing smooth muscle fibers. Nu- 
merous-high papillae covered with stratified flat epithelium are found. The labia 
minora have no hairs, only a few sweat glands, but many sebaceous glands. 
Blood vessels are numerous, and large veins in particular are present, so that a 
certain erection takes place with sexual stimulation, as in the clitoris. The 
nerve supply is the same as that of the latter. 177 
d. The Labia Maj ora 
The labia maj ora have the same structure as the minora, and differ from 
them only in the possession of numerous hairs, unusually long sweat glands and 
much more horny epithelium. The subcutaneous connective tissue contains 
great masses of fat, which together form the adipose tissue of the labia majora. VII. THE ORGANS OF MOVEMENT 
1. THE MUSCLES 
Although each of the three primitive blastoderms is able to produce mus- 
cular fibers, yet most of the muscles originate in the mesoderm. Here we 
find the myotomes, coming from the primitive vertebrae, from which are de- 
rived the great bulk of the muscles of the body. They form rows one behind 
another in the embryonal body, separated by myosepta, and consist of cylin- 
drical cells, the myoblasts. The latter provide the muscle fibers and are 
probably united at first into a synctium by protoplasmic bridges that are not 
confined to any one myotome, but anastomose the myoblasts in neighboring 
myotomes by passing through the myosepta. 
The nuclei within the myoblasts multiply through indirect segmentation. 
The protoplasm contains many coiled threads, the chondrioconts, and little 
granules, the chondriosomes. The chondrioconts group themselves longitudi- 
nally one after another and form the myofibrils, which pass from one myo- 
blast into another and also join neighboring myotomes together. At first they 
are smooth, but later thickenings appear at certain intervals, from which de- 
velop short rods intercalated in the course of the fibrils, and so produce their 
isotropic and anisotropic segments. The formation of fibrils begins in the 
periphery of the cell and soon extends to the entire cell body. 
After all of the myoblasts of the myotome have become fibrillated and trans- 
formed into muscular fibers, many of them degenerate and the remainder multi- 
ply by longitudinal fissuration. The muscular fibers thus produced are 
originally all of the same diameter, but later some increase, while others de- 
crease in circumference. 
The individual muscles may originate either through the fusion of myo- 
tomes that are placed one behind another, or through the outgrowth of myo- 
tomes and groups of myotomes, while still others come from mesoderma that is 
not articulated into myotomes. 
Entire groups of muscles become encased and separated from their 
neighbors by fasciae, dense, firm membranes that consist of several layers of 
connective tissue. Each layer is composed of bundles of connective tissue fibers 
with numerous elastic fibers between them. The direction of the fibers is the 
same in each layer, but varies in the different ones. 
Each individual muscle is enclosed in a perimysium externum, which 
consists of bundles of connective tissue, mixed with elastic fibers, that are longi- 
tudinal for the most part, and contains the larger vessels and nerves that enter 
the muscle (Pl. 68, Fig. 159). The perimysium externum sends into the 
muscle many anastomosing septa that have the same structure but a smaller 
quantity of elastic tissue. These together form the perimysium internum, 
and divide the entire mass of the muscle into bundles that successively become 
178 179 
smaller and smaller. Vessels and nerves run in these connective tissue septa, 
divide repeatedly, and finally reach the primary muscular bundles. Each 
of these consists of a varying number of muscular fibers, encased and 
separated by connective tissue fibers that branch into the bundles from the 
primary septa. 
The arteries commonly enter the muscle perpendicularly to the course of 
the fiber and split up into many branches within the perimysium internum. 
The smallest ones enter the primary muscular bundle perpendicularly and break 
up here into their terminal twigs, which form a long capillary plexus about the 
muscular fibers. The veins are usually two in number, are provided through- 
out with valves and generally follow the course of the arteries. 
The lymphatics accompany the blood vessels and probably also form 
plexuses in the primary muscular bundles. 
The nerves of the muscles consist of mcdullated fibers that come for the 
most part from the anterior roots of the spinal cord; a minority are sensory 
and come from the spinal ganglia. A smallei' number of nonmedullated sym- 
pathetic fibers are always present and are destined for the vessels. Within the 
perimysium internum the nerve trunks break up into twigs that constantly be- 
come finer, which at last enter the primary bundles. They usually cross the 
muscular fibers obliquely and give off their branches at an acute angle (Pl. 69, 
Fig. 161). A mcdullated motor nerve fiber, often accompanied by a sym- 
pathetic one, extends to each muscular fiber and forms on it a motor end 
plate. The axis cylinder suddenly decreases much in thickness near the end 
plate, often splits into two branches and passes over into an antlered formation 
within the sarcolemma (Pl. 70, Fig. 162). This terminal antler is composed 
of fibrils that run at first parallel to each other and then bend about to form 
loops at their ends. The medullary sheath is lost shortly before the entrance 
of the fiber into its end plate, while the sheath of Schwann on the contrary is 
retained and blends with the sarcolemma. The terminal antler lies in the sub- 
stance of the muscular fiber at a place free from myofibrils formed from the 
sarcolemma, the granulosa, or sole plate. Many nuclei are to be found be- 
tween the antlers and in their vicinity. The end plate often bulges the muscle 
fiber outward, for which reason it has been called the neural prominence. Each 
muscular fiber contains only one end plate, as a rule. 
The slightly tortuous sensory nerve fibers often accompany the mus- 
cular ones for long distances, giving off here and there fine, nonmedullated 
varicose lateral twigs that end with little thickenings on the fiber (Pl. 71, Fig. 
163). Another part of the sensory fibers end free in the connective tissue of 
the perimysium with branches that resemble antlers, and there are still other 
fibers that enter encapsuled corpuscles. 
The perimysium internum contains in addition to nerves and blood vessels 
the so-called muscle spindles (PL 68, Figs. 159 and 160), which are to be 
found in almost all the muscles of the human body, with the exception of the 
laryngeal and ocular, varying in number and development, and ranging in 
length from 2 to 10 mm. Each spindle is surrounded by a connective tissue 
sbeatb, and has one or more enlargements in its longitudinal section. In- 
ternally the sheath has a coating of branching cells that anastomose with their 180 
processes and passes through the interior in which striated muscle fibers run 
longitudinally, various numbers entering at one end of the spindle and leaving 
it at the other. On the way these fibers divide dichotomously several times in 
succession, anastomose with their twigs, and thus form a network with fairly 
thin muscular trabecula within the spindle. In many places the nuclei of these 
trabecula are packed very closely together, so that the muscle fiber is completely 
filled with them. They lose their chromatin almost entirely and become little 
vesicles that are given a polyhedral form by the close compression. What re- 
mains of the interior of the spindle is filled with an albuminous fluid. At least 
two nerve fibers enter each spindle, one or more sensory, the others motor, 
and nonmedullated fibers also may be observed in them. The sensory fibers 
form extensive endings about the muscle fibers, twining about them in spirals 
and giving off lateral twigs that end on them in the form of plates or hooks. 
The motor fibers terminate with typical end plates. Opinions are still divided 
concerning the special function of the muscle spindles, yet they are 
doubtless important receptory end organs, which perhaps appertain to the 
muscular sense, and to the adjustment of the condition of contraction of the 
muscle in question. 
2. THE TENDONS 
The muscles are attached to their places of origin and insertion by tendons 
that vary extremely in form. Each tendon is surrounded by a peritenonium, 
which corresponds to, and has the same structure as the perimysium. It sends 
septa into the organ that divide it into bundles of tendon substance ever growing 
smaller, as in the muscle. The smallest, the primary bundle, consists of 
quite a number of tendon fibers with tendon corpuscles between them, as 
described in the first part of this work (Pl. 13, Figs. 39 and 40). The tendon 
fibers run without interruption through the entire tendon and fuse at then* 
distal ends with the connective tissue of the periosteum or perichondrium. At 
their proximal ends they unite with the muscle fibers (Pl. 71, Fig. 164). The 
nature of this union is the subject of a lively scientific discussion. One theory 
is that the tendon fibrils perforate the sarcolemma and are continuous with the 
muscle fibrils. Other authors maintain that the sarcolemma separates the mus- 
cle from the tendon fiber completely, that the last is luted firmly to the first 
by a special substance, and that the peripheral fibrils of each tendon fiber are 
continuous with the connective tissue fibers of the perimysium internum. 
The blood supply of tendons is on the whole very scanty. The vessels 
enter the septa from the peritenonium and break up into capillaries. The in- 
terior of the primary bundle is nonvascular. The blood vessels are accompanied 
by lymphatics- 
The tendon is naturally supplied with sensory nerves alone, which are 
fairly numerous, part of them ending in Pacini's corpuscles, part in organs that 
greatly resemble muscle spindles. The tendon spindles lie in the peri- 
tenonium or in the coarser septa, have capsules, and are passed through by 
tendon fibers on which the nerves end with extensive, bushlike arborizations 
(Pl. 71, Fig. 164). 181 
Wherever tendons pass over joints they are surrounded by special tendon 
sheaths to enable them to glide easily. The tendon sheath is a hollow cylinder 
that is invaginated from below about the tendon, so that the latter comes to 
lie in its axis and is attached to it by the mesotendon. The sheath consists 
of a thick fibrous, and a thin mucous layer, the former being the outer 
envelope, while the latter covers the tendon with its visceral layei' and the inner 
surface of the fibrous sheath with its parietal. Between the visceral and parietal 
layers is the cavity of the sheath, filled with a viscous fluid. The mucous 
layer consists of loose connective tissue with many fibroblasts that form an 
epitheliumlike coat, interrupted in many places, toward the cavity of the sheath. 
3. THE BONES 
The development of bone was dealt with in extenso in the early part of 
this work. Bones are covered externally with periosteum which is divided into 
two layers. The outer of these, the adventitia, consists of bundles of connec- 
tive tissue that cross and interlace, and have running between them many clastic 
fibers and blood vessels. The inner layer, the fibroelastica, is distinguished 
from the preceding by its looser structure, the fineness of its bundles of con- 
nective tissue, the abundance of elastic fibers and of cells, and the small num- 
ber of its blood vessels. On its inner surface, next to bone, it is covered by a 
discontinuous layer of cubojdal cells from which the appositional growth 
and the regeneration of bone proceed and thus become osteoblasts at a 
given time. The adventitia connects the periosteum with the neighboring parts, 
particularly with the tendons of origin and insertion of the muscles, while the 
fibroelastica is joined to the bone substance itself. This union is effected by 
the incoming and outgoing blood vessels, but mainly by bundles of connective 
tissue that bend at right angles from their courses, which in general is parallel 
to the surface, and enter the bone substance, where we have learned to know 
them as Sharpey's fibers. Elastic fibers also may accompany the bundles 
of connective tissue from the periosteum into the bones. 
The bones themselves are composed in their outer segments of compact 
bone substance traversed by Haversian canals, and changes internally into the 
spongy bone substance, a network formed from thin, anastomosing leaves 
of bone. The spongy leaves and trabeculas contain no Haversian canals or 
Haversian lamelhe, but only basic lamellae. In the long bones the compact 
cortex is thickest in the middle of the diaphysis and tapers uniformly toward 
each end. In the flat bones the compact substance occupies a much greater space, 
forming thick outer and inner tables joined together by a thin spongiosa here 
called the diploe. The arrangement of the trabeculae in the spongiosa is char- 
acteristic in each bone and is in accordance with the laws of mechanics. 
The cavities of the spongy substance and the medullary cavity of the 
bone, which lies in the center of the spongiosa, are filled with bone marrow. 
In childhood this is a soft mass stained red by the blood it contains, and is 
known as the red bone marrow. This undergoes many changes in later 
childhood that lead chiefly to the production of fat and transform it into the 
yellow, fatty marrow. In adults the medullary cavities of the large, hoi- 182 
low bones are filled with fatty marrow, while the spongiosa of the smaller bones 
contains red marrow. Yet the former may change back into red marrow at any 
time, especially after great losses of blood. In old age, or sooner under patho- 
logical conditions, a mucous degeneration of the fat takes place in the fatty 
marrow and gives rise to the so-called gelatinous marrow. Under other 
conditions its fibrous constituents may proliferate, and then we speak of a 
fibrous marrow. 
The basis of the red marrow is a reticulated tissue that fills the cavities 
of the bone and is permeated with numerous blood vessels. The fibers are very 
fine, rather coarser about the vessels, and form plexuses. Microchemically 
they resemble the fibers of the reticulum of the lymphoid organs. The cellular 
elements of the marrow lie within their meshes, partly within delicately walled 
blood vessels, partly free, and have important relations to the formation of the 
blood. They are: 
1. The medullary cells, or myeloblasts, medium sized cells with large 
nuclei and narrow, weakly basophilic margins of protoplasm. They originate 
in loco through segmentation and are metamorphosed connective tissue cells 
from the beginning. They may be transformed in two ways. They may wander 
into the blood vessels and become 
2. Erythroblasts. These multiply very much within the vessels, begin to 
elaborate haemoglobin, lose their nuclei, and so become erythrocytes which are 
carried away by the blood current. The red bone marrow must be considered 
the most important center for the formation of erythrocytes in postfetal life. 
The other way in which myeloblasts undergo transformation is for them to 
elaborate granules in their bodies and so to become 
3. Granular leucocytes. We find in the marrow both neutrophilic and 
eosinophilic, as well as plasma cells and mast cells, the characteristic features 
of which have already been described. Part of these likewise enter the circula- 
tion, and the bone marrow consequently is to be regarded as a center for the 
formation of leucocytes. 
4. Giant cells also are produced by the metamorphosis of myeloblasts. 
They are 25 to 30 q in diameter and contain large multilocular nuclei that are 
often basket-shaped (Pl. 2, Fig. 6), with numerous central bodies and an oxy- 
philic, often fibrous protoplasm. Modern researches have made it probable that 
the blood platelets originate as divisional products of the bodies of these cells. 
The development of the fatty marrow from the red takes place in such a 
way that the number of medullary cells gradually decreases, while the cells of 
the reticulum enlarge and fill the meshes. Minute drops of fat appear in the 
bodies of the enlarged cells, gradually coalesce into fat globules and displace 
the cellular elements that take part in the formation of the blood. 
The arteries of the bones are divided into the superficial and the deep. 
The former enter the periosteum and form wide meshed plexuses in the ad- 
ventitia, from which twigs pass everywhere through the fibroelastica and enter 
the compact bone substance in the Haversian canals, through which they run 
to anastomose with the deep vessels. The deep arteries, known as the nutri- 
ent, pass obliquely through the compact substance to the medullary cavity, 
where they break up into large capillaries that assemble again into veins with 183 
very thin walls. The veins accompany the arteries and are generally two in 
number. 
The lymphatics of the periosteum form a plexus that opens internally 
into an interspace between the periosteum and the bone, and is connected in the 
other direction with the interspaces of the bone marrow through the Haversian 
canals. All of these spaces are lined with epithelium, so that the blood vessels 
in the Haversian canals are surrounded by lymphatic sheaths. 
The nerves of the periosteum are both medullated and nonmedullated. The 
former may end in Pacini's corpuscles. Exclusively nonmedullated nerves reach 
the interior of the bone along with the blood vessels, but are destined for the 
muscles of the latter. It is not probable that sensory fibers enter the bones. 
4. THE JOINTS 
The joints originate as clefts in the connective tissue surrounding the pieces 
of the skeleton, still cartilaginous at this time, which are at first peripheral and 
later press in between the ends, while the layer of connective tissue lying next 
to the cavity becomes differentiated into the capsule of the joint and is con- 
tinuous with the perichondrium. 
The articular capsule surrounding each joint is divided into an outer 
stratum Rbrosum, and an inner stratum intimum, or synovial mem- 
brane. The stratum fibrosum blends with the periosteum of the ends of the 
bones that meet to form the joint, and consists of parallel bundles of connective 
tissue packed closely together, which have a rather circular course in the deep 
layers and a longitudinal one in the superficial. Sparse elastic fibers lie between 
the bundles. The intima originates from the cartilage covering the ends of 
the bones and forms folds and villi that project into the joint. It consists of 
loose connective tissue containing many stellate anastomosing fibroblasts, and 
forms a more or less coherent layer over the free surface. 
The articular surfaces of the bones are usually covered with hyaline 
cartilage, which projects without any covering into the cavity of the joint and 
blends at its margin with the intima, with a great multiplication of cells that 
here become stellate. The sternoclaviculai' joint forms an exception, as this 
has a coat of fibrocartilage. The movements of the joint appear to wear away 
the surface of the cartilage with an unraveling of the fibers of its basal sub- 
stance and a disintegration of its capsules. The cartilage capsules are flat and 
lie parallel to the surface in the superficial layers, but deeper in they are round- 
ish and unite in the formation of territories, and still deeper they form the 
cartilage columns. The articular cartilage is separated from the bone by a 
layer of calcified cartilage. 
A small quantity of yellowish, viscid fluid, the synovia, is always to be 
found in the cavity of the joint, containing the destroyed elements from the 
articular surface. Its reaction is alkaline, its chemical composition depends 
upon the condition of the joint as regards rest, because the amount of organic 
constituents is increased when the joint is in active motion. Among the or- 
ganic constituents may be mentioned albuminoids and the synovial mucin. 184 
The intermediate discs, or menisci, found in many joints, consist 
of fibrocartilage, as do also the labia glenoidalia. 
The joints are abundantly supplied with blood vessels, which enter 
the intima and break up there into capillaries. The latter are also found in 
the villi and folds of the intima. 
The lymphatics of the joint form plexuses in the stratum fibrosum and 
the stratum intimum. No connection seems to exist between them and the cavity 
of the joint. 
The joint also has medullated nerves which enter the intima and form 
there a superficial plexus, from which fibers extend to terminate in ovoid end 
corpuscles. 
5. THE SYNARTHROSES 
In the synarthroses two bones are joined either by ligaments of connective 
or elastic tissue, syndesmoses, or by cartilage, synchondroses. The latter 
are found in the vertebral column, where the vertebrae are joined together by 
the intervertebral discs, or fibrocartilagines intervertebrales, each of 
which consists of an external, concentrically laminated annulus fibrosus of 
fibrocartilage and an internal gelatinous nucleus, the nucleus pulposus. 
The latter is composed of masses of vacuolized cells, the remains of the former 
chorda cells. VIII. THE ORGANS OF THE NERVOUS SYSTEM 
1. THE SPINAL CORD 
The neural crest, running in the median line of the ectoderm, forms the 
matrix from which the spinal cord and brain develop. It hollows out to form 
the neural groove, and even at this stage an anterior brain portion and 
a posterior spinal cord portion may be recognized. The closure in of the 
neural groove to form the neural canal begins at the junction of the brain 
and spinal cord portions, and extends thence forward and backward, so that 
by the beginning of the fourth fetal week the entire neural canal is closed in. 
The spinal cord at this time is a quadrangular tube with a broad ventral and 
a narrow dorsal surface, through which passes longitudinally the wide central 
canal, lozenge-shaped on section and divided into right and left halves that 
unite dorsally and ventrally. The central canal is surrounded by an ependy- 
mal layer, outside of which is the cellular mantle layer, and quite ex- 
ternally each half is invested by the narrow marginal film consisting wholly 
of nerve fibers. The cells of the mantle layer, the development of which has 
already been discussed, are completely developed with their neurites and den- 
drites at this time. 
The spinal cord now increases in size and assumes by degrees its definitive 
form. The size of the central canal is diminished by the approximation and 
fusion of the walls of its dorsal half, leaving only the ventral portion that is 
small, roundish, or oval on cross section. The ependymal cells form with their 
processes a septum separating the two dorsal halves of the medulla, the sep- 
tum posterius. At the same time the ependymal layer becomes thinner, while 
the mantel layer, known now as the gray matter, becomes very thick and 
forms the main part of the medulla. The nerve cells are scattered uniformly 
through the gray matter at first, but later collect into quite definite groups. 
Beginning at the end of the second embryonal month the gray matter gradu- 
ally assumes its typical form, hand in hand with the increased growth of the 
neurites into the white matter, the anterior horns appearing first, then the 
posterior. The first medullary sheaths appear in the roots in the fifth month, 
and the formation of medulla extends from here to the dorsal and ventral parts 
of the white matter, leaving the middle portions the longest without medulla. 
We shall now glance over the gross structural relations of the spinal cord 
as shown in a section of the fourth cervical segment (Pl. 73, Fig. 167). 
The section is bounded externally by a thin layer of neuroglia, known as 
the glia capsule, which sinks into a wedge-shaped fissure in the middle of 
the ventral surface, the fissura longitudinalis anterior, and covers its 
walls. It behaves similarly in the middle of the dorsal surface, although we 
can hardly say that there is a fissure at this place. The glia capsule sinks 
deeply into the substance of the spinal cord forming a thin, glial septum, the 
septum longitudinale posterius. Furthermore, numerous branching septa 
185 186 
extend into the spinal cord from the glia capsule; among these there is one 
on each side placed laterally from the posterior longitudinal septum and called 
the septum paramedianum. 
The cross section of the spinal cord is imperfectly divided by the anterior 
fissure and the posterior longitudinal septum into two halves, a right and a 
left, which are connected by a central bridge. Each of these consists of a 
gray nucleus, the gray matter, and an enveloping white mantle, the white 
matter, which are thus named from their appearance in the fresh preparation. 
The white matter owes its color to the fact that it is composed almost ex- 
clusively of medullatcd nerve fibers. The gray matter has a smaller number 
of these and is mainly composed of nerve cells and nonmedullated fibers. 
The two gray halves are connected by a transverse bridge, the gray com- 
missure, which adjoins the white matter on its dorsal side, contains the cen- 
tral canal, and is separated on its ventral side from the bottom of the an- 
terior fissure by a transverse ridge of medullated nerve fibers that connects the 
two white halves of the spinal cord, the white commissure. Dorsally from 
the central canal there also appear fine, transverse, medullated bundles, which 
together form the posterior white commissure. 
In this section of the spinal cord the gray matter is shaped like a butter- 
fly ; in each half it swells ventrally into a hammer-shaped formation, the an- 
terior horn, from the ventral surface of which the slender anterior roots, 
which are hard to see, radiate in little bundles to the periphery. Large nerve 
cells are found in the anterior horn arranged in groups that are known as the 
anterior median, anterior lateral, posterior median, posterior 
lateral, and central. Laterally a pointed process, the lateral horn with 
large nerve cells, is detached indistinctly from the anterior horn. Farther 
dorsally the gray matter is reduced considerably to form the posterior horn, 
which runs in the neighborhood of the periphery with a somewhat dorsal curve, 
and ends at the place where the posterior roots enter. Between the lateral 
and posterior horns, in the so-called lateral tract angle, the gray matter is dis- 
persed by the reticulated fibers of the white matter into little islands to form 
the processus reticularis. We may distinguish in the posterior horn, the 
broad intermediate zone, the narrow neck, and the longitudinally oval 
head. The head is enveloped dorsally by a crescentic mass of gray matter, 
the substantia gelatinosa Rolandi. Peripherally to this is the reticulated 
stratum zonale, which is separated from the surface by a narrow layer of 
longitudinally running medullated fibers, the stratum terminale, or Lis- 
sauer's tract. The nerve cells in the posterior horn are small and can be 
seen only with difficulty. 
The mantle of white matter is readily seen to be divided into three large 
territories on each side by the incoming and outgoing roots; the wedge-shaped 
posterior column between the septum longitudinale posterius and the pos- 
terior root, the lateral column between the posterior and the anterior roots, 
and the anterior column between the anterior root and the anterior fissure. 
In the posterior column the septum paramedianum incompletely separates the 
column of Goll, situated to its median side, from the laterally situated 
column of Burdach. 187 
The grosser changes shown in sections of the spinal cord at different levels; 
consist 1, in the form of the section as a whole; 2, in the form of the gray 
Fig. 35.-Shapes and Sections of the Spinal Cord at Various Levels. 
matter; 3, in the relations between the gray and the white matter; 4, in the 
form of the central canal. The accompanying drawing will convey more 
information concerning all of these points than would an extensive de- 
scription. 
We shall now study more closely the minute structure of the gray matter. 188 
a. The Gray Matter 
The gray matter of the spinal cord is composed of nerve cells and fibers, 
aside from the neuroglia and blood vessels. Part of the fibers are neurites of 
the cells, part enter the gray from the white matter and terminate here. 
The nerve cells form, as we have seen, five well distinguished groups in 
the anterior horn, but we always find scattered cells between these groups. Just 
as the anterior horns of the spinal cord form the anterior columns in the 
white matter, so these individual groups of cells form cell columns, but they 
differ from the former in not being continuous, as distinct interruptions, or 
segmentations, may be seen. 
The cells of the anterior horn are multipolar nerve cells, and the very 
great majority of them have neurites that become surrounded by a medullary 
sheath sooner or later after they have left the cells. They may be divided into 
three large groups in accordance with the further course of these neurites, 
the root cells, the commissural cells, and the columnar cells (Pl. 79, 
Fig. 176). 
The largest cells of the spinal cord are the root cells, measuring 150 p, 
which always contain a vesicular nucleus and large Nissl bodies. They send 
out several strong dendrites, that pass through the gray matter for long 
distances and come into contact with other cells. The always single neurite 
extends from the cell body, or from a dendrite, then suddenly thickens, be- 
comes clothed with a medullary sheath, and arches over to the ventral surface 
of the anterior horn, where it becomes an anterior root fiber (Pl. 81, Fig. 
178). Just before its exit from the gray matter it gives off one or more col- 
laterals to the neighboring cells. Each anterior root fiber is motor and runs 
without interruption from its root cell to the muscular fiber under its control, 
on which it ends in a motor end plate. Hence all of the root cells of the an- 
terior horn are motor. The only exception to this rule is that certain cells may 
rarely be observed to send their neurites through the posterior horn into the 
posterior root. 
While the root cells are distributed among all of the cell groups of the an- 
terior horn, the commissural cells occur mainly in the median groups and 
extend from here into the anterior gray commissure. They are usually smaller 
than the root cells and of a more elongated form. Their neurites pass, after 
they have received their medullary sheaths, into the anterior white com- 
missure, cross the middle line, and reach the anterior column of the op- 
posite side, where each divides into an ascending and a descending branch. 
Thus a constant decussation of fibers takes place in the anterior commissure 
(Pl. 81, Fig. 178). 
The columnar cells are those whose neurites run in the white matter 
of the same side. They are not confined to the anterior horn. They are of 
medium size, are found chiefly in the central groups, and are characterized by 
very strong and very long dendrites. The neurite of each divides, after it has 
been invested with medulla, often within the gray matter, into several branches, 
schizaxons, which usually enter the lateral column, less often the an- 
terior column of the same side, and divide into ascending and descending 
fibers (Pl. 81, Fig. 178). 189 
The cells of the lateral horn and of the formatio reticularis are 
of medium size and are all columnar. They send their neurites into the 
lateral column of the same side. 
The cells of the posterior horn are generally smaller than those of 
the anterior and are not arranged in groups. They are at most of medium 
size and are unevenly distributed in the head and neck. Their neurites enter 
either the lateral or the posterior columns of the same side. Cells of 
Golgi's second type, which have already been described, are found with 
them very often. 
A well differentiated group of cells is to be found only in the median por- 
tion of the base of the posterior horn; these are known as Clarke's cells and 
form the column of Clarke, which extends from the lower cervical down into 
the sacral medulla. Clarke's cells are large and have several thick, gnarled den- 
drites. The neurites assemble into bundles that are often very distinct, 
Flechsig's bundles, which pass transversely through the base of the pos- 
terior horn to enter the lateral column, in which they turn toward the brain 
(Pl. 79, Fig. 176). 
The substantia gelatinosa Rolandi has a homogeneous basal substance 
produced by the degeneration of nerve cells during embryonal life, and con- 
tains in adults three kinds of nerve cells; small, scattered, stellate columnar 
cells, the neurites of which entei' the posterior columns ; elongated, pyramidal 
columnar cells, with their longitudinal axes in the axis of the posterior horn, 
the neurites of which enter the posterior column, and the so-called border 
cells, which are elongated and lie in the periphery of Rolando's substance with 
their longitudinal axes vertical or oblique to the axis of the posterior horn. 
Their neurites run through the latter longitudinally, then bend laterally and 
enter the lateral column of the same side (Pl. 79, Fig. 176). 
If we turn now to the nerve fibers of the gray matter we shall note 
first that a part of them are neurites of cells situated here and as such leave 
the gray matter, while another part enter the gray from the white matter to 
terminate on the cells themselves with end arborizations. Consequently they 
are in part medullated, in part not. Each one of the former variety has a 
nonmedullated initial portion, each of the latter a nonmedullated end piece. 
We have already learned to know the fibers that pass out of the gray 
matter as neurites of the root, commissural, and columnar cells. Those that 
enter the gray matter come, each and all, from the white, and are either end 
sections or collaterals. The fibers running in the white matter give off at 
right angles from place to place lateral branches, or collaterals, which enter 
the gray matter and end on its cells. We may divide them into the collaterals 
of the anterior, lateral, and posterior columns (PL 81, Fig. 178). 
Some enter alone, some in bundles; the latter are met with particularly in the 
posterior horn. They come from the posterior column, pass in parallel, finer 
or coarser bundles through the substantia gelatinosa Rolandi, or in a curved 
course through the median portion of the head of the posterior horn, and 
radiate partly to the cells of the posterior horn, partly to Clarke's, or else pass 
in compact lines through the cervix to enter the anterior and lateral horns and 
to end on their cells. 190 
The gray commissure, which joins the two gray halves of the spinal 
cord, is divided by the central canal into a broad dorsal and a narrow ventral 
part, and consists mainly of neuroglia. We also find in it dendrites of 
the commissural cells, which pass from one half to the other, as well as com- 
missural cells themselves, and medullated fibers. In the dorsal part the lat- 
ter are in fact collaterals of the posterior and lateral columns, which 
unite into little bundles to pass over to the cells of the posterior horn of the 
opposite side. In the ventral part we have similar, but less strong collaterals 
coming from the anterior and lateral columns and passing through the com- 
missure to the cells of the anterior, lateral, and posterior horns of the op- 
posite side. 
b. The White Matter 
Aside from the glia, which will be described presently, the white matter 
consists of medullated nerve fibers running longitudinally and the col- 
laterals that branch from them. The fibers vary much in caliber and have no 
sheaths of Schwann. The thickest ones are in the lateral column, the thinnest 
in the posterior. Various subdivis- 
ions may be made of this mass of 
fibers based partly on embryological, 
partly on experimental grounds, but 
it is difficult or impossible to distin- 
guish them histologically. A brief 
description may enable us to under- 
stand better the extremely important 
relations in the organization of the 
spinal cord. 
The anterior column may be 
divided into two parts, the anterior 
pyramidal tract, and the basal 
fasciculus of the anterior col- 
umn. The anterior pyramidal 
tract bounds the anterior longitu- 
dinal fissure on each side. It is best 
developed in the upper cervical por- 
tion, continuously decreases down- 
ward, and disappears in the dorsal 
portion of the spinal cord. This 
gradual disappearance is due to the 
fact that we have to deal here with 
a long descending tract, the fibers 
of which successively leave the 
column, pass through the anterior commissure, where they lose their medullary 
sheaths, and end on the cells of the opposite anterior horn. The cells from 
which the fibers originate lie in the cerebral cortex. 
What remains after the anterior pyramidal tract has been deducted is 
known as the basal fasciculus of the anterior column, and in compari- 
Fig. 36.-The Columns of the White Matter of 
the Spinal Cord at the Level of the Fifth 
Cervical Nerves. 
In the right half of the figures the descend- 
ing tracts are marked with vertical lines, 
the ascending with horizontal lines, those that 
both ascend and descend with crossbars. In 
the left half the long tracts are dark, the short 
are light. 1, pyramidal tract of the anterior 
column; 2, basal fasciculus of the anterior 
column; 3, Helweg's tract; 4, Gower's tract; 
5, basal fasciculus of the lateral column; 6, 
Monakow's tract; 7, cerebellar tract of the 
lateral column; 8, pyramidal tract of the 
lateral column; 9, zona terminalis; 10, ventral 
bundle of Burdach's column; 11, Schultz's 
comma; 12, middle bundle of Burdach's col- 
umn; 13, dorsal bundle of Burdach's column; 
14, column of Goll. 191 
son with the other is short, containing fibers that originate in cells in the 
anterior horn. Each fiber divides like a T into an ascending and a descend- 
ing branch, which send collaterals into the anterior horn and, after a longer 
or shorter course, bend at right angles and end the same as the collaterals. 
The basal fasciculus of the anterior column passes over into the basal 
fasciculus of the lateral column in the region of the anterior roots. 
This does not reach the periphery in the cervical portion, diminishes in size 
downward, and contains, like that of the anterior column, short tracts, 
neurites of the cells of the anterior and lateral horns. 
Externally to this, close dorsally to the place of exit of the anterior roots, 
we find in the cervical portion a little, triangular district of fibers, Helweg's 
tract. This is a long descending tract, the cells of origin of which 
lie in the olivary bodies of the medulla oblongata, while their fibers end in 
the cells of the anterior and lateral horns. 
Dorsally the periphery of the lateral column is bounded by Gower's 
tract, likewise long, but ascending. Its fibers are the neurites of cells 
situated in the base of the posterior horn. They run upward in the gradu- 
ally swelling tract to the medulla oblongata and finally to the cerebellum. 
Still farther dorsally at the periphery of the spinal cord we meet with the 
important, long, ascending, direct cerebellar tract, which appears first 
in the upper lumbar portion, increases rapidly in strength, to become about 
constant in size in the lower cervical. By far the most of its fibers are the 
neurites of Clarke's cells, and they pass through the medulla oblongata 
to the cerebellum. 
To the median side of the direct cerebellar tract lies in the cervical portion 
a large tract of fibers known as the lateral pyramidal tract, which may be 
followed caudally down into the sacral portion, decreasing much in size, es- 
pecially in the cervical and lumbar swellings. The fibers of the lateral pyrami- 
dal tracts are long and descending; they originate from the cells of the 
cerebral cortex, undergo a total decussation at the junction of the 
brain and spinal cord, and are then gradually distributed to the root cells of 
the anterior horn, on which they break up into terminal arborizations. 
At the place where the preceding adjoins Gower's tract the median part 
of the lateral column is occupied by the long, descending Monakow's 
tract. The cells of origin lie in the red nucleus of the midbrain, their 
neurites soon decussate and may be followed to the sacral portion, ending 
on the cells of the anterior horn. 
The posterior column is divided into a median Goll's and a lateral 
Burdach's column. The column of Goll is wedge-shaped; its base is 
the dorsal periphery of the spinal cord from the septum longitudinale posterius 
to the septum paramedianum, from which it tapers ventrally and terminates 
in a point some way in front of the posterior commissure. It contains ex- 
clusively the slender fibers of the posterior root, which divide like a T, 
the ascending branches running in the column toward the brain. The number 
of fibers in the column of Goll constantly increases upward from the dorsal 
region, and they terminate in the medulla oblongata on the cells of Goll's 
nucleus. It thus contains the ascending portions of the posterior 192 
root fibers; the descending portions lie along the side of the column of Goll 
in the form of a slender, comma-shaped bundle, Schultz's comma. The 
descending portions are short and soon enter the gray matter. 
Burdach's column has fibers that are thicker on an average than Goll's 
and includes all the rest of the posterior column. It is comprised likewise in 
great part of long tracts, especially in its middle segments, the ascend- 
ing parts of the fibers of the posterior roots, which go to the medulla 
oblongata and end there in Burdach's nucleus. But it also contains many 
neurites of columnai* cells of the posterior horn, which are short, and run 
chiefly on the peripheral base and on the opposite zone adjoining the posterior 
commissure. 
c. The Anterior Roots 
The fibers of the anterior roots are, as we have seen, the neurites of the 
root cells situated in the various groups in the anterior horn. They are in- 
vested with medulla at a greater or less distance from the cell, send out their 
motor collaterals and reach their bundles by a curved course. Each bundle 
contains fibers from different groups of cells (Pl. 81, Fig. 178). The bundles 
are quite slender and lie three or four together in the section of the spinal 
cord. They pass through the marginal district between the anterior and the 
lateral columns, at some distance apart are parallel, and converge from the 
so-called sulcus lateralis anterior to the anterior root (Pl. 83, Fig. 181). 
Besides these root fibers the anterior roots contain a smaller number of 
others that originate from the root cells not of the same, but of the opposite 
side, come through the white commissure into the anterior horn and leav^ 
through the anterior root. 
Finally, there are fibers of less caliber than those of the anterior roots, 
which originate in the cells of the lateral horn. These do not go to the 
striated muscle fibers, but enter the sympathetic ganglia, where we shall meet 
with them again. 
d. The Posterior Roots 
The conditions in the posterior roots are considerably more complicated. 
The mcdullated fibers that compose them vary a great deal in caliber and come 
from the spinal ganglia. The posterior root leaves the ganglion as a solid 
cord, soon divides into a considerable number of bundles which pass in the 
sulcus lateralis posterior to the median side of the posterior horn into the 
spinal cord (Pl. 83, Fig. 181). Four subdivisions of each posterior root bun- 
dle may be made from the subsequent course of the fibers. The most median 
subdivisions enter the posterior columns and become posterior column fibers, 
which have been described. Laterally from these are fibers that likewise enter 
the posterior column, but only pass through it in a curvilinear course to enter 
the posterior horn and to end on the cells of Clarke's column. Farther laterally 
are fibers that pass in parallel lines through the substantia gelatinosa to end 
on the cells of the anterior and lateral horns. The most median, finest fibers 
finally enter the stratum terminale to follow a longitudinal course and to end 
on the cells of the posterior horn. 193 
Thus we see that the fibers of the posterior roots, in contrast with those 
of the anterior, are centripetal conductors; they come from the spinal ganglia, 
enter the spinal cord, and continue their courses within it. 
e. The Neuroglia of the Spinal Cord and of the Central Canal 
We have already studied the .minute structure and the development of 
neuroglia and shall deal here only with its essential topographical relations. 
The spinal cord is surrounded by a glia capsule which varies in thickness 
from 2 to 20 p, and is strongest at the dorsal and lumbar swellings. On the 
ventral surface it covers the walls of the anterior longitudinal fissure, while 
dorsally, togethei- with other fibers, it forms the septum longitudinale 
posterius. It consists of strong, longitudinal glia fibers assembled into mani- 
fold bundles which here and there bend at right angles and enter the white mat- 
ter. Part of the bundles of fibers turn outward to enter the surrounding pia. 
Glia septa radiate into the white matter from the glia capsule, branch, 
become thin and for the most part break up before the gray matter is reached. 
These septa divide the white matter into larger and smaller districts, yet have 
little to do with the functional organization. They consist of glia fibers that 
radiate for the most part. The glia capsule is particularly well developed at 
the place of entrance of the posterioi' roots, where it surrounds the nerve fibers 
and penetrates with them into the spinal cord. 
Glia fibers are to be found everywhere in the white matter, part running 
radially, winding between the nerve fibers, part longitudinally and parallel to 
the latter. Yet the mass of neuroglia between the fibers is by no means enough 
to permit us to say that it has an isolating function. Glia cells are also to 
be found everywhere between the fibers, containing their characteristically 
shaped nuclei that are rich in chromatin. 
The gray matter also is abundantly supplied with neuroglia, which varies 
in different parts. It is densest about the central canal, which we shall study 
first (Pl. 80, Fig. 177). The central canal is lined with ependymal 
cells packed closely together, which are cylindrical in childhood and become 
more slender from the canal toward the periphery. Each cell is provided with 
a nucleus, cilia, and basal corpuscles. During youth the central canal begins 
to become obliterated from the sacral region upward, so that by middle life 
a lumen is usually to be found only in the upper cervical portion. The ob- 
literation results from proliferation of the ependymal cells, which assume the 
character of astrocytes, such that the lumen is replaced by a mass of cells 
containing numerous glia fibers. 
About the central canal there is a very dense and powerful accumulation 
of neuroglia, the central mass of glia, the fibers of which have a threefold 
course. The great mass of them are circular, with many longitudinal ones, 
frequently combined into bundles, passing between them. Other fibers, that 
follow a radiating course, are most strongly developed in the ventral and dor- 
sal portions and form in each of these a glia cord from the central canal to the 
bottom of the anterior fissure, or radiating into the septum longitudinale pos- 
terius, the ventral and dorsal ependymal wedges. If these radiating 
fibers are traced toward the center they may often be seen to run to the ependy- 194 
mal cells and to end in their protoplasm. The posterior longitudinal septum 
is therefore composed of fibers that are both ependymal, and from the glia 
capsule. The central mass of glia contains both glia cells and glia fibers, but 
the number of the former is in no true ratio to the enormous mass of the latter. 
The relations of the glia fibers to the blood vessels may be observed very 
well in this region. A fairly thick mantle is formed about them, the mem- 
brana perivascularis, that is separated from the vessel wall by a narrow 
interspace that probably serves for the circulation of the lymph. 
The anterior horn is always provided abundantly with glia fibers, which 
accompany the outgoing anterior roots and form a dense mass at the margin 
of the white matter. A basket of glia fibers is formed about each nerve cell. 
These glia baskets are not immediately contiguous to the bodies of the nerve 
cells, but the two are always separated by an interspace. In the posterior 
horn Clarke's columns are the richest in glia, while Rolando's substance is 
the poorest and contains only quite few glia fibers. 
With regard to the function of the neuroglia, the part that has been 
ascribed to it, that of being a separating substance, can hardly be justified. 
Whether it is merely a filling in material, or is essentially important to the 
nutrition of the spinal cord, needs further elucidation. 
f. The Blood Vessels of the Spinal Cord 
The arteries that supply the spinal cord with blood come partly from 
the vertebral, partly from the paired segmental branches of the abdominal 
aorta and their caudal continuation in the median sacral, the lumbar, and the 
lateral sacral. From the vertebral artery comes the anterior spinal, at first 
paired, later unpaired, which runs downward ventrally from the anterior longi- 
tudinal fissure, and the paired posterior spinal descending along the dorsal 
surface of the cord. The spinal branches pass from the segmental vessels 
through the intervertebral foramina into the vertebral canal, and give off the 
ramuli medii on the cord. An arterial plexus about the spinal cord is pro- 
duced by the formation of a chain of anastomoses between these separate tracts. 
Vertical twigs branch off here and there from the longitudinal vessels and the 
plexus just mentioned, to penetrate into the cord. The most important of 
these are the arterite sulci, about two hundred in number, which are given off 
from the anterior spinal, pass deeply into the anterior longitudinal fissure, then 
bend and enter alternately the right and the left halves of the spinal cord and 
the gray matter on each side of the central canal as the longitudinal central 
arteries. On the dorsal surface the arterite septi posteriores radiate into the 
posterior septum. Radiating branches from the plexus enter the entire 
periphery of the cord, but particularly with the anterior and posterior roots; 
these little vessels when taken together are commonly known as the vasocorona. 
All of these little arteries break up in the spinal cord into capillaries that form 
plexuses about the nerve fibers arranged longitudinally in the white matter. 
Denser capillary plexuses are also formed in the gray matter, especially in the 
region of the cells of the anterior horn and of Clarke's columns. While the 
capillaries of the gray matter come mainly from the arteriae sulci, those of the 
white matter are derived chiefly from the vasocorona. 195 
The veins of the spinal cord form three anastomosing longitudinal trunks 
on both the ventral and the dorsal surfaces, an anterior and a posterior median, 
with two anterior and posterior lateral veins running near the roots. The 
blood flows partly into the intervertebral veins, partly into the plexus venosi 
vertebralis interni. Within the spinal cord itself the veins generally follow 
the course of the arteries. 
g. The Lymphatics of the Spinal Cord 
The lymphatics of the spinal cord are unenclosed spaces about the blood 
vessels that are separated from their surroundings by the glial membranse 
perivasculares. Enclosed lymphatics are totally wanting. These perivas- 
cular spaces within both the white and the gray matter are in open connection 
with a system of clefts between the nerve fibers and about the cells in which 
glia fibers run. They open into the subarachnoidal space. 
2. THE SPINAL GANGLIA 
The first sign of the spinal ganglia is met with very early, at the time when 
the neural plate is transformed into the neural tube. At the place where the 
transition of the neural tube into the ectoderm takes place, cells separate from 
the latter, penetrate between the neural tube and the myotome and form to- 
gether a longitudinal ridge, the ganglionic crest. After this has separated 
from the ectoderm and from the now closing neural tube, it divides into seg- 
ments, i.e., it breaks up into a number of successive cellular masses, each of 
which belongs to a primitive segment. 
The cells composing the spinal ganglia are at first equivalent, but later 
differentiate in two directions. Part of them increase in size, develop neuro- 
fibrils within themselves that extend out of the cells in the form of two pro- 
cesses, and become bipolar nerve cells. The others remain small and sur- 
round the larger ones as amphicytes, or mantle cells. 
One of the processes of the cell passes through the wall of the now closed 
neural tube and becomes a posterior root fiber, the other passes to the 
periphery as a sensory nerve fiber. The primitive bipolarity of the cells 
of the spinal ganglia is lost in man fairly early through the growth of the cell 
body perpendicularly to its longitudinal axis, so as to be first drawn out into 
a pedicle into which both processes open. The further longitudinal growth 
of this pedicle results finally in a unipolar nerve cell, the process of which 
bifurcates like a T into the two nerve fibers. 
Each spinal ganglion in the adult is surrounded by a dense capsule of 
connective tissue, which usually contains fat cells and elastic fibers. The 
capsule sends processes into the substance of the ganglion which anastomose 
and surround groups of cells. The connective tissue of the capsule is con- 
tinued over the posterior root on the one hand, and over the spinal nerves on 
the other. 
The contents of the ganglion consist of nerve cells and nerve fibers. 
The former lie in nests and elongated complexes joined together by bundles 
of medullated nerve fibers (Pl. 83, Fig. 181). Each nerve cell is surrounded 
by a capsule of fine, interwoven connective tissue fibers with flat connective 196 
tissue cells between them. Between the bodies of the nerve cells and the wall 
of the capsule lie a varying number of little cells, the amphicytes (PL 29, 
Fig. 74). These, as we have seen, have the same origin as the nerve cells 
and may either form a layer about the latter, or lie separately. They con- 
tinue over the nerve fiber as it leaves the cell and form upon it the sheath 
of Schwann. 
The cells of the spinal ganglia vary much in size and shape. Usually 
the cell body is spherical or ovoid (Pl. 83, Fig. 182). The nucleus is large, 
vesicular, and exhibits all of the well known characteristics of nuclei of nerve 
cells. The body of the cell contains very many, and usually quite fine Nissl 
bodies scattered about pretty uniformly. They are absent in only one place, 
that of the origin of the process. The great majority of the cells of the spinal 
ganglia have only one process and are therefore unipolar. This process is 
fairly strong, arises from a conical eminence, and soon after its origin is often 
thrown into numerous coils that either embrace the cell body, or form a 
glomerulus situated in a depression of the cell body. Then the process 
breaks through the capsule and bifurcates like a T. It is invested first with 
a sheath of Schwann as a direct continuation of the intracapsular amphicytes, 
and soon after by a medullary sheath. Two medullated nerve fibers 
thus arise from the process, one passing into the spinal cord as a posterior 
root fiber, the other to the periphery in the spinal nerves as a sensory 
nerve fiber. 
It is by no means rare for the protoplasm of the nerve cell to show holes 
or apertures filled with amphicytes at the place of exit of the process, which 
may occupy a fairly large portion of the cell body. 
In addition to the nerve process we find in many cells other similar forma- 
tions, as little intracapsular clubs, or buds, or as longer processes that 
pass through the capsule and end in the neighborhood in a little clubbed thick- 
ening. This end club is always surrounded by a little, separate capsule. 
The nerve fibers issuing from the ganglion unite with the ventral root com- 
ing from the spinal cord to form a spinal nerve, which is connected by a 
ramus communicans with a sympathetic ganglion. In this way numerous non- 
medullated fibers run backward into the spinal ganglia and reach many, 
but not all spinal ganglion cells. Such an afferent fiber passes through the 
capsule and either forms an extremely fine meshed intracapsular plexus about 
the cell, or attaches itself to the cell body as a convolution (Pl. 83, Fig. 182). 
It has been determined by careful computation that the number of medul- 
latcd fibers which enter in the posterior roots is materially less than that of the 
nerve cells in the ganglia, although the two should correspond according to 
what has been said. The explanation of this fact is that medullated fibers may 
also be seen to end within the ganglion with simple clubs. A certain part of 
the nerve cells of the ganglion also are still in a latent, embryonal condition, in 
which they send out no processes. 
The arteries of the spinal ganglia come mainly from the rami spinales of 
the segmental branches of the aorta. They form dense capillary networks in 
the ganglia about the capsules of the cells and the nerve fibers. 
Lymphatics seem to be totally absent. 197 
3. THE BRAIN 
Inasmuch as a description of the structure of the brain that was only ap- 
proximately full and coherent would far exceed the limits of this book, we must 
confine ourselves here to a brief histological analysis of its most important 
parts, and refer the student to special textbooks on neurology for a more com- 
plete discussion. 
a. The Nuclei of the Afterbrain and Midbrain 
In the medulla oblongata the gray matter undergoes a great disruption 
and remodeling, so that it comes to surround the mass of white fibers, and 
forms the floor of the fourth ventricle from the opening of the central canal. 
In addition numerous masses of nerve cells or nuclei appear in the medulla 
oblongata, some of them interpolated as intermediate stations in the cerebro- 
spinal conduction, some serving as centers of origin or of termination for the 
cranial nerves. We shall deal briefly with the most important of these. 
The accessory nucleus is the first that we meet with. It appears in the 
central cervical portion as a group of cells situated dorsolaterally from the 
central cells of the anterior horn, which have the characteristics of root cells, 
except that instead of sending their neurites ventrally into the bundle in the 
anterior root they send them laterally and toward the cerebrum through the 
lateral column (Pl. 75, Fig. 170). Farther up the fibers run more horizontally 
and unite with those that rise from below into the spinal accessory nerve. 
Collaterals from the posterior columns and the pyramids terminate on the cells 
of the accessory nucleus. The nucleus is shoved toward the median line by the 
decussating pyramidal fibers and comes to lie just laterally from the cen- 
tral canal. 
The nuclei of the posterior columns appear at the level of the decus- 
sation of the pyramids, first in the column of Goll, a little later in that of 
Burdach (Pl. 75, Fig. 171). The cells are multipolar, medium sized, with long 
dendrites that have many branches. The fibers of the posterior column end 
on these cells with arborizations, while from them come neurites which, taken 
together, form the lemniscus, which decussates soon after leaving the nucleus. 
This is called the decussation of the pyramids. 
Before the nuclei of the posterioi' column have reached their greatest extent 
and are prominent externally as the clava and the tuberculum cuneatum, the 
anterior pyramidal tract has joined the lateral pyramidal tract, which passed 
through the decussation, to form the pyramids, on the ventral periphery of 
which is the nucleus arcuatus. This stretches for a variable distance 
toward the cerebrum, and consists of multipolar cells of different sizes which 
are intercalated in the course of the fibra? arcuate externa? (Pl. 76, Fig. 172). 
Another motor nucleus appears ventrally from the accessory before the 
central canal has opened; this is the hypoglossal nucleus, which appears 
later in the calamus scriptorius on the floor of the fourth ventricle (Pl. 75, 
Fig. 171; Pl. 76, Fig. 172). The cells resemble exactly the large root cells of 
the anterior horn, and their dense neurites form the root of the hypoglossus, 198 
which leaves the medulla in a ventro-lateral direction, dorsally from the pyra- 
mid. Fibers that come from the cerebral cortex and decussate in the raphe end 
in the nucleus, as well as collaterals from the intramedullary course of the 
trigeminal, glossopharyngeal, and vagus nerves. 
At the caudal beginning of the hypoglossal nucleus appears a formation 
that gives a cross section of the medulla oblongata its characteristic mark, the 
lower, or large olive, or olivary body, the nucleus olivaris inferior 
(Pl. 76, Fig. 172). This is a many folded double lamella of gray matter that 
opens medially, is closed laterally, and occupies the region dorsal to the pyra- 
mids. The entering nerve fibers form a medullary substance between the two 
lamella?, while the latter become bordered by medullated fibers, the stratum 
zonale. The nerve cells that compose the nucleus are small, or at most of 
medium size, spindle-shaped, have several dendrites and one neurite each, which 
pass to the raphe, turn to the opposite side, and pass through the olive to ap- 
pear in the corpus restiforme as the fibres arcuatae internse, or the 
tractus olivo-cerebellaris. Fibers also pass from the vertex of the olive 
into the spinal cord, where we have met with them in the lateral column as 
Helveg's tract. Collaterals from the fibers of the direct cerebellur tracts, 
and fibers coming from the thalamus end on the cells of the olive. 
Besides the large olive we meet with the median accessory olivary nu- 
cleus a little earlier, and dorsal accessory olivary nucleus rather later. 
These have essentially the same structure and the same connections as the large 
olive. 
Medially from the olive toward the dorsum lies the substantia reticu- 
laris, passed through in the median line by the raphe. This contains many 
medium sized and large multipolar nerve cells, the axis cylinders of which turn 
toward either the brain or the spinal cord and are united into little bundles. 
Fibers coming from the cerebellum end on these cells. 
In the dorsolateral parts of the formatio reticularis are large, multipolar 
cells that form the vagoglossopharyngeal nucleus, or the nucleus ambigUUS. 
The neurites of these are hard to see because they always run singly; they join 
the vagoglossopharyngeal root, which will be mentioned later (Pl. 76, Fig. 172). 
The principal nucleus of these nerves is the vagoglossopharyngeal, 
which occupies the region of the ala cinerea. It consists of several groups of 
medium sized cells with few dendrites and slender neurites, which leave the 
medulla oblongata together with the fibers from the nucleus just described. 
Finally we have a third, the sensory vagoglossopharyngeal nucleus, 
which is a thin band of medium sized cells situated laterally and somewhat ven- 
trally from the foregoing, and extending caudally to the level of the decussa- 
tion of the pyramids. The fibers coming from the ganglion of the vagus and 
glossopharyngeal nerves enter the cerebral end of this nucleus and turn toward 
the cauda, forming a roundish bundle known as the tractus solitarius sit- 
uated medially from the nucleus. The fibers end on the cells of the nucleus (Pl. 
76, Fig. 172). 
Somewhat higher we come to the region of the nucleus of the auditory 
nerve (PL 76, Fig. 172; Pl. 77, Fig. 173). As is well known this nerve enters 
the medulla oblongata in two roots, the fibers of which are the neurites of the 199 
ganglia situated in the. internal ear. The fibers of the lateral root of the 
auditory nerve come from the spinal ganglia of the cochlea and enter the 
ventral nucleus, which is situated between the cerebellum and the corpus 
restiforme. This nucleus consists of roundish or polyhedral cells with several 
dendrites and one neurite. The fibers of the lateral root end on them, some 
with special cup-shaped end organs. The neurites of the cells take different 
courses; part of them pass as the striae medullares to the floor of the fourth 
ventricle and through the raphe to the upper olive, or to the lateral lemniscus. 
Another part forms the trapezium cerebri that will be described later. 
Another nucleus of the lateral root of the auditory nerve is the tuber- 
culum acusticum, which is situated dorsally from the ventral nucleus, and 
is very feebly developed in man. It is composed of multipolar, elongated cells 
adjoining the dorsal surface of the restiform body, the neurites of which run 
togethei' with those of the ventral nucleus in the striae medullares. 
The median root of the auditory nerve is composed of neurites of 
the ganglion vestibulare, situated in the internal auditory meatus, and enters 
various nuclei. The first to be mentioned is the extensive dorsal nucleus of 
the auditory nerve, that lies in the floor of the fourth ventricle. This con^ 
sists of cells of various sizes on which the root fibers end, the neurites of which 
decussate in the raphe to then pass toward the brain and into the tegmentum. 
Another part of the median root of the auditory nerve enters Deiter's 
nucleus, bends about caudally and ends on the cells of this nucleus. The 
fibers are collected into little bundles, which give the nucleus its characteristic 
appearance. It lies laterally from the median nucleus of the auditory nerve and 
is composed of multipolar cells that differ greatly in size. Their neurites take 
different courses. Those from the more cerebral parts form the tractus ves- 
tibulo-cerebellaris, and enter the cerebellum through the cerebellar peduncle 
to end in its nuclei. The more caudal cells on the contrary send their neurites 
caudally and thus form a tractus vestibulo-spinalis, which runs in the 
lateral column and may be demonstrated as far down as the lumbar portion of 
the cord. 
Still more laterally, on the medial periphery of the restiform body that is 
now radiating into the cerebellum, is Bechterew's nucleus, which reaches 
up into the roof of the ventricle. A final portion of the median root of the 
auditory nerve enters this. Part of the neurites of its cells enter the tractus 
vestibulo-cerebellaris, part cross the root fibers, pass through Deiter's nucleus, 
and enter the fasciculus longitudinalis medialis ventrally from the dor- 
sal nucleus of the auditory nerve to rise in it into the midbrain, and to end on 
the cells of the nuclei of the ocular muscles. Numerous neurites from Deiter's 
nucleus take the same course. 
The upper olive appears after the lower one has disappeared and lies in 
the same situation. It forms, as we have seen above, an important terminal cen- 
ter for the striae medullares, the fibers of which approach its cells. The latter 
are of medium size, spindle-shaped, send their neurites into the lateral lemniscus, 
and reach with them to the posterior corpora quadrigemina and the internal 
corpus geniculatum (Pl. 77, Fig. 173). 
Just to the median side of the upper olive lies the nucleus trapezoides, 200 
which is developed much better in animals than in man. It is enclosed in the 
mass of fibers that comes down from the ventral auditory nucleus and as a 
whole is known as the trapezium cerebri. The cells of the trapezoid nucleus 
are large and round, their neurites cross the raphe and pass with the lateral 
lemniscus of the opposite side to the medial corpus geniculatum. The ap- 
proaching fibers split and embrace the cell, body, as the hand clasps a ball. 
The facial nucleus appears as the lower olive disappears, and lies later- 
ally and dorsally from it, separated by Monakow's and Gower's tracts from 
the surface of the medulla oblongata. Cerebrally it has only a slight extent 
and consists of large, multipolar nerve cells, the neurites of which are the root 
fibers of the nerve. They unite into a strong bundle that passes in a dorso- 
medial and somewhat cerebral course to the floor of the fourth ventricle close 
to the median line, then suddenly turns at a right angle and runs 5 mm toward 
the cerebrum, bends again laterally and takes a ventrolateral direction, but 
which leads at the same time so far caudally that the exit of the nerve lies at 
the same level as the nucleus. This entire tract is known as the inner genu 
of the facial nerve (PL 77, Fig. 173). Fibers coming from the cerebral 
cortex radiate into the facial nucleus from the pyramids. 
Dorsally from the facial nucleus lie groups of multipolar cells, the neurites 
of which come to light between the facialis and the acusticus and form the 
portio intermedia Wrisbergi. It also contains sensory fibers that arise 
from the cells of the ganglion geniculi, enter with the motor fibers into the 
medulla oblongata, pass over into the tractus solitarius and end on its cells. 
The abducens nucleus, within the genu of the facial nerve on the floor 
of the fourth ventricle, is likewise made up of large, multipolar cells of the 
motor type. Their neurites collect into slender bundles, which take a ventro- 
lateral course, first through the substantia reticularis, then the corpus trape- 
zoides, and finally the pons, laterally from the pyramid, and make an exit. 
Fibers from the pyramids and collaterals of the fasciculus longitudinalis me- 
dialis enter and end in the nucleus. 
We must now discuss briefly the relations of the origins of the trigeminus, 
of which there are four terminal nuclei and the same number of roots. The 
sensory trigeminal nucleus begins as the facial nucleus disappears, lying 
farther ventrally, nearer the floor of the fourth ventricle (Pl. 77, Fig. 173). 
It is composed of little multipolar cells on which a part of the fibers of the 
sensory trigeminal root coming from the Gasserian ganglion end, though the 
greater part of the latter bend down at right angles to form a nerve tract 
running far caudally into the spinal cord, the spinal root of the tri- 
geminus, On it lies a mass that forms the direct caudal continuation of the 
sensory nucleus of the trigeminus and is composed of similar cells. The fibers 
of the spinal root end on the cells of this spinal nucleus of the trigeminus. If 
we follow this root toward the cauda we find it at first to the lateral side of the 
facial nucleus, then ventral to the corpus restiforme, dorsal and somewhat 
lateral from the lower olive, and passed through at this place by the median 
root of the acusticus, and by the fibers of the glossopharyngeus and vagus. 
Later the cellular mass lying medially from the root lightens up very much and 
finally passes into the substantia gelatinosa Rolandi of the posterior horn. 201 
The fibers of the trigeminus may be traced down into the region of the second 
cervical nerves. 
The third nucleus of the trigeminus is motor, lies medially from the sen- 
sory, and is composed of large multipolar cells, the neurites of which form the 
motor root of the trigeminus. 
The fourth and last nucleus of this nerve is composed of large, roundish or 
ovoid, unipolar cells that are situated in the midbrain, laterally from the aque- 
duct, on the outer margin of the central gray matter. The cells are fairly sepa- 
rate and are frequently arranged in rows. The large cell process soon receives 
a medullary sheath and the individual fibers come together so that soon they 
form a fibrous cord, the mesencephalic root of the trigeminus, which 
adjoins the cells on the medial side. The farther caudally it goes, the more 
it turns ventrally until it finally unites with the motor root. It has not yet 
been determined with certainty whether we have to deal here with a motor root, 
or whether the cells are to be looked upon as sensory cerebral ganglion cells 
remaining in the central organ. 
The nuclei of the pons varolii form masses of medium sized multipolar 
cells scattered about between the fibers of the pons. These fibers, which come 
down from the cerebrum, end on them, and the nearby pyramids send numerous 
collaterals to them. The neurites coming from these nuclei form as a whole 
the brachium pontis, which radiates into the cerebellum (Pl. 77, Fig. 173). 
b. The Cortex of the Cerebellum. 
A white medullary ridge and a gray cortex may be differentiated even 
macroscopically in the cross section of every cerebellar convolution. The 
medullary ridge, or lamina medullaris, is made up of medullated nerve fibers 
which either leave the cortex as the neurites of its cells, or come from cells 
outside of the cortex and enter it to end on its cells (Pl. 78, Fig. 174). 
The line of demarcation between the medulla and the cortex is not very 
sharp. The fibers of the medulla radiate in parallel lines into the cortex to 
form a plexus in about its middle, beyond which only a few pass externally. 
Three layers may readily be distinguished in the cortex. Next outward from 
the medulla is the granular layer, the stratum granulosum, which is 
very broad at the top of the convolution, but more narrow in the deeper part. 
On the outer surface of this is the stratum gangliosum, a single layer of 
large nerve cells that send their dendrites into the molecular layer, or 
stratum cinereum, which is of even thickness everywhere and bounds the 
cerebellar cortex externally (Pl. 81, Fig. 179). 
The granular layer is characterized by little, very densely packed gran- 
ular cells, of which only the nuclei are to be seen when ordinary methods of 
staining are employed. They are roundish or angular cells measuring at most 
10 [J, that send out from three to five short, tortuous dendrites, which split 
up into several clawlike terminal branches. The dendrite grasps with each 
terminal claw the body of a neighboring granular cell. The slender neurite 
passes vertically toward the surface into the molecular layer, without receiving 
a medullary sheath, where it divides like a T. Both branches take a course 202 
parallel to the surface and are known here as parallel fibers. We know noth- 
ing more about them. 
Between the little granular cells are scattered the large granular cells, 
measuring up to 40 p, which usually have rather elongated cell bodies and very 
many, much branched dendrites. The very slender neurite splits soon after 
leaving the cell, within the granular layer, into very minute fibrils, hence we 
have here cells of Golgi's second type. 
The cells of the stratum gangliosum we have already become acquainted 
with as Purkinje's cells. They have large pyriform bodies, measuring 70 
by 40 p, the bases of which rest on the granular layer. One or often two 
dendrites extend from these into the molecular layer, branch in a plane vertical 
to the convolution, and form a very characteristic ramification through the en- 
tire thickness of the layer. If the section is made parallel to the course of the 
convolution the tree thus formed may be seen as a relatively small formation 
measuring 50 to 60 p. The cell body presents, especially in its basal portion, 
well developed, coarse Nissl's clods, and encloses a vesicular, large nucleus with 
a large nucleolus. The neurite comes from the base of the cell, passes through 
the granular layer, receives a medullary sheath, and leaves the convolution 
through the medullary layer. 
The most important cells of the molecular layer are the basket cells. 
These range in size from small to medium, are multipolar, and are found 
chiefly in the deeper layers. Their dendrites branch in the molecular layer, 
their neurites become medullatcd fibers which run vertically to the direction 
of the convolution and parallel to its surface, just above the stratum gangliosum, 
and give off here and there a collateral at a right angle to a Purkinje's cell, 
where it forms a terminal basket that closely grasps the body of each cell. 
It follows from this description that the centrifugal fibers which pass 
out from the cortex of the cerebellum are the neurites of Purkinje's cells. They 
all terminate on the cells of the nuclei of the cerebellum, which are yet to be 
described. 
The centripetal fibers that enter the cortex of the cerebellum come 
partly from the spinal cord, the direct cerebellar tract, partly from the me- 
dulla oblongata, the olivo-cerebellar, and the vestibulo-cerebellar tracts, partly 
from the nuclei in the pons, the ponto-cercbcllar tract. 
Numerous medullatcd fibers pass through the granular layer from the 
medullary layer to the cerebellar cortex to form a dense superficial plexus on 
the inner surface of the stratum gangliosum, from which in turn proceed fibers 
that for the most part soon lose their medulla in the molecular layer. In addi- 
tion many so-called moss fibers split up into twigs that end between the 
cells of the granular layer. A final sort of centripetal medullary fibers are 
known as the climbing, which, after they have passed through the granular 
layer, climb up on the bodies and processes of Purkinje's cells, making mani- 
fold loops about them. 
The neuroglia of the cortex of the cerebellum is fairly well de- 
veloped and is represented chiefly by cells lying between and beneath those of 
Purkinje. Their bodies contain a lipoid, their processes and fibers radiate 
vertically to the surface to end here in footlike expansions. As all these feet 203 
adjoin they form a membrana limitans over the outer surface of the cere- 
bellum. The glia fibers coming from ordinary astrocytes are mainly vertical, 
with a few parallel to the surface. 
The cerebellar cortex receives its arteries from the two cerebelli in- 
feriores and the cerebelli superior, the branches of which enter between the 
convolutions and into the latter, where they form a dense capillary plexus in 
the stratum gangliosum and in the granular layer. The capillary meshes run 
longitudinally in the medulla, corresponding to the course of the fibers. The 
venous blood is carried away by the inferior and superior cerebellar veins. 
c. The Nuclei of the Cerebellum 
The cortex of the cerebellum contains several gray masses, the most im- 
portant. of which are the corpus dentatum and the nucleus tecti. 
The corpus dentatum strikingly resembles the lower olive in its con- 
figuration, and like it is a many folded lamina of gray matter with a median 
hilum. The medially placed gray masses of the nucleus globosus and of 
the embolus are nothing else than broken off portions of the corpus dentatum. 
The cells are multipolar. The fibers that enter the nucleus are those already 
known to us as the neurites of Purkinje's cells, together with others coming from 
the olive; they form very dense terminal baskets about the cells. A neurite 
originates from each cell, runs as a medullated fiber in the cerebellar peduncle 
to the red nucleus, and takes part in the formation of the tractus cerebelli 
tegmentalis mesencephali. 
The nucleus tecti lies in the vermiform process just above the roof of the 
ventricle, and consists of materially larger cells than those of the corpus 
dentatum. These cells are spherical and contain a yellow pigment. Their 
neurites enter the medulla oblongata, form the tractus fastigio-bulbaris, 
and extend to the cells of the substantia reticularis as the fibrae arcuatae 
internee. 
d. The Nuclei of the Midbrain and Interbrain 
The nucleus of the trochlearis lies close to the middle line beneath the 
floor of the aqueduct at the level of the inferior corpora quadrigemina. The 
cells of which it is composed are of medium size and multipolar, their neurites 
leave the nucleus as the root fibers of the nerve, take a curved course laterally 
and cerebrally, bend toward the median line in the roof of the aqueduct, and 
finally reach the velum medullare anterius. Here they decussate with those of 
the other side and run laterally. Many fibers that come chiefly from the fas- 
ciculus lonffitudinalis medialis end on the cells of the nucleus, 
o 
The nuclei of the oculomotor nerve form the direct cerebral continua- 
tion of the preceding, and therefore lie in the region of the anterior corpora 
quadrigemina, ventrally from the floor of the aqueduct, and dorsally from the 
median longitudinal fasciculus. They are commonly known as the lateral, 
the median, and the Westphal-Edinger nuclei, which may be broken 
up into large celled and small celled subgroups. We find strikingly small, and 
medium sized to large multipolar cells, which are also to be differentiated by 
the thickness of their neurites, that pass through the fasciculus longitudinalis 204 
medialis, enter the trigonum interpedunculare, and suffer a partial decussa- 
tion. The decussating fibers come chiefly from the lateral nucleus. 
The posterior corpus quadrigeminum consists of an oval gray mass, 
its nucleus enveloped by a mass of medullated fibers, the stratum 
zonale. The nucleus contains numerous nerve cells of various sizes, among 
which are many of Golgi's second type. The neurites of the rest of the cells 
leave the nucleus, turn medially and ventrally, and, after crossing in Mey- 
nert's decussation, run as the tractus tecto-spinalis, ventrally from the 
fasciculus longitudinalis medialis in the medulla oblongata and the upper part 
of the cervical portion of the spinal cord. The stratum zonale arises through 
the splitting of the lateral lemniscus caudoventrally on the posterior corpus 
quadrigeminum, so as to embrace it chiefly on the lateral side, and to send its 
fibers into the nucleus, where they terminate on the cells of the latter. These 
fibers come principally from the upper olive, from the nucleus of the lateral 
lemniscus situated laterally from this, and from the striae medullares, and there- 
fore represent the tractus acustico-tectalis, a part of the cerebral auditory 
tract from the lateral auditory root. 
The anterior corpus quadrigeminum has on the other hand im- 
portant relations to the visual tract. Four different zones may be seen on 
cross section. Most externally is a layer of medullated nerve fibers; 
beneath this is a broad layer of gray matter, the stratum cinereum, con- 
sisting of little multipolar nerve cells on which the fibers just mentioned end. 
Then follows a layer of larger cells and medullated fibers, the stratum opti- 
cum the cells of which belong for the most part to Golgi's second type. The 
medullated fibers that end on them are fibers of the optic tract which arrive 
through the superior peduncle. The fourth and last layer, the inferior 
lemniscus, also contain multipolar cells and medullated fibers. The latter 
are what remain of the fibers of the lateral lemniscus, the most of which 
end in the posterior corpus quadrigeminum as we have seen. To these are to 
be added a part of the middle lemniscus, which brings fibers from the nu- 
clei of the posterior columns and sensory fibers from the trigeminus. Finally 
here also run neurites from the cells of the anterior corpus quadrigeminum, 
part of which reach the tractus tecto-spinalis through the above mentioned 
Meynert's decussation, while part pass through the peduncle of the anterior 
corpus quadrigeminum to the cortex of the occipital brain. 
The red nucleus lies on each side of the middle line ventrally from the 
aqueduct and the fasciculus longitudinalis medialis. It is, as we have seen 
passed through by the root fibers of the oculomotorius and forms the terminal 
center for the neurites of the cells of the corpus dentatum of the cerebellum, 
but in addition it receives a large tract of fibers from the cortex of the fore- 
brain, which end here on little multipolar nerve cells. Together with the latter 
are a much smaller number of large multipolar nerve cells, the neurites of 
which form the tractus rubro-spinalis, that decussates very soon, runs down 
dorsolaterally from the lemniscus into the spinal cord, where we have learned 
to know it as Monakow's bundle in the lateral column. 
The corpora geniculata are connected with two different tracts. A 
part of the fibers of the optic nerve terminate in the lateral corpus geni- 205 
Culatum, while the median one is connected with the auditory nerve. 
The lateral corpus geniculatum consists of varying layers of gray mat- 
ter and medullated nerve fibers. The former contains large and small, 
usually triangular nerve cells on which the optic fibers split up into terminal 
arborizations. All of the optic fibers do not end here, but a part of them enter 
the thalamus, and yet another part pass into the anterior corpus quadrige- 
minum. The neurites from the cells of the lateral corpus geniculatum pass 
through the fasciculus longitudinalis inferior to the cortical visual center, 
situated in the cortex of the occipital lobe. 
The median corpus geniculatum, placed close to the preceding, is 
composed of medium sized, multipolar nerve cells, the neurites of which run 
to the temporal lobes. The fibers of the lateral lemniscus that do not 
enter the posterior corpus quadrigeminum terminate on them, and a part of 
the optic fibers enter the nucleus. 
Quite a number of separate nuclei may be seen in the thalamus opticus, 
covered by the central gray matter, but will not be described here. They 
are separated by lamin® of medullated nerve fibers that form the upper sur- 
face of the stratum zonale, and are separated laterally from the internal 
capsule by the latticed layer. The cells of the nuclei are of medium size, 
multipolar, and have either long radiating, or short fasciculate dendrites. The 
former are called stellate, the latter bush cells. On the cells of the thalamus 
end sensory fibers of the cranial nerves, fibers from the lemniscus, 
fibers from the red nucleus, and particularly fibers coming from the cere- 
bral cortex and known as a whole as the corona radiata. The neurites of 
the cells that compose the various nuclei leave the thalamus in like manner to 
go to the cortex of the cerebrum. 
e. The Hypophysis 
The hypophysis, or pituitary gland, has a double origin. Its anterior 
principal portion is a derivative of the epidermis and appears in embryos about 
3 mm long as a pouchlike diverticulum of the epithelium of the primitive 
stomodaeum, the so-called Rathke's pouch. A little epithelial vesicle is formed 
by the dilatation of its distal end, which for some time remains connected with 
the cavity of the mouth by a duct lined with epithelium. After the appearance 
of the cartilaginous base of the skull this duct disappears. The vesicle settles 
on the base of the interbrain and becomes metamorphosed through proliferation 
of its epithelium into a massive, cellular body with two lateral horns, the cells 
of which are arranged in much twisted trabecula. The second, subordinate 
part of the hypophysis is derived from the floor of the interbrain, where 
a diverticulum of the third ventricle is formed somewhat later than Rathke's 
pouch, the infundibulum. From the distal end of this a solid cone extends, 
approximates the posterior wall of the hypophyseal sac, and is embraced be- 
tween the two horns. This forms the little posterior lobe, the other the 
large anterior lobe of the organ. The anterior and posterior lobes are 
separated in youth by a narrow, cleftlike space, which is what remains of the 
former cavity of the hypophyseal sac. It is lined with ciliated epithelium and 
disappears at the same time of puberty. 206 
The anterior lobe of the hypophysis is covered by a connective tissue 
capsule, which appertains to the dura and continues over the posterior lobe. 
Connective tissue from this capsule enters the lobes with the blood vessels and 
sends out fine fibers that embrace the cellular trabecula. The cells lie 
close together in the trabecula and are of two different kinds (Pl. 41, Fig. 102). 
The majority of them are characterized by the fact that they contain numerous 
granules which stain intensively with acid dyes, and they are consequently 
called chrornophilic cells. The others have bodies that take only a little 
stain and are therefore chromophohic cells. But it is quite probable that 
these two varieties are only cells in different stages of secretion. During preg- 
nancy the organ is considerably enlarged, chiefly because of a multiplication of 
the chromophobic cells. 
In the posterior part of the anterior lobe many cavities appear between 
the cells, which are arranged about them like epithelium. The cavity itself is 
filled with colloid, so that the organ has a marked resemblance to the follicles 
of the thyroid gland. Farther back, in the former cavity of the hypophyseal 
vesicle, vesicular and cystic formations are met with which do not contain 
colloid. This part of the parenchyma interpolated between the anterior and 
posterior lobes is also known as the pars intermedia. 
The smaller posterior lobe of the hypophysis, which has also been named, 
on account of its derivation, the neurohypophysis, has scarcely any cells. 
It consists of connective tissue and neuroglia, and contains brown pigmented 
granules in the meshes of the connective tissue. 
The anterior lobe of the hypophysis is a gland that elaborates an internal 
secretion which is of great importance to the growth of the skeleton. Growth 
is greatly inhibited when it is removed. A pathological enlargement of the 
anterior lobe causes an abnormal development of the hands and feet, as well as 
of the soft parts and bones of the face, producing the condition known as 
acromegaly. The hypophysis also secretes pituitrin, which acts to increase 
the blood pressure. 
The arteries of the hypophysis come from the internal carotid and the 
circulus arteriosus, and form in the anterior lobe a very rich, sinusoidal capillary 
plexus closely adjoining the cellular trabecula. The blood vessels are very 
sparse in the posterior lobe. The veins empty into the cavernous sinus. 
The nerves of the hypophysis are from two different sources. Some enter 
the posterior lobe as neurites of nerve cells situated in the wall of the in- 
fundibulum, pass through it and reach the anterior lobe. There are also many 
sympathetic fibers from the plexus of the pia in the anterior lobe. The nerves 
form plexuses in the latter about all sides of the cellular trabecula. 
f. The Epiphysis 
Three successive diverticula are to be found in the human embryo in the 
roof of the interbrain, the pineal cushion, the parietal vesicle, and the epiphysis. 
While the first two undergo complete involution, the epiphysis, which is at first 
vesicular, becomes a solid body through proliferation of its epithelium, and is 
connected with the stria medullaris through the habenula. 
The epiphysis is enclosed in a connective tissue capsule that sends 207 
numerous anastomosing septa into the organ, which divide the parenchyma into 
larger and smaller lobules. Within these septa are found neuroglia fibers 
and cells together with connective tissue fibers. Each lobule is composed of 
densely packed cells, the bodies of which contain granulations that stain. Still 
other neuroglia cells are interposed between these glandular cells, and certain 
branched ones have been described and pointed out as nerve cells. After the 
epiphysis has reached the acme of its development, in about the seventeenth 
year of life, signs of involution appear, just as in the thymus, which manifest 
themselves first by the appearance of spherical bodies in the nuclei of the cells. 
The connective tissue increases at the cost of the parenchyma, and lime salts 
are deposited in the parenchymatous cells, forming the so-called brain sand, 
little concretions of lime in which a very distinct concentric lamination may 
be seen. 
Little is known as yet concerning the function of the epiphysis, but verv re- 
cent clinical and experimental observations indicate that it is an organ that 
elaborates an internal secretion which exerts an influence over the development 
of the germinal glands and the secondary sexual characteristics. 
The epiphysis is abundantly supplied with blood vessels, which come 
from the tela choroidea superior and form capillary loops about the cells. A 
yellow brown pigment is deposited in their vicinity. 
The epiphysis also contains numerous nerve fibers that enter the organ 
with the blood vessels and form plexuses. 
g. The Cerebral Cortex 
Although the structure of the cortex of the cerebellum is quite uniform in 
all of its parts, that of the cortex of the cerebrum varies a great deal in differ- 
ent parts of the hemisphere, producing many modifications that cannot be taken 
into account here. 
The cerebral cortex has a maximum thickness of 5 mm, and blends inter- 
nally with the medullary substance, from which it is not separated by a 
sharply defined line (Pl. 78, Fig. 175). Six layers of the cells composing the 
cortex are commonly differentiated from their arrangement and structure. 
Most externally is the molecular layer composed of little nerve cells together 
with many glia cells. Among the former are the so-called Cajal's cells, 
slender, spindle-shaped formations that have their longitudinal axes placed 
parallel to the surface and send their processes in the same direction (Pl. 82, 
Fig. 180). These cells, which are particularly numerous in the embryonal 
cortex, seem to have no neurites. The second layer is that of the small 
pyramidal cells, which are small, measuring only 10 p, and do not always 
have a typical pyramidal form, but may be polyhedral, or spindle-shaped. They 
lie very close together, and the deeper we go the larger they become, and the 
more they assume the typical pyramidal shape. We may thus distinguish a 
layer of medium sized, and then a layer of large pyramidal cells be- 
neath the layer of small ones, the domain of which reaches beyond the middle 
of the cortex. These cells are conical, with their bases toward the medulla, 
their apices pointing toward the surface, and they are of all possible sizes up 
to 100 p long and 80 to 40 [J in diameter at the base. The cell body is pro- 208 
longed toward the cortex into the process at the apex, which, after giving off 
lateral twigs horizontally, breaks up in the molecular layer into several branches 
running perpendicularly to the surface. These branches bend suddenly at a 
right angle in the most superficial portion of the molecular layer, run parallel 
to the surface, and gradually become lost. Numerous lateral dendrites pass 
out from the entire body of the pyramidal cell, bifurcate continuously, and 
become lost in the neighborhood; those coming from the base are particularly 
strong. The neurite arises from the base, takes a straight course toward the 
medulla, is invested with a medullary sheath soon after it leaves the cell, and 
enters the medulla as a medullated, centrifugal fibei' giving off numerous col- 
laterals on the way. Between the pyramidal cells are many of Golgi's second 
type, which are especially common in the region of the small pyramids. Next 
to the large pyramids comes the granular layer. Here we have exclusively 
little polyhedral cells, the neurites of which behave the same as those of the 
pyramidal cells. The following layer is characterized by very large pyramidal 
cells, and is known as the layer of giant pyramidal cells, or as the gan- 
glion cell layer. Sixth and last is the spindle cell layer, where we find 
polyhedral and spindle-shaped cells with neurites passing into the medulla, and 
also little, polyhedral, Martinotti's cells, the neurites of which pass up into the 
molecular layer to become tangential fibers. 
The outermost layer of the cortex is quite free from medullated fibers 
(Pl. 78, Fig. 175). In the molecular layer there are very many tangential 
fibers running parallel to the surface, but not easily demonstrable for the most 
part, which cross internally in all directions. It is practically certain that we 
have here, aside from the just mentioned neurites of Martinotti's cells, centri- 
petal fibers that enter from the medulla and end in the cortex. Within the 
othei' regions of the cortex we find a fairly uniform felt of medullated nerve 
fibers, in which parallel lines of fibers placed vertically to the surface are met 
with in the region of the medium sized pyramids, that become stronger as they 
go deeper, and radiate into the fibrous mass of the medulla. These so-called 
radiating bundles are nothing else than the neurites of the cortical cells, 
particularly of the pyramidal, united into bundles. Two very striking stride 
run parallel to the surface. In one of them are many tangential medullated 
fibers, while in the other the fibrous plexus shows a material thickening. These 
are the two Baillarger's stripes. The outer of the two lies in the region 
of the large pyramidal cells, the inner one, more weakly developed, as a rule, 
in the region of the giant pyramidal cells. The tangential fibers running in 
these stripes are chiefly collaterals of the neurites of the pyramidal cells, but 
there end here, just as in the molecular layer, many centripetal fibers ascending 
out of the medulla. 
The neuroglia of the gray matter of the brain is best developed on the sur- 
face, where there is a layer of glia cells that send their processes deeply into 
the tissue, and we find additional glia cells scattered through the entire cortex. 
The glia fibers form a dense felt over the cortex that varies in thickness, from 
which fibers radiate vertically into the tissue. These become fewer the deeper 
they go, although they again become very numerous in the medulla. 
The arteries of the cortex form a wide meshed plexus in the pia, from 209 
which branches pass vertically into the tissue. As these do not anastomose 
they are called terminal arteries. They penetrate down to the medullary 
substance, where they break up into capillaries, having given off on the way 
many lateral branches that supply capillaries to the cortex, forming a plexus 
that is closest about the pyramidal cells. In addition to these long arteries 
that pass through to the medulla there are also short ones that break up into 
capillaries in the gray cortex. The very thin walled and valveless veins of 
the brain follow practically the same intracortical course as the arteries. They 
start in the medulla and as they pass through the cortex they collect from it 
its venous blood. 
h. The Olfactory Bulb 
The olfactory bulb receives the fibers of the olfactory nerve that 
pass through the apertures in the cribriform plate of the ethmoid. This bulb 
is developed to a much greater extent in animals gifted with a sharp sense of 
smell than it is in man, and its layers are sharply differentiated in them. In 
such animals it has a cavity, the ventriculus bulbi olfactorii, which is con- 
nected with the lateral ventricle of the brain, but in man this ventricle is ob- 
literated and replaced by a mass of glia. It is surrounded by the medullary 
substance, medullary nerve fibers that continue backward in the tractus 
olfactorius. Ventrally from the medulla lies the granular layer, consisting 
of bundles of medullated nerve fibers much interwoven together, with little nerve 
cells lying between them. Each of the latter has a neurite that penetrates into 
the medulla, and several dendrites, one of which always enters the succeeding 
layer and branches between its cells. This gelatinous layer, which adjoins 
the granular ventrally, is characterized by large nerve cells that are pyramidal 
in shape, the mitral cells, the bases of which are directed ventrally, the 
apices dorsally. From the apex comes the neurite, which, after giving off 
several collaterals, turns toward the brain into the medulla. The strong den- 
drites arising from the base run ventrally and enter the glomeruli that are 
about to be described, where they split up into many fine fibrils. This layer 
contains smaller nerve cells of Gogli's second type. Next is the glomerulus 
layer, formed of the macroscopically visible glomeruli olfactorii. These 
are spherical corpuscles, measuring about 2 mm, that consist of a very dense 
felt of very minute nonmedullated nerve fibrils. This felt is formed from the 
dendrites of the mitral cells, as has been said, with the addition that every 
fiber from the following, most ventral layer enters a glomerulus and breaks up 
into very minute fibrils that come into intimate contact with the preceding. 
This most ventral layer lies directly on the cribriform plate of the ethmoid, 
and its fibers are those of the olfactory nerve, which thus end in the 
glomeruli olfactorii. 210 
4. THE MEMBRANES OF THE CENTRAL NERVOUS 
SYSTEM 
The spinal cord and brain are encapsuled by three connective tissue mem- 
branes, the dura mater, the arachnoid, and the pia mater. 
The dura, the outermost of these, has an outer and an inner layer. The 
former is attached to the bone by numerous connective tissue fibers and thus 
forms the internal periosteum of the skull and of the vertebral canal, while the 
inner adjoins the arachnoid. Both layers consist of connective tissue fibers 
united into thick bundles that lie close together, and take a uniform course in 
each, though the direction differs in the two. Between the bundles of con- 
nective tissue are elastic fibers that come together to form an outer elastic 
limiting membrane in the outer layer, and an inner elastic limiting membrane in 
the inner. The two layers are connected by a thin lamina of loose connective 
tissue. At the place where the venous sinus is formed the two layers separate 
and enclose the sinus between them. Several layers of fibroblasts, which cor- 
respond to the cells of the fibroclastica of the periosteum, lie outside of the 
external layer, wThile toward the arachnoid the connective tissue of the dura is 
covered by an epithelium, a layer of flat cells, the margins of which may be 
brought out easily by the silver stain. 
The dura is connected with the arachnoid by sparse trabecula? of con- 
nective tissue, and forms with it a sheath about all the nerves that leave the 
central organ. They are so closely approximated in most places that the 
subdural space between them is simply a capillary cleft, but as the outer 
surface of the arachnoid also is covered with epithelium this space has an 
epithelial lining everywhere and is a cavity filled with cerebrospinal fluid. 
It is in open connection with the lymph spaces of the olfactory mucous mem- 
brane, the internal ear, and the eyeball, as it extends with the dura upon the 
olfactory, auditory, and optic nerves. 
The arachnoid is considerably thinner than the dura and has a much looser 
structure. Its bundles of connective tissue form a loose network which is some- 
what thickened both externally and internally. All of the connective tissue 
bundles of the arachnoid are surrounded by fibers that weave about them and 
are covered by epithelial cells. It is connected with the pia by similar, very 
numerous bundles of connective tissue that often are very long, so that an ex- 
tensive cavity lined with epithelium, the subarachnoidal space, exists be- 
tween the two membranes. This space communicates through the foramen of 
Magendie and the aperturae laterales ventriculi with the system of cavities in 
the brain and spinal cord, and like them is filled with cerebrospinal fluid. 
In many places the arachnoid sends villuslike processes into the venous 
lacuna? of the dura, that will be described later. These are the arachnoidal 
villi, or Pacchionian bodies, which have the same structure as the 
arachnoid. 
While the dura and the arachnoid pass smoothly over all of the sulci of the 
brain and spinal cord, the pia mater enters each depression and covers uni- 
formly all of the irregularities of the surface. It also sends processes into 
the brain with the blood vessels to form lymph sheaths about the latter. The 211 
pia consists of a loose connective tissue, is denser and stronger on the spinal 
cord than on the brain, and contains very many connective tissue cells, which 
frequently have pigment granules in their bodies. 
The telae choroideae and plexus choroidei are villuslike diverticula 
of the pia into cavities of the brain. Some arachnoidal tissue also always enters 
into them. In the interior of each arachnoidal villus is a little vascular tree. 
Toward the ventricle they are covered by the brain wall, which is reduced here 
to a simple cuboidal epithelium separated from the connective tissue by a mem- 
brana propria. Filamentous mitochondria have very recently been demon- 
strated in the cells, which break down into minute drops of secretion that are 
extruded from the cells. Lipoid and pigment granules also are met with in the 
latter. 
The secretion produced by the cells is the cerebrospinal fluid, a limpid, 
very slightly alkaline liquid that contains at most only 1% of solid constituents, 
among which are albumose, glucose, and urea. 
The blood vessels of the cerebral membranes are confined to the dura 
and pia, the arachnoid is not vascular. The arteries of the dura mater are 
very abundant, yet send by far the most of the blood they contain to the diploe 
of the skull. Two capillary plexuses are formed in the dura itself, one in the 
inner, the other in the outer layer. The veins form similar plexuses and either 
empty their blood into the sinuses, or accompany the arteries. The dura also 
has in many places, but especially near the sinus longitudinalis superior, cavi- 
ties lined with epithelium that are interpolated between the meningeal veins and 
the sinus, and are known as the venous lacunae. They are in open connec- 
tion with the veins of the brain and of the diploe. 
The pia abounds in capillary plexuses that originate from vessels coming 
out of the brain and spinal cord. 
The nerves of the dura are sensory branches of the trigeminus, together 
with fibers that enter with the vessels. They form a wide meshed plexus from 
■which twigs extend with free ends. Nerves have not been demonstrated with 
certainty in the arachnoid, although the pia is fairly well supplied with them. 
Sympathetic nerves enter it with the vessels, while sensory branches detach 
themselves from the outgoing nerve roots and unite with them. 
5. THE CEREBRAL GANGLIA 
The ganglia of the sensory cranial nerves generally resemble in structure 
the spinal ganglia, yet in many of them bipolar cells are found together with 
the typical unipolar ones, and they form the great majority in the ganglia of 
the auditory nerve (Pl. 29, Fig. 75). Above all the cerebral ganglia are rich 
in the fenestrated forms described in the spinal ganglia, and lobed cell processes 
are found more often in them. Moreover most of the cerebral ganglia contain 
larger or smaller numbers of the sympathetic cells about to be described. 
6. THE SYMPATHETIC GANGLIA 
The coarse structure of the sympathetic ganglia is similar to that of the 
spinal. Each ganglion is enclosed in a connective tissue capsule, in which 212 
fat cells are often present, and contain sympathetic nerve cells, which 
differ in many ways from the nerve cells of the spinal and cerebral ganglia (Pl. 
84, Fig. 184), and are split up into larger and smaller complexes by bundles of 
nerve fibers that pass through the ganglion (Pl. 84, Fig. 183). The most im- 
portant point in which the sympathetic differs from other nerve cells is that 
its neurite becomes in many cases, though not in all, a nonmedullated nerve 
fiber, i.e., one which is invested only with a sheath of Schwann. This is not a 
characteristic of all sympathetic cells, for there are some that have neurites 
invested with medullary sheaths. 
Each sympathetic nerve cell has a capsule. The cell body contains 
one nucleus, rarely two nuclei, which scarcely differ from those of other nerve 
cells. Nissl's clods are found in them and frequently present an arrangement 
that is quite characteristic, the coarser clods lying in the periphery of the cell, 
while those in the interior are fine and dustlike. 
Two principal types may be differentiated from their shapes, motor and 
sensory cells. The motor cells are of a roundish or irregular form. The 
dendrites are strong, but pretty short. The neurite leaves the ganglion as a 
nonmedullated fiber and terminates on a smooth muscle fiber, though it is quite 
possible that it may pass to another sympathetic cell and that it is the neurite 
of this last that reaches the muscle fiber. 
The sensory sympathetic nerve cell is rather more elongated and is 
characterized by very long dendrites which either branch in the same ganglion 
or pass as sympathetic fibers into the epithelium of the viscera, to end there 
as afferent, sensory, sympathetic fibers. The neurite is usually invested with 
a thin medullary sheath, leaves the ganglion, and forms a terminal basket about 
a sympathetic cell in another ganglion. 
As has been mentioned in another place, many medullated fibers of the 
anterior roots pass through the rami communicantes into the spinal ganglia, 
and also into the sympathetic ganglia of the head. These afferent fibers, 
which come from the cells of the lateral horns, or the motor cells of the cranial 
nerves, form terminal baskets about the motor sympathetic cells. Yet such 
fibers may also pass through the ganglia without interruption and accompany 
the sympathetic fibers to their endings. Other afferent fibers are sensory; 
they are the dendrites of the sympathetic cells and conduct to the latter 
stimuli that affect the epithelium of the gastrointestinal mucous membrane. 
Besides the ganglia of the great gangliated cord, and the partes abdominalis 
et pelvina sympathici, the ciliary, sphenopalatine, otic and submaxillary ganglia 
are composed of sympathetic cells. Nerve cells of the sympathetic type are also 
found in true cerebral ganglia, as in the ganglion jugulare. 
7. THE PERIPHERAL NERVES 
Every peripheral nerve is invested with a connective tissue sheath, com- 
parable to the perimysium externum of the muscle, which is called the epi- 
neurium (Pl. 72, Fig. 166). The bundles of connective tissue in it are 
arranged rather loosely and irregularly, are accompanied by many elastic 
fibers, and always include large masses of fat cells. The epineurium everywhere 213 
sends broad longitudinal septa into the nerve, which anastomose and divide it 
into many secondary nerve bundles. Each of these is invested externally 
with a special thin sheath which is easily distinguished from the epineurium 
and is known as the perineural sheath. It likewise consists of connective 
tissue, the very slender bundles of which take part in the formation of fine 
plates concentrically surrounding the secondary nerve bundles, together with 
sparse elastic fibers. Two such plates are always separated by a continuous 
layer of fiat, polygonal, connective tissue cells that lie close together like an 
epithelium. It happens that through this abundance of cells, or of proto- 
plasm, the perineural sheath is quite plainly distinguished from the epineurium 
by its stain (Pl. 71, Fig. 165). 
The perineural sheath in turn sends longitudinal septa into the secondary 
nerve bundles and divides each mass of fibers into the primary nerve 
bundles. This connective tissue, situated within the secondary bundles, is 
called the endoneurium, but this name is also applied to the connective 
tissue that separates the nerve fibers within the primary bundle from one 
another. The endoneural sheath is distinguished from the endoneurium 
in the same way as the perineural sheath is told from the epineurium. 
The structure and dimensions of the nerve fibers that compose the pri- 
mary bundles have already been thoroughly discussed. Those of the cerebro- 
spinal nerves are for the most part medullated and invested with sheaths of 
Schwann, yet nonmedullated fibers are always present. In the same way 
the sympathetic nerves contain medullated along with their nonmedullated 
fibers. 
In its advance toward the periphery the nerve breaks up into separate 
secondary bundles, which later divide again, so that each minute nerve twig is 
always provided with a perineural sheath which invests it completely. As soon 
as the bundle has separated into its individual fibers each of the latter becomes 
invested by an endoneural sheath, in addition to its sheath of Schwann, which 
it loses with its medullary sheath. In the case of sensory fibers that enter 
encapsulated corpuscles this sheath blends with the capsule. In like manner 
it continues into the spinal ganglia and passes over into the capsule of the cell. 
The blood vessels of the nerves, which come from the arterias comites, 
form a large plexus in the epineurium from which twigs perforate the peri- 
neural sheath and break up in the endoneurium into a longitudinal system of 
capillary plexuses. 
Lymphatics are wanting in the nerves. The lymph circulates in them 
within the clefts formed by the connective tissue of the endoneurium in the 
secondary bundles, which are closed externally by the perineural sheath. 
Special nerves, the nervi nervorum, have been demonstrated within the 
epineurium, where they end in free arborizations. IX. THE ORGANS OF SENSE 
1. THE EYE 
The eye is a very complicated structure, to the formation of which the 
nerve plate, the ectoderm and the mesenchyma contribute alike. The percep- 
tive apparatus comes from the nerve plate, the dioptric apparatus 
from the ectoderm, the protective capsule and the extrinsic muscles of 
the eye from the mesenchyma. The first traces of the rudimentary eye 
appear very early in the embryo in the form of a paired optic depression 
on each side in the anterior part of the brain plate. When the latter forms 
the cephalic portion of the medullary tube the depression appears like a 
vesicular diverticulum from its lateral surface, from which develops the optic 
vesicle, connected with the cerebral ventricle by a short canal, the optic 
pedicle. Looked at from the side the vesicle appears flat, rather than like 
a pouch. It now undergoes an uneven development, as its margins grow faster 
than its center, so that in time a cup, or the secondary optic vesicle, is 
formed, the distal layer of which thickens greatly into the cavity, and thus 
gradually approximates the thin proximal layer. Growth is inhibited en- 
tirely in the middle of the ventral wall of the cup, so that the latter presents 
a ventral ocular cleft, which continues as a groove on the optic pedicle. 
In this groove lies the central artery of the retina, which continues through 
the ocular cleft into the interior of the secondary optic vesicle as the hyaloid 
artery. The ocular cleft is closed in finally by adhesion of its walls, together 
with the groove in the distal part of the optic pedicle, and then the central 
artery of the retina enters the optic pedicle obliquely and comes to lie in the 
axis of its distal portion with its continuation, the hyaloid artery, passing 
through the axis of the secondary optic vesicle. 
Simultaneously with these changes the rudiment of the dioptric apparatus 
appears. A zone of thickened epithelium, the lens plate, may be seen very 
early in the epidermis lying opposite the distal wall of the optic vesicle; it 
dips into the lenticular fossa, and this deepens to form the lenticular 
saccule. When separated this becomes the lenticular vesicle, which is 
situated just beneath the epidermis with its proximal wall at first closely 
adjoining the distal, thickened wall of the secondary optic vesicle, but grad- 
ually these two separate, leaving between them the cavity of the vitreous. 
The hyaloid artery passes through this to the proximal wall of the lenticular 
vesicle. All parts of the eye develop from this primitive optic vesicle with 
its pedicle, the lenticular sac, and the adjacent parts of the mesenchyma 
and epidermis, as will be shown in detail. 
a. The Betina 
The retina is the most primitive and oldest part of the entire eye. It 
originates from the epithelial wall of the primitive optic vesicle and is there- 
214 215 
fore really a part of the brain. This wall consists, as we have seen, of two 
layers that blend at the margin of the vesicle. The proximal layer remains 
thin and is a single layer of cuboidal cells in which pigment appears very 
early. The formation of pigment begins at the margin of the cup and grad- 
ually extends proximally until all of the cells are filled with it, and now con- 
stitute the pigment epithelium of the retina. The proximal always closely 
adjoins the distal layer, which is very thick by contrast, and soon becomes 
a membrane having many layers. It is only at the margin of the vesicle that 
the distal layer remains thin, so that the thin margin of the cup, the pars 
caeca retinae, is always sharply defined from the rest of the retina, the 
pars optica retinae. The place of transition, an irregular, circular line, is 
known as the ora serrata. 
The differentiation of the elements that compose the pars optica begins 
quite early, is ended before birth, and follows in general the same laws as in the 
spinal cord and brain. A division takes place into supporting and nerve 
cells, spongioblasts and neuroblasts. The former permeate the entire 
thickness of the organ, the same as the ependymal cells, but unlike the latter 
maintain their form. Only a part of them, confined entirely to the distal part 
of the retina, change into astrocytes. The majority of the nerve cells are 
bipolar, the minority multipolar and unipolar. They are arranged in several 
layers in such a way that every two layers of cells are separated by a layer 
of fibers in which the processes of the cells come in contact. The most distal 
layer, bordering on the vitreous, changes to large, multipolar cells, the neurites 
of which form a continuous layer of nerve fibers on the distal surface. They 
grow into the epithelial optic pedicle, pass in it to the brain, and thus trans- 
form it into the optic nerve. The cells of the most proximal layer develop 
the power of perception of light and form the layer of rods and cones. From 
the proximal end of each a process extends toward the pigment epithelium 
about the third month of pregnancy. 
Temporarily from the place of entrance of the optic nerve, in the optic 
axis, the retina thickens into a central area in embryos of the sixth month. 
This remains for life in most mammals, but in man its center gradually sinks 
in, as the result of a separation of its distal layers, in the last months of preg- 
nancy, creating a pitlike depression in the retina in the optic axis, the fovea 
centralis, the floor of which is formed by the elements of the most proximal 
retinal layer alone. The vicinity of this depression, the area centralis, has also 
been named the macula lutea, because it appears yellow in the eye of the 
cadaver. 
A very distinct striation may be seen in the cross section of the retina 
of an adult, caused by an alteration of layers that are rich and poor in cells 
(Pl. 86, Fig. 187; Pl. 87, Fig. 188). These layers are from without inward: 
1- The pigment epithelium, the proximal, thin layer of the optic 
vesicle that has been described already. This consists of a single layer of 
cuboidal cells, the surfaces of which present a very regular pentagonal or 
hexagonal form (Pl. 7, Fig. 22). The cells are three or four times as broad 
as high and are joined together by a cement. Each sends from its inner, 
distal surface numerous threadlike processes between the elements of the follow- 216 
ing layer. These, as well as the distal part of the cell body, contain granules, 
needles, and rods of a pigment, fuscin, which contains iron, is insoluble in 
water, alcohol and ether, and can be bleached by nascent chlorine (Pl. 7, 
Figs. 23 and 24). It is caused to move by the influence of light. In the 
dark the pigment draws back into the cell body, but in the light it streams 
into the processes. 
2. The layer of rods and cones is also called the neuroepithelium, 
and contains the percipient elements, the rods and cones, which are, as 
we have seen, processes from the most proximal layer of cells of the distal 
lamina of the retina (Pl. 87, Fig. 188). The rods are cylinders about 60 q 
long by 2 to 3 n broad, which may be divided into outer segments, 35 p 
long, and rather shorter inner segments. The outer segment is uni- 
formly thick throughout its entire length, is strongly refractive to light, and 
readily breaks down in consequence of post mortem maceration into numerous 
successive platelets. It has a firm capsule with myelinlike contents. The former 
contains a spiral thread and fine longitudinal fibrils. The outer segment is 
always sharply defined from the inner and is saturated during life with an 
easily destroyed red coloring matter, the visual purple, or rhodopsin, 
that is quickly bleached by the action of light and is regenerated in the dark 
under the influence of the pigment epithelium. The inner segment is always 
shorter than the outer and is not uniformly thick, but is somewhat swollen in 
the middle. It is less refractive than the outer segment, the capsule and 
fibrils of which are continuous over it. It contains in its distal portion many 
short, rigid threads that stain more strongly with most dyes than any of the 
other parts of the rod and form what is known as the rod ellipsoid. 
The cones are shorter than the rods and are likewise divided into inner 
and outer segments. The outei' segment is short and conical, the inner is 
longer, barrel-shaped, and twice as thick as the inner segment of the rod. 
Otherwise all of the structural peculiarities of the rods are repeated in the cones. 
The distal part of the cone is contractile and shortens under the influence of 
strong light; it is therefore known as the cone myoid. The cone ellipsoid, 
situated proximally to it, is always very distinct and well developed. 
3. The limitans externa. The inner segments of the rods and cones 
are bounded distally by the delicate, but sharply defined, membrana limitans 
externa, which is perforated like a sieve to permit the passage of the con- 
tinuations of the rods and cones through its apertures into the next layer. 
The peculiarities of its structure, as well as its importance, will be dealt with 
in the description of the supporting cells. 
4. The outer granular layer is the most conspicuous of all of the 
layers of the retina and exhibits, when stained in the usual manner, six or 
more rows of densely packed nuclei. If specific methods of staining are em- 
ployed, these nuclei may be seen to belong to two different kinds of cells (Pl. 
88, Fig. 189). As indicated above, the inner segment of each cone passes 
through the limitans externa, continues into the outer granular layer, and 
passes over into a cell body lying just beneath this membrane, the cone 
granule, its secretory product. All the cone granules lie in a single row 
just beneath the limitans externa. The cone granule and the inner segment 217 
of the cone are of about the same thickness and differ but little from each 
other. Distally the cone granule suddenly tapers off into a cone fiber, which 
passes in a straight or somewhat curved course through the outer granular 
layer and the succeeding Henle's layer of fibers, to end in an expansion, the 
cone foot, in the outer plexiform layer, from which little terminal twigs 
radiate. The entire formation, the cone with its outer and inner segments, 
the cone granule, cone fiber, and cone foot, we know as the cone optic cell. 
The relations of the rod optic cell are somewhat different. The inner seg- 
ment of the rod continues in a slender fiber on this side of the limitans externa, 
swells out within the outer granular layer into a little cell body with a nucleus, 
the rod granule, to immediately become filiform again and to extend as a 
rod fiber to a little terminal nodule in the outer plexiform layer. The 
rod granules are rather smaller than the cone granules and lie close together 
at all levels in the outer granular layer. 
5. Henle's fibrous layer is formed by the strong cone fibers, and 
consequently exhibits a striation vertical to the surface of the retina. It is 
very narrow, as is also the following. 
6. The outer plexiform layer contains the footlike ends of the cone 
fibers and the knoblike ends of the rod fibers, together with the terminal 
arborizations of the bipolars soon to be described. 
7. The inner granular layer emulates in extent the outer granular 
layer, and like it shows several layers of nuclei by the ordinary methods of 
staining, which are not usually packed so closely together. They belong to 
various kinds of cells, the most important of which are known as the rod 
and cone bipolars (Pl. 88, Fig. 189). The rod bipolar has a spherical 
or pyriform cell body, and passes proximately into a short, necklike process, 
which very soon splits into numerous fibrils in the outer plexiform layer. They 
always have quite a number of end nodules of the rod optic cells between 
them. Distally the cell body changes into a long fiber that passes through the 
inner granular layer and the succeeding inner plexiform layer, breaks up into 
several short processes, and embraces with them the body of a large ganglion 
.cell. The cone bipolar has a similar structure. Its cell body is rather 
smaller, its proximal process rather longer. It breaks up in the outer plexi- 
form layer into a flat, spreading terminal arborization, and comes in contact 
here with the cone foot piece. The distal slender process penetrates for a 
greater or less distance into the inner plexiform layer, and terminates with 
several short, strong end twigs. 
The second kind of cells of the inner granular layer is the amacrine. The 
amacrine cell has a body that is usually rather longer than that of the 
bipolar, and a single process which runs distally to various levels in the inner 
plexiform layer and breaks up into terminal twigs that run horizontally. 
Thus there are found in this layer as many as six sublayers of amacrine ter- 
minal fascicles. Other amacrine cells send theii' terminal twigs through the 
•entire inner plexiform layer, and are known as diffuse amacrines to dis- 
tinguish them from the others which are called stratifying. 
A third variety is the horizontal cells, which lie in the proximal dis- 
tricts of the inner granular layer and may also extend into the outer plexiform 218 
layer (Pl. 88, Fig. 189). Short processes from the elongated cell bodies 
radiate into the latter and unite here with the knoblike ends of the rod optic 
cells. A longer process extends from one end of the cell body horizontally 
through a larger extent of the outer plexiform layer, to finally likewise break 
up into a terminal fascicle, which in turn surrounds a group of the button- 
shaped ends of the rods. 
Mueller's cells form the last variety to which the nuclei of the inner 
granular layer belong. These will be described later. 
8. The inner plexiform layer is also very extensive. It contains the 
distal branchings of the cells of the inner granular layer, which unite here with 
the expansions of the proximal processes of the ganglion cells. 
9. The layer of ganglion cells is formed of typical, large nerve cells 
with distinct Nissl's clods and neurofibrils (Pl. 87, Fig. 188). The cell body 
sends out one or more dendrites into the inner plexiform layer, which either 
pass through or spread out in it horizontally at various levels, so that theii' 
branches form laminae that correspond to the expansion of the amacrines and 
the distal terminal arborizations of the cone bipolars. The neurite usually 
goes off at a right angle from the distal surface of the cell and enters the 
layer of nerve fibers as a nerve fiber. The distal terminal twigs of the rod 
bipolars closely adjoin their cell bodies (Pl. 88, Figs. 189 and 190). 
10. The layer of nerve fibers consists of nonmedullated nerve 
fibers that assemble into bundles and take their way to the papilla in order 
to pass through it into the optic nerve. The great majority of these fibers 
are the neurites of the ganglion cells that have just been described, and hence 
conduct centripetally, but associated with them are centrifugal fibers 
that come through the optic nerve from the cells of the brain, pass through 
the layer of nerve fibers to the inner plexiform layer, and there break up 
into terminal arborizations which come into close relations with the ama- 
crine cells. 
11. The limitans interna is the innermost layer of the retina, and is,, 
like the limitans externa, composed of supporting elements. 
These supporting elements of the retina appear in two different forms, 
as Mueller's, or radiating, fibers and as astrocytes. Mueller's fibers 
are best compared with the embryonal ependymal cells of the medullary tube. 
Like them they are long, cylindrical and rodlike formations that pass through 
the retina from the limitans externa to the limitans interna in a course that 
is perpendicular to the surface. At both ends they spread out in footlike 
expansions that are cemented firmly together so as to form the two limiting 
membranes, the limitans externa and interna, which are therefore not 
independent layers or membranes, but are the connected expansions of these 
fibers (Pl. 87, Fig. 188). Processes extend from the entire length of each 
of Mueller's fibers, some filamentous, some membranous or winglike, which 
penetrate between the nervous elements of the different layers of the retina, 
sheathe them, and anastomose with the processes of neighboring Mueller's 
fibers. A supporting fibrous framework is thus produced that pervades the 
entire retina. Fine fibrils run through the entire length of the cell body and 
spread out like brushes in the expansions. These fibrils resemble fibers of 219 
neuroglia in their staining reactions. From the proximal end of each of 
Mueller's fibers, therefore from the limitans externa, minute fibrils radiate out 
and form fibrillary baskets about the inner segments of the rods and cones. 
The fibrils lie within the homogeneous protoplasm of the cell body, which 
frequently appears indented by the neighboring nerve cells. The nucleus of 
a Mueller's fiber is found within the inner granular layer and commonly stands 
out laterally from the periphery of the cell. 
The astrocytes of the retina are exactly like those of the central nervous 
system. They are found chiefly in the layer of nerve fibers and penetrate 
with these into the optic nerve. They are met with much more rarely in the 
proximal layers. 
Having thus learned the constituent elements of the retina, we have to 
study their relations in the different parts, beginning at the papilla and pro- 
ceeding distally to the ora serrata. The papilla of the optic nerve itself 
contains no nervous elements except the optic fibers, and is therefore known 
as the blind spot (Pl. 86, Fig. 187). The retina here is cut out, as it were 
by a punch. All of the fibers in the layer of nerve fibers converge to this 
hole, pass through it and form the optic nerve. Hence the papilla has a funnel- 
shaped depression in its center, the excavatio papillae nervi optici, from 
the bottom of which the central vessels rise into the retina. 
All of the layers of the retina are best developed in the vicinity of the 
papilla. The layer of nerve fibers is naturally particularly thick, because it 
is here that its elements are most accumulated. The cells of stratum gangli- 
osum form several layers. The rods and cones alternate in the neuroepithelial 
layer, every two cones being separated by one or two rods. 
If we proceed distally toward the temporal side we approach the macula 
lutea, which may be recognized by the fact that the ganglion cells arc heaped 
up still more and form as many as eight layers in the stratum gangliosum. 
The rods become fewer in the neuroepithelium, while the cones become longer 
and assume more nearly the shape of rods. Henle's fibrous layer becomes very 
marked and distinct, thanks to the cone fibers. The entire thickness of the 
retina at this place is about 5 mm. 
The fovea centralis is an oval depression in the retina, measuring 0.2 
by 0.15 mm, the margins of which slope gradually (Pl. 86, Fig. 187). Its 
floor is formed wholly of pigment epithelium and very long cone cells. Rod 
cells are entirely absent. The cone granules lie in two or three layers and 
extend somewhat back of the limitans externa. The cone fibers arch laterally 
from the bottom of the fovea to the cone bipolars in its margin, which are 
arranged obliquely to the surface of the retina. In the wall of the fovea the 
ganglion cells form first a single layer, then more, up to as many as eight 
layers. Mueller's fibers run obliquely to the surface of the retina in the margin 
of the fovea. 
The farther we go distally from the fovea centralis the thinner the retina 
becomes. This thinning affects first and chiefly the layers of nerve fibers and 
of ganglion cells. In the latter the cells very soon form only a single layer 
which presents breaches of continuity that become more frequent the farther 
we go. The cones, which alone were present in the fovea centralis, gradually 220 
become displaced by the rods, though both are always present. Each cone is 
separated from its neighbor at first by one, later by from two to five rods, so 
that the number of rods in the entire retina is twenty-three or twenty-four 
times that of the cones. The gradual disappearance of Henle's fibrous layer 
corresponds to the decrease in the number of the cones. 
The retina in the distal portion of the pars optica has a thickness of only 
0.15 mm; it then suddenly passes into the pars caeca, which is only 0.05 mm 
thick (Pl. 85, Fig. 185). This region of transition is1 called the ora serrata. 
The ganglion cells disappear first, and naturally with them the layer of nerve 
fibers. The outer plexiform layer tapers off quickly, while with its disappear- 
ance the inner and outer granular layers blend and contain many vacuoles that 
are often of considerable size. The nuclei in the blended granular layers belong 
foi' the most part to Mueller's fibers, which are very dense at this place. We 
find no rods and only stunted cones in the neuroepithelium. Finally, there 
remains in the pars caeca only a double layer of cells, the outei' of which is 
formed of cuboidal pigment epithelial cells that are now without processes, 
while the inner consists of a single layer of cylindrical cells. This epithelium 
continues over the ciliary body and is then known as the pars ciliaris retinae. 
b. The Optic Nerve 
As we have seen in the review of the embryology, the optic nerve comes 
from the optic pedicle, which is very short at first, but later elongates a 
great deal. Its lumen communicates with the ventricles of the brain, and its 
wall consists of several layers of cylindrical cells. Inasmuch as the optic cleft 
continues for a distance on the optic pedicle, its ventral surface presents at 
first a groove in which the central artery and vein come to lie. The lumen is 
displaced dorsally, is much contracted, and finally is completely occluded, after 
the closure of the groove, by a lively proliferation of the epithelium, so that 
the optic pedicle is then a massive cellular formation. Before the 
disappearance of the cavity nerve fibers from the young retina grow out 
into the pedicle and use it as a path to conduct them to the brain. The 
farther these fibers progress the more they force apart the epithelial cells of 
the pedicle, which become glial elements between them. Later connective 
tissue also takes part in the structure of the nerve, part of it coming from 
outside, part from the central vessels, and divides the fibers into separate 
bundles. 
The three cerebral membranes combine to form a connective tissue 
investment for the optic nerve at its exit from the skull, that covers it 
throughout its intraorbital course. The dura mater remains distinct and forms 
the outer sheath, while the arachnoid and pia mater blend to form the inner. 
Between the two lies a continuation of the subdural space lined with epithe- 
lium, while within the inner sheath is a system of clefts that communicates with 
the subarachnoidal space of the brain. The two sheaths blend at the 
eyeball and pass over into the sclera (Pl. 86, Fig. 187). 
The inner sheath lies close to the nerve and sends into its substance many 
anastomosing septa which unite with the connective tissue entering the nerve 
with the central vessels in the distal segment. These vessels, the central artery 221 
and vein of the retina, penetrate obliquely into the trunk of the optic 
nerve 7 or 8 mm distant from the eyeball, and then form with the connective 
tissue about them the axis of the nerve. 
The connective tissue septa divide the optic nerve into between eight and 
twelve hundred bundles of unequal size, but do not invade the interior of 
these bundles, each of which consists of medullated optic nerve fibers of 
varying sizes that have no sheaths of Schwann, the same as all central 
nerve fibers. Many astrocytes are to be found within the bundles, with their 
glia fibers running for the most part vertical to the course of the nerve fibers. 
These astrocytes are, as we have seen, the remains of the former epithelial cells 
of the optic pedicle. 
On arriving at the eyeball all of the fibers of the optic nerve lose their 
medullary sheaths, and the nerve trunk consequently becomes considerably 
more slender. At the same time it passes through the so-called lamina cri- 
brosa in a very large number of little bundles of nonmedullated fibers. 
The lamina cribrosa is a cribriform plate of connective tissue that closes the 
aperture in the eyeball for the entrance of the optic nerve, and is situated 
at the place where the sheaths of the optic nerve pass into the sclera. It is 
reinforced by many blood vessels which unite the central artery and vein of 
the retina, before they enter the excavation of the papilla, with the short pos- 
terior ciliary arteries and veins. 
Having arrived at the papilla the fibers radiate in many little bundles in 
the layer of nerve fibers of the retina. One bundle, the papillomacular, goes 
directly to the macula lutea, the rest radiate in all directions and frequently 
interlace. We must remember that these bundles are composed of naked axis 
cylinders with neither medullary nor Schwann's sheaths. 
c. The Choroid 
The epithelial optic vesicle is surrounded very early by numerous blood 
vessels embedded in the mesenchyma. Toward the end of the third or the 
beginning of the fourth embryonal month these vessels present a typical arrange- 
ment, in that the larger ones lie farther out, while the capillaries lie close to the 
pigment epithelium. Pigment granules soon appear in the connective tissue 
cells lying between the vessels, the entire vascular membrane becomes sharply 
defined from the nonvascular connective tissue outside of it, and may then 
be known as the choroid. 
The choroid directly adjoins the outer surface of the retina and contains 
the blood vessels that supply the proximal layers of the latter with nutriment. 
It ends with a fairly sharp margin at the entrance of the optic nerve, and passes 
over into the biliary body at the ora serrata, and presents the following layers 
from within outward: 
The lamina basalis directly adjoins the pigment epithelium, with which 
it is more or less firmly connected, and consists of two layers, the inner of 
which is structureless, while the outer is a very finely fibrillar network of elastic 
fibers known as the stratum supracapillare. Both together are only about 
2 |J thick in young eyes, but may be considerably thicker in old age. 
The lamina choriocapillaris contains exclusively capillary vessels 222 
embedded in a homogeneous, structureless basal substance bounded externally 
by a finely fibrous elastic limiting membrane (Pl. 87, Fig. 188). 
The lamina vasculosa embraces the arteries and veins belonging to 
these capillary vessels, so the coarser vessels lie to the outer side of the finer. 
Membranes formed of bundles of connective tissue lie between the vessels. They 
are abundantly provided with elastic networks, and also contain many con- 
nective tissue cells with bodies permeated with black or brown pigment 
granules. 
The lamina suprachoroidea joins the choroid to the sclera, to which 
it may be said to belong. It consists of several lamellae of connective tissue, 
strongly reinforced by elastic fibers that anastomose. Pigmented connective 
tissue cells lie in the lamellae, and upon them are connective tissue cells con- 
taining no pigment and arranged as in an epithelium. Between the lamellae are 
the perichoroidal spaces, which are filled with lymph. 
d. The Ciliary Body 
The primitive condition of the margin of the optic vesicle is maintained com- 
paratively long. Even at the beginning of the third fetal month it appears as 
a narrow, thin continuation of the retina, the pars caeca retinae. Outside 
of it lies the connective tissue and vessels of the choroid, which more distally is 
continuous with the pupillary membrane to be described later. In the middle 
of the third month a marked longitudinal growth takes place in the pars caeca 
retinae which, together with the great general enlargement of the eyeball, 
simulates a recession of it proximally. The distally developing part of the pars 
caeca is the rudiment of the iris, the ciliary body develops from the rest, to- 
gether with the externally adjoining mesenchyma. The vascular mesenchyma 
proliferates toward the interior of the eyeball in the form of meridionally run- 
ning folds in which each vessel comes to be covered on the inner side by the 
pigmented pars caeca retinae, which now assumes the characteristics of an 
epithelium over these folds. Thus, toward the end of the third month, the 
rudiment of the ciliary processes appears, rising slowly at first from the 
connective tissue of the ciliary body and then rapidly sloping away distally. 
In the fourth month muscular elements appear in the connective tissue of 
the young ciliary body, formed by the development of smooth muscle fibrils in 
the mesenchymatous cells. These have at first a purely meridional course, but 
circular fibers are added later. The ciliary body is enlarged by the develop- 
ment of the ciliary muscle, SO that in cross section it protrudes as a tri- 
angular mass into the interior of the globe. The ciliary processes rise from 
its inner surface, slowly decline proximally into the orbicularis ciliaris and 
slope distally to the root of the iris. 
The ciliary body of the adult is the direct distal continuation of the 
choroid, in which the typical arrangement of the vessels is completely effaced 
by the interpolated ciliary muscle that predominates over the entire structural 
picture (Pl. 85, Fig. 185). There remains of the choroid externally the 
lamina suprachoroidea, which bounds the ciliary body toward the sclera, 
and the lamina basalis internally, which is continuous over the orbicularis 
ciliaris and the ciliary processes next the epithelium. The latter lies upon a 223 
finely fibrous connective tissue carrying the blood vessels, which forms the frame- 
work of the ciliary processes, surrounds the ciliary body, and is known as its 
ground plate. The entire interior of the ciliary body between the supra- 
choroidea and the ground plate is filled by the ciliary muscle. Its bundles 
of muscular fibers are divided by vascular connective tissue, which is a direct 
continuation of the connective tissue of the lamina vasculosa and like it carries 
vessels. 
The ciliary muscle consists of bundles of smooth muscle fibers that 
may be seen to take three courses. The most external ones, adjoining the supra- 
choroidea, arise from the connective tissue of the lamina vasculosa choroideae, 
take a purely meridional course distally and are inserted into the region 
of the scleral protuberance to be described later. This is called the tensor 
choroideae. Farther inward are bundles that arise from the region of the 
root of the iris, radiate apart proximally and inwards and are inserted into the 
ground plate; these are the radiating bundles. Quite internally are bundles 
that have a circular course and, unlike the two just mentioned, are cut across 
in a meridional section; these are the circular bundles, or Mueller's mus- 
cle, and form as a whole the distal part of the muscular ring that surrounds 
the contents of the eyeball. Between the muscular bundles lies fibrillary con- 
nective tissue with numerous elastic fibers, in which we meet with large blood 
vessels and capillaries, as well as extremely numerous nerve fibers and cells. 
The ciliary muscle is very important physiologically as it produces the con- 
dition known as accommodation, which will be discussed under the subject 
of the lens and the zonula ciliaris. 
The connective tissue is continuous internally with the ground plate from 
which the ciliary processes arise. It continues proximally in the tissue of the 
orbiculus ciliaris, which is essentially the lamina vasculosa choroideae, but 
lies directly on the lamina basalis as the lamina choriocapillaris is wanting. 
The inner surface of the orbiculus ciliaris lies in many low, meridional folds 
that blend distally to form seventy or eighty ciliary processes. These rise 
gradually for a distance of 2 or 3 mm, have secondary, foldlike elevations on 
their tops, and suddenly slope away to the root of the iris. Each process con- 
sists of the connective tissue of the ground plate, contains many vessels, and 
is separated from the epithelium by the lamina basalis. 
The epithelium, the pars ciliaris retinae, is a direct continuation of 
the retina in the region of the ora serrata. It covers the entire surface of the 
orbiculus, ciliary body, and ciliary processes, and blends at the root of the iris 
with the epithelium of the posterior surface of the latter, forming the pars 
iridica retinas. It presents a double layer, the outer of which, the continua- 
tion of the pigment epithelium of the retina, is composed of high cuboidal 
cells so densely filled with pigment granules that it is difficult to perceive their 
margins. The inner layer, on the contrary, is free from pigment, is the con- 
tinuation of the distal layer of the retina, and consists of cylindrical cells 
that decrease somewhat in height distally from the orbiculus. Fine fibrils ap- 
pear everywhere within these unpigmented cells, especially in the areas of 
epithelium that cover the depressions between the ciliary processes, continue 
into the interior of the globe, and approach the equator of the lens as the zonula 224 
ciliaris. Mitochondria have been found in these cells that produce a secretion 
which has been demonstrated in the form of minute drops in the cell body. The 
secretion of the aqueous therefore has been ascribed to the inner layer of 
the pars ciliaris retinse. 
e. The Iris 
As shown in the preceding section the iris originates through the growth 
forward of the two layers of the pars caeca retinae toward the lens in about the 
third fetal month. Its lateral surface is covered with mesenchyma that extends 
in a thin layer over the distal surface of the lens and forms the pupillary 
membrane. At the margin of the optic vesicle the nonpigmented distal 
retinal layer passes over into the pigmented proximal layer, yet in such a way 
that the marginal zone has little or no pigment. In the fourth month smooth 
muscle fibrils appear in the distal cells of the proximal layer in the im- 
mediate vicinity of the margin, the young muscle cells separate more and more 
from the epithelium, come to lie in the mesenchymatous tissue as the sphincter 
iridis, and the fibers gradually assemble into several circular bundles. Thus 
we have a muscle that originates from the epithelium of the optic vesicle, and 
therefore from the ectoderm. Toward the end of the fifth month muscular 
fibrils appear throughout the entire region of the proximal pigmented epithe- 
lium of the now much elongated rudimentary iris, at first lying scattered about 
in the cells, latei' assembled on the distal side of the epithelium bordering the 
connective tissue, and form a coherent layer that extends from the root of tha 
iris to the margin of the pupil and is known as the dilatator iridis. Thus 
we see that the epithelium and the muscles of the iris originate from the ecto- 
derm. The distal epithelial layer has no pigment until the sixth month, when 
the formation of pigment begins in its cells and advances from the margin of 
the pupil to the root of the iris. 
To this ectodermal portion is added a mesenchymal, which lies to the 
distal side of the former and is identical with the pupillary membrane. 
This consists of connective tissue that proliferates with blood vessels toward 
the lens and forms a fairly thick layer even in the fifth month. At first it 
passes directly from the margin of the pupil into the pupillary membrane, but 
the relations are changed later as the pupillary margin of the iris grows for- 
ward behind the pupillary membrane, so that the place of attachment of the 
latter is displaced to the anterior surface of the iris. The pupillary membrane 
is adherent at its center to the lens capsule, has an extensive vascularization^ 
and undergoes involution after the eighth month through obliteration of these 
vessels and absorption of its tissue from the center toward the periphery. 
The iris consists of three parts which are, from the distal to the proximal 
(Pl. 85, Fig. 185) : 
1, The iris stroma. This is the mesenchymatous portion and consists of 
very loose, fibrous connective tissue, the bundles of which interlace and leave 
intermediate spaces, some larger, some smaller. The connective tissue is very 
abundantly supplied with branched stroma cells, the processes of which 
anastomose, with the body of each filled with pigment granules in accord- 
ance with the general degree of pigmentation of the person. When the amount 225 
of pigment is great the color of the iris is black, when less it is brown, and when 
the stroma contains no pigment the iris is blue. The stroma of the iris is very 
vascular, and the connective tissue is always better developed about the ves- 
sels. The connective tissue cells form several layers on its distal surface, known 
as the anterior limiting layer. Toward the anterior chamber the cells form 
a more or less uninterrupted coat which is called the anterior epithelium 
of the iris. 
2. The sphincter iridis lies within the stroma in the pupillary portion of 
the iris. It is a narrow, circular band, about 1 mm broad, that lies nearer the 
proximal than the distal surface. Its bundles of smooth muscle fibers are 
circular and exhibit manifold anastomoses with the fibers of the following 
muscle. 
3. The dilatator iridis is a layer of radiating smooth muscle fibers about 
2 |j thick. It lies between the stroma and the posterior epithelium and is also 
called Bruch's membrane. Beyond doubt these muscular fibrils originate 
from the cells of the parietal layer of the retina, and the only question is whether 
they remain permanently connected with thcii' mother cells, as indicated by the 
absence of nuclei, or have a certain independence. 
4. The posterior epithelium of the iris has a double layer. It 
ends at the margin of the pupil, turning back over it like a hook. All of its 
cells are so stuffed with pigment granules that the cell boundaries cannot 
be made out. It is only in albinos that this pigment is absent, and its lack then 
gives the iris a red color. Both layers pass over into the pars ciliaris retinae at 
the root of the iris, where the posterior loses its pigment. 
The iris has the important duty of regulating the quantity of light 
admitted to the eye; the pupil contracts when the light is strong, and dilates 
when it is weak. It also excludes marginal rays that interfere with the produc- 
tion of a distinct image. 
f. The Lens 
We have learned that in a human embryo about 6 mm long the lens is an 
epithelial vesicle cut off from the ectoderm. It lies just beneath the epidermis, 
but gradually becomes separated from it by the developing anterior vitreous 
and the mesenchyma growing in between the two. Very soon after the separa- 
tion the epithelium of the lenticular sac exhibits an irregular growth. The distal 
half of its wall remains low while the cells in the proximal elongate more and 
more until they approach so close to the proximal surface of the distal wall as 
to obliterate the cavity of the vesicle toward the end of the second fetal month. 
We call the low cells of the distal wall the lenticular epithelium, the elon- 
gated ones of the proximal wall the lenticular fibers. The two blend at 
the equator of the lens. When the fibers have attained a certain length 
their capacity for segmentation ceases, they simply grow and leave the pro- 
duction of new fibers to the lenticular epithelium. The cells of the latter di- 
vide in the region of the equator, grow into lenticular fibers, while new fibers 
are formed through constantly renewed cells that are arranged in the form of 
radiating laminae. A cleft, that later becomes a three rayed star, appears on 
the distal and proximal surfaces of the organ where the ends of the fibers meet. 226 
The cells of the lenticular vesicle send out short processes, the so-called 
lenticular cone, from which fine fibrils extend and help to form the primitive 
vitreous surrounding the lens. But the lenticular cone soon disappears, and in 
its place the cells of the lenticulai' vesicle secrete a structureless membrane that 
envelops the young lens on all sides, and is known as the lenticular capsule. 
This is adjoined externally by vascular connective tissue that forms the mem- 
brana vasculosa lentis, which is connected distally with the pupillary 
membrane, and is entered at the proximal pole of the lens by the branches 
of the hyaloid artery. The involution of this begins in the eighth fetal month 
and proceeds simultaneously with that of the pupillary membrane. 
We have to study in the adult lens the capsule, the epithelium, and the 
fibers (Pl. 85, Fig. 185). 
The lenticular capsule is a pellucid, structureless membrane composed 
of separate lamella?. Its greatest thickness is 15 p at the distal pole of the lens, 
from which point it diminishes steadily over the equator to the proximal pole, 
where it is only 5 p thick. The part covering the distal surface is commonly 
called the anterior capsule of the lens, the rest the posterior capsule. It pos- 
sesses a certain degree of elasticity and is closely related chemically to the 
membranae propria1, which it also resembles in its development. An albuminoid 
called membranin has been demonstrated in it. 
The lenticular epithelium is a single layer of cells that covers the proxi- 
mal surface of the anterior lenticular capsule. These cells are only 2 to 3 p 
high in the region of the distal pole of the lens, gradually become higher proxi- 
mally to pass over into the lenticular fibers in the region of the equator. 
They are always placed with their longitudinal axes oblique to the surface of the 
lens, from which their nuclei are distant, and the distal ends of the constantly 
lengthening cells overlap their predecessors until the fibers have a course al- 
most parallel to the surface and give rise in the equatorial region to the so- 
called vortex, or radii lentis. 
The lenticular fibers are long, bandlike formations arranged in radiat- 
ing lamellae. Each lamella consists of a layer of fibers and is connected 
very firmly with its neighbor (Pl. 85, Fig. 186). Mantle fibers, transi- 
tional fibers, and central fibers may be distinguished in each lamella. The 
mantle fibers are fairly regularly hexagonal on cross section, three to four 
times as broad as thick. Each has a nucleus in the region of the equator. 
Toward the interior of the lens the thickness of the fibers increases gradually, 
the cross section becomes irregular, and the nucleated mantles pass over into the 
nonnucleated transitional fibers. The nucleus of the lens is com- 
posed of quite irregular, nonnucleated central fibers. 
The mantle fibers have a soft, semifluid protoplasm that condenses at the 
surface into a firm crusta. In the transitional fibers, and still more in the 
central, the protoplasm becomes more fixed and firm, so that the nucleus of the 
lens is a very resistant, unchangeable mass, while the peripheral layers of fibers 
readily undergo changes of form, at least until manhood, but in old age they 
also become fixed. 
The lenticular fibers consist chemically of about 63 or 64% water and the 227 
balance of solid constituents, mainly albuminoids, albumoid and crystallin, to- 
gether with lecithin, cholesterin, fat, and inorganic salts. 
Each lenticular fiber is an S-shaped band, the two ends of which are 
situated in different planes. It passes from a point in the distal portion to a 
point in another plane in the proximal portion, crossing the equator. In both 
portions of the lens the fibers assemble into radiating surfaces that tend to 
place themselves vertically to these planes. These commissural surfaces appeal' 
in incompletely macerated lenses in the form of two stars, the lenticular stars, 
which are so placed that the radii correspond to the distal, the interradii to the 
proximal. 
The lens, together with the cornea, form the dioptric apparatus of the 
eye, the function of which is to throw a sharply defined, inverted image of any 
given object into the fovea centralis. During rest the eye is adapted to in- 
finity, the ciliary muscle is completely relaxed and thus exerts a strong traction 
on the lens through the zonula that connects the ciliary body with the lenticular 
surface and thereby causes the lens to be flattened. If we wish to see distinctly 
an object at a finite distance, we contract our ciliary muscles and relax the 
zonula sufficiently to allow the elasticity of the lens to curve it enough to throw 
a distinct picture of it into the fovea. The nearer the object is placed, the 
stronger' must be the contraction of the ciliary muscle and the greater the 
curvature of the lens produced. The less curved anterior surface is affected 
more than the posterior so that the difference in curvature between them is 
more nearly equalized. This process is known as accommodation. Its 
amplitude gradually decreases in later life in consequence of the increasing 
rigidity of the mantle fibers, until the eye is more or less without accommodation 
in old age. 
g. The Cornea 
When the lenticular vesicle separates from the ectoderm the latter consists 
of a single row of cuboidal cells that, so far as it lies in the region of the optic 
vesicle, provides the anterior epithelium of the cornea. The narrow space be- 
tween the lenticular vesicle and this epithelium is filled immediately after its 
origin by fine fibrils that come partly from the proximal surface of these epithe- 
lial cells, partly from the cells of the lenticular vesicle, unite and form the 
anterior vitreous, which is continuous proximally with the posterior 
vitreous. The corneal epithelium soon becomes of two layers, shows a distinct 
membrana propria, and at the same time cells from the mesenchyma surround- 
ing the margin of the optic vesicle penetrate between the lens and the epithelium, 
unite with the elements of the anterior vitreous, and gather into several layers 
on the membrana propria. Proximally the pupillary membrana develops from 
the margin of the vesicle to the distal surface of the lens, while between it and 
the layers of mesenchymatous cells the anterior vitreous dissolves and gives 
place to a cavity called the anterior chamber. At this time the cornea 
consists of two or three layers of epithelial cells and several layers of con- 
nective tissue cells. In the most proximal layer of the latter the cells lie 
close together so as to form a single layer of epithelium that bounds the rudi- 
mentary cornea toward the anterior chamber, the posterior corneal epithe- 228 
lium. Proximally the anterior epithelium of the cornea blends with the epithe- 
lium of the conjunctiva, the corneal mesenchyma with the rudiment of the 
sclera. Toward the end of the second fetal month the mesenchymatous cells of 
the corneal rudiment begin the production of collagenous, later of elastic 
fibers, which cause the cornea to increase considerably in thickness. This 
young substantia propria is limited proximally, toward the end of the third 
month, by a homogeneous membrana propria secreted from the cells of the 
posterior corneal epithelium, which soon increases in thickness and becomes 
known as Descemet's membrane. Later it condenses distally against the 
anterior epithelium to form Bowman's membranes. The corneal rudiment 
is nonvascular from the first. 
The individual layers of the adult cornea, thus derived, exhibit the following 
structure: 
The anterior epithelium has an average thickness of 0.1 mm, is strati- 
fied, flat, and consists of from five to seven superimposed layers, the deepest, 
of which is composed of low cylindrical, the next two to four of polyhedral, 
and the final two of flat cells that never cornify (Pl. 10, Fig. 31). In the 
deeper layers the cells are separated by fine intercellular spaces permeated 
by plasmodesmi, but those of the superficial flat layers are closely approximated. 
Mitoses are often met with in the basal cells. Proximally the epithelium passes' 
over into that of the conjunctiva bulbi with a little thickening. 
Bowman's membrane, or the anterior limiting membrane, is, as 
we have seen, nothing else than a condensed, nonnucleated part of the sub- 
stantia propria. Its maximum thickness is 20 p, and it ends at the periphery 
of the cornea (Pl. 85, Fig. 185). 
The substantia propria forms about nine tenths of the entire cornea. 
It is 0.7 mm thick at its apex and 0.9 mm at the periphery. It consists of 
somewhat flattened, long connective tissue fibers about 10 p thick, united 
into about sixty superimposed laminae. Although in general the fibers within 
the laminse are parallel to the surface of the cornea, yet there are many that 
pass from one lamina to another. It is only in the distal layers that we find 
fibers which take an oblique, or even vertical course, pass through several lamel- 
lae and enter Bowman's membrane, the fibrae arcuate. The separate fibers are 
bound together within the laminae by a special cement, which likewise joins 
the individual lamellae. The cornea also contains fine elastic fibers, which 
are most numerous in the proximal layers of the substantia propria. The 
cellular elements of the basal substance of the cornea are the corneal cor- 
puscles (Pl- 89, Fig. 191). These are flat cells with large nuclei, double 
central bodies, and numerous branching processes lying on the connective 
tissue lamellae. The processes of the different cells anastomose with those of 
their neighbors in both the same and neighboring lamellae, and thus form a 
protoplasmic network through the entire substantia propria, situated 
within a system of lymph canals. This system of canals is not completely 
filled by the cells and their processes, but contains with them the lymph that 
serves for the nutrition of the cornea. Wandering cells are commonly met with 
in them. 
Descemet's, or the posterior limiting membrane, is as we have 229 
learned, a secretory product of the cells of the posterior epithelium. It is 6 
or 7 p thick at the apex of the cornea, 10 to 12 p at the periphery, then it 
becomes very thin, enters the framework of the sinus of the anterior chamber 
and disappears. Descemet's membrane is structureless and is very closely 
related chemically to the lenticular capsule, which also is a cellular, secretory 
product. It is very firmly adherent to the substantia propria. 
The posterior epithelium of the cornea is a single layer of flat cells 
that are for the most part regularly pentagonal or hexagonal, and about 2 
or 3 p thick. Each cell contains a slight nucleus that presents many forms 
and is often kidney or bean shaped. Near it lie two central bodies surrounded 
by a basketlike central capsule, the centrophormium. Crystalloids of an 
albuminoid substance also have been found in it. The distal part of the cell 
contains epithelial fibers that pass from one cell into another 
h. The Sclera 
The sclera, the outermost membrane of the eye, is the direct proximal con- 
tinuation of the cornea (Pl. 85, Fig. 185). The sclerocorneal margin runs 
obliquely from without inward, so that the distal lamella? of the cornea end 
sooner than the proximal. The sclera blends with the sheath of the optic nerve 
at the entrance of the latter. Here it attains its greatest thickness, 1 to 2 mm, 
which gradually diminishes distally to 0.3 mm, to increase again to 0.6 mm at 
the insertions of the extrinsic muscles. The fine trabecular formation of 
Tenon's space is given off from its outer surface. Its inner surface is at- 
tached to the choroid by the lamina suprachoroidea. At the distal end of the 
ciliary body the inner surface of the sclera protrudes into the sinus of the an- 
terior chamber as the scleral protuberance, which is bounded distally by a 
depression in the substance of the sclera, the internal scleral sulcus. The 
scleral tissue loosens up distally and passes in separate trabeculae to the sinus 
of the anterior chamber at the internal scleral sulcus, closing in incompletely 
the circular canal of Schlemm, which is in open connection with the an- 
terior ciliary veins. The trabecular formation joins with similar trabeculae 
that rise from the root of the iris to the corneoscleral junction under the name 
of the ligamentum pectinatum iridis, and together they completely fill 
the sinus of the anterior chamber, or Fontana's space, with a loose, spongy 
tissue. 
The sclera, like the cornea, is composed of flat connective tissue fibers, 
but they are not arranged in such regular lamellae. They assemble into bun- 
dles that divide, anastomose, and interlace with each other. The sclera is 
materially richer in elastic fibers than the cornea; they lie on the bundles of 
connective tissue and are particularly abundant in the inner layers and, in the 
region of the scleral prominence. The sclera contains numerous cells that 
differ in shape from the corneal corpuscles. They have membranelike pro- 
cesses with which they ensheathe the connective tissue fibers and thus resemble 
tendon cells. Probably the sclera has a system of lymph spaces comparable to 
that of the cornea in open connection with it and with the spaces of the sinus 
of the anterior chamber. 230 
i. The Vitreous 
The lenticular sac, when it has just separated from the ectoderm, nearly 
fills the entire cavity of the optic vesicle, leaving only a narrow cleft between it 
and the distal layer of the latter. This cleft is filled with an extremely fine 
network of fibrils that come from both the cells of the distal retinal layer and 
the lenticular epithelium, the primitive vitreous. The formation of fibrils 
encroaches also upon the opposing epidermis cells of the lenticular sac, and 
fills the space between them. With the closure of the optic cleft and the en- 
trance of the vessels great numbers of mesenchymatous cells enter the gradually 
widening and deepening cavity of the vitreous, and are reinforced by similar 
elements that enter from the margin of the vesicle. These mesenchymatous 
cells also produce fibrils that mingle with the primary ones from the ectoderm 
and so form the secondary vitreous. The vasa hyaloidea run in the 
axis of the embryonal vitreous from the papilla to the lens. The fibrillary net- 
work is looser about these than elsewhere in the vitreous. After the absorp- 
tion of these vessels an axial space remains, the hyaloid canal. 
The vitreous of the adult fills the entire space between the proximal sur- 
face of the lens and the retina, and thus occupies by far the greater part of 
the interior of the eye. It consists of vitreous tissue and the fluid with which 
this is saturated. The vitreous tissue is a network of extremely minute vitre- 
ous fibrils, that are neither collagenous nor elastic, according to their color 
reactions, but may be classed with the lattice fibers met with in the interior of 
the lobules of the liver. This network of fibrils contains no cells. Peripherally 
it condenses into a limiting layer that closely adjoins, and is firmly adherent 
to the membrana limitans interna of the retina. Distally it adjoins first the 
folds of the orbiculus ciliaris, then the ciliary processes, next the zonula ciliaris, 
and finally the proximal surface of the lenticulai- capsule, where it blends with 
the wall of the hyaloid canal. The latter is not a true cavity, but is filled 
with vitreous tissue that is sharply differentiated from the rest by its far more 
open structure. As stated above, the limiting membrane blends directly with 
the membrana limitans interna of the retina; a special structureless membrana 
hyaloidea does not appear to exist. The vitreous contains only an extremely 
small number of cells, all of which appear to be wandering cells from the retinal 
vessels and lie in the limiting layer. 
The vitreous fluid that saturates the fibrillary network is of alkaline re- 
action and contains less than 1% of solid constituents, including traces of 
albumin, a mucoid substance called hyalomucoid, traces of urea, and inorganic 
salts. 
k. The Zonula Ciliaris 
The zonula ciliaris, also called the zonule of Zinn, and often simply the 
zonule, is a system of fibers stretched between the ciliary body and the 
lens that hold the latter in place and is of great importance in effecting the 
phenomena of accommodation (Pl. 85, Fig. 185). The fibers are not only 
much stronger than the fibrils of the vitreous, but they also differ from them 
in their staining reactions, according to which they resemble neuroglia fibers 231 
and the fibrils of Mueller's fibers of the retina. Part of them arise from the 
surface of the cells of the distal layer of the pars ciliaris retinae, part 
penetrate between them and may be followed proximally to the cells of the 
orbiculus ciliaris. These pass through the depressions between the ciliary pro- 
cesses to the distal surface of the lens, while those from the distal part of the 
ciliary body go to the proximal surface, so that the two sets of fibers cross in 
the region of the equator of the lens, to which special bundles are attached. 
This sort of attachment renders intelligible what has been said in regard to 
accommodation. A flattening of the lens must be produced when the zonule is 
tightened. The interspaces between the fibers of the zonule are filled with 
aqueous and are in open communication with the posterior chamber. 
I. The Blood Vessels of the Eyeball 
All of the arteries of the eyeball come from the ophthalmic, but are 
divided into the central artery, which supplies the optic nerve and the distal 
portions of the retina, and the ciliary arteries, which supply all of the 
other parts. 
The central artery of the retina enters the axis of the optic nerve with 
the vein of the same name about 2 cm from the eyeball and runs in it to the 
papilla, giving off many branches in the septa of the nerve on the way. In 
its passage through the lamina cribrosa it anastomoses with Haller's circular 
arterial plexus and divides in the base of the papilla first into a superior and 
an inferior retinal artery, which immediately divide again into the superior 
temporal, superior nasal, inferior temporal and inferior nasal 
arteries. The temporal arteries give off first a superior and an inferior 
macular artery, and then many branches within the layer of nerve fibers as far 
as the ora serrata, that do not anastomose. Its capillaries do not penetrate be- 
yond the external granular layer. The layer of rods and cones is nonvascular 
and receives its nutrition the same as the pigment epithelium from the choroid. 
The central vein of the retina follows the same course as the artery. 
The ciliary vascular system is divided into a proximal and a distal 
portion, which have many anastomoses. The proximal part is fed by the 
short posterior ciliary arteries, running in the immediate vicinity of the 
optic nerve, which are at first from four to six trunks that divide repeatedly in> 
the neighborhood of the eyeball, pierce the sclera near the optic nerve, and 
enter the lamina vasculosa choroideae. On the way they supply the optic nerve 
and its sheaths with many branches that anastomose with branches of the cen- 
tral artery and form long capillary plexuses about the nerve fibers within the 
bundles. At their passage through the sclera they give off several branches in 
the lamina cribrosa that anastomose with branches from the central artery and 
form Haller's circular arterial plexus. The short posterior ciliary 
arteries supply the entire choroid with blood up to the margin of the orbiculus 
ciliaris, and here connect with the distal ciliary vascular system through the 
recurrent arteries. The larger vessels lie externally in the choroid, while the 
capillaries lie farthest in. Thus the layer of rods and cones of the retina is 
supplied by the short posterior ciliary arteries, and all of the other layers by 
the central artery of the retina. 232 
The distal ciliary vascular system is fed by both the long posterior 
ciliary, and the anterior ciliary arteries. The former run with the 
preceding, one to the temporal, the other to the nasal side, pierce the sclera 
with them and pass distally between the sclera and the choroid. Arriving at 
the ciliary body they receive the recurrent branches of the short posterior 
ciliary arteries and then sink into the circulus arteriosus iridis major, a 
circular plexus at the root of the iris which gives off arteries to the iris and to 
the ciliary body. The former radiate in numerous twigs in the stroma of the 
iris as far as the margin of the pupillary part, forming many anastomoses with 
their neighbors on the way. At this margin a second circular plexus is formed, 
the circulus arteriosus iridis minor, from which twigs radiate again 
toward the pupil. The capillaries of the stroma and muscles of the iris come 
from this plexus. The arteries in the ciliary body form an extensive plexus 
from which capillaries branch to twine about the muscle fibers. 
The anterior ciliary arteries come from those of the ocular muscles 
and pass through the sclera in the region of the orbiculus. They give off first 
the proximally running episcleral vessels which enter the sclera, branch in it, 
and anastomose with the posterior ciliary arteries. Then they give off the 
conjunctival arteries, which pass forward to the margin of the cornea, where 
they bend back so as to form loops. After they have given off these branches 
the arteries perforate the sclera obliquely proximally to the canal of Schlemm 
and sink into the circulus arteriosus major. 
The ciliary veins are less distinctly divided into a proximal and a distal 
system. The main part of the venous blood is carried away by the venae 
vorticosae. The larger veins of the lamina vasculosa choroideae in each quad- 
rant radiate to a point in the region of the equator where they unite into a large 
venous trunk, the vena vorticosa, of which the eyeball contains four, frequently 
five. They pass through the sclera and receive the scleral blood from the 
episcleral veins. The anterior ciliary veins correspond in their course to 
the distal ciliary arteries and receive the venous blood from the conjunctiva 
bulbi, as well as a part of that from the ciliary body and iris, most of which 
goes to the venae vorticosae. The anterior ciliary veins are also in open con- 
nection with Schlemm's canal, but this is never filled with blood. The veins 
corresponding to the short posterior ciliary arteries are developed very poorly. 
m. The Lymphatics of the Eyeball 
Like the brain the eyeball contains no closed lymphatics lined with epithe- 
lium, but has in place of them a system of lymph canals in the cornea 
and sclera, which is in open connection with the lymphatics of the conjunctiva. 
It has also the perichoroidal space, situated within the lamina supra- 
choroidea, that communicates with the subdural and subarachnoidal spaces of 
the sheath of the optic nerve, by means of which a communication exists be- 
tween the two eyes. Finally the large chambers within the globe are to be in- 
cluded among the lymph spaces. The anterior chamber is in open communi- 
cation with the posterior; both are filled with aqueous, an alkaline fluid that 
has a composition similar to that of the vitreous fluid. The aqueous is secreted 
by the nonpigmented cells of the pars ciliaris retinae, and flows away for the 233 
most part through the sinus of the anterior chamber and the canal of Schlemm 
into the anterior ciliary veins. A part is diffused through Descemet's mem- 
brane into the proximal layers of the cornea, and another part is taken up by 
the veins of the iris. 
n. The Nerves of the Eyeball 
The nerves of the eyeball, aside from the optic, are the short ciliary from 
the ciliary ganglion, and the long ciliary from the nasociliary, comprising in 
all seven or eight slender nerve filaments that approach the eyeball as both 
medullated and nonmcdullated fibers. They perforate the sclera in the vicinity 
of the optic nerve, pass forward between it and the choroid, and enter the 
plexus of the ciliary body. On the way they give off branches to the 
choroid, sclera and cornea. 
The nerves of the choroid accompany the blood vessels and are for the 
most part nonmcdullated. They form a very wide meshed plexus within the 
lamina vasculosa, in which little ganglia are imbedded. All of the nerves ter- 
minate on the vessels. 
The nerves of the sclera almost without exception accompany the ves- 
sels and are chiefly vasomotor, yet fibers are also found that end free between 
the bundles of connective tissue. 
The nerves of the cornea, about forty in number, pass distally from 
the ciliary nerves to the region of Schlemm's canal, where they form the cir- 
cular plexus annularis. About sixty little nerve trunks radiate from this 
toward the cornea, lose their medullary sheaths, and enter the substantia pro- 
pria. By continuously dividing and anastomosing they form here an extensive 
basal plexus, which sends out little stems to the epithelium and the basal 
substance. The nerves to the epithelium divide repeatedly and pass obliquely 
through the superficial layers of the basal substance and Bowman's membrane 
to form on the distal surface of the latter the very close meshed subepithelial 
plexus, from which fibrils enter between the epithelial cells, ramify, and have 
free ends. The branches of the basal plexus destined for the substantia propria 
end in little platelike expansions with which they often coapt the corneal 
corpuscles. 
The plexus of the ciliary body, or plexus gangliosus ciliaris, lies 
partly on the outer surface of the ciliary body, partly between its bundles and 
consists of both medullated and nonmcdullated fibers with numerous nerve cells. 
Fibers proceed from this plexus which end free in arborizations between the 
muscular fibers, as well as others that end with little nodules on these fibers, and 
still others that end in a similar manner on the muscles of the vessels. 
The nerves of the iris come from the same plexus and form another one 
which is wide meshed in the ciliary part, close meshed toward the pupil, and 
contains no ganglia. Fibers proceed from this to end in the same way as those 
of the ciliary body on the muscles and vessels of the iris. 
o. The Eyelids and the Conjunctiva 
The eyelids appear in the second fetal month as low, oval folds of skin 
about the cornea. They remain low at the inner and outer angles, while the 234 
parts between these points grow very rapidly, so that at first a biconvex and 
later a slitlike palpebral fissure is formed which is completely closed in the 
third month by an adhesion of the margins of the lids. We have then on the 
distal side of the cornea a fissure-shaped cavity closed by the lids, the con- 
junctival cavity. This extends laterally for a considerable distance beyond 
the margin of the cornea and is lined by a mucous membrane called the con- 
junctiva, the epithelium of which consists at an early period of a superficial 
layer of cylindrical, and one of lower cells which later become several. The 
conjunctiva begins at the margin of the cornea, covers the distal surface of the 
sclera as the conjunctiva bulbi, then bends about to cover the proximal 
surface of the lids as the conjunctiva palpebrarum. Just before arriving 
at the point where the lids are adherent the epithelium passes over into the 
adhering epidermis. A process of cornification begins in the cells of the epi- 
dermis in the eighth fetal month which dissolves the adhesion and permits the 
lids to open. Hairs, and both sweat and sebaceous glands develop from the 
palpebral epidermis. A line of sebaceous glands grows out from the adherent 
epithelium of the inner edge of the lids in the third month, which attain a very 
considerable degree of development, and become known as the Meibomian 
glands. They grow into the connective tissue of the lids and come to lie in 
a zone of dense connective tissue that supports the central part of each lid 
and is called the tarsus. 
The distal surface of the lid of the adult is covered by the skin, the gen- 
eral characteristics of which are the same as those of that of the face. The 
cutis has no papillae, except at the inner edge, but has a smooth surface next 
to the epidermis. The subcutis is not very fat, but is very loose, so that the 
skin of the lid is lifted up into folds very easily. Little woolly hairs, sebaceous 
follicles and sweat glands are found in it. Next proximally to the subcutis lies 
the musculus orbicularis oculi, the circular, striated fibers of which sur- 
round the palpebral fissure. These fibers are assembled into bundles of varying 
size, grouped to form a single layer that extends over the entire breadth of the 
lid from its inner edge (Pl. 90, Fig. 193). 
The cilia project in one or two rows from the outer edge of the lid and 
arch gently outward. Their follicles penetrate deeply into the layer of the 
orbicularis and so separate from it the most central portion, neighboring the 
inner edge of the lid, which is known as the musculus ciliaris, or Riolan's 
muscle. Between the cilia lie the glandulae ciliares, or Moll's glands, 
particularly large sweat glands that open together with the sebaceous, into the 
follicles of the cilia. They have a wider lumen and are not so convoluted as 
the other sweat glands of the lids. 
The Meibomian glands, the most prominent formations of the lids, open 
at the inner edge. They lie in a single row enclosed in the tarsus, and number 
thirty in the upper, twenty-five in the lower lid. They are from 8 to 10 mm 
long in the upper lid, and about half as long in the lower. Each gland begins 
with a single, short excretory duct which is studded with numerous alveoli. 
The duct is lined wuth stratified flat epithelium, while the alveoli are filled with 
polyhedral fatty cells, the most central of which break down and furnish the 
secretion, the sebum palpebrale. 235 
These glands are enclosed in the tarsus, a firm plate of densely felted 
bundles of connective tissue that occupies about the central two thirds of each 
lid, those of the upper and lower lids being connected at the outei' and inner 
canthi. The tarsus appertains to the conjunctiva, supports the lid, and fur- 
nishes a smooth gliding surface for the movements of the latter upon the eye- 
ball. It is separated from the orbicularis by the fascia palpebralis, a tight 
connective tissue into which the tendons of the levator palpebralis superioris 
and the inferior rectus radiate peripherally. Proximally from these tendons 
are smooth muscular fibers which originate in the upper lid between the fibers 
of the levator, in the lower from the connective tissue of the fornix, and are 
inserted into the peripheral end of the tarsus. These smooth muscles are the 
tarsales superior and inferior. 
The conjunctiva palpebrarum is covered by a stratified cylindrical 
epithelium consisting of a superficial layer of cylindrical, and one or two 
deep layers of cuboidal cells. It begins a little way from the inner edge of the 
lid, developing from the gradually thinning epidermis. Many goblet cells 
lie between the cylindrical cells, which are provided with a homogeneous cuti- 
cula. The surface of the mucous membrane is perfectly smooth over the entire 
extent of the tarsus, but toward the fornix it becomes uneven and presents 
many depressions. The propria of the conjunctiva is a firm connective tissue 
that joins the epithelium closely to the tarsus. Its content of lymphocytes 
varies in different individuals. In the fornix are to be found Krause's glands, 
or the glandulae lacrimale accessorise, the excretory ducts of which open 
in to fornix conjunctivae. They resemble the lacrimal glands in their structure. 
In the fornix the palpebral passes over into the conjunctiva bulbi, 
which is joined to the sclera by a loose, subconjunctival connective tissue. Both 
conjunctivae are practically alike in structure. The epithelium is the same, ex- 
cept that a few millimeters from the margin of the cornea it changes into a high, 
stratified flat variety, the direct continuation of which is the epithelium of the 
cornea. Not rarely it contains pigment granules. 
The plica semilunaris, a rudiment of the palpebra tertia found in many 
animals, is a fold of the conjunctiva bulbi and has the same structure. Under 
certain circumstances a little plate of cartilage may appear in its propria. The 
caruncula lacrimalis, situated to its nasal side, has its top covered with 
stratified flat epithelium which changes on its sides to that of the conjunctiva. 
It contains sebaceous and sweat glands and frequently little hairs. 
The arteries of the eyelids, the median and lateral palpebral, form in each 
lid two vascular arches by anastomosis, the arcus palpebralis externus 
and internus. The former lies in front of the peripheral margin of the 
tarsus, the latter in the region of the ciliary muscle. The two arches anasto- 
mose frequently through pretarsal branches and send twigs to the cutaneous 
and conjunctival parts. The conjunctiva has beneath its epithelium a very 
close meshed plexus of very varicose capillaries. The veins form a con- 
junctival and a cutaneous plexus. The former receives the blood from the con- 
junctiva and pours it into the muscle veins. The cutaneous plexus carries off 
most of the blood and empties into the superficial temporal and the angular 
veins. 236 
The lymphatics of the lids form a closed system that follows the course 
of the arteries and is divided into a deep and a superficial portion. The efferent 
passages open laterally into the parotid, medially into the mandibular lym- 
phatic glands. 
The nerves of the eyelids, containing sensory and motor fibers, come from 
various sources that lead back to the ophthalmic, maxillary and facial. They 
form a pretarsal plexus that is particularly well developed in the region of the 
free margin of the lid. Numerous fibers from this penetrate the tarsus, twine 
about the Meibomian glands, and form a second plexus in the propria of the 
conjunctiva. Other fibers run to the muscular bundles of the orbicularis, and 
to the cutis, where they form another plexus, the fibers from which end on the 
muscle fibers and the glands, penetrate into the epithelium, or terminate in end 
clubs. Such a terminal apparatus as the latter is chiefly to be found in the 
conjunctiva bulbi, and at the free margin of the lid just beneath the epithelium. 
p. The Lacrimal Glands 
The lacrimal glands develop in the third fetal month as solid cellular cords 
growing out from the epithelium of the fornix, which soon ramify very 
abundantly, become hollow, and form two separate glands in such a way that 
the excretory duct of the upper is surrounded by the ramifications of the lower. 
At birth the glands are fully developed, but they do not begin to secrete until 
the third month of life. 
The secretion of the lacrimal glands, the tears, is a limpid, alkaline fluid 
having a distinctly salty taste due to the presence of from 1 to 3% of sodium 
chloride. It also contains about 0.5% of albumin. 
The lacrimal are compound branched tubular glands with from 
eight to twelve excretory ducts that open in the temporal part of the fornix. 
The parenchyma of the gland is divided into many large and small lobules by 
septa from the connective tissue capsule. The excretory ducts are lined 
with stratified cylindrical epithelium formed of an inner layer of cylin- 
drical cells, an outer layer of cuboidal, and a homogeneous membrana propria. 
Having entered the gland the ducts branch many times in the interlobular con- 
nective tissue, both layers of cells continuously decrease in height, and the 
lumen becomes very narrow. After entrance into the primary lobules the inter- 
lobular ducts divide into the secretory tubules. These latter also have a 
bilaminate epithelium, though the outer cells are quite flat, branched, and 
force themselves between the membrana propria and the secretory cells. Prob- 
ably these are contractile cells, similar to those found in the sweat glands, 
which are phylogenetically related to the lacrimal. The secretory cells are 
partly cylindrical, partly cuboidal. They contain secretory granules, like 
those of the albumin secreting glands, and two central bodies near the lumen. 
The basal part of the cell is free from secretory granules and contains fibrillary 
radiating mitochondria from which the secretory granules are produced. The 
cells also contain minute drops of fat. The spherical nucleus lies near the base. 
Secretory capillaries appeal' between the cells. 
Branches of the lacrimal arteries enter the lobules and twine about the 237 
tubules and ducts with their capillaries. The veins empty into the ophthalmic, 
or those of the muscles. 
The nerves of the lacrimal gland come from the lacrimal, most of the 
branches of which only pass through its substance. Those that supply the 
tubules are nonmedullated and form one plexus on the membrana propria, an- 
other at the bases of the secretory cells, from the latter of which fibrils enter 
and terminate between the cells. 
q. The Lacrimal Passages 
The formation of the lacrimal passages proceeds from the nasolacrimal 
groove produced by the union of the lateral nasal and maxillary processes. 
From this a solid epithelial ridge separates itself and produces an epithelial 
cord that grows forward on one side toward the nasal cavity, and bifurcates 
on the other toward the median periphery of the circular fold of the eyelids 
which appears at this time. Both branches reach the margins of the lids in 
the third month, but the trunk arrives at the epithelium of the nasal cavity con- 
siderably later. The canalization of the solid epithelial rudiment begins in the 
early part of the fourth month at the place where the canaliculi are given off, 
and progresses from this point toward both the nose and the lids. 
The mucous membrane of the canaliculi is covered by a thick 
stratified flat epithelium resting on a propria that is rich in elastic fibers. 
Between the two is a homogeneous membrana propria. The striated muscular 
fibers, which are circular about the vertical part and longitudinal along the 
horizontal, appertain to the orbicularis oculi. 
The mucous membrane of the lacrimal sac and of the nasolac- 
rimal duct is on the contrary covered with a stratiform epithelium which 
is practically the same as that of the respiratory nasal mucous membrane, ex- 
cept that cilia are absent in places, and that goblet cells are met with less fre- 
quently. The propria also has the character of that of the respiratory mucous 
membrane, as it is infiltrated with many lymphocytes. Little follicles are 
formed in many places, but glands like the glanduale nasales appear only in 
its lower part. 
The arteries of the lacrimal passages come partly from the median palpe- 
bral, partly from the dorsalis nasi and angularis, and form a subepithelial 
capillary plexus. The veins have relations in the nasolacrimal duct that are 
quite similar to those in many parts of the nasal cavity. They form deep 
seated plexuses that present the characteristics of erectile tissue. 
r. The Extrinsic Ocular Muscles 
The muscles that serve to move the eye are composed, like those of the 
body and limbs, of striated muscular fibers, but differ from them in a 
few points. In the first place the fibers are considerably more slender (Pl. 
68, Fig. 159). This is particularly the case with the peripheral fibers of each 
muscle. Furthermore many of the fibers of these muscles are rich in proto- 
plasm. The nuclei also are not so very peripheral as in the fibers of other 
muscles. 
The extrinsic ocular muscles are supplied very abundantly with sensory 238 
nerves that accompany the muscular fibers for long distances and end on 
them with fine nodules and plates (Pl. 71, Fig. 163). There are also fibers that 
wind about the muscle fibers like bands and give off numerous lateral twigs and 
plates. The conditions here are therefore similar to those of the sensory fibers 
of the muscle spindles, which are not to be found in human ocular muscles. 
Tendon spindles are likewise lacking. The motor endings do not differ from 
those of other muscles. 
2. THE EAR 
a. The Internal Ear 
The fh'st sign of the ear appears in the human embryo in about the third 
fetal week as a roundish spot of thickened epithelium situated laterally from 
the posterior cerebral vesicle, known as the ear plate. Quite similarly to the 
later appearing rudiment of the lens, the center of the plate sinks in, causing 
it to take the shape of a cup, and this auditory cup is closed in about the 
beginning of the fourth week to form the auditory vesicle. The latter lies 
close to the hind brain and consists of a low cylindrical epithelium that is higher 
on the median wall. Rostrally to this median wall lies a ganglion from which 
fibers advance to the wall very soon, while a great part of the undifferentiated 
epithelium here is transformed into neuroepithelium. A duct grows out 
from the dorsal part of the now somewhat elongating vesicle that closely ap- 
proximates the hind brain, the ductus endolymphaticus. The portion ly- 
ing dorsally from its mouth we call the pars dorsalis, the rest the pars ventralis 
of the auditory vesicle. A superficial pouch first protrudes dorsally from the 
pars dorsalis, and a similar one laterally a little later. The former, the 
vertical pouch, has at its base a dilatation, the ampulla, into which a part 
of the neuroepithelium enters from the median wall. A similar ampulla is placed 
at the rostral end of the second, horizontal pouch, so that we have in all three 
ampullae supplied with nerves in the pars dorsalis. Two corresponding 
places of the closely approximated epithelial walls fuse in the vertical pouch 
and become absorbed so as to present two windows as it were with a high pillar 
between them. Two semicircular canals thus originate from one pouch, 
an anterior and a posterior. Both start from the remains of the pars 
dorsalis with a broad base, the anterior and posterior ampullae, unite dor- 
sally and open again into the pars dorsalis with a common canal. A similar 
process takes place in the horizontal pouch, except that only one window is 
formed, and therefore only one horizontal semicircular canal. The two 
vertical semicircular canals are at first in the same plane, but later they be- 
come perpendicular to each other, revolving as it were about the common canal 
as an axis. We have now three approximately semicircular canals in the three 
planes of space that start from the pars dorsalis with five openings, three of 
which are dilated into ampulla?, while two are not. The ductus endolymphati- 
cus runs along the median surface of the common canal and is dilated at its 
distal end into the saccus endolymphaticus. 
The transformations of the pars ventralis, part of which are taking place 
at the same time, are as follows. This portion grows forward ventrally, be- 239 
comes narrowed and bends off toward the rostrum. This bent part is called 
the cochlear process. Its proximal segment swells out and takes a position 
against the remains of the pars dorsalis, from which the semicircular canals al- 
ways rise more sharply. The originally median area of epithelium supplied 
by nerves splits up again, one part going to the remains of the pars dorsalis, 
which we now call the utriculus, a second to the saclike dilatation of the 
proximal part of the canal of the cochlea, the sacculus, and another to this 
canal of the cochlea itself. Consequently the canal of the cochlea is still more 
thinned and bends into a spiral as it increases in length. Its initial piece con- 
tracts to a short duct, the canalis reuniens, connecting with the sacculus. 
The mesenchyma in the vicinity of the epithelial auditory vesicle condenses 
at an early period into a blastema which secretes a cartilaginous capsule 
about the organ, that begins to ossify in the fifth fetal month. The original 
mesenchymatous tissue remains preserved between the cartilage and the epithe- 
lial wall, first produces a membrana propria about the epithelium and then 
liquefies for the most part, giving rise to the perilymphatic spaces inter- 
posed between the membranous and the bony labyrinth on the lateral side. The 
bony labyrinth has on the whole the same form as the membranous, but en- 
closes the sacculus and the utriculus with a common capsule, the bony vestibule, 
from which pass on one side the bony semicircular canals, on the other the 
bony cochlea. 
The nasal layer of the membranous labyrinth, leaving out the cochlea, forms 
a membrana propria that is only 4 or 5 p thick in most places, but attains 
a thickness of 100 to 200 p at the places where the nerves end. It is structure- 
less, or presents only a very slight striation, and contains a varying number of 
stellate cells that anastomose through their processes. It is adjoined externally, 
by connective tissue, the fibers of which unite with the periosteum and pass 
through the perilymphatic spaces. Internally the membrana propria is covered 
by an epithelium that consists of a single layer of quite flat, polygonal 
cells, except at the places where the nerves terminate. It is only in the raphe 
of the semicircular canals that the epithelium is a low cylindrical (Pl. 91, 
Fig. 194). 
Excluding the cochlea we have five terminal places for the nerves, a 
crista acustica in each ampulla, a macula acustica in the utriculus and 
in the sacculus, and a macula neglecta in the former. The maculae are 
flat, platelike thickenings of the wall of the labyrinth in which the membrana 
propria is involved to an even greater extent than the epithelium. The cristae 
are crescentic ridges protruding into the lumen of the ampullae that have on 
section the form of a cone. The crest is formed from the greatly thickened 
membrana propria, which presents externally a furrow corresponding to the 
crest, the sulcus transversus. Internally the crest is covered by neuro- 
epithelium, which changes gradually on its sides into flat epithelium. The 
two thickened ends of the crest are surrounded by a zone of simple cylindrical 
epithelium, the planum semilunatum (Pl. 91, Fig. 194). 
The sinus epithelium, or neuroepithelium of the maculae and cristae.; 
is stratiform and consists of two kinds of cells, hair and stay cells. The 
hair cells lie a little distance apart, forming a single layer on the epithelial 240 
surface. Each is bottle-shaped, 25 to 40 p long, and extends down with its 
basal end to about the middle of the epithelium. Its nucleus is large, spherical, 
has a well-developed framework of chromatin and a nucleolus, and lies near the 
base of the cell. Two central bodies are found near the surface. The latter 
is covered by a cuticularized plate from which a number of fine auditory hairs 
extend. It is difficult to determine the length of these as they do not run out 
freely. On the cristas they enter into the formation known as the cupula 
terminalis, which looks in a cross section like a cylindrical hat resting on 
the crista (Pl. 91, Fig. 194), and exhibits a very distinct longitudinal striation. 
Views differ as to its morphological interpretation. On the maculae the auditory 
hairs very soon bend at a right angle, lie close together, and form the abut- 
ments of the otoliths, crystals of carbonate of lime up to 10 p long, that are 
held togethei* by a sort of membrane. The stay cells lie between the hair 
cells, theii* bodies resting on the base of the epithelium, their nuclei forming two 
or three rows. Each cell sends a slender process toward the lumen, which 
reaches the surface between the hair cells. 
The nerves coming to the maculae and cristae are the peripheral processes 
of the bipolar cells of the intumescentia ganglioformis of the internal 
auditory meatus (Pl. 91, Fig. 194; Pl. 29, Fig. 75). They lose their medullary 
sheaths as they pass through the membrana propria and form a plexus beneath 
the epithelium, from which fibers enter the latter, where they run between the 
hail* cells and the bodies of the supporting cells and then turn toward the bases 
of the latter. Each fiber forms a sort of terminal bowl into which a hair cell 
is set like an acorn in its cup. A fiber often divides into several twigs, each of 
which receives the base of a hair cell in its terminal cup. A great number of 
fibrils radiate from the terminal cup, are studded with varicosities, and join 
closely to the protoplasm of the hair cells. 
The membranous cochlea needs a separate description because its 
structure is so complex. It is a closed duct, the ductus cochlearis, blind 
at both ends, which begins in the bony vestibule with the vestibular blind 
sac, rises within the bony cochlea in a spiral of two and a half to two and 
three quarters turns, and terminates with the blind sac of the cupola. The 
very narrow, frequently obliterated, canalis reuniens, or Hensen's canal, 
coming from the sacculus, opens into the ductus cochlearis distally from the 
short, blind sac of the cupola. The ductus cochlearis is triangular on cross 
section (Pl. 91, Figs. 194 and 195). If we think of the cochlea as standing 
upright, although in fact it lies horizontally, and this applies to all that fol- 
lows, the section of the cochlear duct forms a nearly right angled triangle, the 
vertical cathetus of which is not a straight line, but one that is curved out- 
ward. The ductus cochlearis touches the capsule of the cochlea with this verti- 
cal cathetus. The horizontal cathetus extends from the bony outer wall to the 
lamina spiralis ossa of the modiolus cochleae, while the hypotenuse runs obliquely 
through the bony cochlear duct. There are two cavities, one above the coch- 
lear duct, the scala vestibuli, and one beneath it, the scala tympani. The 
latter ascends with the membranous cochlear duct in the bony duct and merges 
with it in the cupola in the so-called helicotrema. The scala tympani ends 
blind at the ventricular blind sac, being closed in by the membrane of the 241 
fenestrum rotundum. The scala vestibuli on the contrary descends into the large 
perilymphatic space of the bony vestibule. 
If we now turn back to the form of the ductus cochlcaris we see that the 
horizontal cathetus of the triangle is shortest in the base of the cochlea, that 
it becomes longer as we go up, and that the reverse is true of the vertical 
cathetus, because the membranous cochlear duct constantly becomes broader 
and lower toward the apex of the cochlea. 
In the initial portion of the vestibular blind sac the horizontal wall, which 
we now prefer to call the tympanal wall on account of its neighborhood to the 
scala tympani, has an extremely complicated terminal nerve apparatus, the 
papilla spiralis, or the organ of Corti. As a whole this is a spiral epithe- 
lial crest that slopes gradually into both the ventricular blind sac and the 
blind sac of the cupola, falls away abruptly on both sides, gradually broadens 
from below upward, and attains its greatest thickness in the middle of the 
cochlea. We will now briefly analyze the separate walls of the canal of the 
cochlea (Pl. 91, Fig. 195). 
The vestibular wall separates the ductus cochlearis from the scala ves- 
tibuli and is commonly known as Reissner's membrane. It extends from 
the crista Reissneri on the outer wall obliquely inward and downward to the 
limbus spiralis covering the lamina spiralis ossea. Its composition is that of 
a double, very thin layer of epithelium. The flat cells blend on the one hand 
with the epithelium of the scala vestibuli, on the other with that of the limbus 
spiralis and the outer wall of the ductus cochlearis. 
The outer wall of the ductus cochlearis is formed by the ligamentum 
spirale, a cushion of connective tissue placed within the cochlear capsule which 
is crescentic in cross section. It reaches up a little way into the scala vestibuli, 
while below it covers almost the entire outer wall of the scala tympani. Its inner 
wall shows three projections, the crista Reissneri, from which Reissner's 
membrane extends, the crista spiralis, which serves for the insertion 
of the tympanal wall of the ductus cochlearis, and a little above this the 
flat prominentia spiralis. The spiral ligament is a much thickened perios- 
teum, but contains a basal substance resembling the membranae proprise. In 
it lie fine connective tissue fibers and numerous stellate or elongated cells with 
anastomosing processes. All of the connective tissue fibers in the region of the 
crista spiralis converge toward the latter. The spiral ligament, so far as it 
lies in the ductus cochlearis, is covered by an epithelium which we may differen- 
tiate into two different portions, the stria vascularis, reaching from the 
crista Reissneri to the prominentia spiralis and adjoining the sulcus spiralis 
externus, which leads over into the epithelium of the tympanal wall. 
The stria vascularis presents a fairly high stratified epithelium, 
the superficial cells of which are flat and stain intensively, the deeper ones 
irregularly cuboidal. The demarcation of the spiral ligament from the con- 
nective tissue is less sharp. The characteristic mark of this epithelium is the 
number of blood capillaries it contains just beneath the most superficial 
layer of cells, because of which the secretion of the endolymph that fills the 
membranous labyrinth is ascribed to the stria vascularis. Embryology teaches 
us that the epithelium of the stria is a mixture of epithelial and connective 242 
tissue, the former grown through by the cells of the latter, as we shall soon 
see in another place. 
On the prominentia spiralis, in the substance of which is a blood vessel, 
the vas prominens, the epithelium thins into a single layer of flat or cuboidal 
cells, and blends with that of the sulcus spiralis externus. This sulcus 
encroaches upon the tympanal wall and is .but slightly developed in the base 
of the cochlea because of the shortness of that wall, but gradually attains a 
considerable breadth. Its epithelium consists of a single layer of cuboidal 
or cylindrical ceils, between which are long connective tissue cells that 
reach the surface in the vicinity of the prominentia spiralis. 
The tympanal wall stretches between the crista spiralis externally and 
the lamina spiralis ossea internally. The latter is composed of two bony 
plates, between which a canal leads into the spiral canal and is covered by a 
plateaulike mass of connective tissue, the limbus spiralis. Toward the 
spindle of the cochlea the limbus slowly blends with the periosteum, in the 
ductus cochlearis it projects like a hook to form the sulcus spiralis internus 
by an arch convex internally to the lamina spiralis. We may thus distinguish 
two lips of the limbus, one vestibular, extending freely into the ductus coch- 
learis, and one tympanal, covering the end of the lamina spiralis ossea and 
blending with the membrana basilaris about to be described. The structure 
of the limbus is quite similar to that of the spiral ligament. Its free surface 
is traversed by radiating furrows that divide it into about twenty-five hun- 
dred ridges, the auditory teeth of Huschke. These do not appear in 
sections as the furrows separating them are perfectly leveled by the epithelium 
covering the limbus. This epithelium blends at the point of the hook with 
the simple cuboidal epithelium of the sulcus spiralis internus. The organ of 
Corti lies between the latter and the sulcus spiralis externus. 
The tympanal wall of the ductus cochlearis is formed by the membrana 
basilaris which is stretched between the tympanal lip of the limbus spiralis 
and the crista spiralis. It consists of radiating, flat fibers of connective tissue, 
the auditory basilar fibers, the length of which increases constantly from 
the base to the apex of the cochlea. They are embedded in a homogeneous 
basal substance. The membrana basilaris is bounded toward the ductus 
cochlearis by the finely striated, nucleated border layer, toward the scala 
tympani by the tympanal cover layer, which consists of several lamin® of 
elongated, loosely placed cells that blend on each side with the single layer of 
low cells of the epithelium of the scala tympani. Within this tympanal cover 
layer may be seen the cross section of the vas spirale. 
The membrana basilaris serves as vibrating membrane on which the per- 
ceptary apparatus of Corti's organ is constructed, grouped into two peculiar 
lines of pillars. These pillars are long, slender, cuticular formations that start 
from the membrana basilaris with broad bases, taper, and then swell out again 
into a knob. They stand with their feet wide apart and with their upper ends 
leaning against each other, so as to enclose a triangular spiral space that extends 
to the top of the cochlea, Corti's tunnel. Its breadth increases from below 
upward, the same as the length of the basilar fibers. The pillars are known as 
the outer and the inner. The outer pillars are longer and slant more 243 
than the inner. Each outer pillar ends above in a roundish head that is 
prolonged outward in a narrow head-plate. The inner pillars are shorter 
and stand more nearly upright. They also have knoblike swellings above, 
but each furnishes a sort of socket in which the head of an outer pillai' is 
received, so as to form a jointlike articulation. The head of the inner pillar 
is prolonged outward in a phalangeal process that covers the head of 
the outer pillai' and closely adjoins the beginning of its headplate. The 
inner pillars are more slender than the outer, and number six thousand to 
forty-five hundred of the latter, so one outer pillar often has to articulate 
with two inner pillars. 
The pillars are cuticular formations secreted by the pillar cells or 
basilar cells, the remains of which we may still see forming a protoplasmic 
coat to Corti's tunnel containing a row of nuclei at the foot of each pillar. 
Fine supporting fibrils pass through the homogeneous substance of the pillars 
and form a dense bundle in the middle portion, from which they radiate like 
a fan into the feet and heads. 
The other parts of the organ lie inside and outside of the framework 
formed by the pillars. The inner division of Corti's organ consists of 
a single row of inner hair cells. These are bulging or cylindrical cells 
with rounded bases that occupy only a little more than half the thickness of 
the epithelium. The longitudinal axis of each is parallel to that of the inner 
pillar. The large, spherical nucleus lies at the base of the cell, the proto- 
plasm is very clcai- and shows between the nucleus and the base a granular 
thickening, Retzius's body, as well as a more filamentous thickening in the 
upper end, Hensen's body. The cell has a cuticularized cover plate on 
its free surface, from which extend ten or twelve short, thick auditory hairs. 
The space beneath the inner hair cells is filled by two high cylindrical cells, 
the outer of which is known as the inner phalangeal cell, the inner as the 
border cell. They hold the inner auditory cell between them and each sends 
to the surface of the epithelium a long process that has a little cuticularized 
plate lying at the same level with the cover plate of the inner hair cell and the 
headplate of the inner pillar. Farther inward the epithelium of Corti's organ 
suddenly declines and blends with that of the sulcus spiralis internus. 
The outer division of Corti's organ is attached to the outer pillar, 
and has in general the same structure as the inner, except that instead of one, 
there are three or four rows of hair cells. Consequently, there are three or 
four times as many outer hair cells as inner, 12,000 of the one to 3,300 of 
the other. Each hair cell is separated from its neighbor by a supporting 
cell, here called a Deiter's cell. The outer hair cells have the same struc- 
ture as the inner, and differ only in being rather longer, more slender, and 
less bulging. Between the first outer hair cells and the outer pillar is an 
open space, called Nuel's space. The supporting cells are long and cylin- 
drical. The upper ends of their bodies grasp each the base of a hair cell and 
extend out in a slender process between two neighboring hair cells to the 
surface of the epithelium, where a knucklelike headplate articulates with it 
between the covers of the hair cells. The last supporting cell, the third 
outer, is particularly strong, and has a long process that is bent inward. 244 
On the other side of this the epithelium again slopes away abruptly and 
blends with that of the sulcus spiralis externus. Each outer supporting cell 
has a bundle of fibrils passing through its longitudinal axis, part of which 
radiate out in a fanlike manner beneath the base of the hair cell, while part 
continue into the process. The nuclei of the supporting cells lie deeper than 
those of the hair cells. 
The cuticularized plates of the inner and outer supporting cells are mov- 
ably connected with the headplates of the outer and the phalangeal processes 
of the inner pillars. Thus there is formed on the surface of the organ of 
Corti a membrane composed of separate pieces movable upon one another, 
which extends from the border cell on the inner side to the last supporting 
cell on the outer, is known as the membrana reticularis, and presents 
roundish apertures in which the hair cells with their cover plates are so inserted 
that no one touches another. 
The nerves entering the organ of Corti come from the spiral ganglia, 
which fills the spiral canal as a coherent mass. Like the intumescentia ganglio- 
formis, it is composed of bipolar cells, the peripheral processes of which are 
invested with medullary sheaths, and penetrate between the two layers of the 
lamina spiralis ossea to Corti's organ, which they enter through apertures 
in the basilar membrane, the foramina nervina. As they pass through 
these apertures the fibers lose their medullary sheaths and divide into two 
cords, one of which runs directly upward to the inner hair cells, while the 
other crosses Corti's tunnel and forms a plexus between the outer supporting 
cells, from which fibers rise to the bases of the hair cells, where they behave 
just the same as those of the maculae and cristae. 
The membrana tectoria, or Corti's membrane, forms a bridge over 
the sulcus spiralis internus from the limbus spiralis. It lies first as a thin 
cuticula on the epithelium of the limbus, then thickens greatly over the sulcus 
spiralis internus, thins again, and bends about like a hook, to end at the last 
supporting cells. It has a fine parallel striation, is cuticular in nature, and 
consequently contains no nuclei. Its position is not always the same; some- 
times it lies close to Corti's organ, sometimes at quite a distance, and probably 
acts as a muffler. 
The arteries of the membranous internal ear come principally from the 
internal auditory through the vestibular, vestibulo-cochlearis, and cochlearis. 
The first supplies the utricle, semicircular canals, and saccule, the last the 
cochlea, while the second sends its blood into both regions. The branches 
pass with the nerves to the membranous labyrinth, where they ramify on its 
outer surface and in the membrana propria. The plexuses in the latter are 
particularly abundant beneath the nerve endings. In the cochlea the arteries 
run out from the modiolus into the bony intermediate wall and enter the 
periosteum of the scalas, the ligamentum spirale, the stria vascularis, and the 
limbus spiralis, but do not penetrate any farther into Corti's organ. The 
venous blood of the cochlea gathers into the vena spiralis modioli. The 
vas spirale is probably one of the root veins and is connected with them by 
branches radiating between the leaves of the lamina spiralis ossea. The rest 
of the venous blood of the membranous labyrinth is carried away by the venae 245 
aqueductus vestibuli and aqueductus cochleae, the latter of which also takes 
a part of the blood from the cochlea. 
The cavities of the internal ear are lymph spaces. The membranous laby- 
rinth is a cavity that is completely closed in on all sides, does not communi- 
cate with the outer world, and is filled with endolymph, a limpid, alkaline 
fluid which contains traces of albumin, mucin and salts. No change of this 
fluid has been demonstrated. The cavity of the membranous labyrinth com- 
municates through the ductus endolymphaticus with the saccus endo- 
lymphaticus situated within the skull between the two layers of the dura. 
Under normal conditions it contains only very slight traces of endolymph. 
The perilymphatic spaces, that surround all parts of the membranous 
labyrinth, are particularly well developed in the bony vestibule, laterally from 
the saccule, where they form the cisterna perilymphatica, into which the 
perilymphatic spaces of the semicircular canals and of the scala vestibuli 
empty. These perilymphatic spaces are not closed like the endolymphatic, but 
are in open communication with the lymph spaces about the bulb of the 
jugular vein through the ductus perilymphaticus, which starts from the 
initial portion of the scala tympani. 
The composition of the perilymph is the same as that of the endolymph, 
except that it is richer in organic constituents. 
b. The Middle Ear 
The middle ear, oi' tympanic cavity, is a diverticulum of the first 
entodermal pharyngeal pouch, which appears about the end of the first 
fetal month as a narrow, prismatic cavity. From it comes by lateral growth 
and contraction of its median portion the tubotympanal canal, which is 
partially occluded for a time by adhesion of its epithelium. Dorsally from this 
lie the auditory ossicles, developing from the second pharyngeal arch and 
the cartilaginous capsule of the labyrinth, and surrounded by an embryonal 
gelatinous tissue. As the latter disappears the tubotympanal space advances 
and surrounds the auditory ossicles, so that finally these, covered with epi- 
thelium, come to lie in the space now called the tympanum, or drum. The 
portion of the tubotympanal space connecting the tympanum with the pharynx 
is drawn out by a marked longitudinal growth into the narrow tubal canal, 
or Eustachian tube, in the median and dorsal walls of which cartilage develops 
in the fourth month. 
The mucous membrane of the middle ear covers all the structures 
in this cavity and blends medially with the mucous membrane of the Eustachian 
tube, laterally with that of the tympanic membrane. The epithelium has a 
single layer of cells that are generally high cuboidal in the recesses and 
depressions, and flatter on the ossicles and the promontory. The cells are 
ciliated in the depressions, particularly on the floor of the tympanum and in 
the sinus tympani. The epithelium rests on a dense connective tissue propria, 
which passes directly into the periosteum. There is no submucosa, and no 
glands are to be found in the tympanum. 
At the ostium tympanicum this passes over into the thicker mucous 
membrane of the tube, which is connected first with the bone and then 246 
with the cartilage by a connective tissue submucosa, and forms longitudinal 
folds, especially along the floor of the tube. The epithelium is a continu- 
ation of that of the nasopharynx, stratiform and ciliated, in which many 
goblet cells may be found. The propria is thin and contains many lympho- 
cytes, which become the more numerous the nearer we approach the ostium 
pharyngeum. Here, also, are lymph follicles, that project into the sub- 
mucosa and constitute as a whole the tonsil of the Eustachian tube. In 
the propria and submucosa of the median wall of the cartilaginous tube are 
mucous glands that form a thick layer toward the pharynx. The hook- 
shaped cartilage lies mainly in the median wall and roof of the tube, and 
encroaches but little on its lateral wall. At birth it consists of hyaline car- 
tilage, but networks of elastic fibers develop in its basal substance later. 
There is very little to say about the ossicles from a histological stand- 
point. They contain a little medulla and are otherwise composed of compact 
bone substance. Their articular ends are provided with cartilages. The manu- 
brium has a cartilaginous coat which is complete in its distal part, but proxi- 
mally is confined to its lateral edge. A meniscus of cartilage is interpolated 
in each of the joints between the ossicles. The margin of the foot plate of 
the stapes is covered with cartilage, as is also the ora fenestras ovalis, while 
the annular ligament of connective tissue forms the junction between the two 
cartilages and allows the stapes only a limited movement. 
The two muscles of the middle ear consist of striated muscular 
fibers, some of which are very fine, some coarser. The stapedius is very rich 
in interfascicular connective tissue. The stapedius is innervated by the facial 
nerve, the tensor tympani by the trigeminus. The structure of the motor end 
plates is typical. Apparently muscle and tendon spindles are to be found 
in them. 
The arteries of the middle ear come from various sources, the stylomas- 
toid, middle meningeal, ascending pharyngeal, internal carotid, and tympanic, 
which furnish a very abundant supply of blood to the tympanum. The larger 
branches run in the periosteum and form a rather dense capillary plexus in the 
propria. Many direct arteriovenous anastomoses also appear to exist. The 
venous blood flows away in the pterygoid plexus, the middle meningeal, and 
the deep auricular veins. 
The lymphatics form a plexus in the propria of the mucous membrane 
and empty into the inferior auricular glands. 
The nerves of the tympanum come from the tympanic plexus in the peri- 
osteum of the promontory, which is formed from the medullated branches of 
the tvmpanic nerve. The plexus is also supplied with medullated fibers through 
the ramus anastomoticus nervi facialis, and with nonmedullated through the 
nervi carotico-tympani. It is plentifully furnished with ganglia and sends the 
minor superficial petrosal nerve to the otic ganglion, as well as numerous 
branches to the tympanic mucous membrane. The latter form a subepithelial 
network from which twigs enter between the epithelial cells, and also a branch 
is sent to the mucous membrane of the Eustachian tube. 247 
c. The External Ear 
The external ear originates from the middle portion of the first pharyn- 
geal sulcus, which does not form a cleft in man, any more than any of the 
other four. It deepens and is surrounded by six little aural eminences, three 
of which appertain to the first visceral arch, and three to the second. While 
these blend with the free aural fold surrounding them caudally to form the 
pinna, the concha sinks in deeply toward the end of the second fetal month 
and forms a passage lined with epithelium, the primitive external auditory 
meatus, at the end of which is formed the roundish epithelial external 
auditory plate, which lies to the medial side of the cartilaginous rudiment of 
the handle of the malleus. The tympanum grows against it from the dorsal 
side and shoves it medially. After the lumen of the external auditory meatus 
has split into two leaves the external auditory plate, the meatus becomes closed 
off from the tympanum by a deeply situated almost horizontal membrane em- 
bracing the handle of the malleus, the tympanic or drum membrane. A 
reticular cartilage develops in the primary part of the meatus and in the pinna, 
while in the secondary part formed by the splitting of the external auditory 
plate the bony auditory meatus comes to lie in postembryonal life, developed 
from the tympanic ring and the adjoining parts of the temporal bone. 
Three parts of the adult membrana tympani may be differentiated from 
without inward, the pars cutanea, the substantia propria, and the pars 
mucosa. The substantia propria forms the basis of the membrane and 
consists of connective tissue drum membrane fibers which blend with the 
annulus tendineus, composed of connective tissue reinforced by elastic 
fibers that peripherally fills the sulcus tympanicus of the tympanic ring. The 
fibers of the drum membrane are united into flat bundles that form two layers. 
The inner bundles radiate from the annulus tendineus to the umbo, or the stria 
malleolaris; the outer, on the contrary, run in circles with the umbo as a center. 
The union of the handle of the malleus with the drum membrane is effected 
mainly by the fibers of the inner layer which form loops about it. But this 
regular arrangement of the bundles of connective tissue is found only in the 
region of the membrana tensa, for in the membrana flaccida the cutaneous and 
mucous parts are separated by a loose connective tissue that appertains to the 
cutis. Between the bundles of connective tissue are stellate connective tissue 
cells, the processes of which anastomose. 
The pars mucosa is covered by a single layer of the epithelium of 
the tympanic cavity, which here is very low, has no cilia, and rests on a 
quite thin connective tissue propria that is joined to the substantia propria. 
The pars cutanea is identical with the skin of the external auditory 
meatus. , 
The external auditory meatus is lined with skin. The epidermis is 
relatively thin and exhibits distinct cornification everywhere. The thickness 
of the corium varies in different places. Papillae are poorly developed and 
entirely absent in the cutaneous coat of the membrana tensa of the drum mem- 
brane. A subcutaneous connective tissue with fat cells is developed only 
in the cartilaginous portion of the meatus, in the bony portion the corium 248 
blends directly with the periosteum. With the exception of the part covering 
the drum membrane the skin of the meatus contains hairs, and both seba- 
ceous and ceruminous glands. The latter are metamorphosed sweat 
glands and empty in infants through narrow openings into the distal parts 
of the hair follicles, but in adults they wind like corkscrews through the epi- 
dermis and their rather funnel-shaped mouths come to lie near the hair follicles. 
In their structure they exactly resemble the sweat glands of the skin, except 
that they are usually much larger, their bodies are more convoluted, and the 
lumens of their tubes are larger. The simple glandular tube also is not rarely 
divided, and then we have to deal with branched tubular glands. 
The cerumen is a whitish, yellowish, or in many races brownish, waxlike 
substance secreted by the sebaceous and ceruminous glands, the latter furnish- 
ing the softening fluid only. It contains minute drops of fat, pigment granules, 
desquamated epithelium, and hairs. 
The cartilages of the external auditory canal are reticular, and 
are enclosed in a perichondrium rich in elastic fibers. 
The pinna is covered by a thin epidermis, beneath which is a thin 
corium with poorly developed papillae. The subcutaneous tissue is better 
developed on the medial than on the lateral surface, so that the skin is more 
easily movable on the former. It is permeated in the lobule with fat cells. 
Minute hairs and sebaceous glands are present everywhere in the skin 
of the pinna. Sweat glands are found mainly in the lobule, on the distal 
surface of the antitragus, and on the eminentia scaphae. The cartilages sup- 
porting the auricle have the same structure as those of the external auditory 
meatus. 
The arteries of the meatus and of the pars cutanea of the drum membrane 
come from the superficial temporal and the tympanic. They form a dense 
capillary plexus in the cutis through radially entering arterial twigs together 
with a larger branch beneath them in the stria malleolaris, known as the malle- 
olar artery. A similar plexus in formed from the vessels of the tympanic cav- 
ity in the propria of the pars mucosa of the drum membrane, and these two 
plexuses anastomose. The veins follow the course of the arteries, form a 
marginal plexus in the drum membrane, and flow into the external jugular and 
the internal maxillary veins, and into the pterygoid plexus. The arteries of 
the auricle come from the anterior and posterior auricular, and the blood is 
carried away in veins bearing the same names. 
The external ear is supplied with many lymphatics that form two dense, 
anastomosing plexuses in the drum membrane, a cutaneous and a mucous. The 
efferent vessels accompany the arteries and arrive at the anterior, posterior, 
and inferior auricular, and the deep cervical glands. 
The nerves of the pars mucosa of the drum membrane come from the 
tympanic plexus, those of the rest of the external ear from the auriculo-tem- 
poral and the auricular branch of the vagus. They form a dense plexus in the 
corium of the drum membrane, from which fibers rise into the epidermis and 
there end free between the cells with arborizations. Other fibers penetrate into 
the substantia propria and form there a second plexus from which fibers with 
arborescent endings run between the bundles of connective tissue. The skin 249 
of the auricle is supplied by the auricularis magnus in addition to the auriculo- 
temporal. The motor nerves of the external ear are branches of the facial. 
3. THE OLFACTORY ORGAN 
The sensations of smell are perceived through the olfactory epithelium 
situated in the olfactory region of the nose, which occupies, as we have al- 
ready learned, only the superior turbinate and the corresponding part of the 
nasal septum. This characteristic epithelium may be demonstrated in the 
human embryo at the end of the first embryonal month in the olfactory fossa. 
Its cells send their central processes into the brain, forming the olfactory 
nerve. The olfactory organ gradually comes to lie in the nasal cavity, but the 
space occupied by it is always smallei' than in many of the lower animals, be- 
cause Jacobson's organ, a protrusion from the median wall of the olfactory 
fossa covered with olfactory epithelium, undergoes an almost complete involu- 
tion in man, while it forms a well developed part of the organ in other animals, 
most of whom have a better sense of smell. 
The olfactory is a neuroepithelium composed of specific, sensory, ol- 
factory cells and supporting cells. It is demarked from the respiratory 
epithelium of the nasal cavity by its yellowish color, and the greater abundance 
of its nuclei. Its thickness, 60 p, is only slightly greater (Pl. 92, Fig. 196). 
The olfactory cells pass through the entire thickness of the epithelium. 
They are very slender, and their bodies present only circumscribed thickenings 
at the places where the nuclei are situated, beyond which they quickly become 
attenuated again and pass into quite slender, often rather tortuous processes 
that form the fibers of the olfactory nerve. The body of the olfactory 
cell rises above the surface of the epithelium in the form of a little vesicle from 
which six to eight quite short olfactory hairs project. The olfactory 
vesicle is developed from the central bodies of the embryonal olfactory cells. 
Each of the olfactory hairs ends with a little nodule in the wall of the olfactory 
vesicle. 
The supporting cells are considerably broader than the olfactory. Their 
nuclei form the deeper layers of the nuclear stratum, while those of the olfactory 
•cells are situated more superficially. The body is provided with numerous 
longitudinal grooves for the reception of the olfactory cells, and with many 
dimples to accommodate the nucleated parts of the latter. It contains granules 
■of a yellowish brown pigment that gives its characteristic color to the olfactory 
region. The distal end of the cell contains a double central body that usually 
extends proximally in several feet. Between the latter lie a third kind of cells, 
the basal, which are small and have anastomosing processes. 
The surface of the olfactory mucous membrane is covered by a homogeneous 
layer secreted by the supporting cells that is bounded distally by a fine cuticula 
into which the vesicles and hairs of the olfactory cells project. 
There is no basal membrane in the olfactory region. 
The propria consists of reticulated tissue in its superficial, of loose con- 
nective tissue in its deeper parts. It contains very many lymphocytes and 
.some little lymph follicles. The adjoining submucosa consists of loose 250 
connective tissue and contains many elastic fibers, which are also met with in 
smaller numbers in the deeper layers of the propria. 
The propria contains many olfactory or Bowman's glands (Pl. 56, 
Fig. 133). These are of the branched tubular variety, with the tubules 
lined with cuboidal or conical cells which contain many albuminoid secre- 
tory granules, like those of the parotid, and are therefore purely serous 
glands. Several tubules unite to form an excretory duct that very soon 
dilates like a bladder lined with a single layer of low cuboidal epithelium, to 
form a reservoir for the secretion. At the margin of the epithelium this 
bladder contracts into a narrow excretory tube that opens on the surface and 
is lined throughout its intraepithelial course with similar low cells. 
The propria is naturally very rich in nerve fibers, for each olfactory 
cell of the epithelium sends one into it, where they assemble into little bundles. 
Larger bundles are formed from the smaller ones, lie close to the bone, and 
run in special grooves to the cribriform plate of the ethmoid, pass through 
this and enter the olfactory bulb. Each bundle is enclosed by a continua- 
tion of the dura, and a perineural sheath may be perceived even within the 
olfactory organ. The fibers that compose these bundles are nonmedullated 
throughout their entire course and are unique, as compared with other non- 
medullated fibers, in that they also lack the sheath of Schwann. Having ar- 
rived at the olfactory bulb each olfactory fiber enters a glomerulus olfac- 
torius and breaks up into a terminal arborization. 
In addition to the olfactory fibers the olfactory mucous membrane con- 
tains sensory fibers from the trigeminus that rise up between the epi- 
thelial cells and end free. 
4. THE ORGAN OF TASTE 
The organs of the sense of taste have been discussed in the description of 
the epithelium of the surface of the tongue. It was stated there that in adults 
taste buds are to be found in the stratified flat epithelium that covers the 
lateral surfaces of the circumvallate papilla? and the opposite part of the 
coronal sulcus. They are found elsewhere in adults only in the epithelium of 
the circumvallate papillae, sparingly in the fungiform, in the epithelium of the 
soft palate, and of the entrance to the larynx, where it has a stratified flat 
epithelium. 
Each bud is barrel-shaped, has an average height of 80 p, a maximum 
breadth of 40 p, and stands with its longitudinal axis always perpendicular to 
the surface of the epithelium. It rests with its base on the propria and its 
distal end in the floor of a little opening on the surface of the epithelium, the 
taste pore, surrounded by the cells of the stratified flat epithelium (Pl. 92, 
Fig. 197). 
Three kinds of cells may be distinguished in each taste bud, two of which 
are supporting, the third sensory. A number of branching cells lie in the 
base of each taste bud with their longitudinal axes parallel to the surface and 
anastomosing with their processes. These are the basal cells. From them 
rise vertically to the surface the second kind of supporting cells, which are 251 
long, extend to the floor of the taste pore and have sometimes slender, some- 
times bulging cell bodies. Each has at the basal end several short footlike pro- 
jections that anastomose with the processes of the basal cells, while it tapers 
at the distal end and forms the floor of the taste pore. Each cell has a fine 
cuticular edge that projects a little into the pore. The supporting cells are 
irregularly distributed through the entire inner space of the bud with their 
nuclei at various levels. 
The sensory or taste cells are long and threadlike, vary in number from 
four to eight in each bud, pass through the entire thickness of the latter, and 
are distributed irregularly between the supporting cells. Their basal ends are 
also connected with the basal cells. Each is very slender and dilates at the 
site of the nucleus after a longer or shorter course. The nucleus is consider- 
ably smaller and stains more darkly than that of the supporting cell. Distally 
from the nucleus the cell body again becomes slender and now runs to the floor 
of the taste pore, into which it sends a long process, the taste rod. The 
taste rods converge distally into the pore, but do not quite reach the surface 
of the epithelium with their ends. 
Open spaces are left within the bud between the supporting and the taste 
cells that are known as intragemmal spaces. They communicate with the 
subgemmal spaces between the basal cells, and the perigemmal spaces 
at the surface of the bud. 
The nerve fibers that supply the taste buds belong to the glossopha- 
ryngeus and are therefore the peripheral processes of the unipolar cells of the 
ganglia superius and petrosum of this nerve. They form an extensive plexus 
studded with multipolar ganglion cells in the propria beneath the bud, from 
which come intergemmal fibers that branch in the stratified epithelium be- 
tween the buds, perigemmal fibers, which are closely adjacent to the sur- 
face of the bud and weave about it, and intragemmal fibers in the bud itself.. 
They branch repeatedly, closely adjoin the sensory cells, and extend freely 
between them. X. THE SKIN 
The entire surface of the human body is covered uniformly by the skin, 
which consists of two parts, an outer derived from the ectoderm, the epi- 
dermis, and an inner from the mesenchyma, the cutis. The epidermis 
in the early stages of embryonal life is an epithelium having two layers, the 
deeper of which, composed of cuboidal cells, is the germinal layer, while the 
superficial covering layer, or periderm, is made up of flat cells. The 
latter exhibits cornification at an early period and consists in many places 
of several superimposed layers. Toward the end of the second month a differ- 
entiation of the epidermis sets in, beginning on the ventral and proceeding 
toward the dorsal surface of the body. The cells of the germinal layer multi- 
ply and provide a third layer that is interposed between it and the periderm, 
the stratum intermedium, which is at first single, but soon becomes strati-* 
tied. While the single layer of cells of the germinal layer now becomes cylin- 
drical, these cells are polyhedral, are joined together by plasmodesmi and 
develop epithelial fibers in their exoplasm that run through several cells, 
passing through these plasmodesmi. Keratohyaline granules appeal- in 
the deepest layers of the periderm, the cells of which are now stratified, about 
the end of the third, or the beginning of the fourth fetal month, and render 
opaque the formerly transparent epidermis. The keratohyalin undergoes a 
change into eleidin in the superimposed layer. 
The cutis lying beneath the epidermis is of mesodermal origin and shows 
at first an organization corresponding to that of the myotomes, but this dis- 
appears soon. Its gelatinous cells develop collagenous and elastic fibers, and 
produce a loose connective tissue that soon divides into a superficial corium 
and a deeper subcutaneous tissue. The surface of the corium is at first 
perfectly smooth, but very soon the connective tissue invades as it were the 
bases of these cells, which now appear fringed or notched, so as to form a 
very firm connection between the epithelium and the corium. Toward the end 
of the second fetal month the epidermis sinks into the corium in the form of 
ridges, but it is not until the fifth month that the ridges of epidermis over 
those of the cutis become visible externally. This formation of ridges begins 
on the volar surface of the fingers and toes and advances proximally. 
The skin serves first as a protective covering for the body, guarding it 
from loss of heat through the development of fat in its deeper layers, and by 
the growth of hair. In addition it eliminates harmful products from the body 
through special glands, and is the seat of the so-called sense of feeling, i.e., 
the perception of touch, temperature, and pain. 
The epidermis is a stratified flat epithelium, the thickness of which 
varies a great deal in different parts of the body and in different individuals. 
252 253 
It is thinnest on the face, abdomen, thigh, and leg, 50 to 90 p, and thickest on 
the plantai' surfaces of the feet and toes, 1 to 3 mm (Pl. 93, Fig. 197). Its 
outer surface is uneven and presents a system of furrows that separates 
quadrangular or more elongated, lozenge-shaped elevations. These furrows 
and fields appear also at the border of the epidermis and corium as ridges that 
project into the latter and are connected by numerous cross ridges, so that the 
detached epidermis, when seen from below, presents a more or less fine network, 
the rete Malpighi, that embraces in its meshes the elevations of the corium. 
At other places, as on the forehead and the auricle, the reverse is the case; the 
corium has a network of ridges on its surface into which the epidermis sinks 
with corresponding elevations. A further complication of this epidermis-corium 
border is that the ridges of the corium are studded wth secondary elevations, 
papillae, which project more or less into the epidermis. These are flat and 
unimportant in the skin of the face, while they may be 200 |j long on the 
glans penis. 
The depressions between the papilla? are filled by the germinal layer, 
or stratum germinativum, of the epidermis. This is covered smoothly 
on the outside by the horny layer, or stratum corneum, which is usually 
the thinner, but the difference is very soon equalized as the entire thickness of 
the epidermis increases, and then the growth of the latter is exclusively through 
a thickening of the horny layer that may finally attain ten times the thickness 
of the germinal. The germinal layer may be divided into the following sub- 
layers (Pl. 93, Fig. 198): 
1. The stratum cylindricum, which consists of a single layer of cylin- 
drical cells of an average height of 15 p. The nucleus lies in the center of the 
cell. The proximal surface with which the latter rests on the corium exhibits 
a fine indentation with the connective tissue of the corium interlocking into the 
intermediate spaces. The cells of this layer are separated from one another 
and from the distal layer by fine intercellular spaces that are traversed by 
cell bridges, SO that the stratum cylindricum and the next distal stratum 
spinosum form a large synctium. Epithelial fibers, described in the first 
part of this work, run in the exoplasm of the cells and pass from cell to cell 
through the cell bridges exhibiting a quite typical course (Pl. 11, Fig. 32). 
Cell divisions are always to be met with in the stratum cylindricum, where 
the superficial cells are replaced as they are destroyed, yet the replacement 
does not take place here alone, as was formerly believed, for mitoses are also 
found in the stratum spinosum. The cylindrical cells always contain fat in 
the form of minute drops about the nucleus. 
2. The stratum spinosum, which fills the depressions between the pa- 
pillae and also covers the latter with several layers of cells. The cells are polyhe- 
dral and this stratum owes its name to the fact that the intercellular spaces 
are broadest and the cell bridges longest here, so that the latter look like 
spines when the cells are isolated. The epithelial fibers that pass through 
these cells take a rather curved course, while those in the cylindrical cells are 
perpendicular to the surface and are therefore parallel to the cell axes. 
3. The stratum granulosum, which lies next to the stratum spinosum 
and usually consists of only a single layei' of elongated cells with their longi- 254 
tudinal axes parallel to the surface. It is composed of several layers only at 
the places where the horny layer is particularly thick, as on the volar surface 
of the fingers and toes. These cells characteristically possess numerous coarse 
granules that stain strongly with basic dyes, arc soluble in acids and strong 
alkalies, and are digested by the gastric juice. These keratohyaline gran- 
ules originate from the cell protoplasm in cooperation with the nucleus, and 
lie only in the central parts of the cell, the peripheral parts of which are free 
from them and contain epithelial fibers. 
4. The stratum lucidum, which includes three or four layers of flat 
cells, and is characterized by its brilliant luster and its anisotropia. This 
characteristic is due to the fat that the cells contain in diffuse form, eleidin, 
a transformation product of keratohyalin, an albuminoid substance soluble in 
water and stained black by osmic acid. The exoplasm of these cells likewise 
contains epithelial fibers, but they lie embedded in a mass of keratin, which 
is characterized by the fact that it is only partially dissolved by strong metallic 
acids and by the gastric juice. It contains tyrosin. Nuclei cannot be demon- 
strated in the cells of this stratum. 
5. The stratum corneum, which, as already stated, varies greatly in 
thickness in different parts of the body, and consists of many superimposed 
layers of flat, squamous, nonnucleated, horny cells. Each cell has a horny 
exoplasm containing epithelial fibers, and a fatty content that is probably 
changed eleidin. The horny exoplasms of neighboring cells blend to form the 
horny lamellae. It is clear from what has been said that the formation of 
keratin is associated with that of keratohyalin, at least in the skin of the adult. 
It is otherwise in the fetus, where keratin is found in the cells of the periderm 
long before the appearance of keratohyalin. 
In the colored races, and to a less degree in Caucasians, the epidermis con- 
tains a pigment, melanin, in the form of minute granules, found only in 
the cells of the stratum cylindricum and the deepest layers of the stratum 
spinosum, which appears for the most part after birth, even in colored people. 
It is secreted by the nucleus of the cells of the epidermis and is also found in 
the intercellular spaces. 
Two layers of the corium may be recognized from the shape and arrange- 
ment of their connective tissue fibers. The superficial layer, the stratum 
papillare, or papillary body, forms the papillae of the corium and con- 
sists of fine interwoven connective tissue fibers. The deeper layer, the stratum 
reticulare, has coarser fibers united into bundles, most of which run parallel 
to the surface, but often cross and interweave. The corium also contains many 
elastic fibers which are very thick and coarse in the stratum reticulare, are 
for the most part attached to the blood vessels, and form a very wide-meshed 
network. Fibers arise from this to the margin of the papillary body, where 
they form a second dense network of fine fibers. From this other fibers ascend 
to the epidermis, become very slender, and form a third network of minute 
fibrils just beneath the cylindrical cells (Pl. 93, Fig. 197). 
Smooth muscular fibers also are present in the corium, where they 
run parallel to the surface in slender bundles, or unite in certain places into 
a muscular network. Striated muscles also are found entering the corium 255 
in little bundles and passing into tendons that are lost between the bundles 
of connective tissue. 
In addition to the flat, fixed connective tissue cells adjoining the 
bundles of connective tissue, the corium contains in many places branched 
pigment cells, chromatophores, and the so-called lipophores, which 
are also branched cells that are usually situated in the papillary body and 
have bodies filled with fat granules. Both they and the chromatophores may 
send their processes between the cells into the epithelium. 
The subcutaneous tissue joins the skin to the subjacent parts, the 
muscle, fascia, or periosteum, and consists of loose connective tissue, more or 
less reinforced by elastic fibers that are strongest in its deepest layers. The 
bundles of connective tissue are either for the most part parallel, or more 
or less vertical to the surface of the skin, thus permitting the attached skin 
to be movable to a greater or less degree. An accumulation of fat is always 
formed in the subcutaneous tissue,v with the exception of a very few parts 
of the surface of the body. The fat cells are grouped in the form of roundish 
or ovoid masses separated by connective tissue septa, called retinacula. This 
panniculus adiposus has a part to play which is by no means unimportant, 
both as a reservoir of fat and as an insulator to prevent loss of heat. Under 
certain circumstances and on certain parts of the body it may attain a thick- 
ness of several centimeters. 
The Hair 
The first stages of the development of the hair may be demonstrated in 
the skin of the face of the human embryo in the second fetal month. The 
cells of the germinal layer, which at this time are cuboidal, become cylindrical 
in circumscribed areas, place themselves with their distal ends obliquely toward 
one another, and bulge out the epidermal margin proximally. This hair 
germ now grows obliquely in the form of a cone into the corium and forms 
the hair plug. This consists of a superficial layer of cylindrical cells enclosing 
a nucleus of polyhedral ones, and forms an acute angle with the surface. On 
its opposite side, in the obtuse angle, it presents a distal and a proximal pro- 
trusion ; the former is the rudiment of the sebaceous follicle, the latter 
the hair swelling. Deeply in the tissue the hair plug strikes a condensed 
place in the connective tissue that is rich in nuclei, which folds in about it on 
all sides like a hat. This is the rudiment of the hair papilla. The cells 
of the bulb plug, as the rudiment of the hair is now termed, adjoining the 
papilla, separate themselves from the rest and unite to form the hair cone, 
which rests on the papilla with a broad base, tapers distally, and passes over 
into a cellular cord that continues into the periderm and consists of horny 
cells, the so-called hair canal. A horny cord, a hair, is set free from the 
peripheral cells of the rudiment by the cornification of the central cells of the 
hair cone advancing in the cells of the hair canal, and extends from the papilla 
to the epidermis in a canal closed distally by the latter and running for a 
little way parallel to the surface. The outermost cells of the hair cone become 
the inner root sheath, while the peripheral cells of the bulb plug provide 
the outer root sheath. 256 
The hair then gradually breaks through the layers of epidermis that cover 
the hair canal and juts out free. All do not break through in this manner, 
for there is a continuous shedding of the hair, that we shall study later. 
We differentiate in the adult the hair itself, the root sheaths, and the 
hair follicle (Pl. 94, Fig. 201). 
The part of the hair that projects from the skin is known as its shaft, 
that within the skin as its root. The latter ends in the corium, or in the 
subcutaneous tissue, in the hair bulb, a knoblike swelling that is embraced 
by the hair papilla. The hair is covered externally by a thin, transparent 
cuticle, which consists of four rows of little rectangular, quite flat, horny 
cells that cover it like shingles from its proximal to its distal end. The cells 
in the shaft and root are without nuclei, but in the vicinity of the bulb they 
become larger and nucleated (Pl. 95, Fig. 202). 
Next, internally to the cuticle comes the cortical substance, consisting 
of nucleated, horny, spindle-shaped, elongated cells firmly joined together in 
many layers and forming the main part of the shaft. The cells in the bulb 
are denser and mixed with those of the cuticle, from which they cannot be 
distinguished. The cells of the cortex contain the pigment of the hair, 
which is found both in the form of intracellular granules and in solution 
saturating the entire cortical substance. It is brought to the cortical cells 
through the branched chromatophores situated in the bulb. The cortical sub- 
stance also contains between its cells very minute bubbles of air, the mul- 
tiplication of which is instrumental in causing the hair to turn gray, although 
the latter is also partly due to a loss of pigment, which is probably taken up 
and carried away by wandering cells. 
The medullary substance forms the axis of the hair, but is developed 
only in its root, and is absent in hairs that are very slender. It consists of 
a double row of cuboidal, nucleated cells, the bodies of which contain granules 
of keratohyalin. The cellular cord broadens out like a funnel in the hair 
bulb and forms the inner coat of the bulb toward the papilla. 
Root sheaths are developed in all hairs. Distally they blend with the 
layers of the epidermis, proximally they first become thin and then form a» 
cellular mass covering the base of the bulb and surrounding the papilla. They 
are divided into an inner and an outer. The former is simply a continuation 
of the germinal layer of the epidermis; the latter begins a little way from 
the surface, adjoining the stratum granulosum of the epidermis, which extends 
as far as this. 
There is little to be said of the outer root sheath; it resembles exactly 
the germinal layer of the epidermis, and like it has epithelial fibers. Prox- 
imally it grows thinner gradually and is lost in the hair bulb. 
The structure of the inner root sheath is more complicated. It has 
three layers. Next to the outer root sheath lies Henle's layer, a single 
or double row of cells that are joined by their epithelial fibers with those of 
the outer root sheath. Beginning distally, at the end of the stratum granu- 
losum, its cells are flat, elongated, without nuclei, and horny; proximally 
they become denser, nuclei appear, cornification disappears, and granules 
of keratohyalin are to be seen in them. Internal to this is Huxley's 257 
layer, composed of two layers of polyhedral cells. These also are cornified 
distally, but the horny cells cease earlier, and those containing nuclei and kera- 
tohyalin begin sooner than in Henle's layer (Pl. 95, Fig. 202). The root 
sheaths are bounded internally by the cuticula, formed of horny scales that 
change proximally into nucleated cells. The cuticula of the root sheaths thus 
directly adjoins that of the hair. In the bulb the cells of the inner root sheaths 
form the main part of those lying peripherally and proximally from the cells 
of the cortical substance. The separate layers can be perceived at first, but 
later they all unite into a uniform mass of cells. 
The hair follicle is found only with the larger hairs, and consists of an 
outer layer of bundles of connective tissue running longitudinally and 
reinforced by elastic fibers, next to which is a second layer in which the bundles 
of connective tissue have a circular course. It is bounded toward the outer 
root sheath by the homogeneous vitreous membrane, in the formation of 
which the connective tissue of the follicle and the cells of the outei* root sheath 
take equal parts. 
A shedding of the hair takes place continually in man; old hairs are 
cast off and replaced by new ones. The casting off of the old hair is intro- 
duced by a process of cornification that starts in the bulb. The shaft of 
the hair is thereby detached, swells at its end like a club, and breaks up into 
fibers. Proximally the hair root, now become solid through the proliferation 
and cornification of its root sheaths, draws out into a slender cord with the 
reduced bulb and the flat papilla hanging on its end. The formation of a 
new hair now proceeds from the hail' swelling already described, which apper- 
tains to the outer root sheath, and practically follows the embryonal type. 
The old hair is thrust out gradually by the pressure of the one newly formed. 
Human hair is provided with a special musculature, the arrectores 
pilorum (Pl. 94, Fig. 201). The cilia, eyebrows, vibrissas of the auricle, 
and the beard are rigid, immovable hairs not provided with these muscles. 
The arrector pili is a roundish bundle of smooth muscle fibers that passes 
obliquely through the obtuse angle formed by the hair and the surface. It 
arises from the stratum papillare, runs backward obliquely to the hair bulb, 
and is inserted into its longitudinal outer layer by means of a short tendon 
reinforced by elastic fibers. Its contraction, which is produced by psychic or 
thermic stimuli, erects the hair and presses it somewhat out from the skin, 
producing what is known as goose-skin. 
The Nails 
The development of the nail begins in the third month on the extensor sur- 
face of the terminal phalanx, the nail field, a place of thickened epidermis 
that becomes more and more distinct from its neighborhood, which gradually 
comes to resemble a wall. This wall turns in at the margin of the nail field, 
forming the nail fold. The substance of the nail begins to be produced 
in the fifth month through cornification of the middle layers of epidermis 
without any preliminary formation of keratohyalin. Thus the nail is covered 
by the peripheral layers of epidermis, the eponychium. The deeper layers 
of the epidermis arrange themselves on the under surface of the nail into longi- 258 
tudinally running ridges which are closei' behind than in front. The ridges 
of the corium that correspond to those of the epidermis have no papillae in the 
greater part of the bed of the nail. The nail is therefore at first invested by 
epidermis, through which it breaks toward the end of embryonal life. 
The nail consists of the stratum corneum and the stratum germina- 
tivum. The former consists of horny, scalelike cells united together into 
several superimposed plates arranged like shingles. Bubbles of air often form 
between the plates and, when large, create whitish spots. The stratum cor- 
neum ceases in the nail fold with laterally sharpened margins, its outer surface 
covered at this place by the corneal layer of the nail wall that continues for 
a way over the free surface as the eponychium. 
The stratum germinativum is a direct continuation of the germinal 
layer of the nail wall, in front of which it is characterized by a regular forma- 
tion of ridges that are considerably narrower in.the back part, which is de- 
limited from the front by a convex line. So far as this back part extends free 
from the nail fold it forms the lunula of the nail. In front, at the place 
where the nail plate separates from the finger tip, the germinal layer of the 
nail is continuous with that of the pulp of the finger. The latter naturally 
also has a stratum corneum, called the hyponychium, which covers the under 
surface of the free nail plate, and therefore consists of the hyponychium and 
a stratum corneum. 
The nail bed is the corium, and has on its surface a system of ridges 
corresponding to those of the stratum germinativum. Papilla? are present 
only in the back part of the root of the nail. The ridges consist of longi- 
tudinally running bundles of connective tissue with vertical ones in the stratum 
reticulare (Pl. 96, Fig. 204). 
The Sweat Glands 
The sweat glands originate as solid protrusions, partly of the free 
ridges of epidermis, partly of the most distal parts of the hair rudiments, 
beginning in the fourth month, and appear first on the volar surfaces of the 
fingers and toes. The cellular plugs that look exactly like hair rudiments 
grow deeply into the tissue and gradually become canalized in various places. 
The young glandular tube then has a double layer of cuboidal epithelium. 
The canal coils up at its proximal end as it continues to grow and forms the 
body of the gland, within which the two layers of cuboidal cells are trans- 
formed into long contractile cells. 
An excretory duct and a body may be differentiated in each sweat gland. 
The excretory duct starts from the sweat pore at the surface of the 
skin, passes through all the layers of the epidermis in corkscrewlike spirals, 
and enters the corium between two of its papillae (Pl. 93, Fig. 198). In this 
first part of its course the canal has no wall of its own, but simply winds 
between the cells of the stratum corneum, but in the deeper layers of the 
stratum spinosum a cellular wall gradually develops, composed of two layers 
of cuboidal cells, the inner of which is particularly marked by its homogeneous, 
strongly staining protoplasm. Within the corium the canal usually runs 
rather obliquely to the surface and has here an outer layer of high cuboidal 259 
cells and an inner one of much lower cuboidal cells. Externally it is sur- 
rounded by a structureless membrana propria. 
The body of the sweat gland lies either in the deeper layers of the 
corium, or in the subcutaneous tissue, and has an average diameter of from 
0.1 to 0.4 mm. In it the duct forms several coils and ends with a rounded 
top. The lumen of the excretory duct becomes not inconsiderably wider at 
its entrance into the body and the entire tube becomes thickened externally. 
The inner, low cuboidal cells disappear, leaving the tube lined with high cuboidal 
or conical ones, that are a continuation of the outer layer of cells of the 
excretory duct. The cells are provided with a thin cuticular border on the 
free surface, while between them secretory capillaries penetrate from the 
lumen and give off lateral branches that pass into the epithelial cells them- 
selves. Each cell contains a spherical or ovoid nucleus situated about in its 
center, and a rodlike central body. The cell body contains mitochondria in 
its basal part, particles of fat, granules of pigment and secretion in its distal 
(Pl. 94, Fig. 200). 
Externally lies a layer of flat, long, secretory cells, which ramify a great 
deal, anastomose, and must be considered from their entire configuration as 
contractile elements. Outside of this the glandular canal is bounded by 
the structureless membrana propria. 
The sweat, or perspiration, the secretion of the sweat glands, is a 
clear, acid fluid that contains on an average 1.5% of solid constituents, in- 
cluding albumin, urea, creatinin, aromatic oxacids, neutral fats and fatty acids. 
It is very poisonous when injected into the veins. 
Among the modified sweat glands are the Moll's glands of the 
eyelids, and the ceruminous glands of the external auditory meatus. The 
axillary glands also should be included, as they differ from the ordinary 
ones in their unusual size and the branching of their excretory ducts. 
The Sebaceous Glands 
Most sebaceous glands develop as diverticula of the most distal parts 
of the hair plugs and remain throughout life connected with the hair follicles. 
They occur alone in only a few parts of the body, such as the red portion 
of the lips and the labia minora. They are branched alveolar glands, 
and each opens with a short neck into the distal end of the hair follicle. The 
short excretory duct soon divides into several branches that are studded with 
many spherical alveoli (Pl. 94, Fig. 201). The epithelium of the excre- 
tory duct is the direct continuation of the outer root sheath of the hair 
follicle, and like it is stratified. It passes over into a mass of large poly- 
hedral cells filling the alveoli, in each of which an exquisitely vacuolized proto- 
plasm encloses a frequently deformed nucleus. Drops of fat lie in the vacuoles. 
The fatty degeneration of the cells proceeds from without inward, and a 
destruction of them takes place in the centers of the alveoli. The most 
peripheral layer of cells contains no fat, their nuclei are round and are very 
often in a condition of mitosis; they serve to replace the cells destroyed during 
secretion. The cell body contains numerous mitochondria, from which the 
drops of fat originate in the central layers through metamorphosis. 260 
The arrector pili, which passes along the outer side of the gland, plays 
an important part in the expulsion of the secretion, the sebaceous matter, 
which is semifluid and contains albumin with a fatty body. 
The Blood Vessels of the Skin 
The arteries of the skin form two extensive superficial plexuses, the 
deeper of which is situated between the subcutaneous tissue and the corium, 
the more superficial beneath the bodies of the papillae. The two anastomose 
freely and the deeper is fed by main trunks rising out of the deeper tissues. 
The branches springing out of the superficial plexus run at first horizontally 
beneath the rows of papillae, sending a twig into each, and terminate in the 
same way themselves. The arteries break up into loops of capillaries in the 
papillae (Pl. 93, Fig. 198). The efferent veins form several plexuses, two 
of which are subpapillary, and two are situated in the deeper layers of the 
corium. The arteries of the hair branch from the arterial plexus of 
the corium and form a capillary plexus in the inner longitudinal layer of the 
hair follicle and an arborization in the papilla (Pl. 95, Fig. 202). About 
the body of each sweat gland is an arterial plexus from which twigs enter 
the body itself and form capillary plexuses about all the coils of the glandu- 
lar tube. 
The Lymphatics of the Skin 
We find enclosed lymphatics lined with epithelium everywhere in the skin. 
They form networks in the corium that penetrate into papillae, but always lie 
beneath the vascular plexuses. It is uncertain as yet whethei' the hair follicles 
contain lymphatics or not. 
The Nerves of the Skin 
The nerves of the skin are very numerous and consist for the most part 
of medullated fibers with a smaller number of nonmedullated. They form 
several plexuses, the deepest of which is in the subcutaneous tissue, the most 
superficial just beneath the papilhe. Part of the fibers coming from these 
plexuses terminate free in the epithelium, part in special end formations, part 
on the cutaneous glands (Pl. 97, Figs. 205 and 206). 
The fibers that enter the epidermis lose their medullary sheaths 
before their entrance and break up into more or less fine fibrils that rise be- 
tween the cells of the epidermis and give off lateral twigs which either run 
out into fine points, or end in little thickenings. Under certain circumstances 
the branches may anastomose so as to form an intraepithelial plexus. Merkel's 
tactile discs are special end formations of the intraepidermal nerve fibers 
that are but sparingly present in man. They occur only in the deepest layers 
of the epidermis, especially in the vicinity of the ducts of the sweat glands, 
and are cup- or dish-shaped, with the concavity directed distally. A twig 
from a nerve fiber ends in the wall of the dish in the form of a plexus pro- 
vided with marked expansions. In the cup lies a spherical cell, which is nothing 
else than a somewhat modified epidermal cell. Several such tactile discs are 
attached to each nerve fiber, which sends a twig to each disc. 261 
Free nerve endings are also met with in the connective tissue por- 
tion of the skin in the form of treelike branchings, the ends of which not 
rarely terminate in end plates. They occur most of all in the bed of the nail, 
but are met with elsewhere. On the hairs the fibers, having lost their medul- 
lary sheaths, form first a circular plexus situated within the connective tissue 
hair follicle, from which other fibers form another plexus internally on the 
vitreous membrane with elongated meshes, from which numerous very minute 
longitudinal fibrils rise and end free. In the hair papilla we meet only spar- 
ingly with nerve fibers going to the musculature of the vessels. 
The encapsulated nerve endings are very much more manifold than 
the free, and we can describe here only a few of the most important. 
The Vater-Pacini corpuscles have a maximum size of 4 x 2 mm, are 
ovoid in shape, and are distributed very widely, especially in the subcutaneous 
tissue of the flexor surface of the hands and feet, fingers and toes, but are also 
found in the periosteum and in other places (Pl. 93, Fig. 197; Pl. 96, Fig. 
204). The axis of the corpuscle is formed by a nerve fiber which loses its 
medullary sheath at its entrance, and swells like a club at its end. The axis 
cylinder is surrounded by a dense protoplasmic sheath with flat cells 
adjoining it internally, enclosed in an extremely fine, close-meshed nervous 
plexus that originates from a second nerve fiber which enters together with 
the first, but differs from it in being extremely slender, and then surrounded 
by a large number of connective tissue concentric sheaths that lie at first 
very close together, then farther apart toward the outer surface, all so filled 
with a serous fluid that the entire corpuscle seems to be tense. The inner 
side of each lamella is lined with epithelial flat cells. 
Krause's end bulbs, found especially in the bulbar conjunctiva, are a 
simpler variety of the Vater-Pacini corpuscles. They likewise consist of an 
axis cylinder surrounded by a dense protoplasmic sheath, but this is enclosed in 
only a single connective tissue sheath, so the formations are more slender and 
cylindrical. 
The Golgi-Mazzoni corpuscles are found in the bed of the nail, but 
mainly on the skin of the external genitals. They are usually similar in shape 
to the Vater-Pacini, but are considerably smaller. The envelope is similar, 
but is commonly composed of only a few lamellae. The interior of the cor- 
puscle is filled with a mass of protoplasm. The entering nerve fiber divides, 
after losing its medullary sheath, into numerous ramifying branches that are 
studded with irregular thickenings and expansions in which they also terminate. 
The thick principal fiber is often accompanied by a thinner one that forms a 
fine plexus on the inner surface of the capsule. 
Meissner's corpuscles enjoy a wide distribution in man. They lie in 
the papillae of the corium just beneath the epidermis, are most numerous in 
the skin of the flexor surface of the fingers and toes, are oval, and have a 
maximum length of 150 p, with a breadth of 100 p. Each is enclosed in a 
thin connective tissue capsule that penetrates into the corpuscle and divides 
it into two to four lobes. One or more medullated fibers enter the corpuscle, 
lose their medullary sheaths, divide, and form in it loops usually placed 
vertically or obliquely to the longitudinal axis, and studded with varicosities. 262 
Between the loop lie cells, some of which are to be considered tactile, some 
to be connective tissue cells. A slender nerve fiber enters Meissner's cor- 
puscle, together with the thick one, and weaves a terminal plexus about 
the loops. 
Many other forms of tactile corpuscles are met with in animals, and espe- 
cially in birds, some of which are simpler, some more complicated (Pl. 97, Figs. 
205 and 206). 
The Mammary Gland 
The mammary is a cutaneous gland and starts as a diverticulum of 
the epidermis. Its rudiment may be seen in embryos of the fourth fetal week 
in the form of the so-called milk stripe, a thickening of the epithelium that 
forms a longitudinal stripe on each side of the body between the rudiments 
of the upper and lower extremities. The milk ridge is formed from this 
in the second fetal month by its becoming more thickened and projecting 
inward. This soon breaks up into separate successive rudiments, of which only 
one survives on each side in man, to undergo further development. It grows 
like a bulb into the corium and sends out shoots during the second half of 
o 
fetal life that are at first solid, but later become hollow. Toward the end 
of pregnancy the bulb itself becomes hollow, dilates, flattens, and then raises 
a nipple, where the milk ducts open singly. The latter are provided with 
vesicular protrusions at their proximal ends. The mammary gland is thus 
developed uniformly in both sexes at the time of birth. It undergoes involu- 
tion during youth in boys, but develops further in girls through the formation 
of new milk ducts. At the time of puberty the entire female breast becomes 
larger and more prominent through a great development of fat. During 
pregnancy a further marked development of the glandular tissue takes place, 
in which the milk ducts divide repeatedly at their ends and end pieces are 
formed. 
The functionating mammary gland of the nursing woman is there- 
fore a conglomerate of seventeen to twenty compound branched alveo- 
tubular glands, each of which has its own milk duct that opens at the 
nipple. Each separate gland forms a lobe of the entire organ, which is 
embedded in a fatty connective tissue that penetrates into the lobes and divides 
them into many lobules. These lobules contain the end pieces (Pl. 98, Fig. 
207). After lactation has continued seven or eight months a great part 
of the end pieces undergo involution, and the loss is made up by newly formed 
more or less fatty connective tissue. 
The secretion of the mammary gland is milk, a neutral fluid that has the 
character of an emulsion. The milk plasma contains dissolved sugar and 
albumin, together with inorganic salts. The sugar is lactose, the albumin 
is chiefly casein, which is not coagulated by heat. In this milk plasma are 
suspended milk globules, drops of fat of variable size consisting mainly of 
neutral fat. Milk also contains very minute granules, probably of calcium 
phosphate. The colostrum secreted at the beginning of lactation differs from 
milk in its yellow color, due to the presence of colostrum corpuscles, and 
the greater amount it contains of lactalbumin and of lactoglobulin, 263 
which cause it to be coagulable by heat. The colostrum corpuscles are mi- 
grated leucocytes, the bodies of which are full of fat granules. 
The milk ducts are first lined with stratified flat epithelium that 
rests on a structureless membrana propria. Just before it opens in the 
nipple it dilates into a milk saccule, in which the epithelium becomes simple 
cylindrical. The excretory duct leading out of the milk saccule has the 
same structure and soon divides into the interlobular ducts, which divide 
again into the intralobular ducts. All of these have the same structure- 
a lining of simple cylindrical epithelium covered externally by a mem- 
brana propria. Between the two are flat nuclei which belong to branched 
contractile basket cells that surround the canal. The intralobular ducts 
pass into the alveolar end pieces, which have the same structure. The 
appearance of the epithelium varies according to the stage of the secretion. 
Cells filled with secretion are high cylindrical or conical and project 
their tops into the lumen. The distal part of the cell contains a granular, 
clear protoplasm that holds numerous larger and smaller drops of fat. The 
proximal part contains one nucleus, often two, surrounded by a darker pro- 
toplasm that holds many tortuous, filamentous mitochondria (Pl. 98, Fig. 
208). 
The expulsion of the secretion from the cell is pictured differently 
by various authors. Some think that the drops of fat and the secretory gran- 
ules simply leave the uninjured cell, while others believe that the entire distal 
part of the cell, with one of the two nuclei, is thrown off into the lumen, so 
that a sort of decapitation of the cell takes place. The nuclei that are de- 
stroyed are replaced by amitosis of those that remain. However this may be, 
the empty cells are low cuboidal, have a nucleus and mitochondria, through 
the breaking down of which and the metamorphosis of its constituents the 
secretion is produced. 
External to the secretory are the basket cells, and outside of these is 
the membrana propria. 
The arteries of the mammary gland come from the internal mammary 
and the intercostals. They branch in the interlobular and intralobular tissue, 
enter the lobules, and form capillary plexuses about the end pieces. The 
veins follow generally the course of the arteries. 
The lymphatics surround the end pieces and form plexuses about the 
milk ducts. They are connected with a plexus in the corium of the skin. 
The efferent vessels run to the pectoral, paramammary, sternal, interpectoral, 
and interclavicular lymphatic glands. 
The nerves come from the intercostals and from branches of the brachial 
and cervical plexuses. Very many of their fibers are nonmcdullatcd, together 
with others that are medullated. They form first a perialveolar plexus within 
the lobule, from which fibers pass through the membrana propria and form 
the plexus intercellularis about the bases of the secretory cells, that send 
very minute fibrils between the cells, where they terminate in little end nodules. 
The skin of the areola of the nipple differs from that about it in 
its thinness, in its pigmentation, and in its plenitude of elastic fibers. It also 
possesses in its papillary bodies a well-developed system of smooth muscle 264 
fibers, part of which surround the milk ducts, part are radiating, and part 
run vertically to the surface. Together they form what is known as the 
areolar muscle, which extends into the nipple and produces by its con- 
traction an erection and a marked protrusion of the latter. The corium in 
the areola of the nipple is peculiarly rich in glands. We find here many 
sweat glands, that are distinguished by their size, and well-developed seba- 
ceous glands, each with a rudimentary hair follicle, that enlarge consid- 
erably during pregnancy and lactation. Finally, we find here the accessory 
milk glands, or Montgomery's glands, which, to the number of twelve 
or fifteen, lie in the subcutaneous tissue and are nothing else than miniature 
mammary glands, each corresponding to a single lobe of the larger organ. 
The skin of the areola of the nipple and of the nipple itself is very richly 
supplied with sensory nerves, part of which terminate free in the epidermis, 
part in special end formations. Among the latter we find Meissner's cor- 
puscles within the papillae of the corium, which are very high, and the Vater- 
Pacini corpuscles in the deeper layers of the corium. 
THE COPYRIGHTS OF THIS BOOK, IN ALL ENGLISH-SPEAKING COUNTRIES, 
ARE OWNED BY REBMAN COMPANY, NEW YORK. INDEX 
Accommodation, 223, 227 
Accretion lines, 38 
Achromatic framework, 6 
membrane, 6 
substance, 6 
Acramines, 217 
diffuse, 217 
stratifying, 217 
Adenoid tissue, 82 
Adrenalin, 150 
Afterbrain, 197 
nuclei, 197 
olive, 198 
pyramids, 197 
Aggregated follicle, 80 
Albuginea, 153, 163 
definitive, 165 
primary, 165 
Alveolar passages, 127 
septa, 128 
Alveoli, 127 
Amitosis, 11 
Amoeboid movement, 5 
Amphicytes, 195 
Amphipyrenin, 6 
Amylopsin, 117 
Anaphase, 12, 14 
Annulus fibrosus, 77, 184 
tendineus, 247 
Appendix, 108 
Appositional growth, 60, 181 
Aqueous, 224 
Arachnoid, 210 
Arachnoidal space, 210 
villi, 210 
Archispermatocytes, 154 
Archoplasm, 7, 13 
Areola, inner cell, 59 
outer cell, 59 
Arrectores pilorum, 257 
Arteriolse rectse spuriae, 143 
verse, 143 
Arterioventricular bundle, 77 
valves, 78 
Artery, coronary, 78 
penicillate, 83 
precapillary, 73 
thecal, 83 
Astrocytes, 47, 218 
Atresia of follicles, 169 
Auditory basilar fibers, 242 
canal, 248 
cup, 238 
hairs, 240, 243 
meatus, 247 
nerve, 198, 210 
ossicles, 245 
plate, 247 
Auditory teeth of Huschke, 242 
vesicle, 238 
Auerbach's plexus, 110 
Aural folds, 247 
eminences, 247 
Axis cylinder, 43, 45 
filaments, 160 
Axoplasm, 46 
Bone cells, 64, 89 
compact, 63, 181 
corpuscles, 65 
lamellar structure, 63 
lymphatics, 183 
marrow, 181 
nerves, 183 
physical properties, 63 
primary, 61 
regeneration, 181 
secondary, 61 
substance, compact, 62 
spongy, 62 
Bowman's capsule, 138, 140 
glands, 250 
membranes, 228 
Brain, 39, 197 
sand, 207 
Bronchi, 125 
extrapulmonary, 125 
intrapulmonary, 126 
lateral, 127 
main, 127 
Bronchioles, 127 
intralobular, 127 
respiratory, 127 
Brown's molecular movement, 
196 
Bruch's membrane, 225 
Brunner's glands, 109 
Bundles of His, 39 
Burdach's column, 188, 191, 192 
nucleus, 192 
Baillarger's stripes, 208 
Bartholin's glands, 176 
Basal filament, 97 
homogeneous, 242 
membrane, 107, 125, 130 
substance, 89 
Basic substance, 57, 59 
cartilage capsule, 59 
Basophilic chromatin, 5 
Bechterew's nucleus, 199 
Bile, 114 
Biliary capillary, 114 
pigments, 113 
salts, 113 
Bilirubin, 113 
Biliverdin, 113 
Bladder, 145 
adventitia, 146 
arteries, 146 
crypts, 146 
epithelium, 145 
glands, 146 
lymph follicles, 146 
lymphatics, 146 
membrane, 146 
mucus, 146 
muscularis, 146 
nerve fibers, 146 
propria, 146 
submucosa, 146 
veins, 146 
Blandin-Nuhn glands, 93 
Blind spot, 219 
Blood, 66 
corpuscles, 66 
primitive, 66 
red, 66, 67, 82 
white, 67, 68 
corpuscular elements, 71 
functional, 114 
islands, 66 
nerves, 76 
plasma, 66 
platelets, 70 
primitive, 72 
secondary, 73 
shoots, 72 
vessels, 72, 76 
Bone, 181 
arteries, 182 
basic substance, 62, 64 
canaliculi, 64 
cavities, 64, 89 
Cajal's cells, 207 
Calcareous epithelium, 18 
Calcification points, 62 
Calices majores, 137 
minores, 137 
Canalis reuniens, 239 
Capillary, 73 
basket, 104 
intracellular, 31 
secretory, 31, 97 
Carcinoma, 4 
Cardiac muscle fiber, 38 
Cardiogenous plate, 76 
plexus, 79 
Carotid gland, 151 
Cartilage, 58, 60 
capsule, 59 
cavity, 58 
cell, 58 
elastic, 58, 61 
fibro, 58 
hyaline, 58 
reticular, 68, 61 
Caruncula lacrimalis, 235 
Casein, 262 
Caudal section, 171 
265 266 
Cavity of vitreous, 214 
primary medullary, 62 
Cell, 1 
basket, 97 
biological properties, 7 
bipolar, 41 
body, 2, 14, 59 
bridge, 19, 21 
budding, 15 
cartilage, 58 
chief, 131 
chordal, 50 
chromaffin, 149 
colloid, 131 
constriction, 15 
construction, 2 
cover, 22 
crenated, 22 
cuboidal, 242 
cylindrical, 5, 172, 242 
ependymal, 47 
fat, 53 
filiform, 24 
foot, 154 
form, 1 
giant, 4 
goblet, 18, 108 
hair, 24 
manifestation of life in, 8 
mast, 52, 54 
membrane formation, 2 
metabolism, 8 
motor, 41 
multipolar, 41 
muscular, 32 
nucleus, 2, 4 
oxyphilic, 133 
peg, 108 
pigment, 10, 53 
plasma, 52, 54 
plates, 14 
polynucleated, 4, 15 
prickle, 22 
primitive spermatic, 154 
propagation of, 11 
pyramidal, 189 
reaction of, to external stim 
uli, 10 
reticular apparatus, 2 
rod, 83 
segmentation, 8, 15 
indirect, 12 
sensory, 41 
size, 1 
smooth muscle, 73 
spermatic mother, 154 
stellate, 188 
striated muscle, 33 
territories, 59 
umbrella, 22 
unipolar, 41 
wandering, 52, 53 
water in, 9 
Cement, 19, 63, 87, 88, 89 
edges, 19, 20 
lines, 64 
striae, 38 
Central bodies, 2, 6, 44, 54 
canal, 193 
corpuscles, 33 
Central formation, 2 
spindle fibers, 13 
Centrodesmose, 6 
Centrophornium, 229 
Centrosome, 7 
Centrosphere, 7 
Cerebellum, 201 
arteries, 203 
cells, 201 
cortex, 201 
fibers, 202 
neuroglia, 202 
nuclei, 203 
Cerebral cortex, 190 
Cerebrospinal fluid, 210 
Cerumen, 248 
Choledochus, 116 
Chondral stage, 58 
Chondrin, 60 
Chondriosomes, 2, 7, 12 
Chondroblast, 57 
Chondroid connective tissue, 61 
Chondroitin-sulphuric acid, 57, 
60 
Chorda dorsalis, 50 
Chordal epithelium, 50 
sheath, 50 
Choroid, 221 
blood vessels, 221 
lamina, 221 
Chromatic membrane, 5 
substance, 6 
Chromatin, 5, 8 
oxyphilic, 5 
Chromatophores, 255 
Cilia, 234 
'Ciliary body, 222 
muscle, 222 
processes, 222 
vascular system, 231 
Circulus arteriosus iridis major, 
232 • 
Cisterna perilymphatica, 245 
Clarke's cells, 189, 191 
Clasmocytes, 52, 54 
Climacterium, 173 
Clitoris, 176 
glans, 176 
prepuce, 176 
Cochlear process, 239 
Cohnheim's fields, 35 
Collaterals, 42 
Collecting tubules, 137, 139 
Coliegen, 55 
Colloid, 131 
Colostrum, 262 
corpuscles, 262 
Column of Burdach, 186, 191 
Clarke, 189 
Goll, 186, 191 
Commissure, gray, 186, 190 
white, 186 
Cone, 216 
bipolar, 217 
ellipsoid, 216 
fiber, 217 
foot, 217 
granule, 216 
myoid, 216 
optic cell, 217 
Conjunctiva, 233 
arteries, 235 
bulbi, 234 
cavity, 234 
lymphatics, 236 
nerves, 236 
palpebrarum, 234 
veins, 235 
Contraction nodes, 33 
Convoluted tubule, 138, 140 
Corium, 248, 252 
Cornea, 227 
anterior chamber, 227 
cells, 227 
connective tissue, 228 
epithelium, 227 
fibers, 228 
lymph canals, 228 
Corneal corpuscles, 228 
Cornification, 252 
Corona radiata, 168, 205 
Corpus albicans, 169 
cavernosum, 162, 176 
dentatum, 203 
geniculatum, 204 
luteum, 169 
quadrigeminum, 204 
Corpuscular elements, 79 
Cortex, cerebral, 207 
arteries, 208 
cells, 207 
fibers, 208 
neuroglia, 208 
Cortical substance, 134 
Corti's membrane, 244 
organ, 241, 242, 243 
tunnel, 242 
Cowper's glands, 159 
Crusta, 4 
Cumulus oviger us, 167 
Cupola terminalis, 240 
Cuticula, 87 
dentis, 88, 89 
Cutis, 252 
Cylindo-conical segments, 47 
Cytomitome, 3 
Daughter stars, 14 
Decussation of the pyramids, 
197 
Better's cell, 243 
nucleus, 199 
type, 42 
Dental canaliculi, 88 
groove, 86 
papilla, 86 
pulp, 90 
blood vessels, 90 
lymphatics, 90 
nerves, 90 
root, 87 
tubules, 88 
Dentine, 65, 87, 88 
basal substance, 88 
fiber, 90 
globules, 88 
interglobular spaces, 88 
Dentrites, 39 
Descemet's membrane, 228 
Deutoplasm, 3, 4 267 
Diaphragm, 130 
arteries, 130 
innervation, 130 
lymphatics, 130 
muscular part, 130 
tendinous part, 130 
veins, 130 
Digestive organs, 84 
Dilator iridis, 224 
Dimples, primitive gastric, 101 
Dioptric apparatus, 214, 227 
Diploe, 181 
Directing bodies, 168 
Discs, 35 
Distal layer, 214 
Drum, 245 
Ductus cysticus, 116 
endolymphaticus, 238, 245 
epididymitis, 156 
perilymphaticus, 245 
Dura mater, 210 
Ectoplasm, 4 
Elastic cartilage, 58 
network, 130 
Elastica externa, 75 
interna, 74, 75 
Elastin, 55 
Electrical force, 11 
Eleidin, 254 
Embolus, 203 
Enamel, 18, 87, 89 
cells, 87 
cuticle, 89 
membrane, 87 
organ, 86 
prism, 88, 89 
pulp, 87 
Endocardium, 77, 78 
Endolymph, 245 
Endometrium, 172 
Endoneural sheath, 213 
Endoneurium, 213 
Endothelium, 20 
Enterokinase, 111 
Entodermal cloaca, 145 
Enzymes, 4 
Epidermis, 22, 252 
Epididymis, 156 
Epineurium, 212 
Epiphysis, 206 
Epithelial cells, 19 
fibers, 22 
Epithelium, 20, 76 
cardiac, 77 
enamel, 87 
epicardial, 78 
flat, 129 
glandular, 24 
respiratory, 128 
sensory, 24 
simple cuboidal, 20 
cylindrical, 21 
flat, 20 
stratified, 21 
cylindrical, 22 
flat, 21 
stratiform, 23 
ciliated, 23 
transitional, 22 
vascular, 73 
Eponychium, 257 
Epoophoron, 166 
Equatorial plate, 13 
Erepsin, 111 
Erythroblasts, 182 
Erythrocytes, 66, 67 
Eustachian tube, 246 
Exoskeleton, 61 
osseous, 61 
External female genital organs, 
176 
Eye, 214 
cells, 218 
fibers, 218 
ganglion, 218 
muscles, 214 
Eyeball, 231 
blood vessels, 231 
lymphatics, 232 
nerves, 233 
Eyelids, 233 
Fallopian tube, 170 
arteries, 171 
cilia, 171 
epithelium, 170 
folds, 170 
lymphatics, 171 
membrane, 170 
muscularis, 171 
nerves, 171 
propria, 171 
secretion granules, 171 
submucosa, 171 
veins, 171 
Fascia palpebralis, 199 
penis, 163 
Fasciculus longitudinalis media- 
lis, 199 
Fat, 3, 59 
Fatty capsule, 57 
Female genital organs, 165 
Fenestrated membranes, 55 
Ferment profibrin, 71 
Ferments, 103, 111 
Fiber, branched muscle, 34 
cardiac muscle, 77 
collagenous, 54, 73 
connective tissue, 54 
elastic, 54, 61, 73, 125 
firm, 47 
medullated sensory, 76 
muscular, 130 
nonmedullated, 76 
radiating elastic, 74 
smooth muscle, 74, 166 
striated muscle, 34 
supporting, 47 
tendinous, 130 
Fibrse arcuatae internae, 198, 
203 
Fibrillary network, 51 
Fibrils, 3, 38, 51 
collagenous, 59, 63 
contractile, 32, 34 
internal, 33 
limiting, 33 
transversely striated, 38 
Fibroblast, 52 
Fibrocartilage, 58, 60 
Fibrocartilagines interverte- 
brales, 184 
Fibroelastica, T81 
Fimbriae, 170 
Fissura longitudinalis anterior, 
186 
Flagella, 11 
Flechsig's bundles, 189 
Follicle, 30 
cortical, 81 
lingual, 92 
lymph, 92 
primary, 165 
solitary, 80 
Follicular cells, 167 
epithelium, 166 
Fontana's space, 229 
Foramina nervina, 244 
Foreskin, 163 
Fornix, 236 
Fovea centralis, 215, 219 
Framework, 59 
Ear, external, 247 
arteries, 248 
cartilage, 248 
connective tissue, 247 
epidermis, 247 
epithelium, 247 
glands, 248 
hairs, 248 
lymphatics, 248 
membrane, 247 
mucosa, 247 
nerves, 248 
propria, 247 
veins, 248 
Ear, internal, 238 
ampulla, 238 
arteries, 244 
blood capillaries, 241 
cells, 244 
crista spiralis, 241 
ganglion, 238 
membranes, 240 
nerves, 240, 244 
pars dorsalis, 238 
perilymphatic spaces, 245 
plate, 238 
prominentia spiralis, 241 
spiral ganglia, 244 
veinous blood, 244 
ventralis, 238 
vertical pouch, 238 
vestibular wall, 241 
Ear, middle, 245 
arteries, 246 
cartilage, 246 
epithelium, 245 
fibers, 246 
glands, 246 
lymph follicles, 246 
lymphocytes, 246 
membrane, 245 
muscles, 246 
nerves, 246 
ossicles, 246 
propria, 245 
veins, 246 
Ebner's glands, 93 
Ectoderm, 149, 214, 224, 256 268 
Frenulum, 162 
clitoridis, 176 
Frog, 5 
Fuscin, 216 
Glomus coccygeum, 151 
Glue, 55 
Glycogen, 3, 36, 59, 113, 114 
Golgi-Mazzoni corpuscles, 261 
Golgi's type cells, 42 
secondary, 189, 208 
Gott's nucleus, 191 
Gower's tract, 191 
Graafian follicles, 167 
Granules, 30, 54 
Granulosa, 179 
Gray matter, 188 
columnar cells, 188 
commissural cells, 188 
nerve cells, 188 
nerve fibers, 188 
root cells, 188 
Ground plate, 223 
Gums, 88, 90 
Hippuric acid, 138 
His' bundle, 77 
Hollow spaces, 167 
Huxley's layer, 256 
Hyaline cartilage, 58 
Hyaloid canal, 230 
Hyatoplasm, 3 
Hydatids, pedunculated, 170 
Hydrochloric acid, 103 
Hymen, 175 
Hyponychium, 258 
Hypophysis, 205 
arteries, 206 
cells, 206 
nerves, 206 
veins, 206 
Gall bladder, 115 
epithelium, 116 
membrane, 116 
muscularis, 116 
nerves, 1J6 
propria, 116 
Ganglia, 39 
cerebral, 211 
Ganglionic crest, 195 
Gastric dimples, 102 
glands, 103 
mucus, 102 
Gelatin, 55 
Gelatinous nucleus of interver- 
tebral discs, 50 
Genital cord, 165, 170 
eminence, 162, 176 
folds, 162, 176 
prominences, 162, 176 
Germinal center, 80 
epithelium, 166 
spot, 167 
vesicle, 5, 167 
Gianuzzi's crescents, 97 
Gland, 24 
acinous, 28 
alveolar, 25, 27, 28 
alveotubular, 25, 28, 29 
base, 26 
body, 25 
branched, 25 
cardiac, 104 
cervical, 173 
ciliary, 234 
closed, 25, 29 
compound, 25 
convoluted, 25 
fundus, 103 
gastric, 102 
haemolopoietic, 25 
labial, 85 
lacrimal, 235 
lingual, 92, 93 
neck, 25 
ovarian, 166 
palatine, 94 
parotid, 95 
pyloric, 103 
serous, 93 
simple, 25 
solid, 25, 29, 30 
sublingual, 95 
submaxillary, 95 
true, 25 
tubular, 25, 26 
uterine, 172 
vestibular, 176 
Glandular cavity, 24 
cells, 24 
Glia cell, 47 
fiber, 41, 47, 48 
tissue, 41, 47 
Glisson's capsule, 113 
Glomerulus, 140, 142 
olfactorius, 209 
Idiosome, 154 
Infundibulum, 127 
Interbrain, 203 
floor, 205 
Intercalares fibrocartilagines, 
60 
Intercellular bridges, 19 
clefts, 19, 20 
spaces, 19, 21 
substance, 19, 49, 54 
Interlobular ducts, 113 
Intermediate disc, 47, 184 
Interpolated sections, 38 
Interstitial granules, 38 
growth, 60 
substance, 36 
Intervening tendons, 38 
Intestinal juice, 111 
Intestine, 105 
arteries, 109 
epithelium, 106 
folds, 106 
lymphatics, 110 
membrane, 106 
muscularis, 109 
nerves, 110 
propria, 106 
submucosa, 106, 109 
Invertine, 111 
Involucrum, 161 
Iodine, 131 
Iris, 222, 224 
epithelium, 225 
nerves, 233 
pigment, 225 
stroma, 224 
Irregular leaves, 38 
Hasmoglobin, 66 
crystals, 67 
Haemokonia, 71 
Haemotoxylin, 5 
Hair, 255 
canal, 255 
cells, 256 
cone, 255 
connective tissue, 257 
cortical substance, 256 
cuticle, 256 
fiber, 257 
follicle, 255 
germ, 255 
medullary substance, 256 
musculature, 257 
papilla, 255 
pigment, 256 
plug, 255 
root sheath, 255 
shedding, 257 
swelling, 255 
vitreous membrane, 257 
Hatter's circular arterial plex- 
us, 231 
Hassel's corpuscles, 134 
Haustra, 109 
Haversian canals, 62, 64, 89 
lamellae, 64 
Heart, 76 
ganglia, 79 
muscle fiber, 77 
nerves, 78 
skeleton, 77 
valves, 78 
vesicular supporting tissue, 
77 
Heat, 11 
Heidenhain's rods, 96 
Helicotrema, 240 
Helweg's tract, 191, 198 
Henle's fibrous layer, 217 
layer, 256 
loop, 137, 139 
Hensen's body, 243 
canal, 240 
Hepatic duct, 113 
lobules, 113 
Highmore's corpus, 153 
Hilus of the gland, 81 
Jacobson's organ, 249 
Joints, 183 
articular capsules, 183 
surfaces, 183 
blood vessels, 184 
intima, 183 
lymphatics, 184 
nerves, 184 
Keratin, 18, 254 
Keratohyaline granules, 252, 
256 
Kerkring's folds, 106 
valves, 106 269 
Kidney, 136 
arteries, 142 
cortex, 141 
definitive, 136 
lymphatics, 143 
medulla, 141 
nerves, 143 
pelvis, 137, 144 
stroma, 142 
veinous blood, 143 
veins, 143 
Kidney pelvis, 144 
adventitia, 144 
arteries, 144 
lymphatics, 145 
membrane, 144 
nerves, 145 
propria, 144 
transitional epithelium, 144 
Kinase, 71 
Krause's corpuscles, 86, 94 
end bulbs, 261 
glands, 235 
Kupfer's vacuoles, 114 
Lens, 225 
equator, 225 
nucleus, 226 
plate, 214 
Lenticular capsule, 226 
cone, 226 
epithelium, 225 
fibers, 225 
fossa, 214 
saccule, 214 
stars, 227 
vesicle, 214 
Leucocytes, 67, 68 
amoeboid movement, 71 
eosinophilic, 4, 54, 134 
granular, 182 
granules, 68 
neutrophilic, 4, 79 
Leydig's cells, 155 
Lieberkuehn's crypts, 108 
Ligamentum pectinatum iridis, 
229 
spirale, 241 
Light, 11 
Limbus spiralis, 242 
Lime, salts, 60, 63 
Limitans externa, 216 
interna, 218 
Limiting cell lines, 38 
Linin, 6 
Lipoid, 4, 158 
Lipophores, 255 
Lips, 84 
blood vessels, 86 
cutaneous portion, 84 
epidermis, 84 
epithelium, 84 
lymphatics, 86 
mucous membranous portion, 
84 
muscular stratum, 85 
skin, 84 
Liquor folliculi, 168 
Lissauer's tract, 186 
Littre's glands, 148 
Liver, 111 
cells, 114 
cell trabecula, 114 
lymphatics, 115 
nerves, 115 
tubule, 114 
Lungs, 126 
basal membrane, 128 
epithelium, 128 
glands, 128 
lymphatics, 129 
mucous membrane, 128 
musculature, 128 
nerves, 129, 130 
propria, 128 
Lunula, 258 
Lutein, 169 
Lymph, 79 
sinus, 80, 81 
Lymphatic follicles, 80 
glands, 80 
nodules, 80 
Lymphatics, 78, 79 
blood vessels, 80 
capillaries, 79 
Lymphatics, larger, 80 
roots, 79 
small, 79 
valves, 80 
Lymphoblasts, 69, 70, 72 
Lymphocytes, 4, 5, 69, 79, 82, 
125 
eosinophilic, 69 
mast cells, 70 
monophyletic, 70 
neutrophilic, 69 
polyphyletic, 70 
transition form, 69 
Lymphoid cords, 81 
organs, 80 
Macrophagi, 71, 72, 82, 107 
Macula acustica, 239 
lutea, 215, 219 
neglecta, 239 
Male sexual organs, 152 
Malpighian corpuscle, 83, 140 
Maltase, 111 
Mantle fibers, 13 
Marginal areola, 44 
Marrow, 182 
Massa intermedia, 161 
Media, 74, 75 
Mediastinum testis, 153 
Medulla, 81 
Medullary fasciculi, 81 
rays, 141 
sheath, 45, 46 
Meibomian glands, 234 
Meisner's corpuscles, 261 
plexus, 110 
Melanin, 254 
Membrana basilaris, 242 
propria, 85 
reticularis, 244 
tectoria, 244 
vasculosa lentis, 226 
Membrane, basal, 85 
Mammary gland, 262 
arteries, 263 
cells, 263 
epithelium, 263 
fibers, 264 
lobe, 262 
lymphatics, 263 
muscles, 264 
nerves, 263, 264 
veins, 263 
Menisci, 184 
Menopause, 168, 173 
Menstrual period, 173 
Menstruation, 168, 173 
Merkel's tactile discs, 260 
Mesamoeboids, ichthyoid stage, 
66 
primitive, 66 
sauroid stage, 66 
Mesenchyma, 49, 214, 224, 252 
Mesentery, 57, 120 
Mesoderm, 178 
Mesotendon, 181 
Metabolism, 8 
Metachromasia, 54 
Metaphase, 12, 13 
Meynert's decussation, 204 
Labia glenoidalia, 60, 184 
majora, 176, 177 
minora, 176 
Labyrinth, 241, 244, 245 
Lacrimal glands, 236 
arteries, 236 
cells, 236 
nerves, 237 
Lacrimal passages, 237 
erectile tissue, 237 
Lactalbumin, 262 
Lactase, 111 
Lactation, 262 
Lactoglobulin, 262 
Lactose, 262 
Lacunae, 64 
Lamellae, 63 
basis, 64 
Haversian, 64 
interposed, 64 
interstitial, 64 
Lamina cribrosa, 221 
vitrea, 168 
Langerhans' islands, 117, 118 
Lanthanin, 6 
Larynx, 123 
arteries, 125 
basal membrane, 124 
cartilage, 124 
elastic fiber, 124 
entrance, 124 
epithelium, 123 
glands, 124 
goblet cells, 123 
lymph follicles, 124 
lymphatics, 125 
mucous membrane, 123 
muscles, 124 
nerves, 125 
propria, 124 
submucosa, 124 
veins, 125 
Lattice fibers, 113 
Lecithin, 37 
Lemniscus, 204 270 
Microphagi, 72 
Microscopic anatomy of or- 
gans, 66 
Microsomes, 3, 30 
Midbrain, 197, 203 
Milk, 262 
ducts, 262 
emulsion, 262 
globules, 262 
plasma, 262 
ridge, 262 
saccule, 263 
stripe, 262 
Mitochondria, 2, 8, 59, 162 
Mitochondrial sheath, 161 
Mitosis, 11 
barrel stage, 14 
Moll's glands, 234, 259 
Monakow's bundle, 204 
Monaster, 13 
Montgomery's glands, 264 
Moss fibers, 202 
Mother star, 13 
Motor end plate, 179 
nerve fiber, 179 
Mouth, mucous membrane 
Movement, organs of, 178 
Mucus, 4 
Mueller's cells, 218 
duct, 152, 170, 171 
fibers, 218 
muscle, 223 
Muscle, animal, 32 
cardiac, 38 
columns, 35 
heart, 32 
involuntary, 32 
orbicular, 234 
organic, 32 
red, 37 
smooth, 32 
spindle, 179 
striated, 32 
voluntary, 32 
Muscles, 178 
arteries, 179 
fasciae, 178 
lymphatics, 179 
nerves, 179 
veins, 179 
Muscular bundles, 179 
compartment, 35 
fibers, 178 
network, 34, 38 
stratum, 84 
tissue, 32 
Myelin, 46 
Myeloblasts, 70, 72, 182 
Myelocytes, 72 
Myoblasts, 178 
Myocardium, 77 
Myofibrils, 178 
Myometrium, 172 
Myosepta, 178 
Myotomes, 178 
Nail substance, 257 
Nasal cavity, 121 
basal membrane, 122 
blood vessels, 122 
cement edges, 122 
connective tissue corium, 121 
epithelium, 121 
erectile tissue, 122 
glands, 122 
goblet cells, 122 
lymph follicles, 122 
lymphatics, 122 
mucous membrane, 121, 123 
nerves, 122 
olfactory region, 121 
papillae, 121 
respiratory region, 121 
vestibule, 121 
Nasolacrimal groove, 237 
Naso-pharynx, 99 
Neoplasms, 4 
syphilitic, 4 
tuberculous, 4 
Nerve cell, 41 
cranial, 46 
ends, 46 
fiber, 40, 45, 46 
medulla, 46 
nucleus, 45 
olfactory, 46 
origins, 46 
spinal, 46 
structure, 42 
Nervi nervorum, 213 
Nervous system, 46 
arteries, 211 
blood vessels, 211 
membranes, 210 
nerves, 211 
organs of, 185 
plate, 214 
sympathetic, 46 
veins, 211 
tissue, 39 
development of, 48 
Neumann's sheath, 89 
Neural canal, 185 
crest, 185 
groove, 185 
Neurilemma, 47 
Neurites, 39 
Neuroblasts, 48, 215 
Neuroepithelium, 216 
Neurofibrils, 39, 42, 46 
Neuroglia, 47, 190 
Neurokeratin, 46 
Neuron, 41 
Nicol prism, 34 
Nipple, 262 
skin of areola, 263 
Nissl's clods, 44 
granules, 44 
Noduli, 78 
Normoblasts, 71 
Nuclear fluid, 6 
Nuclei, resting, 14 
Nuclein base, 5 
Nucleolus, 5, 45 
Nucleus, 59 
accessory, 7 
Nucleus, direct division of, 11 
indirect division of, 11 
gelatinous, 181 
globosus, 203 
pulposus, 184 
red, 204 
tecti, 203 
trochlearis, 203 
Nuel's space, 243 
Ocular cleft, 214 
muscles, 237 
fibers, 237 
nerves, 238 
Oculomotor nerve, 263 
Odontoblast, 87, 90 
(Esophagus, 100 
blood vessels, 101 
branches, 101 
fibrous membrane, 101 
glands, 100 
lymph follicles, 101 
lymphatics, 101 
propria, J00 
mucous membrane, 100 
muscularis mucosae, 100 
Olfactory bulb, 209, 250 
cells, 209, 249 
epithelium, 249 
fibers, 209, 249 
fossa, 249 
glands, 250 
glomerulus, 250 
hairs, 249 
lymph follicles, 249 
lymphocytes, 249 
nerve, 2*09, 210, 249 
organ, 249 
propria, 249 
region, 249 
vesicles, 249 
Olivary body, 198 
Olive, large, 198 
upper, 199 
Omentum, 57 
Oocytes, 165, 166, 167 
Oogonia, 165 
Optic artery, 221 
connective tissue, 220 
depression, 214 
fibers, 205, 220 
glia, 220 
nerve, 204, 210, 220 
pedicle, 214, 220 
tract, 204 
vein, 221 
vesicle, 214 
Ora serrata, 215 
Orbicularis ciliaris, 223 
Ossein, 64 
Ossification, 62 
directing trabeculae; 62 
endochondrial, 62 
perichondrial, 62 
Osteoblasts, 61 
Osteoclasts, 4, 62 
Otoliths, 240 
Ovarian interstitial gland, 166 
vessel, 165 
Nail, 257 
bed, 257 
field, 257 
fold, 257 271 
Ovary, 165 
arteries, 169 
cortex, 166 
epithelial cells, 165 
germinal cells, 165 
germinal rudiment, 165 
lymphatics, 169 
medulla, 166 
medullary substance, 165 
nerves, 169 
Oviduct, 170 
Ovulation, 168 
Ovum, 167 
Oxygen, 9 
Parotid gland, 98 
nerves, 98 
Pars caeca retinae, 215, 222 
ciliaris retinae, 220, 231 
optica, 215 
Penicilli, 83 
Penis, 162 
fascia, 163 
lymphatics, 164 
nerves, 164 
veins, 164 
Perceptive apparatus, 214 
Percipient elements, 216 
Perforatorium, 160 
Perichondrium, 60 
Perichoroidal spaces, 222 
Periderm, 252 
Perilymph, 245 
Perimetrium, 172 
Perimysium externum, 178 
internum, 178 
Perineural sheath, 213 
Perinuclear ring, 44 
Periodental membrane, 88, 90 
Periodontium, 181 
Periostium, 181 
Peripheral blood vessels, 213 
lymphatics, 213 
nerve fibers, 213 
nerves, 212 
Peritenorium, 180 
Peritoneum, 105, 119, 130, 171 
basal membrane, 120 
epithelium, 119 
nerves, 120 
phagocytic properties, 120 
propria, 120 
subserosa, 120 
vessels, 120 
Perspiration, 259 
Peyer's patches, 108 
Phagocytes, 54, 72 
Phalangeal process, 243 
Pharynx, 99 
glands, 99 
hypophysis, 99 
lymphatics, 99 
muscular coat, 99 
nerves, 99 
tunica adventitia, 99 
Pia mater, 210 
Pigment, 44 
epithelium, 215 
Pillars, 242 
Pinna, 248 
Pituitary gland, 205 
Pituitrin, 206 
Plasma, 79 
Plasmodesmi, 252 
Plasmosomes, 3, 59 
Pleochroism, 63 
Pleura, 129, 130 
Plexus choroidei, 211 
fundamentalis, 76 
intramuscularis, 76 
perimuscularis, 76 
Plica semilunaris, 107, 235 
Polar field, 12 
spindle fibers, 13 
Pons Varolii, 201 
Portio vaginalis uteri, 172 
Postmenstrual period, 173 
Predentine, 87 
Premenstrual period, 173 
Prepuce, 162 
of clitoris, 176 
Prespermatids, 154 
Primary bundle, 180 
Primitive segment columns, 136 
Primordial skeleton, 58 
Processus reticularis, 186 
Prochondrial stage, 57 
Pronephros, 136 
Prophase, 12 
Propria, 85 
Prostate, 157 
Protochondrial stage, 58 
Protoplasm, 2 
granular, 3 
Proximal layer, 214 
Pulp, 87, 88* 90 
cavity, 87, 88, 90 
Pupillary membrane, 224 
Purkinje's cells, 202 
fibers, 39 
threads, 78 
Pacchionian bodies, 210 
Palate, 94 
blood vessels, 95 
hard, 94 
lymphatics, 95 
mucous glands, 94, 95 
mucous membrane, 94 
muscular layer, 94 
nerves, 95 
soft, 94 
Pancreas, 116 
accessory nucleus, 118 
arteries, 119 
basal filaments, 118 
centroacinar cells, 117 
connective tissue capsule, 117 
end pieces, 117 
interlobular ducts, 117 
intermediate pieces, 117 
lymphatics, 119 
mucous gland, 117 
mucous membrane, 117 
muscularis, 117 
nerves, 119 
propria, 117 
secretion, 117 
secretory capillaries, 118 
secretory cells, 117 
simple cylindrical epithelium, 
117 ' 
Pancreatic juice, 117 
Panethian cell, 108 
Panniculus adiposus, 108 
Papilla, 219 
spiralis, 241 
Papillae, 85 
circumvallate, 91 
filiform, 91 
foliate, 92 
fungiform, 91 
renal, 137 
Papillary ducts, 137, 138 
Papillomacular bundle, 221 
Paraganglion intercaroticum, 
151 
Paraplasm, 3 
Parathyroid, 132 
blood vessels, 133 
glands, 132 
nerves, 133 
parenchyma, 132 
Parenchyma, 81, 132 
Paroophoron, 170 
Parotid duct, 98 
Radii lentis, 226 
Ranvier's cross, 47 
nodes, 46 
rings, 46 
Rathki's pouch, 205 
Receptory end organs, 180 
Reducing segmentation, 154 
Reissner's crusta, 241 
membrane, 241 
Remak's fibers, 46 
Renal capsule, 142 
column, 141 
labyrinth, 141 
lobules, 141 
pyramids, 141 
tubules, 141 
Rennin, 103, 117 
Respiration, 9 
Respiratory organs, 121 
Rete Malpighii, 253 
ovarii, 166 
testis, 153, 155 
Reteblast, 152, 165 
Reticular apparatus, 7, 44 
cartilage, 58 
Retina, 24, 214 
central area, 215 
distal layer, 215 
pigment, 215 
proximal layer, 215 
Retinacula, 255 
Retzius' body, 243 
Rhodopsin, 216 
Riolan's muscle, 234 
Rod. bipolar, 217 
ellipsoid, 216 
fiber, 217 
granule, 217 
optic cell, 217 
Rods, 216 
Root canal, 87, 88, 90 
Ruffin corpuscle, 94 272 
Sacculus, 239 
Saccus endolymphaticus, 245 
Saliva, 95 
Salivary corpuscles, 95 
glands, 95 
tubules, 96 
Sarcolemma, 33, 37, 39 
Sarcoplasm, 32, 33, 36, 38 
Scala tympani, 240 
vestibuli, 240 
Schizaxons, 188 
Schlemm's canal, 229, 232 
Schultz's comma, 192 
Schwann's sheath, 45, 47, 196, 
221. 
Schweger's striae, 89 
Sclera, 229 
cells, 229 
connective tissue, 229 
fibers, 229 
Scleral protuberance, 229 
sulcus, 229 
Scrotum, 162 
Sebaceous alveoli, 259 
epithelium, 259 
glands, 259 
Sebum palpebrale, 234 
Secretion granules, 171 
Semen, 159 
formation, 154 
Seminal vesicles, 157 
Semilunar valves, 78 
Sense of feeling, 252 
organs, 214 
Sensory nerve fibers, 179 
Septa, 81 
Septula testis, 153 
Septum longitudinare posteri- 
us, 185 
membranaceum, 77 
paramedianum, 185 
penis, 163 
urorectale, 145 
Serosa, 173 
Sertoli's cells, 154 
Sharpey's fibers, 63, 89, 181 
Sheath, adventitial, 73 
limiting, 64 
Schwann's, 45, 47, 196, 211 
Sinus urogenitalis, 145 
Skin, 252 
blood vessels, 260 
cells, 253, 254 
connective tissue, 255 
covering layer, 252 
epithelium, 252 
fat, 255 
fibers, 252, 254 
germinal layer, 252 
horny layer, 253 
intercellular spaces, 253 
lamellae, 254 
lymphatics, 260 
muscles, 254 
nerves, 260 
papillae, 253 
pigment, 254 
subcutaneous tissue, 255 
Soleplate, 179 
Solitary follicle, 80 
Sperm, 159 
crystals, 159 
Spermatic duct, 156 
Spermatids, 154 
Spermatocytes, 154 
Spermatogenesis, 154 
Spermatogonia, 154 
Spermatozoa, 24, 155, 159 
Sphincter iridis, 224 
papillae, 144 
vesicae, 146 
Spinal anterior column, 186 
horn, 186 
roots, 186, 192 
blood vessels, 194 
brain portion, 185 
central canal, 185 
cord, 39, 185 
ependymal layer, 185 
ganglion, 195 
glia capsule, 185 
gray matter, 185, 188 
lateral column, 186 
horn, 186 
lymphatics, 195 
mantle layer, 185 
marginal fibers, 185 
nerve, 196 
neuroglia, 193 
posterior column, 186 
horn, 186 
roots, 186, 192 
Spiral thread, 161, 162 
Spleen, 82 
arteries, 83 
capsule, 82 
nerves, 83 
Splenic pulp, 82 
Splenocytes, 83 
Spongioblasts, 48 
S-shaped band, 227 
Stairs, 38 
Steapsin, 103, 117 
Stellulae Verheynii, 143 
Stomach, 101 
arteries, 105 
folds, 102 
lymphatics, 105 
mother cells, 102 
nerves, 105 
parietal cells, 102 
propria, 102 
veins, 105 
Stratum cinereum, 204 
corneum, 253, 254, 258 
cylindricum, 253 
germinativum, 253, 258 
granulosum, 253 
intermedium, 252 
intimum, 183 
lucidum, 254 
opticum, 204 
papillare, 254 
reticulare, 254 
spinosum, 253 
submucosum, 173 
subserosum, 173 
supravasculare, 173 
terminale, 186 
vasculare, 173 
Stratum zonale, 186, 198, 204 
Stria vascularis, 241 
Striae medullares, 199 
of Retzius, 89 
Stroma, 68, 166 
Subdural space, 210 
Submaxillary blood vessels, 97 
excretory duct, 96 
gland, 96 
lymphatics, 97 
nerves, 97 
Substantia gelatina Rolandi, 
186, 188 
Sulcus coronarius, 63 
spiralis externus, 241, 242 
internus, 242 
transversus, 239 
Suprarenal adrenal, 149 
arteries, 153 
capsules, 149 
cellular cords, 150 
epithelial cortex, 149 
internal organ, 149 
lymphatics, 150 
medulla, 150 
nerve cells, 149 
nerves, 151 
septa, 149 
solid cellular cords, 149 
Sweat, 259 
glands, 258, 259 
pore, 258 
Sympathetic afferent fibers, 212 
ganglion, 211 
motor cells, 212 
nerve, 76 
nerve cells, 212 
sensory, 212 
Synarthroses, 184 
Synchondrosis, 184 
Synctium, 19, 119 
Syndesmoses, 184 
Synovia, 183 
Synovial membrane, 183 
Taenia, 109 
Tarsus, 234 
Taste, organ of, 250 
buds, 92, 124, 250 
cells, 251 
fibers, 251 
germinal spaces, 251 
nerves, 251 
pore, 250 
rod, 251 
Tavara's nodes, 78 
Tears, 236 
Teeth, 86 
Telae choroideae, 211 
Tendon, 56, 180 
blood supply, 180 
corpuscles, 56, 180 
fibers, 180 
lymphatics, 180 
sensory nerves, 180 
sheath, 181 
spindles, 180 
Tenon's space, 229 
Tensor choroideae, 223 273 
Terminal chambers, 85 
nodule, 217 
Testicle, 152 
arteries, 155 
ejaculatory ducts, 157 
epididymis, 156 
epithelium, 154 
excretory passages, 156 
genital ridge, 152 
rudiment, 152 
germinal cells, 152 
cords, 152, 153 
epithelium, 152 
intermediate cells, 155 
interstitial glands, 155 
lymphatics, 155 
nerves, 155 
sexual rudiment, 152 
veins, 155 
Thalamus opticus, 205 
Theca externa, 168 
folliculi, 166, 167 
interna, 168 
Thecal lutein cells, 168 
Thoracic duct, 80 
Thrombogen, 71 
Thymus, 133 
arteries, 134 
cells, 133 
lymphatics, 135 
nerves, 135 
parenchyma, 133 
physiology, 134 
veins, 135 
Thyroglobulin, 131 
Thyroid gland, 131 
arteries, 132 
follicles, 131 
lymphatics, 132 
nerves, 132 
veins, 132 
Tigroid substance, 44 
Tissue, 16 
absorption, 18 
adenoid, 51 
basal corpuscle, 17 
base, 17 
bristle band, 18 
cartilaginous, 50, 57 
chordal, 49, 50 
cilia, 17 
collagenous, 74 
connective 50, 52, 74 
capsule, 80, 95, 132 
corium, 84 
fibrillary, 52 
loose, 56, 129 
submucous, 85 
tight, 56 
yellow, 57 
cuticular band, 17 
elastic, 56, 74 
epithelial, 17 
excretion, 17 
fat, 57 
flagellated cell, 17 
free surface, 17 
gelatinous, 49, 51 
glands, 17 
hair cells, 17 
Tissue, lymphoid, 151 
membrane, 17 
metanephrogenous, 136 
mucinogen granules, 18 
mucus, 18 
muscular, 32, 75 
muscular striated, 33 
nervous, 39 
neuroglia, 50, 52 
osseous, 50, 61 
pigment, 57 
protection, 17 
reticular, 51 
reticulated, 180 
rod band, 18 
root of cilia, 17 
secretion, 17, 18 
subcutaneous, 84 
subendocardial, 77 
subepicardial, 78 
subpericardial, 78 
subpleural, 130 
supporting, 49 
adenoid, 50 
reticulated, 50 
vesicular, 49, 50, 77 
vitreous, 49, 51 
Tomes' fiber, 90 
granular layer, 88 
process, 87 
Tongue, 91 
arteries, 93 
ganglia, 94 
lymphatics, 93 
mucous membrane, 91 
musculature, 91, 93 
nerve supply, 93 
submucosa, 91 
veins, 93 
Tonsil, 95 
pharyngeal, 99 
tube, 99 
Tooth band, 86 
crown, 88 
rudiment of permanent, 88 
Trabecula, 81, 82 
Trachea, 125 
arteries, 125 
cartilaginous rings, 125 
epithelium, 125 
glands, 125 
lymphatics, 125 
mucous membrane, 125 
musculature, 125 
nerves, 125 
propria, 125 
submucosa, 125 
Tractus acustico-tectalis, 204 
cerebelli tegmentalis mesen- 
cephali, 203 
fastigio-bulbaris, 203 
olivo-cerebellaris, 198 
rubro-spinalis, 204 
solitarius, 198 
tecto-spinalis, 204 
vestibulo-cerebellaris, 199 
vestibulo-spinalis, 199 
Transition portion, 84 
Trigeminus, 200 
Trigona fibrosa, 77 
Trypsin, 117 
Tubal canal, 245 
Tuberculi contorti, 153 
recti, 153, 155 
Tuberculum acusticum, 199 
Tubotympanal canal, 245 
Tunica adventitia, 73 
albuginea, 153, 166 
dartos, 163 
intima, 73 
media, 73 
propria, 84 
submucosa, 85 
vaginalis propria, 153 
Tympanal cover layer, 242 
wall, 242 
Tympanum, 245 
Urea, 113, 138 
Ureter, 136, 137, 144 
Ureteral sheath, 144 
Urethra, 146 
cloaca, 146 
female, 148 
male, 147, 162 
primary, 146 
Uric acid, 138 
Urinary bladder, 145 
organ, 136 
Urine, 138 
Urogenital sinus, 162, 172 
Uterine glands, 172 
Uterovaginal canal, 170, 171 
Uterus, 17 
arteries, 174 
lymphatics, 174 
membrana propria, 172 
mucous membrane, 172 
muscularis, 172 
nerves, 174 
peritoneal coat, 172 
smooth muscle fiber, 172 
Utriculus, 239 
prostaticus, 158 
Vacuoles, 59 
Vagina, 174 
adventitia, 175 
arteries, 175 
flat epithelium, 175 
lymphatics, 175 
mucous membrane, 175 
muscularis, 175 
nerves, 175 
propria, 175 
submucosa, 175 
veins, 175 
vestibule, 176 
Valvulae conniventes, 106 
Vas afferens, 142 
deferens, 156 
efferens, 82, 142 
spirale, 242 
Vasa hyaloidea, 230 
vasorum, 76 
Vascular system, 66 
Vasoglossopharyngeal nerve, 
198 
nucleus, 198 274 
Vater-Pacini's corpuscles, 86, 
94, 261 
Veins, 75 
adventitia, 75 
connective tissue, 75 
media, 75 
muscular tissue, 75 
valves, 75 
Venae corticales, 143 
rectae, 143 
stellatae, 143 
Venulae rectae, 143 
Venulae rectae corticales, 143 
Vermiform appendix, 108 
Vessel, central chyle, 107 
lacteal, 107 
Villi, 106, 107 
Villous epithelium, 107 
Visual center, 205 
Visual purple, 216 
tract, 204 
Vitelli of embryonal cells, 4 
Vitreous, 230 
anterior, 227 
fibrils, 230 
fluid, 230 
humor, 51 
membrane, 168 
tissue, 230 
Vocal cords, 124 
false, 124 
true, 124 
Vortex lentis, 226 
White matter, 190 
nerve fibers, 190 
Wolffian body, 136, 153 
duct, 136,'146, 153, 166 
Wrisbergi portio intermedia, 
200 
Yolk globules, 167, 168 
Zona fasciculata, 149 
glomerulosa, 149 
parenchymatosa, 166 
pellucida, 167 
reticularis, 149 
vasculosa, 166 
Zonula ciliaris, 230 
cells, 231 
fibers, 230 
Zonule of Zinn, 230 
Wandering cells, 52 
Westphal-Edinger nuclei, 203 
Wharton jelly, 51