THE HISTORY 
OF 
THE HUMAN BODY 
BY 
HARRIS HAWTHORNE WILDER, 
11» 
Professor of Zoology in Smith College 
V Second Edition; revised 
NEW YORK 
HENRY HOLT AND COMPANY 
1923 Copyright, 1909, 1923; 
By 
HENRY HOLT AND COMPANY 
August, 1023 
PRINTED IN 
UNITED STATES OF AMERICA To the Memory of 
Kobert PREFACE TO THE SECOND EDITION 
In preparing this second edition of “ The History of the 
Human Body ” there is but little more to add to the first 
preface. Aside from the correction of numerous slight errors, 
revealed through the use of the book as a text-book with many 
classes of College students, the new edition contains many 
additions in which the results of the researches between the 
dates of 1909 and 1923 have been embodied, and in some of 
which the subjects have been entirely or partially rewritten. 
These latter have been mainly in the fields of the nervous 
system, the lymphatics, and the muscles; the first embodying 
the results of the labors of Herrick, of Johnston, and of 
Parker, the second those of Miss Sabin, and of the opposing 
school, led by Huntington and McClure, and the third based 
upon the morphological treatment of the muscles by Mc- 
Murrich. There has been also considerable revision of the 
endoskeleton, especially the skull, the mandible, and the 
shoulder-girdle, in which the work of the paleontologists has 
thrown fundamental light upon these parts. 
The chapters on the Nervous System and the Sense-Organs 
have been entirely recast by my wife, Inez Whipple Wilder, 
whose services I wish here gratefully to acknowledge. Any 
way in which this part of the book may prove a help will be 
wholly due to her earnest work in the usually thankless task 
of revising a part of another’s book, although in this case it 
has awakened nothing but the profoundest gratitude. 
The author wishes also to express his thanks to his col- 
league, Dr. Emmet Reid Dunn, especially for aid in the 
chapters on the skeleton, and to many of his other friends for 
encouragement and advice. 
Smith College, Northampton, Mass. 
February 15, 1923. 
V PREFACE TO THE FIRST EDITION 
This book has a twofold purpose: first, to present the re- 
sults of modern anatomical and embryological research rela- 
tive to the human structure in a form accessible to the general 
student, and, secondly, to furnish students of technical human 
anatomy with a basis upon which to rest their knowledge of 
details. 
Regarding the first of these purposes, it may be said that, 
while many of the phases of the doctrine of evolution have 
been thoroughly exploited, and their general teaching has be- 
come the property of the general scholar, the contribution 
to thought furnished by anatomy has been considered of too 
technical a character for popular presentation. It is true that 
this science necessarily rests upon a material basis, and in- 
volves a mass of extremely intricate details, and it is also true 
that a more or less complete knowledge of these is absolutely 
necessary before the contribution of this science to evolution- 
ary thought can be appreciated; but in these respects anatomy 
does not differ from other branches of natural science, the 
essential teachings of which are already a matter of general 
knowledge. If, then, the technical difficulties have been sur- 
mounted in the case of Geology, Astronomy and general 
Zoology, it is not too much to hope that in the course of the 
next few years the mission of Anatomy may also become gen- 
erally known, especially since its results touch human interests 
more closely than do those of any of the kindred sciences. 
Concerning the second purpose, that of assisting in the 
technical study of human anatomy, it is hardly necessary to 
present an argument, since the great advantages of studying 
human anatomy in connection with both comparative anatomy 
and embryology are patent to all who have employed this 
method. While there are still a few human anatomists who 
present the old argument that the science is too full of detail 
already to allow the assumption of additional facts, the ex- 
VII VIII 
PREFACE TO THE FIRST EDITION 
perience of everyone who has learned the parts of some com- 
plicated organ like the brain, by the old method, and has had 
it later elucidated by the new, is a sufficient refutation of 
such a position. It takes but a little experience with anatomy, 
as taught by the modern comparative method, to see that this 
latter furnishes a rational basis for an absolute knowledge of 
the fundamental relationships, while the old method is largely 
an intricate system of mnemonics. A student of the older anat- 
omy must needs remember arbitrarily that two given parts 
are related in a certain way and not in the reverse way, and 
if his memory is inadequate to the task he has nothing to 
save him, while a student, furnished with a morphological 
basis for his knowledge and able to refer the parts back to 
a time in which they were in a much simpler condition, will 
know that they must be related in a certain definite way, and 
cannot be otherwise arranged. 
The present work has especially the needs of the medical 
student in mind, since it is not a general comparative anatomy, 
but, as its title signifies, a “ history of the human body,” in 
which the structure of the lower vertebrates is expounded 
only so far as is needed to throw light upon the relations 
found in Man. Thus the lines that do not lead in this direc- 
tion, but represent specialized side-branches, like those of 
birds or snakes, are barely touched upon, other than as illus- 
trations of principles similar to those under consideration, 
although certain exceptional modes of development or eccen- 
tric specializations are often mentioned on account of their 
general interest. 
The technical terms of human anatomy employed in this 
work conform in general to the list prepared by the Basle 
Anatomical Nomenclature (BNA), but in cases where these 
terms differ widely from those in common use in America the 
latter are placed in brackets after the BNA term. In cases 
where the BNA nomenclature is not in accord with morpho- 
logical principles, these terms are rejected, but are indicated 
in brackets or otherwise. Of these, the most important are 
the following: PREFACE TO THE FIRST EDITION 
IX 
1. In the case of the bones of the carpus and tarsus. For 
these the BN A nomenclature employs the terms used on the 
Continent, and especially Germany (e.g., triquetrum, multan- 
gulum majus, etc.), instead of those to which the Americans 
and English are accustomed. The synonomy of these terms is 
presented in the form of a table, but as both sets are purely 
arbitrary and describe the shapes and relative sizes as found 
in Man alone, there seems no reason why one should be pre- 
ferred to the other, or, indeed, why either should be longer 
perpetuated, in preference to the simple system employed by 
comparative morphologists. 
2. In several cases in which terms of orientation are still 
employed with reference to Man in a standing position (e.g., 
superior and inferior instead of anterior and posterior; an- 
terior and posterior instead of ventral and dorsal). Thus, in 
the case of the columns of the spinal cord, it is thought best 
to reject the BNA terms posterior and anterior in favor of 
the more natural dorsal and ventral, as employed in the case 
of all other animals. In the same way the two venae cavae are 
referred to as anterior and posterior instead of superior and 
inferior. 
3. In the case of the pads of the palm and sole. Here the 
principle involved is one of use rather than position, and the 
point at issue depends upon the true function of these parts. 
The two views held at present are (1) that their function is 
tactile, and (2) that it is mechanical, preventing the tendency 
to slip by presenting a surface covered by ridges, [cf. Chap- 
ter IV.] The BNA term for these pads is toruli tactiles, a 
term which does not accord with the view expressed here. 
In a few cases the adoption of the new nomenclature in- 
volves changes in well-established terms; for example, ductus 
[mr] deferens, stratum germinativum \_mucosum], and renal 
[Malpighian] corpuscles; and in some there is a slight change 
in spelling, as thyreoid and chorioid, but as these are all in the 
interest of exactness and do not violate morphological princi- 
ples, they are employed here. 
In the case of a work which, like the present one, attempts X 
PREFACE TO THE FIRST EDITION 
to cover a large field, in each and every point of which there 
are opposing views, both as to the facts themselves and to 
their interpretation, errors and misinterpretations are inev- 
itable, and the writer craves the indulgence of those who have 
directed their special attention to any one of the subjects 
touched upon here. The book is primarily intended as an 
interpretation of the work of the specialists in anatomy, 
especially during the last half-century, and its mission will be 
accomplished if it serves to render the facts obtained more 
accessible to the general reader. 
Dryads’ Green, Northampton, 
May, 1909 CONTENTS 
PAGE 
Preface to the First Edition v 
Preface to the Second Edition ix 
chapter 
I. The Continuity of Life i 
II. The Phylogenesis of Vertebrates 26 
III. The Ontogenesis of Vertebrates 48 
IV. The Integument and the Exoskeleton 78 
V. The Axial Skeleton 124 
VI. The Visceral Skeleton 168 
VII. The Appendicular Skeleton 182 
VIII. The Muscular System 217 
IX. The Digestive and Respiratory Systems 286 
X. The Vascular System 347 
XI. The Urinogenital System 396 
XII. The Nervous System 437 
XIII. The Sense-Organs 521 
XIV. The Ancestry of the Vertebrates 564 
Appendix; The Classification of Vertebrates 597 
XI PLATES 
• Page 
Plate I. Diagrams showing Vertebrate development; 
stages I and II. Based upon a stereogram by 
Kingsley 62 
Plate II. Diagrams showing Vertebrate development; 
stages III and IV. Based upon a stereogram by 
Kingsley 63 
Plate III. Development of the urinogenital system in Am- 
niotes from stage of sexual indifference (a) to 
male (b), and to female (c). In part after 
Gegenbaur 418 
Plate IV. Longitudinal median sections of Vertebrate 
brains corresponding to the first half of the 
series in Fig. 153 in the text, [(b) and (c) 
after Edinger] 464 
Plate V. Longitudinal median sections of Vertebrate 
brains corresponding to the second half of 
the series in Fig. 153. [After Edinger]. . 465 
Plate VI. Diagram of cranial nerves in Anamnia. 
[After Wiedersheim] . .... 502 
Plate VII. Diagram of cranial nerves in Amniota. 
[After Wiedersheim]. .... 503 
Plate VIII. Inter-relation of Trigeminus, Facialis, Glosso- 
pharyngeus, and Vagus, together with the sym- 
pathetic ganglia, in Man. Based upon diagrams 
by several anatomists (Arnold, Gray, 
Gegenbaur). . . . . . .510 
XIII “Man still bears in his bodily frame the indelible 
stamp of his lowly origin 
Charles Darwin : “ Descent of Man ’* 
(closing- sentence) CHAPTER I 
THE CONTINUITY OF LIFE 
“ Ich sage immer und wiederhole es, die Welt konntfe 
nicht bestehen, wenn sie nicht so einfach ware.” 
Johann Wolfgang Goethe, in Eckermann, 
Gesprdche mit Goethe, n Apr., 1827. 
One of the grandest generalizations formulated by modern 
biological science is that of the continuity of life; that the 
protoplasmic activity within the body of each living being 
now on earth has continued without cessation from the remote 
beginnings of life upon our planet, and that from that period 
until the present no single organism has ever arisen save in 
the form of a bit of living protoplasm detached from a pre- 
existing portion; that the eternal flame of life, once kindled 
upon this earth, has passed from organism to organism, and 
is still going on, existing and propagating, incarnated within 
the myriad animal and plant forms of the present day. Built 
up of carbon, hydrogen, oxygen, nitrogen, together with 
traces of a few other elements, yet of a complexity of struc- 
ture that has hitherto resisted all attempts at complete 
analysis, protoplasm is at once the most enduring and the 
most easily destroyed of substances; its molecules are con- 
stantly breaking down to furnish the power for the manifesta- 
tions of vital phenomena, and yet, through its remarkable 
property of assimilation, a power possessed by nothing else 
upon earth, it as constantly builds up its substance anew from 
the surrounding medium, usually in excess of that lost by dis- 
integration, and possessed of qualities identical with those of 
the parent mass. The continuity, then, is not one of ma- 
terial, but of qualities, and it is this that makes an organism 
the same from birth till death. An acorn, a sapling, an oak 2 
THE HISTORY OF THE HUMAN BODY 
—all are the same organism, although the bulk of the acorn 
is but the hundredth part of the sapling, and that the thou- 
sandth part of the oak, and although every particle that con- 
stituted the organism in an early stage may have been elim- 
inated long before the next stage is reached. Upon the at- 
tainment of a certain size-limit, the most or the whole of the 
constantly accumulating excess is freed from the parent or- 
ganism, in the form of germinal particles, each of which, still 
continuing the process of assimilation, wrests building ma- 
terial from its surroundings, from other organisms as well as 
from inorganic substances, and, if successful, develops into a 
new organism, which often to the minutest details reproduces 
the parent from which it arose. 
Through this power of assimilation there is a constant en- 
croachment of the organic upon the inorganic, a constant 
attempt to convert all available material into living substance, 
and to indefinitely multiply the total number of individual 
organisms. This tendency receives a check, however, from 
two sources: from the forces of the inorganic world, since 
each organism is particularly sensitive to surrounding con- 
ditions, and, secondly, from other organisms. It has been to 
offset these that all variations in organisms have taken place, 
changes which have furnished a great power of adaptation to 
various conditions and have resulted in the invasion and occu- 
pancy of all environments in which the conditions do not 
absolutely prohibit protoplasmic activity. 
Thus have developed all the plant and animal forms which 
have ever appeared on the earth, and since no one of these 
can have arisen spontaneously, but depends for its develop- 
ment upon a bit of living protoplasm thrown off from a pre- 
viously existing organism, it follows that all living beings may 
be traced back through continuous though converging lines 
of life to the first beginning of all life—the primordial proto- 
plasm. Difficult as this may be to follow in the case of the 
more complex organisms, those which, through constant 
modification, have departed most widely from the original 
condition, this continuity of life is easily seen in the one- THE CONTINUITY OF LIFE 
3 
celled organisms, or Protozoa, which are the simplest in struc- 
ture of all living things. The essential body substance con- 
sists of a minute mass of semi-fluid protoplasm, in the interior 
of which lies a denser portion which constitutes its most im- 
portant organ, the nucleus. This latter is the physiological 
center for the control of all the vital functions of the animal, 
and is undoubtedly extremely complex in structure, even in 
the simplest members of the group. In some protozoans the 
protoplasm is enclosed by a thin but fine cell-membrane, which 
preserves for the animal a more or less definite shape; in other 
cases there is no such membrane, and the protoplasm is free 
to assume an irregular and constantly changing outline, each 
species, however, still preserving a certain characteristic range 
of form. 
Through the intaking of other organisms, either alive or in 
a state of disintegration, the protoplasm of all Protozoa has 
the power of adding to its bulk, through assimilation; a pro- 
cess perhaps more than all others characteristic of life and 
not imitated in any way by lifeless matter. For this process 
a nucleus is absolutely essential, for it has been experimentally 
proven that non-nucleated fragments of the simpler Protozoa 
are capable of continuing their existence for some time, and 
can even receive foreign materials, yet have no power of 
assimilation. A fragment containing a nucleus, on the other 
hand, will continue to grow and will ultimately completely 
restore the lost part. 
This process of growth is limited, however, not by any 
failure in the vital process, but by the mathematical law of the 
ratio of surface to mass.* The intaking of both food and oxy- 
gen, and also the expulsion of all waste products, take place on 
the external surface only, or, in the case of those covered by a 
cell-membrane, over a restricted portion of that area, but on 
* This law is that the surfaces of homologous solids are to each other 
as the squares, and their masses as the cubes, of their homologous dimen- 
sions. A protozoan which has increased to twice its normal size, i. e., twice 
its original diameter, has increased its surface four times and its mass eight 
times. It has therefore reduced its proportionate surface by one half, and 
its supply of food and oxygen in the same degree. 4 
THE HISTORY OF THE HUMAN BODY 
account of the law just mentioned, the mass of a growing 
animal increases faster than its external surface, and the time 
is soon reached at which it is in danger both of starving and 
of suffocation. To offset this, recourse is had to a process 
called fission, which effects at the same time a relief from the 
physiological difficulty and a multiplication of the individual. 
In its simplest form this reproduction by fission, as it is 
termed, is inaugurated by (1) a lengthening of the nucleus; 
(2) a contraction of its middle portion, producing a form 
Fig. 1. Simple fission. Diagrams based on the infusorian Paramcecium. 
In all the figures the macronucleus is on the left, the micronucleus on the 
right. The division of the micronucleus is effected by mitosis, that of the macro- 
nucleus is direct. 
like an hour-glass, and (3) a separation of the two halves, 
forming two independent nuclei, each half of the original 
size. A similar subdivision of the body of the cell follows, 
the arrangement being such that each piece becomes supplied 
with one of the two nuclei, and is capable of beginning an 
independent existence. In certain other cases the proceeding 
is more complicated. The organism surrounds itself with a 
shell or cyst, secreted by the protoplasm, and after a quiescent 
period, breaks up, not into two, but a larger number, usually 
four, eight or sixteen, which become released by the bursting 
of the cyst and swim out into the water, each in its turn to> 
assimilate foreign matter until of the size for another encyst- 
ment. 
It is but natural to refer to the undivided organism as the THE CONTINUITY OF LIFE 
5 
“ parent,” and to the resultant organisms, whether two or 
more, as the “ offspring,” yet it is here plain that we do not 
have to do with either parent or children in the usual sense. 
The “ parent,” as such, ceases to exist the moment it becomes 
divided; yet no death has ensued, for there is no dead body. 
The vital activity of protoplasm has been perpetuated, without 
an interruption, from the undivided mass to each piece result- 
ing from the fission, or in other words, the life is continuous. 
In a restricted sense, then, a protozoan is immortal: its 
Fig. 2. Multiple fission as shown by the parasite of malaria, Hama- 
mceba malaria. [After Ross and Fielding-Ould.] 
The enclosing outline represents a human blood corpuscle, within which the 
transformation takes place. 
(a) Young amoeboid stage formed from a sporozoid. (b) Older amoeboid stage, 
showing growth, (c) Beginning of multiple fission. ,(d) Division of the mass 
into eight sporozoids. At this stage the sporozoids become liberated through the 
distintegration of the remains of the corpuscle, and invade the plasma. From this 
they enter new corpuscles, and assume the amoeboid form as at a, thus completing 
the cycle. 
vital activities have been continuous, without interruption from 
the beginning of life upon the planet. It is not meant, of 
course, that a protozoan is indestructible, for countless num- 
bers of them are continually succumbing to mechanical or 
chemical injury; but each accident of this sort extinguishes a 
life which has existed without cessation from the first life of 
all. The actual material particles are constantly changing, 
even while a protozoan is retaining its identity as an indi- 
vidual, yet that which is continuous from moment to moment 
in such an individual, is equally so during and after each 
fission, and is perpetuated without interruption, in each piece, 
so long as it does not meet with conditions which destroy it. 
Aside from the phenomenon of reproduction by fission, there 
is another procedure which has been observed in many forms 6 
THE HISTORY OF THE HUMAN BODY 
of Protozoa, and while in the present state of knowledge it 
cannot be asserted that it is a universal procedure, existing in 
all species, it is very likely that this or a similar process is oc- 
Fig. 3. Conjugation. Diagrams based on the infusorian Paramcecium. 
Here the two gametes are of the same size and the fusion is temporary 
with similar results in the case of each. 
(a) The two micronuclei are forming mitotic figures preparatory to division, 
(b) The two micronuclei have elongated; the macronuclei are disintegrating. (c) 
One-half of each micronucleus passes into the other individual through the 
mouth, (d) Fusion occurs in each individual between the half nucleus 
that originally belonged to it and the half nucleus that has come from 
the other. This forms a fusion-nucleus, (e) The fusion-nuclei form mitotic figures 
preparatory to division. At about this time the two individuals separate. (f), (g), 
(h) The fusion-nucleus divides three times in succession, eventually forming eight 
nuclei. (i) Four of the eight nuclei enlarge and form macronuclei, and four re- 
main small and become micronuclei. These become associated in pairs, one micro- 
and one macro-nucleus, and are distributed to four individuals that result from 
two successive divisions. Each of these, evidently as the result of the conjugation, 
has a renewed power of fission, and multiplication continues in this way [cf. Fig. i] 
until the power becomes diminished, when it is renewed by a new conjugation 
[cf. Fig. s (a)]. 
casionally undergone in all cases. This is the process of con- 
jugation [Fig. 3] which, in the cases best studied, seems to 
bear a definite relation to the process of reproduction by 
fission. In these cases the number of fissions which can occur 
in succession appears to be limited, for after a series of these THE CONTINUITY OF LIFE 
7 
it seems that the reproductive force becomes lessened, causing 
longer pauses between successive fissions, and ultimately the; 
death of the organisms. It is at this time, when the fissions 
are farther between and carried on with less activity, that 
conjugation appears. This consists typically of the temporary 
fusion of two individuals, during which there is a mutual inter- 
change of certain of the elements of the nuclei. When this 
is accomplished the two individuals, or gametes, as they are 
here termed, separate, and begin anew a fresh series of fissions 
as at first. 
The purpose of the process thus seems to be something like 
a rejuvenescence, by means of which the reproductive activity 
may be renewed; yet, that the action is chemical rather than 
physiological is suggested by experiments in which a similar 
increase of activity, taking the place of conjugation, may be 
induced by the addition of food-substances like beef broth to 
the water containing the species under investigation. 
In many cases the process of conjugation is rendered more 
complicated by the differentiation of two sorts of individuals, 
macro- and micro-gametes, which are evidently produced for 
this especial purpose by a variation in the usual course of the 
fission process. In this case the two may unite perma- 
nently and form a zygote, which becomes thus endowed with 
special reproductive activity. [Fig. 4.] 
In multicellular organisms the matter becomes still more 
complicated, but is essentially the same so far as concerns pro- 
toplasmic continuity. Here only certain cells, which are called 
germ-cells, act as gametes and conjugate, producing the new 
organisms by their repeated divisions, while the remainder, 
often vastly preponderating over the former in actual bulk, 
build up a body or soma, which forms a shelter and protection 
for the germ cells. Somata possess a high degree of adapt- 
ability to external conditions, and become modified to fit them, 
so that in this way they and the germ-cells contained within 
them may come to be developed in places and under circum- 
stances where otherwise they could not possibly exist. 
In this way all animal and plant forms have been produced, 8 
THE HISTORY OF THE HUMAN BODY 
Fig. 4. Carchesium, a sessile protozoan colony, showing conjugation. 
[Diagram in part after Butschli and Schewiakoff.] 
a a Macrozooids, which, by their division produce either b, other macrozooids, or 
c microzooids, which eventually become free, d Free-swimming microzooids, per- 
haps from another colony, d microzooid (here a microgamete) in conjugation with 
a macrozooid (macrogamete). In each of the above individuals may be seen a 
vermiform marcronucleus and a spherical micronucleus, e Detail of conjugation. 
The macronucleus of each component is shown broken into fragments previous to dis- 
solution; the two micronuclei are dividing mitotically into two halves, one-half of each 
destined to pass into the other component. The micro- and macro-gametes are 
designated, respectively, as male and female. [Subsequent stages similar to those 
shown in Fig. 3.] THE CONTINUITY OF LIFE 
9 
each being but the temporary dress of a proliferating mass of 
protoplasm; a detached mass of tissue, which feeds, breathes, 
and often moves and perceives, for the better support and pro- 
tection of the continuous lizhng protoplasm. The soma is 
mortal, and after a longer or shorter period loses its vitality 
and goes to dissolution; the germ, in the restricted sense of 
being coeval with life upon the earth, is immortal; and yet, in 
spite of the far greater value of the latter, the two are very 
closely associated. As the soma becomes modified, the germ 
becomes equally so, since each germ, as it develops, repro- 
Fig. 5. Diagrams illustrating the life cycle in unicellular and multicel- 
lular organisms. 
The round dots represent cells. In (b) and (c) the germ cells are gray, the 
somatic cells black. In all cases the destruction of a cell is indicated by a heavy 
black bar placed beneath it. 
(a) Protozoan type, with equivalent gametes. The series begins with a con- 
jugation, after which the gametes separate and a series of simple fissions follows in 
the case of each gamete. After several generations of these, in which many of the 
individuals produced are destroyed, conjugation again appears, completing the cycle. 
(b) Life cycle in a male Metazoan. The cycle begins with a conjugation between 
a macro- and a micro-gamete (ovum and spermatozoon), after which there follows 
a series of simple fissions, which differ from those of (a) in the perpetual union 
of the components thus formed, represented here by connecting lines. There is 
thus built up an interdependent cell-colony, the soma, shown in the fifth row from 
the top. Certain of the somatic cells become microgametes (=spermatozoa), destined 
for conjugation, and capable of independent existence when separated from the 
rest. The remaining somatic cells perish simultaneously. 
(c) Same as (b), but the gametes produced are macrogametes (=ova), and the 
soma is consequently female. The conjugation of these cells with microgametes is 
shown in the lower row, thus completing the life cycle. The bars placed beneath 
the completed germ cells in (b) and (c) suggest the probable proportion of accidental 
destruction. 
duces a soma almost identical with that from which it came; 
a result which can be explained only by supposing that each 
germ contains a controlling mechanism, directing and de- 
termining the development of every individual part in the 
future soma. 10 
THE HISTORY OF THE HUMAN BODY 
The differences between unicellular and multicellular organ- 
isms in these respects may be graphically expressed in the 
accompanying diagrams. [Fig. 5.] 
In the first of these (a), which represents the condition in 
the simpler unicellular animals, a cycle of cell generations 
begins with a conjugation, a procedure during which a part 
of the nuclear material of the two conjugating individuals is 
mutually exchanged, the result seeming to be an increased 
activity of division for some time. The resulting cell genera- 
tions are followed in the diagram in the case of but one of 
the two conjugating individuals, that of the other being sim- 
ilar. Several generations are indicated, as also the chance 
mortality of individuals, the result of this last being to keep 
the total number of individuals in each generation approxi- 
mately the same in spite of the geometrical ratio in which the 
individual cells tend to increase. 
The two other diagrams (b) and (c) represent a similar 
cycle of cell generations in two multicellular organisms, male 
and female, respectively. In these, the cycle begins with the 
union of a male and female germ-cell, that is, a permanent 
conjugation between a micro- and a macro-gamete, forming 
a fertilized ovum. Because of the cellular differentiation due 
to a necessary adaptation, the male cell is small and active and 
equipped with a locomotive organ in the form of a vibratile 
flagellum, while the female cell is more or less immobile and 
furnished with a large amount of yolk, the food supply for 
the embryo during its early development, when it cannot ob- 
tain its own nourishment. After the conjugation there 
ensues a series of cell generations, as in the other case, with 
the essential difference that here they remain in organic con- 
tinuity with one another and form, not independent indi- 
viduals, but the component parts of a multicellular organism. 
The number of such generations is often very great, certainly 
much greater than here represented, and the cells early begin 
a differentiation of form and function which leads eventually 
to the formation of all the tissues necessary to build up the 
adult body or soma. Among those early cells are the THE CONTINUITY OF LIFE 
11 
primordial germ-cells, differently marked in the diagrams, 
which seem to retain the general qualities of the first egg-cell 
and to resist the tendency to specialization seen in the others. 
From these the final germ-cells develop, small and mobile in 
the case of the male, large and provided with yolk in the case 
of the female. These, liberating themselves from the soma, 
unite in pairs to form another cycle like the first, while all the 
generations of the somatic cells are sooner or later brought 
to an end simultaneously, the death of the individual. 
This organic connection between the cells, which constitutes 
the essential difference between unicellular and multicellular 
organisms, has its advantages as well as its disadvantages. 
The chief among the first is the great power of differentiation 
among individual cells or cell-groups, with the resultant di- 
vision of labor; a great disadvantage lies in the fact that 
through this very specialization of function, any vital accident 
occurring in one part drags down to death all the other cells 
of the organism. The germ-cells alone are the immortal 
parts, the continuous principle which survives the destruction 
of the soma, and each contains within itself, expressed in the 
form of an ultra-complex mechanism, the ability to reproduce 
in its cell descendants every detail of the soma from which 
it originated. 
In this is seen the primary value of the soma, which be- 
comes clear when taken in connection with the struggle on the 
part of nature to develop as much protoplasm as possible. 
The soma is a mass of protective cells, capable of a high 
degree of specialisation, and thus able to adapt itself in accord- 
ance with the needs of every environment in which it is 
possible for organic beings to exist. Even its death is an 
adaptation, for by this means new and perfect somata are con- 
stantly taking the place of those whose usefulness as guardians 
of the germ-cells has become impaired by the inevitable injury 
to which organisms are constantly exposed. Life is con- 
tinuous in the germ-cells from generation to generation and 
has been carried into all environments and protected and 
multiplied through a constant succession of perishable somata. 12 
THE HISTORY OF THE HUMAN BODY 
The adaptations of the soma are extremely gradual, and 
thus, if all forms that have ever existed could be arranged in 
order, they would form a continuous series, not in the form of 
a straight line, but in that of a profusely branching tree, since 
from one parent form two or more varieties are constantly 
arising, capable of inhabiting a slightly different environment, 
and, if successful, continuing along separate lines of develop- 
ment. As a matter of fact, however, the fauna and flora of 
the world at present represent, for the most part, but isolated 
units in the great system, and while a careful study of the 
structure of every known form has led to the restoration of 
many portions of this tree, there are in other places great gaps 
filled thus far only by inferences, and therefore matters of 
continual controversy. 
This continuity of all life and the recognition of animals 
and plants as no more than the countless adaptive forms of 
the plastic soma, enable the zoologist to trace out with con- 
siderable accuracy the history of those series of which the 
records are the best preserved, a history which, while lying in 
the past, is represented in the present by forms which arose in 
earlier periods, the complete adaptation of which has allowed 
them to successfully struggle with their competitors and thus 
to survive with but little change to the present day. It is in 
this sense, then, that there can be a history of the human body, 
the history of the struggles and successes and failures of our 
remote ancestors, as they successively encountered the various 
environments wherein this history has been enacted. The 
ocean, the marsh, the prairie, the forest, each has formed the 
complex stage-setting of an historic period and has contributed 
to the formation of the human soma. Man’s body was, like all 
others, not made new, but adapted, and this not once, but 
repeatedly. Old organs have been readapted to new uses or 
are retained as merely functionless rudiments; new organs have 
arisen through the change of function of some preexisting 
part; the body has in all its details been molded and shaped 
with each new change to the end of producing the highest 
degree of physiological efficiency; and this always with sole THE CONTINUITY OF LIFE 
13 
reference to the problem in hand and with no regard to the 
future inconveniences which may arise from a certain form 
or arrangement. 
To learn this history we must turn to the comparative 
anatomy of vertebrates. Some of them are still so similar 
to the early stages of our own development that we may 
almost look upon them as our former selves; others represent 
development along other lines to which their environment and 
its necessities have brought them, and they show us what we 
might have been, had chance led us in their direction. 
The first period of vertebrate history was an aquatic one, 
in which the environment was represented, not merely by the 
water, which developed a certain kind of respiration, and al- 
lowed a style of locomotive organs inadmissible on land, but 
also by the vast hordes of carnivorous enemies generated in 
the depth of the ocean; yet, through these struggles was 
gained an exoskeletal armor with which to ward off the 
attacks of the powerful molluscs and crustaceans of the 
Silurian seas; and of the armor plates thus obtained the relics 
are still retained in the cranial region, forming the dermal 
bones of the skull (frontals, parietals, squamosals, etc.). 
Profound changes became necessary when our ancestors 
left the ocean and sought refuge in the marshes and upon 
land; changes not merely in the mode of respiration, but in 
the entire skeletal and muscular system, owing to the great 
difference in specific gravity between water and air. Differ- 
ences in food caused modifications in the digestive system, 
and all surfaces exposed to the air developed glands in pro- 
fusion to resist the drying effect of sun and wind. During 
this period were acquired pentadactylous extremities, lungs 
and larynx, and the salivary and lacrimal glands. The or- 
ganism became modified in countless ways, as the attempts to 
inhabit dry lands, apart from the marshes, ushered in the next 
great period, that of the rocks and plains. 
Here began a complete aerial respiration, the development 
of the permanent kidneys, which replaced the Wolffian bodies 
of amphibians, and the formation of a cornified epidermis, with 14 
THE HISTORY OF THE HUMAN BODY 
its proliferations in the form of scales, horns and claws. The 
great increase in the size and strength of the limbs, begun in 
the previous period, reached here a high degree of perfection, 
and towards the end of this period vertebrates were for the 
first time enabled by the help of these to lift their bodies com- 
pletely from the ground and exchange the crawling move- 
ments for a definite walk. 
But the most important of all the changes produced by a 
land environment has been the rapid increase in the size and 
efficiency of the central nervous system, which became de- 
veloped in part through the need of controlling the larger limb 
muscles, and in part in response to the far more varied 
environment afforded by the land surfaces and the consequent 
necessity of recording a larger number of sensory impressions. 
By a curious and indirect method this development, especially 
that of the perceptive centers of the brain, has been still more 
encouraged in a certain group of rather generalized mammals 
through the occupation of an arboreal environment. The 
direct result of this was, that in these animals, which were, in 
the main, large enough to grasp the boughs in climbing, a 
prehensile paw with an opposable first digit was developed on 
both anterior and posterior limbs, and this new tool, especially 
the anterior set, which became hands, from now on allowed 
the animals to grasp all sorts of objects, and expose them to 
a more careful scrutiny, thus causing a continually greater 
development of the recording centers of the brain. 
As this arboreal environment has been the latest in the 
line of human history previous to the assumption of a strictly 
terrestrial life, there are still in man’s body more evidences 
of this than of the earlier stages, but these, because they are 
the latest, are also the most superficial, and consist of such 
characters as the flattened nails, the pectoral position of the 
mammae, and the opposable thumbs. 
The latest change of all, the assumption of an erect position 
and the emancipation of the anterior limbs from all locomotive 
functions, has necessitated a few modifications, especially 
changes in the pelvic girdle and in the relative size and THE CONTINUITY OF LIFE 
15 
strength of the muscles of the legs, but has effected little in 
the way of actual change of structure, so that anatomically 
man still stands very near his arboreal kinsmen that represent 
the immediate past in the history of human development. 
This study of the succession of forms upon the earth is 
termed race history or phylogenesis, and forms one of the two 
sources from which the past history of animal development 
may be obtained. The other is the sequence of stages re- 
corded during the embryonic development of each individual, 
and is termed the developmental history or ontogenesis. By 
what is at once the most natural and the most mysterious law 
of nature each individual animal inherits, not only the struc- 
ture of its immediate parents, the attainment of which means 
the end of its development, but also that of its entire line of 
ancestors, which appear in approximately the natural order 
of succession and constitute the stages of its ontogenetic de- 
velopment. 
As a residt of this it follows that the two records, phylo- 
genetic and ontogenetic, run closely parallel, and each serves 
in many places to bridge a gap or explain an obscure period 
in the other. This parallelism of the two records lies at the 
basis of all morphological speculation, and forms what is 
often termed the law of biogenesis * It must not be expected, 
however, that the correspondence in the two records is com- 
plete, since numerous disturbing causes must be taken into 
consideration which tend to modify each record quite inde- 
pendently of the other. In the race history there are many 
gaps caused by extinction, and the forms that have come 
down to us from earlier periods have become much changed 
from their former condition and represent their ancestors in 
a qualified sense only; while in the individual development 
there are many characters that are in no sense historic, and 
have to do with such immediate environmental problems as 
nutrition or protection. These latter characteristics, which 
* The “ Biogenetisches Grundgesetz ” of Haeckel; formulated by him 
as follows: “Die Ontogenie (Keimesgeschichte) ist eine kurse Wieder- 
holung der Phylogenie (Stammesgeschichte)(1874). 16 
THE HISTORY OF THE HUMAN BODY 
are called cccnogenetic, or modern, are clearly of no importance 
in such inquiries as the present, and must be carefully distin- 
guished from those that are palingenetic, that is, actual repe- 
titions of past history. 
It is essential, then, in order to interpret correctly the two 
records, phylogenetic and ontogenetic, and from them to re- 
produce the past history of our race, with its solutions of the 
details of man’s structure, that the nature of each form of 
record be thoroughly understood. The phylogenetic or race- 
history is the plainer and more direct of the two, and presents 
fewer technical difficulties to the student, but it contains at 
present extensive gaps, not yet filled in by the discovery of 
fossil remains; the manuscript is plain and clear, but has suf- 
fered much from the ravages of time and is fragmentary at 
best: the ontogenetic, on the other hand, presents a more con- 
tinuous story, but the difficulties in the way of investigation 
are very great; here the manuscript is written in a micro- 
scopic hand, and is, moreover, a palimpsest, scribbled over with 
extraneous material, added at late dates and connected with 
the exigencies of development. 
The characteristics of the phylogenetic record may be made 
clear by the aid of the accompanying diagram [Fig. 6], which 
represents a purely hypothetical case, and the conditions in- 
volved may be presented in the form of laws, as follows: 
I. Development has not been in a single direction, but in 
many, since the constant rivalry between allied forms causes 
them to continually push their zvay into new environments, 
the gradual adaptation to which causes a greater and greater 
divergence between the descendants of those that entered the 
new environment and those that remained in the old. 
To illustrate this by the diagram, suppose 29 to represent a 
terrestrial carnivorous animal, living on the border of the 
ocean and preying upon the forms of life found upon the 
shore, or within shallow water. Pressed by the struggle for 
existence, in this instance represented by the scarcity of this 
sort of food, certain individuals venture farther out into 
deeper water and attempt to capture fish. Thus begins the THE CONTINUITY OF LIFE 
17 
establishment of a group which becomes more and more 
aquatic, as represented by the divergent line leading to 34, 
until finally a completely aquatic fish-eating animal or group 
of animals is the result, the form at the end of the line, 34, 
representing the highest point of specialization attained. 
The remaining descendants of 29, continuing to live in pre- 
cisely the same habitat as their ancestors, as is here indicated 
Extinct forms are represented by open circles, living forms by solid black ones. 
The same number distinguished by exponent letters signifies a close relationship. 
The dotted areas suggest some special environment, the inhabitants of which show 
“ adaptive resemblance ” although representing several unrelated lines. 
Fig. 6. Hypothetical tree illustrating the interrelations of organisms. 
by the continuance of the line 29-35, in the same direction as 
28-29, either remain exactly as their ancestors, or probably 
become more highly specialized in the same direction. Con- 18 
THE HISTORY OF THE HUMAN BODY 
tinual divergencies on the part of animals, as they seek new 
environments in this way, produce the numerous divergent 
branches, the relative time of the divergence being expressed 
by the position of the intersection and the amount of the mod- 
ification by the length of the line. 
II. Although animal forms are not related to one another 
as members of a single linear series, they yet form a con- 
tinuum, and any two living forms, however great the struc- 
tural difference between them, are connected to one another 
by a continuous chain of animals, a connection which will 
become apparent by tracing the lines backwards along the 
ancestral course of each until they meet at their earliest com- 
mon ancestor. 
Thus, in tracing the relationships of 9 and 14, neither form 
is ancestral to the other, but both arose from the common 
ancestor 7, back of which their history is identical. As the 
ancestral forms are now wholly extinct, they are no longer 
available for study save when found in the fossil state, but 
their place may often be supplied by modern forms which are 
but little modified from the condition of the actual ancestors. 
Thus the recent forms 8a and 7a are almost as useful in re- 
producing this part of the phylogenetic history as 8 and 7 
would be, and through them the interrelationship of 9 and 
14 may be readily traced. This may be stated as a third law: 
III. Although the actual ancestral forms lying at the fork- 
ing of the branches no longer exist and have seldom been 
found in a fossil state, many clews to their structure may be 
obtained by the study of those of their descendants which 
have retained most completely the ancestral environment, and 
which have, therefore, kept many or most of the ancestral 
characteristics. 
Thus in studying the relationships and comparing the struc- 
ture of two such divergent forms as 32 and 34, the living form 
30a would be of the greatest assistance, as it would enable 
the investigator to see what was the common structural 
heritage from which, through two lines of modification, the 
two forms in question have developed. 
IV. Among the fossil remains of extinct forms which geo- THE CONTINUITY OF LIFE 
19 
logical investigation has unearthed, many forms have been 
found which are the actual ancestors of groups nozu distinct, 
and they have thus been of the greatest value in tracing out 
phylogenetic relationships. Others, however, represent a 
series of forms which developed, culminated and became ex- 
tinct before modern times, thus presenting a group of great 
value to the student, but haznng no bearing upon the present 
discussion.. 
Perhaps the most famous of the ancestral forms found in 
a fossil state is the Archccopteryx, a definite transition be- 
tween reptiles and birds. Of this, two specimens were discov- 
ered in the lithographic slate quarry at Solenhofen, Germany. 
Others, of almost equal importance, have assisted greatly in 
suggesting the relationship between amphibians and reptiles, 
and have furnished clews to the proper arrangement of the 
orders of living mammals. As illustrations of large groups 
of animals whose history lies wholly in the past may be men- 
tioned the trilobites, a group of crustacean-like articulates, 
which became wholly extinct at the end of the Palaeozoic Age, 
and the ammonites, an extinct group of cephalopod molluscs. 
V. The relative amount of structural difference between 
any two divergent forms is proportionate to the amount of 
contrast between their environments, and not necessarily to 
the amount of time that has elapsed since their divergence 
from the common ancestor. 
That time has in itself no power to modify an animal 
species is shown by the slight differences that exist in some 
cases between certain living forms and their fossil allies. 
Perhaps the most conspicuous example of this is the brachio- 
pod, Lingula, a worm enclosed in a bivalve shell. This form 
has existed from the earliest Silurian times to the present day, 
and yet there are hardly sufficient differences between the 
earliest fossil Lingulae and those now alive to allow them to 
be treated as distinct species. As a rule, however, successive 
geological periods show almost a complete change in their 
fauna and flora, and most of the modern forms are quite 
recent in origin. 
The persistence of ancestral types in a slightly modified 20 
THE HISTORY OF THE HUMAN BODY 
condition is indicated in the diagram by such forms as 3a or 
30a where the shortness of the line connecting the living form 
with its ancestor indicates but little change from the earlier 
condition. 
VI. As a given environment tends to exert a similar influ- 
ence upon all of its occupants, members of quite distantly 
related groups which become associated in the same environ- 
ment often become so similarly influenced as to bear, super- 
ficially, at least, a great resemblance to one another. This is 
called “ analogical resemblance ” and has been productive of 
many mistakes in the attempt to clear up phylogenetic rela- 
tionships. 
Many striking examples of this are found among verte- 
brates. Thus a pelagic environment, as seen among the 
extinct ichthyosaurs and the modern Cetacea, has changed 
the fore-limbs into fin-like paddles, reduced the hind-limbs to 
functionless rudiments, shortened the neck, and given head 
and body a piscine form; limbless, attenuated forms occur 
among fishes, amphibians and several groups of reptiles other 
than snakes; and a grazing habit produced in the herbivorous 
reptilian group of the Ceratopsia a close resemblance to the 
large ungulate mammals of a later day. 
This law is illustrated in the diagram by the forms included 
by the dotted line, which represents a given environment, in- 
vaded by members of several groups. Here the descendants, 
not only of related forms like 5 and 6, but of those of quite 
distant ancestors, as 23 and 30, have become similarly modi- 
fied, until they may resemble one another so closely as to de- 
ceive the casual observer. Forms 20 and 33, representing 
totally distinct stocks, may thus bear so close a superficial 
resemblance as to be popularly classed together under the 
same general term.* 
* Thus whales and porpoises are vulgarly supposed to be fishes; shrew- 
moles, mice; and bats, birds. Salamanders are usually confused with 
lizards; and certain blind and limbless lizards (Rhineura) which occur in 
Florida and burrow in the earth, so closely resemble earth-worms as to 
deceive at first glance a professional naturalist. THE CONTINUITY OF LIFE 
21 
In the above exposition of phylogenesis there can be seen 
at once both its advantages and its disadvantages as an his- 
torical record. 
In cases in which a line of descent is well represented by a 
series of adult animals, the advantage of being able to study 
large forms with functional parts is obvious; but where the 
extinction of intermediate forms has obliterated the record 
at some important point, the phylogenetic data fail completely 
and must be supplied by the parallel history found in the indi- 
vidual development of the nearest allied forms. The great- 
est assistance has often been furnished by palaeontology, but 
as the hard parts alone leave their imprint in the rocks, they 
are of little or no assistance in the history of many of the sys- 
tems. Again, through the metamorphosis of the earlier geo- 
logical formations and the consequent obliteration of all 
organic remains occurring in them, the palaeontological record 
has lost beyond hope of recall all of its early stages, and at the 
period of the first fossiliferous strata, the main classes of 
animals as we have them at present, had already become 
established. 
It is here that the study of comparative embryology lends 
its assistance, since in the embryological record the earliest 
stages are preserved, although often overlaid with secondary 
modifications. By its aid may be traced, not only the lines 
connecting any two forms (Rule II. above), but it furnishes 
faint though definite clews to the early history of animal de- 
velopment previous to the beginning of the palaeontological 
record. Its defects, though many, are not the same as those 
of the phylogenetic record, and the two thus reinforce one 
another to a remarkable degree, each completing the gaps left 
in the other, and corresponding closely in those places in 
which both records are preserved. 
The exposition of developmental history, or ontogenesis, 
may be given in the form of laws as in the former case. 
I. The developmental history of an animal includes all 
stages from that of the fertilized egg (ovum) to that of the 
sexually mature adult, and is not in any way interrupted by 22 
THE HISTORY OF THE HUMAN BODY 
the act of birth or hatching. These latter are purely external 
phenomena and mark no important stage in the development 
of the animal save in the line of certain necessary adaptations. 
The birth period often varies considerably in allied forms. 
These external phenomena are wholly adaptive and are 
regulated by the conditions imposed by the struggle for exist- 
ence. Thus aquatic salamanders lay eggs which pass through 
all the stages from the beginning outside of the body of the 
parent, but in certain more terrestrial species, although closely 
allied to the foregoing, the eggs are detained in the oviducts 
of the mother, where development continues throughout the 
larval period and the young are produced in a practically adult 
condition. 
It is advantageous to some species to produce a large num- 
ber of immature offspring, relying upon chance for the sur- 
vival of a few of them; under other circumstances it has been 
proven the better course to produce a small number of well- 
developed young, furnished with a better equipment for fight- 
ing the battle of life. 
II. In developmental history a given species reproduces in 
miniature its own ancestral history, and thus passes through 
those stages only through which its actual ancestors have also 
passed. 
Thus, in the diagram, form 18b has passed through the 
stages 18a, 18, 17, 6, 5, 4, 3, 2, and 1 as well as the innumer- 
able stages between these points as represented by the lines 
connecting them, but would not reproduce any stage in the 
history of some allied form through which the latter has 
passed since the divergence, such as 7 or 19. 
The only stages common to any two recent forms, allied or 
not, are those below the point represented by their latest com- 
mon ancestor. This may be formulated as follows: 
III. In any two given forms only those developmental 
stages which represent common ancestors are the same in 
both. From the point at which their ancestors diverged their 
developmental histories are distinct and different. It follows 
from this that the more closely allied the two forms, the more THE CONTINUITY OF LIFE 
23 
completely will their embryonic development coincide, and 
conversely, in forms widely apart the divergence begins very 
early and only the first stages of the two developmental his- 
tories will be coincident. 
To illustrate those points: if the development of 16 and 34 
be compared, only the early stages 1, 2 and 3 will be seen to 
coincide; if, however, the developmental histories of 21 and 
16 be taken, they will be found coincident as far as their last 
common ancestor, 5. I11 closely allied forms, such as 37/ and 
37g, almost the entire embryological history in the two animals 
will closely correspond, differences being noted only at the last. 
IV. The more highly specialized the animal, the more 
changes its ancestors have passed through; and therefore so 
much the more is to be rccapitidated onto genetically. This is 
effected in part by lengthening the embryonic period and in 
part by sliding over or dropping out some of the stages. 
In the fish, for example, after the development of a simple 
circulation designed for a water-breathing vertebrate, there is 
nothing farther to do than to perfect and to mature it as it is; 
in the mammal, however, the circulatory system, which is at 
first like that of the embryonic fish, must become successively 
modified as amphibian, reptilian, and finally mammalian; a 
much longer history, which involves numerous changes and 
adaptations. 
V. The different historic stages are not given the same time 
value, but the earlier the stage, the more it is accelerated. 
The earlier stages also lose in distinctness and detail and are 
more often lost than the later ones. It follows from this that 
the early part of the history is best learned from the lower 
forms, in which the stages sought are not very remote from 
the adult condition. 
The approximate time values of the developmental stages 
are seen in the development of the hen’s egg; the segmentation 
stages, and the formation of blastula and gastrula, which rep- 
resent all the earlier invertebrate portion of the history, are 
passed through in a few hours; the establishment of the meso- 
dermic somites (myomeres), which makes it a vertebrate, is 24 
THE HISTORY OF THE HUMAN BODY 
well marked by the end of the second day; at the age of four 
days the embryo is sauropsidan, at five or six definitely avian, 
and the remaining fifteen days are spent in perfecting the 
details first of a gallinaceous bird, and lastly of the particular 
species to which it belongs. Furthermore, the remainder of 
the history, until the adult stage is reached, that is, the latest 
historical period, requires many months. The value of the 
study of the more primitive forms is well seen by the forma- 
tion of the mesoderm, and especially that part of it which 
give rise to the myomeres or primitive muscle segments. In 
Amphioxus, a form considerably below the fishes, the mesoderm 
arises from the primordial intestine in the form of paired di- 
verticula, from the dorsal part of which the myomeres arise; 
in fishes and amphibians these elements are not distinct 
diverticula, but still possess cavities or the rudiments of them; 
and in birds and mammals the myomeres arise as solid cubes 
cut from an indifferent cell mass, and give absolutely no clew 
to their early history. 
VI. In studying an embryological record one must con- 
stantly distinguish between palingenetic characters, or those 
which are true repetitions of the past history, and caenogenetic 
characters, or those which have been more recently acquired 
as the result of some special adaptation. One of the most 
universal among these latter is the presence of yolk, a food 
supply for the embryo, which lies between or within the cells 
and, when excessive, causes misleading distortions in the pro- 
portion of parts and effects the obliteration of many important 
features. 
In general the actual size of an egg is due to the amount 
of yolk it contains, and thus the historic records are reproduced 
with greater faithfulness in very small ones. This is well 
shown by the comparison of the almost yolkless egg of Am- 
phioxus with that of the bird, which represents the other 
extreme. In the one the cylindrical form of the primitive 
vertebrate is well preserved and appears almost at the begin- 
ning; in the other the dorsal portion of the future body lies 
for a time almost flat on the surface of an enormous sphere 
of yolk, and is enabled later to assume the cylindrical form THE CONTINUITY OF LIFE 
25 
only through a secondary adaptation by which the embryonic 
and vitelline (yolk) portions of the egg become nearly sep- 
arated from one another, the connection being retained through 
a narrow stalk. 
It will be seen by the above exposition of the two historical 
records, phylogenetic and ontogenetic, that they are by no 
means complete and that the fragments that exist are often 
difficult to interpret. This has necessarily occasioned a large 
amount of controversy among morphologists, not alone in 
the interpretation of the facts, but even in some cases in the 
recognition of the facts themselves, owing to the great me- 
chanical difficulties in the way of their examination. As in 
all earnest investigation, however, the differences grow less 
as the work progresses, and at the present time there is a prac- 
tical agreement upon the main features of vertebrate history, 
the differences being confined mainly to details. In some 
cases in the following chapters attempts have been made to 
set forth divergent views, but, for the most part, both for the 
sake of clearness and in order to present the matter within 
suitable limits, the selection has been made of that theory 
which, in the judgment of the writer, possesses the greatest 
probability. 
The significance of an anatomical fact depends upon the 
phylogenetic position of the animal studied, yet at the same 
time it must be remembered that the only criterion we possess 
for making the phylogenetic arrangement is that of the an- 
atomical structure, so that the two lines of investigation are 
mutually dependent and are likely to become equally modified 
by the presentation of each new fact. As a basis for this 
history of the human body, which is at the same time a history 
of vertebrates, especially of those that lie in the direct line of 
human ancestry, it is thus necessary to consider the various 
vertebrate groups, both living and extinct, so far as we know 
them, and study their mutual relationships as deduced from 
their structure and development. This is, in fact, a brief study 
of vertebrate phylogenesis, and will be considered in the next 
chapter. CHAPTER II 
THE PHYLOGENESIS OF VERTEBRATES * 
“ The Epicureans, according to whom animals had no creation, 
doe suppose that by mutation of one into another, they were 
first made; for they are the substantial part of the world; like 
as Anaxagoras and Euripides affirme in these tearmes: nothing 
dieth, but in changing as they doe one for another they show 
sundry formes.” 
Plutarch’s Morals; transl. by Philemon Holland, 1603, p. 846. 
Although no great subdivision of animals with the pos- 
sible exception of the echinoderms (star-fish, sea-urchins, 
etc.), possesses a more isolated position than do the verte- 
brates, this latter group is connected in an obscure way with 
the invertebrate world through a series of animal forms of 
uncertain position themselves and usually grouped together 
under the name of Prevertebrata or Protochordata. These 
comprise a worm-like form, Balanoglossus, that burrows in the 
mud along the sea-coasts, the sac-like tunicates, and the small 
and slender Amphioxus. Formerly classed at great distances 
from one another among molluscs, worms and even plants 
(e.gsessile tunicates), they are now united, owing to the 
common possession of pharyngeal gill-slits, a dorsal nervous 
system, and an internal skeletal rod, the notochord, although 
in some cases these two latter characteristics are transitory 
structures that appear only during the early steps of development. 
The highest of these animals, and consequently the one 
nearest the true vertebrates, is Amphioxus (Branchiostoma) ,** 
a small marine creature something like a headless fish, which 
* For a detailed classification of vertebrates, to accompany this chapter, 
the reader is referred to the Appendix. 
** The correct name of this animal, as known to taxonomists, is Branchio- 
stoma, but it has so long been known as Atnphioxus, the name which it 
held for years, and so much of the classical literature concerning it has 
employed this latter name, that it has seemed best to use it in this book. 
“ Amphioxus ” has thus become the common or vernacular name of the 
animal, the scientific name of which is Branchiostoma. 
26 THE PHYLOGENESIS OF VERTEBRATES 
27 
is found in the shore water of the warmer seas, usually buried 
in the sand in a vertical position, with the anterior end project- 
ing into the water, expanded into a sort of hood for the collec- 
tion of its food. When fully grown it is about two inches in 
length and is in the form of a cylinder, flattened laterally, and 
pointed at either end. It is divided into a succession of body 
segments, somites, by V-shaped lines, which represent the 
edges of the partitions of connective tissue, the myocommata. 
These run through the masses of body muscles, and divide 
them into segmental portions, the myomeres. The internal 
skeletal axis, which forms one of the chief characteristics of 
the group of vertebrates, is here represented by a flexible 
cylindrical rod of a substance resembling cartilage, running 
through the body from tip to tip. This rod, the notochord, 
shows no trace of segmentation, and it is thus seen, as is 
also the case in all vertebrate embryos, that the segmentation 
so fundamentally characteristic of vertebrates, and so well 
marked in their internal skeleton (vertebrae, ribs, etc.), was 
acquired first by the muscular system, perhaps as an adapta- 
tion to facilitate the flexibility of the body, and that it was 
secondarily carried over to the skeleton. 
In arranging a phylogenetic tree of the vertebrates, Amphi- 
oxus should be placed at the bottom, although, if absolute 
accuracy is demanded, neither Amphioxus nor any modern 
animal, with its later modifications, should be placed at any 
point along the main stems of the phylogenetic tree, but all 
should be placed at the termini of branches; proximity to the 
ancestral line being indicated by the shortness of the branch. 
If, however, later modifications, since they have undoubt- 
edly affected all modern forms to a greater or less extent, 
may be left out of account, and if the successive animal forms 
may be placed in the positions occupied by their direct ances- 
tors, we may thus form a phylogenetic tree like the one given 
here, which expresses the relationships of modern forms to 
one another in a simple and essentially correct manner. 
Above Amphioxus ensues a great gap, the greatest in the 
entire series, bridged over by no forms, either living or fos- 28 
THE HISTORY OF THE HUMAN BODY 
sil, with which we are acquainted, and only suggested in part by 
the members of the next higher group, the cyclostomes. This 
group comprises eel-like forms, to be carefully distinguished, 
however, from true eels or from any of the true vertebrates, 
Double underscoring indicates an extinct group; single underscoring one that has 
but a few living representatives. The boundaries of the Classes are represented by- 
dotted lines. 
Fig. 7. Phylogenetic tree of vertebrates. 
since they possess neither jaws nor teeth in the sense of those of 
the higher vertebrates, but have the mouth surrounded by a 
circular lip which is capable of being extended so as to re- 
mind one of the hood possessed by Amphioxus. Within this 
mouth there are variously shaped spines or plates which serve THE PHYLOGENESIS OF VERTEBRATES 
29 
as teeth. A most important distinction between Amphioxus 
and the cyclostomes, however, lies in the fact that the latter 
possess a definite head, with brain and sense organs, parts 
which exist only in a rudimentary or potential sense in Am- 
phioxus. 
In distinction from the cyclostomes, or “ round-mouths,” 
are the true vertebrates, which are termed gnathostomes, or 
“ jaw-mouths,” the possession of jaws being a constant char- 
acteristic of the entire group. The lowest class of gnathos- 
tomes is that of the fishes (Pisces), but these are in turn 
subdivided into several groups, some of which represent 
lateral branches, that is, specializations along definite direc- 
tions, and thus not in the direct line of human history. The 
most primitive group is that of selachians, which comprises 
the sharks and dog-fish, and the skates or rays. This group 
of animals is absolutely fundamental for the morphologist 
and represents the first great stage in the main line of verte- 
brate history. Selachians have a wholly cartilaginous skele- 
ton, the mouth upon the lower side of the head and not at the 
anterior end, as in other fish, and five gill-slits which open 
separately and free, not covered by an operculum (gill-flap). 
Their position in the tree is clearly in the main line above 
the cyclostomes. 
The ganoid fishes are also of great importance to us. 
They represent a few remnants of what was the dominant 
group during the Devonian epoch and are the direct de- 
scendants of the selachians. As in the case of all such rem- 
nants, they are extremely diverse in structure among them- 
selves and are placed in a single group rather more for con- 
venience than because of any very close relationship to one 
another. They are characterized by the tendency of the 
scales to fuse into bony plates, or scutes, a tendency which in 
the past resulted in the development of a special group, the 
placoderms, which were entirely covered by a suit of mail 
formed in this way. Similar plates cover the head in all mod- 
ern ganoids and they occur in rows along the body in a few 
forms (sturgeons). The skeleton is mainly cartilaginous in 30 
THE HISTORY OF THE HUMAN BODY 
the lower representatives of this group, but becomes more or 
less bony in the higher. The gill-slits no longer open directly 
and separately to the outside, as in their selachian ancestors, 
but are grouped together and covered by a gill-flap or oper- 
culum. 
The two remaining groups of fishes, teleosts and dipnoans, 
represent independent lateral branches that have specialized 
in accordance with certain definite lines and are consequently 
not in the direct line of man’s ancestry. Such groups often 
form collateral testimony of considerable morphological value 
and are thus not without importance even in the present line 
of speculation. The teleosts have an almost completely ossi- 
fied skeleton and are the descendants of the bony ganoids, with 
which they are so closely connected through intermediate 
forms that the separation between them is mainly an artificial 
one.* They are essentially a modern group and constitute 
the great majority of the fishes in the world to-day, thus 
taking the place of the ganoids of earlier times. The dipnoi 
are represented by but three forms, one found in Africa, one 
in Australia and one in South America. They are fresh-water 
fishes and are remarkable for their power of sustaining long 
periods of drought by digging into the mud, and breathing 
air through a modified air-bladder. They were thus for- 
merly considered the link between fishes and amphibians, but 
later researches into their structure do not confirm this view. 
As a matter of fact the amphibians seem to have come from 
the ganoids, although by means of forms now lost, and to have 
developed first into the Stegocephali, a group wholly extinct 
but well represented by fossil remains occurring in and about 
the coal deposits. These had many of the characteristics of 
our modern amphibians, but possessed scales arranged 
in definite rows, organs which are entirely lacking in all 
living representatives of this Class, with the exception of the 
* Although the employment of the two terms “ ganoid ” and “ teleost ” 
is a convenient one in comparative anatomy, modern ichthyologists tend 
strongly to the rejection of both terms and the fusion of the two groups 
into a single one, the Teleostomi. Cf. Appendix. THE PHYLOGENESIS OF VERTEBRATES 
31 
Coecilia or Gymnophiona, which still possess scale rudiments, 
not visible externally. The Stegocephali are of extreme im- 
portance, since they were the ancestors both of the present- 
day amphibians and of the two main reptilian lines, and the 
survival of a single representative would have been of price- 
less value to morphologists. As it is, however, we are in 
possession of a large number of fossil remains, many of them 
extremely well preserved, and representing four distinct Sub- 
orders; and further discovery along this line may well be ex- 
pected at any time. Of the soft parts the fossil imprints fur- 
nish but little evidence, a lack which must be supplied by 
the study of the urodeles, undoubtedly their nearest living 
allies and presumably not very different in the essential in- 
ternal features. 
These latter animals, though not quite in the direct line 
of human ancestry, are thus of the greatest importance as the 
best representatives of what may be called the amphibian 
stage. The urodeles comprise the tailed amphibians, their 
most typical representatives being the forms known as sala- 
manders and newts; called also in many sections, unfortu- 
nately, “ lizards,” owing to their superficial resemblance to 
these latter animals. The more primitive members of this 
group are often large (10-40 cm.), and the giant Crypto- 
branchus of Japan, the largest of all living amphibians, attains 
the length of a meter. 
The Amir a, or tailless amphibians, include frogs, toads and 
tree-toads, and attain their tailless condition in part by a 
retrogressive development of the caudal region and in part 
through the excessive development of the ilia and the thigh 
muscles, a feature connected with their jumping habits. The 
Coecilia are blind subterranean forms, burrowing in the 
earth like earth-worms, to which they bear considerable re- 
semblance. They are much attenuated, are without external 
limbs, and have their bodies clearly marked off into annular 
segments. They occur only in the warmer parts of the world 
and consist of but few forms, all highly specialized. 
There are “ at least four main divisions of the class Rep- 32 
THE HISTORY OF THE HUMAN BODY 
tilia, all beginning in Paleozoic times, and all represented by 
their direct descendants today. The birds, crocodiles and 
tuatara of the Diapsida; the mammals of the Synapsida; the 
lizards and snakes of the Parapsida, and the turtles of the 
Anapsida* 
Of these the Anapsida contain several of the most primitive 
reptiles known, like Seymouria, and Limnoscelis, and since 
the turtles (Chelonia) are also members of this Sub-Class, al- 
though highly specialized through the formation of an endo- 
skeletal armor, they may yet be relied upon for furnishing the 
details of the soft parts of this primitive group, of which, 
otherwise, only the skeleton would be known, and this im- 
perfectly. 
The modern lizards and snakes are sufficient to supply us 
with knowledge concerning the Parapsida, but are so much 
alike that many taxonomists unite them into a single Order, 
the Squamata. The other Parapsids are extinct, and consist 
of the Ichthyosaurs and Mosasaurs. 
The Diapsida are represented in modern times by the croco- 
diles, and by a single species, resembling a lizard, and found 
in New Zealand. This is Sphenodon (Hatteria), the only 
living representative of the Rhynchocephalia. This Sub- 
Class is further of interest, since it is the ancestral group of 
modern birds, which may be considered as an overgrown 
branch of the Diapsida, specialized to a phenomenal extent, 
and isolated by complete extinction of the intermediary forms. 
This entire line, although it is not even remotely connected 
with the mammals ancestrally, and thus has nothing to offer in 
explanation of mammalian structure, is yet of the greatest 
interest because of its isolation, its high degree of specializa- 
tion, and the numberless anatomical adaptations, affecting 
every organ and every system, which are rendered necessary 
in order to make possible the attainment of so strange an 
environment as the air. 
The group of birds long remained unusually isolated, with 
characteristics which applied to each member of the entire 
* Williston, in Journ. of Geology. 1917. THE PHYLOGENESIS OF VERTEBRATES 
33 
group, but which gave no definite indication of relationship 
to others, until by the unexpected discoveries, in 1857 and 
again in 1861, of two specimens of the famous Archeopteryx, 
a transition animal, half a bird and half a reptile, revealing 
the reptilian affinities of the Avian line. Traced in the litho- 
graphic limestone of Solenhofen, Germany, this animal, about 
the size of a robin, was found to bear a set of contour feathers 
along the outer sides of its fore legs, and a similar set of 
tail feathers applied along the entire length of a long, verte- 
brated tail. The remainder of the animal was clothed with 
scales, and the toes, three of which were still distinct and 
separate on the fore leg or wing, were equipped with long, 
curved claws. Along the jaws was placed a row of sharp, 
conical teeth, suitable for eating insects and other small 
creatures, and the animal, evidently a tree-dweller, walked 
and perched on the limbs and occasionally flew rather heavily 
from bough to bough. That this creature was reptilian, in- 
deed almost a reptile still, was assured from the first, but it 
has only recently been established that Archeopteryx has 
close affinities to the Dinosaurs, and especially to the sub- 
division of this group which shows much similarity to birds 
in its pelvic bones, the Ornithischia. 
There remains the Sub-Class of the Synapsida, and this 
is of the most vital interest and value to us, because it is di- 
rectly ancestral to the group of mammals. Unfortunately the 
entire group has become extinct, the only Sub-Class among 
the Reptilia thus fated, and even the extinct members are 
known very incompletely. There is thus some doubt whether 
mammals have descended from the Theromorpha, largely 
represented by American fossils, or whether the allied The- 
rapsida from South Africa may not be more directly their an- 
cestors, but at present the trend of opinion points to the 
Therapsids. Some members of this latter group are so near the 
mammals in many particulars that it has been only with the 
greatest care, and with the consideration of all the available 
parts, that their reptilian nature has been definitely estab- 
lished. In studying the remains of these forms, the most of 34 
THE HISTORY OF THE HUMAN BODY 
which were small animals like the earliest mammals, which 
were their contemporaries, it seems impossible not to either 
assign them also to the mammals, or to assign the early mam- 
mals to the reptiles, or to place all in a third group, midway 
between the two. 
The earliest mammalian remains are contemporary with 
those of the Therapsids, and are those of small forms, like the 
most mammalian of the reptilian remains. These are ap- 
parently nearly related to the monotremes, the lowest living 
mammals, which are represented by two forms occurring in 
Australia and Tasmania, the Duck-bill Platypus (Ornith- 
orhynchus) and the spiny ant-eater (Echidna). The latter 
has no connection with the true ant-eaters (Myrmecophagidce) 
of South America, which are placental mammals. The mono- 
tremes are strongly reptilian in certain skeletal features; like 
true reptiles and unlike all other mammals, they possess a 
single terminal orifice, that of a common cloaca, into which 
open the alimentary canal, the ureters and the genital 
ducts; and they actually lay eggs, that is, very immature em- 
bryos, surrounded by a thin, cornified shell. The mammary 
glands, one of the essential characteristics of the class of mam- 
mals, are seen here in a very simple condition. They consist 
of two lateral groups of integumental glands, apparently of the 
tubular type, which open separately in the bottom of an oval 
depression, the mammary pocket. There are no teats, and 
the young obtain the secretion either directly from the de- 
pressions or by sucking at the hair in this region. 
The next group above the monotremes are the marsupials, 
with the exception of the opossum also confined to the Aus- 
tralian region. As in the previous group, the young are born 
in an immature state, but are unprotected by an egg-shell, and 
are matured in an external abdominal pouch (marsupium) 
until able to care for themselves. The relation between the 
monotremes and the modern marsupials is hardly close enough 
to justify an immediate succession, but suggests that each 
group, as we now know it, has descended from more primitive 
ancestors that were thus related; that is, that the ancestor of THE PHYLOGENESIS OF VERTEBRATES 
35 
modern marsupials was a direct descendant of the ancestor of 
the monotremes. 
Beyond the marsupials all the mammals are placental, that 
is, the embryos are retained for a longer time within the 
uterus of the parent and are nourished by means of an organ 
formed in part from the mucous membrane of the uterus and 
in part from tissue furnished by the embryo but not included 
within its body. This organ is termed the placenta and is 
connected with the body of the embryo through an umbilical 
cord. This cord contains fetal blood vessels which connect 
proximally with the main circulatory system of the embryo 
and develop distally into a system of capillaries that lie in villi 
in the embryonal portion of the placenta, obtaining their 
nourishment and effecting the interchange of respiratory gases 
through osmotic transmission. There is thus no direct organic 
continuity between mother and offspring, and neither nerves 
nor blood vessels are continuous from one to the other. In- 
deed, in the lower placental mammals the connection be- 
tween the maternal and embryonal portions of the placenta 
is very loose and the two easily separate at birth, although in 
the higher forms the connection becomes more intimate and 
the separation takes place between the muscular and mucous 
coat of the uterus, thus involving an actual loss of maternal 
tissue. 
The placental mammals, although their appearance was 
comparatively recent, geologically speaking, have specialized 
in all directions, and now occupy almost every available en- 
vironment, not only of the land, but of the water also. Some 
are fitted to pursue and drag down large herbivorous animals, 
while others feast upon dead bodies or suck the blood of the 
living after the manner of parasites. Many are specially 
adapted to the capture of insects, either on or beneath the sur- 
face of the ground, or on trees, and some have even developed 
the power of flight by which they may follow their prey 
through the air. The hosts of the vegetable feeders are as 
highly differentiated and become specially adapted to feed 
either upon low herbage or the leaves of trees, roots, bark or 36 
THE HISTORY OF THE HUMAN BODY 
fruits, and have even developed one group of oceanic forms, 
fitted to browse upon the sea-weeds and other submerged 
vegetation. 
These various lines of specialization, together with the usual 
extinction of intermediate forms, have produced a series of 
more or less isolated groups, or Orders, the interrelationships 
of which have been deciphered in part by the labors of anat- 
omists, in part by those of palaeontologists, but are still more 
or less uncertain. A suggestion of this is shown in the ac- 
companying phylogenetic tree of mammals (Fig. 8), which 
takes into consideration both living and extinct groups, so 
far as known. 
/ 
The earliest mammalian forms, of which we possess only 
fragmentary remains, were more like the reptiles, and espe- 
cially the Therapsids, than any now extant, but possessed 
many of the characters of the monotremes, which may be con- 
sidered their somewhat highly specialized descendants. To 
this group has been given the name Pantotheria, and as the 
ancestors of all the rest they may form the main trunk of 
the phylogenetic tree of mammals. The monotremes are the 
nearest living descendants, and they have been derived from 
them through an ancient and closely related group, the Multi- 
tuberculata. All three of these groups were reptilian in struc- 
ture, and may be classed together and in contrast to all the 
other mammals, as the Sub-Class Prototheria. 
While still primitive, however, the Pantotheria began to 
differentiate along two lines, the one somewhat resembling the 
marsupials, the other the insectivores, and thus early these 
two lines of development became inaugurated. Eventually the 
reptilian characters were dropped, and the animals, passing 
over into the Sub-Class Eutheria, or typical mammals, be- 
came respectively the Didelphia, or marsupials, and the Mono- 
delphia, or placentals. The first of these lines then differ- 
entiated into the marsupialian Orders of the present time, dis- 
tinguished mainly by variations in the dentition, and the 
second, which resembled the present-day Insectivora, passed 
over into that Order. THE PHYLOGENESIS OF VERTEBRATES 
37 
This insectivorous stem, in addition to perfecting its own 
type along the narrow lines first laid down, developed several 
PROTOTHERIA f 
EUTHERIA 
Fig. 8. Phylogenetic tree of mammals. 
A branch that terminates in an arrow point still possesses living representatives, one that ends in 
a short cross bar is extinct. 
lines of differentiation, and it was from these that all the 
higher placental mammals have arisen. A very primitive 38 
THE HISTORY OF THE HUMAN BODY 
stem is that of the Rodentia, of which the extinct group of 
Tillodontia may have been the first; succeeded by the Du- 
plicidentata or gnawing animals, like the rabbits, in which, 
back of the two sharp upper incisors, there is a second re- 
duced pair, and later by the Simplicidentata, like squirrels, 
rats, mice, and beavers, in which the upper incisors consist 
of a single pair. 
The branch represented here as immediately above the last, 
suggesting a little less primitive character, is that leading to 
the group usually called the Edentata, and consisting of the 
sloths, armadilloes, ant-eaters, besides several extinct forms, 
such as the Megatherium, Megalonyx, and Glyptodon, the 
first two like the sloths, the last like an armadillo. In the 
more specialized of these there is a peculiar joint between 
two of the vertebrae of the back, and these are called the 
Xenarthra in contrast to those in which this joint is normal, 
the Nomarthra. The Edentates have always been exclusively 
American, the living forms mainly South American. 
From this same generalized group, the Insectivora, there 
have developed two distinct lines of flying or soaring forms, 
the Chiroptera or bats, and the Galeopithecus, a single species 
found in the East Indies and the Philippines, but not related to 
any of the other stems except, perhaps, the Primates. 
By far the most prolific of the stems proceeding from the 
Insectivora is that which started with the extinct group of 
Creodonta. These animals were at first small, generalized 
mammals, scarcely distinguishable from the parent insecti- 
vores, but they gradually took on special characters which 
suggest the modern Carnivora, which are considered their 
direct descendants. Before specializing along this line, how- 
ever, some of them began to differentiate in several other di- 
rections and thus gave origin to the Primates, the Condy- 
larthra, a generalized form of ungulates, and probably a line 
of aquatic carnivorous forms, destined to become the most 
erratic and singular of all mammals, the Cetacea, or whales 
and porpoises. These earliest ancestors of divergent lines 
were very much alike, and the early primate, carnivorous, THE PHYLOGENESIS OF VERTEBRATES 
39 
and hoofed forms, were all very generalized, and without the 
differential characteristics that their descendants later de- 
veloped. The most primitive of the Primates were a group 
called the Mesodonta, of which the modern lemurs are the 
most direct descendants. Very early, however, forms like the 
modern monkeys, Anthropoidea, began to make their appear- 
ance, forms in which the orbit was entirely separated from 
the temporal fossa, and in which the dentition was the same 
as in the monkeys of the Old World and in Man; and in these 
we find the direct human ancestors. 
The creodont stem developed, as stated above, the modern 
Carnivora, including the cats, dogs, bears, and weasels, and 
from this at an early date, there probably arose a carnivorous 
line that adapted itself to the sea. This is the Pinnipedia, or 
those with fin-like feet, the seals, the walrus, the sea-lion, etc. 
The remaining stem, that of the Condylarthra, was per- 
haps the most prolific of all in respect to the amount of vari- 
ation, and the extent of modification, for it has produced the 
Sirenia, aquatic forms, nearly as highly specialized as the 
whale; the Proboscidia, or elephants, with an excessive modifi- 
cation of the nose; and an enormous variety of animals 
with a reduction of toes, the series reaching its absolute limit 
in the horse, which has lost all the digits but one, this be- 
coming greatly strengthened to serve the purpose of an en- 
tire foot. 
The original Condylarthra have long been extinct, as well 
as the earlier derivatives, the Amblypoda, Ancylopoda, Taxe- 
opoda, and Litopterna; but, fortunately, of all these there is 
left a single solitary Genus, Procavia or Hyrax, for which 
the Order Hyracoidea has been made. This is a little animal 
of about the size of a rabbit; the one referred to in the King 
James Bible as the “ coney.” It frequents Syria and the ad- 
joining countries, and a related species is found in South 
Africa; the sole survivors of the early ungulates. 
The modern ungulate forms, aside from Hyrax, may be 
represented by two stems, the one leading to the Proboscidea 
and Sirenia, the other branching immediately into the Peris- 40 
THE HISTORY OF THE HUMAN BODY 
sodactyla, with an odd number of functional digits, and the 
Artiodactyla, with an even number. The Proboscidea include 
the two species of living elephants, besides several extinct 
ones, like the mammoth, the mastodon, and the dinotherium, 
and the Sirenia consist of two living genera of unwieldly 
aquatic herbivores, the manatee or sea-cow, and the dugong, 
which subsist on sea-weeds and consequently do not wander 
far from the coasts. The Perissodactyla include the three 
lines represented by the tapir, the rhinoceros, and the horse; 
and the Artiodactyla embrace the non-ruminant pigs and 
hippopotami, and the almost numberless species of ruminants, 
such as cattle, sheep, antelopes, and deer. Of these perhaps 
the most distinct are the giraffes and the Tylopoda, or camels. 
In reviewing the two phylogenetic trees as given in Figs. 
7 and 8, it will be seen that it is precisely those forms that 
are the most needed to show the interrelationship of groups 
that have suffered the most from the extinction of their species, 
which is but another way of expressing the fact that general- 
ized and transition forms are not as well fitted for the struggle 
for existence as are their more specialized and better adapted 
descendants, and are hence often exterminated by the very 
races which have developed from them. This extermination 
tends to isolate the terminal groups and thus to disguise the 
plan of development, as may be seen by reference to Fig. 7, 
in which the distinction is shown between living and extinct 
groups. The effect of extinction will here be shown if the 
reader imagines the extinct groups completely blotted out, 
which will leave the modern orders entirely cut off from one 
another. 
The same principle may be seen also in the second diagram, 
the phylogenetic tree of mammals (Fig. 8). Here the groups 
of the greatest importance in showing relationships are the 
primitive Insectivora, the Mesodonta, the Condylarthra, and 
the Creodonta, and although the existence of the first could 
be surmised from their modern descendants, the discovery of 
the fossil remains of the others were absolutely essential to 
the reconstruction of the original relations between the three THE PHYLOGENESIS OF VERTEBRATES 
41 
great groups of primates, carnivores, and ungulates. It is 
thus not surprising that the various orders of mammals have, 
until recently, been treated like isolated groups, and that, even 
yet, any scheme that may be offered must be looked upon 
as provisional and liable to be modified by the bringing to 
light of new evidence, especially that from palaeontological 
sources. 
It will be noticed that the branch leading to the Primates, 
the order to which Man belongs, is represented in the dia- 
gram as one of the shortest and least specialized, a presenta- 
tion which, although opposed to the prevailing opinion, is in 
strict accord with the facts; since in anatomical structure these 
animals show comparatively little deviation from the primi- 
tive mammalian type and do not exhibit the extreme special- 
ization displayed by the groups representing most of the other 
terminal branches. Such aberrant orders as those of the bats, 
whales, and horses, which have departed farthest from the 
original mammalian environment, show in consequence the 
greatest modifications and are thus the most specialized; cer- 
tain other groups, the peculiarities of which are not so strik- 
ing, are still greatly modified in comparison with the Primates. 
Thus the majority of the ungulates show a reduction in the 
original number of digits, the extremes resulting in either two, 
as in the camels and deer, or one, as in the horse; but the 
Primates, together with the rodents and modern insectivores, 
preserve the original number of five, inherited directly from the 
amphibians and reptiles. The teeth of ungulates are character- 
ized by a great complexity in the folding of the enamel layer, 
and in the number and arrangement of the cusps; those of ro- 
dents are specialized for the purpose of gnawing, and in the 
Cetacea they are either secondarily reduced to the form of 
simple cusps, all alike, or are lost altogether; the Primates, 
however, are very simply constructed in these particulars 
and remain close to the lower type as shown in the marsupials. 
The possession of well-developed clavicles, and of separate 
ulna and radius, of tibia and fibula, with their power of prona- 
tion and supination, are also primitive characters, not met 42 
THE HISTORY OF THE HUMAN BODY 
with in the more specialized groups, but persisting in insecti- 
vores and many rodents. 
Primates are also primitive in their muscular system, pos- 
sessing in many instances a single undifferentated muscle- 
mass where the members of other Orders show a complex 
group of muscular units. 
Aside from the adaptation of their extremities to an ar- 
boreal life, the one line of development by which the Primates 
have become differentiated from all the rest is in that of their 
central nervous system, and especially that of the cerebrum, 
which has given them a far greater capacity for recording 
their sensory impressions, and thus of profiting by experience, 
the basis for the development of reason. It is chiefly in this 
respect that the human species has developed so jar beyond 
the condition of the other Primates that the world has long, 
and perhaps willingly, been deceived in regard to their true 
relationship. In spite of all prejudice, however, man is, ana- 
tomically speaking, a typical primate, closely related, even in 
many of the smaller details, to the rest, and the only way 
in which he has proved superior, through the excessive de- 
velopment of the cerebral hemispheres, is not a modification 
calculated to produce important correlated changes in the 
other parts. Of the two living Sub-orders, the Lemuroidea and 
the Anthropoidea, the former are the more primitive and more 
nearly represent the generalized Mesodonta from which the 
race sprung. In the completeness of the partition which sepa- 
rates the orbital and temporal fossse, Man is seen to be an An- 
thropoid; and in important characters, such as the reduction 
of the premolars from three to two, he agrees with the Catar- 
rhine division of this Sub-order. If we employ the usual 
schedule of values to be attached to points of structural differ- 
ence, as used for the purpose of classification, we cannot fairly 
place him in a Family apart from the large tailless apes of the 
Old World, and aside from this we have several intermediate 
links, which the researches of the past few years have brought 
to light, and which reduce even the slight gap formerly con- 
sidered to exist between them. THE PHYLOGENESIS OF VERTEBRATES 
43 
The date of Man’s appearance on the earth has been pushed 
back many thousands of years beyond what was formerly be- 
lieved to be possible, and this has been absolutely proven by 
the most indisputable facts. Crania of the present human 
type have been discovered in Europe in association with the re- 
mains of such extinct forms as the cave-bear and the hairy 
mammoth, and numerous carvings and incised drawings have 
been discovered in which the latter animal has been portrayed 
by an eye-witness and with much artistic ability. This brings 
the present species, Homo sapiens, with proportions like that 
of the modern European, back to the end of the last glacial 
epoch, or, as some think, to a time contemporary with it, per- 
haps two or three hundred thousand years ago. 
Aside from this, there have also been found, dating from 
about the same period, remains of men, or man-like creatures, 
of proportions unknown at the present time and constituting 
several distinct species of the Genus Homo; there have also 
been found remains sufficiently unlike man to be classed as 
representatives of distinct Genera. The first new species of 
man to be determined was Homo neandertalensis, so named 
from the type specimen, which was discovered in 1857, 
valley of the river Neander, near Dusseldorf, northwestern 
Germany; but since then many other specimens, some of 
them much better than the original skeleton, have been found, 
mainly in Western Europe, so that we have now details, not 
only of head and face but of all parts of the body. The head 
was large, the forehead low and retreating, the arms were 
short, and the legs were bent at the knees. Perhaps the 
most striking characteristic was formed by the presence of an 
enormous ridge across the forehead above the eyes, as in the 
Gorilla and Chimpanzee, although the other features, both of 
head and body, were not simian. 
A recent kinsman of Homo neandertalensis was found in 
an unused mine at Broken Hill, Northern Rhodesia, South 
Africa, which, if a distinct species, as some think, may be 
called Homo rhodesiensis. Its characters are much like that 
of the previous species. 44 
THE HISTORY OF THE HUMAN BODY 
Other forms, men or man’s near relatives, are seen in Homo 
heidelbergensis, which consist thus far of a single, though al- 
most perfect, mandible found in a gravel pit near Heidelberg 
in 1907, and the “ Piltdown Man,” assembled from many 
fragments from a gravel pit near Sussex, England, and named 
Eoanthropus dawsoni, after its discoverer. There is also the 
slightly known Hesperanthropus haroldcooki, based upon a 
single tooth from the Snake Creek Beds, Sioux Co.. Nebraska, 
of which we are in great need of more evidence, although 
the inference from the tooth seems correctly deduced. Lastly, 
in the chain of human pedigree comes the celebrated “ ape- 
man,” or “ man-ape ” from Java; a femur, a skull-cap, and 
three teeth, found in 1891. This form, although with an ex- 
ceedingly low forehead, and with overhanging brows even 
more prominent than H. neandertalensis, yet walked quite 
erect, like modern man, by virtue of its straight femur, and 
deserved the name of Pithecanthropus erectus, given it by its 
discoverer. Much further study in the same locality and 
several subsequent expeditions, sent out for the purpose, have 
failed to find other remains of this species, other than a fourth 
tooth, found at the original spot. 
Concerning the ancestry of Pithecanthropus, and its rela- 
tionship to the modern apes the widest opinions still prevail, 
but the trend of opinion leads to the rejection of the four 
living anthropoids (gorilla, chimpanzee, orang and gibbon) as 
direct ancestral forms. Owing to the modifications time is 
apt to produce in animal species it seems more logical to ex- 
pect to find the connection in some extinct type, as, for ex- 
ample, the European Dryopithecus of the middle Miocene. 
As the case stands at present, however, there are few animal 
species concerning which so many of the intermediate links 
have been preserved as in the case of man, and to the scientist 
the “ missing-links,” the discovery of which would be of the 
greatest importance, are not those representing intermediate 
anthropoidal forms, but those lying in the far greater gaps 
lower down, as, for example, between lemurs and primitive 
insectivores, or between the Pantotheria and the Therapsida THE PHYLOGENESIS OF VERTEBRATES 
45 
which would throw further light upon the reptilian ancestry 
of the Mammalia. 
Naturally the phylogenetic stages which lie in the direct line 
of human ancestry are of the most value as historical records, 
and as such form the main subject of study for the mor- 
phologist, but collateral lines furnish many helpful sugges- 
tions, and in cases where a group of animals which repre- 
sents an ancestral line has become wholly extinct, dependence 
must be placed upon the nearest related group, although not 
directly in the line of descent. With this in mind it will be 
seen from the foregoing that the phylogenetic stages of the 
greatest value in the present discussion are the following: 
1. Amphioxus. 
2. Cylostomes. 
3. Selachians. 
4. Ganoids (rather than Teleosts). 
5. Urodeles (as a substitute for the Stegocephali). 
6. Reptiles (preferably the Chelonians as representatives of 
the most primitive group of the Reptilia). 
7. Monotrernes (the nearest living allies of the Pantotheria). 
8. Marsupials (probably not very near the direct line, but 
suggestive of the conditions in the primitive Insec- 
tivora). 
9. Insectivora (of the modern type, still quite primitive. 
The rodents are valuable here also as collateral lines, 
descended from the primitive Insectivora). 
10. Lemurs (practically modern Mesodonta, and hence repre- 
senting fairly well the immediate ancestors of the an- 
thropoids). 
11. Cercopithecidae (tailed monkeys of the Old World). 
12. The large tailless apes of the Old World (Gorilla, Chim- 
panzee, Orang, Gibbon). 
13. Pithecanthropus (extinct). 
14. Homo primigenius (extinct). 
15. Homo sapiens. 
In this list an attempt has been made to enumerate only 
living forms, specimens that are still available to the anatomist 46 
THE HISTORY OF THE HUMAN BODY 
for dissection and full comparison. In two cases, however, 
13 and 14, this resolution was broken, owing to the vital 
importance of these forms. It must also be remembered 
that we possess at least a partial skeletal record of some of 
the extinct groups that lie in the direct line of ancestry, and 
that these records, although extremely fragmentary, are of 
the utmost value. It will also be seen that these stages are not 
those of coordinate groups, but that they grade from Classes 
to Orders, then to Families, and finally to Genera and Species; 
this is, however, the natural manner of considering an an- 
cestry, for the early stages are the less detailed and are ex- 
pressed equally well by all the members of a large group, 
while the finishing touches, which separate genera from gen- 
era and species from species, consist of slight differences, 
more recent and superficial in character. 
A similar gradation is seen in the developmental history, as 
studied in comparative embryology, in which the earliest fea- 
tures laid down are those of the main subdivisions; then come 
in succession those of the Class, the Order, the Family, and 
so on until the distinguishing characters of the Species make 
their appearance, the latter usually not fully expressed until 
maturity. 
The truth of this actual recapitulation of the history became 
apparent to the early morphologists, one of the greatest of 
whom thus expressed his feelings while gradually tracing back 
from the adult condition the developmental history of the 
skull of the common fowl: “Whilst at work I seemed to 
myself to have been endeavoring to decipher a palimpsest, and 
not one erased and written upon again just once, but five or 
six times over. Having erased, as it were, the characters of 
the culminating type, — that of the gaudy Indian bird, — I 
seemed to be among the sombre Grouse; and then, towards 
the end of incubation, the characters of the Sand-grouse and 
Hemipod stood out before me. Rubbing these away, in my 
downward work, the form of the Tinamou looked me in the 
face; then the aberrant Ostrich seemed to be described in 
large archaic characters; a little while, and these faded into THE PHYLOGENESIS OF VERTEBRATES 
47 
what could just be read off as pertaining to the sea-turtle; 
whilst, underlying the whole, the Fish in its simplest Myxi- 
noid form could be traced in morphological hieroglyphics.” * 
In following out the historical development of the different 
systems, as outlined in the ensuing chapters, both embryonic 
and phylogenetic records have been drawn upon as the pri- 
mary sources from which this history may be deduced, and the 
conclusions which have the corroboration of both may be 
naturally considered the most trustworthy ones. Each of these 
two records has its advantages and its disadvantages; in the 
former the stages are continuous, although the early ones are 
obscure, and all parts of the record are apt to be overlaid and 
mystified by csenogenetic changes; in the latter the record is 
far more fragmentary and its stages are discontinuous, but the 
facts are usually plainer and more easily read. An adult lower 
animal which represents a phylogenetic stage in the history of 
a higher shows the parts in full physiological efficiency, while 
in an embryonic stage the organs are at the best not wholly 
functional, and often render it difficult to imagine an adult 
animal with the same relationship of parts; on the other hand, 
in places where a long historic period has no known living or 
fossil representative among adult animals, the only clew is 
that furnished by embryology. It will be seen that in a few 
cases, notably in the history of the transition from fins to 
walking limbs between fishes and amphibians, both records 
are unsatisfactory, and in such cases the only hope of a definite 
solution lies in the future discovery of some extinct form 
which may bridge the gap and thus furnish a clew by which 
the two discontinuous threads may be united. 
* W. K. Parker, in Trans. Roy. Philos. Soc., 1869, pp. 803-804. CHAPTER III 
THE ONTOGENESIS OF VERTEBRATES 
“ . . . the embryological record, as it is usually 
presented to us, is both imperfect and misleading. It 
may be compared to an ancient manuscript, with many 
of the sheets lost, others displaced, and with spurious 
passages interpolated by a later hand. . . . Like 
the scholar with his manuscript, the embryologist has 
by a process of careful and critical examination to 
determine where the gaps are present, to detect the 
later insertions, and to place in order what has been 
misplaced.” 
Francis Balfour, Comparative Embryology. 
Vol. I, p. 3- 
With the exception of a few cases of asexual reproduction, 
that is, cases in which an individual arises from a single parent, 
every multicellular organism results from a conjugation be- 
tween a macro- and a micro-gamete. These are called the 
ovum and spermatozoon, respectively, and are the product of 
two distinct parent individuals. Precisely the same phenome- 
non occurs frequently among colonial unicellular organisms, 
where an entire colony produces gametes of only one sort, 
and in this case the distinction between such a colony and 
the mass of cells which constitute the body of a simple 
Metazoan is extremely slight and depends solely upon the 
amount of differentiation between the individual cells and the 
consequent degree of mutual interdependence attained. In 
both cases the cell mass, aside from the gametes, constitutes a 
soma, composed in the one case of homogenous, in the other 
of heterogenous, cells. The soma, or cell colony, is perishable 
and restricted to a definite time of existence; the gametes by 
their conjugation produce zygotes, each of which, by its re- 
peated division, may form a new soma, that is, the colony, or 
the individual of the succeeding generation. 
48 THE ONTOGENESIS OF VERTEBRATES 
49 
Among- multicellular organisms, the gametes are produced 
in definite organs, the gonads, or germ-glands, which pro- 
duce but a single sort of gamete, either macro-gametes (ova) 
or micro-gametes (spermatozoa). Those glands are termed 
respectively ovaries and testes, and may occur in the same or 
in different individuals. In the latter case the individuals 
are said to be of separate sexes and are termed male and fe- 
male, the former secreting the micro-, the latter the macro- 
gametes. Individuals possessing both sorts of gonads are 
termed hermaphroditic or bisexual, but owing to the fact 
that usually the two sorts of organs are functionally active 
at different times, the organisms are seldom functionally bi- 
sexual, but alternately male and female. Such hermaphroditic 
forms are frequent among invertebrates, and occur regularly 
in certain classes, but in vertebrates they are found only 
; among the Cyclostomes (Myxinoids), although in all cases the 
curious homology between the parts in the two sexes (Cf. 
1 Chap. X) suggests that the phenomenon may have been wide- 
spread or even universal among the ancestors of modern 
vertebrates. 
In most aquatic animals the gametes are liberated in the 
-water and conjugation takes place without any act on the 
-part of the parents, through the motor action of the micro- 
rgametes themselves, exactly as in Protozoa; in terrestrial 
forms, however, since the gametes need a liquid medium, this 
latter is supplied by glands, and the seminal fluid of the male, 
in which the micro-gametes swim actively, is conveyed to the 
female by some form of copulation. 
Since the superficial phenomena are so obvious that they are 
universally recognized without technical study, while the es- 
sential details require for their detection the care and patience 
of an experienced microscopist, and since especially the 
parallel phenomena occurring among the Protozoa have re- 
mained unknown until within comparatively recent years, it 
may be easily comprehended that the terms in common use 
relative to these phenomena fail to express the underlying bio- 
logical principles and are not of universal applicability. Thus 50 
THE HISTORY OF THE HUMAN BODY 
the micro-gametes, first discovered in the seminal fluid of 
mammals,* were termed spermato-zoa, or sperm animals, a 
term expressing the view held at that time that they were 
parasitic or adventitious organisms occurring in a fertilizing 
or quickening fluid, and from this the act of mixing the 
spermatic fluid with the ova was termed fertilization. This term 
is now applied technically to the entrance of the spermatozoon 
The upper row represents the egg of Sycandra, a calcareous sponge [after F. E. 
Schulze]; the lower row represents that of the rabbit [after Bischoff], In the 
rabbit the egg is surrounded by a thick capsule, the zona pellucida. The egg of the 
sponge is without this and floats freely in the water. 
Fig. 9. Earliest stages of Metazoan development. 
into the ovum, i.e., to the union of the two gametes, 
and is thus similar to conjugation applied to Metazoa. 
Furthermore, since, in the majority of cases, the bulk of the 
ovum so far exceeds that of the spermatozoon that the latter 
appears to be lost in the process, the term ovum, or egg, is com- 
monly used to designate not only the macro-gamete (the un- 
fertilized egg), but also the double cell resulting from the 
conjugation (the fertilized egg), a use of terms which neces- 
sitates constant watchfulness in order to guard against con- 
fusion. Ovum and spermatozoon, the macro- and micro-ga- 
* Discovered in 1677 by Ludwig Hamm, a pupil of Leeuwenhoek. THE ONTOGENESIS OF VERTEBRATES 
51 
metes respectively of a conjugation, are essentially Protozoa, 
and thus the first stage in the development of multicellular 
animals is an historic repetition representing the first and 
simplest of organisms. The spermatozoon with its motor 
organ still retains its protozoan character even in the highest 
of the vertebrates, but the ovum, loaded down with yolk, 
bears for the most part little resemblance to an active or- 
ganism. Even here, however, in certain sponges and hydroid 
polyps, a more primitive form of ovum is still preserved, for 
it is amoeboid in form and possesses functional pseudopodia, 
being often impossible to distinguish from genuine Amoebse, 
the simplest of Protozoa. This is a good illustration of Rule 
V of ontogenesis as given in the previous chapter, since it 
is to be expected that here, among the lowest and simplest of 
the Metazoa, the early stages would receive the fullest atten- 
tion in the ontogenetic recapitulation. 
In size ova vary greatly, but the difference is due mainly 
to the actual amount of food stuff, or yolk, required in 
each case: this in turn is proportional, not to the size of 
the adult animal, but to the degree of maturity at which it is 
most advantageous for the young animal to begin its free 
existence. Some animals produce a few very large eggs and 
thus use up their reproductive energy in developing yolk; 
others produce large quantities of tiny eggs which will develop 
into innumerable minute larvae. Both extremes and all in- 
termediate grades are the result of adaptation to the various 
conditions that surround the different organisms and thus 
regulate the size of the egg, as well as the size and shape of 
the parts in the adult. Thus, for example, the ova of jelly- 
fish, earth-worms, many molluscs, star-fish, and most mam- 
mals, are very small, almost microscopic; those of insects, 
crustaceans and fishes are usually of an appreciable size, those 
of frogs and of certain fish are still larger, while the eggs of 
reptiles and birds are enormous, those of the latter having 
reached the extreme limit relative to the size of the parent. 
In the eggs of placental mammals, which are practically 
yolkless, there is no great difference in actual size between 52 
THE HISTORY OF THE HUMAN BODY 
such extremes as those of the elephant and the mouse; in 
the birds, on the other hand, the true egg, i. e., the yellow 
sphere usually termed the “ yolk,” is approximately propor- 
tionate to the size of the parent. This difference is due to 
the fact that in mammals the egg is little more than the first 
cell of the new individual, since the food supply comes en- 
tirely from outside sources, while in birds the food is placed 
wholly within the egg and is the only source available to the 
young bird. 
The spermatozoon, never having yolk to give it bulk, is al- 
ways small, usually far beyond the limits of the unaided eye. 
Its form is typically that of an oval cell-body or “ head ” 
to which is attached a locomotive flagellum, which may at- 
tain an appreciable dimension in respect to length, but is al- 
ways extremely delicate. 
When the seminal fluid and the ova are brought together 
there is always a vast excess of spermatozoa, and in cases in 
which direct observation has been possible, as in aquatic forms, 
in which the mingling of the elements occurs freely in the 
water, the eggs are seen to be assailed by dozens of active 
spermatozoa, each endeavoring to effect an entrance. To 
permit the entrance of one and only one of the entire number, 
several devices are made use of by the eggs of various species; 
one of these is the encasement of the entire ovum in a shell, 
in which there is a single minute opening, the micropyle, 
through which a single spermatozoon enters and in so doing 
effectually blocks the way for all successors. In other cases 
the entrance of a spermatozoon seems to cause some chemical 
or physical change which renders the egg substance impervious 
to the other male cells or incompatible with their continued ex- 
istence. In the eggs of echinoderms (star-fish, sea-urchins, 
etc.) the stimulus of an entering spermatozoon causes the im- 
mediate formation of an external membrane which effectually 
prevents any farther entrance. In mammals it is probable 
that the zona radiata proves an impassable barrier to all sper- 
matozoa except those that approach it in a direction perpen- 
dicular to its surface, thus greatly reducing the number that THE ONTOGENESIS OF VERTEBRATES 
53 
are in condition to enter the egg. It is also likely that here, 
as in many other cases, several spermatozoa may actually 
enter the egg substance, but that all except one are simply 
added to the yolk and serve as food. 
The spermatozoon, after the entrance into the egg is once 
effected, drops its locomotor apparatus and becomes merely 
a nucleus, which fuses with the one belonging to the egg, a 
procedure similar to conjugation in the Protozoa. The egg 
cell thus becomes furnished with a fusion-nucleus, and may 
be considered from now on the first cell of a new organism. 
From it arise all the cells of the developing animal through 
the process of fission, and, since a division of the nucleus al- 
ways precedes the division of the cell, it follows that this 
fusion-nucleus is in the same way the primary one from which 
all later nuclei are to be derived. Since nozv, as has been shown 
by direct observation, this fusion-nucleus becomes divided 
in such a way as to effect an exactly equal division of both 
maternal and paternal components, and since the process has 
been found to continue as far as the investigators have been 
able to follow it, it is extremely probable that the nucleus of 
each and every cell of the adult organism contains an element 
derived from each of its parents. 
Herein lies a material basis for the phenomena of heredity, 
and it thus becomes evident that all hereditary traits and char- 
acters are perpetuated through the direct transmission and 
growth of a bit of material furnished by each parent 
and handed down to each cell of the organism. Although 
this is still the mystery of mysteries to the biologist, the 
careful study of the past fifty years, directed upon this very 
point, has revealed much, but in so doing has added more 54 
THE HISTORY OF THE HUMAN BODY 
that is still unknown. It has shown the nucleus to be a mi- 
crocosm of extraordinary complexity, and has opened up a 
new world, the very existence of which has until lately re- 
mained unsuspected. 
What seems to be the essential element of all nuclei, found 
alike in plants and animals, is a substance which, from its ex- 
treme susceptibility to staining fluids when artificially treated 
for purposes of microscopic examination, has been designated 
by the non-committal term of chromatin. During functional 
Fig. io. Diagrams representing normal mitosis. 
In I the nucleus is “resting”; the centrosome is seen by its side. In II the 
spireme appears, which in III becomes separated into chromosomes. In IV the 
centrosomes have become placed at opposite poles, while the chromosomes form 
an equatorial plate midway between them. Each chromosome divides longitudinally 
in V, and in VI and VII becomes drawn to the two opposite poles. In VIII the 
cell divides into two. 
Stages II—IV are spoken of as the Prophases of the Mitotic process; Stage V, where the chromo- 
somes split, is called the Metaphase; Stages VI and VII, which effect the separation of the two sets 
of chromosomes, constitute the Anaphase; and the final phenomena, beginning with VIII, where the 
new cell-wall is forming, and ending with the massing together of the separate chromosomes of each of 
the daughter cells, and the loss of individuality of the separate chromosomes, are the Telophases. 
activity this substance is diffused throughout the nucleus in 
little, irregular masses, but assumes the form of a continuous 
thread or chain preparatory to a cell division, and eventually 
becomes separated into a definite number of bodies, the 
chromosomes. 
Owing to the minute size of these bodies, to their habit 
of frequently attaching themselves to each other, and to their THE ONTOGENESIS OF VERTEBRATES 
55 
considerable number in many cases, the exact number of 
chromosomes in a given cell has been difficult or impossible 
to estimate exactly, but even in the early days of the study 
of chromosomes the idea became established that the num- 
ber in each cell of a given species of animal or plant was 
constant. Lists were formed of the number character- 
Fig. ii. Arrangement of the chromosomes in several species of the genus 
Drosophila. 
Here there are always two sex chromosomes, but only one is equivalent to the usual X-chromosome. 
In all of these the female possesses the two X-chromosomes, which are the two lower ones in a and b 
and the heavier ones on the left in c. In d they are indistinguishable from the others, and in e, which 
is a male, the two sex chromosomes are at the bottom, only one being the X, the other, here bent 
into the form of a U, is called Y. [From Sturtevant, after Metz.] 
istic of many species of organisms. Thus the number 4 oc- 
curs in the pin-worm (Ascaris)\ 8 in certain other Nema- 
todes; and 12 in the mole-cricket (Gryllotalpa); the number 
16 is found in the rat, as also in the pine and the onion. 
Cyclops, a minute Crustacean, was found to have 24, the 
same number that was found in the frogs, mice, snails, a cer- 
tain lily and a fern (Osmunda). The earthworm has 32, 56 
THE HISTORY OF THE HUMAN BODY 
the torpedo, 36; and a small shrimp (Artemia) was found to 
have the largest number known, 168. 
While in general this conception of a constant number of 
chromosomes in a given species has been corroborated by 
later research, and, with certain modifications about to be 
explained, is one of the basic facts of cytology, a definite 
modification has been necessitated by the discovery of a sex- 
ual difference in the number, due to the presence or absence 
of a certain peculiar chromosome (or chromosome group), 
the x-chromosome, or sex-chromosome. Typically all cells 
of normal individuals, both of germ and soma, possess one of 
these special chromosomes in one sex, and two in the other, 
while the other chromosomes, called autosomes, are constant 
in number in both. In the vast majority a single x-chromo- 
some is possessed by the male and a pair by the female, but 
in a very few, like birds and butterflies, this rule is reversed, 
and it is the female that has the single x-chromosome. In 
some cases there are variations of this, and the single 
x-chromosome may be represented by a group, one group in 
the male and two in the female. In others there are two 
x-chromosomes in each sex, one large, and one small, in the 
sex that has the lesser amount, (usually the male) and two 
large ones in the other. The male will thus have two chromo- 
somes, x and y, while the female has two x’s. Again the 
x-chromosome may be found attached to one of the regular 
chromosomes, and thus the total number by actual count, 
but without further analysis, will be found the same in the 
two sexes. 
Thus, while the autosomes are still considered to be abso- 
lutely constant in the cells of a given species, the eccentric 
behavior of the x-chromosomes makes a definite total count 
without regard to the sex of the individual no longer possible, 
or even desirable. We can say, however, in any given animal 
that every cell has a autosomes, plus 1 x-chromosome in one 
sex and a autosomes, plus 2 x-chromosomes in the other; also 
that occasionally the single x-chromosome, or' chromosomes, 
is replaced by a group. It is also to be remembered that THE ONTOGENESIS OF VERTEBRATES 
57 
in a few instances the female has the minimum number of 
x-chromosomes, and the male the maximum, instead of the 
reverse. 
While the somatic cells of any Metazoon bear the above 
outlined relationship of their chromosomes, differing funda- 
mentally in the two sexes, the germ-cells, or gametes are 
different. In them, whether ova or spermatozoa, whether 
Oogenesis 
S per ma.togene$i£ 
{Oogonium 
Spermatogonium 
First reductive division 
(First jab) 
Second reductive division 
(Second JdU^ 
Fig. 12. Diagram illustrating the differential behavior of the two sexes in Gam- 
etogenesis. 
In the female the egg is formed from one of the four potential gametes, which takes all of the yolk 
provided by the mother, while the other three abort, as the polar bodies. In the male, where there 
is no necessity for providing yolk, all four gametes are equally effective in fertilization. Of these 
four two contain an x-chromosome, while two are without it. The eggs are always thus provided. 
The zygote, resulting from the union of an egg with either kind of sperm, has an equal chance of being 
provided with one or two of these. In the former case it becomes a male; in the latter a female. 
they arise from female or male individuals the germ-cells 
possess just half the number of autosomes characteristic of 
the species from which they originate. They possess the 
haploid number; the somatic cells the diploid. This is brought 
about by a special procedure, employing three cell-generations, 
and known as Gametogenesis. This is seen in two forms, 
Oogenesis, or the development of the ova, and Spermatogene- 
sis, or the development of the spermatozoa. Of these two 
procedures spermatogenesis is the simpler, and the easier to 
explain. 58 
THE HISTORY OF THE HUMAN BODY 
In the spermatogonia, or primordial male germ-cells, the 
chromosomes arrange themselves in pairs, one of each pair 
derived from each parent. In some cases the chromosomes 
of each pair are slightly different in shape from the others, 
and may be readily distinguished; in others this principle, 
underlying the pairing, may be assumed. This procedure is 
termed Synapsis. Then follow, in rapid succession, two di- 
visions of the cells, the reductive divisions, eventually result- 
ing in the production of four, during which each pair of 
chromosomes of which the components become no longer 
distinguishable, distribute one-fourth of their substance 
to each of the four. Each of these four germ-cells then 
goes through a metamorphosis, and becomes a spermato- 
zoon. 
During this division of each autosome into four parts, so 
that each of the spermatozoa becomes equally equipped, the 
x-chromosomes behave somewhat differently. At the first 
of the two reductive divisions the single x-chromosome divides 
like the others, but at the second these two are sent bodily 
into two of the four, while the two others are not so furnished. 
The two first, if used in fertilization, will unite with an ovum 
and produce females; the others will produce males. 
In Oogenesis, or the production of ova, the process is funda- 
mentally the same, but with this fundamental difference, that 
here there is to be considered a food substance, called yolk, 
essential to the formation of a true egg, and all of this sub- 
stance, instead of being distributed equally through the four 
final cells, is placed in one. This one becomes by this act 
the ovum, or egg, while the other three abort, and are ex- 
pelled as the “ polar bodies ” or “ directive spheres.” 
As the decision as to which one of the four becomes the 
definite egg is made at the first reductive division, the egg 
always contains one of the two x-chromosomes; the other 
cell of the first reductive division contains the other, but that 
is not of any importance because it is cast off from the egg. 
Now, as the ovum always contains an x-chromosome, the final 
sex of the zygote depends entirely upon which of the two THE ONTOGENESIS OF VERTEBRATES 
59 
kinds of spermatozoa is used to fertilize it, one with an 
x-chromosome, or one without. If the spermatozoon has one, 
the egg, having one to start with, comes to have two, and 
becomes a female; if the spermatozoon has none, the one 
x-chromosome which the ovum had before it was fertilized, 
has to suffice, and the resulting zygote is a male. Finally, as 
the spermatozoa have a 50% chance of either having an 
x-chromosome or not, the same proportion exists between the 
sexes. 
Evidently the reason why Nature insures not only an equal 
division of the chromatin in general, but a similar care in 
insuring the division of each pair of chromosomes, must be 
that in the chromosomes lies the inheritable substance, 
which transmits to the offspring the characteristics of the1 
parents, and of each parent equally. Neither the preponder- 
ating bulk of the ovum (macro-gamete) nor the flagellum and 
other locomotor apparatus of the spermatozoon (micro- 
gamete) are of any significance in hereditary transmission, 
but are mere adaptive characters, of provisional functional 
importance, and without influence in directing the develop- 
ment of the new organism; while the chromosomes, to effect 
the union and equal division of which the other parts have 
been developed, form the true germ-plasm, transmitted in 
direct continuity from both parents and entering every cell 
as it develops, directing both the architectural plan which 
these cells assume and also their gradual differentiation into 
the tissues which form the adult soma of the succeeding gen- 
eration. 
In this “ continuity of the germ-plasm ” is found the ma- 
terial basis also for the recapitulation theory, the law of bio- 
genesis explained in the first chapter; for the continuously 
living chromatin, which pervades each cell of an organism, 
has in its own existence actually experienced all the somatic 
modifications of its entire past history, traces of which it must 
retain in some form of structural expression, enabling it to 
control the development of the soma during every stage of 
its existence. How this is effected is far beyond our present 60 
THE HISTORY OF THE HUMAN BODY 
means of observation, and perhaps of experiment, but the re- 
sults presuppose an inconceivably complex structure in the 
chromatin in order to render such results possible. 
The first stage in the development of all Metazoa, that of 
the fertilized ovum or zygote, is followed, in most cases imme- 
diately after fertilization, by a succession of cell-divisions, 
or cleavages, as they are here termed, which, in typical cases, 
follow a general geometrical plan and result in the formation 
of a mass of cells that shape themselves into a definite embryo- 
logical stage, that of the blastula. As the various geometrical 
Fig. 13. Early Metazoan development; typical. [After models of Am- 
phioxus by Hatschek.] 
I, the egg. II, III, and IV. cleavage stages. V and VI, blastula: in VI, which represents a some- 
what older stage than V; one-half has been removed. VII represents the beginning of the gastrular 
invagination, and VIII is the completed gastrula, both sectioned as in VI. 
forms assumed by the cells during the cleavage stages are all 
represented among colonial one-celled organisms, so there 
are also a few such that, in the arrangement of their cells, 
closely resemble the blastula. In this stage the cells form a 
hollow sphere, one cell in thickness, and in cases in which the 
blastula floats freely in the water, as in that of many of the 
invertebrates, each cell is provided with long vibratile flagella, THE ONTOGENESIS OF VERTEBRATES 
61 
by which the colony is moved. This larval form is closely imi- 
tated by such an organism as Volvox, which is usually reck- 
oned as a plant, but serves to show a physiologically functional 
adult organism in the corresponding stage. The folding in, 
or invagination of one portion of the blastula, as in the dia- 
gram here (Fig. 13, VI, VII, VIII) produces a two-layered cup 
which forms the next important ontogenetic stage, the gastrula, 
and in attaining this the embryo passes beyond the Protozoa in 
its imitative repetition and assumes the essential form of the 
simplest of the Metazoa, the Ccelenterata. A typical gastrula 
is radiate in structure, and possesses a central axis with two 
poles, oral and apical, the former characterized by the pres- 
ence of the gastrular mouth or protostome. This latter leads 
into the large central cavity, the gastrocoele, which has de- 
veloped from the exterior at the expense of the cavity of the 
blastula, the blastocoele. In some gastrulae this latter cavity 
is completely obliterated by the completion of the process of 
invagination, but often remains as a space between the two 
layers, the ectoderm and endoderm. 
When this type is completed and becomes an adult animal 
it often assumes a considerable complexity of structure but 
never gets far away from the original plan and does not de- 
velop more than the two primary layers. The fresh-water 
hydra is an example of one of the simplest coelenterates or 
gastrula-animals, and the coral polyps and medusae represent 
the more complex ones. In none of these does a blastocoele 
appear, in the simpler forms ectoderm and endoderm are 
everywhere in contact, and in the more complex medusae the 
space between them is filled by a gelatinous tissue developed 
from the other layers, and termed mesenchyme.* 
Up to this point the course of development is the same for 
* This is carefully to be distinguished from the mesoderm, or middle 
layer, which appears first in animals above the ccelenterates and is always 
in the form of a definite layer. The mesenchyme never appears as a layer, 
but its cells serve to fill in the spaces between the true germ layers, and 
the structures formed from this source are thus determined by the form of 
the surrounding tissues. 62 
THE HISTORY OF THE HUMAN BODY 
all Metazoa, allowing for the adaptive modifications always 
met with in the application of a general plan to a group of 
organisms. 
From this point on, however, there is a divergence in the 
course of development, and the various branches of the higher 
Metazoa proceed along different paths; yet all develop, al- 
though through different means, the three following attributes, 
which differentiate them from the lower Metazoa, the Ccelen- 
terata: 
1. The formation of a third germ, element, the mesoderm, 
situated between ectoderm and endoderm. 
2. The formation of a new cavity or system of cavities, the 
metaccele, lined wholly by the mesoderm. 
3. The attainment of a new body axis, and a bilateral, in- 
stead of a radiate, symmetry. 
Omitting all further reference to the other branches, it ap- 
pears that in the branch leading to the vertebrates the gas- 
trula assumes the form and position shown in Fig. 14, in 
which it becomes placed horizontally, with the apical pole for- 
ward and the protostome posterior and dorsal and in the 
median line. The plan of structure is a bilateral one, with 
dorsal, ventral and two lateral aspects. If this metamorphosis 
has any biogenetic value, that is, if it is indicative of a genu- 
ine historic stage in the phylogeny of vertebrates, it suggests 
an ancestral gastrula that sank to the bottom, lay upon its 
side and exchanged a free swimming for a crawling mode of 
locomotion, apical pole forward. Such an hypothetical form 
as this corresponds, however, to nothing known at the present 
time, but may well have disappeared without trace, since a 
similar fate has happened to the transition forms linking 
the vertebrates to the other Metazoa, leaving the group un- 
usually isolated. [See Chapter XIV.] 
There now occur several simultaneous changes which inau- 
gurate the essential vertebrate structure and are best explained 
by the help of the accompanying diagrams. [Plates I and 
II.] The gastrula has now become considerably elongated in 
the direction of the newly acquired secondary axis and is rep- Plate I. Diagrams showing Vertebrate development, ex- 
plained in the text; stages I and II. Based upon a stereogram by 
Kingsley. Plate II. Diagrams showing Vertebrate development; 
stages III and IV. Based upon a stereogram by Kingsley. THE ONTOGENESIS OF VERTEBRATES 
63 
resented as cut transversely across, the diagram representing 
the posterior portion and showing the cross-section as well as 
Fig. 14. Diagrams of gastrulse. [Based on models of Amphioxus by 
Hatschek.] 
(a) Typical gastrula, as in Fig. 12, VIII, but differently placed, for comparison 
with the others. (b) Early gastrula of Amphioxus, a probable ancestor of the 
vertebrates, (c) Later embryo of the same. 
xy, primary axis, i. e., that of the gastrula; ab, secondary axis, that of the adult 
Amphioxus; cn, neural canal; cne, neurenteric canal; np, neuropore; ntc, notochord. 
a portion of the length. The most superficial of these changes 
involves a longitudinal mid-dorsal stripe, which becomes grad- 64 
THE HISTORY OF THE HUMAN BODY 
ually turned in, forming a trough. Through the fusion in the 
median line of the edges of the trough, the turned-in portion 
becomes a tube, which ultimately frees itself from its attach- 
ment to the rest of the ectoderm, and forms the neural tube, 
the anlage* of the nervous system. The walls of this tube, by 
an excessive thickening of certain definite portions, become the 
brain and spinal cord, and the lumen is perpetuated as the ven- 
tricles of the brain and the canalis centralis of the cord, the 
embryonal communication between these cavities being re- 
tained throughout life. 
A somewhat similar structure, also median, arises from the 
dorsal wall of the endoderm. This appears at first as an in- 
verted trough, and possesses a narrow lumen, but it eventually 
becomes pinched off from its place of origin, not as a tube, 
but as a solid rod of cells, the notochord, which forms the 
precursor of the vertebral column. 
A third procedure, more complicated than the other two, is 
that involved in the formation of the mesoderm. Like the 
notochord, this arises also from the endoderm, and appears 
typically in the form of paired, lateral pockets, the mesodermic 
diverticula. There is reason to suppose that originally, that 
is, in certain of the lost forms between the creeping gastrula 
and Amphioxus, these diverticula were used as gonads, or 
sac-like cavities, in the lining of which the germ cells were 
developed, but in the vertebrates this function is retained by 
but a very small portion of their surface, as will be shown 
later. These diverticula soon separate themselves from the 
intestine and expand until they fill practically the entire space 
between ectoderm and endoderm and lie in close contact with 
one another. They thus form a series of paired cavities, the 
metacoeles, those of each side separated by transverse par- 
* The word Anlage is borrowed from the German to express a concep- 
tion for which there is no generally recognized English equivalent. It signifies 
the first visible indication of a part that appears in the embryo, and may thus 
signify either a definite cell-mass or a slight change in the arrangement of 
cells. B. G. Wilder has suggested the word proton for this, Kingsley uses 
fundament, and Lillie has of late used primordium. All have now a restricted 
use, but are far from universal. THE ONTOGENESIS OF VERTEBRATES 
65 
titions composed of the walls of adjacent diverticula, and 
those of the two lateral series similarly separated by longi- 
tudinal partitions which lie in the median line above and be- 
low the intestine. The early loss of the transverse partitions 
converts the segmental series of lateral cavities into a single 
pair, one for each side, while a similar reduction of the greater 
portion of the ventral longitudinal partition throws the two 
cavities together and forms eventually a single large metacoele 
or body cavity, lined by the mesoderm. One layer of this 
Fig. 15. Diagrammatic cross sections through vertebrate embryos, based 
upon the conditions found in selachians. [Modified, after van WijheJ 
(a) Earlier stage, in which the three parts of the mesodermic diverticula are still continuous, 
(b) Later stage, in which the epimeres of the mesodermic diverticula have separated from the meso- 
hypomeres and form a continuous layer around the body, interrupted only at the mid-dorsal and the 
mid-ventral lines. 
In all the figures the ectoderm is represented by square cells, the endoderm by crossing diagonal 
lines, the mesoderm by solid black, the mesenchyme by dots. I, epimere; II, mesomere; III, hypo- 
mere; a, aorta; g, gonad; i, intestine; m, myotome of epimere; me, metaccele (the definite coelom); 
n, nerve cord; nc, notochord; nph, nephridium; sk, sklerotome, the anlage of the axial skeleton; 
w, pronephric duct (Wolffian duct). 
invests the outer body wall, the other the intestine, the parie- 
tal and visceral layers respectively. The longitudinal parti- 
tions, both dorsal and ventral, serve as suspensory ligaments 66 
THE HISTORY OF THE HUMAN BODY 
in the intestine and are termed mesenteries. The dorsal one 
is retained throughout its entire extent; the ventral one disap- 
pears posterior to the liver. It will be noticed that the meso- 
dermic diverticula during their development have expanded 
at the expense of the protocoele, the original cavity included 
between ectoderm and endoderm, and thus at the completion 
of the process the protocoele has become reduced to a com- 
plicated system of narrow spaces lying everywhere between 
the other layers. The protocoele is thus called the primary 
and the metacoele the secondary body-cavity, and it is this 
latter, the one lined by the mesoderm and included between its 
two layers, that forms the permanent body-cavity of verte- 
brates, the so-called coelom or pleuro-peritoneal cavity. The 
narrowed spaces of the protocoele become filled with embry- 
onal connective tissue cells, the mesenchyme, which never as- 
sume the form of a definite layer, and are produced by pro- 
liferation from the mesoderm, and perhaps from the other 
two layers as well. Canals are left here and there which in 
time are built up into a continuous system of vessels, with 
walls of connective tissue, and form the vascular system 
(blood-vessels and lymphatics). 
The arrangement of the various embryonic elements at this 
point is shown in the accompanying diagrams based upon 
selachian embryos, and exhibiting the actual proportions as 
they exist in a rather primitive vertebrate. [Fig. 15.] The 
general arrangement of parts in an adult dog-fish is not ma- 
terially different from the last of these. Through the forma- 
tion of a restricted middle area, the mesodermic diverticula 
become divided into dorsal, middle and ventral portions, the 
epimere, mesomere and hypomere respectively, each with a 
distinct, separate history. 
The epimere, the inner wall of which becomes greatly thick- 
ened, eventually cuts itself off from the remaining meso-hypo- 
mere, and expands both dorsally and ventrally between the 
latter and the ectoderm until it meets the opposite one in the 
mid-dorsal and mid-ventral lines, separated only by thin strips 
of connective tissue. From the thickened inner wall of this THE ONTOGENESIS OF VERTEBRATES 
67 
develop the voluntary muscles of the body, the segmentation 
of which is retained among the fishes throughout the greater 
part of the body, and still appears in unmistakable traces 
among the highest forms. The connective tissue 
partition separating the muscle masses of the two sides be- 
comes the linea alba, a conspicuous white line, which persists 
in all vertebrates. The cavity of the epimere becomes sup- 
pressed by the growth of the inner wall and thus comes to 
nothing. 
The consecutive meso-hypomeres soon lose their independ- 
ence through the breaking down of the transverse partitions, 
as described above, but the metameric repetition found among 
the parts derived from them continue to suggest their origin 
as separate diverticula. From the narrowed mesomere there 
arise the essential organs of the urogenital system, many parts 
of which retain throughout life the indications of a segmental 
origin. The cavities of the mesomere become those of the 
systems derived from it. 
The hypomeres, fused into a single bag or sac, form the 
definite coelom, or pleuro-peritoneal cavity, of which they fur- 
nish the lining membrane, the peritoneum. The outer layer 
(parietal mesoderm) lines the body wall; the inner (visceral 
mesoderm) invests the primary intestine and, later on, its 
derivative organs, as lungs, pancreas and liver. In all below 
the mammals the coelom is a single cavity, and its investing 
membrane is continuous. In the mammals, however, the de- 
velopment of a muscular partition across this cavity, anterior 
to the middle, the diaphragm, separates the entire body cavity 
into two parts, the thoracic and abdominal cavities, and also 
breaks the continuity of their lining. That of the thoracic 
cavity, investing the lungs, is called the pleura, and that of 
the abdominal cavity, investing the alimentary canal and its 
associated organs the peritoneum. As cavities and lining were 
named first in mammals, they are often spoken of in the lower 
forms, when the two are not distinct, as the pleuro-peritoneal 
cavity and the pleuro-peritoneum respectively. 
Although the above sketch represents the underlying plan 68 
THE HISTORY OF THE HUMAN BODY 
upon which the development of all vertebrates is based, it is 
not found in an unmodified condition save in the lowest classes. 
It is most typically represented in the development of Am- 
phioxus, for which the foregoing description, save in a few 
points, might well be used; in the selachians, also, the modi- 
fications are not very great and the plan may be easily traced. 
In the amphibians, however, the plan is so much obscured, 
especially in its earlier stages, that for a long time, during the 
early history of the science of embryology, the homologies 
were not recognized. These modifications become still greater 
in the Sauropsida and Mammalia, in which, without the help 
of the amphibians, which here, as elsewhere, form a valuable 
connecting link, the recognition of the early stages would be 
hardly possible. The principal disturbing factor, at least in 
amphibians and the sauropsida, is the presence of increasingly 
greater quantities of yolk, which presents numerous mechan- 
ical problems, and its influence is felt with equal emphasis in 
the case of placental Mammals, where the egg, although yolk- 
less, has evidently become so through a secondary reduction 
and still follows in its development that of the yolk-filled eggs 
of the Sauropsidan type. 
One of the most important modifications in the develop- 
mental history of the higher classes concerns the appearance 
and subsequent development of the mesoderm and the forma- 
tion of the definite coelom. In Amphioxus the pairs of di- 
verticula arise in quite typical fashion from the sides of the 
primitive intestine, and this procedure is almost as easily 
recognized in the case of the selachians. The amphibians show 
considerable modification, and these are the last in the ascend- 
ing scale in which the diverticula are provided from the first 
with vestiges of a definite lumen, in the form of an irregular 
crack between the outer and inner cell layers. 
Above this class the mesoderm appears first in the form of 
an irregular cell layer which starts at the sides of the noto- 
chord and invades the space between ectoderm and endoderm. 
In it the region of the epimeres becomes easily distinguished 
by a great increase in the thickness of the layer, and the in- THE ONTOGENESIS OF VERTEBRATES 
69 
dications of the separate diverticula appear through a series of 
transverse fissures, which divide the mass into separate square 
blocks, the so-called mesodermic somites. These first appear 
at about the middle of the body, and are added to progres- 
sively both anteriorly and posteriorly until the full number is 
reached. The mesoderm of the meso-hypomeric region 
Fig. 16. Four cross-sections of vertebrate embryos showing develop- 
ment of the mesoderm. 
(a), Amphioxus [after Hatschek]; (b) Triton (a salamander) [after Hert- 
wig]; (c) bird, diagrammatic; (d) mole [after Heape]. 
k, ectoderm; n, endoderm; m, mesoderm; mt, parietal layer of the mesoderm; 
mp, visceral layer of the mesoderm; v, nerve cord; t, notochord; w, Wolffian duct; 
g, gastroccele. 
remains for a time as a single undivided layer, but ulti- 
mately splits into two, outer and inner, containing between 70 
THE HISTORY OF THE HUMAN BODY 
them a single undivided space, the future coelom. This lat- 
ter is here called a schizocoele, in respect to its mode of 
origin. 
There is thus attained in the higher vertebrates a much 
shortened and greatly modified method of producing the ele- 
ments for the later developments, hardly recognizable on com- 
paring it with the more expanded and simple form found 
among the low types. Aside from such modifications as 
those mentioned, which are explained through mechanical 
exigencies, there appear to be differences in the origin of the 
Fig. 17. Three early vertebrate embryos, showing mesodermic somites. 
(a) turtle [after Mitsukuri]. (6) chick [after Duval], (c) pig [after Keibel]. 
nt, nerve cord (brain); nt', nerve cord (spinal cord); ms, mesodermic somites; e, ear; 
yv, yolk veins. 
Incidentally this figure, containing figures of embryos at the same stage, representing a reptile, a 
bird, and a mammal, and drawn by members of three different nations, without any possible collu- 
sion, well illustrates the ancestral repetition of the same stage in very different vertebrates. 
first mesoderm cells themselves, differences which tend to 
shake our faith in the absolute homology of the germ layers. THE ONTOGENESIS OF VERTEBRATES 
71 
Since, however, in spite of such variation in the early history, 
the same embryonic elements eventually appear in all cases, 
so that the anlagen of the principal organs are the same for 
all, it is hardly possible that the early modifications, however 
profound, have any deeper significance than that of csenoge- 
netic adaptions to the various changed conditions of develop- 
ment. 
The presence of yolk has a great modifying influence, both 
on the general shape of the early embryo and upon the definite- 
ness of its stages. Yolk is an inert substance, the presence of 
which in large quantities within the cells interferes with their 
normal division and with their assumption of normal propor- 
tions. Beyond a certain amount, in fact, complete cell division 
is impossible, and the egg comes to consist of two portions, (i) 
the protoplasmic area, in which all cell divisions take place, and 
which ultimately becomes developed into the embryo, and (2) 
the yolk. The extreme of disproportion is seen in the bird’s egg, 
where the protoplasmic area is represented by the light yellow 
embryonal disc, about 4-5 mm. in diameter, which floats on the 
upper surface of the huge, non-cellular yolk mass. In such 
cases, the embryo, when passing through the early stages, or 
until after the establishment of the mesodermic somites and the 
formation of head and tail, is spread out on the surface of the 
spherical yolk, in proportion to which it is so small as to be 
almost flat, but later on becomes nearly separated from it, re- 
taining its connection by a narrow yolk-stalk attached in the 
umbilical region. The embryo grows at the expense of the 
yolk, and as the former increases in size, the latter dimin- 
ishes, so that by the time the animal assumes a free life the 
yolk has nearly or wholly disappeared. 
The embryos of the higher vertebrates differ from those 
of the lower in one very conspicuous feature, and that is, in 
the possession of fetal membranes, external to the embryo and 
designed in part for protection and in part for the obtaining 
of oxygen and food. The two essential membranes are the 
amnion and the allantois, which are present in reptiles, birds 72 
THE HISTORY OF THE HUMAN BODY 
Fig. 18. Diagrams of Amniotes, showing the relation of the extra- 
embryonal membranes. 
(A) Sauropsidan, with functional yolk-sac and respiratory allantois. (B) Mam- 
mal, with functionless yolk-sac and with the allantois converted into an umbilical 
cord and placenta. THE ONTOGENESIS OF VERTEBRATES 
73 
and mammals and absent in fishes and amphibians, a difference 
which is expressed in the two terms Amniota and Anamnia 
(with and without amnion), applied respectively to the two 
divisions in question. 
The amnion appears to be solely for the protection of the 
embryo. It is a thin transparent membrane, composed of 
parietal mesoderm and ectoderm, and is formed by the growth 
of folds about the embryo. It invests the latter on all sides and 
forms about it an enclosed space, the amniotic cavity, in which 
the embryo lies, immersed in a colorless amniotic fluid, of 
about the same specific gravity as the embryo itself. The 
allantois is in the form of an empty sac, composed of two 
layers, visceral mesoderm and endoderm, and develops from 
the umbilical region of the embryo. In reptiles and birds it 
pushes its folded edges between yolk-sac and amnion on the 
inner, and the shell on the outer, side, and thus comes to com- 
pletely invest the former and line the latter with a double 
membrane. In this there develop two large allantoic (umbili- 
cal) arteries and two allantoic veins, and the organ thus serves 
as an excellent respiratory organ, affecting the interchange of 
gases through the porous shell. In placental mammals the 
egg-shell is replaced by a membranous chorion, and the allan- 
tois effects a close union with this, either involving the entire 
surface or more generally a restricted area of it, and this sur- 
face, entering into a more or less intimate relationship with the 
mucous membrane of the maternal uterus, forms the essential 
organ of nutrition, the placenta. That portion of the chorion 
which is involved in the formation of a placenta is covered by 
branching processes, the chorionic villi, forming a surface 
known as the chorion jrondosum in distinction from the smooth 
area, the chorion Iceve. A diffuse placenta, where the villi 
cover the entire external surface of the chorion, is the most 
primitive type, and is found in pigs, horses, whales and por- 
poises; if a small portion of the chorion is left smooth, the 
placenta is bell-shaped, as in some edentates and lemurs. By 
a continuation of this process, that is, by a farther extension 
of the smooth area, the placenta becomes discoidal, which is 74 
THE HISTORY OF THE HUMAN BODY 
the form characteristic of Man and the higher anthropoids, 
insectivores, bats and rodents; in the lower monkeys there are 
two such discs, placed at opposite poles, the placenta discoidea 
duplex. It is the single discoidal type, as found in man, that 
gave the name “ placenta ” to this organ, as the word signi- 
fies a round, flat cake. 
If there are two smooth areas at opposite poles, with pla- 
cental villi between them, the zonary placenta is formed, the 
type characteristic of all carnivores, elephants, Hyrax and the 
Sirenia. A very distinct type of placentation is the cotyle- 
donal, characteristic of ruminants. Here the placental struc- 
ture is confined to small nodules or cotyledons scattered over 
the entire surface of the chorion, and varying in number from 
three to five in the deer to more than a hundred in the sheep 
and cow. 
All of the above forms of placentation are easily derived 
from the primitive diffuse type, and as a rule actually pass 
through the changes during early development, the form finally 
assumed being attained through the growth of smooth areas 
(chorion Iceve). 
The methods of placentation may be again divided with 
reference to the relationship to the uterine mucous membrane; 
in one type the villi at birth are simply drawn out of the ma- 
ternal portion, leaving pits, and in the other type the union 
between fetal and maternal elements is more intimate and the 
separation occurs between the mucous and muscular walls of 
the uterus itself, thus involving the loss of maternal mucous 
membrane, called in this connection, the decidua. The latter 
of these types, in which the placenta becomes a far more spe- 
cialized organ, is termed deciduate, the former indeciduate. 
In ungulates and in many of the edentates the placenta is in- 
deciduate, in most others it is deciduate. 
When the fertilized egg of a placental mammal first enters 
the uterus it does not at once become fixed, and development 
proceeds for some time before there is any attempt at the for- 
mation of a placenta. Meanwhile the egg passes through a 
series of typical cleavage stages and attains the condition of a THE ONTOGENESIS OF VERTEBRATES 
75 
hollow sphere, similar to the blastula of more typical onto- 
genesis. This, however, is not a blastula, but the blastodermic 
vesicle, upon one side of which there develops an embryonal 
area similar to that of the bird, that is, spread out in the form 
of a flattened disc, and not cylindrical as in the case of other 
yolkless eggs. This apparently useless circumlocution can be 
understood only on the ground, supported also by the early 
development in the marsupials and monotremes, that mammals 
have been derived from ancestors having large, yolk-filled 
eggs and that the secondary reduction of this substance has 
been too recent to effect a corresponding modification in the 
course of development. Adhesion to the walls of the uterus 
occurs through the formation of chorionic villi over the 
surface of the blastodermic vesicle, in which the form of 
placentation characteristic of the species soon becomes 
manifest. 
The later developmental history of vertebrates subsequent 
to the formation of the germ layers and the establishment of 
the anlagen of the various systems, belongs to that division 
of the subject known as organogeny, or the development of 
the various organs, and cannot be followed further in this 
place; it receives a fuller treatment, however, in the ensuing 
chapters, where the systems are considered separately and 
where embryological facts are made use of in so far as they 
are needed to explain the history of the several organs. Most 
of the systems arise from a single germ layer, often, indeed, 
from a definitely restricted locality in one of them, the anlage 
of which appears at an early period, and there is thus a time 
at which an organ, however complex and difficult to understand 
as it exists in the adult, is exceedingly simple. This primitive 
condition furnishes the best possible starting point from which 
to follow its gradual modifications step by step until the adult 
form is reached. The derivation and original anlage of most 
of the systems have been given above and are expressed graph- 
ically in several of the diagrams [Plates I and II; Fig. 15], 
but it may be also useful to introduce the chapters on 76 
THE HISTORY OF THE HUMAN BODY 
the several systems by a table which shows the derivation of 
each. 
Embryonal Element. Derivative 
I. Ectoderm Epidermis; including that of the entire exter- 
nal surface, as well as the more external 
parts of mouth cavity, rectum, and other 
cavities opening to the exterior. 
Epidermic structures; including all glands of 
the integument, nails and claws, hair and 
feathers, horny scales, the enamel of the 
teeth and the crystalline lens. 
Nervous System; including brain and cord; 
peripheral nerves and sympathetic system 
with the ganglia associated with each; the 
epithelium of the sense organs, and the 
tapetum of the eye. 
II. Endoderm Alimentary canal, that is, its essential layer, 
the mucous membrane; also all organs de- 
rived from this, as thymus and thyreoid 
glands, larynx, trachea and lungs, liver and 
pancreas. 
Notochord; the anlage about which the ver- 
tebrae (mesenchymatous structure) are 
formed. 
III. Mesoderm. 
a. Epimeres. .. .Voluntary muscles, except those of jaw, hyoid 
and branchial arches. 
b. Mesomeres. . Urogenital system, including the germ glands. 
c. Hypomeres.. Peritoneum; including pleura of mammals; 
germ-glands; voluntary muscles of jaw, 
hyoid and branchial arches, including the 
muscles of the larynx. THE ONTOGENESIS OF VERTEBRATES 
77 
IV. Mesenchyme Connective tissues; including those in the 
strict sense, also cartilage and bone, and 
the corium. Involuntary muscles of the 
viscera and of the skin. 
Vascular system; including heart, blood-vessels 
and blood; lymphatics; and the septum of 
the diaphragm. CHAPTER IV 
THE INTEGUMENT AND THE EXOSKELETON 
“ Seit Huxley seine Schrift ‘ Zeugnisse fiir die Stel- 
lung des Menschen in der Natur ’ veroffentlicht hat, 
sind 31 Jahre vergangen, und wenn man erwagt, was 
in diesem Zeitraum auf dem Gebiet der physischen 
Anthropologie, der Embryologie und Morphologie 
uberhaupt gearbeitet und erreicht worden ist, so ist es, 
meine ich, an der Zeit, den Blick wieder einmal riick- 
wiirts zu richten, das zu einem einheitlichen Ganzen 
zusammenzufassen, was an vielen Orten zerstreut 
liegt, un daraus endlich zu ersehen, was der Mensch 
war, was er ist, und was er sein wird.” 
Robert Wiedersheim, Der Bau des Menschen, 
1893, P- 3- 
The usual invertebrate form of integument is composed of 
a single layer of epidermic cells, the external surface of which 
is covered by a non-cellular structure formed from the cell 
walls. This outer element is often a transparent cuticula * or 
in other cases may consist of vibratile cilia. Beneath the in- 
tegument, and separated from it by a thin layer of connective 
tissue, lie the muscles. 
The integument of Amphioxus conforms to this general 
type, but in all true vertebrates important changes take place, 
rendering it quite different in structure and of far greater 
complexity. The epidermis becomes many-layered and loses 
the external cuticula, although cilia persist in a few early 
larval forms, and the underlying connective tissue becomes 
thick, often much exceeding the epidermis in this respect. As 
this latter layer, the corium [cutis], is almost indissolubly as- 
* The flattened outer cells of the vertebrate epidermis, which form the stra- 
tum corneum, are, under certain circumstances, easily separated from the 
next, and form a thin layer often termed the “ cuticle.” The use of the word 
in this connection is questionable, on account of its liability of being con- 
fused with the non-cellular cuticula of invertebrates. 
78 THE INTEGUMENT AND THE EXOSKELETON 
79 
sociated with the epidermis, while very loosely attached on 
its under side to the parts which it covers, the two form to- 
gether an easily detachable part, known as the skin or hide, 
similar in general function to the integument of invertebrates, 
but far more complex in structure. The vertebrate integument 
is further characterized by a great variety of secondary struc- 
tures, involving one or both layers and either remaining be- 
neath the surface, as is the case with glands and pigment, or 
projecting conspicuously beyond, as in hairs, feathers and 
scales. 
Concerning the integument itself, in so far as it can be 
treated apart from its accessory organs, it may be noted that 
the epidermis is always several cells deep and is in constant 
growth, being renewed from the innermost layer in about the 
same proportion as it is worn off at the surface. This inner 
layer is a fairly definite one and is termed the stratum germ- 
inativum mucosum or Malpighii~\. Its cells are con- 
stantly proliferating and the older cell generations are grad- 
ually pushed toward the surface, becoming flattened and more 
cornified as they progress. They thus form a protective cov- 
ering for the more delicate cells that lie beneath them, and 
compose a layer, which, in distinction to the stratum germ- 
inativum, is called stratum corneum. Some authorities dis- 
tinguish also for convenience a stratum lucidum, lying between 
the two, although the exact limits of none except the stratum 
germinativum are definitely fixed. 
It is evident that, in order to avoid an excessive growth of 
these upper layers, there must be some way by which they 
may be continually removed. This is accomplished in reptiles 
and amphibians by periodic moults or ecdyses, through which 
the entire surface layer is cast off by a single process, and 
quite often in one continuous piece, after the formation of a 
new layer beneath it. In many forms with a cornified skin, like 
snakes and lizards, these cast-off “ skins,” the exuvice, are 
matters of common observation, and are seen, to reproduce 
most faithfully every scale, horn or other protuberance charac- 
teristic of the animal; in certain other cases the cast-off skin 80 
THE HISTORY OF THE HUMAN BODY 
is eaten by the owner. In birds and mammals there is no 
periodic moult, so far as the skin is concerned, and no con- 
tinuous layer cast off, but the dead and dried cells are con- 
stantly being worn from the surface and pass away unnoticed. 
In these animals, however, there is usually a definite period 
for the renewal of the accessory parts, the feathers and hairs, 
a form of moult to be carefully distinguished from the fore- 
going. 
The corium, in common with other connective tissues and 
in contrast to the epidermis, is not composed wholly of cells, 
but consists in great measure of fibers, which run in all direc- 
tions between the cells and are produced through their agency. 
These fibers, which, though not the formative element of the 
corium, are the most important structural ones, form a rather 
loose and often very elastic felting, which, in many vertebrates, 
notably mammals, forms the main bulk of the skin. In fact, 
it is this layer alone, which, artificially thickened by the action 
of tannin, is used for leather, the epidermis being first removed 
by maceration. The corium is the thickest in mammals, but 
is also fairly thick in amphibians and in many fishes. In rep- 
tiles and birds it is thin, the amount of protection thus lost 
being compensated for by the dense and firm covering afforded 
by the accessory epidermic structure, scales and feathers re- 
spectively. Birds have the thinnest corium of all vertebrates, 
a condition undoubtedly correlated with the development of 
the feather coat, which renders the protection of a thick corium 
superfluous. 
In the formation of the accessory organs each of the two 
layers furnishes materials characteristic of itself, and, although 
in later growth a structure that originates in one layer can, 
and generally does, invade the province of the other, there is 
a definite place of origin for each element involved. Thus 
from the epidermis come integnmental glands of all sorts, al- 
though they usually dip down into the corium from which they 
receive a fibrous investment. Pigment may be derived from 
either layer, but more usually from the corium, and when 
found in the epidermis, as it commonly is, it is more likely to THE INTEGUMENT AND THE EXOSKELETON 
81 
have wandered in from the corium than to have originated 
in place. Blood-vessels, with the single exception of the 
pharyngeal mucous membrane of lungless salamanders,* are 
entirely confined to the corium. Sensory nerve endings of the 
simplest type are distributed freely through the epidermis, but 
the more specialized forms remain in the corium, although 
they may be located in papillae pushed up into the epidermic 
zone. The epidermis thus forms a bloodless covering with but 
slight sensitiveness, the main function of which is to protect 
the more delicate structures included in the lower layer. Aside 
from this general protection afforded by the unmodified epi- 
dermis, both layers of the skin have the power of originating 
hard parts, which enter into the formation of certain acces- 
sory external structures that form a more or less complete 
exoskeleton. Thus the corium produces true bone, with the 
haversian canals and other osseous characters, while the epi- 
dermis forms horn and enamel, the latter superficially resem- 
bling bone, but harder and with a different structure. The 
structures formed from these may be composed of one sub- 
stance and involve but a single layer in their formation, al- 
though the other usually cooperates in some other way, or 
again they may be composites formed from material furnished 
by both layers. 
Thus exclusively horny structures, such as hairs or feathers, 
are formed from the epidermis alone, but, through the neces- 
sity of nourishment, they dip down into the richly vascular 
corium, which forms special organs to further this result. The 
dermal scutes of ganoids, and the dermal bones of higher 
forms arise wholly within the corium, while a tooth is a com- 
posite structure composed of dentine, a hard sort of bone, 
from the corium, overlaid with enamel from the epidermis. 
As exoskeletal structures are universal among vertebrates, 
and often form their most obvious characteristics, and espe- 
cially as they have interesting morphological histories of their 
own, they deserve special treatment, and will be taken up in 
the order of their appearance. 
* See Chapter IX, under Respiration. 82 
THE HISTORY OF THE HUMAN BODY 
The indifferent or generalized condition that serves as the 
starting point for all exoskeletal elements is found in the body 
covering of the dog-fish, which consists of imbricated rows of 
pointed scales, that is, rows arranged in such a way that the 
scales of one row cover the intervals of the one behind it. This 
typical arrangement is seen also in the scales of other fishes 
and reptiles and in the feather papillae on the skin of a plucked 
Str.cor.' 
Str.mucJ 
Cut. — 
Str.cor. . 
Str.muc.T 
Cut.. 
Fig. 19. Comparison in development and structure between a placoid 
scale and a tooth. 
(a), (b), and (c) represent the scale; (d), (e), and (f) the tooth. In all the 
figures the stratum corneum is dotted, the stratum germinativum is represented by a 
layer of large cells with nuclei; and the cutis is presented as composed of fibers 
with scattered cells. 
x, enamel membrane; y, cutis papilla; e, enamel; d, dentine; p, pulp cavity. 
bird. A similar, though less obvious, plan underlies the ar- 
rangement of the hair in mammals, as will be shown later. 
The scales in the dog-fish are of the form known as placoid, 
each consisting of an approximately flat base from which rises 
a sharp-pointed cusp, inclined in the direction of the free edge 
of the scale, or posteriorly when the scale is in place. This 
scale is somewhat complex in structure and consists of a basis 
or core of dentine overlaid by a layer of enamel, especially 
thick over the cusp, which is almost wholly composed of it. 
The scale is hollow beneath and a nutrient papilla formed 
from the corium finds its way into the interior. It arises in THE INTEGUMENT AND THE EXOSKELETON 
83 
the skin of the embryo from a fold which involves about 
equally both layers, and the scale develops between them, the 
dentine being formed from the corium and the enamel from 
the epidermis. 
In selachians the jaws are equipped with several rows of 
pointed teeth, usually arranged like the scales which cover the 
surface, and as the lining of the mouth cavity has exactly the 
same embryonic history as the external skin and is composed 
of the same two layers, it must be concluded that the teeth 
were once simple placoid scales like the rest, and that their 
later modifications have been due to the difference of the 
function to which they have become subjected, an inference 
sufficient to account for their slight changes in form as well 
as for their increased size and hardness, which is correlated 
with the greater amount of work to be accomplished. These 
teeth, seen here almost at their point of departure from gen- 
eralized placoid scales, are inherited by all higher vertebrates, 
although in some cases, like turtles and birds, they have be- 
come secondarily lost. Aside from the correspondence in form, 
arrangement and structure, the homology is clearly shown by 
the development, which proceeds in all cases from a fold, 
involving both corium and epidermis, in which the tooth sub- 
sequently appears. These organs, when once acquired, are 
subjected to great variations as an accommodation for the 
prehension and mastication of the innumerable kinds of food; 
they develop as pointed needles, fangs for inoculating poison, 
sharp-edged chisels, flat surfaces for grinding, and ornamental 
tusks, in all retaining the general structure characteristic of 
placoid scales. The morphology of the teeth will be taken 
up somewhat more at length in the chapter on the digestive 
system, with which those parts become so early associated. 
In ganoids, to which, as the lineal descendants of sela- 
chians, one should look for the next phase of this history, the 
scales develop from the corium alone, the epidermis remain- 
ing passive. There is thus formed a type of scale that is com- 
posed entirely of dentine, and lacks all trace of enamel. This 
dentine, however, is very fine and hard in character and usu- 84 
THE HISTORY OF THE HUMAN BODY 
ally presents an extremely smooth and polished surface which 
has been often referred to as genuine enamel. Scales of this 
(A) sturgeon (Acipenser). (B) salamander (Amolysloma). (C) turtle. (D) 
sea-lion (Otaria). 
ROS, rostral plates; N, nasal; F, frontal; Pr. F, pre-frontal Post. Fr, post-fron- 
tal; PMX, pre-maxillary; MX, maxillary; J, jugal; QJ, quadrato-jugal; P, parietal; 
SQ, squamosal; PT, pterygoid; PO, pro-otic; 00, opisth-otic; SO, supra-occipital; 
Oc. Lat., lateral occipital; OP, opercular; 5'. Cl., supra-clavicle. 
Fig. 20. Dorsal views of various skulls, showing the dermal bones. 
type, termed ganoid (i. e., shining; from which the Order re- 
ceives its name), are usually rhomboid in shape and lack the THE INTEGUMENT AND THE EXOSKELETON 
85 
cusp or point of the placoid type. In the sturgeon the scales 
consolidate into large, bony shields or scutes, and this principle 
was carried to the extreme in the long extinct and nearly re- 
lated groups of placoderms, where the entire fish was covered 
with a heavy suit of mail, probably as a protection from the 
huge molluscan forms which then thronged the seas. In the 
sturgeon, however, these plates are not continuous, but are 
arranged in longitudinal rows along the back and sides, leav- 
ing large areas unprotected. In all ganoids similar scutes 
cover the entire head, and fit together by their edges, forming 
sutures, but leaving no appreciable intervals. These are fairly 
definite in number and arrangement in the different species 
and form the so-called dermal bones of the skull. [Cf. Chap. 
V.] Certain of these, as the frontals, parietals, maxillaries 
and squamosals, persist in the highest groups; others, like the 
opercular and rostral series, disappear completely, while of an 
extensive orbital series one alone persists as the lacrimal. 
The dermal bones that line the mouth cavity, such as the 
vomers, palatines and parabasal, retain the indications of their 
origin longer than do the others, since, in many cases among 
both fishes and amphibians, they are covered with teeth which 
are arranged in imbricated series over a considerable area; 
occasionally, even, as in the vomers of the frog larva, these 
elements begin as separate conical teeth which fuse secondarily 
to form the plate, thus repeating ontogenetically their mode of 
origin. Nearly always, however, the history is curtailed, and 
the dermal bones first appear as thin, lace-like structures, lying 
in the sub-cutaneous connective tissue, and enlarge from defi- 
nitely located “ centers of ossification ” by marginal additions. 
The scales of teleosts are developmentally and structurally 
like those of ganoids, the ancestors of the group, but in form, 
although often rhomboid in the young, they become approxi- 
mately circular, and are hence termed cycloid. Ctenoid scales 
are a variety of this in which the inner margin attached to the 
skin is extended into numerous small processes like the teeth 
of a comb. 
Modern amphibians have a soft, slimy skin, without exo- 
skeletal structures of any kind, save in the rare order of coecil- 86 
THE HISTORY OF THE HUMAN BODY 
ians (Gymnophiona), in which scale rudiments lie in pits sunk 
beneath the surface. In the extinct group of Stegocephali, 
which possessed many amphibian characters, the body was 
covered, at least ventrally, with well-developed, imbricated 
scales. These facts together furnish sufficient proof that am- 
phibians were originally scaly and that the present naked 
condition is due to a secondary reduction. These scales were 
probably bony, like those of ganoids and teleosts. 
It is an abrupt transition from the scales of fishes to those of 
reptiles, since, in this latter class, the scales are purely epi- 
dermic in origin and are composed of horn {keratin), a sub- 
stance allied to enamel, without trace of bone. The corium, 
it is true, nourishes the scales by means of richly vascular 
papillae placed beneath each, but furnishes none of the hard 
parts. There is no doubt that in some way these scales must 
be related to the bony ones of ganoids and teleosts, but the 
relation appears to be an indirect one. They may have had 
a common origin in scales which, like those of the placoid 
type, possess both elements, the one emphasizing the epidermic 
portion, the other that of the corium. This would seem to con- 
flict with the direct derivation of reptiles from the ganoids 
as we know them, and shows the incompleteness of our records 
at this point. 
Aside from scales the reptilian integument possesses a great 
variety of other exoskeletal forms, such as spines, combs, and 
claws, all made of keratin, and equally unlike anything in 
ganoids or amphibians. In this wealth of horny exoskeletal 
elements the reptiles are closely followed by their lineal de- 
scendants, the birds, where the scales are represented by the 
far more elaborate, but strictly homologous, feathers, and 
where beak and feet are encased in horny coverings. The cov- 
ering for the beak has evidently replaced teeth, as in turtles, 
and is undoubtedly a recently acquired character, since fossil 
birds occur in the Cretaceous formation, in all respects like 
modern birds save in the presence of conical teeth set in 
sockets; furthermore, tooth germs have been found in the jaws 
of the embryos of several species of modern birds, transitory THE INTEGUMENT AND THE EXOSKELETON 
87 
in character and never developing far enough to break through 
the gums. 
The scales of mammals are commonly little emphasized, 
owing to the conspicuous nature of the hairy coat, principally 
associated with them, but they are, nevertheless, of great mor- 
phological value. They occur in definite regions and only in 
certain forms, but are so widely distributed that, were all 
other reasons absent, their former more extensive distribu- 
tion would be strongly suggested. In most cases they are 
found only on tails and paws, but in the Manidce, an edentate 
group, the entire dorsal surface of the body and limbs is cov- 
ered with large, imbricate scales, and in the closely related 
armadillos, similar scales fuse to form a dorsal carapace, as 
well as shields for the head, tail and limbs. Scale formation 
on paws and tail occurs mainly in marsupials, rodents and 
insectivores, and may be seen particularly well on the dorsal 
surface of the paws of moles and shrews, or on the flat tails 
of the beaver and muskrat, in which the scales are usually 
rounded and regularly imbricated. Where the tail is cylin- 
drical, as in the rats and mice, the scales are arranged in rings, 
those of one row standing in imbricated relation to the one 
which it overlaps. 
In structure the scales are epidermic, like those of reptiles, 
underlaid by corium papillae. They usually remain more or less 
embryonic, and the epidermis, though cornified, does not de- 
velop definite hard parts, but in the Manidce distinct horn 
scales are produced, as thick and heavy as those of reptiles, 
the main difference being that there are here no periodic 
ecdyses, and the scales are shed and renewed singly, as occa- 
sion requires. In young armadillos the scales that form the 
carapace and shields are like those of Manis, but they become 
soon reinforced by ossifications of the corium, one for each 
scale, which enlarge and finally fuse to form an osseous sub- 
structure. These corium elements are plainly secondary struc- 
tures and are not to be considered as primary elements of the 
mammalian scale, which, as stated above, is entirely epidermic. 
Aside from the sporadic occurrence of scaled areas in 88 
THE HISTORY OF THE HUMAN BODY 
various mammals, as previously noticed, a more definite proof 
of the former completeness of the scaly coat is found in the 
relationship between scales and hairs in scaled areas, and in 
Fig. 2i. Hair arrangement in various mammals. Diagrammatic. [After 
de Meijere.] 
(a) Myopotamus (South American rodent). Tail, with scales and hairs, (b) Midas 
(Brazilian monkey). Back, (c) Sus vittatus (pig). Back. The finer bristles are 
left out upon the right side of the picture, (d) Ccelogenys paca (the “ paca,” a 
South American rodent). Back, (e) Dasyurus viverrinus (Australian marsupial). 
Back, (f) Loncheres cristata (South American rodent, allied to Myopotamus). Back. 
the arrangement of the hair in other places. If almost any 
scaled surface be examined, the tail of the rat for example, it THE INTEGUMENT AND THE EXOSKELETON 
89 
will be noticed that scattered hairs appear among the scales 
in a definite relationship, and that a group of three hairs, one 
median and two lateral, projects from beneath the margin of 
each scale, the median hair being somewhat longer and stouter 
than the others. It further appears that there is a similar ar- 
rangement of hairs, usually in groups of three, upon hair areas 
not associated with scales, the hair groups being arranged in 
imbricated series, and that this arrangement is general, even in 
mammals without trace of scales. There are some modifica- 
Fig. 22. Hair arrangement 
in various mammals. 
(a) Ursus arctos (brown 
bear). Front of chest. Dia- 
grammatic. [After De Mei- 
jere.] (b) Canis familiaris 
(dog). Four developmental 
stages. The adult arrange- 
ment is like that of (a). 
[After De Meijere.] (c) 
Homo. Scalp of negro. Cam- 
era drawing from the actual 
object. 
tions of this, due to secondary changes, such as the need of a 
thick fur, but even in these modifications the original plan 
is still apparent. Thus, in the pig, there are two sets of bris- 
tles, a coarser set arranged in imbricated groups of three, and 
a finer set, filling the intervening areas without definite ar- 
rangement; to obtain a thick fur, as in the rabbit, each hair 
in the group may become a bundle of hairs, the bundles being 
arranged in groups of three as in the typical case; even the 
number three is not always kept, and groups of five occur, 90 
THE HISTORY OF THE HUMAN BODY 
with two lateral hairs on each side, or groups of seven with 
three. Occasionally, as in the dog and cat, the plan becomes 
partly obscured in the adult, but is evident during development, 
Fig. 23. Formation of friction ridges from single rows of epidermic 
warts. [After Miss Whipple (Mrs. H. H. Wilder).] 
(a) Midas rosalia (Brazilian monkey). Proximal portion of hypothenar pad. (b) 
Midas rosalia. Apical pad. (c) Homo. Advanced fetus. Side of finger in tran- 
sition region. The dotted lines indicate the position of sweat-glands. 
the three-hair group being definitely marked in the advanced 
fetus. 
A still further corroboration of the former presence of scales 
in mammals may be obtained from the study of the lower 
surfaces of the paws, where, except in such extreme modifica- 
tions as the ungulates, scales either still exist or have left a 
permanent record in a peculiar configuration of the epidermis, THE INTEGUMENT AND THE EXOSKELETON 
91 
Fig. 24.. Formation of friction ridges in pairs from rings formed by 
the confluence of epidermic warts. [After Miss Whipple (Mrs. H. H. 
Wilder).] 
All the figures are taken from Lemur macaco (semi-ape from Madagascar). 
(a) Detail of area below the interspace between index and medius. Here are 
seen individual isolated warts w; groups of these forming rings g; fully formed 
rings r; also the formation of ridges in pairs by the lengthening of rings in one 
direction (best shown at the right); the single isolated ring enclosed by the ridges 
of the pattern is also suggestive, (b) A portion of the interdigital pad. (c) Apical 
pad. (d) Detail showing two methods of formation of three ridges from the rings. 92 
THE HISTORY OF THE HUMAN BODY 
directly traceable to them. In their simplest form the scales 
or scale rudiments are in the form of rounded, wart-like epi- 
dermic organs, which cover the entire surface in Ornitho- 
rhynchns and large portions of it in marsupials, insectivores 
and lemurs. They possess a more or less imbricated arrange- 
ment, and their identity with scales is shown, not merely by 
their structure and development, but by a comparison with the 
scaled dorsal surface of the paw in such cases as that of the 
shrew or the star-nosed mole, where the transitions from one 
form to the other may be seen along the edges of the paw. 
This primitive condition is modified in most cases by the pres- 
ence of characteristic mammalian organs, the pads, which 
are used as contact surfaces, and are typically eleven in num- 
ber, five for the tips of the digits, four for the distal margin 
of palm or sole, below the interdigital intervals, and two near 
the wrist or ankle. Upon these the scale rudiments become 
arranged in rows, and by their fusion form friction ridges, 
so called from their use, which is to prevent slipping, like the 
parallel ridges seen on the handles of certain steel instruments. 
These friction ridges are always arranged at right angles to 
the direction in which there is the greatest tendency to slip, 
that is, directly across the pads in walking forms, but arranged 
in concentric circles about the highest part of the pad in the 
arboreal lemurs and monkeys where slipping in all directions 
is equally to be expected. Owing to the general principles that 
the separate scale rudiments form friction ridges on the ac- 
tual contact surfaces only, it follows that when the pads re- 
main high their surfaces alone are ridged, while the depressed 
areas are covered with separate units, but when, as in lemurs 
and monkeys, there is a progressive tendency to utilize the 
entire surface for contact, the ridged areas spread in exact 
correspondence with the acquirement of contact surface, un- 
til, in the higher primates, the entire ventral surface of the 
paws becomes covered with ridges, leaving separate scale rudi- 
ments only along the boundaries, where this modified skin 
meets that of the dorsal surface. 
Had the friction ridges, which completely cover the palmar THE INTEGUMENT AND THE EXOSKELETON 
93 
and plantar surfaces in Man and the other higher primates 
developed primarily in forms in which the entire surface was 
used for contact, it may be assumed that they would have taken 
some simple form, designed with reference to the area as a 
whole; since, however, they have passed through the longer 
and more complex history caused by the introduction and 
secondary reduction of various pads, they have preserved the 
indications of the former relief by an arrangement otherwise 
zvithout cause or meaning. This may be seen by a compari- 
son of the lower surface of the paw in some animal in which 
the pad system is in full function with that of one in which 
the inequalities of the surface have become secondarily re- 
duced. (Fig. 25.) The one is an actual relief, the other a 
flat sketch; the one possesses raised pads surrounded by folds 
of skin which diverge in three directions from points known as 
triradii, the other indicates the former location of the pads by 
whorls and other patterns, and that of the folds by the arms 
of embracing triradii. Thus in the field-mouse (Fig. 25, a) 
there are present four interdigital pads, the first situated im- 
mediately below the interval between thumb and index, the 
second below the interval between the latter and digit III, 
and so on. Each of these is inclosed by folds of skin which 
diverge in three directions from points known as triradii, and 
there are three triradii about each pad except the third, which 
possesses a fourth one, located between digits III and IV. 
Below these lie the thenar and hypothenar pads, the folds of 
which are often well marked, though not especially so in this 
case. The apical pads at the ends of the digits also possess 
folds, not well shown in the figure, with two triradii, one upon 
each side. Turning now to the paws of Macacus, a small 
monkey (Fig. 25, b), in which the relief has been reduced to 
a flat surface, each of the above features (except the thenar in 
this especial case) is expressed by the configuration of the 
ridges, as indicated in the figure. The ridges essential in 
marking the palm are indicated by solid lines, although in 
reality not different from the rest. Each pad is represented 
by a figure or pattern, of which the four interdigital are the 94 
THE HISTORY OF THE HUMAN BODY 
most typical, and are in the form of concentric circles, the 
center coincident with the summit of the pad. The double loop 
Fig. 25. Ventral surface of anterior chiridium of an insectivore and of 
a primate showing correspondence between relief and arrangement of 
friction ridges. [After Miss Whipple (Mrs. H. H. Wilder).] 
(a) Crocidura ctrrulea (shrew-mouse). Fore paw showing walking-pads enclosed 
by triangular folds of skin, (b) Macacus sp? (Old World monkey). Hand, covered 
by friction ridges, the arrangement of which corresponds to the relief of (a). 
The pads are represented by concentric circles, and the triangular folds by triradii. 
These latter features are here designated by heavy lines, although in the real object 
they are not more conspicuous than the others. 
of the hypothenar is a degeneration from the primitive type, 
to which it is connected by the existence of transitional forms, 
either in other individuals of the same species or in different 
species. The thenar pattern has here become entirely re- THE INTEGUMENT AND THE EXOSKELETON 
95 
duced, but is often present. The apical patterns are also 
modified, but in a lateral view would show the triradii. 
Inasmuch, however, as in the Primate hand and foot the 
ridges are still of considerable functional importance, they 
are apt to become further modified at each point of the sur- 
face in accordance with the use of that point, and it thus 
happens that in different species and in different parts of the 
surface there are varying degrees of faithfulness to the ancient 
records. Thus in Macacus the use of the hand is such that 
thenar, hypothenar and apical pads tend to degenerate, while 
the interdigitals are preserved in their typical relations, while 
in the human hand the reverse is the case, and the apical 
patterns are nearly always well marked, and often in the form 
of typical whorls with two lateral triradii; while the inter- 
digital patterns are usually lost or obscurely indicated. A 
hypothenar pattern is frequent, especially in the white race, 
and occasionally occurs in its primitive form as a whorl with 
three triradii; but the thenar is of rare occurrence, and then 
usually associated with the first interdigital. These changes 
are in part explained by the tendency of the ridges to assume 
an approximately transverse direction, a tendency in which 
the right hand has surpassed the left, owing to the long pref- 
erential use of the former. 
In the human foot the apical patterns are about as well 
marked as in the hand, but with a smaller percentage of the 
primitive whorl type; the four interdigital pads are fairly well 
indicated and often appear in infants as rounded elevations. 
Of these the most constant is the first, placed on the ball of 
the foot below the great toe, and is frequently of the whorl 
type, occasionally with three triradii; the primitive condition 
again corresponding to the functional importance of the region 
which here bears the main force of the body during a portion 
of each step. The hypothenar is occasionally indicated by 
a loop on the outer edge, but the thenar is practically lost. 
An additional loop, of uncertain morphological significance, 
occasionally occurs on the heel. 96 
THE HISTORY OF THE HUMAN BODY 
Fig. 26. Print of right hand of boy (Anglo-American), showing a com- 
plete set of friction-skin patterns. 
I, II, III, IV, the four interdigitals; a, the thenar; b, the hypothenar; the five 
apical patterns (not lettered) are seen on the finger-tips. THE INTEGUMENT AND THE EXOSKELETON 
97 
It thus appears that, aside from the sporadic distribution of 
scales in various mammals, the palmar and plantar surfaces, 
save in the most modified cases (Ungulates, Cetacea), are 
covered with scale elements, either distinct or united in rows 
to form ridges; and, furthermore, that in other parts of the 
body the hair follicles occur in definite groups, arranged in 
alternate series; facts that can be interpreted only as indica- 
tive of the former presence of a scaly coat. 
That this stage is actually passed through in embryo mam- 
mals has not as yet been definitely determined, but some cir- 
cumstances seem to indicate that the vestiges of this covering 
may be looked for in the epitrichium, which is a superficial 
epidermic formation without definite structure so far as is 
known. This at one time covers the surface, but save in the 
palmar and plantar regions disintegrates and disappears; con- 
tributing in man to the formation of the vernix caseosa, found 
upon the surface of the new-born infant. Upon the palms 
and soles, however, at least in man, where it has been mainly 
studied, it appears to persist and take part in the formation 
of the friction ridges. 
This brings with it the suggestion that the epitrichium rep- 
resents the primitive scaly coat of ancestral mammals, the 
greater part of which becomes lost by an embryonal ecdysis. 
How this epitrichium appears and what its fate is on surfaces 
where scales persist, other than the palms and soles, or in the 
few scaled mammals, is not known; but in the sloths and 
ant-eaters, nearly related to the last, it is especially firm and 
remains until birth as a definite covering. In many mammals 
the similarity to a moulting external layer is increased by the 
presence of a thick layer over the nails or claws, continuous 
with the epitrichium, and cast off with it, the eponychium. 
The hair, which forms the characteristic coat of present-day 
mammals, may be safely considered as once accessory to a 
covering of scales, which it has secondarily replaced, as ex- 
plained in the foregoing, but this does not account for its 
origin, or suggest its primary function. An attempt has been 
made to homologize hairs with the integumental sense organs 98 
THE HISTORY OF THE HUMAN BODY 
of amphibians, owing to a similarity in the early stages of 
development; but although this view may receive some little 
extra support from the considerable degree of sensitiveness 
Str.C. I 
Sir. muc7 
Cutis < 
Layer! 
Str.C 
Strmut 
Cutis < 
StrC. 
Str. muc. 
Cutis 
StnC.fi 
StemikP 
'S eb 
Fol 
,-Out 
In 
Cutis- 
Pap 
Fig. 27. Development of hair. 
sir. c, stratum corneum; str. muc., stratum germinativum; seb, sebaceous gland; 
fol, follicle; out, outer root sheath; in, inner root sheath; G, hair germ; Gi, beginning 
hair; pap, corium papilla. 
which some hairs attain, the evidence in favor of it is slight, 
and the idea does not receive general credence. It seems 
likely that the hair developed subsequently to the scales and THE INTEGUMENT AND THE EXOSKELETON 
99 
not before them, and may have exercised some function of 
protection, possibly that of a fringe upon or near the free 
edges to prevent the accumulation of dirt in the folds where 
they overlap, a purpose for which organs of similar appear- 
ance but of different origin are frequently employed in in- 
sects and crustaceans. 
In development the hair is wholly epidermic, formed by 
the stratum germinativum, but dips down into the corium 
in the form of a solid column of rapidly proliferating cells, 
the outer layer of which soon differentiates into a sheath or 
follicle, while the inner cells become horny and form a shaft 
which projects beyond the surface and becomes the hair. 
Growth is constantly kept up at the bottom of the follicle, and 
proceeds from a small area of actively proliferating cells which 
are nourished by a corium papilla and form the true root, or 
matrix, of the hair. From an inspection of the accompany- 
ing figure (Fig. 27), it becomes evident that this matrix 
is merely a specialized portion of the stratum germinativum 
and that the hair consists of the upper layers derived from it, 
and renewed from beneath as in the superficial skin. When 
a hair is pulled out, the break usually occurs immediately above 
the matrix, and the lost portion involves the hair, the epider- 
mic sheath, and quite often the follicular sheath as well, parts 
that are easily regenerated so long as the matrix remains. 
Associated with this structure are typically two sorts of glands, 
tubular and acinous, which are formed as outpushings from 
the sides of the follicle and grow down into the corium. These 
develop in various mammals to subserve many different pur- 
poses, often becoming dissociated from the original connec- 
tion with the hair. To these two types all forms of integu- 
mental glands occurring in mammals may be referred. Their 
modifications and transformations may be considered later. 
The occurrence and distribution of the hair are in strict ac- 
cordance with the needs of the animal, and show great dif- 
ferences, corresponding to the various environments to which 
mammals have become adapted. The hair may differ in 
length, in caliber, in thickness (i. e., the number of groups in 100 
THE HISTORY OF THE HUMAN BODY 
a given area), in texture, or in form; it may be increased to a 
thick, matted wool, or may show every degree of reduction 
down to a total loss. It may develop into bristles, as in the 
hog, or even form spines, as in the hedgehog and porcupine, 
although this latter result is usually brought about by the con- 
fluence of numerous individual hairs. The “ horn ” of the 
rhinoceros is such a structure, and not a true horn. Many 
variations in thickness are brought about by modifications in 
the hair groups, or by the interpolation of supernumerary hairs 
independent of the group system. In the former case the num- 
ber of single hairs in each group may be increased, or each 
primary hair may be represented by a bundle; or again, each 
primary hair may be accompanied by a series of accessory 
hairs, arranged as satellites about the former. In the latter 
case there is usually a marked difference between the hairs 
that are included in the primary system and those that are not, 
as is seen in the case of the hog, in which there are two sizes 
of bristles, coarse ones in groups, and finer ones interpolated 
without system (Fig. 21, c). 
It may be said in general that arctic forms and those living 
at high altitudes are the most plentifully supplied with hair, 
while tropical and sub-tropical forms are sparsely covered. An 
aquatic life tends to reduce the hair coat; if the animal is 
but semi-aquatic, as seals and otters, the hair is reduced to 
the form of a fine plush, but in the Cetacea and Sirenia, 
which are wholly aquatic, the reduction is almost a complete 
one. Many apes are but scantily supplied with hair, the ven- 
tral side of body and limbs being but sparsely covered, while 
the upper part of the face and ears are nearly bare. The same 
tendency is continued farther in Man, who shows considerable 
racial variation, ranging from the hairy Ainus, the native Aus- 
tralians, and certain hairy individuals in the white race to the 
smooth and beardless Malays. That Man was formerly sup- 
plied with a thick coat of hair, however, is shown by the fetal 
condition, at one stage of which the entire body, not excepting 
the face, is covered by a coat of fine down, the lanugo. This 
mainly disappears before birth, and becomes eventually re- THE INTEGUMENT AND THE EXOSKELETON 
101 
placed by the permanent coat, which usually shows but slight 
development save in certain definite localities. The lanugo 
persists in a reduced condition on the face, especially in females, 
forming the down which gives to the cheeks their character- 
istic bloom. Abnormal hairiness in man, or hypertrichosis, 
is fortunately rare, and is of two kinds; the one, hypertricho- 
sis ve'ra, is due to an excessive growth of the permanent coat 
which replaces the lanugo; the other, pseudohypertrichosis, is 
the result of the persistence of the lanugo. 
Localized hypertrophy in various mammals in the form of 
manes, crests or tufts of hair, is of frequent occurrence and 
is used for various purposes, such as defense from flies or 
other noxious insects, attraction of the other sex, or as a pro- 
tection from the teeth of rivals. Under this general head 
come also the beard of man, which corresponds in position 
and direction to that found in other primates, and the long 
hair of the head. The other locations in Man in which long 
hair occurs, the axillary and pubic regions, do not seem to 
belong here, and probably represent portions that escaped 
reduction rather than hypertrophy. The obvious function of 
the cranial hair is a protection from the sun, and its location 
suggests that it is developed with reference to the erect and 
not the quadrupedal position, in which latter case it would 
have extended farther down the back. The axillary and pubic 
tufts may be for lessening the friction between the limbs dur- 
ing motion; it has been also suggested that they possessed a use 
in transitional forms in furnishing places to which the infant 
might cling, thus leaving the arms of the parent free for 
climbing. In support of this latter view it may be noticed that 
the distances between these locations correspond approxi- 
mately to the proportions of a normal infant, and that an in- 
fant thus attached is also in the right position for nursing. 
Aside from differences in caliber and length, the hair of vari- 
ous mammals differs markedly in structure, in color, in the 
shape of its cross-sections in various places and in the shape 
assumed by each hair. In structure a hair consists of a firmer 
cortex of varying thickness enclosing a softer medulla; a 102 
THE HISTORY OF THE HUMAN BODY 
single layer of epidermic cells covers the cortex externally. 
Differences in color and luster are due to the amount of pig- 
ment in the cortex, the sculpture of the epidermic covering and 
the presence or absence of air in the medulla. 
The cells of the epidermic covering may fit smoothly upon 
one another or may project like scales. A typical illustration 
of this latter case is that found in wool, and by virtue of this 
peculiarity the separate hairs may be made to cling together 
by causing the minute teeth to interlock, a result effected 
throught the act of spinning. To this peculiarity the possi- 
bility of wool as a textile fabric is due. 
In Man there is much racial variation in the hair of the 
head, a character of considerable value in ethnology. The 
degree of waviness or curliness is due to the shape of the 
single hairs; if they are cylindrical, that is, circular in cross- 
section, they are perfectly straight, as in the typical Mongo- 
lian ; a slight degree of flatness with an elliptical cross-section, 
allows the hairs to become wavy, as in many Europeans; 
if more flat, they are curly, and if very flat, the hairs are 
woolly, as in the Negroes. In this last class there are two 
subdivisions, the Eriocomi, where the hair is evenly dis- 
tributed, making a solid mat, and the Lopliocomi, the “ che- 
veux en grains de poivre,” in which the hair is collected 
into little tufts with partings between them. ' This latter pe- 
culiarity is seen in adult Bushmen and Hottentots and in the 
children of most other negro races. 
The degree of flatness of the cross-section is expressed by 
an index in which the longer diameter is considered unity 
and the shorter is compared with it in the form of a decimal 
fraction. Thus, in a perfectly cylindrical hair, the index 
would be ioo, in one in which the breadth of the oval is half 
the length the index would be 50. As a matter of fact there 
is no index so high as 100, but it ranges between 85, that of 
the Japanese, and 40-50, that of the Hottentots. In Euro- 
peans it varies between 62 and 72. In length the hair varies 
greatly, straight hair being the longest and woolly hair the 
shortest. In races with either extreme (straight or woolly) THE INTEGUMENT AND THE EXOSKELETON 
103 
the hair of the two sexes is of equal length, but in those with 
wavy or curly hair that of the female considerably exceeds in 
length that of the male. 
The hair exhibits a definite slant or direction of growth, 
which varies in different parts of the mammalian body, so that 
one may speak of hair-streams or hair-currents. This direction 
is the one shown by the follicle and by the hair immediately 
after its emergence from the skin, and is entirely unrelated to 
the various directions which the free masses may temporarily 
assume under the influences of gravitation, wind, or other ex- 
ternal forces. It is thus best seen in animals with a coat of short 
appressed hair, like horses or short-haired dogs, and is often 
quite obscured in those with long hair, or in those with soft, 
plush-like fur, like seals and moles. In these latter, however, 
it may be accurately revealed by shaving or clipping the hair. 
In general it may be said that a given area shows a defi- 
nite direction, the lines of which may be parallel or some- 
what divergent, two adjacent areas being separated either 
by a parting, where the streams diverge from one another, or 
by a raised crest or seam where they converge. At certain 
points special features may be noticed, the most important 
of which are the vortex or whorl, the rhomboid and the feath- 
ering. In the vortex variously directed hair currents unite 
to form a spiral figure, which either converges to form a 
central tuft, convergent vortex, or starts at the center and 
diverges, divergent vortex. The first type of vortex often 
marks a point at which some projecting organ is later to ap- 
pear, as at the corners of the forehead in the calf before 
the appearance of the horns; or else one where a former pro- 
jecting organ has disappeared, as at the umbilicus indi- 
cating the former presence of the umbilical cord; but, on the 
other hand, there are numerous instances where such a rela- 
tionship cannot be established. The significance of the second 
type is unknown. Either type may form either a right- or a 
left-handed spiral (clockwise or contra-clockwise). A rhom- 
boid is an open space of the shape designated by the name, 
and appears where the corners of four areas meet. It is thus 104 
THE HISTORY OF THE HUMAN BODY 
The black lines designate the lines of parting, the arrows show the direction )t 
the hair currents. Rhomboids and vortices are also shown. 
Fig. 28. Hair direction in human fetus. [After Voigt.] THE INTEGUMENT AND THE EXOSKELETON 
105 
always so arranged that the hairs converge at two opposite 
corners and diverge at the other two. A feathering is a spe- 
cial form of area, usually more extensive than the two last, 
occurring only in association with a vortex, of which it forms 
a continuation in one direction. It is in the form of a long 
and narrow ellipse, and the hair currents run along a central 
axis and diverge to the margin. 
All of the above forms may be readily seen upon our do- 
mestic animals, and are often well marked in man, especially 
in individuals whose skin is covered with very short appressed 
hairs. An especially good object is the broad, square chest 
of the bull-dog, on which are usually three vortices and three 
rhomboids; a vortex above and a rhomboid below in the me- 
dian line, a lateral rhomboid on each side of the vortex, and 
a lateral vortex on each side of the rhomboid. Aside from 
these there occurs a convergent vortex on each elbow, usually 
one on each side of the neck, and upon the hinder parts a 
pair of especially conspicuous vortices, above which, at the 
base of the tail, are two rhomboids. Individual variation may 
show departures from this description. 
In Man the various features are present and often well 
marked, but as they require for their expression a certain 
grade of pilosity, they are usually overlooked. Here, also, 
as in other animals, there is considerable individual variation, 
and a feature marked on one person may be absent on another; 
the two sides, also, are not necessarily symmetrical. The 
most conspicuous vortex is the one at the crown of the head, 
easily observed in boys with short hair. This may be either 
clockwise or contra-clockwise, and seems to follow no rule 
in this respect. Other vortices occur above the angle of the 
jaw and in front of the axilla. Rhomboids occur along 
the mid-ventral line; one of them is situated at the angle 
between the throat and the chin, immediately above the thy- 
reoid protuberance, a second at the anterior end of the sternum, 
and a third on the abdomen, midway between the umbilicus 
and the pubic eminence. A rhomboid is found constantly upon 
the lower part of the ulna, a little above the wrist. 106 
THE HISTORY OF THE HUMAN BODY 
The study of hair direction has excited an occasional inter- 
est among- morphologists, and a number of theories have been 
advanced to explain the origin of the various features, but 
there has been as yet too little morphological work in this field 
to allow much theorizing or to serve as a basis for definite con- 
clusions. The general tendency of the hair to slope backwards 
from the point of the nose to the end of the tail suggests the 
influence of the air-currents upon a rapidly moving body, or 
at least an adaptation to them, the same phenomenon being 
strikingly exhibited by the direction of feathers in birds, and 
that of scales in reptiles; in the same way the general down- 
ward slope of the hair along the sides of quadrupeds suggests 
the influence of gravitation, especially when taken in connec- 
tion with the apparent hair direction in the sloth, which shows 
a parting along the mid-ventral line from which the hairs are 
directed ventro-dorsally, as if in correlation with the custom- 
ary inverted position of the animal. In opposition to this, 
however, it may be pointed out that in certain areas the di- 
rection is the reverse of that which either of the above forces 
would produce, and as for the case of the sloth, the direction 
observed may be that assumed by the long hair after emerging 
from the surface, since the direction of the follicles seems 
never to have been investigated. Darwin’s well-known at- 
tempt to attribute the hair direction on the human arms to the 
direct influence of tropical rains upon the arms of simians, 
when held above the head for protection, is at variance with 
the facts, and must be dismissed from the discussion. 
Recently a new line of explanation has been sought in the 
influence of underlying parts, especially that of the sub-cutane- 
ous muscles, the constant traction of which influences the hair 
follicles over definite areas, but this idea cannot as yet be con- 
sidered to have passed the stage of a vague hypothesis, 
especially since many of the observations are fallacious, and 
hence have no weight in establishing the conclusions. 
It seems likely, since the hairs originated in association 
with a complete coat of scales and at a time which must be 
designated as premammalian, and since the original hair direc- THE INTEGUMENT AND THE EXOSKELETON 
107 
tion must have been the same as that of the scales which pre- 
ceded them, that this original direction would have been 
retained after the loss of the scales, and that the hereditary 
transmission of this may account for at least a general plan 
underlying the variations occurring in the mammals of the 
present day. The existence of individual variations, known 
to be considerable in man and certain domestic animals, points 
to a diminution of the original functional importance, which 
has become no longer sufficient to retain the various features 
at a definite standard. 
Aside from the formation of horny scales, feathers, and hair, 
the epidermis produces numerous other organs composed of 
keratin, and fitted for various uses. The most of these appear 
as isolated instances to subserve a particular purpose in a 
small group of animals, but in one case, that of claws or 
nails, the organs are possessed by both Sauropsida and Mam- 
malia and form a strictly homologous series throughout, which 
presents some interesting modifications. 
The first employment of this substance in this locality 
appears to be in the dog-fish, where the fins are lengthened 
beyond the limits of the fish skeleton by numerous horn 
threads, set close together and forming two series, overlapping 
the cartilaginous rays on each side. Otherwise there is little 
use of keratin among fishes and almost none at all among 
amphibians, unless there be included a certain form of wart 
found in toads and due to the local thickening of the stratum 
corneum. One species of salamander also (Siren) possesses 
horny plates in the mouth, serving the purpose of teeth. In 
turtles, a dorsal carapace and a ventral plastron are formed 
from parts of the endoskeleton, with the addition of dermal 
elements, and these are covered by large plates of keratin, the 
so-called “ tortoise-shell.” The jaws of the same animal are 
also covered with horny plates equipped with a sharp cutting 
edge, and a precisely similar formation produces the charac- 
teristic beak of birds, although it is hardly to be supposed that 
the two structures are genetically connected. Aside from 
the coat of imbricated scales, many reptiles possess horns. 108 
THE HISTORY OF THE HUMAN BODY 
crests and other cornified structures, many of which are un- 
doubtedly scale modifications; and in birds there are occa- 
sionally horny structures, often with a core of bone, like the 
spurs of the game cock, of doubtful morphological value. The 
lower legs and feet of birds are encased by a horny epidermis, 
a part of which is covered by definite scales, while other parts 
of it are divided by grooves into square or polygonal areas. 
The skin of crocodiles is marked in much the same way and 
does not form overlapping scales, yet it is highly probable that 
in both cases the areas separated are the equivalent of scales, 
since overlapping is not a necessary characteristic of these 
organs. 
In mammals there are many special organs composed of 
keratin. The “ whalebone ” of whales is derived from the 
epidermis of the hard palate and forms a thick fringe which 
hangs from the upper jaw and is employed as a strainer. 
There are three types of horns: that of the rhinoceros, formed 
by a coalescence of numerous keratin fibers, probably the 
morphological equivalent of hairs; the hollow type found in 
some ruminants, in which a hollow keratin structure is fitted 
over a core of bone; and, thirdly, the solid horn of deer and 
antelopes, where the final structure is composed of the bony 
core alone, the epidermis being represented by the “ velvet,” 
an external covering which atrophies after the horn is com- 
pleted and is rubbed off by the animal. Thus this last, in its 
final condition, that of an antler, cannot be counted among 
epidermic structures. 
In reptiles, birds and mammals the ends of the digits are 
armed by horny structures, strictly homologous throughout, 
although variously denominated as claws, nails or hoofs, in 
accordance with their shape. In the typical claw (Fig. 29, a) 
the parts to be noted are the convex dorsal plate, the concave 
ventral plate and the apical pads of the digit. In the saurop- 
sidan claw (a) the two plates are of about equal importance 
and the terminal pad is represented by an unmodified scale 
or by several scales. In the typical mammalian claw (b) the 
ventral plate is somewhat reduced and the terminal pad is THE INTEGUMENT AND THE EXOSKELETON 
109 
well developed and covered with friction-ridges. In monkeys 
(d) the dorsal plate is flatter and broader as an adaptation to 
the prehensile hand or foot and does not project much beyond 
the end of the digit; the ventral plate is much reduced in 
extent and is not very horny, and the terminal pad has de- 
creased in volume and is indicated mainly by the friction- 
ridges, which are in the form of a loop or whorl. The ex- 
Fig. 29. Diagrammatic longitudinal sections through digits of various 
mammals, to illustrate the morphology of claws, hoofs, and nails, [(a), 
after Gegenbaur; (b)-(e), after Boas.] 
(a) Echidna, (b) Typical unguiculate. (c) Horse, (d) Monkey, (e) Man. 
The dorsal plate is represented by solid black; the ventral plate is striped. The 
bones are dotted. 
treme of this line of development is reached by man (e) in 
which the last remnant of the ventral plate appears in the 
narrow strip of skin between the inner surface of the nail 
(i.e., the dorsal plate) and a terminal fold where the friction- 
ridges commence. The terminal pad is much as in monkeys. 
In the hoofed quadruped another line of development is shown 
(c) in which the ventral plate forms a horny, though rather 
soft, surface for contact with the ground. There is no pad, 
and the soft integument representing that area lies behind the 
hoof, continuous with the ventral plate. 
Glands occur in the integument of all vertebrates, profusely 
in fishes, amphibians and mammals, rarely and strictly local- 
ized in the Sauropsida. They are always derived from the 
stratum germinativum of the epidermis and vary greatly in 110 
THE HISTORY OF THE HUMAN BODY 
complexity of development and in the nature of their secre- 
tion. The principles underlying gland formation are very 
simple, and may be briefly considered in this place before 
taking up in detail their occurrence and distribution in verte- 
Fig. 30. Diagrams of various types of glands, shown as invaginations 
from a layer of indifferent epithelium. 
(a) represents a region in which certain of the surface cells are differentiated 
as unicellular glands. (b) is a simple tubular gland and (c) a simple acinous 
gland, each formed from a complex of gland cells. Tubuiar glands may become coiled 
(d), or branched (e). Acinous glands may consist of a single acinus, as in (c), 
or of several, as in (f). A still greater complexity is seen in (g), where each 
acinus possesses its own excurrent duct, all being collected into a common duct 
which leads to a single outlet. 
brate integument. The protoplasm of all cells has the power 
of storing up some form of secondary material, metaplasm, 
extracted from the materials supplied to it, and a gland cell 
differs from others mainly in the fact that its metaplasm is 
of use to some other part of the organism and that its chief 
value to the organism lies in the material which it produces. THE INTEGUMENT AND THE EXOSKELETON 
111 
Usually also a gland cell, specialized as it were in this direc- 
tion, secretes these products in greater abundance than in the 
case of other cells. As a single cell may thus have all the attri- 
butes of a gland, the simplest glands are composed of but one 
such elementary unit and are unicellular. Such simple glands 
are of extensive occurrence among animals and are generally 
used where a surface is to be kept uniformly moistened with 
some secretion, as a protection against water or air, and where 
there is no special auxiliary structure, like the eyelids of land 
vertebrates, to insure an even distribution. The majority of 
glands, however, are multicellular and represent various solu- 
tions of the problems of how to increase the physiological 
efficiency within a definite space, i.e., how to increase the 
effective secreting surface without increasing the mass. The 
diagrams in Fig. 30 represent various solutions of this prob- 
lem, as well as varying degrees of physiological efficiency, the 
most complex form being in general the most successful. Be- 
ginning with single cells opening upon a free surface it is 
evident that the efficiency increases with the number of gland 
cells in a given space, the limit of this type being reached 
when all the cells have become thus employed. If, however, 
the problem allows the utilization of a certain amount of depth 
the efficiency may become much increased by folding or 
invaginating portions of the original surface below the general 
level, either in the form of tubules (b) or flask-shaped glob- 
ules, acini (c). Each of these primary types may become 
still further complicated in several ways. The tubular form 
may become convoluted (d) or branched (e), and the acinous 
form may develop secondary acini (f). Through a slight 
cellular differentiation the cells nearest the outlet of the gland 
may become flattened and form a non-secreting duct through 
which may pass the fluid manufactured in the secretory por- 
tion. This principle may be extended to the secondary acini, 
and when these latter become profusely multiplied, the result 
is a definitely localized and very effective organ, as in (g). 
These varied forms are not sharply defined, and even the 
fundamental types of tubular and acinous glands may grade 112 
THE HISTORY OF THE HUMAN BODY 
into one another in such a way as to make the classification 
indeterminate. 
Glands may be also divided according to their method of 
furnishing the secretion, since some cells, when surcharged, 
liberate their fluid by bursting, and thus become destroyed, 
while others allow their secretion to pass through their walls, 
retaining their physiological life for an indefinite period. In 
the former case the supply of cells is kept up by a constant 
proliferation from a zone of growth; in the other case the 
community of cells retains its identity. The glands in the 
former case are termed necrobiotic, in the other they are 
vitally secretory. This physiological distinction is often of 
use in determining homologies at times when the structure is 
non-committal or misleading. 
Unlike most other structures, the integumental glands of 
vertebrates do not appear to have a continuous history in 
the various Classes, but are developed in each Class, or even 
in specific cases, to suit the needs of particular environment 
or habits. In fishes and amphibians the main function of 
integumental glands is to secrete a protective slime, to defend 
the surface from the action of the water, to which, as a 
secondary function, probably accidental at first, there is often 
added to the secretion some acrid or even actively poisonous 
quality, for defense against predaceous animals. The glands 
supplying this function are often of the unicellular type, with 
a narrowed neck at the surface, and called beaker cells from 
their shape; the simple acinous type, in the form of flask- 
shaped glands, is widely distributed among amphibians, where 
the glands often occur in clusters, causing a conspicuous pro- 
tuberance. The integument of the Sauropsida is characterized 
by an almost complete absence of glands, certain special ones 
appearing in definite localities and employed for some special 
purpose. Such are the cloacal glands of certain snakes, which 
secrete for defensive purposes a milky fluid having a nauseating 
odor, and the musk glands of certain turtles, which may be 
defensive or used as a sexual allurement. In the males of 
certain lizards a single line of glands opens along a definite row THE INTEGUMENT AND THE EXOSKELETON 
113 
of scales on the inner aspect of the femora; at the time of pair- 
ing these secrete a gummy fluid which hardens into short spines 
or teeth, employed during copulation. In birds the sole in- 
tegumental gland is the uropygial, a compound mass situated 
‘Stratum corneum 
Stratum lucidum 
Stratum mucosum 
Epidermis 
Corium 
Fig. 31. Typical mammalian hair with its accessory parts. [After 
Weber.] 
Gl. ac., acinous gland, usually sebaceous; Gl. tb„ tubular gland, usually per- 
spiratory in function; M, ar. pil., arrector pilarum muscle. 
on the dorsal aspect of the tail rudiment, and secreting an oily 
fluid used for anointing the feathers. 
In mammals, the skin is, as a rule, profusely glandular, and 
the glands possess the highest degree of physiological dif- 
ferentiation. In spite of their diversity, however, they may 114 
THE HISTORY OF THE HUMAN BODY 
all be referred to two primary forms, each originally de- 
veloped in association with a hair. This primitive condition 
is still common, though often with some slight modifications, 
and is shown in diagrammatic form in Fig. 31. The glands 
arise as outpushings from the wall of the follicle, into which 
they empty. One of these types is a long, slender tube, often 
convoluted at its free end, a tubular gland; the other short 
and somewhat lobed, an acinous gland. Aside from this 
morphological distinction the tubular gland is vitally secre- 
tory, the acinous necrobiotic. As a secondary modification 
either type may exist without a hair, but such cases are excep- 
tional. It seems likely that such a complex as that represented 
in the figure was originally associated, as are the hairs, with 
the primary scales, one for each, the glands being associated 
with the median hair only, but this cannot as yet be definitely 
asserted. 
Each of the two glandular elements is capable of great mod- 
ifications, both morphologically and physiologically. The 
tubular type is not always convoluted, but may be straight 
and simple, or branched. Its characteristic secretion is a thin, 
colorless, watery fluid, the perspiration or sweat; this is 
viscous and reddish in the hippopotamus and albuminous and 
of a blue color in Cephalophus, a South African antelope. A 
much modified form of these glands furnishes the thick and 
oily ear-wax. In distribution these glands are often found 
over the entire body (hippopotamus, bear), but may be 
strictly localized, as in most rodents, where they are found 
mostly on the ventral surface of the paws. They are entirely 
wanting in the two aquatic orders of Sirenia and Cetacea, also 
in Manis, in a sloth (Choloepus), and an insectivore (Chryso- 
chloris). They are usually found on the palmar and plantar 
surfaces, where, in man and the monkeys, their openings are 
readily seen at regular intervals along the middle line of the 
friction ridges. In this location, by moistening the ridges, they 
undoubtedly assist the firmness of the grasp, often a factor of 
vital importance in an arboreal animal. When distributed 
over the general surface and yielding the customary colorless THE INTEGUMENT AND THE EXOSKELETON 
115 
watery fluid, these glands are usually called sweat glands 
(glandules sudoriparce), but enough has been said to show that 
this term is too limited to be employed in general. In Man, 
where they fulfill this function, they do not appear to be 
evenly distributed, and there seem to be individual differences 
in the extent and copiousness of the secretion. As it has been 
found that these glands rarely occur in the integument of the 
Fuegians, there are undoubtedly marked racial differences, 
but there is, at present, little knowledge upon this point. 
The primary use of the second, or acinous, type of integu- 
mental gland seems to be to furnish an oily secretion for the 
lubrication of the hair, forming the sebaceous glands; and cor- 
responding to this use, they appear less inclined than does the 
other type to become disassociated from a hair follicle. 
Modified forms do occur, however, often unconnected with 
hair follicles, and modified in their secretion to subserve some 
special use. Such are the tarsal (meibomian) glands of the 
human eyelid, which are properly the hypertrophied sebaceous 
glands of the eyelashes, whose purpose it is to supply a line 
of oil for the edges of the lids and thus prevent the overflow 
of tears. Other modified forms of this type are found at the 
orifices of the body, as the lips, the anus, and upon the ex- 
ternal genitals (Tyson’s glands, preputial glands, etc.). Cor- 
responding to their primary function as sebaceous glands, 
they are wanting in Cetacea and in adult Sirenia; the scale- 
covered Manis retains only the modified orificial glands. They 
are, however, wholly wanting in some sloths (Choloepus) and 
in an African insectivore (Chrysochloris), the same animals 
in which the glands of the tubular type are also wanting. 
Aside from these small, generally distributed, glands, the 
integument of mammals is especially characterized by the oc- 
currence of localized masses of glands, often voluminous in 
size and furnishing a secretion intended for a special purpose. 
The elements of which these masses are composed are some- 
times of the tubular and sometimes of the acinous type, or of 
both sorts together. Of these the anal sacs are widely dis- 
tributed, composed mainly of tubular glands, and forming 116 
THE HISTORY OF THE HUMAN BODY 
from two to five sacs which open into the rectum immediately 
within the anus, from which in some cases they may be pro- 
truded by being turned inside out. The best known of these 
are the two lateral ones of the skunk, weasel, and allied forms, 
which are covered with a muscular layer derived from the 
levator ani muscle, and secrete an ill-smelling fluid as de- 
fense. Other similar glands secrete odoriferous fluids em- 
ployed as a mode of sexual attraction, some of which are 
agreeable to man and are used in the manufacture of perfumes 
(musk, civet). 
In their simplest form such glands open separately, but near 
together, the surface covered by the opening being designated 
as a glandular area, usually free from hair or nearly so; but in 
many cases this glandular area becomes depressed and forms a 
sac or bursa sunken beneath the surface and serving at times 
as a reservoir for the secretion. It is as such a structure that 
the mammary or milk glands, so characteristic of the Class 
of Mammalia, have arisen, and their appearance in the mono- 
treme, Echidna, exhibits nearly their original condition. The 
female of this animal possesses upon the ventral side an 
integumental pouch, the marsupium, in which the eggs are 
placed, and in which the young are nurtured when hatched. 
Opening into the sides of this pouch is a pair of glandular sacs 
or pouches, supplied with glands which are probably of the 
tubular type, although long supposed to be acinous. These 
sacs are the mammary pockets, at the bottom of which lies the 
glandular area with its numerous openings. The secretion, 
which is a form of milk, pours out into the pockets, where it 
is taken up by the young. There are no traces of nipples, but 
the lips of the sac, the corium wall, fit around the nose of the 
young and prevent loss (Fig. 32, a). A slight advance is 
seen, in the young Halmaturus, a marsupial, where the 
mammary pocket is deeper, and a rudimentary nipple is 
formed by the elevation of the middle of the glandular area 
at the bottom (d). This structure is still further developed 
in the young opossum (e), and in this latter animal the 
functional activity of these organs causes the complete ex- THE INTEGUMENT AND THE EXOSKELETON 
117 
trusion of the nipples, and the mammary pocket is lost (f). 
This type of nipple, in reality a mammary pocket turned in- 
side out, is the most usual among the higher animals. In it 
the glandular area forms the nipple itself, while the corium 
wall, the rampart-like lip of the pocket, becomes the areola, 
a circular area of modified skin surrounding the nipple. Quite 
another type of nipple, though derived equally with the former 
from the mammary pocket of the Echidna, is that occurring 
(a) Primary condition, as in Echidna. (b) Embryo calf, comparable with the 
condition seen in (a). (c) Cow (adult) showing “ false ” nipple produced by the 
prolongation of the cutis wall, (d) Halmaturus (a marsupial) previous to lactation, 
(e) Didelphys (a marsupial) previous to lactation, (f) Didelphys during lactation, 
showing “ true ” nipple, produced by the eversion of the mammary pocket. 
In all the figures the area glandularis, i. e., the surface of the mammary pocket, 
is represented by a dotted linej _the cutis wall by a full line. The branching lines 
opening either at the bottom of the depression or at the summit of the elevation, are 
the milk glands. 
Fig. 32. Morphology of nipples. [After Weber.] 
in ruminants. In this the glandular area remains at the 
bottom of the pocket, while the surrounding corium wall be- 
comes elevated more and more until a long pendulous nipple 
is formed from that (Fig. 32, b and c). The mammary- 
pocket is here retained as a long lactiferous duct running 
through the nipple. In many placental mammals the earliest 
embryonic indication of the mammary glands consists of a 118 
THE HISTORY OF THE HUMAN BODY 
lateral ridge, extending from axilla to groin, occupying the 
position of a future row of nipples. By the suppression of 
this mammary ridge at regular intervals there arises a series 
of elevations which at first sight appear to be the nipples, but 
which become secondarily reduced and eventually come to 
form actual depressions. These are evidently the ontogenetic 
repetitions of the mammary pockets, since from these, after 
the manner detailed above, the true nipples arise, faithfully 
repeating the stages shown in Fig. 32. The mammary ridge 
perhaps represents the wall of the marsupium or pouch, thus 
suggesting that the placental mammals have been derived 
from ancestors which possessed a marsupium. The conclu- 
sion is not necessary, however, that these ancestors were the 
Didelphia of the present day, but that the common ancestors 
of both modern types of mammals, marsupial and placental, 
possessed this organ, and that the Didelphia have retained it 
as a functional organ, while in the Monodelphia but few traces 
remain. The occurrence of a marsupium among the mono- 
tremes, the only living representatives of the Prototheria, 
points to the same thing. (See Fig. 8, and the accompanying 
text.) 
The number and position of the nipples vary much in the 
different groups of monodelphic mammals, and furnish a 
series of illustrations of adaptation, both to the habits of 
life and the number of the young. In that type which ap- 
pears to be the most primitive, there is a series of nipples 
arranged in a lateral row upon either side and extending 
from axilla to groin. In this case, as in pigs, most carnivora, 
and many rodents, the animal lies upon one side while nurs- 
ing. By the suppression of the anterior end of this series 
inguinal mammae are produced, as in ungulates, which nurse 
their young while standing erect. In the Cetacea the single 
pair of inguinal nipples lies in the bottom of a pocket, not 
the primitive one of the Echidna, but one secondarily de- 
veloped to solve the problem of nursing under water. The 
lips of this pocket fit tightly about the snout of the young, 
which can suckle beneath the surface being at the same time THE INTEGUMENT AND THE EXOSKELETON 
119 
able to breathe through the nostrils, which in these animals 
have migrated backward from their primary position. By a 
suppression of the posterior portion of this series, pectoral 
mammae are produced, either two pairs, as in some lemurs, 
or a single pair, as in the majority of primates, and in bats. 
In the aquatic Sirenia, also, the mammse are pectoral. As 
they bear but a single young at a time and nurse it by clasping 
it in the flippers while standing upright in the water, these 
animals, as suggested by the name, are probably the real 
origin of the well-nigh universal mermaid myth. The pectoral 
position is the most convenient for arboreal animals like 
the Anthropoidea and enables them to carry the offspring in 
one arm and leave the other free for climbing. 
In animals with a restricted number of mammae there 
have been frequently observed cases of supernumerary nipples 
or supernumerary mammae. These cases are termed respec- 
tively hyperthelism and hypermastism, and are looked upon 
as atavistic of the former development of a complete series, 
of which those normally developed form a part. They are 
often noted in the adult human subject, and the anlagen of 
numerous pairs of nipples occur regularly in the embryo. 
The occurrence of six-nippled sheep that have a tendency 
to cast two young at a birth has been recently made the sub- 
ject of experiment with a view to perpetuating the latter 
peculiarities, and thus form a race adapted to countries with 
a short summer, like Canada. Whether there is a definite cor- 
relation between these two characters, or whether in Man 
there is any correspondence between hypermastism and a 
tendency to produce twins, has never been determined. 
The occasional occurrence of mammae in unusual positions, 
as on the thigh or the back, as has been noted in the human 
subject, is a displacement, and not a reversion, and hence has 
no normal morphological meaning. 
The occurrence of rudimentary nipples in the male is the 
rule among placental mammals, but seems not to be the case 
in monotremes and marsupials. If this be true, this is a 
definite instance of the hereditary transmission to the male 120 
THE HISTORY OF THE HUMAN BODY 
sex of parts that developed first in the female, and formed 
for a long time an exclusive characteristic. Since neither 
natural selection nor sexual selection could have a part in this 
transmission, it has been brought forward as a case of the 
direct transmission of an acquired characteristic. It may, 
however, be a phenomenon similar to the strange series of 
homologies of the various parts of the reproductive organs, 
treated in full in Chapter XI, where such an explanation is 
inadmissable, although at the present time no satisfactory 
one can be offered. Although usually the only parts of the 
mammary apparatus to occur in the male are the rudimentary 
nipples, yet cases of so-called gynecomastism are known, in 
which well-defined and even .functional mammae occur in 
persons of the male sex, unaccompanied by any sexual ab- 
normality. 
Pigment is a coloring matter, occurring in the form of 
granules, and existing in certain cells as a form of metaplasm 
secreted by them and retained within their substance. These 
pigment cells are found in both epithelium and connective 
tissue. Pigment often occurs in the interior of the body, 
notably in the peritoneum of amphibians, where it lines the 
coelom and invests certain organs with a brown or even black 
covering. It occurs in the integument or the integumental 
structures of all vertebrates except certain white animals, and 
in albinos, the peculiarity of which consists of a total absence 
of pigment from all parts of the body. These two cases may 
be readily distinguished by observing the iris of the eye, which 
retains its pigment in normally white animals, but lacks it in 
albinos, giving the eyes a pinkish cast. As a general rule the 
integument of vertebrates is pigmented when without ac- 
cessory structures, or when these form an insufficient covering, 
but in birds and mammals, in which the feathers or hair 
are respectively sufficient to entirely conceal the skin, these ac- 
cessory parts receive the color and the integument is unpig- 
mented. This rule is further emphasized by the fact that bare 
places, like the head and neck of vultures and the ischial cal- 
losities of monkeys, or scantily covered places like the entire THE INTEGUMENT AND THE EXOSKELETON 
121 
integument of elephants and rhinoceroses, are pigmented, and 
occasionally highly colored. 
The pigment of vertebrate integument is usually contained 
in branching connective tissue cells, produced in the corium, 
but capable of wandering into the epidermis. In some cases, 
as in Man and monkeys, the stratum germinativum of the epi- 
dermis is pigmented, and varying degrees of this are respon- 
sible for the great variety of skin color in Man. Although 
the connection is hard to prove, it is a general truth that 
human races that live in the tropics are the darkest and that 
the skin grows gradually paler in races nearing the poles, 
there being a correlation in this respect between skin color and 
hair color. As instances of this, there may be recalled the sooty 
blackness of the Sudanese, and the dark color of the aborigi- 
nes of India, which may be compared with the lighter color 
of Europeans and northern Asiatics. More convincing cases 
of this are seen in representatives of the same race; such as is 
shown by the contrast between the Italians and Spaniards on 
the one hand and the Scandinavians on the other, between 
the North and South Chinese, or between the American In- 
dians in Canada and those in Mexico. A similar reduction in 
pigment is found in people living at high altitudes in com- 
parison with the same race living in the bordering lowlands. 
The importance of these correlations has been repeatedly de- 
nied, and there are numerous instances of exceptions, often 
conspicuous ones like the dark skin and hair of the Eskimo, 
and at the present state of our knowledge too much cannot be 
urged on this point. 
Another point of interest lies in the regions of the body in 
which the pigmentation is the densest. As a general rule it 
may be observed, not in vertebrates alone but in invertebrates 
as well, that the darkest and most deeply colored parts are 
those that lie uppermost, exposed to the light, while the under 
parts are lacking in pigment. That this bears no relation to 
the architecture of the body may be seen in the case of the 
flounder, a fish that is much compressed laterally and has the 
habit of lying upon one side at the bottom in rather shallow 122 
THE HISTORY OF THE HUMAN BODY 
water. This habitual lower side, which is sometimes the left, 
sometimes the right, half of the body, is entirely colorless, 
while the upper side is marked with a complicated pattern 
resembling the sandy or muddy bottom on which the fish 
lies. 
This principle is a general one and applies to the distribu- 
tion of pigment, whether in the integument itself or in its 
accessory structures, a fact that may be readily seen by com- 
paring a form with a naked skin, like a frog or a porpoise, 
with one covered with hair. In terrestrial mammals the dif- 
ference is not always a marked one and is apt to be greater in 
short-legged forms that creep close to the ground; in birds, 
which in the majority of cases expose nearly the entire surface 
to the light, the body may apparently be of uniform color, but 
here the deeper pigmentation is confined to the exposed sur- 
faces of the feathers, while the portions which are shielded 
from the sun are less deeply colored or even without pigment. 
The white ventral surface of aquatic birds like snipe and gulls 
is a protective coloring and comes under another principle. 
In man the distribution of pigment is also unequal, the darker 
areas being, in all races, the back and the dorsal aspect of arms 
and legs, while the chest and abdomen, the ventral aspect of 
the limbs, and especially the palms and soles, are lighter in 
color. This distribution, it will be noticed, corresponds to 
the influence of the light, when man assumes the quadrupedal, 
and not the usual human, position. Aside from these general 
areas, a deep local pigmentation occurs in the axilla and groin, 
about the anus and the external genitals, and upon the nip- 
ples and areola, where it is evident that the pigmentation is 
for some other purpose and can bear no relation to the distribu- 
tion of light. In this general connection between a darker 
color and the increase of the sun’s light and heat, there must 
be some physiological advantage which a dense pigmentation 
gives its possessor, an advantage which seems to be a real one 
whenever there is a chance for comparison between a black 
man and a white man in the tropics in regard to their relative 
power of enduring the heat of the sun. Although the subject THE INTEGUMENT AND THE EXOSKELETON 
123 
is still an obscure one, it seems probable that the presence of a 
dense layer of pigment in the stratum germinativum effectu- 
ally prevents the direct action of the light upon the surface 
capillaries, thus allowing them to expand and retain the blood 
at the surface where the excess of heat can be constantly 
thrown off. Thus repeated observations have been made in 
Samoa* when whites and natives have been together and doing 
the same work, that the skin of the latter would be dry and 
glowing as in a fever, while that of the former was cold and 
damp. Under these circumstances the Samoans would be 
constantly giving off heat while the whites were compelled to 
retain theirs. 
In the presentation of the above facts in connection with 
one another, the conclusion seems almost unavoidable that the 
various conditions are directly due to the solar action upon 
each individual, and to the propagation of the conditions thus 
acquired until the physiological advantages become inborn in 
the race. Although this may seem at first the simpler expla- 
nation, there are numerous biological facts associated with 
heredity that seem to render impossible so direct a transmis- 
sion of somatic peculiarities, and point to a more indirect 
method of attaining the same end through individual variation 
and the selection for survival in each generation of those in 
which the desired peculiarity is the most marked. This ex- 
planation, however, is in many points as unsatisfactory as the 
other when applied to this case, since we know that the strug- 
gle for existence in man has never been severe enough to 
compel the extinction of individuals differing from others by 
a shade of color; neither is sexual selection operative here, 
since among a primitive people all who are not physically unfit 
become the propagators of the race. The matter must be left 
at present as one in which the facts are evident but the ex- 
planation of them obscure; the problem is to be solved some- 
time, and when solved will offer an explanation of the relation 
of structure to environment everywhere. 
* According to Dr. A. Kramer. — Die Samoa Inseln, Stuttgart, 1903. CHAPTER V 
THE AXIAL SKELETON 
. . our ‘ physic ’ and ‘ anatomy ’ have embraced 
such infinite varieties of being, have laid open such 
new worlds in time and space, have grappled, not 
unsuccessfully, with such complex problems, that the 
eyes of Vesalius and of Harvey might be dazzled by 
the sight of the tree that has grown out of their grain 
of mustard seed.” 
Thomas Henry Huxley, in his essay: On the 
advisableness of improving natural knowledge. 
An endoskeleton or internal framework for the support of 
the muscles and the protection of the viscera is one of the dis- 
tinguishing characteristics of vertebrates, for with the excep- 
tion of a few sporadic cases in which internal skeletal parts 
occur, invertebrates are without such a system. The verte- 
brate endoskeleton is a part of the connective tissue system of 
the body and, in its usual sense, includes only bone and cartil- 
age, although both developmentally and physiologically the 
associated ligaments and other connective tissues belong with 
the former. 
Primarily the endoskeleton consists of three systems, 
originally distinct from one another, the axial, the visceral, and 
the appendicular. The axial includes the vertebral column 
and a large part of the skull; the visceral includes the lower 
jaw, certain elements in and about the upper jaw, the hyoid 
apparatus, and the branchial or gill arches; and the appendic- 
ular includes the shoulder and hip girdles and the skeleton of 
the free limbs. 
Of these, the axial is the oldest, and is represented in its 
simplest form by the notochord, although this organ soon 
yields in functional importance to the connective tissue sheath 
which enwraps it, from which, in higher forms, the main ele- 
124 THE AXIAL SKELETON 
125 
ments of the vertebrae are derived. The notochord, which 
is endodermic and arises from the primitive alimentary canal 
in the manner related above, is the only portion of the endo- 
skeleton not formed from the mesenchyme, and is hence not a 
Fig. 33. Diagram of vertebrate, showing relation of skeleton to soft 
parts. 
Axial Skeleton: tr, trabecula; p, parachordal; d, dermal bones of skull; nt, 
notochord; np, neuropophyses; hp, haemapophyses. 
Visceral Skeleton: m, mandible; br, branchial arches. 
Appendicular Skeleton: ga, anterior; d, its dermal element; gp, posterior 
girdle; x, anterior free limb; y, posterior free limb. 
Soft Parts: nv, nervous system; hy, hypophysis; a, aorta; v, sub-intestinal vein 
(an embryonic organ); int, intestine. 
connective tissue; but this becomes gradually replaced by 
skeletal elements formed from true mesenchymatous tissue, so 
that in the higher forms the adult skeleton is wholly from this 
latter source. The notochord seems to have been a very 
ancient form of endoskeleton', antedating that formed of con- 
nective tissue and functioning in the immediate ancestors of 
the present-day vertebrates. It is present in what may be 
nearly its original condition in Amphioxus, where it appears 
as a cylindrical rod, extending through the longitudinal axis 
of the body from end to end. This rod furnishes a certain 
degree of rigidity and allows the animal to maintain a fixed 
length, while permitting a considerable amount of flexibility 
through its elasticity. In about the same condition it appears 
as a constant organ during the early embryonic life of every 
vertebrate; and, although it is usually replaced in great measure 
by mesenchymatous elements, yet in some fishes, even in those 126 
THE HISTORY OF THE HUMAN BODY 
as high as ganoids, it retains much of its original appearance 
and function. Both in Amphioxus and in these forms, as well 
as in all embryos, it is formed of a semi-gelatinous tissue, 
often called pre-cartilage, and is surrounded by a firm sheath 
of connective tissue, which, in those adult forms with a per- 
sistent notochord, supplies the necessary firmness and rigidity 
in which particular the notochord alone would be inadequate. 
This sheath becomes, in fact, of far greater importance than 
the notochord, and the cartilaginous and osseous tissues 
formed from it come to encroach more and more upon the 
yielding tissue within, and eventually supply the main elements 
used in the construction of the vertebrae. 
The first stage in this advance is seen in the lamprey and 
other cyclostomes, where there appear pairs of little cartilages, 
lying upon the side of the notochord sheath and projecting 
upwards to protect the nerve cord, which lies along the dorsal 
side of the notochord. These little cartilages are of two kinds. 
The primary ones develop from the edges of the intermuscular 
septa, and are hence intersegmental, a point which is important 
to remember in connection with the relative position of the 
vertebrae in higher forms. A secondary set alternate with 
these, and form intercalary pieces, protecting the intervals 
between the first set. 
A second advance over the condition found in Amphioxus 
is seen in the formation of a head, which, since here the noto- 
chord no longer extends to the tip of the snout but ends a 
little behind the plane of the eyes, has been supposed to be in 
part a transformation of the anterior end, and in part a new 
formation added anterior to this. Whether this may be safely 
assumed or not, the anterior termination of the notochord 
forms in the embryo an important topographical point, the 
portion of the head along the sides of the notochord being 
referred to as parachordal, and that anterior to it as prce- 
chordal. The hypophysis, an organ lying in the median line 
and depending from the lower surface of the brain, lies at the 
anterior point of the notochord, and will thus serve to mark 
the boundary between these two portions of the head. THE AXIAL SKELETON 
127 
The next few stages in the history of these parts, lying 
between the condition above described and definite vertebrae, 
are still somewhat a matter of controversy, since, in the various 
Fig. 34. Diagrams illustrating a theory of the development of the ver- 
tebrate. 
(a) Condition previous to the formation of vertebral anlagen (caudal region). 
The body is divided into segments by transverse myocommata through which run the 
notochord, the nerve cord, and the aorta. In the region of the coelom the myocom- 
mata open ventrally and allow the alimentary canal to pass. There is no trace 
of bone or cartilage, (b) Later stage, in which skeletal bridges have formed along 
the edges of the myocommata, both dorsally and ventrally. (c) Detail of still later 
stage, in which the sheath of the notochord has chrondrified (or ossified) at the 
points where the bridges come in contact with it. (d) Completed vertebras, formed 
by the fusion of the elements shown in (c). 
types of fish, where these stages should be sought, numerous 
modifications have taken place which are to be explained as 128 
THE HISTORY OF THE HUMAN BODY 
special adaptations, admirably fitted to the habits and environ- 
ment of the various species, but covering- up the original race- 
history which we are seeking to interpret. While, then, one 
cannot be dogmatic about this portion of the history, the fol- 
lowing seems to be the most likely course of development, and 
its various stages are to be found, with some little modifica- 
tion, in living species. 
It would seem, then, that the primary pairs of neural 
processes, for the purpose of better fulfilling their mission of 
protecting the nerve cord, became more elongated until they 
finally met in the middle line above the nerve cord, thus form- 
ing a series of intersegmental neural arches, shaped like inverted 
Vs; and since the aorta, lying immediately beneath the noto- 
chord, needed a similar protection, other arches became 
developed for this purpose, situated immediately below the 
former, and with their points directed downwards. 
When these two systems of arches were well established, 
they would naturally seek to secure a firmer attachment to the 
notochordal sheath by spreading out their bases, and as the 
two sets of arches, neural and hcemal, were opposite each 
other, each neural arch being associated with its corresponding 
haemal arch, the enlarged bases would grow together. To 
support the increasing weight of these parts, the notochordal 
sheath would then chondrify or ossify beneath these bases in 
the form of rings, and the fusion of three elements, a neural 
and a haemal arch and a notochordal ring, would form a 
vertebra of the type found in the ordinary bony fish. The 
condition just described, with neural and haemal arches alike, 
is that found in the tail region, posterior to the visceral cavity, 
while in the trunk the haemal arches are open and their halves 
widely divergent, forming the ribs, which embrace the visceral 
cavity. The rings, which are formed from the notochordal 
sheath, begin in the center of the future vertebrae, and as they 
grow, expand along both edges until they come in contact 
with the preceding and succeeding ones. At the same time, 
however, the ossification has proceeded inwards as well, re- 
stricting the notochord, and as the central portion of the ring THE AXIAL SKELETON 
129 
is the oldest, that part becomes the most restricted, often com- 
pletely severing the notochord at this point, or intra-verte- 
brally; while the notochord is retained at practically its original 
caliber at the newest edges of the ring, or inter-vertebrally. 
The completed ring, which forms the body or centrum of the 
vertebra, is cylindrical, with concave ends like the interior of 
conical cups. Such vertebrae are called amphiccelous (=both 
ends hollow), and the hollows of the adjacent vertebrae enclose 
masses of notochord in the form of two cones, placed base to 
base. 
Although the above sketch is, in a way, hypothetical, many 
of the stages described actually occur as the adult condition in 
various fishes, especially ganoids, and the final condition is 
exactly shown by the teleosts, as one may have frequent occa- 
sion to observe. That the evolution of the vertebral column 
up to this point has been somewhat after the plan here given, 
may be generally conceded, and it is hoped that important 
links in the history may be discovered in the field of palaeontol- 
ogy, which has already furnished us so many valuable records 
and bridged so many gaps. 
Above the fish the development of the vertebral column has 
been, not so much in the acquirement of new elements, as in 
the regional modification of those already possessed. This is 
strikingly shown by the comparison of the vertebral column 
of a fish with that of a reptile or mammal; in the first of these 
the vertebrae are all very much alike, while in the second they 
are differentiated into successive groups, and in cases in which 
this differentiation has reached its extreme each vertebra may 
be sufficiently unlike the rest to be identified by the anatomist 
when isolated, a feat impossible in the former case. 
This regional differentiation is due chiefly though indirectly 
to the change of environment from water to land, a change 
which necessitates the replacement of soft and weak fins by 
two pairs of firm limbs, and substitutes for the evenly dis- 
tributed buoyancy of the water a fixed support at two points, 
the shoulder and hip girdles. In the first land animals the 
limbs were small and weak, and progress was attained through 130 
THE HISTORY OF THE HUMAN BODY 
a sinuous motion of the body. Even here, however, the influ- 
ence of the limbs would be felt, since they would cling to the 
surface and thus furnish definite fixed points where the sinu- 
ous motion of the vertebrae would be lessened. By a gradual 
increase in the size and strength of the limbs, the animal would 
attain the power of crawling, that is, of dragging the body 
over the ground through the action of the limb muscles as well 
as those of the back, and finally the limbs would become strong 
enough to bear the entire weight and the body would be lifted 
wholly above the surface of the ground, thus changing the 
crawling motion into a true walk. This gradual development 
of the free limbs is accompanied by important correlated 
changes in the vertebral column, due in the main to two causes. 
The first of these is the increased size and functional impor- 
tance of the limb girdles, or those parts of the limb skeleton 
enclosed within the body, to which the free part is movably 
attached, usually by a ball-and-socket joint; and the second is 
the increase in size of the proximal limb muscles. As the 
limbs become larger and stronger, their girdles, i. e., the proxi- 
mal portion of their skeleton, feel the need of a stronger sup- 
port and a more intimate association with the axial skeleton, 
a need especially felt by the hip-girdle, since here the greater 
weight is sustained. This girdle, seeking the necessary sup- 
port, grows dorsally around the body, until it meets the ends 
of a pair of ribs with which it articulates. In the lower 
forms the ribs are very short and are borne upon the end of 
short transverse processes, and the girdle with its dorsally 
developing process, known as the ilium, the rib, the transverse 
process, and the vertebra, form a complete chain around the 
body. 
At first this association involves but a single vertebra, the 
location of which is apt to vary. Thus in Necturus, the most 
primitive salamander now in existence, the hip-girdle is usually 
attached to the 19th vertebra, counting from the head, but the 
20th is occasionally employed instead, and two cases have been 
reported in which the attachment was to the 18th. Cases are 
also known in which the attachment is oblique, either to the THE AXIAL SKELETON 
131 
19th on one side and the 20th on the other, or to the 18th on 
one side and the 19th on the other. 
In these low forms this sacral vertebra shows no special 
modification save that it may be slightly stouter than its fel- 
lows, and have longer transverse processes and stouter ribs, 
but as the posterior limbs increase in size and functional power 
their girdle increases with them and may form similar attach- 
ment to two or more adjacent vertebrae, which may become 
more and more modified and form a more or less complete 
Fig. 35. Variations in the composition of the human sacrum. [After 
Gegenbaur.] 
fusion into a single piece, the sacrum. This anchylosis of 
adjacent sacral vertebrae is the most complete in birds and in 
Man, and for the same reason, namely, the employment of 
the hind limbs alone for the support of the body; although in 
the two cases the number and arrangement of the associated 
parts differs very considerably. 
Variation in the sacral region is not confined to the lower 
forms, although it is more frequent in these latter (e. g., 
Necturus) and becomes relatively stable in the higher and 
more specialized classes. As shown in Fig. 35, there is varia- 
tion both in the point of attachment of the hip bones and in 
the number of vertebrae involved in the composition of the 
human sacrum, and similar variations have been noted in other 
mammals. These, like the variation in the number of ribs and 
in the groups of vertebrae, not infrequent in the human subject, 
should serve to dispel the idea that the body of man or any 132 
THE HISTORY OF THE HUMAN BODY 
other animal is formed in accordance with a definite pattern 
or is constructed upon any other principle save those of hered- 
ity and environment. 
The anterior, or pectoral, girdle never becomes directly at- 
tached to the vertebral column, and consequently the latter 
receives no direct modification through the development of the 
former, but the use of this region as a secondary center of 
support causes a division of function between the vertebrae 
that lie anterior and posterior to it, the first forming the neck. 
By the establishment of this point and the sacrum, the two 
centers of support, the vertebral column becomes divided into 
regions, the differentiation of which depends upon the degree 
of development of the limbs and the amount of difference in 
the function to which these parts are subjected. Beginning 
anteriorly the vertebrae anterior to the first center of support 
are the cervical or neck vertebrae, the first one or two of which 
are especially modified to bear the head and allow of its special 
motions. The vertebrae between the shoulder-girdle and the 
sacrum are spoken of in general as the trunk vertebra, and in 
birds and mammals allow a further subdivision into thoracic 
and lumbar, the former being provided with free ribs, and the 
latter being without them. Then follow the sacral vertebra, 
usually more than one in forms above the amphibia, followed 
by the caudal vertebra or tail. The correlation between the 
regional differentiation and the development of the limbs is 
especially emphasized by such forms as the whales, which have 
secondarily lost the hind limbs, and snakes, which have lost 
both pairs, since in the former the deprived region, and in the 
latter the entire vertebral column, have lost all trace of such 
differentiation. 
The second cause of modification of the vertebral column is 
correlated with the first and is directly due to the increase in 
size of the limb muscles and their consequent need of broader 
and stronger points of origin. The limb muscles, in the case 
of animals with well-developed limbs, are usually broad, fan- 
shaped sheets, like the trapezius and latissimus dorsi, attached 
wholly or in part to certain processes of the vertebral column, THE AXIAL SKELETON 
133 
and cause much local differentiation in the varying degrees of 
development of these processes. Although topographically 
related to the trunk, and classed with trunk muscles in works 
on anatomy, they belong morphologically to the limbs, and 
when these latter are small, as in salamanders, the muscles are 
small also, seldom extending to the vertebral column, and 
thus exercise little or no modifying influence upon it. 
Other modifications of the vertebral column are due to 
the movement of the body as a whole and to the separate and 
more or less specialized motions of the head and tail. Thus, 
to perform the crawling movement of salamanders and most 
reptiles, where a sinuous motion of the body axis forms the 
principal mode of locomotion, there must be a large amount 
of flexibility, especially in regard to lateral movements, be- 
tween the separate vertebrae; and thus the amphiccelous form 
of intervertebral articulation, the restricted motion of which 
proves sufficient for fishes, becomes converted into true ball- 
and-socket joints by the ossification (or chondrification) of 
the ball of notochord contained in the cavities between each 
pair of adjacent cups, and by its anchylosis to one of the con- 
tiguous vertebrae. This forms the ball; the unmodified cup- 
shaped end of the other vertebra serves as a socket. The 
anchylosis of the notochordal balls may take place with either 
the preceding or the succeeding vertebra; in the former case 
each vertebra of the series will have the cup at its anterior, 
and the ball at its posterior end, forming the type known as 
proccclous, while in the latter case the reverse condition is the 
result, such vertebrae being designated as opisthoccelons. 
Both of these conditions are common among amphibians 
and reptiles, but with the attainment of limbs sufficiently stout 
to entirely sustain the weight of the body such a flexibility of 
the vertebral column is not only unnecessary but becomes a 
positive detriment, and thus in mammals the vertebras become 
accelons, that is, the articulations are reduced to mere flat 
contact surfaces, and the notochordal balls are transformed 
into the intervertebral cartilages that serve as cushions. In 
the cervical vertebrae of many mammals the opisthocoelous 134 
THE HISTORY OF THE HUMAN BODY 
type of articulation is retained. In birds even this restricted 
motion is undesirable except in neck and tail, owing to the use 
of the entire body as an air-ship which must be held in a rigid 
position, and the trunk-vertebrae ossify into two completely 
anchylosed pieces, the first including the thoracic, and the sec- 
ond the lumbar, sacral, and a part of the caudal, vertebrae. 
The head is responsible for many modifications of the verte- 
bral column, developed in part in response to the necessity of 
turning it in all directions, and in part to the need of lifting 
it from the ground, or even sustaining it above the level of the 
rest of the body. Like many others, these problems are as- 
sociated with a terrestrial environment and are not experienced 
by fishes, in which the main endeavor is to retain the head in 
a rigid state as the direct anterior extension of the body axis, 
since a pliant head would render a change of direction while 
swimming of almost momentary occurrence and would entirely 
forbid those quick, arrow-like propulsions upon which most 
fishes depend for safety and for the successful pursuit of their 
prey. In the first experience of a terrestrial life, however, all 
this becomes changed. The turning of the head not only gives 
an increased power of observation, but is necessary in attack 
and defense, and thus the vertebrse lying between the skull 
and the place of support for the anterior limbs become differ- 
entiated to form a cervical or neck region, the main en- 
deavor of which is to gain flexibility and increase the mobility 
of the head. 
Although in some of the higher terrestrial vertebrates this 
power is but little used, in others it develops to an extraordi- 
nary extent, notably among the birds, in which this is the 
only part of the vertebral column, except the tail, to which 
motion is allowed. Here, in some instances, the neck not only 
becomes extremely flexible, but greatly elongated, accompanied 
by extraordinary modifications of the trachea and the blood- 
vessels, in order to accommodate themselves to the rapid 
changes of shape and position of which the neck becomes 
capable. 
On the other hand, certain mammals, like the whales and THE AXIAL SKELETON 
135 
porpoises, and the dugongs or sea-cows, which, having de- 
scended from terrestrial ancestors, have become secondarily 
adapted to an aquatic life, form a remarkable corroboration 
of the statement that a movable neck is incompatible with a 
natatory habit, since in these the seven cervical vertebrae, typi- 
cal of the Mammalia, have become greatly flattened antero- 
posteriorly, and are either fitted so closely together that but 
little motion is possible between them, or are even anchylosed 
into a single piece, thus not only reducing the length of the 
neck region and approximating the head to the shoulders, 
but depriving it of motion, two important piscine charac- 
teristics. 
Not only are the intervertebral articulations in the cervical 
region extremely flexible in general, but that of the first with 
the skull and the first with the second become especially modi- 
fied, changes which often profoundly affect the shape of these 
vertebrse. The first of these articulations is a double modified 
ball-and-socket joint, the protuberances, or occipital condyles, 
occurring upon the occipital region of the skull along the 
lateral edges of the foramen magnum, and fitting into saucer- 
shaped depressions on the anterior face of the first vertebra, 
or atlas. In Amphibia and Mammalia these condyles are wide 
apart and distant from one another, while in reptiles and birds; 
they coalesce in the mid-ventral line and form what appears 
to be a single median condyle, the two components being 
usually indicated by a median groove. In all cases the motion 
between the skull and the atlas is in one plane only and 
imparts to the head the bowing motion. 
The turning from side to side is effected by the articulation 
of the first vertebra with the second, and is due to a curious 
modification by which the body of the first vertebra remains 
disconnected from its own neural arch and anchyloses with 
that of the second vertebra, the axis, forming its pivot-shaped 
odontoid process, around which the ring-shaped atlas may 
rotate. This typical relation of the first two vertebrse, occur- 
ring in reptiles, birds and mammals, is modified in amphibians 
through a secondary inclusion of the elements of the atlas 136 
THE HISTORY OF THE HUMAN BODY 
within the skull, leaving the axis with its pivot to serve as 
the first free vertebra. This bone secondarily acquires artic- 
ular surfaces to articulate with the lateral condyles. 
In animals whose limbs are strong enough to sustain the 
body above the ground the weight of the head and the 
necessity of holding it in place beyond the anterior center 
of support causes considerable modification of the vertebrae, 
the influences sometimes reaching beyond the middle of the 
body. In these cases the head is held up in part by muscles, 
but in mammals there is also an important auxiliary appa- 
ratus in the form of a strong ligament, the ligamentum 
nucha?, which extends between the occipital region of the skull 
and the spinous processes of the cervical and dorsal vertebrae 
on a principle similiar to that of a check .rein. 
In mammals with large and heavy heads, especially when 
the weight is augmented by voluminous horns or large tusks, 
the weight sustained by this ligament becomes enormous, and 
not only is the ligament developed in proportion, but so, also, 
are the occipital crests and the spinal processes of the 
vertebrae, which serve it as points of attachment, the pro- 
cesses especially involved being those of the anterior dorsal 
region opposite the shoulders. This correlation between a 
heavy head and exaggerated spinous processes is such that 
from a slight indication of the one in a fossil the other may 
be assumed. In the Cetacea, which have the buoyancy of 
the water to assist them, and in Man, through the erect po- 
sition of whom the head becomes almost perfectly balanced 
upon the summit of the vertebral column, this entire appara- 
tus, including the ligament and its points of attachment, be- 
comes much reduced, but from a totally different cause in the 
two instances. 
The tail, or post-sacral region of the vertebral column, is 
developed strictly in correlation with the needs of the animal 
and varies in development from a voluminous portion of the 
body, containing a large number of vertebrae and furnished 
with metameric muscles, to a mere rudiment, invisible ex- THE AXIAL SKELETON 
137 
ternally. Examples of the former may be seen in salamanders 
and in many snakes, in which the caudal region, that posterior 
to the cloacal orifice, may be even more extensive than the 
remainder of the body; the opposite condition appears in the 
frog, where the long caudal notochord of the tadpole becomes 
in the adult consolidated into an unsegmented urostyle, situ- 
ated between the two elongated ilia and entirely enclosed by 
the soft parts. Similar reductions are found in birds, in which 
the tail skeleton consists of six free and six anchylosed verte- 
brae, and in the higher anthropoids, in which the 3-5 embryonic 
vertebrae become in the adult consolidated into a single piece 
(coccyx). 
There are two distinct sets of ribs developed among Ver- 
tebrates, having a slightly different history, but subserving the 
same general purpose, that of protecting the viscera, and of 
furnishing attachments for the muscles. Since one set is suf- 
ficient for use in the same animal, both do not occur simul- 
taneously save in a single instance, but the one set is, with some 
exceptions, characteristic of fishes, the other of higher forms. 
The origin of the first of these sets has been already explained 
in the discussion of vertebrae, where they were described in 
teleost fishes as expansions of the lower or haemal arches. 
The other ribs have in their origin no direct connection with 
the vertebrae, that is, they are not derived from portions of 
them, but develop from the free edges of the intermuscular 
septa, the myocommata, where they border upon the visceral 
cavity. This process of rib formation does not necessarily 
involve the entire free edge of the septa, but is confined in 
lower forms to the extreme dorsal region, bordering on the 
vertebrae, with which they articulate. Thus in selachians, 
almost the only fish that possess this sort of rib, and in amphib- 
ians, they are very short, being functionally scarcely more 
than movable tips for the transverse processes and of no value 
for the protection of the viscera. They make no attempt to 
reach around the body, and thus never come into relation with 
a sternum. This latter, to us the typical relation for ribs to 
assume, appears first in reptiles, which thus form the prototype 138 
THE HISTORY OF THE HUMAN BODY 
of the later development in birds and mammals. In these 
latter a typical rib possesses two well-marked segments, a 
dorsal and a ventral, often bent at an angle to each other; both 
Fig. 36. Morphology of ribs. [After Wiedersheim.] 
(a) Ganoid, (b) Dipnoan. (c) Teleost. (d) Selachian, (e) Polypterus (a spe- 
cial case among ganoids), (f) Urodele. 
In the three first the condition in both trunk and tail is given. In all the figures 
the “ fish rib ” is striped, the myocommatous rib is black, and the basal stumps are 
outlined. 
may be fully ossified, as in birds, or the ventral segments 
may remain cartilaginous, forming the so-called “ costal car- 
tilagescharacteristic of mammals. In birds the dorsal seg- 
ments possess flat uncinate processes, which extend backwards 
from their posterior edges and overlap the succeeding rib, thus THE AXIAL SKELETON 
139 
effecting here the rigidity necessary in all parts of the body in 
adaptation to flight while allowing for play of the respiratory 
motions. 
The distribution of the two types of ribs among vertebrates 
is a little unusual, since the second or myocommatous type 
appears, not only in amphibians and amniotes, the higher 
groups, but also in the selachians, one of the most primitive. 
This is one of the many indications of kinship between these 
and the higher forms, and suggests the direct descent of the 
amphibians from selachian-like ancestors, thus disposing of 
the remaining fishes as collateral lines, in which the piscine 
type attains its special line of development, without relation- 
ship to the higher classes, save through a common ancestor. 
The haemal arch ribs, or true “ fish-ribs,” are characteristic of 
teleosts, dipnoi, and most ganoids; in one of the latter, Polyp- 
terus, both sets appear simultaneously, the myocommatous set 
being functional, while the haemal arch set is rudimentary, not 
attached to the vertebrae, and hence of little use to the fish, 
but of great significance to the anatomical historian. 
In a strict sense it cannot be said that the fish-ribs or haemal 
arch ribs are in all cases exactly homologous with one another, 
or even that they are in all cases formed mainly from the 
haemal arches, since recent investigation has demonstrated the 
existence of other elements, derived directly from the bodies 
of the vertebrae, and normally supporting the haemal arches, 
which are often concerned in the formation of the ribs; but 
not only would an exposition of this lead us too far into details, 
but would take us away from the main inquiry, since the phe- 
nomena do not occur on the direct road traced in our present 
history. The conception of these ribs as expanded haemal 
arches is not in any case far from the truth, since the other 
elements concerned are themselves functionally if not mor- 
phologically parts of the haemal arches. 
As shown by amphibians and reptiles every vertebra be- 
tween the second (axis) and the sacrum is typically fur- 
nished with a pair of ribs, which in these Classes are usually 
free. In birds and mammals certain of these become anchy- 140 
THE HISTORY OF THE HUMAN BODY 
losed to the vertebrae which bear them, leaving a set of 
thoracic vertebrae (the “ dorsal ” vertebrae of the older termin- 
ology) interpolated between two groups, cervical and lumbar, 
in which the rib elements are fused. In the cervical vertebrae 
these fused ribs form the ventral element of the plainly double 
transverse processes, and enclose between themselves and the 
original transverse process (diapophysis) the vertebral for- 
amina. In the lumbar vertebrae the rib elements form the 
large wing-like transverse processes (pleurapophyses), and 
are thus seen to be not equivalent to the processes of the same 
name in other regions. 
The number, both of free ribs and of vertebrae forming 
each group, differs considerably, not only in different mam- 
mals, but even in different individuals of the same species. 
Thus in Man, although twelve pairs of free ribs is the rule, 
the rib element of the last (7th) cervical vertebra is occasion- 
ally free, “ cervical rib,” and, more commonly, a free rib ap- 
pears on the first lumbar vertebra. As this is perhaps the 
rule rather than the exception in the gorilla, one of Man’s 
nearest living allies, this anomaly is often called the “ gorilla 
rib.” Variation in the sacral vertebrae has already been 
noticed [Cf. Fig. 35 and accompanying text]. 
The origin of the sternum is still a matter of controversy. 
Fishes, as adults, lack this part entirely, but the whole case 
has recently been brought up afresh by the recognition of cer- 
tain appearances in the embryonic shoulder-girdle of Sela- 
chians as a presumptive sternum. This matter will be dis- 
cussed later in the chapter on the Appendicular Skeleton. 
Above the fishes a sternum is typically present, the excep- 
tions being rare, and then connected with some other reduc- 
tion, usually that of the girdle and anterior limbs. In snakes, 
in which both pairs of limbs are lost, the sternum has gone 
also, and this loss has rendered possible two striking adapta- 
tions of these animals, the mode of locomotion by the use of 
the ends of the very numerous ribs, and the habit of swallow- 
ing huge mouthfuls, far too large to pass through the ring 
formed by the vertebrae, ribs, and sternum, in more typical THE AXIAL SKELETON 
141 
vertebrates. What is apparently the first indication of a 
sternum is seen in the salamander Necturus, perhaps the lowest 
Fig. 37- Morphology of the sternum. 
(a) Necturus (a primitive salamander), (b) A higher salamander, (c) Frog, 
(d) Lacerta (European lizard), (e) Cat. 
c, coracoid; d, epicoracoid; e, episternum; f, clavicle; g, scapula; h, suprascapula; 
p, procoracoid; st, sternum; m, manubrium; stb, sternebrae; x, xiphisternum. 
amphibian, in which from three to five of the thoracic myocom- 
mata chondrify in the ventral region, forming small V-shaped 142 
THE HISTORY OF THE HUMAN BODY 
elements, indefinite in shape and irregular in occurrence, as is 
usually found in an organ at its beginning, before the type has 
become fixed. If we seek the reason for their appearance we 
shall probably find it in an attempt to lessen the pressure upon 
an important point. One of the sternal elements, usually 
the largest, lies in the fourth myocomma, in close connection 
with the overlapping coracoids, and as at the same point in 
higher salamanders there is a definite sternal plate of a rhom- 
boid shape, this latter may have developed from the element 
in question, while the others have been lost. This may also be 
the same piece found in frogs and other tailless amphibians, 
again in the same relationship to the coracoids, and entering 
into a more or less complete connection with the two halves of 
the shoulder-girdle in forming the skeletal armature that covers 
the pectoral region. As the ribs of all amphibia are very short 
and rudimentary, and do not reach even half way around the 
body, there is never the slightest attempt at a connection be- 
tween them and the sternal piece: but in the Amniota a close 
attachment of one or more of the anterior ribs with the lateral 
borders of the sternum is universal. This has naturally 
suggested a genetic connection between the two structures, 
that in these animals certain of the ribs, extending around the 
body, have fused in the mid-ventral line, forming a series of 
median elements, the sternebrae, a pair for each pair of ribs 
involved. As no such origin is suggested for the Amphibia 
there has developed the hypothesis of two sterna, an archi- 
sternum, not connected in any way with ribs, for the Am- 
phibia, and a neosternum of costal origin characteristic of the 
Amniota. 
The recently established fact that the connection between 
ribs and sternum is in all cases a secondary one at once re- 
moves the necessity of the double origin hypothesis, and we 
now see in the sternum of all Vertebrates the various modifi- 
cations of a single organ. The origin of this part in the first 
place is still in dispute, but may well be sought, not in a series 
of myocommatous elements, as given above, but in the THE AXIAL SKELETON 
143 
shoulder-girdle. This theory will be considered further on 
under the discussion of the shoulder-girdle (p. 199). 
The confinement of the sternum to the thoracic region 
leaves the ventral abdominal surface unprotected, an affair 
of no great moment so long as an animal remains small, or 
not very much elongated, but when, as in the Crocodilia, the 
elongation of the body greatly increases the extent of the 
unprotected surface, while at the same time the increase in 
size renders the body ponderous, the pressure exerted on the 
abdominal viscera by the weight of the body as the animal 
crawls, or even lies passively on the surface of the ground, 
must be very great. It is evidently to overcome this in part 
and furnish some protection for the soft parts that there de- 
velops in this region a series of skeletal elements precisely 
similar in origin to the primitive sternal pieces of Necturus, 
formed by the ossification of the ventral portion of the ab- 
dominal myocommata. Developing along the mid-ventral 
line also, many of the pieces become connected together and 
form a system of “ abdominal ribs,” as they have been called, 
better known as the parasternum. As these do not appear to 
be represented in any other Order, they are of no phylogenetic 
value, but serve to explain the reason for the origin of the 
sternum in the salamanders by furnishing an exact physio- 
logical parallel. 
The ontogenetic history of the skull, a complex of skeletal 
elements developed at the anterior end of the notochord, is 
singularly constant in all classes, and we may thus feel con- 
fident that we have in this a repetition of stages once passed 
through by the adult ancestors of the present-day vertebrates. 
It is true that the early stages thus indicated do not correspond 
with the adult condition of any form now living, but of the two 
types in which we might expect to find a correspondence 
with this period of the history, Amphioxus has no head, and, 
of course, no skull, and the cyclostomes with their parasitic 
habit are too much modified to be reliable; there is, moreover, 
an enormous gap between the two, and a second, almost as 
great, between the latter and the selachians, so that it may 144 
THE HISTORY OF THE HUMAN BODY 
well be conceded that adult animals representing the stages 
indicated by the embryonic history once existed in the places 
now left vacant. Nothing could fit better into this onto- 
genetic history at a later period than the selachian skull, as 
will be shown further on, thus verifying the record at an im- 
portant point, and rendering it more probable that the earlier 
Fig. 38. Diagrams showing the development of the primordial skull. 
Since this organ develops primarily beneath the brain as a support the 
figures represent the ventral aspect. 
(A) Early stage, before the appearance of cartilage. The notochord is seen lying 
along the nerve cord as far forward as the hypophysis. The three sense-organs, 
nose, eye, and ear, have already appeared. (B) This stage shows the trabeculae [t], 
the parachordals [p], and the capsules around the sense-organs. (C) In this the 
trabeculae, the parachordals, and the nasal and otic capsules have fused into a single 
mass, the primordial skull, or chondrocranium. The anterior end of the notochord 
is imbedded in this. The cartilaginous capsule of the eye remains free to allow 
the necessary movements of the eyeball. 
embryonic stages, so constant in appearance in all vertebrates, 
are equally accurate in reproducing the conditions once found 
in forms now lost to us. To outline the history, then, with the 
help of embryology, it appears that the ancestral vertebrate, 
after the acquirement of the prechordal addition to its head, 
developed several pairs of external sense organs in the cephalic 
region, three of which, the nasal sacs, the eyes, and the (inner) 
ears, have persisted. Of others there are indications in early THE AXIAL SKELETON 
145 
embryonic life, such as those arising between the eye and ear 
and supplied by the Seventh nerve, and there are reasons to 
believe that the original sense organ of the Second pair was 
not the eye as we have it now, but the lens alone, in the form 
of a simple capsule; but these matters hardly belong in this 
place and are suggested merely as indications of the elaborate 
past history of the head, entirely gone from the world of 
adult life, but now restored in part by the labors of a gener- 
ation of embryologists. At this time the notochord, termi- 
nating at the hypophysis, a downgrowth of the brain just an- 
terior to the ear capsules, was the only skeletal element in 
the head, and could have had little value as an organ of sup- 
port, and none whatever as an organ of protection. This 
condition of affairs is represented in Fig. 38, A, which, it 
must be noted, represents the head as seen, not from above, 
as it is more usually drawn, but from below, a view that en- 
ables one to see the notochord and its termination behind 
the hypophysis. To this condition there are added, but no 
one yet knows how or from what source, two pairs of later- 
ally placed cartilages (Fig. 38, B), the one alongside the noto- 
chord and the other anterior to it, the parachordal and pre- 
chordal elements respectively. The former are rather flat, of 
an elongated crescentic shape, filling in the space between the 
notochord and the ear capsules; the latter are elongated and 
rod-like or beam-like, hence often termed the trabeculae (di- 
minutive of trabs, a beam), and lie a little beneath the eyes 
and nearer the median line. 
At the same time the three persisting pairs of sense organs 
become enclosed by cartilaginous capsules, differing some- 
what in their development, according to the needs of the organ. 
Thus the nasal capsules remain open anteriorly for the free 
admission of the fluids to be tested, the eye-capsules involve 
the sclera alone, while the otic capsules usually become entirely 
closed and develop fairly thick walls, since sound vibrations 
can pass easily through solids and do not need a special open- 
ing. As these several cartilaginous elements, the para- and 
prechordals and the sense capsules, increase in size, they fuse 146 
THE HISTORY OF THE HUMAN BODY 
together in about the following manner: The two trabeculae 
expand anteriorly and fuse with each other across the middle 
line and with the nasal capsules as well; extending backwards, 
they fuse with the parachordals. These latter, growing in 
width, fuse both with the otic capsules and with each other, 
including in this fusion the anterior end of the notochord, 
which becomes lost in the general mass. 
There is thus formed a single, curiously shaped piece of 
continuous cartilage, composed of all the elementary pieces, 
with the natural exception of the optic capsules, which must 
This skull is still very primitive, covered with scutes of nearly the same size and shape, suggestive 
of the original unmodified scales, yet with certain of the typical bones already recognizable from their 
position and relationship to the others. These elements may be, as Gregory says, “probably analogous 
rather than homologous.” 
Fig. 39. Dorsal view of the skull of Dipterus, a Devonian Dipnoan. 
remain free to allow the turning of the eyeball (Fig. 38, C). 
These pieces fuse so completely that all boundaries are lost, 
and we can speak only of a parachordal or a trabecular re- 
gion, and so on, without assigning definite boundaries. This 
consolidated piece is termed the primordial skull or chondro- 
cranium, and remains at this stage in selachians, where it is 
characteristic of the entire Order, being throughout life with- THE AXIAL SKELETON 
147 
out trace of ossification and with such slight modifications 
only as are necessary for the adaptations of the various adult 
forms. It is a natural supposition drawn from common ex- 
perience that a skull is intended for the protection of the 
brain, but in this case the function is rather that of support, 
since it lies laterally to and in part beneath the brain, leaving 
practically the entire dorsal and the anterior part of the ven- 
tral aspects without protection. In the adult selachians, in- 
deed, these deficiencies are made up in part by the formation 
of firm membranes, continuous with the cartilage and closing 
in the open fontanelles, but they are plainly secondary modifi- 
cations, for use during the active adult life of these animals, 
and do not appear in the embryonic history of higher forms. 
This stage of the chondrocranium, permanent and final in 
the Elasmobranchs, is passed through with during the develop- 
ment of all higher vertebrates, and although the shape and 
proportions of the parts often differ widely in anticipation of 
the various needs of the adults, they all possess in common an 
origin from the same elemental parts, and the characteristic 
regions may in all cases be readily recognized. 
Beginning with this universal chondrocranium, or “ prim- 
ordial skull,” the true cranium of all higher vertebrates is 
acquired by the addition of true bone, which arises from two 
sources of origin. The first are the cartilage bones; the 
second, dermal bones. Cartilage bones appear as centers of 
ossification within the cartilage of the primordial cranium it- 
self, developing at those spots where either the strain of a 
movable joint or that of some other mechanical force causes 
stress, in other words, their origin seems to be due to physio- 
logical reasons. Thus the quadrate and the articulare, be- 
tween skull and lower jaw, form the joint between these parts; 
the exoccipitals develop between the skull and the first verte- 
bra, and form the condyles which serve as the joint between 
head and neck. In other cases a cartilage bone serves to 
protect particularly delicate parts, as the pro-otic and opis- 
thotic, which form caps about the otic capsule, or the orbito- 148 
THE HISTORY OF THE HUMAN BODY 
sphenoid, which protects the exit of the optic nerve from the 
cranium. The physiological needs which have called the carti- 
lage bones into existence, have retained the original topo- 
graphical relationships of these elements, and thus render 
the tracing of the homologies of cartilage bones throughout 
all vertebrates definite and positive. 
The dermal bones, on the other hand, have no close rela- 
tionship with the primordial cranium, but arise from the ex- 
ternal covering of scales, which at first cover the exterior and 
also line the mouth-cavity and pharynx. At their first ap- 
pearance these dermal bones are more nearly uniform in 
size, but even here they are relatively much larger than placoid 
scales would be supposed to be, and are more probably to 
be considered, not simply modified scales, but scutes or plates 
formed by the coalescence of many scales or scale bases, quite 
the same structures as the similar scutes, which, in several 
longitudinal rows, protect the otherwise naked skin of the 
sturgeon. In Ganoids, and throughout the higher fishes, these 
external scutes of the head and gill region form a complete out- 
side skull, within which the older cartilaginous skull is en- 
cased, strengthened by a more or less continuous set of carti- 
lage bones, an external bony skull encasing an inner one that 
is partly cartilaginous. 
Presumably, as is seen in certain Devonian Dipnoi (Dipterus, 
Fig. 39), the dermal skull of the earlier fishes was composed 
of a large number of rather small scutes, approximately of 
the same size, and hexagonal in outline, but even here there 
was shown a tendency to emphasize the scutes in certain places, 
which in their descendants became fixed and constant. Thus, 
among the earliest of the Teleostomes, there are found the 
nasals, frontals, and parietals, placed in pairs down the mid- 
dorsal line in the order named, from before backward, and in 
all higher forms without exception they are met with in much 
the same relation. Around the eye, defining the orbit, the 
earliest forms show a ring of circum-orbital scutes, certain 
ones of which become perpetuated in higher forms as the THE AXIAL SKELETON 
149 
lacrimal, jugal, and pre- and post-frontals. Upon the ventral 
side of the skull the extensive parasphenoid covers a large part 
in fishes and amphibians, but is not perpetuated in higher 
forms; upon this side there develop also two sets of jaw 
arches or arcades, composed of dermal elements. The lateral 
arch consists of the pre-maxillary and maxillary, the medial 
of palatine and pterygoid. 
In the modern Amphibia this dermal skull, consisting of 
bony plates or scutes, never pre-formed in cartilage, and aris- 
Fig. 40. Two views of the skull of Cryptobranchus allegheniensis, a 
primitive salamander, a little higher than Necturus. 
CA) Dorsal. (B) Ventral. 
Dermal Bones: pm, premaxillary; mx, maxillary; n, nasal; f, frontal; pr. f, 
pre-frontal; p, parietal; sq, squamosal; pt, pterygoid; vp, vomero-palatine; pb, para- 
basal. 
Cartilage Bones: os, orbitosphenoid; q, quadrate; ex. o, exoccipital op, operculum, 
Other Parts: col, columella; nas, nasal capsule; ec, eye capsule; ot, otic cap- 
sule. 
In both figures the dermal bones have been retained on the right side of the 
skull and removed on the left. 
ing directly from connective tissue without any cartilage stage, 
remain beneath the skin, and enter more or less closely 
into relationship with the chondrocranium and the bones that 
are formed within it; and as the ultimate purpose of all animal 
development is to adapt each animal more fully to its in- 150 
THE HISTORY OF THE HUMAN BODY 
dividual needs, and not in any way to preserve early history, 
physiological necessity may fuse cartilaginous and dermal 
elements into a single piece, or may divide an originally 
single element into two or more. Especially is this indepen- 
dent adaptation of a line of development seen in one so isolated 
Fig. 41. Dorsal view of the skull of Osteolepis, a Devonian Rhipidistian Ganoid. 
[After Gregory.] 
as the frogs, which have at present no near relatives, while 
if, as is strongly suspected, the members of what now seem 
similar, like the snakes, or the rodents, have really become 
similar through similar habits, though from different, un- 
related stocks, we cannot rely too strongly upon the similarity 
of a bone, either in shape or relationship, as a proof of actual 
identity. 
Probably no single problem of Vertebrate anatomy has 
aroused so much interest as that of the homology of the 
separate bones of the skull, especially at the hands of the 
paleontologist and the systematist, for by means of the differ- 
ent relationships of these the latter seeks the key to the ar- 
rangement of these animals into groups, while for the former THE AXIAL SKELETON 
151 
the imperfect impressions of the skull bones and teeth are 
often the only parts of extinct animals that are left. 
In all such studies it is of the greatest importance to es- 
tablish definite homologies for the separate bony elements, 
yet the vagueness of our ideas as to what constitute homol- 
Fig. 42. Lateral view of the skull of Osteolepis, a Devonian Rhipidistian 
Ganoid. [After Gregory.] 
ogies leads to endless discussions and the framing and relin- 
quishment of many an opposing theory. 
In the early days of comparative anatomy, when a general 
similarity of shape and position was sufficient ground for con- 
sidering an element found in two separate skulls homologous, 
without reference to its development in the two cases, the 
ground for a supposed homology was based merely upon ap- 
pearance, often with laughable results. These were the days 
of Cuvier and Owen, whose entire schemes of the homology 
of the skull bones were shattered in 1858 by Huxley, who 
first pointed out the two fundamentally different groups of 
bones; those pre-formed in cartilage and appearing as centers 
of ossification in the parts of the chondrocranium, and those 
arising subcutaneously from connective tissue without trace 
of cartilage in any stage of their development, the cartilage 
bones and the dermal bones respectively. By the establish- 
ment of these two distinct groups the problem was greatly 
clarified, and an element from one group could never more be 
homologized with any one from the other, thus dividing the 
problem in two. 
A complete homologizing of the cartilage bones, based upon 152 
THE HISTORY OF THE HUMAN BODY 
the embryological history of the skulls in the various Verte- 
brate types, seems at once possible, especially since these ele- 
ments are not very numerous; they show, moreover, certain 
definite relationships to the separate cranial nerves, and 
other soft parts, and to the elements of the primordial cra- 
nium. They also exhibit certain physiological activities, and 
Fig. 43. Diagram of the skull of a typical Teleost, based on the cod or 
haddock. 
appear wherever there is special mechanical stress, as about 
the joint between the mandible and the cranium, or about 
the junction of the skull and the vertebrae. 
There remain, however, the much more numerous dermal 
elements, which formed originally an encasing external skull, 
enclosing the entire chondrocranium, and fitting closely upon 
its outer surface. This structure is formed of thin, over- 
lapping plates, which, as some see them, show only a general 
similarity in plan, in which only the most obvious and uni- 
versally present may be definitely homologized, while others, 
at the other end of the series, believe in the complete homol- 
ogizing of every piece. 
Some claim for the dermal bones an original condition, to 
be found among the early Ganoids and Dipnoi, in which all THE AXIAL SKELETON 
153 
the existing dermal elements to be found in any modern skull, 
were present, and account for each modern form through cer- 
tain modifications of this original type. They account for 
finding a single bone in a certain case where there are two in 
the type by supposing that an early fusion has taken place; 
for finding two or more bones in the place of a single typical 
Fig. 44. Dorsal view of the skull of Stegops, a Stegocephalian. [After Hoodie.] 
piece as the result of a splitting or fragmentation of the 
original; they affirm that by increasing the size of a typical 
piece through growth, it may increase to any size, while by 
the opposite tendency, reduction, an original element may 
become decreased to the point of complete loss. However 
likely this idea may be, it must be confessed that they re- 
mind us strongly of the doctrine of archetypes of Goethe, of 
Owen, or of Agassiz; the search of an earlier perfect or com- 
plete condition, to which we may refer all existing conditions 
as more or less complete modifications; a philosophical yearn- 
ing for the Golden Age. 
Others, at the opposite extreme, focus their attention upon 
the great amount of plasticity shown by bony structures, and 
show how susceptible these structures are to respond to any 
and every strain or stress imposed upon them by environment 154 
THE HISTORY OF THE HUMAN BODY 
or habit. To them a single bone in an ancestor may readily 
become two or more in a descendant, if found advantageous, 
and with no cause other than the need of the animal con- 
sidered. To them there has been, indeed, a history, but the 
recent modifications of a given form may have produced re- 
sults that seem fundamental, but are not so. 
If, now, we take into consideration-these two extreme views, 
and assemble the numerous forms in which the vertebrate 
skull has appeared, not only as shown by present-day forms, 
but in the past also; if we add to the skulls of the living 
animals the more or less imperfect records of the extinct 
ones; although we are not in possession of a definite criterion 
for exact homologies, it is still possible to compile a descrip- 
tion of the separate bony elements of the vertebrate skull, 
a description which may serve as a guide to the determina- 
tion of the probable elements in definite cases. The elements 
of the mandible, the hyoid, and the larynx, treated elsewhere 
as parts of the visceral skeleton, need not be considered 
here, save as they may be, in some forms, anchylosed to other 
parts; the bones of cartilaginous and dermal origin may be 
carefully distinguished also; but the general arrangement will 
be topographical, as best fitting a description. 
Elements of the Vertebrate Skull 
The roof. In all Vertebrate skulls, above the Elasmobranchs, 
except in a few cases of unusual form, or in those cases with 
the eyes reduced or wanting, the centers of the orbits form 
a good starting-point, and a line drawn straight across the top, 
connecting the two, crosses a pair of dermal bones, in con- 
tact with each other in the median line, the two frontals. In 
front of these, also in contact with each other medially, are 
the two nasals, while behind the frontals are the two parietals. 
Behind these latter there is either a pair of dermal supra- 
occipitals (early Dipnoi, most Stegocephali), or, more com- 
monly, a single median piece, in either case bordering upon the 
anterior dorsal lip of the occipital foramen. Slipped in be- THE AXIAL SKELETON 
155 
tween the two parietals, and placed between these and the 
supra-occipital, may often be found an interparietal, while a 
preparietal may occur in a 
similar position between the 
parietals and the frontals. The 
nasals border, with their an- 
terior margin, the external 
nares. 
The orbital ring. In many 
Ganoids and Teleosts the orbit 
is entirely surrounded by a ring 
of circum-orbital elements, the 
two dorsal ones of which, in 
contact with the frontal, be- 
come known respectively as the 
pre- and post-frontals, and per- 
sist here and there, especially 
in Stegocephals and in reptiles, 
while the antorbital element, in 
front of the orbit, and origi- 
nally upon the face, becomes 
the lacrimal. The post-orbital, 
the one behind the orbit, is found, as a well-recognized 
element, in many amphibians and reptiles, while the remain- 
ing one or two, beneath the orbit, continue in all higher ani- 
mals as the Jugal, and become a very important bone in mam- 
mals with heavy jaws, the main element in the zygomatic 
arch to which the masseteric muscle is attached. 
The otic region. The otic region, lying lateral to the parie- 
tal, is one of the most complicated of the entire skull, owing 
to the assemblage here of both dermal and cartilaginous ele- 
ments, and to the inclusion also of several parts of the visceral 
skeleton, which participate in the formation of the jaw and in 
biting and chewing. This region is based fundamentally 
upon a cartilaginous capsule, originally distinct, of oval shape, 
in which there early appear several centers of ossification, 
Fig. 45. Dorsal view of the skull 
of Archegosaurus, a temnospondylous 
Stegocephalian. [After Credner.] 156 
THE HISTORY OF THE HUMAN BODY 
typical cartilage bones. The most constant of these, the pro- 
otic and opisthotic (par occipital), appear in the form of hollow 
cones, capping respectively the anterior and posterior ends 
of the capsule. These are associated topographically with the 
exoccipital, also a cartilage bone, developed to assist the 
movement of the head upon the first of the vertebrae, and 
Fig. 46. Dorsal view of the 
skull of Desmognathus fusca, a sala- 
mander. [After Wiedersheim.], 
Fig. 47. Ventral view of the 
skull of Desmognathus fusca, a sal- 
amander. [After Wiedersheim.] 
frequently either the opisthotic and exoccipital are fused, or 
the pro-otic is also added to form a single mass, the 
“ petrosum.” 
The operculum. Posteriorly and laterally, in Ganoids and 
Teleosts, there develops over the region of the gills a gill- 
flap or operculum, and this part becomes stiffened by the 
formation of four dermal bones, pre-operculum, operculum, 
inter-operculum, and sub-operculum. Of these the first alone 
persists above the fishes, and, under the name of the squamo- 
sum, covers the anterior portion of the otic region. Otherwise 
this opercular flap disappears, with the bones which support 
them, the squamosum alone persisting in all higher forms (ex- THE AXIAL SKELETON 
157 
cept snakes), more or less covering the anterior part of the 
otic region. 
The sides of the head. Between the parietal bones on the 
inside and the opercular series (or the squamosum) on the 
outside several dermal elements may be expected, especially in 
Ganoids and Teleosts, filling up the long crack that would 
Fig. 48. Dorsal view of the skull 
of a turtle. 
Fig. 49. Ventral view of the 
skull of a turtle. 
otherwise exist between these two, and disposed usually in a 
single row, from before backwards. Beginning at the posterior 
rim of the orbit with the post-orbital, previously mentioned as 
one of the circumorbital ring, the line continues as the inter- 
temporal and supratemporal (pterotic), well shown in the 
Stegocephali, and ends at the posterior margin of the skull 
with the tabulare. 
The attachment of the skull with the vertebral column. The 
attachment of the skull with the vertebral column is of two 
kinds, direct and indirect. The direct is originally the cartilag- 
inous continuation of the row of vertebral bodies into the 158 
THE HISTORY OF THE HUMAN BODY 
base of the chondrocranium, as is shown in simple form in 
the skull of the Elasmobranchs, but where this is improved 
upon it is done by the ossification of the posterior part of this 
region into the basi-occipital, and, anterior to this, the basi- 
sphenoid and pre-sphenoid, making a median row of bones, 
Fig. 50. Lateral view of the skull of a turtle. 
continuing the vertebral bodies well into the head. These 
elements are rather late in appearing, being universal in reptiles 
but not found in amphibians. The indirect attachment is 
that made by the two exoccipitals, each of which develops a 
strong condyle, which is received into a pair of lateral cups 
of the first vertebra, and in mammals forms one of the char- 
acteristics of the atlas. 
The outer arcade. The upper jaw of the Elasmobranchs, 
originally a part of the mandibular arch, and hence a part of 
the visceral skeleton, becomes in Ganoids, and from thence 
throughout, replaced by a series of dermal bones, supplied 
typically with teeth, and occupying the outer margin as far as 
the suspension of the mandible. These, beginning at the an- 
terior end, are the pre-maxillary, maxillary, jugal, and quad- 
rato-jugal, and the series terminates at the quadrate, a cartilage 
bone, formed by the proximal (posterior) end of the Elasmo- 
branch upper jaw. This quadrate bone reaches the otic region, 
and adds to the general confusion in amphibians and Saurop- 
sida, while in mammals a new and unexpected feature produces 
a fundamental revolution. In Elasmobranchs a visceral ele- 
ment, derived originally from the second visceral arch, the 
hyo-mandibular, is used to hang the two jaws upon, the first THE AXIAL SKELETON 
159 
visceral or original mandibular arch, but, when the quadrate 
bone usurps this function, the hyo-mandibular gets drawn into 
the ear, and becomes a thin rod, the columella auris. This 
functions to reinforce the sound vibrations and intensify the 
power of hearing, while, in all Tetrapods, as far as mammals, 
the quadrate suspends the mandible, attaching directly to the 
proximal end of the mandibular (Meckel’s) cartilage, the two 
forming the articulation of the jaw. This proximal end gains 
a center of ossification, and becomes known as the articulare 
of the jaw. 
The auditory ossicles. Suddenly, so far as we know, at the 
very first appearance of actual mammals, both the quadrate 
and the articulare become drawn into the middle ear, joining 
the columella, that had so long preceded them in this otic 
migration, and become the two outer ossicles of the series of 
ear bones, the incus and malleus respectively, while the old 
columella, now known as the stapes, forms the innermost one 
of the row. The mandible itself, now composed mainly of the 
dentary, and with an almost complete suppression of the 
cartilaginous element, has obtained an entirely new articula- 
tion, one with the petrosum. 
The inner arcade. While a secondary upper jaw is thus 
formed from an outer row of dermal bones, the original 
cartilaginous upper jaw becomes pushed in nearer the median 
line, and becomes covered, in exactly the same way as the 
lower jaw does, by a series of investing pieces; flat, dermal 
bones. 
Upon each side, beginning anteriorly just behind the pre- 
maxillary, are found in turn the vomer, palatine, and ptery- 
goid, which run in an approximately parallel row to the outer 
one, premaxillary, maxillary, etc. These are placed thus in 
the form of two “ arcades,” which together constitute the bulk 
of the bones that show upon the anterior half of the skull as 
seen from the ventral aspect. Both arcades converge posteri- 
orly, and terminate at the quadrate. Between these two sets 
of arches the roof of the mouth is covered, in fishes and am- 
phibians, by a large dermal piece, often very extensive, the 160 
THE HISTORY OF THE HUMAN BODY 
parasphenoid, but in Amniota this piece seems to have wholly 
disappeared, while its place is taken, topographically and 
functionally, by cartilage bones, some of which have been 
met before. When in their full panoply, in the role of a com- 
plete floor for the cranium, these bones are nine in number, 
along the median line, from before backwards, pre-sphenoid, 
Fig. 51. Dorsal view of the skull 
of Lacerta viridis. [After Wieder- 
sheim.] 
Fig. 52. Ventral view of the skull 
of Lacerta viridis. [After Wiedersheim.J 
basi-sphenoid, and basi-occipital, and upon the sides of each 
a pair of lateral wings, respectively the orbito-sphenoids, alt- 
sphenoids, and ex-occipitals, 
This ends the enumeration of the usual bones of the skull, 
but in order to complete the description of what may be met 
with, including both recent and extinct forms, a few more 
elements need to be mentioned, although their homologies are THE AXIAL SKELETON 
161 
by no means sure. These are mainly to be found in the 
pterygoid and otic regions, and occur as distinct elements only 
in a few groups. For example, the full set of pterygoid bones 
includes, as found in fishes, the metapterygoid, while the con- 
ventional pterygoid is divided into two, ectopterygoid and ento- 
pterygoid. An “ os transversum” a conspicuous dermal bone 
Fig. 53. Lateral view of the skull of Lacerta viridis. E After Wiedersheih.] 
in alligators, snakes, lizards, and Stegocephali, extends across 
from pterygoid to maxilla, which is probably the same as the 
piscine ectopterygoid; and an epipterygoid, a rod-like bone 
which in some lizards extends across the skull from the pari- 
etal on the roof through to the pterygoid on the ventral side, 
probably belongs also to the pterygoid series. Teleostomes 
possess several extra bones in the otic region, the epi-otic, 
sphen-otic and pter-otic, elements which apparently developed 
in this group, and have no descendants to which to perpetuate 
them. Finally, there are still some unsolved problems in the 
nasal region, where opinions are still at variance. There is, 
for example, some difference of opinion concerning the vomers, 
which, in the Anamnia, are always paired, and in mammals 
and turtles median, and to relieve the embarrassment of which 
some consider the two not homologous, but suppose the median 
vomer to be the remnant of the old ichthyopsidan parasphen- 
oid. Others, on the other hand, point out that the single 
vomer, in the opossum at least, is laid down in the embryo 
with a paired anlage, suggesting that it is primarily a paired 
element in all cases. Another bone that has aroused some 
recent interest is the septo-maxillary, a minute nodule of bone, 162 
THE HISTORY OF THE HUMAN BODY 
found in frogs and in many salamanders, developing as an 
ossific center in the midst of the complicated nasal capsule 
of these animals. This element has been frequently mistaken 
for the lacrimal or even the nasal; it has been more fre- 
quently overlooked entirely, and has attracted interest by a 
very recent claim that it is distinctly traceable in mammals. 
Other inter-nasal elements. In the anterior part of the 
chondrocranium, between the two nasal capsules, there are 
found, in all Classes of Vertebrates, centers of ossification, the 
homologies of which have not been satisfactorily traced, but 
which bear the general names of ethmoid, and which become 
in the mammals very important accessory organs to the sense 
of smell, serving as a surface upon which to spread the ol- 
factory mucous membrane. Thus in the mammals the eth- 
moid complex consists of a median ethmoid and two lateral 
ect-ethmoids, the first of which unites closely to the median 
vomer, and forms the septum, separating the nostrils. The 
septo-maxillary, mentioned in the preceding paragraph, and 
found as a cartilage bone in the midst of the very complicated 
cartilaginous nasal network of frogs and toads, evidently may 
be considered here. 
In many mammals, in which the nasal cavities frequently 
develop scrolls or other devices for increasing the olfactory 
surface, two extra cartilage bones, the spheno-turbinals and 
inferior turbinals, sometimes appear, arising from separate 
centers; and in a few cases, where the nasal cavities are ex- 
traordinarily complex, still other separate pieces appear in 
the conchae, derived from the ect-ethmoid. 
The separate elements of the Vertebrate skull, to which are 
added those of the mandible and the hyo-branchial apparatus, 
are given here, arranged in the form of an alphabetical list, 
with the abbreviations which are used in all the illustrations 
involving these parts. Elements of the parts last mentioned, 
parts of the visceral skeleton, are designated by an as- 
terisk (*). The cartilage bones are in italics, and both here 
and elsewhere all abbreviations designating parts pre-formed THE AXIAL SKELETON 
163 
in cartilage, or those often met with in a cartilaginous con- 
dition, are in capitals. Small letters, on the other hand, 
signify dermal, or membrane, bones. 
Skeletal elements oj the skull and hyo-branchial apparatus. 
Ali-sphenoid AS 
Angulare * ang 
Articulare * ART 
Basi-branchial * BB 
Basi-hyal * BH 
Basi-occipital BO 
Basi-sphenoid BS 
Cerato-branchial * CB 
Cerato-hyal * CH 
Circum-orbital ring cr 
Columella auris * COL 
Coronoid * c 
Dentare * d 
Dermal supra-occipital dso 
Ect-ethmoid (lateral 
ethmoid) EE 
Ecto-pterygoid ( — os 
transversum tr 
Ento-glossal* EG 
Ento-pterygoid enpt 
Epi-branchial * EB 
Epi-hyal * EH 
Epi-otic ( = tabulare) t 
Epi-pterygoid (part of 
quadrate) EPPT 
Ethmoid E 
Ex-occipital EXO 
Frontal f 
Glosso-hyal* (Ento- 
glossal) GH 
Gular gu 
Hyo-mandibulare * (Col- 
umella) HM 
Hypo-branchial * HB 
Hypo-pharyngeal * (lower 
pharyngeal HPH 
Hypo-hyal * HH 
Incus ( = Quadrate ) I 
Inferior turbinal (Mam- 
mals) IT 
Intercoronoid * inc 
Inter-hyal * (Teleo- 
stomes) IH 
Inter-opercular (Teleo- 
stomes) inop 
Inter-parietal inp 
Inter-temporal intm 
Jugal j 
Lacrimal la 
Lateral ethmoid ( = Ect- 
ethmoid) EE 
Malleus ( = Articulare)* M 
Maxillare mx 
Median ethmoid (Meseth- 
(moid) ME 
Mentale (Mento- 
meckelian) * MN 
Mes-cthmoid ME 
Meso-pterygoid (Ecto- 
pterygoid) ecpt 
Meta-pterygoid mpt 
Nasal n 
Operculare (Teleostomes; 
in gill-flap) op 
Operculum (in otic 
capsule) O 164 
THE HISTORY OF THE HUMAN BODY 
Opisth-otic ( = Par- 
occipital) 00 
Orbito-sphenoid OS 
Palatine pi 
Para-glossal * (birds) PG 
Para-sphenoid ps 
Parietal p 
Petrosum ( = PO -f- 
00 -f EXO, etc.) PET 
Pharyngo-branchial * PHB 
Post-frontal pstf 
Post-orbital pstor 
Post-splenial * pspl 
Pre-articulare * prart 
Pre-coronoid * prc 
Pre-dentare (Dinosaurs) prd 
Pre-frontal prf 
Pre-maxillare pmx 
Pre-operculare 
( = Squamosal) prop 
pre-parietal pp 
Pre-sphenoid PS 
Pro-otic PO 
Pter-otic pto 
Pterygoid pt 
Quadrato-jugal qj 
Quandrate* Q 
Rostrale (Dinosaurs) r 
Septo-maxillare SM 
Sphen-ethmoid (Anura) SE 
Sphen-otic (Teleo- 
stomes) sphnot 
Spheno-turbinal (Mam- 
mals) ST 
Spleniale * spl 
Squamosal ( = Pre- 
operculare) sq 
Stapes * (Columella -J- 
Operculum) S 
Stylo-hyal* (Mammals) SH 
Sub-operculare sbop 
Supra-angulare * spang 
Supra-occipital (cartilagi- 
nous) SO 
Supra-temporal (Pter- 
otic) pto 
Symplectic * (Teleo- 
stomes) SYM 
Tabulare (Epi-otic) t 
Thyro-hyal (Mammals) TH 
Thryroideum, os (= Basi- 
branchial) BB 
Transversum, os (Ecto- 
pterygoid) tr 
Tympanicum T 
Tympano-hyal * (Mam- 
mals) TYH 
Uro-hyal * (Teleo- 
stomes) UH 
Vomer v 
A fundamental difference in the skulls of Anamnia and 
Amniota is seen in the bones that compose the floor of the 
cranial cavity. In the former this essential portion of the 
skull is formed largely by the parasphenoid, a dermal 
bone, derived from the roof of the pharynx which is here 
derived from the ectoderm (stomatodeum). In Teleostomes 
and in frogs this bone forms a conspicuous cross or T, while 
in Urodeles it practically covers the entire mouth in the form THE AXIAL SKELETON 
165 
of an enormous oval plate. In Amniotes, on the other hand, 
the parasphenoid seems to be entirely wanting, and its place is 
functionally replaced by a series of cartilage bones, some of 
which have appeared before, while others are new. Along 
the median line, beginning anteriorly, there are the pre- 
sphenoid, basi-sphenoid, and basi-occipital, and each of these 
is accompanied by a pair of lateral elements, respectively the 
orbito-sphenoids, ali-sphenoids, and ex-occipitals, thus forming 
a complex of nine cartilage bones. 
The sudden and total dropping out of so large and im- 
portant an element as the parasphenoid, a dermal bone, and 
the equally sudden appearance of a new cranial floor, com- 
posed of nine cartilage bones, some of which are new, is 
certainly a most unusual event, and many attempts have been 
made to account for it. The appearance of a new cartilage 
bone is not unprecedented, for in several portions of the 
cranium we have parts of the cartilaginous cranium still with 
us, and wherever there is cartilage a sufficiently urgent neces- 
sity may always call into existence new centers of ossification, 
but to adequately explain the reasons for a sudden total loss 
of a part that, up to its complete disappearance, has always 
been as large and important as the parasphenoid, is harder. 
Perhaps the best suggestion along this line is that which sees 
the continued presence of the parasphenoid in the Amniote 
vomer, which is seldom a paired element, as in Anamnia, but 
is most frequently median, and thus possibly not homologous 
with the paired vomer. However, this hypothesis has to ac- 
count for the loss of this latter part, which may be as hard 
to do as in the case of the parasphenoid, and, as an additional 
embarrassment, the vomer, in the embryo of the opossum, 
which is a marsupial and therefore very primitive, is found 
to come from paired anlagen, which subsequently fuse, bring- 
ing the vomer everywhere into line as an homologous element 
throughout. This conclusion, although highly satisfactory to 
the vomer, leaves the sudden and complete loss of so im- 
portant a part as the parasphenoid as unaccountable as ever. 
Another characteristic of vertebrate skulls, especially seen 166 
THE HISTORY OF THE HUMAN BODY 
in those where certain peculiar modifications change the 
stress that falls on certain parts, is the secondary fusion of 
originally distinct elements, a phenomenon that occurs wholly 
in response to physiological needs, and consequently shows 
no respect whatsoever to the preservation of the phylogenetic 
history. It oftentimes involves both cartilage and dermal 
bones, and may even include parts of the visceral skeleton, 
uniting these elements into a single bone-complex, wherever a 
special resistance may be needed. Each of these bone-com- 
plexes is, in the adult, a morphological enigma, many of which 
have yielded to modern research, while for others there are 
as yet no certain clews. 
Among mammals these consolidations are very extensive, 
especially among the primates, but even this condition is 
eclipsed by that in birds, where the fusion reaches its ex- 
treme and nearly all of the bones of the cranium proper are 
fused in the adult into a single piece, so that, in order to 
properly study the skull, observation must be made upon a 
newly-hatched fledgeling or even upon an advanced embryo 
extracted from the egg. 
Almost as good an example of the fusion of the original 
elements as that of birds is found in the skulls of Man and 
other Primates, where an extensive fusion of originally dis- 
tinct parts is the response to the development of so heavy a 
brain. Excellent illustrations of this are found in the “ sphe- 
noid ” and “ temporal ” bones of human anatomy, the first 
composed of eight separate elements, the second of six, and 
including, in its inner chamber, three more. The “ sphenoid ” 
is composed of the pre-sphenoid, with its two lateral pieces, 
the orbito-sphenoids; the basi-sphenoid, with its accompany- 
ing ali-sphenoids, and in addition the two pterygoids, which 
form its outer “ pterygoid plates.” The “ temporal ” is com- 
posed of the squamosal and the petrosal, the latter resolvable 
into pro-otic and opisth-otic, and to this, externally, is added 
the tympanic, a mammalian bone developed in the meatus of 
the ear. Two hyoid pieces, the tympano-hyal and stylo-hyal, 
are closely attached to the petrosal, and in later life usually THE AXIAL SKELETON 
167 
fuse with it. Lastly, with the building of the middle ear the 
three auditory ossicles, malleus, incus, and stapes, the last 
double (operculum columella) become so intimately as- 
sociated with the “ temporal ” as to be practically considered 
with it. 
In the skulls of both amphibians and amniotes, even in the 
most completely ossified ones, there remain certain portions 
of the unossified chondrocranium, especially about the nose 
and internal ears. In mammals there develop from this the 
cartilages of the external nose which, although often highly 
specialized, are to be considered in respect to origin the most 
ancient parts of the skull. 
In giving the account of the earlier stages in the develop- 
ment of the chondrocranium, nothing was said concerning the 
subsequent history of the cartilage of the eye-ball, which 
could not unite in the formation of the skull. In many fish 
the outer coating of the eye-ball (sclera) remains cartilaginous 
and occasionally becomes very thick and heavy (e.g., sword- 
fish). There are also traces of cartilage in the sclera of many 
salamanders. Both cartilage and bone occur in the sclera of 
certain birds, notably hawks and owls, in the latter of which 
a series of long, palisade-like ossicles forms an elongated tube, 
shaped something like the tubes of an opera-glass. Whether 
these are the direct descendants of the primitive capsule seems 
very doubtful, and it is more probable that these develop- 
ments have been called out de novo in response to functional 
necessity. CHAPTER VI 
THE VISCERAL SKELETON 
“ Multum restat adhuc, multumque restabit, et post multa 
saecula nemini occasio praecludetur aliquid adjiciendi.” 
Seneca. 
The visceral skeleton is associated with the anterior portion 
of the alimentary canal and the parts derived from it, and 
seems to have developed primarily along the sides of the 
pharynx for the regulation of those most primitive of verte- 
brate respiratory organs, the gill-slits. Although Amphioxus 
possesses a very regular and quite complicated system of 
skeletal bars to support its eighty or ninety pairs of gill-slits, 
these cannot as yet be brought into definite relationship with 
the visceral skeleton of the higher vertebrates; the same may 
be said of the visceral skeleton of the cyclostomes, which is in 
the form of a complicated pharyngeal basket, bearing little 
apparent resemblance either to the skeletal bars of Amphioxus 
or to the succession of simple arches characteristic of the 
fishes. It is in the selachians that we first meet with a vis- 
ceral skeleton to which that of the higher forms can be cer- 
tainly referred, and it is here, therefore, that the morphologi- 
cal history of the vertebrate visceral skeleton must start. 
This consists of a series of pairs of cartilages, more or less 
modified from the form of simple rods, and alternating with 
gill-slits that open from the exterior into the pharyngeal 
cavity. In most selachians there are seven well-developed 
pairs of these, besides a few cartilages that may represent 
rudiments of others, but as in two very primitive genera 
there are, respectively, eight and nine regular pairs, besides 
possible rudiments of others, eleven or twelve original pairs 
can be accounted for, thus suggesting a former condition with 
a large number of gill-slits, a supposition that compares well 
168 THE VISCERAL SKELETON 
169 
with the testimony furnished by Amphioxus. The selachian 
condition, showing all the pieces that may be accredited to the 
visceral skeleton, together with the chondrocranium is given 
Fig. 54. Skull and bran- 
chial skeleton of Squalus acan- 
thias, the dog-fish, after draw- 
ings by students. * 
(A) Lateral view. [C. E. Hep- 
burn.] (B) Ventral view. [J. L. 
Whitney.] 
PPQ, palato-pterygo-quadrate; 
Mk. Meckel’s cartilage; K, l. the 
two sets of labial cartilages; HM, 
hyomandibular; CH, cerato-hyal; 
Sp, spiracular cartilage; BB, basi- 
branchial; HB, hypo-branchial; CB, 
cerato-branchial; EB, epi-branchial; 
PB, pharyngo-branchial. The sub- 
script numerals designate the sepa- 
rate gill arches, the Roman numer- 
als the visceral arches. 
*In these two figures the rela- 
tionships of hypo- and cerato-bran- 
chials is incorrectly shown. CBi 
does not articulate with any of 
the hypo-branchials, but ends 
freely medially, while the cerato- 
branchial that really connects with 
HBi, is CB2. In the same way 
CB3 articulates with HB2, and CB4 
with HB3. Cf. Miss Hosford’s 
paper in Anat. Record for April, 
1920. 
in Fig. 54, somewhat diagrammatized. It may be safely as- 
sumed that at one time these visceral arches were all similar 
to one another, associated with similar gill-slits and all gill- 170 
THE HISTORY OF THE HUMAN BODY 
bearing, although important modifications have now taken 
place in certain ones of them. These modifications are of the 
highest importance in this history and may now be considered, 
with constant reference to the figure. 
The first, or mandibular pair, which is the most modified 
of all, becomes folded about the mouth in such a way as to 
make a serviceable pair of jaws, both upper and lower, with 
each of which several rows of pointed placoid scales are as- 
sociated to serve as teeth. In short, one hardly knows whether 
to describe these parts as gill-arches covered with placoid 
scales, or as jaws equipped with teeth, since their origin as 
the first, and their present and future function as the second, 
are both so clearly indicated. 
We have thus revealed in these organs a valuable bit of 
history, since we can here observe the jaws and teeth of ver- 
tebrates almost at their birth, and can learn the source from 
which the material was derived and how it was first trans- 
formed. It cannot be said, however, that the jaws as used 
here have developed directly into those of the higher verte- 
brates, since in the intermediate history there is much addition 
of new material and replacement of old, with the one object 
in view of increased physiological effectiveness. 
The assumption of an upper and lower jaw marks an epoch 
in early vertebrate history, since these organs, once acquired, 
replace forever the circular jawless mouth, and hood-like lip, 
characteristic of both Amphioxus and cyclostomes. So radi- 
cal must have been the change that some think that even the 
mouth opening is a different one, that the one associated 
with the new jaws was once merely a gill-slit like the rest, 
through which some ancestor acquired the habit of admitting 
food, and that its manifest advantage over the other mouth 
in its convenient skeletal equipment, caused the disappearance 
of the old one and the perfection of the new; there are, how- 
ever, certain indications that point to the former possession 
of a still older mouth, the palceostoma, homologous with that 
of the tunicate, and with this both the hood-like mouth of 
the cyclostomes and the slit form of other vertebrates may be THE VISCERAL SKELETON 
171 
contrasted as the neostoma, or secondary mouth. This last 
expression applies simply to the opening, which is probably 
homologous in all vertebrates, but becomes completely meta- 
morphosed by the addition of jaws. [Cf. below, Chap. XII, 
sub i'Iypophysis.\ 
The use of placoid scales as teeth is also a new idea, and 
they are certainly a great advance over the horny epidermic 
excrescences that arm the circular lip of the cyclostomes. Pla- 
coid scales are composite structures, composed of dentine cov- 
ered over by enamel, and they are so perfectly fitted for the 
purpose that they have had no rivals for the office; thus, al- 
though the changes in form and arrangement have been num- 
berless, the teeth of even the highest vertebrates, composed of 
dentine overlaid by enamel, attest their origin from placoid 
scales. Aside from the testimony of comparative anatomy, 
the embryonic history of the teeth, even in the highest form, 
is a direct corroborative testimony to this mode of origin. 
That the primitive Selachian, having once hit upon the idea of 
receiving food through a gill-slit, and having equipped it by 
folding up a gill-arch to serve as its skeleton, should have 
annexed also the scales which ran along the border of the 
slit, as accessory organs for the retention and dismemberment 
of the food, would follow as a natural consequence, and thus 
the teeth of the present-day Selachians may be so considered; 
simply placoid scales, still arranged in several rows, like those 
that cover the external surface. That in these rows the teeth 
are arranged opposite each other, and not in alternate series, 
as elsewhere, may be considered merely a detail resulting from 
the change of function. 
Associated with the first pair of visceral arches, the primi- 
tive jaws, are the cartilages of the second pair, the hyoid, 
also emancipated from the function of bearing gills and modi- 
fied in part to assist in the action of the jaws. Like the first, 
these arches also consist of two pieces, a dorsal and a ventral 
one. The first, called the hyomandibular, is more or less de- 
tached from the other and forms an intermediate piece, tech- 
nically called a suspensorium, between the cranium and the 172 
THE HISTORY OF THE HUMAN BODY 
true jaw. To this is also attached the ventral piece, or hyoid 
proper. 
Aside from these definite and well-developed visceral 
arches, as they may be called collectively, there are the rudi- 
ments mentioned above, and functionally of little importance, 
held by some to be the reduced remnants of still other arches. 
Such are the labial cartilages, lying at the angle of the mouth, 
and used to strengthen the integumental folds of the lips; an- 
other of these is the spiracular cartilage, crescentic in shape, 
and surrounding the spiraculum or blow-hole, perhaps a modi- 
fied gill-slit, anterior to the others. These, if admitted to the 
series, will considerably increase the number of original vis- 
ceral arches, and render more probable their descent from a 
form like Amphioxus. 
The remaining five arches of the Selachians are all much 
alike, and are gill-bearing, associated with gill-slits. Each 
typically consists of four pieces, named, beginning ventrally, 
hypo-branchial, cerato-branchial, epi-branchial and pharyngo- 
branchial, and along the mid-ventral line the four pieces of each 
side are united by a median basi-branchial. This would make 
each gill-arch a ring of nine pieces, surrounding the pharynx, 
and providing for it a skeletal framework surrounding the 
mucous membrane, composed of forty-five separate pieces; 
in actual cases, at least in modern times, this framework is 
no longer complete, and suffers many modifications of the 
original plan. 
In the other Orders of fishes the visceral skeleton becomes 
somewhat modified, but the seven pairs of visceral arches are 
recognizable in all cases. The most marked changes are those 
affecting the jaws, and are primarily due to the extensive de- 
velopment of dermal bones, which reinforce the cartilaginous 
bars, as they do in the case of the chondrocranium. The lower 
jaw becomes almost entirely encased by them, the principal 
dermal elements being the dentary, which covers the outside 
and bears the most or all of the teeth, and the angulare, which 
covers the inner side. 
Lying within these, as if bound in splints, is the cartilagi- THE VISCERAL SKELETON 
173 
nous lower jaw, the original first visceral arch, which is des- 
tined from now on to lose its functional importance save at 
its posterior end, which here emerges from the splints and pre- 
sents a rounded articular surface. This piece is called the 
articulare, and sometimes ossifies, forming a cartilage bone. 
The entire cartilaginous arch, thus subordinated, the mandibu- 
lar cartilage, is also called Meckel’s cartilage, the name com- 
memorating the distinguished German anatomist, Johann 
Friedrich Meckel [1781-1833], who first saw it in the em- 
bryo human jaw, lying encased in the dermal bones, much as 
in the adult ganoid or teleost. 
As in the case of the skull proper, the mandible was also 
more complex, and was possessed of more parts, in early 
days, than is found in any form still living. From fossil im- 
Fig. 55. Mandible of Trimerorhachis, a Stegocephalian, exhibiting an 
unusually large number of membrane bones. 
(a) inner asnect, (b) outer aspect. 
There are three coronoids, which bear the most of the teeth, two splenials, a supra-angulare 
and a pre-articulare. [After Williston.] 
pressions of early fishes, amphibians, and reptiles, together 
with an occasional glimpse of an element not usually found, 
we may reconstruct the original condition of the mandible, 
when it consisted of a mandibular cartilage, overlaid by a 
complete panoply of dermal bones. Upon the outer aspect 
the mandible was largely covered by the dentare, usually 
dentigerous, overlapped near the proximal end by the supra- 174 
THE HISTORY OF THE HUMAN BODY 
angulare. Below this latter, and covering the ventral edge 
of the jaw, so that it might be seen from either the inner or 
outer aspect, is the angulare. Upon the inner aspect, which 
is somewhat more complex than the outer, the angulare is 
seen again, wrapped around the ventral edge at the proximal 
end, above which comes the pre-articulare, usually the largest 
Fig. 56. Mandible of Labidosaurus, a Cotylosaurian reptile, showing a 
somewhat simpler condition than Trimerorhachis. 
(a) inner aspect, (b) outer aspect. 
The coronoid series is reduced to a single one, there is but one splenial, but the supra-angulare 
and the pre-articulare are still distinct. [After Williston.] 
element of the inner side. Anterior to this comes the spleniale, 
while above the pre-articulare, forming the large process of 
attachment for the temporalis muscle, the coronale is placed. 
In a cross-section of a mandible taken posteriorly over the 
supra-angulare and coronale, the dermal bones make a four- 
sided box, enclosing the mandibular cartilage, while in a cross- 
section taken through the jaw in its anterior half, the dermal 
elements consist of only two, the dentare and spleniale, ar- 
ranged somewhat like the capital letter D. 
While the primitive mandible, as developed in the first 
animals that exhibited dermal bones, may be considered as 
having had about the above elements, there are some cases, 
mainly in extinct forms, in which the condition is still more 
complex, owing doubtless to a secondary subdivision of the 
original elements. Thus a jaw may have two, or three, or THE VISCERAL SKELETON 
175 
even more, coronalia, two splenialia, and so on. Also it must 
be remembered that usually, at least above the amphibians, 
the articulare, instead of being left as a ball of cartilage, is 
ossified from a center, becoming a cartilage bone, adding 
another bone to the jaw complex. 
The original upper jaw, however, loses still more prestige, 
since its function as a jaw is entirely usurped by a set of 
Fig. 57. Diagrammatic cross-sections through a typical mandible of 
a reptile or amphibian. 
(a) proximal, (b) distal. 
In the first, the two sides of the sheath, which contains the mandibular cartilage, is composed of 
the pre-articular (prart) within, and the supra-angular (spang) without. The base is formed by the angu- 
lar e (ang) and the cover, which bears teeth, is the coronoid. (c) In the distal section the inner and outer 
sides are made respectively of the spleniale (spl) and denlare (d), the latter curving around to form 
also the base and cover. 
dermal bones, the prxmaxillary and maxillary, placed paral- 
lel to, but outside of it. In spite of this, however, it becomes 
directly encased by other dermal bones, the palatine and the 
pterygoid, and is retained as an accessory upper jaw in some 
fishes and a few salamanders. Its posterior end, like that of 
the lower jaw, remains free and, after the reduction of the 
anterior portion, fits in between the hyomandibular and the 
articulare of the lower jaw as an extra suspensory piece. In 
later development it ossifies as a cartilage bone, under the 
name of quadrate. As for the ultimate fate of the anterior 
portion, the dermal palatine and pterygoid enlarge at its ex- 
pense, and it is retained as an unimportant bit of cartilage 
in some amphibians, but beyond these it is not seen. The two 
dermal pieces, which originally encased it, however, retain con- 176 
THE HISTORY OF THE HUMAN BODY 
siderable importance and usually appear in the skulls of am- 
niotes along a curve approximately parallel to that of the 
maxillaries, but interior to it, thus marking the former po- 
sition of the lost cartilage. These bones are often tooth- 
bearing in fishes and amphibians, retaining this much of their 
former function. 
Of the five pairs of true branchial arches, the first four 
are retained in ganoids and teleosts as gill-bearers, while the 
fifth lies in the floor of the pharynx and is often covered 
with sharp teeth arranged in several rows (= “inferior 
pharyngeal bones ”). 
A notable modification, which, however, is mainly an ex- 
ternal one, is found in the development of the operculum or 
gill-flap, which appears first as a fold of skin and becomes 
reinforced by special dermal bone. This ultimately develops 
posteriorly so as to cover all the gill-slits in such a way that 
they seem to be internal, and are not visible from the exterior, 
as in most selachians. 
The subsequent history of the visceral skeleton and its fate 
in amphibians, reptiles, birds, and mammals may be quickly 
outlined. These Classes are fundamentally terrestrial, and 
never possess genuine internal gills, and thus the main changes 
are due to a loss of function, which, by throwing the branchial 
arches out of employment, would have caused them to disap- 
pear, were it not that they could be in part employed to sub- 
serve some other necessary function. It will be seen that in 
most cases this has been their fate, but this very change of 
function, although it saves them from complete destruction, 
of necessity causes considerable modification, oftentimes a pro- 
found one. 
Beginning with the most anterior arches, the mandibular 
and hyoid, the hyomandibular piece seems to disappear en- 
tirely, although doubtfully identified by some morphologists 
with certain other elements in the otic region, and leaves to the 
quadrate the responsible role of being the only suspensory 
piece for the mandible. As a cartilage bone the quadrate per- THE VISCERAL SKELETON 
177 
sists in amphibians, reptiles, and birds. The several elements 
of the mandible remain distinct in amphibians and reptiles, 
but consolidate in birds, the proximal end, which articulates 
with the quadrate, being in all cases the free posterior end of 
Meckel’s cartilage ( — articulare). In mammals a great change 
takes place in these parts, the history of which is repeated in 
the developing embryo, through which the facts first came to 
light. Here both quadrate and articulare, external at first, as 
in amphibians and reptiles, become drawn into the tympanic 
cavity (middle ear) where, still retaining approximately their 
original shape, though proportionately reduced in size, they be- 
come the incus and malleus, respectively, while the mandible, 
each half consolidated into a single bone, forms a new articula- 
tion directly with the skull in the petrosal region. The old 
articulation of the mandible, that between quadrate and articu- 
lare, now incus and malleus, after having served so long and 
well in the mastication of food, emancipated from this coarse 
work and remaining almost embryonic in point of size, be- 
comes attuned to sound waves and assists in their transmission! 
The third and innermost bone of the tympanic cavity, the 
stapes, has been for a long time a true bone of contention, in 
spite of its small size. Some authorities have attempted to 
identify it with the missing hyomandibular, the dorsal half of 
the second visceral arch, which disappears above the fish. It 
appears, however, to have had a double origin, one for the 
loop, the other for the base. The first is the visceral element, 
the missing hyomandibular, while the base seems to have been 
originally derived in amphibians from the cartilaginous wall of 
the otic capsule and to be thus a part of the chondrocranium, 
and not an element of the visceral system. The foramen in 
this minute bone, to which it owes its stirrup-like shape in 
man and in some other mammals, transmits an artery which 
in man disappears in the embryo, but in insectivores and 
rodents is retained throughout life, the arteria stapedialis. 
The remaining arches subserve in part the original function 
of gill-bearers so long as there is opportunity, which occurs 178 
THE HISTORY OF THE HUMAN BODY 
Fig. 58. Morphology of the visceral skeleton. 
(A) Selachian. (B) Amphibian. (C) Sauropsidan. (D) Mammal. 
In all the figures the visceral arches are designated by Roman numerals; in the 
case of the first two the dorsal and ventral segments are further designated by ex- 
ponent letters (d and v). Other designations are, lb, labial cartilage; s, spiracular 
cartilage; o, operculum. In arch VII the arytaenoid and tracheal cartilages are 
designated by the exponents a and b, respectively. THE VISCERAL SKELETON 
179 
only in a few amphibians, and then mainly during larval life, 
and are otherwise modified to assist in the functions performed 
by two organs that develop in the region, the tongue and the 
larynx. The latter makes its appearance in a very simple 
condition, associated with two bag-like lungs, in the most 
primitive of the salamanders, and utilizes as the first laryngeal 
cartilage, cartilago lateralis, the last of the gill-arches (5th 
branchial or 7th visceral arch). This element, which, by sub- 
division and metamorphosis, develops into a pair of arytcenoid 
cartilages and a series of lateral tracheal pieces, shows itself 
capable of becoming a complicated mechanism, sufficient for 
the needs of amphibians; but in the reptiles the 4th gill-arch 
(6th visceral) becomes associated with it as the epiglottis, 
which here appears for the first time. The reptilian larynx, 
with but little modification, is employed by the birds, but in 
mammals the next two gill-arches, counting anteriorly, the 
2nd and 3rd (4th and 5th visceral) become associated together 
in front of the larynx and form the protecting shield-like 
piece, the thyreoid, which in the lowest Order (Monotremata) 
still appears like two pairs of arches covering the larynx 
ventrally. 
It may be said in general that in all the Classes, from the 
amphibians on, those visceral arches not employed as jaws 
or as laryngeal cartilages form a hyoid or hyo-branchial com- 
plex and furnish a skeletal equipment for the tongue, an organ 
which often develops voluminously, fitted for very special 
work. Thus, in the amphibians in general this complex con- 
sists of the 2nd to the 6th visceral arches, inclusive; in reptiles 
and birds of the 2nd to the 5th, and in mammals of the 2nd 
and 3rd only. In the latter these two remaining arches form 
a complex consisting of a median piece, the basi-hyal, and 
two pairs of cornua, the anterior pair representing the 2nd 
visceral arch, the true hyoid of fishes and the posterior pair 
the 3rd visceral or 1st gill-arch. In most mammals the an- 
terior cornua of this hyoid complex consist of a chain of 
small bones, which, enumerated from the basi-hyal (“ body of 180 
THE HISTORY OF THE HUMAN BODY 
the hyoid”), are: cerato-hyal, epi-hyal, stylo-hyal, and 
tympano-hyal, the last closely associated with the external 
opening of the ear. In man and the higher apes the two 
latter are fused with the skull to form the “ styloid-process of 
the temporal bone,” and this is connected with the cerato- 
hyal ( = “ lesser or anterior cornua ”) by the stylo-hyoid 
ligament, which replaces the missing epi-hyal. Occasionally 
a rudiment of this latter bone is found in the middle of the 
ligament. 
A singular metamorphosis of a portion of a visceral arch 
is that by which in mammals the outer end of the 2nd or hyoid 
arch, naturally located near the external opening of the ear, 
segments off and becomes transformed into the cartilage of 
the external ear-flap, the auricula or pinna, which in the 
various Orders responds readily to the varied environments of 
different mammals and exhibits a great range of variation in 
shape and size. 
Of what a series of changes and unexpected metamorpho- 
ses has the visceral skeleton shown itself capable, and what 
vicissitudes have the several elements experienced! Begin- 
ning as a set of similar arches, regulating the opening and clos- 
ing of gill-slits, they become jaws, vocal organs, supports for 
the tongue, suspensory pieces for the mandible, tympanic 
ossicles and flapping external ears. Indeed, a well-recognized, 
though not generally accepted, theory derives from them also 
the skeleton of the free limbs, including the shoulder and 
hip girdles, and even the long bones of the limbs and the 
numerous smaller pieces in carpus and tarsus and in the digits. 
But even without this latter theory, which appears untenable, 
the subject presents a remarkable history of repeated change 
of function, of the adaptation of old material to new purposes, 
of the dethronement of the old systems and the employment 
of their organs for the development of the new; we see here 
Nature constantly exploring new environments, and then 
adapting function to environment, and material to function, 
constantly making over old organs in obedience to mechanical THE VISCERAL SKELETON 
181 
laws and seldom originating or creating new ones de novo. The 
results are thus often imperfect and incomplete, and are fre- 
quently obtained by a very indirect and circuitous route, 
and although many an animal form and many myriads of 
individuals have succumbed as the result of some mechan- 
ical disadvantage which the skill of a simple human mechanic 
could have remedied, there is never the least sign or indica- 
tion of such an interference. Everything develops as an in- 
evitable result of natural law, as a part of the general plan 
which is broad enough to include the entire universe and which 
is willing to sacrifice countless hecatombs of lives rather than 
submit to a single exception to its laws. CHAPTER VII 
THE APPENDICULAR SKELETON 
“ the two organs whose contact with the environment is 
most intimate are the feet and teeth, and these are seen to 
suffer the most profound changes with the passage of 
time and the consequent changing of the environment. 
. . . Thus it is by a study of feet and teeth so much of 
an animal’s life conditions and consequent habits can be 
deduced.” 
Richard Swann Lull, Organic Evolution, 1917, p. 549. 
In about the same degree as the visceral skeleton of the true 
vertebrates is suggested by the gill armature of Amphioxus, 
so do its fin-folds and the rows of spines enclosed by them 
suggest a simple condition from which may be derived the 
appendicular or limb skeleton. In Amphioxus a continuous 
though very low fin-fold begins near the anterior end, runs 
the entire length of the animal, and is continuous around the 
tail with a median ventral one as far forward as the atriopore, 
where it divides into two, which, as the metapleural folds, 
continue almost to the mouth. 
This fold system is supported throughout its entire length 
by a skeleton of gelatinous fin-spines, thus forming an ap- 
paratus, which, if developed slightly more than in Amphioxus, 
would be very serviceable in retaining the equilibrium while 
swimming, serving as dorsal and ventral keels. The skeletal 
fin-rays would become developed as well as the external folds, 
and the general appearance would be not unlike that sug- 
gested by Fig. 59, a. It will be noticed that in Amphioxus 
certain parts of the caudal fin are wider than the rest, showing 
how responsive this fold is to a localized increase of function, 
and this allows one to draw the hypothetical ancestor with 
certain areas of the fin-fold marked by a greater width, corre- 
sponding to the places where the greatest stress would be apt 
182 THE APPENDICULAR SKELETON 
183 
to come in an actively swimming animal. As a farther de- 
parture from Amphioxus, the division of the median ventral 
fin takes place at the anus, and not at an atriopore, since this 
latter is probably a special organ developed to supply the 
needs of Amphioxus, and not transmitted to any of its de- 
scendants. 
Fig. 59. Diagrams illustrating the fin-fold theory. 
(a) and (b) [after Wiedersheim] represent the unmodified theory. A continuous 
fin-fold, stiffened by skeletal rays, extends along the median dorsal line, around the 
tail, and along the mid-ventral line as far as the cloaca, where it divides into two 
lateral folds that extend along the sides of the trunk. The retention of portions 
of this fold and the loss of the intermediate portions results in the formation of both 
median and paired fins. In (c) [after Rabl] is shown Rabl’s modification of this 
theory. The two lateral folds are from the beginning distinct from the median one, 
and are hence subjected to external influences, especially at their free anterior and 
posterior ends, thus modifying the first and last rays the most, and the others in 
a progressively decreasing series towards the middle of the fold. When, later, the 
first and the last portions become set off (along the dotted lines) and the interme- 
diate portion suppressed, they form fins, of which the anterior one shows greater 
modifications along its anterior, the other along its posterior border, precisely as 
is the actual case among fishes. 
Judging from the final results as we see them in the great 
Class of fishes, it may be supposed that the inequalities in the 
development of this fin-fold increased through localized func- 
tional activity, until certain portions became especially well 
developed, while the intermediate portions were entirely lost 
(Fig. 59, b). The significance of this is seen if this figure be 184 
THE HISTORY OF THE HUMAN BODY 
now compared with any good, typical fish, which shows the 
perfected type resulting from this process. Here the fins are 
all alike in structure, proving their derivation from a common 
origin, but are divided into two groups in accordance with 
their position, median and paired. Of the median fins the 
dorsal (one or more) and the anal function as keels to re- 
tain the equilibrium and prevent the body from rolling side- 
ways, and the caudal fin is the main organ of propulsion, but 
may act in part also as a rudder to regulate the direction in 
which the animal moves. The paired fins, which bear the in- 
appropriate names of pectorals and ventrals, act as subsidiary 
oars or paddles and seem mainly to guide and maintain the 
course. 
A recent suggestion, which serves as an addition to the fin- 
fold theory, is given in Fig. 59, c, in which it is supposed that 
the lateral folds are primarily distinct from the median one, 
and that the paired fins develop from their free extremities, 
where the stress of motion naturally comes. This furnishes 
a reason other than chance for the formation of two, and only 
two, pairs of fins, and also explains a sort of symmetry shown 
in the two sets of fins of many fishes, since the free edge, 
and hence, the strongest development, is anterior in the for- 
ward fin and posterior in the back one. 
The median fins, being of use only in the water, disappear 
above the fish, although the necessity of similar organs for 
aquatic life is well shown by the fact that in secondarily 
aquatic higher vertebrates which have returned to the water 
although derived from a terrestrial ancestry, some new form of 
median fins becomes developed. Such animals have lost the 
serviceable median fins of fishes, with their strong skeletal 
spines, and, as they cannot recall them, are forced to develop 
some makeshift arrangement to serve the purpose. Thus, 
aquatic amphibians develop a tail fin of integument without 
skeletal support, the tail of the crocodile is supplied with 
keels formed of projecting scales, and the whale has manu- 
factured perfect dorsal and caudal fins out of whole cloth, as THE APPENDICULAR SKELETON 
185 
it were, since they are made from thick, though hairless, 
mammalian integument, reinforced by connective tissue. The 
dorsal fin of this latter, as is the case with other external 
details, is wonderfully fish-like, but the caudal fin is flattened 
the other way, and extends laterally, instead of up and down, 
as in fishes. 
To summarize, then, the original median fins of fishes have 
developed to subserve the needs of an aquatic life, and disap- 
pear forever with the assumption of a terrestrial environment, 
although, in secondarily aquatic forms, similar organs are de- 
veloped from other sources. The paired fins, on the other 
hand, useful in fishes, assume their highest importance on 
land, and become the two pairs of free limbs. To these more 
than to any other organ, the higher vertebrates owe their ex- 
traordinary development, and their high degree of success 
in occupying so many sorts of environment, since by this means 
they have been able to possess the surface of the earth, to oc- 
cupy the trees, to burrow in the ground, to return to the sea 
and even to conquer the problem of aerial navigation. It is, 
moreover, probable that the emancipation of the fore limbs 
from the function of locomotion and the acquirement by them 
of prehensile powers, which enable them to seize objects and 
bring them to the immediate attention of the sense organs, 
have been the chief causes of the excessive brain development 
which has achieved for the Primates the greatest success thus 
far attained in the domination of the world. 
Corresponding to the great variety of functions of which 
the paired limbs are capable, there is an equally vast series of 
modifications of structure, presenting an army of forms which 
include various sorts of ambulatory limbs, paddles, grasping 
organs, tools for excavating the earth, and wings of several 
sorts. Among the modifications there must be included also 
the numerous cases of limb reduction, which may affect the 
fore or hind limbs or both, and exhibit every stage of reduc- 
tion down to total loss. Thus the amphibian Siren has lost 
its hind legs totally, while retaining its fore legs, and simi- 186 
THE HISTORY OF THE HUMAN BODY 
larly in the case of the whale the fore legs become developed 
into enormous flippers or paddles, while the hind legs are re- 
duced to useless rudiments entirely concealed beneath the 
skin. The opposite tendency is seen in the ostrich, where the 
legs are very stout and heavy, while the wings are much re- 
duced and almost without function, and is still better exhibited 
in the kiwi-kiwi of New Zealand, in which the wings have 
totally disappeared. In several groups the complete or nearly 
complete reduction of both pairs is correlated with an exces- 
sive lengthening of the body, locomotion being effected by 
an undulatory movement of the entire animal. In snakes, 
which progress mainly through the action of their very numer- 
ous ribs, the loss of both pairs of limbs is usually a total 
one, but in the boas the posterior limbs appear as spur-like 
rudiments, situated upon either side of the cloacal orifice. 
Aside from the snakes a similar form is assumed by several 
group of reptiles and amphibians, the adaptation fitting them 
in some cases for a life similar to that of snakes, and in others 
for a subterranean existence. These latter, which include at 
least one group of lizards and one of amphibians, burrow in 
the earth like earth-worms, and as in the process of this 
adaptation they have lost their eyes, reduced their head and 
arranged their scales in the form of annular segments, they 
resemble these latter animals almost to the point of deception. 
Through all their vicissitudes, however, the number of free 
limbs is constant in all vertebrates, except when secondarily 
reduced, and consists of two pairs, corresponding, as we sup- 
pose, to the number of original points at which the primitive 
fin-fold became hypertrophied. Although this number may 
be reduced as a special adaptation, it can never be increased, 
and the favorite mythological conceptions of human and other 
vertebrate forms with supernumerary limbs are far more im- 
possible and absurd than is usually recognized, since they are 
generally held to be merely contrary to experience, but are 
here seen to violate the fundamental principles of develop- THE APPENDICULAR SKELETON 
187 
ment.* It might, indeed, have been possible in the first place 
for the lateral fin-folds to have hypertrophied in three or more 
places instead of two, a result which some slight change of 
environment or habit would have then easily effected, but the 
time for accomplishing this is now long past and, the number 
of limb-anlagen once established, no subsequent change is pos- 
sible. Here again the principle is clearly enunciated that 
there is never any anticipation for the future in the history 
of development, and that each step is taken with sole refer- 
ence to the needs of the animal that takes it. Although a 
given form is destined to be the ancestor of a great group of 
higher animals, yet there is no prescience exhibited in the 
details of its structure other than the needs of its own economy, 
and the task of laying down the lines in accordance with which 
its numberless descendants are to be constructed is left to 
the chance of the necessary adaptations. These conditioning 
characteristics may or may not be the best for the future, but 
in either case they are transmitted to posterity, to grant them 
success or failure, as the case may be. 
The form assumed by the free paired limbs throughout the 
Class of fishes is that of a fin or ichthyopterygium, a type con- 
sisting of a thin double membrane supported by a variable 
number of fin-rays; a single type also underlies the countless 
modifications exhibited by the higher vertebrates, the hand- 
form or chiropterygium, a type consisting of three main di- 
visions, proximal, medial, and distal, the last terminating in 
five digits. Simple as it is to refer all modifications existing 
in higher vertebrates to the latter, and in the fishes to the 
former type, no satisfactory explanation has thus far been 
forthcoming to bridge the wide gap existing between the two. 
* Cases of monstrosities with extra limbs, in which the total number ex- 
ceeds four, are not violations of this priniciple, but are anomalies due to a 
multiplication of certain of the anlagen, like monsters with two heads or 
two bodies. The cause of such redundancy is as yet imperfectly under- 
stood, but enough has been already proven to show that it lies in the 
germ, in which the abnormality probably exists in the form of redundant 
germinal elements. 188 
THE HISTORY OF THE HUMAN BODY 
That the two possess an independent origin would involve the 
total suppression of the appendicular apparatus formed from 
the fin-fold and the development de novo of two pairs of ap- 
pendages in the same place, and for the same or a similar 
purpose, a supposition which involves too much improbability 
to be considered for a moment. The free limbs of the one 
type must be strictly homologous with those of the other, and 
the fact of the present distinctness of the two types is un- 
doubtedly due to the extinction of transition forms. This 
Fig. 6o. Diagrams illustrating the development of the fin skeleton; 
based on that of selachians. [After Wiedersheim.] In (a) and (c) 
the right side shows a slightly older stage than the left. 
transition must have taken place at the epoch at which the 
vertebrates first attained the land, a transition which must 
have been a comparatively abrupt one, characterized by a 
rapid adjustment to the needs of a terrestrial life. It thus 
follows that the transition forms themselves must have been 
put at a disadvantage when in competition both with their im- 
mediate descendants, which were better fitted for the land, 
and with their immediate ancestors, which had never left 
the water, and their rapid extinction was a necessary conse- 
quence. The deficiency in the historical record at this place, 
however, has not prevented speculation on this subject; on the 
contrary, it has proved an especially attractive field for the 
anatomical philosopher, and the discussion of some of the 
leading theories of this question will be considered farther pn 
in the present chapter. 
Aside from this problem, however, the history of the d^ THE APPENDICULAR SKELETON 
189 
velopment of the limbs is by no means clear in other respects, 
and although the faith in their complex homology throughout 
is universal, the manner of their development and the relation 
of the various forms to one another cannot be agreed upon. 
Embryology, which is usually so suggestive, is practically 
silent here, since the record seems in all cases to be much 
abbreviated. The best that can be done, therefore, is to 
arrange a sequence of adult forms which seem to show transi- 
tions from one type to another, paying as much regard as 
possible to the lines of descent as indicated by the other parts. 
In this way have been sketched the histories which follow, 
and in reading them it must be remembered that a history 
founded merely on a succession of adult forms, and remain- 
ing unsubstantiated by a parallel series of embryonic stages, 
rests upon an insecure foundation and is liable to receive con- 
siderable modification through the discovery of additional 
facts. In tracing these histories the anterior and posterior 
limbs must be treated separately, since, although the free 
limbs are plainly serially homologous, and often correspond 
quite closely, part for part, the girdles, although equally a por- 
tion of the appendicular skeleton, differ fundamentally from 
one another and must have had a somewhat different early 
history. 
Beginning with the posterior limb, which is more conserva- 
tive than the anterior, and probably retains more primitive 
characteristics, it may safely be supposed that at its origin as 
a localized flap of a once continuous fin-fold, its skeleton con- 
sisted of a series of spines or rays, independent of one another, 
and somewhat larger at their bases, tapering to their free 
extremities (Fig. 60, a). To insure strength and to gain a 
concerted action, a very natural step would be to widen these 
at their bases still farther until they fuse, forming a piece 
something like a comb, with long teeth far apart (Fig. 60, b). 
As these organs become of still greater importance and need 
a firmer support, the basal portions, corresponding to the 
backs of the combs, would be likely to grow inwards until 190 
THE HISTORY OF THE HUMAN BODY 
they meet and fuse across the mid-ventral line, thus forming 
a very primitive girdle, with which the free part would become 
movably articulated (Fig. 60, c). This last case, is, however, 
not a hypothetical one, but drawn directly from the posterior 
girdle and free limb skeleton of the dog-fish, a typical se- 
lachian, in which the free limb consists of a basipterygium, 
bearing a series of rays, the whole being movably attached to a 
girdle in the form of a ventral band. Although there is at 
present an impassable space between this free limb and that 
of even the simplest amphibian, the girdles of the two forms 
are not so far apart, since a broadening of the middle piece 
into a plate, and the extension of its ends dorsally until they 
come in contact with a pair of ribs, would convert the one 
into the other. (Cf. Fig. 61, d, Necturus). 
The fault in the above theory is that, while offering a direct 
transition from the selachian to the amphibian form, it leaves 
no solution for the various conditions that occur in ganoids, 
some of which, at least, ought to be included in the line of 
descent. For these a plausible solution is offered in Fig. 61. 
The first of these figures is that of Acipenser, the sturgeon 
(Fig. 61, a), and still represents the primitive condition of 
parallel fin-rays, save that several of the anterior ones have 
fused in order to meet the greater strain imposed upon them. 
This tendency has progressed still farther in a related ganoid, 
Scaphyrhynchus (Fig. 61, b), where the proximal portions of 
nearly all the rays are included in the heavy basal piece, 
which bears both the distal portions of the rays of which it 
is composed as well as the few original rays which have not 
entered into its formation. In this an important step is the 
formation of a pair of little pieces, which are segmented 
off from the proximal ends of the two basal pieces, and which 
serve to interpret the condition found in Polypterus, in many 
ways the highest of the living ganoids and the one nearest 
the amphibians (Fig. 61, c). On this the basal piece of 
each fin has partly ossified and becomes a long limb-bone 
highly suggestive of a femur, and the two are attached to a THE APPENDICULAR SKELETON 
191 
small mid-ventral piece, a rudimentary girdle, which is divided 
by a suture into two portions, plainly the same as the two 
Fig. 6i. Series illustrating a theory of the phylogenetic development 
of the pelvic girdle. [Mainly after Wiedersheim.] 
(a) Acipenser (sturgeon), (b) Scaphyrhynchus (shovel-nosed ganoid), (c) Polyp• 
terus (ganoid). (c) Necturus (primitive salamander). (e) Dactylethra (South 
African frog), (f) Turtle. 
In (a) the part m is formed by a fusion of the anterior- rays. The pieces kk, 
segmented off from m in (e), form in (c) a rhomboidal plate. In (d) this plate 
has grown large and bears a pair of ossified ilia, i, and a pair of centers of ossi- 
fication, the ischia, h. In (e) appear two more ossific centers, the pubes, g. (f) 
is a typical pelvic girdle, with all its parts. The epipubis, e in (e), is incidental, 
and unimportant in this connection. 
small inner pieces of Scaphyrhynchus, here united to form a 
rhomboid plate. 192 
THE HISTORY OF THE HUMAN BODY 
The derivation from this of the condition found in the 
urodele Necturus becomes at once evident in a comparison 
of the two (Fig. 61, c and d), in the latter of which the steps 
in advance consist of the enlargement of the plate, its con- 
nection with the vertebral column through two small proc- 
esses, the ilia, that extend dorsally, and the appearance of 
two centers of ossification posteriorly. In this the real transi- 
tion from the fish type to that characteristic of the higher 
vertebrates consists of the direct connection between the hip- 
girdle and the vertebral column, a condition never found in 
fishes. That this attachment has been newly acquired at the 
stage represented by Necturus is evidenced by the lack of 
difference between the sacral vertebra, to which the attachment 
is made, and the adjacent ones, as well as by the frequency of 
variation in the vertebra selected for this attachment, as ex- 
plained above. 
Within the plate itself are two ossifications, which represent 
the first appearance of the ischiadic bones, destined to become 
an important element in the hip-girdles of higher forms; and 
in some of the higher amphibians; another pair of osseous 
elements, the pubic bones, also appear in anurans (Fig. 61, 
e). 
From the latter form of girdle to that of a reptile (Fig. 
61, f), the step is a smaller one, the main difference being in 
the formation of a large obturator foramen between the ven- 
tral elements, pubis and ischium, a foramen present, though 
insignificant in Necturus and other amphibia (Fig. 61, d). 
In some reptiles the obturator foramina becomes confluent, 
forming a heart-shaped foramen cordiforme. 
Allowing for considerable variation in form and proportion, 
the pelvic girdle of mammals is similar to that of reptiles, 
and consists of the three elements, ilium, ischium, and pubis, 
the first dorsal, the other two ventral. The ilium is attached 
to the sacrum, which varies somewhat in the number of ver- 
tebrae involved in its formation, and the ischium and pubis of 
the two sides usually unite in the mid-ventral line to form THE APPENDICULAR SKELETON 
193 
a symphysis, although in man the symphysis involves the 
pubic bones alone, the ischia being wide apart. This is doubt- 
less in correlation with the enormous size of the head of the 
human infant, for the passage of which through the pelvic 
outlet provision must be made. A similar case is found in 
birds, where, with the single exception of the African ostrich, 
not the ischia alone, but the pubes also are wide apart to 
allow for the passage of the enormous eggs, characteristic of 
the Class, and out of all proportion to anything that exists 
elsewhere in nature. 
The history of the shoulder-girdle is like that of the skull 
in consisting of two distinct elements, dermal and cartilagi- 
nous. The dermal girdle consists of cleithrum, clavicle, and 
interclavicle; the cartilaginous of scapula, pro-coracoid, and 
Fig. 62. Diagram of the shoulder-girdle of a primitive reptile, showing 
the complete series of elements found in the vertebrates above the fishes. 
Certain of the Permian extinct reptiles realise this scheme very nearly. 
Dermal elements: CLTH, cleithrum; CL, clavicle; IC, interclavicle. Cartilaginous elements: PC, 
procoracoid [anterior coracoid]: C, coracoid [posterior coracoid]; S, scapula. 
coracoid. It is likely that the prosternum, and perhaps the 
remainder of that bone belongs with the other cartilaginous 
parts just named, and hence to be classed as a girdle ele- 
ment (Fig. 62). 
As in the skull, the cartilaginous shoulder-girdle is the ear- 
lier, being found in its primitive condition in the selachians, 
where it consists of a single, horse-shoe-shaped piece, fitted 
like a U around the body in the pectoral region, the break 
corresponding to the region where the vertebral column lies. 
In the embryo it differentiates from a band of mesenchyme 
into a mid-ventral sternum, followed along each side by cora- 
coid and scapula, passing dorsally in succession. Between the 194 
THE HISTORY OF THE HUMAN BODY 
two last the pectoral fin is inserted. Eventually, in the adult, 
the entire chain fuses into a single piece of cartilage, in which 
the separate elements are no longer distinct. The occasional 
cutting off of the free dorsal end as a separate cartilage is 
without morphological significance. 
Upon this cartilaginous core as a base the dermal girdle 
is applied. In the teleosts this element becomes voluminous, 
but in urodeles it is entirely wanting, the shoulder-girdle 
consisting, as in selachians, of cartilage with the addition of 
Fig. 63. Shoulder-girdle of fishes. 
(a) Dog-fish (selachian). (b) Polypterus (ganoid). (c) Cod (teleost). (d) 
Ceratodus (dipnoan). 
s, scapula; ss, suprascapula; c, coracoid; cv, clavicle; ct, cleithrum; xx, supra- 
cleithra. 
a single cartilage bone, the bony scapula. In the Amniota 
both clavicle and interclavicle are to be expected, but the 
cleithrum is present only in a few early fossils and in many 
mammals the entire dermal girdle is much reduced. The 
history of the separate dermal elements in detail is as fol- 
lows: 
The interclavicle is the only median dermal piece, shaped 
like a capital T or a cross, its two lateral arms extending to 
and often overlapping the clavicles. Not occurring in fishes 
or in amphibians, it becomes a conspicuous element of the 
shoulder-girdle in Permian reptiles, and has continued in 
modern lizards. Fused with the clavicles in birds it appears THE APPENDICULAR SKELETON 
195 
as the middle part of the jurcula or “ wish-bone ” and often 
forms a conspicuous head. In the reptilian mammals, the 
monotremes, it forms a conspicuous bone just anterior to the 
prosternum, with the two clavicles free from it but closely 
Fig. 64. Sternum and shoulder-girdle of mammals. [After W. K. 
Parker.] 
(a) Ornithorhynchus. (b) Human embryo. 
c, coracoid; d, epicoracoid; e, episternum; f, clavicle; g, scapula; h, suprascapula; 
tn, manubrium; stb, sternebrae; x, xiphisternum. 
applied to it. In the ottier mammals it has usually dis- 
appeared, but in a few rodents and insectivores certain 
problematic elements occur, among which there may be pos- 
sible homologs. If, however, such parts are shown to be 
of cartilaginous origin the homology would not hold true. 
The whole subject is one which demands further embryo- 
logical investigation. (Fig. 65.) 
The cleithrum and clavicle are the two lateral dermal ele- 
ments. In many ganoids they are about equal in size, and are 
placed so that the clavicles are medial and the cleithra lateral. 
In teleosts, the cleithra are large, and often so surpass all the 
other elements of the shoulder-girdle as to appear to function 
as the entire part, the rest being scarcely more than adjuncts 196 
THE HISTORY OF THE HUMAN BODY 
to the joint of the fin. Present only in the extinct Stegocepha- 
lians among the Amphibia, the cleithrum shows itself for the 
last time in a few of the most primitive reptiles (Moschops, 
Eryops) and then vanishes from the scene. 
The clavicle is the more medially placed of the two lateral 
dermal elements, with a history that even yet is but partially 
Fig. 65(a) 
Fig. 65(b) 
Fig. 65(e) 
Fig. 65(d) 
Fig. 65(c) 
Fig. 6,?. Possible homologies of the mammalian interclavicle. 
(a) Ornithorhynchus [from Weber after Wiedersheim], (6) Ericulus, a Madagascar Insect- 
ivore [from Weber after Leche]. (c) Mus sylvaticus [from Reynolds after W. K. Parker], 
(d) Cricetus vulgaris [from Weber after W. K. Parker], (e) Sorex [from Flower after W. K. 
Parker], ic interclavicle; pc procoracoid; cl clavicle; m manubrium; c coracoid (posterior 
coracoid). 
known. In the anurous Amphibia it becomes closely asso- 
ciated with the procoracoid, a cartilage bone, and in reptiles, THE APPENDICULAR SKELETON 
197 
birds, and mammals, although the clavicle is usually present 
and often conspicuous, one is not always certain if the bone 
that is called the clavicle is really this dermal element alone, 
or a fusion of the two elements, the presence of cartilage cells 
in the embryo being taken as an indication of the presence of 
both a clavicular and a procoracoid element. 
In birds the clavicles, fused with the interclavicle, form the 
jurcula, or “ wish-bone ” and in mammals the clavicles, prob- 
ably of double origin, are best shown in the more primitive 
groups, disappearing more or less completely in such special- 
ized animals as the ungulates. 
The cartilaginous shoulder-girdle begins in the selachians 
as a single piece, but with the potential elements of scapula, 
coracoid, and perhaps sternum. In amphibians the coracoid 
element appears as two distinct parts, anterior and posterior, 
meeting or overlapping in the mid-ventral line. Of these the 
posterior is called the coracoid, the anterior, the procoracoid. 
This division is plainly seen in Permian reptiles, where the 
procoracoid is usually termed the epicoracoid, and the coracoid 
the metacoracoid. In the monotremes, whose reptilian char- 
acter is so conspicuously seen in other respects, these two 
coracoid elements are both present, but the procoracoid is 
small and of little functional importance, while the coracoid 
extends across the ventral part of the girdle from scapula 
to prosternum. This reduction of the procoracoid is already 
shown by the theraspids, that group of ancient reptiles from 
whom the mammals are presumed to have descended, and it 
is not surprising to find that in the higher mammals the pro- 
coracoid has been entirely lost while the definite coracoid has 
developed from the posterior element. In modern reptiles, 
however, and probably in birds, that coracoid element which 
has been kept is the anterior one (procoracoid), while the 
posterior one has been lost. (Fig. 66.) A further modification 
occurs in most mammals (marsupials and placentals), which 
consists in the reduction of the true coracoid, the posterior 
element, to a small process, which becomes attached to the 
scapula as the “ coracoid process ” 198 
THE HISTORY OF THE HUMAN BODY 
G. Adult Marsupials 
and Placentals 
(Homo) 
Cor 
F Foetal Marsupials 
(Trichosurus) 
Mammals: 
homologies 
within the 
class are 
not disputed 
Cor 
/Epic 
EL. Monotrernes 
{Or nithorhy nchus) 
Reptiles: ho- 
*molo<jies with- 
sSsir» the class 
\arc not 
is pu- 
tted 
Cot 
•Epic 
D. Therapsids 
C Dicy no don) 
Cor= 
Post 0*1 
a? Epic1 
Tjj i 
AntC. 
C. Many 'Permian \ 
Reptiles 
CDim e fro don)Co r _ 
PostCtfl 
Epic= 
J An+C- 
Epic nr 
4 = £ 
/Ant. C. 
B. Modern 
Reptiles 
(Iguana!) 
A.Certain Permian 
Reptiles 
(5 eymouria) 
•Epic = 
AntC. 
Fig. 66. Homologies of the Coracoids. 
The series begins with the Permian reptile, Dimetrodon, which possesses both coracoids, anterior 
and posterior; from these the mammalian and modern reptilian lines start, the former retaining the 
posterior, the latter the anterior. The piece labelled epic (epicoracoid) is the one referred to in this 
book as the pro- (or pre-) coracoid, traces of which are retained by many modern mammals. [After 
Romer.] THE APPENDICULAR SKELETON 
199 
The scapula is the most important and the most conserva- 
tive element of the cartilaginous shoulder-girdle. In its most 
reduced condition, that found in the hoofed mammals, the 
shoulder-girdle consists of scapula and coracoid only, the 
latter reduced to a simple process upon the former, but even 
here the scapula is large and well-developed, and does not 
lose the characteristic form of this part. 
Concerning the origin of the sternum opinions differ widely, 
and only a few of the many theories connect it even remotely 
with the shoulder-girdle. As certain of these theories do so, 
these will be considered in this place, while the morphology 
of this part is discussed again with the axial skeleton (p. 140). 
Starting with the median ventral element found in the 
embryonic girdle of selachians, this part, separated from the 
rest, and skipping the other fishes, which possess no trace 
of it, is easily comparable with the rhomboid, plate-like ster- 
num found in the urodeles, fitting into the angle formed by the 
posterior edges of the two overlapping coracoids. This same 
element, ossified through the middle, but remaining cartilagi- 
nous at both ends, becomes the posterior sternum of the Anura, 
and sustains the identical relationship to the coracoids. Cer- 
tain forms from this group, for example, the frogs, possess 
also an anterior sternum, similar in structure, but harder to 
explain on the basis of this theory. 
In modern reptiles the sternum is most often a flat plate 
of rhomboid form, like the sternum of the Urodela, but at- 
tached to several pairs of ribs, and in some cases there de- 
velop from this rhomboid plate a pair of posteriorly extended 
processes. These diverge from each other in many cases, but 
in others they meet closely together in the middle line, and 
form a long posterior extension of the rhomboid plate, per- 
haps comparable with the prosternum and mesosternum of 
mammals. (Fig. 67.) 
The fact that in the Amniota the sternum, with or without 
the posterior extension, is usually met by the ventral pro- 
longation of several pairs of ribs, has naturally been responsible 200 
THE HISTORY OF THE HUMAN BODY 
for the belief that in some way the two parts, ribs and ster- 
num, are genetically connected, and that the sternum has 
arisen as a result of the ventral fusion of ribs. This idea 
Fig. 67. Homologies of the Interclavicle and Sternum in Reptiles. 
(a) Trachydosaurus. (b) Chirotes. (c) Crocodilus. ic, interclavicle; pc, procoracoid; c, 
clavicle; c, coracoid (anterior). [After Hanson.] 
is, however, no longer tenable, since, during development, 
the ribs have an origin independently of the sternum, and be- 
come attached to it secondarily, while in certain vertebrates 
(amphibians) they never reach it. This has necessitated the 
assumption of two sterna, an archisternum, having no con- 
nection with ribs, the sternum of the Amphibia, and the neo- 
sternum, arising from the ventral fusion of ribs, the sternum 
of the Amniotes. The removal of the ribs from all primary 
connection with the sternum in any case clears the situation 
by making the sternum probably homologous throughout. 
Aside from the theory above outlined, which derives the 
entire sternum, pro- and meso-, from the mid-ventral piece of 
the selachians, some derive the prosternum alone from this, 
and seek an independent origin for the posteriorly directed 
processes which form the meso-sternum. In the present lack 
of knowledge, especially embryological, the decision between THE APPENDICULAR SKELETON 
201 
these theories cannot be reached; and the reader is reminded 
also that other theories, which do not derive the sternum 
from the shoulder-girdle at all, but from axial parts, are still 
tenable, and are discussed above in the chapter on the Axial 
Skeleton. 
(A) Dog-fish (selachian). (B) Ceratodus (dipnoan). 
s, scapula; ss, suprascapula; c, coracoid; p, propterygium; ms, mesopterygium; 
mt, raetapterygium; rad, radials. In B there are radials on both sides of a central 
axis. 
Fig. 68. Anterior fins of fishes. 
The early history of the free limbs has not been wholly 
deciphered. The fins of fishes exhibit a great variety of form, 
based upon a series of fin-rays, either distinct or united to 
basal pieces. Of these latter the posterior limb of selachians 
shows one, the basi-pterygium, and the anterior limb three, 
named in order, beginning at the front, pro-, meso-, and meta- 
pterygium (Fig. 68, A). These latter pieces have often been 
considered as primitive, and various attempts have been made 
to derive from them the long bones of the limbs in terrestrial 
forms; but they are not always found in ganoids, which ought 
to show here, as elsewhere, transitions to the higher verte- 202 
THE HISTORY OF THE HUMAN BODY 
brates. The Dipnoi show a beautifully symmetrical type of 
fin-skeleton, which consists of a jointed central axis with rays 
upon either side, a type which many have regarded as the 
primitive form from which all the other cases have been de- 
rived, and have named it accordingly the archipterygium 
(Fig. 68, B). The highly specialized Dipnoi, however, are 
Fig. 69. Diagrams of typical free limb skeleton, 
(A) According to the usual nomenclature; names belonging to the posterior limb 
are bracketed. (B) Suggestion for a common nomenclature for both limbs. 
In (A) the separate carpal and tarsal pieces are as follows: a, radiale [tibiale]; 
bj ulnare [fibulare]; x, intermedium; y, centrale; i—5, carpalia [tarsalia]. 
In both diagrams the more constant sesamoids are indicated by dotted lines. 
not the proper animals to which to look for primitive condi- 
tions, and it is also true that as a rule it is unsymmetrical and 
not symmetrical forms that show the early conditions of a 
part. 
Immediately above the fishes the chiropterygium or hand- 
form appears, a type so definite and fixed in character that all 
limbs from the amphibians on may be directly referred to it, THE APPENDICULAR SKELETON 
203 
while it appears in almost its typical condition in animals 
widely separated. (Fig. 69.) It consists of a proximal joint 
formed of a single bone, a second joint of two, followed by a 
series of several small pieces from which extend five digits, 
each composed of several bones. Unfortunately the parts were 
originally named without much reference to the striking cor- 
respondence (serial homology) between the anterior and pos- 
terior limbs, and thus some of the corresponding parts have 
received very different names, leading to a redundancy of 
terms. The nomenclature and correspondence are indicated 
in the following table: 
Anterior Limb 
Posterior Limb 
Humerus 
Ulna (outer) 
Radius (inner) 
Carpus 
Tarsus 
Metacarpus 
Phalanges 
The digits are designated either by number, I-V, or else 
the Latin names for the fingers are used, pollex, index, medius, 
annularis, minimus. These names are applied equally to both 
members, except that for digit I of the posterior limb the term 
hallux is employed instead of pollex. 
The nomenclature of the bones of carpus and tarsus has 
caused by far the most difficulty, as they are extremely vari- 
able and liable to fuse with one another or to disappear. Still, 
in spite of much irregularity, they are reducible to a type or 
pattern, as given in the diagram, to which the individual cases 
may be referred. In this typical form (Fig. 69, A), there is 
a piece at the distal end of each of the two limb bones of the 
second joint, and five at the proximal ends of the five meta- 
carpals [or metatarsals]. These are called radiale and ulnare 
[tibiale and fibulare], and the five carpalia [or tarsalia] re- 
spectively. Of the latter set the individual bones are desig- 
nated by a number, as tar sale IV, car pale 11, car pale V, 204 
THE HISTORY OF THE HUMAN BODY 
etc. Aside from these there are two median pieces: the inter- 
medium, lying between the two proximal elements, and the 
centrale, distal to it and enclosed by both rows. 
The close similarity that exists between the anterior and 
posterior limb skeleton, and the frequency with which the two 
need to be compared, piece by piece, leads one frequently to 
wish that the nomenclature of the two should be unified 
throughout, as has already been done in part in the distal por- 
tion. Probably the chief objection to this lies in the diverse 
ideas which still exist concerning the serial homology of the 
parts, yet the practical advantage that has already resulted 
from a partial uniform nomenclature in carpus and tarsus 
shows the possibility of completing such a scheme as a working 
hypothesis. Such a scheme is shown in Fig. 69, B, which 
may be compared with A of the same figure, that shows the 
nomenclature now in use. 
In addition to the definite carpal and tarsal bones, which 
belong to the primary limb skeleton, there are certain other 
elements of sporadic occurrence, that are situated in or about 
the tendons of muscles and serve some mechanical purpose in 
connection with the action of those parts. These are sesamoid 
bones, and are suggested in the diagram by dotted lines. Of 
these the most usual are a radial and an ulnar one [tibial and 
fibular] placed upon the free edges of the carpus [or tarsus]. 
Sesamoids also occur in other portions of the limb skeleton; 
thus the patella, constantly found in birds and mammals, oc- 
curs in the posterior limb between the first and second long 
joints, and forms the protuberance at the knee. Small sesa- 
moids, often associated in pairs, are found on the flexor side 
of the digits between the phalanges. 
Although the above scheme for a typical carpus or tarsus 
and its nomenclature seems to serve the purpose of naming 
the parts in all cases (Fig. 70), there are many facts which 
forbid us from imagining that it is really a primitive condi- 
tion. Thus the animal in which it comes to its most perfect 
realization is the turtle, the carpus of which is almost dia- THE APPENDICULAR SKELETON 
205 
grammatic, while the salamanders, where a more primitive 
type is to be expected, depart almost as widely from the dia- 
grams as do the mammals. From certain indications it seems 
probable that in the early carpus and tarsus there were two 
centralia (Fig. 70, a), and that the separate bones were ar- 
Fig. 70. Various forms of carpus. Figures (a)-(c) after Elisa 
Norsa; figure (e), after Flower. 
(a) Sphenodon (Hatteria), a New Zealand lizard, (b) Chick embryo, early 
stage, (c) Chick embryo, later stage, (d) Lacerta, a European lizard, (e) Talpa, 
European mole, (f) Pig. 
R, radius; U, ulnar; r, radiale; u, ulnare; i, intermedium; c, centrale; 1-5, car- 
palia; p, pisiforme; f, os falciforme; I-V digits. 
ranged, not symmetrically, but in oblique rows continued more 
or less directly to the digits, and suggesting the derivation of 
both carpus [and tarsus] and digits from long fin-rays, divided 
into numerous joints. (Fig. 74, f.) 
In the nomenclature of the carpal and tarsal bones em- 
ployed in human anatomy we have an unusually good illustra- 
tion of parts named with reference merely to a single animal 
form, for the names given them, e.g., cuneiform, trapezium, 206 
THE HISTORY OF THE HUMAN BODY 
multangulum majus, os magnum, etc., are wholly relative and 
might not apply even to closely allied animals. Since, how- 
ever, these older names are still more or less used, it may be 
well to compare them with the more unusual nomenclature 
given above, a comparison which may be best shown in the 
form of a table as follows: 
Older Nomenclature 
Morphological 
Names 
Older Nomenclature 
Hand 
Hand 
Foot 
Foot 
scaphoides (naviculare) * 
radiale 
tibiale 
f os calcis (calcaneus) 
{ astragalus (talus) 
l (value undetermined) 
lunare (lunatum) 
intermedium 
cuboides (triquetrum) 
ulnare 
fibulare 
anlage in embryo, 
with occasional 
persistence 
centrale 
naviculare 
trapezium (multangulum 
majus) 
carpale I 
tarsale I 
entocuneiforme 
trapezoides (mult- 
angulum minus) 
carpale II 
tarsale II 
mesocuneiforme 
os magnum (capitatum) 
carpale III 
tarsale III 
ectocuneiforme 
unciforme (hamatum) 
carpale IV 
carpale V 
tarsale IV 
tarsale V 
os cuboideum 
radial 
sesamoid 
tibial 
sesamoid 
pisiforme 
ulnar 
sesamoid 
fibular 
sesamoid 
* Synonyms used more frequently by European anatomists, are given in 
parentheses. These latter have been adopted by the BNA, but the substitution 
of them in America for the more familiar terms placed first in the above list, will 
be difficult to accomplish, and it is a question if it be desirable, since neither set 
of terms rests upon a morphological basis. 
Aside from the normal bones in carpus and tarsus there 
occur occasionally supernumerary elements of greater or less THE APPENDICULAR SKELETON 
207 
frequency. Thus in Man, in which the subject has been nat- 
urally investigated the most thoroughly, there have been re- 
corded for the carpus fifteen or sixteen such elements, aside 
from the occasional occurrence of the division of a normal 
element into two (bipartite). The summary of such elements, 
so far as known, resting upon the investigation of several 
Fig. 71. Diagrams of the supernumerary carpal bones. [After 
Pittzner.] 
r, radiale externum; ce, centrale; l, (epilunatum); z, hypolunatum; c, triquetrum secundarium; 
P, epipyramis; y, praetrapezium; s, styloideum; i, parastyloideum; e, metastyloideum; m, capi- 
tatum secondarium; g, ossiculum Gruberi; k, os hamuli proprius; v, os Vesalianum; ps, pisiforme 
secundarium. 
Of the normal carpal bones the scaphoides and the cuboides are represented as bipartite. 
This peculiarity has been also observed in the lunare and the trapezoides. 
thousand human carpi, is shown in Fig. 71. Similar super- 
numerary elements occur in the tarsus; the most important of 
which are the trigonum, associated with the astragalus, and 
present in 8% of the cases studied; the tibiale externum (n- 
12%); the peroneum (8-9%); and an intermetatarseum (8- 
9%), a derivative, sometimes of the first, sometimes of the 
second, metatarsal. 208 
THE HISTORY OF THE HUMAN BODY 
The typical number of digits is five, but this number is fre- 
quently reduced by the loss of digits at either end of the series. 
Instances of reduction, usually with vestiges of the missing 
digits, are of frequent occurrence and range from cases with 
the loss of the first alone, as in salamanders, or of the last 
alone, as in the feet of birds, to the extreme case exhibited 
by the horse, in which the middle digit is alone developed, 
accompanied by vestiges of II and IV. 
In the pig and the ox two digits, II and V, considerably 
reduced in size, are set behind the two well-developed ones, 
III and IV, and terminate in horny spurs that do not touch 
ground. Digit I is entirely wanting. 
The occasional occurrence of hyperdactylism, or cases with 
supernumerary digits, in all groups of tetrapod vertebrates, 
together with the presence, in numerous cases, of extra bones, 
sesamoid and otherwise, beyond and at the sides of the true 
digits (e.g., os falciforme in Fig. 70, f), have often been in- 
terpreted as pointing to a previous condition with more than 
five digits, a condition that would well accord with the deri- 
vation of the hand from a fin, since in most cases the latter 
structures possess more than five fin-rays. These phenomena 
are not, however, so interpreted by all morphologists, and the 
subject is a controversial one at present. Such an hypotheti- 
cal digit placed before the thumb or great toe is called a proe- 
pollex or pr os-hallux respectively; the one continuing the series 
beyond the fifth is a post-minimus. 
There is a general opinion that the original chiropterygium 
was a pentadactyl one, or even that it was possessed of 
one or two more than five, and certainly such an opinion 
lies at the basis of most of the theories concerning the 
method of transition from the fin-form to the hand-form, but 
if one would allow himself to enter the field without preju- 
dice, and would resort to the plain teachings of embryology, 
as is usual in cases involving doubt, he would start with a 
didactyl form. 
If one should simply observe the external development of 
the limb-buds of a salamander, he would find that the pro- THE APPENDICULAR SKELETON 
209 
cedure is inaugurated by the formation of a cleft in the distal 
end of the bud, and that the developing chiropterygium shapes 
itself into a hand-form with two digits, the first and the second. 
Somewhat later the third rises from the outer edge of the 
second by budding, and the fourth in the same way from the 
third. All amphibians have at most only four digits in the 
fore-limb, but in the case of a posterior limb a fifth digit may 
be looked for as a further bud from the fourth. There is 
thus a fundamental difference in the origin of Amphibian 
digits; the first two are primary, and are derived directly 
from the free limb, while the last three are secondary, de- 
rived by budding from the others in succession. Thus, if 
we hold this sequence of events to actually relate the history of 
the chiropterygium, we must begin 
with a didactyl, and not a pentadactyl, 
primitive form. It follows that there 
is no reason for trying to find an 
ancient form with more than five digits, 
and quite relieves us of all talk about 
a prehallux or a postminimus. 
Naturally the test of the historical 
accuracy of a developmental procedure 
comes from a close correspondence be- 
tween the successive stages and cer- 
tain adult animals which are assumed 
to be ancestral, and in this connection 
we cannot fail to be struck with the 
footprint of Thinopus, the oldest land 
Vertebrate known. We do not know 
whether it would be classed as amphib- 
ian or fish, except that it controlled the 
land and walked. We do know, how- 
ever, judging from this single foot- 
track from the Devonian, that it possessed practically a 
didactyl foot, with a third digit budding out from the outer 
edge, closely corresponding to a developmental stage in 
salamanders. (Fig. 72.) 
Fig. 72. Foot-print of 
Thinopus, from the Devon- 
ian. 
This is the oldest land verte- 
brate known, and consists of a 
single foot-print in the museum at 
Yale University. No other re- 
mains of this animal are known. 
This print was evidently made by 
a foot with two well-developed 
toes and a partially developed 
third one borne by the second. 
[After Lull.] 210 
THE HISTORY OF THE HUMAN BODY 
The chiridial appendage above described, although it forms 
the universal plan upon which is based that of all tetrapod 
vertebrates, is yet capable of great modification and has en- 
abled its various possessors to adapt it to a great variety of 
uses. The detailed consideration of this belongs to the field 
of special comparative anatomy and does not come within the 
(d) Necturus, a primitive salamander. (b) Ichthyosaurus, an extinct marine 
lizard, (c) Globicephalus, a cetacean (dolphin), (d) Pterodactyl, an extinct flying 
reptile, (e) Bird, (f) Bat. 
Figure (a) represents an unmodified limb skeleton; (b) and (c), limbs modified 
for swimming; (d), (e), and (£), those modified for flight. Designations as in the 
previous figure. 
Fig. 73. Modifications of the fore limb. 
scope of this work, yet as an illustration of the general prin- 
ciple of adaptation a few cases may be mentioned which show 
certain of the modifications acquired in the successful adap- 
tation of the free limb to locomotion in the two difficult ele- 
ments of water and air (Fig. 73). The chiridial appendage 
(chiropterygium) is primarily a walking leg (Fig. 73, a), de- 
signed to aid in pushing the body along the ground or, when 
extremely developed, to support the body above the surface 
and assume the entire function of locomotion. In becoming 
modified to subserve the needs of an aquatic life it needs to THE APPENDICULAR SKELETON 
211 
change into a paddle, which it does by elongating the digits, 
pressing them close together, and surrounding them by a web 
of integument. The digits also often become hyperphalangeal, 
that is, they develop an unusual number of phalanges, as is 
seen both in the marine lizard Ichthyosaurus and in the en- 
tirely unrelated branch represented by the Cetacea. [Fig. 
73; compare (b) and (c).] 
In Ichthyosaurus the five regular digits are retained, and 
there is also a row of small bones along the outer edge, con- 
sidered by some to be a sixth digit, a “ post-minimus.” It is 
probably a row of sesamoids employed here to widen the 
paddle and thus increase its effectiveness. In the cetaceans 
the external digits suffer some reduction while the two middle 
ones, II and III, are lengthened. 
In adaptation to flight the chiridial appendage forms a 
framework for a thin surface, without appreciable weight, and 
formed either of integument or of integumental structures of 
some sort. There have been at least three independent and 
perfectly successful attempts to solve this most difficult me- 
chanical problem, each one involving profound changes in the 
limb skeleton. In the extinct pterodactyls the principal modi- 
fication consisted of an extreme lengthening of the little finger 
to form a framework for the wing membrane; in the bats a 
similar result has been attained by lengthening all the digits 
except the pollex; and in the birds, where the development of 
feathers necessitates the formation of a firm framework with- 
out motion between the parts, there is formed a bone complex 
composed of carpal and metacarpal elements and several 
phalanges. [Fig. 73; (d) to (f).] 
At the conclusion of this subject it may not be without in- 
terest to review briefly the subject of the transition between 
the two types of free limb, the ichthyopterygium and the 
chiropterygium, in which, although as yet no theory has 
gained general credence, or has even passed beyond the stage 
of an ingenious speculation, many interesting suggestions 
have been advanced, some of which may be near the truth, as 
may at any time be shown by the discovery of the fossil re- 
mains of transition animals, unknown at present. 212 
THE HISTORY OF THE HUMAN BODY 
In general the similarity between fin-rays and digits is seen 
by everyone, and this probable homology is rendered more 
likely by the fact that the bones of the digits in animals with 
the hand-form of limb are preformed in cartilage, thus sug- 
gesting the fin-rays of the selachians and ganoids. Both are 
divided by joints into movable segments; both are supplied 
with flexor and extensor muscles; and in some fishes the rays 
are prolonged by the addition of horn threads, in structure 
and position recalling the claws or nails of the hand-form, al- 
though placed on both sides instead of one. Even the usually 
excessive number of the fin-rays is no real objection, since in 
some fishes this number is a very limited one. Thus in a way 
the derivation of hand or foot from the fish-fin may be ac- 
counted for; but an insuperable difficulty lies in the presence 
of the two lengths of limb bones interposed between the girdle 
and the distal complex, which seem to correspond with nothing 
found in the fin. It is to be remembered, however, that these 
are not especially long in the more primitive forms, like 
salamanders, so that the main obstacle lies not so much in 
their length as in their very existence, which has never re- 
ceived a satisfactory explanation. 
An early suggestion along this line was based upon the 
anterior fin of selachians with its three basal pieces (Fig. 
74, a). The mesopterygium is usually much the largest and 
shows a tendency to form the sole connection with the shoul- 
der-girdle. If this becomes established, the mesopterygium be- 
comes a humerus, and the pro- and metapterygium, slipping 
away from the girdle, and bearing the free rays, would become 
respectively radius and ulna (Fig. 74, b and c). This theory 
seemed for a time to receive especial corroboration from the 
structure of the paddle of the extinct sea-lizards, Ichthyosaurus 
and Plesiosaurus, but the time is now passed for drawing 
such broad conclusions from a chance resemblance in some 
highly specialized form, aside from which it must be noted 
that the theory is based upon the anterior fin alone, leaving 
the more primitive posterior one out of the question. 
When, a little after this, the biserial dipnoid fin was heralded THE APPENDICULAR SKELETON 
213 
as the primitive type, and named in consequence the “ archi- 
pterygium,” it turned thought in a new direction, and an effort 
Radius £ 
Ulna 
Radius 
Ulna 
[Fibula] 
> [Tibia] 
Fig. 74. Theories of the derivation of the hand-form (chiropterygium) 
from the fin-form (ichthyopterygium). 
a-c, theory of Huxley; d-e, theory of Pollard; f, a common theory which seeks to base the hand- 
form upon the archipterygium. (a) Cestracion, a primitive selachian; (b) Ichthyosaurus, an extinct 
lizard, (c) Neclurus, a primitive salamander; (d) Chlamydoselachus, a primitive salachian; (e) Po- 
ly pier us, a cross-opterygian ganoid; (f) Ranodon, an Asiatic salamander. Note that in this last the 
member figured is a posterior one; in the rest it is an anterior one. 
was made to seek in the more primitive cases of the hand- 
form a central axis with lateral rays proceeding from it. In 214 
THE HISTORY OF THE HUMAN BODY 
one such attempt the central axis was formed by humerus, 
ulna, ulnare, carpale V and the fifth digit, to which the re- 
maining bones served as lateral rays upon the inner or radial 
side, thus forming a uniserial form, as in the selachians; in 
another the hind limb of the salamander Ranodon was used as 
a basis, and femur, fibula, intermedium, the two centralia, 
tarsale II and the second digit were taken as the central 
axis, leaving one digit upon the inner, and three upon the 
outer side to serve as lateral rays. Aside from the unsafe 
basis, however, upon which all such theories are founded, 
their use as an argument is nullified by the ease with which, 
in turn, each digit, with its associated bones, may serve either 
as a hypothetical central axis or as a lateral ray. 
As opposed to the theories thus far recorded, which seek 
to derive the entire free limb from the fin, is a more recent 
one, based upon the anterior fin of the ganoid Polypterus, in 
which ulna and radius are derived from the fin, while the 
humerus represents an element to be detached later from the 
shoulder-girdle. In this fin (Fig. 74, e), the pro- and meta- 
pterygium, already in the form of long bones, articulate with 
a projecting point of the girdle, while the meso-pterygium, 
reduced to a small, disc-shaped element, lies between them 
but detached from both. If now the long pro- and meta-pte- 
rygia are taken as radius and ulna respectively, ignoring the 
meso-pterygium for the present, the projecting process of the 
shoulder-girdle, which articulates with both, and which might 
easily become detached from the main mass if of functional 
advantage, would become a humerus. The mesopterygium 
would thus become intermedium or intermedium and centrale, 
leaving the remaining parts of the carpus, the metacarpus and 
phalanges, to be formed from fin-rays. This is in many ways 
the most satisfactory solution thus far, and the fact that it 
rests upon the condition found in a single species is no real 
objection, since it is altogether probable that terrestrial verte- 
brates were originally derived from a single form, perhaps a 
single species, and that Polypterus, with its close anatomical 
correspondence with the lowest urodelous amphibia, is nearly THE APPENDICULAR SKELETON 
215 
allied to that form. A more serious objection lies in the fact 
that the posterior fin cannot be as easily developed into a 
chiropterygium as can the anterior one; still the anterior and 
A Crossopterygian Ganoid, showing a close similarity to the hand type. [Drawn from the fossil by 
Hussakoff. After Bertram Smith.] 
Fig. 75. Skeleton of the pectoral fin of Sauripterus taylori. 
posterior limbs may not have had exactly the same origin or 
early history, since a later similarity of use would cause a 
convergence in anatomical structure. Moreover, as a mat- 
ter of fact, the pelvic fin of Polypterus exhibits proximally 
a single long bone, showing at least a tendency in a similar 
direction. Here the matter must rest for the present from lack 216 
THE HISTORY OF THE HUMAN BODY 
of further evidence, but the solution may be found at any 
time, as has happened in so many other cases, through the 
discovery of the fossil remains of some transition form. The 
pick of the geologist may unearth the key to this problem, a 
true palaeontological Rosetta stone, by the aid of which the 
discrepant records written on fin and foot may be deciphered 
and brought into harmony.1 
Since the last sentence was written at the time of the first 
edition (1909) several matters relating to the question of the 
transition from the ichthyopterygium to the chiropterygium 
have come to light. The rediscovery of the foot-print of 
Thinopus, with its two digits, in the basement of the Yale 
University museum has been already mentioned, and com- 
mented upon in reference to the testimony of the embryology 
of salamanders; a second rediscovery is that of the Crosso- 
pterygian Ganoid, Sauripterus (Fig. 75), which certainly 
gives a strong suggestion of a hand-type of free limb, although 
without doubt still a fin. Yet even now (1923) the question 
remains unsettled, and neither Thinopus nor Sauripterus solves 
the problem. The transition from the ichthyopterygium to the 
chiropterygium remains an open question. 
1 A number of fossil Polypteri have been discovered in the Triassic beds 
along the shores of Lake Tanganyika, in the waters of which their living 
descendants are still found. This would seem a favorable locality from which 
to expect results. CHAPTER VIII 
THE MUSCULAR SYSTEM 
“ When we no longer look at an organic being as a 
savage looks at a ship, as something wholly beyond his 
comprehension; when we regard every production of 
nature as one which has had a long history; when we 
contemplate every complex structure and instinct as the 
summing up of many contrivances, each useful to the 
possessor, in the same way as any great mechanical 
invention is the summing up of the labor, the experience, 
the reason, and even the blunders of numerous workmen; 
when we thus view each organic being, how far more 
interesting . . . does the study of natural history become! ” 
Ch. Darwin, Origin of Species. Chapter XV. 
Vertebrates possess two sorts of muscular tissue, unstriated 
or involuntary, in the form of masses of elongated single cells, 
and striated or voluntary made up of long fibers usually 
formed from cell complexes or syncytia. The cells of the in- 
voluntary type are mesenchymatous in origin and are usually 
associated together to form the muscular coats in the walls of 
hollow organs, thus providing them with their power of mo- 
tion. Voluntary muscles on the other hand are derived directly 
from the mesoderm and the delicate cytoplasmic fibrils which 
constitute the active contractile structures of all muscle cells, 
show in this form of muscle a high degree of differentiation 
vwhich involves a cross-striated structure of each fibril and 
thus gives to the muscle fibre as a whole the cross-striated ap- 
pearance from which its name is derived. Although the mus- 
cles of this type are in general under the control of the will 
and are hence termed voluntary, there are limited regions 
such as the walls of the pharynx and oesophagus, where gen- 
uine striated muscle may be involuntary. 
The musculature of the heart in some respects apparently 
intermediate between the two sorts of muscle tissue, since its 
217 218 
THE HISTORY OF THE HUMAN BODY 
fibrils possess cross-striations, is in reality a modification of 
the involuntary type the cells of which form a reticulated 
syncytium. Its derivation is rendered clear through the study 
of its embryological development which shows it to be the 
hypertrophied muscular coat of an expanded blood vessel. 
As muscles of the first, or involuntary, type are formed as 
portions of other organs, their morphological history is that 
of the organs of which they form a part, and is considered 
elsewhere. The voluntary muscles, on the other hand, con- 
stitute a distinct system of organs and it is their morphology 
that forms the subject of the present chapter. 
These organs fall naturally into three anatomical divisions, 
corresponding to those of the skeleton; axial, appendicular, 
and visceral, except that, as a matter of nomenclature, the word 
“ visceral ” has been long applied to the involuntary muscles 
of the viscera, and hence is here inappropriate. For this di- 
vision of the muscles, that is, for those that are associated with 
the visceral skeleton, in order to avoid this confusion the 
term “ branchiomeric ” is employed, thus making the three 
divisions of the voluntary muscles, corresponding to the skele- 
ton, the axial, appendicular, and branchiomeric. 
In the embryo the appendicular muscles are derived directly 
from the axial, and both arise from the dorsal portion of 
successive mesodermic somites, the epimere; while the 
branchiomeric muscles, limited to the anterior part of the 
body, arise from the ventral portion, the hypomere, the ele- 
ment which in the remainder of the body develops into the 
pleuro-peritoneum, and furnishes no voluntary muscles. As 
opposed to the branchiomeric muscle, the remainder com- 
prising the axial and appendicular, are termed metameric. 
The metameric muscles of a primitive vertebrate, like a fish, 
or the embryo of a higher form, presents the entire mass 
of body muscles in a strictly segmented series. The human 
embryo, in a somewhat diagrammatic form, appears like Fig. 
76, with three rather isolated muscle-segments representing 
the original muscles of the eyeball, then three segments in 
the neck, innervated by the Hypoglossal nerve, after which THE MUSCULAR SYSTEM 
219 
follows the axial system of the body. The hypomeric muscles, 
or branchiomeric as they are more usually called, are not 
shown here. 
Aside from these three primary divisions of straited mus- 
cles, there occur in almost all vertebrates certain superficial 
Fig. 76. Metameric muscles of the human embryo, diagrammatic. 
[After PiersolJ 
muscular elements, usually in the form of subcutaneous layers 
and intimately associated with the corium. These have often 
been treated as a distinct group of muscles, but recent investi- 220 
THE HISTORY OF THE HUMAN BODY 
Fig. 77. Necturus, with the integument removed, showing the primi- 
tive myotomic muscles and the differentiation in the regions of the limbs 
and the visceral skeleton. THE MUSCULAR SYSTEM 
221 
gation has placed it beyond doubt that we have here to do with 
muscular elements which have developed independently in dif- 
ferent animals in response to certain physiological needs and 
have been derived from the most convenient subjacent skeletal 
muscles. However, in spite of their secondary origin from 
other muscular groups, they have differentiated so far struc- 
turally that it is more convenient to treat them as a dis- 
tinct group, the integumental muscles, rather than to con- 
sider them with the various muscles from which they were 
originally derived. The order of treatment, therefore, of the 
muscles, as described in this chapter, will follow the plan 
just outlined. 
The primary groups of skeletal muscles, with the exception 
of the integumental, can be well shown in a condition ap- 
proaching the primitive one by carefully removing the skin 
from a salamander and inspecting the muscles as they lie in 
their natural position (Fig. 77). In this preparation the axial 
muscles form the bulk of the muscular system, and are seen 
to consist of a series of muscle somites, or myotomes, separated 
by thin perpendicular planes of connective tissue, the myo- 
commata. These axial muscles are divided by a horizontal 
furrow, which runs along each side, into dorsal and ventral 
masses, a distinction which is of fundamental topographical 
importance, as these masses are innerved respectively from the 
dorsal and ventral branches of the spinal nerves. The groove 
dividing them marks the place of the lateral line of fishes, in 
which is located a row of specialized sense organs. The 
axial muscles begin at the base of the skull and continue to the 
end of the tail, but show some interruption of their course in 
two places, corresponding to the attachments of the two pairs 
of limbs. That of the anterior limbs, however, is seen to be 
more superficial in character, and when the appendicular mus- 
cles, that spread out in thin fan-like sheets over the trunk 
myotomes, are removed, the myotomic muscles are displayed 
in an almost uninterrupted sequence. In the case of the pos- 
terior limb, however, the skeletal girdle lies imbedded within 
the body muscles, and this, together with the cloaca, causes 222 
THE HISTORY OF THE HUMAN BODY 
a considerable hiatus in the sequence of the myotomes 
on the ventral side, although dorsally the sequence is unbroken. 
The muscles attached to the free limbs and their girdles form 
the appendicular group, of little proportional importance here, 
but destined in the higher vertebrates, with the development 
of larger and more powerful limbs, to assume a far greater 
bulk, and in some cases to even surpass that of the axial 
muscles. There will be noticed this difference in the proximal 
muscles of the anterior and posterior limbs, that while those 
of the latter arise almost exclusively from the girdle itself, 
those of the anterior limbs are spread out fan-like over the 
axial myotomes, and arise either from the myocommata or 
from the integument. In the higher forms this difference re- 
ceives much greater emphasis, and in both birds and mammals 
the appendicular muscles of the fore limbs completely enwrap 
the body both dorsally and ventrally, thus concealing the axial 
muscles entirely, from the waist to the neck, except in the ven- 
tral abdominal region. 
Corresponding to the extensive development of the visceral 
skeleton in the animal under consideration, the branchiomeric 
musculature is also large and well shown. This occupies the 
ventral and lateral regions of the head and neck, and includes 
the massive muscles of the mandible, which extend over the 
top and sides of the skull. In the higher forms these muscles, 
with the exception of those of the mandible, lose in bulk, but 
perhaps gain in complexity, supplying pharynx, and the 
laryngeal region. 
Although thus far the facts presented rest upon a secure 
morphological basis, and although it is comparatively easy to 
follow the fate of the primary muscle masses as a whole in 
the separate vertebrate Classes, the further emphasis of the 
separate units which differentiate from the primary masses, 
that is to say, the homology of the individual muscles, is a 
subject fraught with especial difficulties, and is one in which 
the ground is uncertain, owing to the lack of fixed principles 
to direct the investigation. To begin with the axial muscles 
in their undifferentiated condition as a series of similar myo- THE MUSCULAR SYSTEM 
223 
tomes and follow out the various fate of their derivatives 
throughout vertebrates as the parts become modified to sub- 
serve countless special uses; to start with the limb muscles in 
the form of myotomic buds and follow the transformation 
outwardly expressed by the varied shape of fin, wing or leg; 
and finally to discover a fundamental plan in the complicated 
branchiomeric muscular system of selachians, and carry out 
the history of these elements as they gradually assume control 
of such different parts as the jaws, the auditory ossicles and the 
laryngeal cartilages; such would be the history of the muscu- 
lar system as it may sometime be written, a history even the 
outlines of which are in many places still waiting to be es- 
tablished. 
' A great barrier in the way of morphological study of the 
muscles lies in the fact that there is no definite criterion of 
homology, no simple way of absolutely proving that a given 
muscle in a certain animal is morphologically the same as a 
similarly related one in another species. This can be proven 
in a fairly satisfactory way by tracing the race history through 
a series of forms, provided that no extensive hiatus occurs in 
the series; but more often the animals to be considered are 
isolated forms, separated by wide gaps from their nearest liv- 
ing allies. With other organs the embryological record fur- 
nishes valuable clews and often traces a continuous history, by 
the aid of which the condition in isolated adult forms may be 
interpreted: but here not only are the embryological condi- 
tions extremely difficult to interpret, but in the majority of 
cases the historic stages are not there, and the final condition 
is seen to arise suddenly from a mass of apparently undiffer- 
entiated cells. 
In attempting to establish a basis of homology it is to be 
remembered that a muscle is not always a definite organ like a 
bone or blood vessel, but is rather a mass of muscular fibers 
set apart for a more or less distinct purpose and differentiated 
from the surrounding muscle masses in proportion to the defi- 
niteness and precision of its action. One muscle may be en- 
tirely isolated from the adjacent fibers by a firm cover of con- 224 
THE HISTORY OF THE HUMAN BODY 
nective tissue and provided with a special tendon attached to 
a definite skeletal process; another may be a bundle of fibers 
but partially separated from a larger mass and acting only in 
connection with it. Often, too, the action produced by a mus- 
cle is not precise enough to prevent numerous variations, and 
thus may vary considerably in different individuals of a single 
species or even in the two sides of the same individual. It is 
thus necessary to study all possible individual as well as 
specific variations of a muscle or a muscle group before laying 
down the outlines of its history. 
When the study of comparative myology was in its infancy, 
muscles were homologized and named from their general 
location, appearance and use, without regard to their develop- 
mental history, a method which, while fairly safe when ap- 
plied to closely allied forms, was apt to be very misleading 
when applied to animals as different, for example, as members 
of the different vertebrate Classes. Thus, if the starting point 
were the human subject, as was the former universal custom, 
it would be quite easy to recognize such a muscle as the deltoid 
in the apes and monkeys, which possess prehensile arms of a 
similar shape and used in a similar way, but it would be much 
more difficult to determine the same muscle in the ox, which 
uses its fore legs so differently, and the difficulty might be- 
come insurmountable in a form as different as a turtle or a 
frog. 
Somewhat more reliable as criteria for homology than posi- 
tion or use are the origin and insertion, the points at which 
the muscle fibers are attached, although these are altered by 
increase or decrease in volume or by a slight change in use; 
and, if the change is marked, may attain quite different re- 
lationships to the skeletal parts. There is, however, some dif- 
ference in the relative value of these two points, the origin 
being the more constant in certain regions, the insertion in 
others. It has long been considered an axiom of comparative 
myology to give the credit for greater constancy to the origin 
in all cases, but in the muscles of the appendicular skeleton the 
reverse seems to be the case, since here the insertions are at THE MUSCULAR SYSTEM 
225 
the distal end and are effected through narrow tendons, the 
precise attachment of which is a matter of great importance 
in the action of the muscle, while the origins occupy large and 
rather indefinite areas, the extent of which is relative to the 
degree of development in each case. 
Undoubtedly the most reliable criterion for muscular ho- 
mology is that based upon the constant relation between a 
given muscle and the nerve which supplies it, since the nerve 
which originally supplies a given primitive element, such as a 
myotome or a limb-element, never jorsakes it, but follows it 
through all its vicissitudes and continues to supply its deriva- 
tives, of whatever complexity or form, through all the changes 
of relation, which are often very great. A conspicuous ex- 
ample of this is the Facial nerve (Vllth cranial), which, in 
spite of its name, is not, so far as it is a motor nerve, origi- 
nally associated with the face but with the hyoid region of the 
neck. In the lower mammals this nerve supplies an integu- 
mental muscle covering the side of the neck, of which the 
human platysma is a remnant, and as it happens that in higher 
forms this sheet becomes extended over the face and differen- 
tiates into the various slips that form the mimetic musculature, 
its nerve follows it, multiplying its branches in strict accord- 
ance with the growth and differentiation of the muscle, until 
it covers the entire face with its ramification and earns the 
name of Facialis. Another branch of the same nerve supplies 
the digastric muscle of the mandible in the lower vertebrates 
(the equivalent of the posterior belly of the like-named mus- 
cle of mammals), and when in reptiles a small slip detaches 
itself from this muscle and wanders into the middle ear to 
become the stapedius, a minute branch of the nerve in ques- 
tion follows it to its ultimate location and furnishes it with its 
nerve supply. 
Were it possible to follow each motor nerve fiber from its 
origin to its connection with its muscle, it would probably 
serve as an absolute criterion for muscular homology, but 
there is much chance of error in the fact that an anatomical 
nerve is not a single fiber, but a bundle of them, and while 226 
THE HISTORY OF THE HUMAN BODY 
each fiber is presumably constant in its nerve-supply, there is 
some variation in the way in which they are put into bundles, 
so that no one can be sure that a given nerve is always quite 
homologous with one in a like location in another animal. 
This is especially true of the innervation of the limbs, where 
the nerve supply, after proceeding from a certain fairly definite 
number of nerve roots, passes into a plexus, in which the 
fibers become divided up and reunited in new combinations; 
and although in two allied forms the final nerves that emanate 
from the plexus may be constant in number and position, no 
one can be quite sure that their make-up in individual fibers 
is the same, and indeed it is more than likely that they are 
not. Furthermore, in cases in which a single muscular ele- 
ment has differentiated into a group of well-separated muscles, 
each with a specific action, all will be supplied by branches 
of the same nerve, and the innervation will furnish no clew 
to homology, beyond that of identifying them as members 
of the same limited group. 
Thus the criterion of nerve supply, although in theory an 
accurate and definite method, often fails in its application 
through variation in the make-up of the separate nerve bun- 
dles, and while undoubtedly the best criterion we possess, it 
cannot be employed in all cases. It is the most reliable in its 
application to the axial muscles, where there has been the least 
amount of differentiation, and where the primitive segmenta- 
tion is still evident or but slightly disguised; it has also proven 
of value in the muscles of the branchiomeric system, especially 
in the case of fishes and amphibians, but in such cases as the 
highly differentiated limb muscles, this method is difficult of 
application, and cannot be followed beyond the homologizing 
of the larger groups. Here the character most to be depended 
on is the insertion, since these are in most cases by narrow 
tendons and hence very definite, while the origins spread over 
a greater or less area in proportion to the size of the muscular 
“ belly,” or fleshy mass, and are thus somewhat inconstant in 
individuals of the same species, or probably in the same indi- 
vidual at different periods of its life. THE MUSCULAR SYSTEM 
227 
Although, in the study of the morphology of the muscles, 
through the incompleteness of the embryological record and 
the technical difficulties in the way of examining it, reliance 
has hitherto been placed mainly on the comparison of adult 
forms, much may undoubtedly be learned concerning the his- 
tory of individual muscles and muscle groups from their on- 
togeny, especially in those cases in which the animal passes 
through an active larval period, and hence exhibits the earlier 
stages in junctional activity and consequently in greater com- 
pleteness. 
The muscles are grouped as follows to show their morpho- 
logical relationship. As this is also the order in which they 
are taken up in this chapter this list may serve as a handy 
table of contents for the remainder of the chapter. 
I. Metameric muscles 
(a) Axial 
Trunk 
Dorsal (episkeletal) 
Ventral 
Hyposkeletal 
Oblique 
Rectus 
Head 
Eyeball group 
Hypoglossal group 
(b) Appendicular 
II. Branchiomeric muscles 
Trigeminus group 
Facialis group 
Glossopharyngeus group 
Vago-accessorius group 
III. Integumental (a topographical sub-division, the muscles of 
which belong primarily in other groups, chiefly branchio- 
meric) 
Sphincter colli; 
Platysma, Mimetic muscles of the face 
Panniculus carnosus 
Sternalis 
Axillary arch 228 
THE HISTORY OF THE HUMAN BODY 
Both phylogenetically and ontogenetically the axial muscles 
of the metameric system are the first to appear, and forcibly 
suggest some lost ancestor in the form of a segmented worm, 
the muscles of which were mainly longitudinal in direction 
and repeated themselves metamerically. The appearance of 
these, in the form of mesodermic somites, is one of the earliest 
post-gastrular stages in the vertebrate embryo, and is the first 
suggestion of segmentation. In the lower vertebrates the 
muscle somites develop through the longer process of the 
formation and extension of pairs of lateral diverticula and the 
subsequent separation of the epimeres, as in the theoretical 
sketch given above [Chap. Ill], but in the Sauropsida and 
Mammalia, through acceleration of development, the epi- 
meres are cut off from an indifferent mesoderm in the form 
of approximately cubical blocks, arranged in pairs on each 
side of the nerve-cord and notochord, and were long considered 
to represent “ primordial vertebrae,” a name occasionally used 
even at the present time, although its literal meaning has been 
long since discarded. From these the myotomes develop 
through the formation of longitudinal fibers, and the mesen- 
chyma of the intervals between the mesodermic somites be- 
comes transformed into the myocommata, to which, on either 
side, the muscle fibers are attached. 
The primary subdivision of the axial muscles is seen in 
a transverse section through the body region of any of the 
more typical lower vertebrates. The motor spinal nerves, 
through their division into a dorsal and a ventral branch, 
suggest a corresponding division of the muscle masses, di- 
vided at the plane of the lateral line. A further division of 
the ventral nerves supplies three separate masses, the hypo- 
skeletal, the oblique and the rectus groups, the two last con- 
stituting in the trunk region the muscular layers, which form 
the lateral and ventral body walls. 
The rectus muscles, the most ventral, retain their fibers in 
their original longitudinal position, and may be expected to 
show at least traces of the original myocommata. In Urodeles 
this mass extends the entire length of the body and tail, THE MUSCULAR SYSTEM 
229 
the pubo-thoracicus, but in mammals is restricted to the 
rectus-abdominis, retaining some of the myocommata, under 
the name of “ tendinous inscriptions.” In the region of neck 
Fig. 78. Cross-section of a Vertebrate (diagrammatic), showing the 
primary sub-divisions of the metameric muscles. 
(a) Dorsal (episkeletal) set. (b) Ventral, (bi) hyposkeletal set. 
(b2) obliquus set. (b3) rectus set. 
and larynx there are a few further remnants of this mass, the 
sterno-hyoideus, sterno-thyroideus and thyro-hyoideus, in 
which occasional tendinous inscriptions may still be seen. 230 
THE HISTORY OF THE HUMAN BODY 
The obliquus mass occurs in the form of thin sheets, often, 
at least in the thoracic region, interrupted along the myocom- 
mata by the ribs. Thus, in man, the external obliquus of the 
abdominal wall becomes continued anteriorly by the external 
intercostals, and the internal obliquus is continued into the 
internal intercostals, while both are continued along the neck 
into the scaleni. 
Lastly the hyposkeletal muscle mass is found in discontinu- 
ous masses; the psoas magnus and parvus, and the longus 
colli. 
The dorsal, or episkeletal, muscle mass, innerved by the 
dorsal branch of the spinal nerves, is responsible for the true 
trunk muscles of the back, from which the muscles of the 
appendages have to be carefully eliminated. 
This mass of episkeletal muscles fills the spaces on either 
side of the median line, lateral to the vertebrae, and dorsal to 
the series of ribs and transverse processes; here they enter 
into an intimate relationship with the succession of meta- 
merically repeated skeletal parts which they find beneath and 
about them, and with which they become attached through 
complicated series of tendons. 
But little has been done with the muscles of this group from 
the standpoint of comparative anatomy, and thus their mor- 
phological history cannot as yet be attempted. It must suf- 
fice here to treat them to a rapid review as they exist in man, 
with some attempt at a morphological arrangement of their 
several elements, after which may be considered what is known 
of their history. 
In man the back is covered superficially with several layers 
of muscles which are dorsal in a topographical sense only, 
having secondarily invaded this territory from other places 
of origin. The most superficial of these, 'trapezius and latissi- 
mus dorsi, form a thin covering over almost the entire back, 
effectually concealing all beneath them. 
Of these the first reveals itself from its innervation to be 
in part a branchiomeric muscle, and the second shows clearly 
by its relationship to be appendicular. Both of these possess THE MUSCULAR SYSTEM 
231 
an important function in the motion of the arm, but, cor- 
responding to the small size of the arm in the salamanders, 
they appear in those animals as small, fan-shaped muscles 
which extend dorsally from shoulder and humerus respectively, 
and cover but a small portion of the trunk myotomes. With 
the gradual increase in the size and importance of the limbs 
which they supply they have enlarged to the extent found 
in man and in most of the mammals. The rhomboidei, lying 
beneath these and of less extent, belong wholly to the ap- 
pendicular group. Still deeper, beneath all of the appen- 
dicular muscles, are the two serrati posteriores, the re- 
mains of a continuous sheet in certain primitive mammals. 
Their innervation from ventral branches of the spinal nerves 
shows that they, too, are originally strangers to the dorsal 
region and belong rather with the ventral axial muscles. 
The removal of all the above exposes the genuine dorsal 
axial muscles, the derivatives of the, trunk myotomes, the 
system innerved by the dorsal branches of the spinal nerves. 
They lie lodged in the space embraced by the spinous and 
transverse processes of the vertebrae and run in the main in 
a longitudinal direction. They are beset with metameric 
series of tendons, which attach themselves to the correspond- 
ing portions of successive vertebrae or ribs, and plainly suggest 
a segmental origin. Proceeding inwards from the more super- 
ficial series they may be divided as follows, although it must 
be remembered that these muscles are often closely attached 
to one another and are not as distinct as in most other 
regions. 
I. Spino-transversalis System. — This system, the fibers 
of which arise from spinous and insert on transverse processes, 
consists of a single muscle, the splenius, confined to the an- 
terior portion of the trunk, and unrepresented posterior to 
about the middle of the thoracic vertebrae. In form it is a 
thin sheet of oblique fibers, a portion of which, splenius 
cervicis [colli], inserts into the transverse processes of cervical 
vertebrae, while the remainder, splenius capitis, inserts into the 
base of the skull. (Fig. 86.) 232 
THE HISTORY OF THE HUMAN BODY 
II. Sacro-transverso-transversalis System.— This arises 
from a large mass of muscular fibers filling in the space 
between the hip-girdle and the most posterior ribs. These 
fibers take their origin from sacrum and ilium, and from the 
Icmgissimus capitis 
bs [trachelo-mastoid] 
lon&issimus cervias 
b, [iransversalis colli] 
ilio-costalis eervicis 
[cervicalis ascendens] a. 
ilio-costalis dorsi 
!m. accessorius ad] _ 
sacro-lumbalem j 
longissimus dorsi, bj 
ilio-costalis lumbonnn 
[ sacro- lumbalis ] a, 
anultifidus 
Fig. 79. Muscles of the human back. 
The superficial layers .belonging to the appendicular system have been removed. 
Upon the right are shown the three portions of the ilio-costalis system externally, 
and within this the longissimus dorsi. Upon the left below is the multifidus; and 
above, the remainder of the longissimus system. THE MUSCULAR SYSTEM 
233 
lumbo-dorsal fascia. As the fibers issuing from this origin are 
not sufficient to supply the entire vertebral column, they are 
reinforced by others which arise from the transverse processes 
of the vertebrae, beginning with the lowest lumbar. The 
metameric insertions are in two longitudinal rows, or series, 
the outer into the ribs, and the inner into the transverse pro- 
cesses. A bundle of the more anterior fibers of the latter 
inserts into the base of the skull. The system may thus be 
designated sacro-transverso-transversalis, the first two ele- 
ments designating the origin, the third, taken in the broad 
sense and including the ribs, the insertion. 
The outer series begins posteriorly as the sacro-lumbalis, 
and becomes continued in the region of the ribs by a series of 
muscular slips arising from lower ribs and inserting in upper, 
musculus accessorius ad sacro-lumbalem. A still further con- 
tinuation, which takes the series into the cervical region, is 
formed of slips that arise from the upper ribs and insert on 
the transverse processes of the lower cervical vertebrae, the 
cervicalis ascendens. As these three muscles, the names of 
which have come down to us from premorphological times, are 
clearly portions of a single series, they are best referred to 
under the name of ilio-costalls, the three portions being dis- 
tinguished respectively as ilio-costaiis lumborum, ilio-costalis 
dor si and ilio-costalis cervicis. [BNA.] 
The inner series, posteriorly blended with the former, as- 
cends along the lumbar and lower thoracic regions under the 
name of longissimus dorsi, and is reinforced anteriorly by the 
transversalis colli and the trachelo-mastoid, the latter insert- 
ing on the mastoid process of the skull. In placing the nomen- 
clature on a morphological basis the series may retain the 
name of its most important member, the longissimus, the parts 
which are longissimus dorsi, longissimus cervicis and longis- 
simus capitis. [BNA.] (Fig. 79.) 
In tabular form the parts of this system with their synonyms 
are as follows: 234 
THE HISTORY OF THE HUMAN BODY 
Sacro-transverso-transversalis System. 
Outer series. 
ilio-costalis lumborum (sacro-lumbalis) 
ilio-costalis dorsi (accessorius ad sacro-lumbalem) 
ilio-costalis cervicis (cervicalis ascendens) 
Inner series. 
longissimus dorsi 
longissimus cervicis (transversalis colli s. cervicis) 
longissimus capitis (trachelo-mastoideus s. com plexus 
minor) 
III. Spino-spinalis System. — This a single series lying 
beneath the preceding close to the median line and consisting 
of slips that arise from more posterior spinous processes and 
insert on more anterior ones, skipping at least one vertebra 
between origin and insertion. This series is, like the two pre- 
ceding, an interrupted one, and is divided into two constant 
portions, spinalis dorsi and spinalis cervicis. (Fig. 80.) A 
few origins from spinous processes of cervical vertebrae and 
associated with the semi-spinalis of the next system, are of 
occasional occurrence, and represent a spinalis capitis. 
IV. Transverso-spinalis System. — This is a complex 
system, the elements of which arise from transverse processes 
and insert upon the spinous processes of vertebrae situated 
more anteriorly. It is in close contact with the previous sys- 
tem, by which it is covered, and may be imperfectly divided 
into three series or layers, superficial, middle and deep, called 
respectively semi-spinalis, multifidus and rotatores. These 
layers differ not merely in position, but in the course of their 
separate slips, since those of the first pass over 4-6 vertebrae 
between origin and insertion, those of the second pass over 
2-3, while those of the deep layers are themselves subdivided 
into two series, rotatores longi et breves, of which the outer 
pass over a single vertebra, while the inner attach to adjacent 
vertebrae. 
The semi-spinalis is divisible into three portions, dorsi, 
cervicis and capitis. The last of these, semi-spinalis capitis, is THE MUSCULAR SYSTEM 
235 
a large and well-developed muscle, divided longitudinally into 
a median and a lateral bundle; the inner one of these is trav- 
ersed by a myocomma, dividing the muscle into two fleshy 
pemispinalis f coinplexus (major) 
capitis '[biventer cervicis 1 
spinalis colli [cervicis] 
Semispinalis colli 
[cervicis] 
SUpraspinatusi 
infraspinatus 
teres minor 
teres major 
semispinalis 
dorsi 
Spmah'^dorsi 
guadratus 
lumborum , 
I intertransversarii 
piriformis 
f gemellus superior 
' obturator interims 
'] gemellus inferior 
quadratus femoris 
Fig. 8o. Muscles of the human back; the muscles lying beneath those 
shown in Fig. 79. 
Certain of the deeper muscles of shoulder and hip are also shown. 236 
THE HISTORY OF THE HUMAN BODY 
bellies, from which comes the older name of biventer cervicis. 
The outer portion forms the complexus (major) of the older 
anatomists. (Figs. 79, 80.) 
V. Intervertebral System. — Still beneath the multifidus 
and rotatores are several series of short muscles, stretching 
between adjacent vertebrae, and probably representing a few 
fibers of the primitive myotomes, which have remained in their 
original condition. Of these the most extensive are the inter- 
Fig. 8i. Muscles of the posterior cervical region (human) 
rj, rectus capitis posterior major; rn, rectus capitis posterior minor; os, obliquus 
capitis superior; oi, obliquus capitis inferior, rl, rectus lateralis. Areas of origin on 
the skull are indicated as follows: x, splenius; y, semi-spinalis colli; s, trapezius; os, 
obliquus capitis superior. The cervical vertebrae are indicated by Roman numerals. 
spindles, lying on either side of the median line, and extend- 
ing between adjacent spinous processes. Typically associated 
with all the intervertebral intervals, they occur in man mainly 
in the cervical and lumbar regions, including the first and last 
of the dorsal vertebrae, and are wanting through the middle 
of the back. A second set, the intertransversarii, between ad- 
jacent transverse processes, occur in lumbar and cervical 
regions, but in the thoracic are represented by tendons without THE MUSCULAR SYSTEM 
237 
contractile fibers. The intertransversarii of the lumbar region 
are divided into medial and lateral portions, a division probably 
without morphological significance; on the other hand, those of 
the cervical region are divided the other way into ventral and 
dorsal portions, which belong respectively to the like-named 
divisions of the axial musculature and are hence morpholog- 
ically distinct. The ventral cervical intertransversarii are in- 
nerved by ventral branches, and are attached to the ventral or 
rib element of the complex “ transverse process ” of the cervi- 
cal vertebrae. They are thus in the same series as the inter- 
costals, and have been already treated in that connection. The 
dorsal cervical intertransversarii, on the other hand, are in- 
nerved by dorsal nerves and are thus serially homologous with 
both portions of the intertransversarii of the lumbar region and 
with the tendons which have replaced them in the thoracic 
region. 
Between the axis and the skull, corresponding to the special- 
ized motions needed in this place, there has developed a com- 
plex group of little muscles, which are seemingly differentia- 
tions from the intervertebral system. The rectus capitis 
posterior minor, in the form of a pair of small slips, extends 
between the rudimentary spine of the atlas and the occipital 
bone, and a similar but somewhat larger pair, rectus capitis 
posterior major, extends from the spine of the axis to the 
occipital bone, embracing the other pair. An obliquus capitis 
inferior extends from the spine of the axis to the transverse 
process of the atlas, and an obliquus capitis superior continues 
from this point to the occipital bone. 
This entire group develops from what appears in reptiles as 
a single mass. The dorsal branch of the second cervical nerve 
runs through this mass and divides it into medial and lateral 
portions. From the former both recti (major and minor) 
develop, through a longitudinal separation of their fibers, and 
from the latter arise both obliqui, which become separated 
from one another through the outward growth of the transverse 
process of the atlas, which has divided the muscle across its 
fibers. 238 
THE HISTORY OF THE HUMAN BODY 
Posterior to the pubo-ischiadic symphysis and the cloacal 
orifice, structures which make an hiatus in the ventral series 
of axial muscles, both dorsal and ventral masses become re- 
duced in size and taper down to form the musculature of the 
tail. But little morphological research has been devoted to 
this region, and even in mammals, where the conditions seem 
the best known, there is much to be done to complete the sub- 
ject. In long-tailed mammals there are typically two exten- 
sor es caudce upon each side of the mid-dorsal line; an extensor 
caudce medialis, which is a direct continuation of the multifidus 
and an extensor caudce lateralis, which appears between multi- 
fidus and longissimus, but does not seem to be a continuation 
of either. Upon the sides are two abductores caudce, dorsalis 
and ventralis, the former being short and of lesser functional 
importance, the latter assuming the function of the principal 
abductor. Ventrally there is a single pair of flexor es {de- 
pressor es) caudce. Of the above, as shown by the innerva- 
tion, the extensors and dorsal abductors belong to the dorsal 
system, the ventral abductors and the flexors to the ventral. 
Aside from these there is found a series of intertransversarii 
caudce, presumably belonging to the dorsal system as in the 
case of the lumbar muscles of the same name. 
In the tailles apes and in man, corresponding to the great 
reduction of caudal (coccygeal) vertebrae, the muscles are 
greatly reduced, and of uncertain occurrence, yet traces of all 
except the intertransversarii have been detected, and certain of 
them are fairly constant. The sacro-coccygei posterior es (ex- 
ten sores coccygis) are found upon the dorsal side of sacrum 
and coccyx, and in individual cases may represent either of 
the two extensors or the dorsal abductor, or any combination 
of these. Thus in ioo human bodies the medial extensor oc- 
curred 53 times, the lateral extensor 43 times and the dorsal 
abductor 87 times, there being but six cases in which indica- 
tions of the group are wholly absent. Upon the ventral side 
of the coccyx are found rudiments of the flexores caudce, de- 
scribed under the names of sacro-coccygei anteriores, s. curva- THE MUSCULAR SYSTEM 
239 
tores coccygis, and of the ventral abductor, here called the 
coccygeus or abductor coccygis. The former was found in 
102 out of no cases; the latter appears to be constant, al- 
though often tendinous. 
(A) Ventral. (B) Dorsal. 
pir, piriformis; acv, abductor caudae ventralis; acd, abductor caudae dorsalis; eel, 
extensor caudae lateralis; sea, sacrococcygeus anterior ( = flexor caudae). The lumbar 
vertebrae are designated by Roman numerals, the coccygeal by Arabic. 
Fig. 82. Human caudal muscles. [After Lartschneider.] 240 
THE HISTORY OF THE HUMAN BODY 
Two further caudal muscles, pubo-coccygeus and ilio-coccyg- 
eus, are found in long-tailed mammals, stretching partly 
across the pelvic floor, and sustaining some connection with 
the rectum at its termination. These muscles, although 
rather small, possess much morphological interest, since, in the 
anthropoid apes and in man, they come to lie transversely 
across the posterior pelvic opening and form the levator ani, a 
muscle, which, in connection with the coccygeus, is the prin- 
cipal element concerned in the construction of the so-called 
diaphragma pelvis. This is a muscular and tendinous floor or 
partition, closing the posterior outlet of the pelvic cavity, and 
its formation is unquestionably an adaptatio7i to the erect posi- 
tion of the anthropoids and man, thus strengthening what 
would otherwise become from this position a point of weak- 
ness. 
Although the exact homologies of pubo- and ilio-coccygeus 
(levator ani) are still somewhat obscure, they probably belong 
to the ventral axial system, and are not connected otherwise 
than by contiguity with the other muscles of the perinaeum, 
and the genital organs, such as the sphincter ani, ischio-cav- 
ernosus, etc., which like the sphincter marsupii and sphincter 
cloacae of monotremes (cf. p. 277 below) may be modifications 
of integumental muscles. 
Although, as previously stated, the morphological history of 
the dorsal trunk muscles is almost unknown, there are yet a 
few points of interest which may prove suggestive. Among 
the amphibians there is a suggestion of a longitudinal sub- 
division of this mass into a medial and a lateral portion, a 
change which in the reptiles becomes complete and definite. In 
the mammals this condition is still evident, with but a few sec- 
ondary modifications which tend to obscure the plan somewhat. 
Thus the spino-spinalis and transverso-spinalis systems, the 
latter with all of its subdivisions, belong to the medial portion, 
while the sacro-transverso-transversalis system, on the other 
hand, with its two main subdivisions of ilio-costalis and longis- 
simus, may be referred to the lateral portion. Of these two THE MUSCULAR SYSTEM 
241 
systems it may be said in general that the separate slips of the 
medial portion are inclined towards the median line and become 
inserted into spinous processes, while those of the lateral por- 
tion are inclined outwards (laterally) and become inserted 
either into transverse processes or into ribs, the two being 
genetically the same. The most important exception to this 
is seen in the longissimus, which does not indeed violate the 
principle just laid down, but which possesses a few origins 
from spinous processes, and hence from the median line; these, 
however, have been proven to be of secondary origin, arising 
through an association of a part of the longissimus with the 
spinalis, a relation that is not established until during a fairly 
late embryonic period. The only other doubtful case is that of 
the splenius, which arises from spinous processes as though 
belonging to the medial portion, but inserts in part laterally. 
This muscle, however, is found only in mammals, and may 
thus be considered to have developed long after the division 
into the two portions had become established. A similar rela- 
tionship in the obliquus capitis, which, although belonging to 
the medial portion, has its origin from a spinous process and its 
insertion into a transverse process, has been explained above 
as a secondary condition induced by an extension of the trans- 
verse process of the atlas. 
To complete the discussion of the axial muscles there still 
remains a group to be considered, and that is, the muscles of 
the eyeball, for the proper estimation of which one must look 
to embryology, especially that of selachians and amphibians. 
During development certain pairs of mesodermic somites ap- 
pear in the head as well as throughout the trunk and tail, and 
in embryos of such forms as those named, in which the repe- 
tition of the early stages is best given, these epimeres appear 
for a time as hollow cavities, the so-called “ head cavities.” 
With several of these head cavities dorsal (motor) elements 
from the cranial nerves become associated, and although cer- 
tain of them become abortive, after continuing up to the stage 
of possessing a nerve supply, three pairs remain, those asso- 242 
THE HISTORY OF THE HUMAN BODY 
ciated with the third, jourth and sixth pairs of cranial nerves 
respectively, all motor nerves. From these develop the muscles 
of the eyeball, in accordance with their innervation; that is, 
from the walls of the head cavity associated with the third 
nerve (Oculomotor) develop rectus superior, rectus internus 
(medialis), rectus inferior and obliquus inferior; from that as- 
sociated with the fourth nerve (Trochlearis) there develops the 
obliquus superior alone; and from that supplied by the sixth 
nerve (Abducens) arises the rectus externus (lateralis). The 
retractor bulbi, a muscle which appears first in the amphibians, 
develops from rectus externus and is consequently innerved by 
the sixth nerve; it lies enclosed by the recti, and often develops 
into a hollow cone or a system of slips which act almost as 
independent muscles. It is well developed in most mammals, 
but is rudimentary or lacking in the primates. 
Aside from the head cavities from which the muscles of the 
eyeball are derived, a matter which is in many points still 
controversal, there are several other pairs of head cavities 
found only in the embryo. These furnish one of the many 
proofs of the segmental character of the primitive head, both 
of the parachordal and prechordal portions and that originally 
we may look back to an ancestor in which the head was well 
equipped with a successive series of metameric muscles, and 
that of these the muscles of the eyeball are the only survivors. 
In this place we may consider the muscles innerved by the 
Hypoglossal nerve. This nerve does not occur in the An- 
amnia, but appears as such in the Amniota, in which it is 
universal. Its sudden appearance in the series is considered 
to be due to a fusion of several of the first vertebral elements 
with the occipital region of the skull, thus causing the first 
two or three of the spinal nerves to pass out through foramina 
in the skull and to form a compound nerve which is topo- 
graphically one of the Cranial series. This is the Hypo- 
glossal and the muscles innerved by it are naturally meta- 
meric muscles and belong in the same series as the muscles 
of the eyeball and those of the trunk. These include the THE MUSCULAR SYSTEM 
243 
Hyo-glossus, Stylo-glossus, Genio-glossus, and in mammals 
the muscle of the tongue, the Lingualis. These relationships 
are well shown by the diagrammatic figure of a human em- 
bryo showing the metameres. (Cf. Fig. 76.) 
The appendicular muscles are the direct derivatives of cer- 
tain axial metameres, as is especially well shown by the on- 
togenetic repetition seen in selachian embryos. Here the 
first indication of a lateral fin is in the form of a mass of tissue, 
appearing externally as a slight, shelf-like projection, flattened 
dorso-ventrally. That this may be considered as a remnant of 
a once continuous fin-fold, which included both lateral and 
median fins, has been explained above in the chapter on the 
appendicular skeleton. Into this mass there grows from the 
ventral portions of the myotomes a series of what have been 
aptly termed myotomic buds, becoming, through bifurcation, 
two for each somite. These continue their development until 
the fin is supplied with a segmental series of muscle bundles, in 
number twice that of the somites from which they are derived. 
The actual number of myotomes thus involved differs in dif- 
ferent fishes, but in all there are certain similar principles 
which may be brought out by the more careful study of the 
case before us. By comparing the three diagrams in succession 
(Fig. 83) it will be noticed that of the contribution of the first 
trunk myotome only the second of the two buds succeeds 
in developing, and that that one becomes abortive and fails 
to furnish a permanent contribution to the fin. Beyond this 
the contributions appear chronologically in regular order from 
the front backwards, so that in A the two buds of the seventh 
myotome and the first of the eighth have reached the fin, 
while the second of the eighth and those of the ninth are still 
some distance away and less completely developed. The second 
figure, B, shows the complete disintegration of the contribu- 
tion of the first myotome and the further development of those 
from the second to the sixth inclusive, that is, buds II-XIII, 
while buds XIV-XVII, or those of myotomes 8 and 9, have 
barely reached the goal. In C is seen a single bud belonging  THE HISTORY OF THE HUMAN BODY 
244 
Fig. 83. Development of the anterior (pectoral) fin in the dog-fish. 
[After Braus.] 
The shaded portion represents the anlage of the fin skeleton, into which the 
muscle elements are growing in the form of myotomic buds. THE MUSCULAR SYSTEM 
245 
to the tenth myotome, which seems here far from the fin, 
but even this eventually reaches the fin, as does also a second 
one from the same myotome, forming buds XVIII and XIX 
respectively. 
The cause of this apparent struggle to reach the fin on the 
part of the most posterior myotomic buds, is one which ex- 
plains also certain other characteristic features of the develop- 
ment, and is found in the unequal rate of growth of fin anlage 
and of body axis, the latter considerably surpassing the former. 
There results from this the concentration or bunching together 
of the nerves of the free limb, especially noticeable in Fig. 83, 
C, a circumstance favorable to the formation of a nerve plexus, 
and as this concentration of a number of pairs of nerves to' 
form those supplying the limbs is also seen in the case of all 
higher vertebrates, it is a convincing proof of the derivation of 
the limb muscles from a more extensive series of myotomes 
than that indicated by the adult breadth of the limb. 
From this sketch of the development of a limb as seen in 
the selachians it becomes apparent that, were it possible in 
each group of vertebrates to trace the derivation of each limb 
muscle to a given myotomic bud, or, in other words, were it 
possible to follow the later history of each separate myotomic 
bud to the complex conditions of higher forms, a sure and 
certain homology of the limb muscles could be carried out; as 
a matter of fact, however, the primitive history in the develop- 
ment of limb muscle is found only in the cartilaginous fishes, 
which, in their adult state, are scarcely beyond the last of 
the three stages shown here, while in all higher vertebrates 
these early stages are dropped out completely, and in a 
developing limb, in which for a time the cells seem exactly 
alike, and without differentiation of any kind, the first indica- 
tion of any definite arrangement is the collection of these ap- 
parently indifferent cells into masses that suggest the parts as 
they exist in the adult. 
In such a case, then, the only recourse lies in the comparison 
of adult forms, and here, owing to the complexity of the sub- 246 
THE HISTORY OF THE HUMAN BODY 
ject and the technical difficulties in the way of such investiga- 
tion, much remains to be accomplished. There has, indeed, 
been a large amount of anatomical work done on the subject, 
but little has yet been attained in the study of the phylo- 
genetic development of the separate muscles or muscle groups, 
and the morphological history of the limb muscles is as yet 
far from complete. 
The key to the interpretation of the muscles associated with 
the hand type of limb {chiropterygium) must be found, if at 
all, among the tailed amphibians, which are the first animals 
to possess a true chiropterygium, that is, a free appendage 
furnished with digits instead of fin-rays, and here, in fact, are 
found many highly suggestive conditions, showing many of 
the most characteristic muscles of the higher type still in 
partial connection with the myotomes from which they have 
arisen. On the other hand, a direct comparison of these 
muscles with those of man and other mammals is by no means 
impossible and yields many interesting results, since the dis- 
tance between these two groups of animals is much less than is 
commonly supposed, and the intermediate stages do not in- 
clude either the tailless amphibians, the birds, or even the 
majority of reptiles, since all these have specialized along 
lateral lines. Indeed, man himself is far more primitive in 
the condition of his limbs, with their ancient inheritance of 
pentadactylism, than are either the salient Anura, with their 
four anterior digits and their specialized hip-girdle, or such 
reptiles as the turtles, in which both girdles have become much 
modified in connection with the formation of carapace and 
plastron. 
Probably the most primitive living vertebrate, above the 
fishes, is a large aquatic salamander, Necturus, generally dis- 
tributed throughout the United States, except the North- 
eastern States and the extreme South. It may thus be as- 
sumed to represent in the muscles of its free limbs the earliest 
condition of chiropterygial musculature yet remaining to us, 
and is consequently of the utmost importance in the present THE MUSCULAR SYSTEM 
247 
inquiry. It will be remembered that in most vertebrates there 
exists a certain close correspondence in the skeletal parts of 
anterior and posterior limbs, a so-called serial homology, and 
in many cases this correspondence is at least suggested in 
certain details of the muscular system. Here, in Necturus, 
however, the correspondence of the free limbs from elbow or 
knee down is practically an exact one and includes not only 
the skeletal parts but the muscles, arteries and nerves, precisely 
what would be expected in this primitive form if the serial 
homology is really fundamental and not due to secondary 
modification through a similarity of use. 
Proximal to the elbow and knee, however, there is little if 
any correspondence or even similarity in the musculature, cor- 
responding in this respect to the great differences in the two 
Fig. 84. Lateral view of shoulder muscles of Necturus. 
Id, latissimus dorsi; ds, dorso-scapularis; t, trapezius; j, omohyoid; k, levator 
anguli scapulae; la, levatores arcuura; d, dorso-trachealis; ph, procoraco-humeralis; as, 
anconeus scapularis; al, anconeus lateralis. 
girdles, and therefore for descriptive purposes the limb may 
be divided into unlike proximal portions, to be treated sepa- 
rately, and similar distal portions, which are almost identical 
and may thus be considered together. The first will include 
the girdle and the proximal joint of the free limb, the upper 
arm or thigh, and the second the remainder, or that from 
elbow or knee to the end. 
The principal muscles of the proximal portion of the an- 
terior limb in Necturus are shown in figures 84, 87, 88, and 
89. Of these certain are extrinsic and extend from trunk or 248 
THE HISTORY OF THE HUMAN BODY 
head to the appendicular skeleton; others are intrinsic, both 
origin and insertion being upon the latter part. 
Conspicuous among the first is the latissimus dorsi, certain 
parts of which are here seen to arise from myocommata in the 
form of elements in the act of separating themselves from the 
axial musculature, while other fibers, the anterior portion, no 
longer show their segmental origin. Ventral to these are seen 
two slips clearly derived from the long superficial rectus, and 
still farther ventral, covering the chest region, lies the volumi- 
nous pectoralis, still in part composed of slips attached to the 
myocommata. Beneath these superficial layers are deeper 
muscles, like the levator scapulae and the serratus magnus, here 
plainly derived from the axial muscles in the form of separating 
slips. 
Turning to the intrinsic muscles, it is seen that the outer 
surface of the three portions of the girdle is covered with fan- 
shaped sheets that converge to the head of the humerus, where 
they insert near together. Of these the dorsalis scapulae cov- 
ers the scapula, the procoraco-humeralis the procoracoid, and 
the supracoracoideus the coracoid. Distal to these come 
the muscles, the bellies of which occupy the region of 
the upper arm, and which may thus form a third group. 
Of these there are three that occupy the flexor aspect, and 
a complex one with several heads that lies upon the ex- 
tensor aspect. Of the first the two coraco-brachiales, longus 
and brevis, arise from the coracoid and insert on the humerus. 
These lie on the medial side. On the outer or lateral side lies 
the humero-antebranchialis, which arises along the humerus 
and inserts by a tendon into the proximal end of the radius. 
The complex muscle on the extensor side, the anconeus, is 
constant in the character of arising from several heads and in 
the insertion of all by a common tendon into the olecranon 
process of the ulna, although the name of “ triceps,” applied 
to this muscle in man, is objectionable, since the number of 
heads is variable and three is by no means the typical number. 
Thus here in Necturus there are four, a central superficial one THE MUSCULAR SYSTEM 
249 
from the scapula, a median and a lateral one from the humerus 
and a median one from the coracoid. The term anconeus, 
bearing no suggestion of the number of points of origin, but 
referring to its insertion alone (ayx&v, elbow, ulna), is much 
preferable. 
In mammals, corresponding to the great difference of struc- 
ture shown among the different Orders, there is a great di- 
versity in the appendicular musculature, but if there be taken 
for comparison with the above any of the more primitive 
pentadactylous quadrupeds, such as a marsupial, a rodent or 
a lower primate, the majority of the muscles in the two can be 
readily homologized (Figs. 85, 86). Latissimus dorsi and 
trapezius have greatly increased in extent; the former, having 
reached the mid-dorsal line, no longer possesses a free dorsal 
margin anteriorly, and posteriorly shows no trace of the primi- 
tive myotomic slips of which it was originally composed. A 
slip, segmented off from its anterior edge, has become a sep- 
arate muscle, with the name of teres major. The trapezius 
extends from the occipital region of the skull, a point which it 
attains in the higher salamanders, along the mid-dorsal line, to 
a point considerably posterior to the scapula, where it overlaps 
the latissimus. It may either be divided into three distinct 
slips, anterior, middle and posterior, or may be in the form of 
an unbroken sheet; and in climbing arboreal forms, like the 
monkeys and apes, and in man, is of enormous extent, the two 
covering the entire upper half of the back and prolonged 
posteriorly into a median point like a monk’s hood, whence the 
alternative name of cucullaris, employed by European anato- 
mists. From its anterior margin a bundle of fibers is set off 
and becomes the sterno-cleido mastoideus, a muscle running 
obliquely across the side of the neck from the anterior end of 
the sternum to the skull just behind the ear, and conspicuous 
in man. 
These three associated muscles possess a curious double in- 
nervation, which show definitely that they are composed of 
fibers from two original systems. Their innervation from 250 
THE HISTORY OF THE HUMAN BODY 
Fig. 85. Diagram of human muscles, showing their relation to the 
skeleton. [After Eustachius.] 
This figure and the next were drawn originally by the gifted Italian anatomist, 
Bartolomeo Eustachio, who died in 1574. His very numerous anatomical drawings, 
embracing all parts of the body, were neglected for nearly two centuries, and were 
finally collected and published, first, by Lancisi in 1714, and later by Albinus in 1761. 
These figures are taken from the latter edition. 
aa, attollens auris; oc, occipitalis; t, trapezius; d, deltoid; rr, rhomboideus, major 
and minor; Id, latissimus dorsi; gj, glutaeus maximus; gd, glutaeus medius; gl, gracilis; 
st, semitendinosus; b, biceps femoris. THE MUSCULAR SYSTEM 
251 
Fic. 86. Diagram of human muscles, showing their relation to the 
skeleton. [After Eustachius. See explanation of Fig. 85.] 
te, temporalis; spl, splenius; las, levator anguli scapulae; spa, serratus posterior 
superior; spp, serratus posterior inferior; sa, serratus anterior (magnus); t, triceps; 
tm, teres major; gn, glutaeus minimus; p, piriformis; st, semitendinosus, origin; bl, 
long head of biceps femoris, origin; bs, short head of biceps femoris; ve, vastus 
lateralis; vi, vastus medialis; sm, semimembranosus; am, adductor magnus; cr, vastus 
intermedius; po, popliteus. 252 
THE HISTORY OF THE HUMAN BODY 
Cranial nerves (Vagus and Accessorius) indicate clearly a 
branchiomeric element, while a second innervation from 
Cervical nerves of the Spinal system suggest equally an origin 
of certain elements from metameres. Topographically and 
functionally they rank undoubtedly as Appendicular muscles. 
The deeper layer of extrinsic muscles, levator scapulae and 
serratus anterior \magnus\, have increased meanwhile, prob- 
ably by the addition of intermediate slips that arise from the 
myotomes between the two, and become in most mammals a 
continuous layer, the primary metamerism being expressed in 
its slips of origin, which form “ digitations,” or separate 
pointed slips that arise from the successive ribs, or from their 
equivalent processes in the cervical region. Aside from these a 
second series of slips, more superficial than the above, appears 
in the higher amphibians, and these develop in mammals into 
the rhomboideus system. This inserts into the scapula and con- 
sists primarly of a slip from the occipital bone, rhomboideus 
capitis, and one from the vertebral spines in the interscapular 
region, rhomboideus dorsi. Both of these occur in most mam- 
mals, but in man rhomboideus dorsi alone is normally present, 
sub-divided into two slips, major and minor, while rhomboideus 
capitis appears only as a rare anomaly. The pectoralis in 
many mammals forms a complex system of distinct and semi- 
distinct portions, showing at least a superficial and a deep 
layer. In man and the anthropoids these two layers are repre- 
sented by two muscles, pectoralis major and minor respectively. 
The subclavius is a differentiation from the deeper layer. 
Of the intrinsic group the dorsalis scapulae becomes mainly 
the deltoid, often divided into several portions, spino-deltoid, 
acromio-deltoid, etc., but single in man. A small portion of 
this muscle becomes the teres minor, topographically associated 
with the teres major, derived from the latissimus. The supra- 
coracoideus is probably represented by the supra- and in fra- 
spinati, which have extended dorsally over the scapula, pushing 
their way beneath the deltoid, as this muscle has gradually 
lifted itself up from the general outer surface of that bone. THE MUSCULAR SYSTEM 
253 
The procoraco-humeralis seems to have become lost, together 
with the axial slips that insert into the procoracoid. 
The anconeus, the extensor muscle of the upper arm, varies 
mainly in the number and position of its heads, and not in 
its insertion or general position. Its identity with the human 
triceps has been already commented on. On the flexor side of 
the upper arm the history is not as plain. In the mammals 
there are two long muscles that insert into the forearm, the 
biceps brachii, that arises from the shoulder girdle and inserts 
by a tendon into the proximal portion of the radius, and the 
brachialis [anticus] that arises along the shaft of the humerus, 
and inserts into the proximal end of the ulna. Aside from 
these, there is a coraco-brachialis, from the coracoid process 
to the shaft of the humerus. These do not homologize readily 
with the muscles of the same region in urodeles, but the last 
muscle, which appears in some mammals as two, compares well 
with that of like name in Necturus. This leaves the humero- 
antebrackialis to be compared with both biceps and brachialis, 
and it may well be the ancestral form from which both have 
originated. In origin it is like the latter, and shows no sim- 
ilarity with the biceps, which arises, usually by a single head, 
from the scapula and the coracoid process; it is, however, pre- 
cisely like the biceps in its mode of insertion, and must be at 
least in part homologous with this latter muscle. 
The second region to be considered is the proximal portion 
of the posterior limb and includes the muscles of the pelvic 
girdle and thigh. As in the skeleton there is in the muscula- 
ture little suggestion of serial homology between the two pairs 
of appendages, although in Necturus the two limbs closely cor- 
respond in the distal portion. A fundamental difference in the 
muscles of the two girdles is that in the posterior limb they 
are nearly all intrinsic, and arise from the appendicular skel- 
eton, while in the anterior limb an extensive system of extrinsic 
muscles controls in part both the girdle as a whole and the 
proximal part of the free limb. This difference is undoubtedly 
correlated with the definite attachment of the posterior girdle 254 
THE HISTORY OF THE HUMAN BODY 
to the axial skeleton through the formation of a sacrum, while 
in the anterior girdle there is either no attachment to the verte- 
bral column, or a freely movable one through clavicle, 
sternum and ribs. 
In Necturns (Fig. 87), in which the pelvic girdle is in the 
form of a flat pubo-ischiadic plate and a narrow ilium, the 
muscles are naturally divided into those of the outer (ventral), 
pife, pubo-ischio-femoralis externus; pifi, pubo-ischio-femoralis internus; pit, pubo- 
ischio-tibialis; pt, pubo-tibialis; isf, ischio-femoralis; isc, ischio-caudahs; cpit, caudali- 
pubo-ischio-tibialis; cf, caudali-fermoralis; ra, rectus abdominis. Other designations: 
gl, cl., cloacal gland; Pub., pubic portion of pubo-ischiadic plate; isch., ischiadic 
portion of the same. 
Fig. 87. Ventral view of the pelvic muscles of Necturus. 
and those of the inner (dorsal) side of the plate, and, thirdly, 
those which arise from the ilium. Of the first there are two, 
forming as many layers on the outer side of the plate; the 
pubo-ischio-tibialis which runs down the inner side of the leg, 
passes the femur and inserts into the proximal end of the tibia, 
and the pubo-ischio-jemoralis externus, which inserts into the 
femur. Upon the inner side of the plate there is a single large 
muscle, the pubo-ischio-jemoralis internus, the fibers of which THE MUSCULAR SYSTEM 
255 
part to accommodate the ilium, but reunite again upon the 
outer side of this obstacle. This also inserts into the femur. 
Aside from these, a narrow band, the pubo-tibialis, arises from 
the lateral edge of the pubo-ischiadic plate, and inserts in the 
tibia. From the ilium arise an ilio-extensorius, which inserts 
into the femur, an ilio-jemoralis, also to the femur, and an 
ilio-fibularis, a narrow band, which inserts into the proximal 
end of the fibula. A femoro-fibularis, also band-like, arises 
from the flexor side of the femur, and inserts into the fibula, 
near the last. The only extrinsic muscles are found in a set 
of three caudal muscles, which arise along the sides of the tail 
not far below the pelvis, and run in a sheath anteriorly to the 
posterior limbs, inserting the one into the ischium, another 
into the femur, and the third into the margin of the pubo- 
ischio-tibialis. 
Unlike the anterior limb, in which the most of the muscles 
occuring in Necturus may be readily recognized in mammals, 
the homologies of the proximal portion of the pelvic limb are 
all more or less doubtful. As a beginning, there are found 
upon the outer side of the pubo-ischium in mammals the 
obturator externus, the adductores, the gracilis, which belongs 
with the adductor group, and possibly the sartorius. Of these 
the obturator is probably the homologue of the pubo-ischio- 
jemoralis externus, and the remainder may be derivatives of 
the pubo-ischio-tibialis, in spite of the difference in respect to 
insertion. The pubo-ischio-jemoralis internus seems to give 
rise to the obturator internus with the two associated gemelli. 
The ilio-extensorius is probably the prototype of the great 
complex of the front of the thigh, quadriceps femoris, com- 
posed of the three vasti, externus, medialis [crureus] and 
internus, and the rectus femoris. The glutcei are probably 
derived from the ilio-femoralis. 
Of the muscles of the posterior aspect of the thigh, enor- 
mously developed in mammals, the inner ones, Mm. semimem- 
branosus and semitendinosus, are probably also derived from 
the pubo-ischio-tibialis, while the two heads of the outer one, 256 
THE HISTORY OF THE HUMAN BODY 
the biceps femoris, come from two distinct sources, and in 
many mammals are separate muscles. The derivation of the 
long head is uncertain, but it may be homologous with the ilio- 
fibularis of urodeles, in spite of the difference in origin. The 
short head, on the other hand, is derived from the glutaeal 
group, and is identical with the long narrow band, described 
in many mammals as the tenuissimus. Since it is associated 
with the long head to form a “ biceps ” muscle in a few mam- 
mals only, including man and several apes, this slip is best 
considered as a separate muscle of the glutaeal group, under 
the name of glutceo-cruralis. The caudal group of muscles, 
which extends in urodeles from the sides of the tail to the 
ischium and femur, persists with some modifications in mam- 
mals; in man and the higher anthropoids, in which the reduc- 
tion of the caudal vertebrae restricts the origin, the group is 
represented by a single muscle, the piriformis, extending from 
the coccygeal region across to the femur. 
The muscles of the distal portion of the vertebrate chirop- 
terygium, that is, from elbow or knee on, aside from the mod- 
ifications imposed upon them by the varying shapes of the 
limbs themselves, and the great difference in their use, are, in 
their essential features, quite similar in all living tetrapods, and 
in their differences show the modifications of a primary type 
due to environment rather than the suggestion of an historic 
development of that type. The study is, therefore, one mainly 
of the adaptations of a given set of elements, rather than a 
phylogenetic history, which latter, as is the case also with the 
bones of the same region, must be sought in the gap separating 
fin and hand, that is, in the phylogenetic stages represented 
by lost forms of ganoids, stegocephali, and their allies. The 
salamander Necturus, probably the nearest approach to this 
series represented by living fauna, offers in its distal muscles 
some few suggestions of an earlier phylogenetic stage, and is 
thus of fundamental importance in the present inquiry. The 
well-nigh complete correspondence in the fore and hind limb 
as regards not only bones and muscles, but other parts as well, THE MUSCULAR SYSTEM 
257 
has been commented on above and presents a noteworthy 
example of serial homology, to be considered later. There 
are, also, as is the case with higher forms, some traces of a 
correspondence between the dorsal and ventral surfaces of a 
single paw, giving a suggestion of the derivation of the chiridial 
musculature from a fin-like precursor, in which the jointed 
rays (digits) were supplied by similar muscular elements ap- 
plied both dorsally and ventrally, as in present-day fishes. 
The following description is that of the anterior limb, but with 
the substitution of the terms tibia and fibula for radius and 
ulna, tarsus for carpus, and so on, it will be found almost 
equally applicable to the posterior one. In a few cases a muscle 
which is well developed in the anterior limb is small or want- 
ing in the posterior, and thus the former is a little more 
typical.* 
The dorsal aspect of the antebrachium (Fig. 88, a and b) is 
largely taken up superficially by a single muscular mass, M. 
dorsalis antebrachii (da.) which arises from the distal end of 
the humerus. This separates, spreads out over the ante- 
brachium, and divides distally into four slips, three for the in- 
terdigital spaces and one for the ulnar side of digit V. Each 
of these in turn divides into two, which insert by tendons into 
the bases of the adjoining metacarpals. The muscle is thus an 
abductor-adductor complex, furnishing the digits with lateral 
motions, but without any power in extending them. The radial 
aspect of digit II is alone unsupplied from this system, and 
this deficiency is made up by the supinator (5), a muscle which 
* In one point the free limb of Necturus diverges from what is gener- 
ally believed to be the typical chiropterygium, and that is, it possesses but 
four digits in each extremity instead of the canonical five which is usually 
considered primitive. Since the nearest ally of this species, the cave form, 
Proteus, exhibits a still greater reduction of digits (anterior, 3; posterior, 
2), it has been presumed that this is in both cases a secondary reduction. 
Certain facts, however, lead one to think that the first land vertebrates 
possessed a smaller number of digits than five, and if this be so, the condi- 
tion in these two salamanders is primitive, and not a secondary reduction. 
According to the reduction theory digit I is assumed to be the one lost, 
and in accordance with this the four digits present are designated here, both 
in text and illustrations, as II-V. 258 
THE HISTORY OF THE HUMAN BODY 
underlies the former, arising from the ulnar side of the carpus. 
It crosses the limb obliquely, and inserts into the internal or 
free aspect of metacarpal II. Extension of the digits is effected 
by four short muscles, Mm. extensores breves (x, x), which 
(a) Superficial muscles, (b) Deeper muscles. 
da, dorsalis antebrachii, its separate insertions into the metacarpa'lia are shown 
in (b); er, radial extensor; eu, ulnar extensor; fu, ulnar flexor, showing from the 
other side; xx, extensores breves; s, supinator; zz, intermetacarpales. 
Fig. 88. Muscles of the fore-paw of Necturus; dorsal aspect 
arise from the distal row of carpalia and become continued 
into tendons that lie along the dorsum of the separate digits 
and are inserted into the bases of the terminal phalanges. 
Partly along the sides of the dorsalis, and partly covered by it, 
thus forming a deeper layer, are two long muscles, associated 
respectively with radius and ulna, Mm. extensor radialis and 
extensor ulnaris {er. and eu.). These arise from the humerus THE MUSCULAR SYSTEM 
259 
with the dorsalis and are inserted, the one along the shaft 
of the radius and into certain of the radial carpals, the other 
along the ulna and into ulnar carpals. 
The ventral (palmar) aspect of the limb is more complicated 
in respect to its muscles. These are covered superficially by a 
dense palmar fascia or aponeurosis (fp), to which many of the 
ventral muscles are attached. This aponeurosis is a continu- 
Fig. 89. Muscles of the fore-paw of Necturus; ventral aspect 
(a) Superficial muscles, (b) Deeper muscles. 
fp, palmar fascia; ps, palmaris superficialis; pp, palmaris profundus; uc, ulno- 
carpalis; fu, ulnar flexor; fr, radial flexor; pr, pronator; yy, flexores breves super- 
ficiales; tt, terminal tendons of the palmar fascia. 
ation of the fascia covering the ventral muscles of the forearm 
and appears at its thickest and densest as it passes over the car- 
pal and metacarpal regions. At the separation of the digits this 
aponeurosis also is divided into four bands which run along 
the ventral surface of the separate digits and are inserted into 
the terminal phalanges. These slips are functionally and prob- 
ably morphologically the long flexor tendons (tt.), and corre- 
spond in a way to the long extensor tendons of the dorsal side, 260 
THE HISTORY OF THE HUMAN BODY 
but it must be remembered that here they are the continuation, 
not of muscular bellies, but of a non-contractile aponeurosis. 
The entire aponeurosis, however, is caused to move by serving 
as point of insertion of several muscles, more proximally placed, 
the palmaris superficialis (ps), which inserts along its proximal 
edge, or, more exactly, between its two layers, which thus 
invest the muscle, and the palmaris profundus (pp), which is 
entirely covered by the aponeurosis and is inserted into its 
dorsal (internal) side. The action of these muscles upon the 
aponeurosis causes it to act indirectly, through its digital slips, 
upon the separate digits, and cause a complete flexion. Aside 
from the palmaris system, the physiological action of which is 
that of a system of flexors, there are two sets of short flexors, 
Mm. flexores breves superpciales and flexores breves profundi, 
each consisting of four muscles, one to each digit. The super- 
pciales arise from the distal row of carpalia, and pass into ten- 
dons, which, encountering the long slips of the aponeurosis, 
divide into two lateral tendons and become inserted upon the 
sides of the penultimate phalanges. The flexores profundi lie 
close to the bone, arise beneath the former, and become in- 
serted into the bases of the proximal phalanges. There are 
here also, as on the dorsal side, two long muscles which arise 
from the humerus and are inserted along the shafts of radius 
and ulna and into the corresponding sides of the carpals, serv- 
ing as flexors of antebrachium and manus as a whole. These are 
respectively the flexor radialis and flexor ulnaris (fr and fu). 
The ventral muscles thus far enumerated, act either directly 
or indirectly as flexors, but beneath all of these is a set of short 
abductors and adductors of the metacarpals, abductores and 
adductores breves, which correspond in function to the large 
muscle mass of the dorsal aspect, M. dorsalis antebrachii, with 
its abductor and adductor tendons. These extend across the 
interval between the distal carpalia and the metacarpals, and 
like those of the dorsal mass, supply both sides of the two inner 
digits, III and IV, and the inner sides of II and V. As in 
the case of the abductor and adductor system of the dorsal THE MUSCULAR SYSTEM 
261 
side, M. dorsalis antebrachii, the internal (radial) side of digit 
II remains unsupplied from this system and the deficiency is 
made good by the pronator (pr), a muscle which lies obliquely 
across the antebrachium and is related to the skeletal parts 
precisely as is the supinator of the dorsal side. Like the 
latter it arises from the shaft of the ulna and passes obliquely 
downwards to the radial side of the limb, where it becomes 
inserted by a tendon into the radial side of the base of meta- 
carpal II. 
Deepest of all, beneath the short abductors and adductors, 
and reached equally well from either dorsal or ventral aspect, 
a set of three inter metacar pales stretch their fibers across the 
interspaces between the separate metacarpals and act either as 
abductors or adductors of the separate digits (Fig. 88, b, 
zz). 
Reviewing the conditions in this, probably the most primi- 
tive chiropterygium now left to us, several interesting points 
become manifest. The digits are moved in two ways, either 
flexed and extended or moved sideways, but while the system 
which provides for this latter form of motion is extremely well 
perfected, that for flexion and extension is not. For abduc- 
tion and adduction there are typically five separate muscles for 
each digit, that is, two ventral, two dorsal and one intermeta- 
carpal, while for flexion and extension, aside from the system 
supplied by an aponeurosis, and evidently a newly introduced 
feature, there are but three. This extreme perjection of the 
sideways movement of the digits in the most primitive chirid- 
ium known, together with the weak and makeshift arrange- 
ments for bending and straightening the digits, strongly sug- 
gest the derivation of the chiridial type from one in which the 
digits {fin-rays?) required to be constantly opened and shut 
by lateral movements, precisely as in the case of the fins of 
most fishes. 
During later phylogenetic history there is an evident tend- 
ency to increase the efficiency of the flexor-extensor system and 
diminish that of the abductors and adductors, except in the 262 
THE HISTORY OF THE HUMAN BODY 
case of the two digits that form the ends of the series (I and 
V), and the most of these changes have already occurred 
among the higher urodeles. Thus in Cryptobranchus the dor- 
salis antebrachii, which in Necturus serves as an abductor- 
adductor system and terminates at the base of the metacarpals, 
is continued into four long tendons, which insert into the ter- 
minal phalanges, and thus becomes the extensor communis digi- 
torum, although in the hind limb at least, from the sides of 
the long tendons, small lateral slips extend still to the sides 
of the metacarpals, the remains of the abductor-adductor sys- 
tem. The short extensors become more complicated than in 
Necturus, but insert along the proximal part of the digits 
and are no longer continued into long tendons to the ter- 
minal phalanges, so that function has been usurped by the 
other muscle. 
Upon the ventral side a more intimate connection between 
the tendons of the palmar aponeurosis and the associated 
muscles gives rise to the system of long flexors, as found in the 
higher vertebrates. In the arm, where this history has been 
most completely followed, the palmaris muscles, superficial and 
deep, uniting with the palmar fascia and its long tendons, 
form the two long flexors characteristic of mammals, flexor 
digitorum sublimis and profundus. In the monotremes the 
bellies form a common mass, flexor communis digitorum, al- 
though with double tendons to the separate digits, a deep 
tendon which inserts on the terminal phalanx and a superficial 
tendon which forks. The two resulting parts insert upon the 
edges of the penultimate phalanx, and allow the deep tendons 
to pass through between them. A later differentiation of the 
belly divides it into a flexor profundus, continued into the deep 
tendon, and a flexor sublimis, continued into the superficial 
tendon. The most superficial of the fibers separate into the 
somewhat inconstant “ palmaris longus.” The flexor pollicis 
longus of man belongs with the profundus. 
The tendons of the short superficial flexors of amphibians 
become mainly employed in the formation of the profundus THE MUSCULAR SYSTEM 
263 
tendons, while the bellies degenerate, but those associated with 
digits I and V develop in the mammalian hand and foot into 
special muscles connected with those digits, such as the abduc- 
tors of pollex and minimus, the opponentes of the same, and 
the flexor brevis minimi digiti. The short, deep flexors of the 
amphibians, flexores breves projundi, become the mammalian 
lumbricales; and the still deeper set of abductors and adduc- 
tors, together with the intermetacarpales, become the two sets 
of interossei, palmares and dorsales, the latter arising upon the 
ventral aspect and coming through to the dorsal side during 
development. 
The four muscles which in Necturus lie along the ulnar and 
radial sides of the antebrachium on both dorsal and palmar 
aspect, and furnish a flexor and an extensor for each side, are 
continued with some modifications in higher animals. The 
origin from the distal end of the humerus remains the same, 
but the insertions along the shafts of ulna and radius are given 
up, and are either confined to the carpal bones of the corre- 
sponding sides or a new tendinous insertion is acquired which 
extends to the base of some metacarpal, the muscles becoming 
flexor carpi radialis, extensor carpi ulnaris, and so on. The 
extensor carpi radialis of mammals becomes divided into two 
similar muscles, longus and brevis, which insert into the bases 
of metacarpals II and III respectively. 
A final group of limb muscles are the pronator and supinator, 
which give the limb the power of turning about its axis, thus 
crossing or tending to cross the two bones of the forearm or 
lower leg. Of these the pronator lies upon the flexor, the 
supinator upon the extensor aspect of the limb, their fibers 
extending diagonally across from one bone to the other. In 
Necturus there is one upon each aspect, the character of which 
suggests their derivation from the primary system of abductors 
and adductors. 
The striking correspondence in many features between the 
anterior and posterior limb, especially shown in cases in which 
the two are used in a similar way, has naturally led to the 264 
THE HISTORY OF THE HUMAN BODY 
theory of the serial homology between them, that is, an original 
homology, not between different animals, as is usually meant 
by the term, but between different parts of the same animal. 
The theory presupposes a time at which both sets of limbs were 
exactly alike, part for part, and thus the final results, however 
unlike in the two cases, are referable to a single ground plan 
from which both have been derived. It would be thus possible 
to homologize bone for bone, muscle for muscle, and to extend 
the parallelism to the vessels and nerves as well. 
A strong proof of this is afforded by the close similarity of 
the two limbs in the lowest of the amphibians, as stated above, 
for here, as shown especially in that form which is probably 
the most primitive of all, the correspondence is very remark- 
able. It must be noted, however, that this serial homology is 
clear only in the case of the distal portion of the limb, the part 
beyond the elbow or knee, while in the portion proximal to this 
point, there is very little suggestion of such a parallelism. 
From this may be drawn the following conclusions: Granting 
that both limbs have arisen from a similar origin, and were 
alike at the start, it is allowable to suppose that the distal 
portions, being used in a similar manner, have either retained 
their primitive structure, or have differentiated alike, up to the 
point exhibited by the present-day urodeles; the proximal por- 
tions, facing from the start radically different problems con- 
nected with the poise of the body, the varied action of different 
parts of the trunk, and other differentiating factors, have 
become modified along different lines, and have attained results 
that suggest little of the original homology. 
The fin-fold theory of the origin of the limbs, given in 
a former chapter, throws but little light upon the theory of 
serial homology, and, it must be confessed, even stands some- 
what in the way of such an hypothesis, since although in its 
primitive form, the fin-fold may be considered to have been 
made up of similar elements, repeating themselves metameri- 
cally, and appearing probably as skeletal rays supplied with 
muscles from the trunk musculature in the form of “ myotomic THE MUSCULAR SYSTEM 
265 
buds,” yet there is no suggestion that an identical number of 
these elements was originally taken in the case of the two sets 
of limbs or that the strictly pentadactylous character of the 
hand form could have been in any sense primitive, or could 
have existed at the time at which the two limbs might be sup- 
posed to have been strictly identical. 
However, as opposed to all theory in the matter, and it must 
be remembered that the fin-fold theory itself rests upon very 
little actual evidence, here is the fact of the actual close corre- 
spondence in the fore and hind limbs of urodeles in general, 
and especially in the case of Necturus, the particular form 
which from other reasons is considered especially primitive, 
perhaps even the oldest living representative of all animals 
possessing the hand form of appendage. 
Among mammals the limbs of the primates, even in the 
slightly modified form possessed by bipedal man, are quite 
primitive, and, together with those of the allied insectivores 
and rodents, retain the typical five digits, a character in which 
most of the Carnivora and nearly all of the ungulate groups 
show a much greater specialization. Thus the distance sep- 
arating primates from the amphibians is not very great, and 
it is therefore not surprising that in the distal portion an homol- 
ogy, not merely of the bones, but of the muscles as well, is 
still quite evident. In the chapter on the appendicular skeleton 
the close correspondence was pointed out between the bones of 
the arm and hand and those of the leg and foot; here the simi- 
lar correspondence may be considered between the muscles of 
these parts, the subject being confined in this case, however, to 
the distal portion, that is, the portion from the elbow or knee 
on. 
Take in the first place the set of four radial and ulnar 
muscles, which in their final form in the primates become five, 
namely, the flexor carpi ulnaris, flexor carpi f-adialis, extensor 
carpi ulnaris and the two extensores carpi radiates, longus and 
brevis. 
Their homologues in the leg have naturally become modified 266 
THE HISTORY OF THE HUMAN BODY 
through the difference in function and the formation of a heel, 
and their determination is perhaps the least clear of any of the 
distal limb muscles. There are, however, to correspond to the 
two flexors, the tibialis posterior and the soleus-gastrocnemius 
complex, of which the first, a tibial flexor, represents the radial 
one, and the second, primarily a fibular flexor, the correspond- 
ing muscle on the ulnar side. Upon the extensor aspect, the 
tibialis anterior may be the serial homologue of both radial 
extensors, while the ulnar extensor is represented by the two 
peronoei, longus and brevis. In tabular form the above homol- 
ogies appear as follows: 
Arm 
Leg 
Flexor carpi radialis 
Flexor carpi ulnaris 
Extensor carpi radialis longus ) 
Extensor carpi radialis brevis $ 
Extensor carpi ulnaris 
_ (longus 
} brevis 
The set of supinators and pronators may next be considered, 
and in Necturus in which in each limb there is a single supi- 
nator on the extensor side and a single pronator on the flexor 
side, the homologies are evident. In all cases they extend 
from a more proximal origin upon the outer side (ulnar or 
fibular) to a more distal insertion upon the inner (radial or 
tibial). In the human arm there are two pronators, teres and 
quadratus, but as these are continuous in many marsupials and 
carnivores, they may be considered as derivatives of the single 
urodele muscle. Their homologue in the leg is undoubtedly 
the popliteus. Upon the dorsal side most works on human 
anatomy record two supinators, longus and brevis, but as the 
longus [ = brachioradialis BNA] is really a portion of M. 
brachialis, and belongs with the upper arm, the only true 
supinator is the one designated brevis [= supinator, BNA], 
undoubtedly the same as that in the urodeles. Its homologue 
in the leg seems to have disappeared. THE MUSCULAR SYSTEM 
267 
The remaining muscles, those controlling the action of the 
separate digits, are still more in accord, and that too in spite of 
Abd.min.di4. 
Fig. 90. Diagrams of the muscles of the human hand and foot, illustrating the correspondence 
between them.* 
’Perhaps the two sesamoids of Digit I in both hand and foot, should not be represented as definitely the points 
of insertion of the several muscles concerned. 
I’T . = 
quadraftra 
Abductor 
hallucis 
(brevis) 
(A) Hand; (B) Foot. 
AbdJOin.di& 
Abductor 
pollicis 
longus 
Abductor 
poDicis 
brevis 
the great difference in use between hand and foot, especially 
in civilized man, suggesting the conservatism of these parts. 268 
THE HISTORY OF THE HUMAN BODY 
and the fact that it is easier to keep a complicated structure, 
when once obtained, even when not used in all its parts, than 
to replace it with a simple structure without unnecessary parts, 
provided only that the more complicated structure is in no case 
detrimental to the effective working of the organ in its simpli- 
fied function. It is often presumed that the reason why such 
changes as these have not occurred is that the time has been 
insufficient to effect it, but this is not the case. The only 
reason for an adaptation lies, not in the lapse of time, which 
in itself is powerless to effect even the slightest change, but in 
the question of expediency for the animal, that is, whether the 
part comes within the power of natural selection or not. In 
the present case the foot of man and his immediate ancestors 
has borne its present shape from inter-glacial times, a period 
given by conservative estimates at 100,000 to 200,000 years, 
and has as yet undergone but little change along the line of 
reduction. A needful progressive change has indeed taken 
place, namely, the development of the peromzus tertius, a 
muscle for lifting the outer edge of the foot and thus counter- 
acting the tendency to walk exclusively upon this portion; but 
the change in the line of reduction of a needless complexity 
of parts has been but slight, simply because there has been 
no need of such a modification. 
To review the complete serial homology of the muscles of the 
hand and foot would prove too long a task for the present 
work, but a large part of the correspondence may be presented 
in the form of a diagram (Fig. 90), which gives all the muscles 
of the flexor surface, excepting the lumbricales and the inter- 
ossei. In both there is seen a double system of flexor tendons, 
perforantes and perforati; the first digit has the perforans 
alone, in both hand and foot, and in the foot a perforatus is 
wanting in digit V, possibly a regressive change. In the 
anterior limbs both of these systems are long muscles, their 
bellies lying along the forearm, while in the foot the belly be- 
longing to the three perforated tendons of digits II to IV 
is a short one, confined to the foot region. The two outer THE MUSCULAR SYSTEM 
269 
digits are richly supplied with individual muscles, in which 
there is a remarkable correspondence between hand and foot, 
and that too in spite of the loss of independent action in the 
case of the little toe. This fact of the rich supply of muscles 
to the marginal digits, I and V, is made much of by supporters 
of the theory of the prce-pollex and post-minimus, theoretical 
digits that may have once existed at either end of the present 
series of five. The muscles in question are interpreted as the 
musculature of these extra digits, remaining after the loss of 
the skeletal parts to which they were originally attached. It is 
noteworthy also that the opponens hallucis is absent in man 
and has to be supplied in the diagram from the orang and other 
apes in which it is present, and that a similar loss or, at least, 
lack of individuality, is observable in the appearance of the 
little toe, two further regressive characters suggestive of a 
slight simplification through reduction. 
The subject of the homology of the limbs cannot be complete 
without reference to the various methods of comparison which 
have been proposed by numerous investigators, and which 
depart more or less radically from the one given here. Thus 
the torsion to which the limbs have been plainly subjected ap- 
pears to many a hindrance to a direct comparison of similar 
parts as given above and leads them to make the comparison 
in other ways; thus, in the earliest of these theories, more than 
a century ago, the right arm was compared, not with the right 
leg, but with the left; the thumb became thus the homologue 
of the little toe, radius was compared with fibula, and ulna 
with tibia (Fig. 91, B). That this theory, fantastic as it may 
seem, is not merely a vague speculation, but one to the aid 
of which many facts may be invoked, is shown by its per- 
sistence in one form or another even to the present day. 
In fact, the theorists on the subject of limb homology have 
been well divided into two schools, syntropists and antitropists, 
the former making a direct comparison of the limbs of the same 
side, with the digits in their usual order, the latter changing 
the order either by reversal, by comparing the limbs upon 270 
THE HISTORY OF THE HUMAN BODY 
opposite sides of the body, or by some other unusual means. 
Of this there is every possible variation; one theory considers 
in the first place the limbs of the same side to be the symmetri- 
cal equivalent of each other, and that thus the ulna is the 
Fig. 91. Diagrams explanatory of various theories of limb homology. 
(A) Syntropist theory,—members of the same side homologized. (B) Antitropist 
theory,—members of opposite sides homologized. (C) Homology between the spinal 
nerves involved in the antitropist theory. The cervical and dorsal nerves going 
posteriorly, are compared with the lumbo-sacral nerves going anteriorly. Thus, the 
fourth cervical nerve (C<) is the homolog of the second sacral (S2), and so on. (D) 
Theory of Foltz, 1863. The first digit of each limb is bivalent, and the equivalent 
of digits 4+5 of the other. (E) Theory of Eisler, 1895. Here the relation is an- 
titropic, but the homology applies to the three inner digits only in each member, 
leaving no homologue for digits 4 and 5 in each case. 
homologue of the tibia and the radius that of the fibula, and 
considers also the three radial fingers to be the equivalent of 
the three tibial toes, but in the reverse direction, as indicated THE MUSCULAR SYSTEM 
271 
in the diagram at Fig. 91, E. This leaves the two outer digits 
of each member without correspondence in the other. Another 
theory compares the digits, also in the reverse order, but con- 
siders both the thumb and the great toe bivalent, that is, equal 
to two digits, and thus compares each with two other digits of 
the other member, Fig. 91, D. This comparison of the digits in 
the reversed direction, however, when carried to its con- 
clusion, leads also to the homologizing in the reversed 
direction, of the spinal nerves that supply the limbs. Thus 
the nerves of the brachial plexus proceeding posteriorly, 
must be the homologues of the nerves of the lumbo-sacral 
plexus, proceeding anteriorly, as in Fig. 91, C. Perhaps 
the most recent of the theories in which there is a reversal 
of any part is one in which the limbs of the same 
side are taken for the comparison, and in the normal position 
as far as the knees, but which assumes that in the distal portion 
there has been a torsion of both arm and leg, thus causing the 
original extensor muscles to become flexors and vice versa. 
This homologizes the flexors of the forearm with the exten- 
sors of the lower leg, but allows in the proximal portion a 
direct comparison of the flexors with flexors and extensors 
with extensors. 
As already suggested, the theories just enumerated are not 
mere vague surmises, but rest in most cases upon careful study 
of the anatomical details; as they are not in accord with one 
another, however, they cannot all be right, and the remarkable 
degree of correspondence in bones and muscles, not merely in 
the salamanders, but also in many mammals, including man, a 
correspondence that is obvious and easily apparent, is a strong 
argument in favor of a natural syntropic comparison, as given 
here. The embryological history, moreover, so far as it is 
given, shows no sign of such a torsion or reversal as is de- 
manded by the antitropists, but presents as the first stage of the 
fore and hind limb, two pairs of lateral flaps, each with a 
cranial and a caudal border and a dorsal (outer) and a ventral 
(inner) surface. Of these the cranial border becomes respec- 272 
THE HISTORY OF THE HUMAN BODY 
tively the radial and tibial sides of the future limb, the caudal 
border the ulnar and fibular. The muscles of the original 
dorsal and ventral surfaces remain in their primary position 
and may be compared in the two limbs; in the distal portion 
the dorsal muscles become extensors, the ventral, flexors, in 
each limb. The embryological history thus furnishes a definite 
proof in favor of the hypothesis of syntropism, or that of 
direct comparison, limb with limb, in the normal position, and 
this theory is espoused at the present time by the majority 
of investigators. 
Fig. 92. Diagrams of primitive branchiomeric muscles. 
(A) Typical form, hypothetical. (B) Condition based upon that of the urodele 
Siren, with a few details supplied from Ne'turus. 
I-VII&, levators of the arches; I-VIIv, depressors of the arches; m, mandibular 
arch; h, hyoid arch; bi to b7, branchial arches; t, trigeminus; fa, facialis; gl, glosso- 
pharyngeus; v, to vt vagus (pneumogastric) elements; x, temporalis; y, masseter; s, 
digastricus; la—1-4, levatores arcuum; dl, dorso-laryngis and dorso-trachealis; a, inter- 
mandibularis anterior; c, intermandibularis posterior; d, hyo-pharyngeus, anterior por- 
tion; e, hyo-pharyngeus, posterior portion; f, laryngei. 
The branchiomeric musculature differs from the metameric, 
thus far considered, in its derivation from the ventral portions 
of the mesoderm, that is, from the hypomeres instead of from 
the epimeres. The skeletal parts with which it is associated THE MUSCULAR SYSTEM 
273 
are those of the visceral arches and their derivatives, including 
the jaw and the hyoid, and, in the higher forms, the numerous 
cartilages of the larynx and the auditory ossicles. This sys- 
tem has thus its most extensive though perhaps not its most 
specialized development among the fishes, for here the gill- 
arches are functional and need to be regulated by systems of 
levators, depressors, constrictors, dilatators and so on, which 
often attain a high degree of complexity. In the amphibians, 
where in spite of the existence of gill-slits, at least in the larva, 
there is but little need of controlling the movements of the 
separate arches so precisely, the branchiomeric musculature 
appears in a greatly simplified form, and the few muscles that 
persist enter into the service of aerial respiration and regulate 
the opening and closing of the pharyngeal cavity and the larynx. 
Among them appear two well-defined series of muscles, the 
one dorsal and the other ventral to the visceral arches, that act 
respectively as levators and depressors of those parts. Their 
condition in urodeles, together with a diagram representing an 
hypothetical ancestor from which they may have been derived, 
are given in Fig. 92. 
In the diagram A the seven visceral arches, including the 
mandible, are given in order, representing as many somites, 
with their motor nerve supply. For the first or mandibular 
somite this latter is the mandibular branch of the trigeminus; 
for the second or hyoid, the facialis; for the third, the glosso- 
pharyngeus; and for the remaining four, a like number of 
branches from the vagus, which is a complex of several original 
elements. The dorsal muscles attached to the arches are 
levators, the ventral depressors. In the second figure, B, is 
shown the actual condition in a primitive urodele. 
Beginning with the levator series, the first becomes the equiv- 
alent of the adductor mandibulce of fishes, here differentiated 
into temporalis and masseter, the muscles of mastication. The 
second, leaving its primary conection with the hyoid arch, be- 
comes also attached to the mandible, but in such a way that it 
opens it, thus acting as the antagonist to the first. This mus- 274 
THE HISTORY OF THE HUMAN BODY 
cle is usually referred to as the “ digastric,” a name taken from 
human anatomy, but it is probably homologous with the pos- 
terior belly alone of the mammalian muscle of the same name. 
The next four muscles are those associated with the first 
four gill-arches, and function in the lower urodeles and in the 
larvae of the more specialized ones as the levatores arcuum; the 
next and last belongs plainly in the same series, but as its arch 
has become specialized as the primary laryngeal cartilage 
[Chapter VII], it extends ventrally to meet it. On account of 
this relationship it has received the name of dorso-laryngeus. 
The ventral series consists of flat sheets, arising from the 
mid-ventral line, where they meet in pairs. Of these, the first 
two, the intermandibulares, anterior and posterior, form the 
muscular floor of the mouth and are attached respectively to 
the mandible and the hyoid. Of the next two, those associated 
with the two first gill-arches, there is no trace, and the next, 
the fifth, is present only in Necturus and its ally, Proteus. 
The sixth, under the often inappropriate name of hyo-laryn- 
geus, is generally present, and the seventh, stretching between 
the two lateral laryngeal cartilages, becomes a set of true 
laryngeal muscles, the dorsal and ventral laryngei (laryngeus 
dorsalis and laryngeus ventralis). 
Above this stage the further phylogenetic history of the 
visceral muscles has been followed only in part, and the con- 
clusions drawn are those which are the most obvious. 
The two masticatory muscles, temporalis and masseter, occur 
in all higher forms and are homologous throughout, save that 
two further slips, the pterygoideus externus and internus, be- 
come differentiated from them, probably from the original 
masseter. In mammals a small slip from the pterygoideus 
internus, becomes the tensor tympani of the middle ear. The 
second levator becomes associated with a muscular slip from 
the first depressor, intermandibularis anterior, and forms a 
part of the digastricus of mammals, the two elements being 
united by a tendon. The diploneuric character of this muscle, 
that is, the innervation of the anterior belly from the trigeminus THE MUSCULAR SYSTEM 
275 
and that of the posterior from the facialis, receives thus an 
explanation. A portion of the posterior belly, that is, of the 
second levator, becomes separated from it in reptiles, and 
follows the stapes into the middle ear, whence it becomes the 
stapedius muscle, innerved by a special branch of the facialis. 
The ventral muscles of these same first two segments are per- 
petuated, the first in part as the anterior belly of the digastricus 
just mentioned and in part as the mylo-hyoideus; the second 
as the stylo-hyoideus. Beyond this, however, the history is 
not clear. In the previous chapter it was shown that the 
various gill-arches, beginning posteriorily, become associated 
with the original pair of laryngeal cartilages to form the com- 
plicated larynx of higher forms, but whether the muscular 
elements primarily associated with them assist in the forma- 
tion of the musculature of the final organ, or whether this 
musculature is derived entirely from the muscles primarily be- 
longing to the seventh arch, that is, dorso-laryngeus and the 
laryngei, cannot yet be definitely stated. 
The muscles which, in the Amniota are innerved by the 
Cranial elements posterior to the Vagus, form two distinct 
problems. The eleventh cranial nerve, the Accessorius, be- 
comes formed from extra bundles of the Vagus, and hence the 
muscles which it supplies must belong to the branchiomeric 
series. These are but two in number, the Sterno-cleido-mas- 
toideus and the Trapezius. The first, like the nerve, belongs 
to the Amniotes only, and its inclusion in this series is quite 
plausible, but the second, so clearly a muscle of the fore- 
limb, and already well developed in the Amphibia, seems 
hardly to belong here. Both of these muscles, however, are 
also innerved by cervical nerves, and may thus possess a 
double origin. 
Unlike the Accessorius the Hypoglossal nerve consists 
primarily of the most anterior spinal nerves, and owes its 
appearance among the cranial series to a simple event that 
occurred in vertebrate history when the Anamnia became 
Amniota. This was nothing more nor less than the fusion 276 
THE HISTORY OF THE HUMAN BODY 
of the first two or three vertebrae with the occipital region 
of the skull. This naturally caused these nerves to issue 
through cranial foramina, and to become cranial nerves, 
forming, when united, the Hypoglossal. The muscles in- 
Fig. 93. Cross-section of the human neck (diagrammatic), to show the 
morphological grouping of the muscles in that region. 
Metameric muscles: (a) Dorsal (episkeletal) set; mainly complexus, splenius, semi-spinalis, and 
multifidus. (b) Ventral (bi) hyposkeletal set; longus colli (b2) obliquus set; the three scaleni 
(b3) rectus set; sterno-hyoid, sterno-thyreoid, omo-hyoid. Branchiomeric muscles: (x) pharyngeal 
muscles (y) instrinsic laryngeal muscles (s) Sternocleidomastoid (t) Trapezius (u) Platysma 
myoides. [After Piersol, modified.] 
nerved by it are thus fundamentally of the metameric series, 
and include the muscles of the tongue; such as the Hyo- 
glossus, Stylo-glossus, Genio-glossus, and in mammals the soft, 
fleshy tongue, itself, the Lingualis. (Cf. pp. 242, 243, above.) 
Superficial to the muscular systems already described, and 
lying directly beneath the integument there are found in many THE MUSCULAR SYSTEM 
277 
vertebrates muscular elements, usually in the form of sheets 
or layers, and connected with the integument, which thus ac- 
quires locally some power of movement. These muscles form 
what may be conveniently termed the integumental system, 
although there are included here contributions from several 
wholly unrelated systems, independently developed in the dif- 
ferent groups of animals to subserve special functions and 
therefore restricted in their occurrence. They usually possess 
at one end a firm attachment to some skeletal part or at 
least to skeletal muscles, while at the other end, or perhaps 
over an extended surface, they adhere to the inner side of 
the integument, thus furnishing the skin area involved with 
the degree of motion required. 
In both birds and mammals certain shoulder muscles furnish 
an important contribution to the integumental system, but, 
as would be expected, the two cases are totally independent 
of one another. In birds the integumental area involved is 
the patagium or web, extending across the angles of the axilla 
and elbow and increasing the resisting surface of the wing. 
This is regulated by a series of patagial muscles, strictly in- 
tegumental in their relations, but derived from the various 
muscles of shoulder and arm; of these the most important 
is M. propatagialis, derived from the anterior portion of the 
pectoralis, with an associated slip from the biceps. 
In mammals an extensive layer, derived from latissimus and 
pectoralis, spreads over the side of the body, and in some 
cases the two extend to the mid-dorsal and mid-ventral lines, 
encasing the trunk in a sub-cutaneous muscular sheet. This 
is the panniculus carnosus, and is primarily employed in mov- 
ing and wrinkling the skin as a defense against insects. In 
the monotremes the portion derived from the pectoralis ex- 
tends over the entire ventral aspect of the body, and, where it 
meets the marsupial pouch and the cloacal orifice, forms from 
its fibers certain more specialized slips to serve as sphincters 
{sphincter marsupii and sphincter cloacae). 
A panniculus carnosus, perhaps here mainly a contribution 278 
THE HISTORY OF THE HUMAN BODY 
from the latissimus, is also present in marsupials (Fig. 94, A) 
and covers the flanks with fibers that converge to an insertion 
into the humerus. From these it is directly continued to the 
Insectivora and Carnivora, and to other Orders of mammals. 
Its action is seen in the shaking of the skin of a wet dog or 
the twitching along the outer portion of the legs of horses 
(A) Macropus bennett (kangaroo). (B) Cynocephalus Itacmadryas. (C) Cerco- 
pithecus sabaeus. (D) Cerco pit he cus cephus. 
Fig. 94. Phylogenesis of the panniculus carnosus. [After Tobler.] 
and cattle when these surfaces are stimulated by the bite of 
an insect. In the lower primates, the panniculus appears as 
a broad sheet upon each side, much as in marsupials (Fig. 
94, B), but within this Order it is seen to separate into axil- 
lary and inguinal portions (Fig. 94, C and D) and in the an- 
thropoids the former alone remains, much reduced in size 
(Fig. 95). In man there appear to be normally no traces of 
this muscle, but there occurs occasionally a system of slips in 
the axillary region, the axillary arch, associated with both 
latissimus and pectoralis and very variable in appearance. THE MUSCULAR SYSTEM 
279 
a typical characteristic of a rudiment. In association 
with this, there occasionally develops a more posterior 
pectoralis slip, the pectoralis abdominis, a relic from a remote 
past. Still another rudiment of the panniculus system is seen 
in the sternalis muscle, an element of rare occurrence and 
Fig. 95. Anterior remnant of the panniculus carnosus, “ achselbogen,” 
in the gorilla. [After Tobler.] 
pmj, pectoralis major; fcb, coraco-brachialis fascia; Id, latissimus dorsi; pa, 
“ pectoralis quartus,” a part of the panniculus; x, tendinous fibers from the 
latter. 
proven to belong here by its occasional relationship with both 
pectoralis abdominis and the elements of the axillary arch, 
as well as by its innervation from the anterior thoracic nerve, 
in common with the foregoing (Fig. 96). The sternalis mus- 
cle has been recently shown to occur much more frequently 
in the Japanese than in Europeans (13 per cent, against 4 280 
THE HISTORY OF THE HUMAN BODY 
"Wilder: History of the Human Body. 
Holt & Co., N.V., 1923, p. 280" 
Fig. 96. Two cases of axillary panniculis rudiments in man. 
(A) [After Tobler], (B) [After Gehry], pmj, pectoralis major; pmn, pec* 
toralis minor; d, deltoid; Id, latissimus; pa, “pectoralis quartus,” a part of the 
panniculus; x, the definite “ achselbogen ”; st, the sternalis, a rare muscular 
anomaly, also a part of the panniculus. THE MUSCULAR SYSTEM 
281 
per cent.). It lies superficial to the pectoralis major, and when 
well developed may be so contracted as to be plainly visible 
from the exterior. 
"Wilder: History of the Human Body. Holt & Co., 
N.Y., 1923, p. 281" 
Fig. 97. Stemalis muscle in a living Japanese. 
From a photograph. [After Adachi.] 
Another system of integumental muscles is originally lo- 
cated in the neck region in reptiles and birds, but extends over 
a great part of the face in mammals. It is innervated by the 
Facialis, and hence must be a derivative of the branchiomeric 
system of muscles. In turtles, and in birds this muscle is in 
the form of a sheet, which enwraps the neck region with 
tranverse fibers, the sphincter colli. In mammals this sheet 282 
THE HISTORY OF THE HUMAN BODY 
differentiates into two layers, a more extensive superficial layer, 
the platysma, and a smaller and deeper layer, which retains 
the original name of sphincter colli. The fibers of these two 
sheets run primarily at right angles to one another, those of the 
platysma being directed upwards and towards both snout and 
ear, those of the sphincter in more nearly the original direc- 
tion across and around the neck. 
In following the phylogenetic series through marsupials 
and lemurs to primates, a considerable extension of both of 
these layers over the face and head is noticed, and as they 
meet the eyes, nose, ears, and lips there is seen a pronounced 
tendency to form special slips for the regulation of these parts, 
a tendency precisely similar to that of the ventral panniculus 
in the case of the marsupial pouch and the cloaca of the mono- 
tremes. There is thus formed the extensive system of facial 
muscles, often termed the “ mimetic ” muscles, which become 
so highly differentiated in the apes and in man, and this grad- 
ual differentiation can be clearly followed in the phylogenetic 
series (Fig. 98). 
The superficial sheet or platysma extends upwards across 
the side of the neck, and, reaching the ear, divides into two 
sheets, dorsal and ventral. The dorsal sheet, auriculo-occipi- 
talis, subdivides once more and furnishes the auricularis pos- 
terior [retrahens auris] and the occipitalis (i.e., the occipital 
portion of the “ occipito-frontalis ” of human anatomy), and 
from the latter are derived the intrinsic muscles of the dorsal 
surface of the ear-flap, rudimentary in man. The ventral or 
facial portion gives off along the sides of the mandible the two 
slips, levator menti and quadratus labii inferioris [depressor 
labii inferioris'], which latter becomes attached to the bone, and 
is continued over the face as M. sub-cutaneus faciei. The ulti- 
mate differentiations of this latter are quite complex and 
concern three portions into which the sheet divides itself. 
Of these an auriculo-labialis inferior furnishes the intrinsic 
muscles upon the ventral or forward surface of the ear-flap 
and an auriculo-labialis superior differentiates into the zygo- THE MUSCULAR SYSTEM 
283 
Front 
Ansup 
Zyg 
°P 
-Occ. 
Nas.' 
Als. 
Au.post 
Ali. 
Antite 
Tri. 
Auant. 
Plat 
Front 
Nas.^ 
Mx-lab^ 
O.or.^ 
Plat. 
Buce. 
Fig. 98. Mimetic muscles in Ateles (a South American monkey). 
[After Ruge.] 
(A) Superficial layer. (B) Deep layer. 
Front, frontalis; Au. sup, auricularis superior; O. p, orbicularis palpebrarum; Zyg, 
zygomaticus; Nas, nasalis; A, l, s, auriculo-labialis superior; A. 1. i., auriculo-labialis 
inferior; Au. ant, auricularis anterior; Au. post, auricularis posterior; Antiir, anti- 
tragicus; Plat, platysma myoides; Occ, occipitalis; Tri, triangularis; Mx, lab, maxillo- 
labialis; O. or, orbicularis oris; Bucc, buccinator. 284 
THE HISTORY OF THE HUMAN BODY 
maticus [major], the orbicularis oculi [palpebrarum] and the 
levators of the lip and side of the nose. Finally a third ele- 
ment, the orbito-auricularis, furnishes two of the extrinsic ear 
muscles, auricular is anterior and superior [attrahens and attol- 
lens auris], and the frontalis, which in the apes comes nearly 
in contact with the occipitalis previously mentioned, the two 
becoming connected by a fascia. The gradual lifting of the 
cranial dome and the formation of a forehead, culminating 
in man, spreads apart the two muscular elements of this 
occipitalis-frontalis sheet and extends the intervening fascia 
to become the galea aponeurotica, so extensive in Man. That 
portion of the platysma which covers the sides of the neck 
in Man remains in its original undifferentiated condition, and, 
although quite variable in its occurrence and in the control 
over it, is yet often capable of throwing the skin into longi- 
tudinal folds, its original function. 
The deeper layer, the sphincter colli proper, extends also to 
the face, but is mainly confined to the region about the mouth, 
where it gives rise to orbicularis oris, caninus [levator anguli 
oris], buccinator and the intrinsic muscles of the nose. 
The original sphincter colli, lying within the province of the 
seventh cranial nerve, exhibits a superb example of the con- 
stancy of a muscular innervation. The branches of this nerve 
expand with the muscles which it supplies, and with the 
migration of the latter to the face there comes also the nerve. 
It thus happens that this element, originally the motor nerve 
of the hyoid region, comes to be called the “ facialis,” 
corresponding to the region in which it is met with in Man, 
whose anatomy first attracted especial attention. 
That this system of facial muscles was primarily developed 
for the purpose of regulating the orifices of the mouth and 
the organs of special sense there can be no question, but the 
high degree of specialization attained in the higher primates 
suggests a totally distinct function, that of communication 
of the moods of the animal to its associates, that is, a language. 
That some outward expression for the developing power of THE MUSCULAR SYSTEM 
285 
thought should show itself pari passu with the development 
of brain was to be expected, and it appears that at about the 
point at which this muscular differentiation became apparent, 
that is, among the lemurs, the various cries produced by the 
larynx became insufficient and were supplemented by the de- 
velopment of mimetic muscles, through the medium of which 
far more subtle shades of meaning could be expressed. For 
a time, therefore, in the anthropoid precursors of man, both 
forms of intercommunication must have existed side by side, 
and have been of about equal value or with some advantage 
in favor of the mimetic muscles, as in the apes of the present 
day; but when, by the shortening of the snout and the con- 
sequent flattening of the dental arcade a greater differentia- 
tion of articulate sounds became possible, these latter became 
more and more employed as the better medium of intercom- 
munication, and the help of the facial muscles became less and 
less necessary. Corresponding to this change, many of the 
mimetic muscles, such as those of the ears, the nose, and the 
scalp, show in man less power than in the apes, while those of 
the cheeks and lips, employed as auxiliary to the production 
of articulate sounds, have reached a still higher degree of 
development. CHAPTER IX 
THE DIGESTIVE AND RESPIRATORY SYSTEMS 
“ Wie in jeder Wissenschaft aus den Thatsachen 
Schliiss sich ergeben, welche das werthvollste 
Ergebnis der Forschung darstellen, so sind auch fur 
die vergleichende Anatomie die geistige Verwerthung 
der Thatsachen durch ihre Verkniipfung das wissen- 
schaftliche Ziel. Was kann es nutzen, unendlich die 
Organisation betreffende Erfahrungen zu sammeln, 
wenn daraus nicht eine Einsicht in jene erwachst, ihr 
allmahliches Werden verstandlich wird, indem es sich 
in mannigfachen, aber auseinander hervorgegangenen 
Zustanden darstellt, die ihre Verwandtschaft unter 
einander in der Organisation zum Ausdruck kom- 
men lassen.” 
Carl Gegenbaur, Lehrbuch d. vergl, Anat. 
1898 ed., p. 27. 
The first step in the evolution of the Metazoa from pro- 
tozoan cell colonies, that is, the procedure which initiated the 
transformation of a colony of similar, one-celled organisms 
into a single organism of many cells, was the inpushing of its 
walls at a given point, resulting in the formation of a two- 
layered cup, the gastrula. From that moment on, the inner 
and outer cells became placed in a different position relative 
to the entire organism, and were thus subjected to different 
experiences. The outer layer was interposed between the 
organism and the external world; the inner dealt entirely 
with the material received into the cavity formed by it and 
used for food. It is obvious that this difference of experience 
would result in a physical differentiation of the cells, and such 
was, indeed, the case. The cells of the outer layer, the ecto- 
derm, became in part protective and in part receptive of ex- 
ternal stimuli, differentiations later to result in the formation 
of an exo-skeleton and a nervous system; those of the inner 
layer, the endoderm, developed the power of obtaining, ab- 
286 THE DIGESTIVE AND RESPIRATORY SYSTEM 
287 
sorbing-, and assimilating the nutritive qualities of the food, 
and thus formed the digestive cavity, the first portion of the 
organism to differentiate as a distinct system. This digestive 
cavity, or gastroccele, remains in the lower invertebrates as a 
blind cavity with but a single opening, and first among the 
worms (Vermes), it becomes converted into a complete canal 
by the formation of an anal orifice, thus obviating the necessity 
of employing the same orifice for both the intaking and the 
expulsion of the contents of the cavity. 
A further advance in the development of the endodermic 
portion of the organism is seen in the higher invertebrates 
(articulates, echinoderms, etc.), and in the vertebrates, where 
certain lateral diverticula become divided off from the primary 
alimentary canal, and form a definite body cavity, the coelom, 
so that the ultimate alimentary canal of these animals is but 
a part of the canal of the lower organisms. In vertebrates 
the canal suffers a still farther loss by the formation and later 
separation of the notochord. Another departure from the 
primary condition is seen in the mouth and anus of vertebrates, 
which are shown by their development not to be homologous 
with the similarly named cavities of lower forms but new 
formations, involving other morphological relations, and 
formed by contributions from the ectoderm. In the develop- 
ment of many invertebrates the primary mouth of the gastrula 
becomes the permanent one of the adult organism and an 
anus is formed by continuing the blind end of the gastrular 
invagination until it meets the surface ectoderm at a point 
opposite that of the mouth; in the vertebrate embryo, how- 
ever, the gastrular mouth lies postero-dorsally with reference 
to the future animal and thus bears no relation to either 
mouth or anus of the perfected form (Fig. 71, A). During 
the development of the nervous system, however, there comes 
a curious and inexplicable connection between the lumen 
of the neural tube and the gastrular mouth, which effects a 
temporary connection between this cavity and that of the 
gastroccele through the so-called neurenteric canal (Fig. 71, 
B), but this connection is only transitory and the entire struc- 288 
THE HISTORY OF THE HUMAN BODY 
ture soon disappears, leaving the gastrocoele as a closed sac, 
with neither oral nor anal orifices. These are formed secon- 
darily through inpushings of the ectoderm, the blind ends of 
which come in contact with the endoderm and later break 
through at the point of contact, thus completing the canal 
(Fig. 71, B, in and an). The functional alimentary canal 
Fig. 99. Diagrams showing the formation of the vertebrate alimentary 
canal and nerve cord, and the early relation between them. 
(A) Early embryo, immediately after the gastrular stage, based on Amphioxus. 
Compare this with Fig. 13 (c). (B) later stage, based on the embryo of the frog. 
g, gastrocoele (—cavity of alimentary canal); n, neuroccele (—cavity of the 
neural tube); ne, neurenteric canal; np, neuropore; b, blastopore; an, developing 
proctodaeum; m, point where the stomatodaeal invagination will take place; d, liver 
invagination; h, heart; nc, notochord. 
thus comes to be formed of three elements, an anterior ecto- 
dermic one, the stomatodceum, a middle endodermic one, the 
mesodceum, and a posterior portion, also ectodermic, the 
proctodccum. In the articulates (crustaceans, insects, and 
spiders), in which the alimentary canal is formed in much 
the same way, the proportion of the functional digestive tract THE DIGESTIVE AND RESPIRATORY SYSTEM 
289 
formed by stomato- and proctodseum, and therefore ectoder- 
mic, is extremely large, and the mesodseum, though volumi- 
nous, is employed mainly in the formation of laterally placed 
digestive glands; but in the vertebrate the canal is mainly 
mesodseal, and therefore endodermic, the ectodermic oral and 
anal contributions being much restricted. 
With the exception of a small number of auxiliary organs 
like the jaws, teeth, and tongue, the entire digestive system is 
derived from this simple tube, and all the parts which appear 
in even the most complicated cases develop from this by 
means of such mechanical principles as increase in length, local 
enlargement, foldings, outpushings, and inpushings, in short, 
such principles as are employed in developmental history every- 
where. More than this, from the anterior portion of this canal 
there develop the two principal respiratory systems of verte- 
brates, the branchial or gill system for aquatic breathing, and 
the pulmonary or lung system for air. This close association 
between digestive and respiratory systems is essentially a ver- 
tebrate characteristic and is hardly known among other ani- 
mals, save in the cases of Amphioxus, the tunicates and the 
Enteropneusta, which in other respects also show their close 
affinity to the Vertebrata. [See Chap. XIV.] Although so 
closely related anatomically, the digestive and respiratory sys- 
tems are best disassociated in treatment and will be considered 
separately as far as possible. 
Although essentially and in its origin an endodermic organ, 
the alimentary canal always becomes reinforced by other tis- 
sues which form layers outside of the primary endodermic one. 
Including the latter the layers are usually considered four in 
number, named in order, beginning with the inner one: mu- 
cosa, submucosa, musculosa, and serosa. The mucosa is the 
primary agent in digestion and develops glands for the pro- 
duction of the various necessary digestive juices; it also 
contains a thin layer of involuntary muscular fibers and is 
permeated with blood and lymphatic vessels for absorbing and 
carrying away the nutriment when in a proper condition for 
assimilation. The submucosa is a thin layer of connective 290 
THE HISTORY OF THE HUMAN BODY 
tissue, needed to give support to the more delicate mucous 
layer. The musculosa, or muscular coat, may vary much in 
the different regions, but consists typically of involuntary mus- 
cular cells arranged in two layers, circular and longitudinal, 
the former internal. By the contraction of the circular layer 
the caliber of the tube is lessened and its length increased, 
while by the contraction of the longitudinal fibers the tube 
is shortened and thickened. Various combined actions of these 
fibers produce the peristaltic movements which occur during 
active digestion, and furnish an important mechanical aid in 
the process. The serosa, or serous covering for the tube, is in 
reality a reflexed portion of the peritoneum, which lines the 
coelom, and which is attached in such a way that, besides 
covering the canal itself, its reflexions form broad, supporting 
membranes known as mesenteries, which attach the tube 
loosely to the body wall and hold it in place. 
In no living vertebrate is the canal, when fully developed, 
in the form of a straight, undifferentiated tube, but becomes 
modified in several ways. In the first place, during the nor- 
mal process of digestion, it necessarily becomes divided into 
regions, each of which is devoted to the performance of a 
certain physiological function, either mechanical or chemical. 
These portions are furthermore differentiated from one 
another in shape and size, and vary from long, attenuated 
tubes, to short and wide sacs; some grade into one another 
without definite boundaries; others are quite sharply set apart 
by a sudden change of external shape, by a localized restric- 
tion in the caliber of the tube, or by the entrance at a definite 
point of some new digestive juice. 
A second cause for modification in the primary simple 
digestive tube lies in the mathematical law of the ratio of 
surface to mass, whereby the surfaces of two homologous 
solids are as their squares, the masses as the cubes, of their 
homologous dimensions. If, for example, an animal posses- 
sing a straight alimentary canal with a smooth mucous lining 
were to increase to twice its original length, its bulk would 
be increased eight times, but the square surface of the in- THE DIGESTIVE AND RESPIRATORY SYSTEM 
291 
terior of the alimentary canal but.four times; in other words, 
it would have but half as much digestive power, and would 
be in danger of starving were not some means employed to 
proportionately increase its digestive surface. As the physio- 
logical digestive membrane is the mucosa, this layer is the one 
primarily concerned in these modifications, and increase in its 
surface is gained, (i) by lengthening the entire canal and al- 
lowing it to fold or coil in some more or less definite fashion, 
(2) by the formation of diverticula, or blind pockets, usually 
long and narrow like the canal itself, and (3) by various meth- 
ods of folding or wrinkling the mucosa itself, with or without 
the other modifications. Independently of the above law, vari- 
ations in the amount of digestive surface, and especially in 
the capacity of those portions of the canal used as temporary 
reservoirs, are correlated with the quality of the food habitu- 
ally taken, an innutritious food requiring a greater capacity 
and probably a greater mucous area than a more concentrated 
one. 
A third necessary tendency of the mucosa is the formation 
of glands for the elaboration of the various digestive juices 
needed in the case of different kinds of foods and in different 
stages of the process; and in this are shown again the prin- 
ciples of gland formation as treated above in the case of the 
integument. Thus there is a widespread occurrence of beaker 
cells and of simple tubular glands, which dip a short distance 
below the surface, and as they are usually placed close to- 
gether they form a thick mucosa, the thickness of which is 
that of the length of the glands composing it. In more com- 
plicated cases the glands may become too large to be included 
within the mucosa and push their way outward to the serosa, 
beneath which they appear as localized swellings, as in the case 
of the pancreas of many fishes; a still farther extension of 
this principle produces an accessory organ like the liver or 
like the pancreas of higher vertebrates, beyond the bounds of 
the alimentary canal, but connected to it by one or more ducts 
and still invested by the serosa (peritoneum). 
The application of these principles and the gradual attain- 292 
THE HISTORY OF THE HUMAN BODY 
ment of a complicated alimentary canal is shown by the com- 
parison of various phylogenetic stages (Fig. ioo). In fishes the 
canal does not much exceed the body in length, and its wind- 
ings consist of a few open curves, although in actual cases the 
length may vary as the quality of the food, and it may thus 
happen that in two closely allied forms considerable differ- 
Fig. ioo. Comparative diagrams of the alimentary canal. 
(A) Fish. (B) Bird. (C) Mammal. 
I, pharynx; II, oesophagus; III, stomach; IV, duodenum; V, intestine; VI, cloaca; 
g, salivary glands; r, thyreoid gland; m, thymus gland; /, bronchi, leading to the 
lungs; h, liver; j, pancreas; k, spleen; y, pyloric diverticula (in fishes); z, cloacal 
diverticula (in birds); x, intestinal diverticulum, the coecum (in mammals); a, ap- 
pendix (the narrowed free end of the latter in man). In B the stomach, III, is in 
two parts, glandular and muscular; in C the intestine is differentiated into Va, 
small intestine; Vb, ascending colon; Vc, transverse colon; Vd, descending colon; 
and Ve, the rectum. 
ence may be seen in this particular. Indeed, a great contrast 
in the length of the canal may occur in various stages of 
the same animal, as in the frog, the vegetarian tadpole of 
which possesses a spiral, much coiled intestine, while that of THE DIGESTIVE AND RESPIRATORY SYSTEM 
293 
the carnivorous adult shows but a few windings, the change 
taking place within a few weeks or even days in correlation 
with the change of food (Fig. ioi). Allowing for a few iso- 
lated cases, however, the canal of fishes and amphibians is 
Fig. ioi. Comparison of alimentary canal in (A), tadpole and (B), 
adult frog. [After the Leuckart wall charts.] 
short and becomes considerably lengthened in the Sauropsida 
and Mammalia, where the intestine, the part mainly involved 
in the increase of length, becomes disposed in complicated 
folds and windings. These, although apparently wholly ir- 294 
THE HISTORY OF THE HUMAN BODY 
regular in their disposition, may be referred to a definite sys- 
tem, as may be made clear by a study of the various steps in 
the process. In a simple, straight intestine, the starting point 
of all forms, the tube is enwrapped by the peritoneum (serosa), 
which becomes reflected along the mid-dorsal line, the two 
layers becoming applied to one another to form a suspensory 
ligament, the mesentery, which in turn is attached along the 
median line of the body wall ventral to the vertebral column. 
As the tube elongates it lengthens the free edge of this mes- 
entery, the effect of which is to throw it into sinuous curves 
directed alternately to right and left, which in extreme cases 
fall from side to side in the form of long and narrow loops. 
The intestinal windings are never seen in their typical form, 
however, owing to the peristaltic movements of the musculosa 
which cause the folds and loops to constantly change their po- 
sition so that their disposition in an animal is never the same 
at two intervals of time.* 
The subdivisions into which the canal is divided anatomi- 
cally for descriptive purposes depend upon localized enlarge- 
ments or constrictions, the formation of diverticula, or the 
presence of definite digestive glands, and become more definite 
and numerous in higher forms as these features gradually 
appear and become more emphasized. The first portions to 
differentiate are the pharynx and stomach, the former being 
a funnel-shaped enlargement of the anterior end, characterized 
by pairs of lateral diverticula, the pharyngeal pouches, which 
may break through to the exterior and form slits; the latter 
a spindle-shaped or sac-shaped compartment for the reception 
of food of all sorts in about the condition in which it is swal- 
lowed. The narrowed portion between these forms the oesopha- 
* This constant change of appearance and endless variety in arrangement 
is exactly suited to the demands of divination, which always depends upon 
a large amount of chance variation of some object; and it is very probable 
that the Roman augurs, who manufactured prophecies from the inspection 
of the entrails of the sacrificial animals, were possessed of as definite a 
system as is seen today in the case of palmistry, a “ science ” founded like 
the other upon the erroneous and utterly baseless assumption of a causal 
relation of two unrelated sets of phenomena. THE DIGESTIVE AND RESPIRATORY SYSTEM 
295 
gus. At its lower end the stomach is bounded by a restricted 
portion, the pylorus, which, by a specialization of the circular 
muscles, forms a valve capable of closing, thus converting the 
stomach temporarily into a closed sac. 
The remainder of the canal may be comprehendingly desig- 
nated intestine, the regional differentiation of which does not 
appear as early and is never so marked as in the anterior por- 
tion. A little below the pylorus it develops from its mucosa 
two enormous glands or gland-complexes, liver and pancreas, 
which usually grow out far beyond the limits of the walls of 
the canal, retaining their connection with their place of origin 
through ducts. The portion of the intestine between the 
pylorus and the orifices of these ducts receives the special name 
of duodenum. A second early and better marked subdivision 
of the intestine is a terminal enlargement which generally re- 
ceives the openings of the urinary and reproductive systems, 
and is hence termed the cloacal chamber, or simply cloaca. 
This portion retains its importance in fishes, amphibians and 
the Sauropsida, but in mammals it plays a subordinate role, 
appearing as a distinct organ in the lowest mammals alone, 
the monotremes. 
In certain definite portions of its length the alimentary 
canal shows a tendency to throw out diverticula, sac-like or 
tubular in shape and designed apparently to increase the 
general surface. Of those the most important are the lateral 
pharyngeal pouches previously mentioned; the pyloric coeca, 
found in fishes, and often very numerous; colic coeca, at the 
beginning of the large intestine in mammals; and cloacal coeca, 
found in birds. 
Following this general sketch of the alimentary canal, its 
development and its differentiation, the separate portions may 
be considered in greater detail [Cf. Fig. ioo]. The most an- 
terior of these are the mouth cavity and pharynx, usually fused 
into one, the stomato-pharyngeal cavity, although in mammals 
the development of the soft palate forms an incomplete separa- 
tion between the two. This cavity opens to the exterior 296 
THE HISTORY OF THE HUMAN BODY 
through the mouth opening or stoma, which appears to be of 
two types in accordance with its surroundings and equipment. 
The first of these, the cyclostoma, is that seen in the lam- 
prey and allied forms, which on account of it, are termed the 
Cyclostomata. This type is similar to that of Amphioxus and 
may be related to it; it is circular in shape and equipped with 
horny, epidermic teeth. 
The second, or gnathostoma, is furnished with a movable 
pair of skeletal jaws, equipped with teeth formed of dentine 
overlaid with enamel. In origin these jaws are a pair of vis- 
ceral arches and the teeth are locally modified placoid scales 
(See Chaps. IV and VII), and it may be held either that the 
gnathostoma or jaw-mouth is the same as the first, to which 
the gill-arches with their associated teeth have become added; 
or that it is a new opening, originally a pair of gill-slits, which 
have become fused in the mid-ventral line, and that the first 
mouth has become lost. In favor of this latter view is the 
position of the gnathostome in the selachians, where it may 
be expected to show the most primitive condition, and where 
it is not at the anterior end but on the ventral side with a 
long rostrum anterior to it. Later on, as is shown in some 
ganoids, it attains secondarily an anterior terminal position. 
If from the evidence presented an hypothetical sketch be 
permissible it may be allowed that in early vertebrates there 
was a circular jawless mouth, provided with a hood and situ- 
ated at the anterior end of the body; that in some form mid- 
way between the lamprey eel and the shark the habit arose of 
seizing and taking in food by the anterior gill-slits, the edges 
of which, provided with sharp, pointed scales, served better 
for the retention of their living prey than did the oral hood 
and horny teeth of the actual mouth. The continuance of this 
habit would perfect the tools employed, which in this case were 
movable gill-arches armed with placoid scales, and the new 
mouth, formed by the ventral fusion of two lateral slits and 
furnished with superior organs of prehension, eventually 
usurped the function of the old one, which thus became re- 
duced and finally disappeared. In one point alone, that of THE DIGESTIVE AND RESPIRATORY SYSTEM 
297 
position, was the old mouth superior, and the final step in 
the perfection of the new one was its gradual migration to the 
anterior end, the various steps in the attainment of which may 
be seen among the ganoids. 
Behind the mouth on each side there develops a row of out- 
pushings, the pharyngeal pockets, which meet a corresponding 
set of inpushings from the outside (Fig. 102). In fishes these 
(a) Ventral aspect of early embryo, with the front part of the lower jaw and the 
gill arches removed, (b) Inner view, looking ventrally, of the lower jaw and gill- 
arches, corresponding to the part removed from (a), but taken from a somewhat 
younger embryo, (c) Similar to (b), but taken from an older embryo, not far 
from the age of (a). 
In (a) the most anterior of the arches sectioned is the mandibular (=lower jaw), 
succeeding which are the gill-arches in order, three being definitely formed. An- 
terior to the sectioned arches are seen the two superior maxillary processes, which, 
by their later union form the upper jaw. Between and a little above these is the 
fronto-nasal process. In (b) and (c) the most anterior arch is the mandibular, with 
the gill-arches succeeding it in order. The round median mass is the tuberculum ini- 
par, which, with the eminences immediately behind it, form the tongue. The thy- 
reoid anlage and the glottis and epiglottis are seen in (c). 
Fig. 102. Pharynx and visceral arches in human embryo. [After His.] 
break through at the points of contact, and form the gill-slits, 
a series of permanent openings (4-8 in number) that form a 
communication between pharynx and exterior and allow the 
escape of the water constantly taken in at the mouth and 
used in respiration. One or more of these slits appear in 
amphibian larvae and in a few forms persist throughout life, 298 
THE HISTORY OF THE HUMAN BODY 
but in reptiles, birds, and mammals, although the pharyngeal 
pockets and their corresponding external depressions form 
during embryonic life, only two or three ever break through 
and then for a very short period. The most anterior of these, 
however, which, in selachians, appears as the spiraculum, or 
blow-hole, persists in all higher vertebrates, as the Eustachian 
tube \tuba auditiva BN A] and the middle ear. The remain- 
ing pockets disappear as such, but various accessory structures, 
such as cartilages, muscles, arteries, and glands, arise in the 
embryo in association with them and afterwards become modi- 
fied to subserve numerous important purposes.* A not un- 
common malformation in man, a few cases of which have been 
reported in other mammals, is that of a cervical fistula, which 
forms an open communication between pharynx and exterior, 
usually upon one side alone. This is nothing more or less 
than a permanent gill-slit and may be considered as a case of 
arrested development, or the retention of what is designed to 
be a transitory stage. 
The nasal cavities, which lie above the anterior part of the 
stomato-pharyngeal cavity, are in fishes quite independent 
of the latter, but come into direct communication with it in 
Amphibia by the formation of a pair of openings, the posterior 
nares or choance, which appear in the roof of the mouth. This 
communication was apparently one of the changes inaugurated 
during the transition from water to land, and allows the in- 
gress and egress of air to the pharynx and thence to the lungs 
without opening the mouth, since this action, although harm- 
less for an animal immersed in water, would soon cause the 
drying up of the mucous membrane lining the mouth cavity 
if resorted to in air with anywhere near the same frequency. 
In the case of the nasal cavities this is prevented in part by 
the small size of the external openings, but still more by the 
formation of slime glands capable of producing an abundant 
* Cf. Chap. VII under Visceral skeleton; Chap. VIII under Branchiomeric 
muscles, Chap. X under Arterial arches; and the present chapter farther 
on under Thymus and Thyreoid. THE DIGESTIVE AND RESPIRATORY SYSTEM 
299 
secretion. The waste lacrimal fluid conveyed from the eyes 
to the nose, is undoubtedly also of assistance in this respect. 
The posterior part of the pharynx shows a strong tendency 
to form median diverticula, either dorsal or ventral, which ex- 
pand into large sacs or reservoirs and either retain or lose 
their communication with the parent cavity. Such are the 
various sorts of air-bladders found among fish; these are gen- 
erally situated dorsally with respect to the pharynx, but ven- 
tral ones occur in a few ganoids. Occasionally these reser- 
voirs possess an attenuated pneumatic duct, communicating 
with the pharynx, and the supply of air is regulated by the 
fish coming to the surface and making a snapping or swallow- 
ing movement; but in the majority of cases the air-bladder 
is a closed sac, filled by gases extracted from the blood. In 
terrestrial vertebrates there appears in the embryo a mid- 
ventral diverticulum which opens from the floor of the pharynx, 
grows posteriorly and forks into two lateral branches. This 
forms the pulmonary system, the lateral sacs becoming lungs 
and bronchi, the median duct the trachea, and the opening 
in the pharynx the glottis, which becomes regulated by a series 
of cartilages, and muscles derived from the branchiomeric 
system, and forming the larynx. 
The idea naturally suggests itself that this ventral pulmo- 
nary system is a direct inheritance from a similarly situated 
air-bladder, such as actually occurs in some ganoids, and al- 
though there is no direct proof of this, it seems very probable. 
It has also been noted that the diverticulum which produces 
it lies immediately back of the converging line of pharyngeal 
pockets and it has thus been interpreted by some as a continua- 
tion of this system, the forking into the two lungs being taken 
as proof of the formation of the median diverticulum from the 
confluence of two lateral pockets. In fact the air-bladder of 
certain ganoids is richly supplied with respiratory blood- 
vessels, and thus forms a better lung physiologically than that 
of the more primitive amphibians, which are often simple 
sacs, yet a direct continuity from one to the other cannot 300 
THE HISTORY OF THE HUMAN BODY 
be traced, since the lungs are always paired and an air-bladder 
is always single, except in Dipnoi. 
The nasal cavities in mammals and birds are separated from 
the pharynx by an approximately flat plate of bone that forms 
the roof of the mouth, or hard palate. This is formed by 
horizontal plates directed inward from the maxillaries and 
premaxillaries. They arise in the embryo upon the sides of the 
head and grow towards the center, finally uniting in the me- 
dian line. In some groups of birds the two halves remain 
dissociated, thus forming a palatine clejt by which a di- 
rect communication is retained between the nasal and 
pharyngeal cavities. This failure to unite may occur as an 
abnormality in mammals, producing the malformations known 
as hare-lip and cleft palate, malformations thus attributable to 
the principle of arrested development. In certain mammals, 
as the cat and dog and especially rabbit, the remains of the 
closure are permanently shown in the form of a deep median 
groove, the philthrum, which partly divides the upper lip and 
runs along the septum of the nose externally. 
In mammals the bony palate (hard palate), which is com- 
posed of horizontal processes from premaxillaries and maxil- 
laries and a portion of the palatines, is continued posteriorly 
into a membranous soft palate or velum palati. This is a dupli- 
cation of the pharyngeal mucous membrane and is supplied 
with semi-voluntary muscular fibers from the sheets surround- 
ing the pharynx. 
Accompanying the stomato-pharyngeal division of the ali- 
mentary canal are several important auxiliary organs derived 
from various sources. These are the teeth, tongue, tonsils, the 
glands of the mouth cavity, and the glands of the pharyngeal 
pockets which will be considered in the order given. 
Excepting the horny formations in the mouth of the Cyclo- 
stomata, which are isolated structures of epidermic origin, 
the teeth of the vertebrates are strictly homologous organs 
throughout. They were originally placoid scales which dif- 
fered in no respect from those that cover the exterior of the THE DIGESTIVE AND RESPIRATORY SYSTEM 
301 
present-day selachians, and have been modified in form through 
the change of function due to their position in and about the 
mouth cavity. Teeth and placoid scales correspond closely in 
structure and development, and consist of a basis of dentine, 
or “ ivory,” as it is often called, overlaid by enamel, the first 
being formed from the corium, the latter from the epidermis. 
In their original distribution as seen in fishes and amphibi- 
ans they occur not only along the edges of the jaw, but also 
in patches over the roof and floor of the mouth cavity, co- 
extensive with the stomatodseum; but in most reptiles and in 
mammals they are confined to a single row in each jaw. Cor- 
responding to their origin the most primitive arrangement is 
that of an imbricated pattern as in other scales, a condition 
shown in many of the areas within the mouth cavity, and in 
the jaw teeth of many selachians, where they appear in several 
rows. Primarily, at the stage in which the jaws and skull 
are cartilaginous, as in modern selachians, the teeth are sepa- 
rated from one another, but in a slightly higher stage the 
bases fuse for mutual support, thus forming a flat plate of 
bone upon which the separate tooth elements appear as pro- 
jecting points or cusps. This proceeding is repeated ontoge- 
netically in a few cases, as in the paired vomers of the frog; 
although usually by a shortening of the development the stage 
at which the scales are separate is dropped out and the bony 
plate develops as a single structure, so that in cases where the 
cusps have become lost there is no indication of their dermal 
origin. Thus are formed the flat bones that line the mouth 
cavity, such as the vomers, palatines, pterygoids and parabasal, 
which in lower forms often retain their dentigerous character; 
such is also the origin of the premaxillaries and maxillaries 
that form the upper jaw, as well as that of the splint-like 
dentare, angulare, etc., that enclose Meckel’s cartilage 
and form the mandible. (See Chap. VII.) The cusps are 
thus originally an integral part of the bony splints which bear 
them, and in fishes, amphibians, and most reptiles the two 
remain continuous, but in crocodiles and mammals, as well 302 
THE HISTORY OF THE HUMAN BODY 
as in the fossil toothed birds, the two become detached, and the 
teeth are inserted by what is termed a thecodont articulation 
into deep pits in the bone called thecce or alveoli (Fig. 103). 
That portion of the tooth which fits into an alveolus is 
termed the root and is covered by a sort of bone called cement, 
while that which appears above the gum is called the crown, 
and is the only part overlaid by enamel. 
All teeth are hollow and contain a central pulp cavity, which 
encloses a nutrient corium papilla; in cases where the cusp is a 
part of the bone the pulp cavity is open along one side to 
admit the passage of nerves and blood-vessels. In thecodont 
teeth, however, the latter come up through the bottom of the 
theca and enter the pulp cavity at the inner end of the root. 
Fig. 103. Types of teeth. 
(a), pleurodont; (b), acrodont; (c) thecodont with open root; (d) thecodont 
with closed root. 
In this respect there are two kinds of roots, open and closed, 
in the first of which the root is widely opened and not dis- 
tinct in structure from the crown, while in the latter the root 
is nearly solid and its lumen is restricted to a fine canal 
through which the vessels may reach the pulp. In both cases 
layers of new dentine are constantly, though slowly, added to 
the rest through the agency of a layer of cells called odonto- 
blasts, which cover the pulp and are firmly applied to the inner 
walls of the pulp cavity. In the first type, in which the root 
is widely open and the entire tooth fits like a cap over the 
pulp, the tooth is gradually pushed upward by the addition 
of new layers underneath, and is thus continually elongating, THE DIGESTIVE AND RESPIRATORY SYSTEM 
303 
while in the second or closed type the addition of new layers 
merely diminishes the size of the pulp cavity without increas- 
ing the length of the tooth as a whole. Both methods are in 
a way a provision against the loss of substance due to constant 
use; in the first type the outward growth and the loss through 
wear usually balance one another so that the tooth remains 
of about the same length throughout life; in the second the 
tooth grows gradually shorter while the addition of new layers 
beneath merely protects the sensitive pulp by keeping a con- 
stant thickness of dentine between it and the free surface. 
Thus, in man, whose teeth are of the second type, the chewing 
surface in old age is at a level which in youth would lay bare 
the pulp. Illustrations of the first type are seen in the teeth 
of the hippopotamus, the tusks of swine and elephants, and 
the chisel-like incisors of rodents; in the latter case the 
front side only is covered with enamel, and as this substance is 
harder than dentine, it wears away more slowly, and constantly 
presents a projecting edge, thus keeping the teeth sharp. 
Such teeth depend upon constant use in order to be kept at 
the proper length, and in abnormal cases in which some ir- 
regularity of the jaw prevents the meeting of opposite teeth, 
they grow past one another and become eventually disposed in 
coils and other eccentric forms which may prove the death of 
their possessor through an inability to feed properly. Similar, 
although here perfectly normal, instances are those of the or- 
namental tusks of elephants and wild swine, and in one of 
these latter, the babyroussa of the East Indies, an* upper tooth 
upon each side bores upward through the lip and erects a 
curved point high above the snout. 
The primitive form for a tooth is that of an elongated cone, 
a type which occurs with slight variation in proportions among 
all the lower vertebrates. In mammals, however, the teeth 
are characterized by the introduction of numerous modifica- 
tions, sometimes very complex in nature, resulting in a large 
number of variations, not merely in different species, but in 
different regions of the same jaw, which correspond to a differ- 
entiation in use between the front teeth, that are in a position 304 
THE HISTORY OF THE HUMAN BODY 
for grasping the food, and the back teeth, which come under 
the immediate control of the masticatory muscles. In the 
Cetacea alone the teeth are all alike (homodont) and of the 
simple conical type, and here it may be considered certain 
that this condition is a secondary one and that the teeth have 
lost the usual complexity through disuse; since an aquatic 
animal cannot chew and employs its teeth merely for the pur- 
pose of holding and retaining the food. In all other mammals 
the teeth are heterodont, and possess several distinct forms, 
which have been conveniently divided into three types, incisors, 
canines, and molars, with a possible subdivision of the last 
into premolars and molars proper. 
Of these types the incisors are the most anterior and con- 
sist in the upper jaw of those borne by the premaxillary bones 
and in the lower jaw of those opposite the former. There may 
be five of these in each upper half jaw, and four in each lower 
half, but the usual number is three or two. Corresponding 
to the most frequent function of teeth thus situated their 
typical shape is that of chisels for cutting or biting, a form 
easily derived from the primitive conical forms by a flattening 
in a labio-lingual direction. Beyond these there is in each 
half jaw a single canine, typically in the form of a pointed 
cusp elongated beyond the level of the other teeth and best 
preserving the primitive shape. The canines of the lower 
jaw lie anterior to those of the upper jaw and in the case 
of elongated canines slip past one another. As their chief 
policy is that of piercing and tearing, they become reduced 
or are entirely wanting in animals in which this function is 
superfluous. The remaining teeth may be classed together as 
molars, or cheek-teeth, or a distinction may be made in most 
cases between an anterior group of premolars, in which the 
first teeth are usually replaced by a second set, and a posterior 
group of true molars, which develop later than the rest and 
are not replaced. That this distinction is in part an artificial 
one and often difficult or impossible of application is shown 
by the study of the subject of replacement, considered below. THE DIGESTIVE AND RESPIRATORY SYSTEM 
305 
Of premolars the greatest number that occurs in mammals is 
four in each half-jaw, of definite molars, five. 
The number of each kind of teeth occurring in a given 
mammal is briefly and graphically expressed by a dental for- 
mula, which may be written in a variety of ways. Thus the 
entire dentition may be expressed as in the following formula 
for the cat, in which the upper and lower rows represent the 
two jaws, the mid-ventral line is marked by the short per- 
pendicular and the number of each group of teeth is desig- 
nated by a digit: 
This formula shows at once that there are in each jaw six 
incisors flanked on each side by a canine, and that there are 
four cheek teeth above and three below on each side, only the 
last one of which is not replaced and is therefore a true molar. 
Except for the sake of symmetry, however, one half alone 
may be given, as in the following for the Bovidce, the family 
to which cattle and sheep belong, a formula showing a total 
loss of canines and of upper incisors: 
From the study of the dentition in all mammals and espe- 
cially that of marsupials and the more primitive placental 
mammals, the following hypothetical dentition has been de- 
duced for the ancestral type, to which all existing dentitions 
may be referred: 
This formula provides for ten incisors and eighteen cheek 
teeth in each jaw, with a total of 60 teeth, 30 in each jaw. 
This formula is nearly attained by some marsupials, where the 
most primitive condition would be expected, but in placental 
mammals there is always a reduction of incisors and molars, 306 
THE HISTORY OF THE HUMAN BODY 
the former being never more than three upon each side. Thus, 
in the opossum (a marsupial) the dental formula is: 
the last figure representing the total number of cheek teeth, 
without attempting a distinction between premolars and mo- 
lars. Among placental mammals the Insectivora show the 
most primitive dentition; for example, that of Talpa (the 
mole) is: 
Certain cases, on the other hand, show an extraordinary re- 
duction, as in the mouse-like rodent, Hydromys, where the 
formula is: 
The greatest reduction is found among some Cetacea, and in 
certain edentates (Myrmecophaga). In certain of the first 
group the tooth germs never develop but remain within the 
gums, while feeding is carried on through the development of 
horny fringes (the so-called “ whale-bone ”) depending from 
the upper jaw, and forming a filter, which transforms the 
mouth into a scoop-net for the apprehension of shoals of 
marine creatures of various sorts. In the duck whale, Hy- 
peroodon, there are but four teeth, all in the lower jaw, the an- 
terior ones of conical shape, behind which are two imbedded 
in the gums; in the narwhal, Monodon, there are numer- 
ous small teeth that fall out before maturity, leaving the jaw 
toothless, but in the male a single upper tooth, usually that 
of the left side, develops into a long, projecting horn so large 
that it renders the skull asymmetrical. In certain of the 
smaller Cetacea, dolphins and porpoises, the jaws are fur- 
nished with conical teeth, nearly or quite homodont, which 
often surpass in number those of any other mammals (in the THE DIGESTIVE AND RESPIRATORY SYSTEM 
307 
Dolphin, Delphinus, 47-65 in each half-jaw). Both this ex- 
cessive number, which is at variance with the general formula 
for mammalian dentition, and their homodont character, must 
be looked upon as secondary adaptations to aquatic conditions. 
In the majority of primates there are two incisors upon each 
side and a well-developed canine. In the monkeys of the 
Western Hemisphere (Platyrrhini) there are three premolars 
and either two or three molars, but in those of the Eastern 
Hemisphere (Catarrhini) there are always two of the former 
and three of the latter, giving the constant formula of: 
Of these the five medial teeth of each jaw are replaced by a 
second set, the three molars not, the formula for the milk 
dentition being: 
In these points man corresponds completely with the other 
Catarrhini. 
Regarding the evolution of the various shapes of mammalian 
teeth from the primitive conical type, the canines and incisors 
present a simple problem, since the first retain almost their 
typical form, and the second show merely a labio-lingual 
flattening. The derivation of the complex cheek teeth, how- 
ever, presents a serious problem, for the solution of which two 
main theories have been offered. According to the first, or 
tritubercular, theory, the fundamental postulate must be laid 
down that every tooth, no matter how complex, represents a 
single primary element, and that the modifications are due 
to the development of additional cusps for the purpose of in- 
suring a better articulation with the opposing surfaces of the 
teeth of the other jaw. The number of cusps which may thus 
develop on the contact surface of a tooth is six, the first of 
which to appear is the protocone, or primary cusp. Associated 308 
THE HISTORY OF THE HUMAN BODY 
with the protocone are 
two secondary ones, the 
paracone, which is an- 
terior to it, and the 
metacone, which is 
posterior (Fig. 104, I 
and II). These may 
become connected by 
crests or ridges and the 
tooth may become still 
more complicated by 
the bending of the 
crests and cusps into a 
V-shaped figure, the 
trigon (Fig. 104, III). 
A tooth, or a dentition, 
in which the three 
cusps, para-, proto-, 
and meta-cone, still lie 
in a straight line fol- 
lowing the line of the 
jaw, is called tricono- 
dont, one in which the 
bending has taken 
place is trigonodont. 
This tri - tubercular 
tooth, still under the 
influences of the oppos- 
ing tooth surfaces, may 
develop a lateral spur, 
the hypocone, upon 
which 1-3 tertiary 
cups, the conuli, may 
develop, or else the 
hypocone may form 
the point of a project- 
ing spur, the talon 
Fig. 104. Phyletic history of the molar 
cusps. [After H. F. Osborn.] 
I. Reptilian stage, haplodont; Permian. II. 
Triconodont stage. III. Tritubercular stage. 
IV. Tritubercular-tuberculo-sectorial; lower Ju- 
rassic. V. The same, upper Jurassic. VI. The 
same, upper Cretaceous. VII. The same, Puerco, 
lower Eocene. VIII. Sexitubercular-sexitubercu- 
lar, Puerco. IX. Human lower molar. 
Pt, protocone; P, paracone; m, metacone; h 
hypocone; Ptd, protoconid; Pd, paraconid; md, 
metaconid; hd, hypoconid; hid. hypoconulid; ed, 
entoconid. THE DIGESTIVE AND RESPIRATORY SYSTEM 
309 
The nomenclature given here is that of the upper teeth, the 
corresponding parts of the lower jaw being distinguished by 
the addition of the suffix -id, thus: protoconid, hypoconid, 
talonid, etc. In favor of this theory may be urged its complete 
applicability to all known forms, both living and fossil, as a 
system of nomenclature, and its correspondence in sequence 
of stages to that shown by the extinct forms in consecutive 
geological periods. The embryological record, also, with a few 
exceptions, which may be explained in other ways, is in ac- 
cord with it. 
A second theory to account for the complex form of molars 
is that they are in reality, as they are often termed, “ double 
teeth,” and arise from the fusion of several primary germs. 
This concresence theory is much older than the first, and had 
been generally abandoned in favor of the other when its 
probability in the case of certain forms was recently reasserted, 
owing to the testimony of embryology. It is thus possible 
that both theories may be true as applied to different cases, 
the tritubercular method being the more general. 
A widespread phenomenon among mammals is that of the 
replacement at a definite period of certain of the anterior 
teeth by a second set, and in respect to this power mammals 
are classed as diphyodont, in which such a replacement oc- 
curs, and monophyodont, where but one set appears. Al- 
though in man and in many domestic animals, in which this 
procedure was first studied, the distinction between the two 
sets is clear and definite, such is not the case among certain 
other mammals, and careful embryological records which 
show numerous cases of rudimentary tooth germs both pre- 
ceding and succeeding the definite set, has caused a revision 
of the entire subject. The matter becomes more compre- 
hensible by referring to the lower vertebrates, especially rep- 
tiles, where the papilla of each physiologically active tooth is 
associated with a succession of additional tooth germs in dif- 
ferent stages of maturity, and designed to replace the func- 
tional tooth in case of injury (Fig. 105). 
In a vitally important tooth, as in the poison fangs of ser- 310 
THE HISTORY OF THE HUMAN BODY 
pents, at least one replacement tooth, nearly ready for use, 
lies continually by the side of the functional one, and may 
develop almost at a moment’s notice in case of the loss of the 
latter. This power, limited to a single generation of replace- 
ment teeth, restricted to that part of the jaw anterior to the 
true molars, and arranged to assert itself at a certain definite 
period in the case of each tooth, would result in the two den- 
titions of the typical diphyodont Mammalia (Fig. 106, A), and 
such has been undoubtedly the origin of this phenomenon, save 
Str. cor. 
Str. muc 
Cutis 
Fig. 105. Succession of teeth in reptiles. 
(A) Section through Jaw of Phyllodactylus (a lizard) showing functional tooth (7) and 
several replacement teeth (II, III); m, Meckel’s cartilage; n. nerve. [After Gegenbaur.] 
(B) Diagram of integument of gum, to explain the succession of teeth, str. cor., stratum 
corneum; sir. muc., stratum germinativum; 7, II, etc., the succession of tooth germs. 
that it is not necessary to assume that all the teeth of either set 
belong to the same original generation, especially as they do 
not develop simultaneously. Thus some have seen in the suc- 
cession of mammalian teeth the remains of five separate tooth 
generations, individuals from several uniting in a specific case 
to form a set, either the milk or the permanent one. Others,, 
admitting that each of the two definite sets represents a single 
tooth generation, find in certain cases traces of tooth germs 
that precede the milk dentition, and others that succeed the 
permanent set; they thus consider that mammals have in- THE DIGESTIVE AND RESPIRATORY SYSTEM 
311 
herited from their reptilian ancestors four tooth generations, 
prelacteal, lacteal, permanent and post-permanent, of which 
(A) Dentition of a lion cub of six and a half months. [After Weber.] Milk 
dentition functional, replacement teeth still within the jaw and shown as shaded 
areas. (B) Upper jaw of new-born seal, with replacement teeth almost ready for 
function. The milk dentition is reduced to useless rudiments cast off immediately 
after birth. [From Hertwig, after Burckhardt.] 
Fig. 106. Figures representing diphyodont dentition. 
the second and third have become generally established.* The 
* It is asserted that in Nasodon, a Tertiary ungulate, a pair of prelacteal 
incisors becomes developed. 312 
THE HISTORY OF THE HUMAN BODY 
first developed in the premammalian ancestors, the last may 
come to full development in the future. 
Regarding the occurrence of the two dentitions there is 
much variation. In certain marsupials but one tooth is re- 
placed, the last upper premolar, this tooth alone constituting a 
second set, while otherwise the milk set is retained through life. 
They are thus almost monophyodont, with a permanent milk 
dentition, and are in sharp contrast to other monophyodont 
mammals in which the milk set is represented by useless rudi- 
ments that are either absorbed before birth or expelled soon 
after. In view of all facts thus far presented it seems that 
the monophyodont condition has been secondarily acquired as 
a special adaptation in certain forms by the suppression of 
one or the other of the two typical generations (Fig. 106, B). 
Although fishes possess no functional tongue the material 
out of which a tongue is to be constructed is present in the 
form of the anterior part of the hyo-branchial apparatus. 
The anterior border of this complex lies in the floor of the 
mouth cavity, following the outlines of the jaw, and by certain 
actions of the muscles may be projected upwards so as to 
form a noticeable elevation. When in amphibians these parts 
become relieved from the gill-bearing function, they form the 
basis of the tongue, often fusing into a complex, moved by 
muscles and supporting a fleshy organ of some sort. In that 
type of tongue which leads to the higher vertebrates the 
skeletal basis consists of two to four of the visceral arches, 
beginning with the hyoid, and, although admitting of many 
varieties, possesses as essential a median basi-branchial piece, 
called here the os entoglossum, and two posteriorly projecting 
cornua. The tongue of the Sauropsida is a direct continuation 
of this, and in both cases the principal motion is a protrusion 
and withdrawal of the organ as a whole, which is effected by 
means of the two posterior cornua, which lie in sheaths from 
which they may be everted. When the tongue is unusually 
long the sheaths and the enclosed cornua are of correspond- 
ing length, and their dispositon when retracted becomes a 
problem variously solved in different cases. Thus in a cer- THE DIGESTIVE AND RESPIRATORY SYSTEM 
313 
tain salamander, Spelerpes juscus, the sheaths of the cornua 
run down the sides of the body and are attached to the ilia, 
and in the woodpecker they curve around the occipital region, 
pass over the top of the head and terminate at the base of the 
upper beak, near the anterior nares. In these cases the ends 
of the cornua are attached to the bottom of the sheaths and as 
they are withdrawn the sheath is turned inside out and adds 
to the total length. 
In mammals the hyo-branchial support of the tongue is 
reduced to a complex composed of a basi-hyal (body) and two 
pairs of cornua, of which the anterior are typically the longer 
and are formed by a chain of four skeletal pieces, cerato-, epi-, 
stylo-, and tympano-hyal, the latter attached to the tympanic 
region of the skull. The posterior cornua consist each of a 
single piece, thyreo-hyal, connecting the body with the thyre- 
oid cartilage of the larynx. In man the anterior cornua show 
an unusual modification, the tympano- and stylo-hyals are 
fused with the otic region of the skull, forming the styloid 
process, the cerato-hyal is reduced to a rudiment which con- 
nects with the styloid process by a ligament in which no trace 
of the epi-hyal is seen. Thus the anterior cornua, though 
typically longer and more complex than the others, are spoken 
of in man as the “ lesser,” an inheritance from the earlier 
anatomical science in which comparison with other mammals 
played no part. 
Inserted upon this skeletal complex as a basis there is de- 
veloped in mammals a fleshy tongue composed of interlaced 
muscular fibers, a part of which are intrinsic and belong to 
the tongue itself, while others are extrinsic and consist of the 
terminal fibers of other muscles. This organ is thus a struc- 
ture quite unlike the tongue of most other vertebrates, in 
which the skeletal support reaches through the organ and in 
which motion is confined to a simple protrusion and retraction, 
and resembles rather the fleshy lobe which appears in a few 
cases appended to the other structure, as in the frog. More- 
over, in some mammals, notably marsupials and lemurs, there 
exists, beneath the fleshy organ, an accessory tongue-like struc- 314 
THE HISTORY OF THE HUMAN BODY 
ture, the sub-lingua, which possesses many attributes of the 
tongue of the Sauropsida and like it is supported by a car- 
tilaginous piece which may represent the os entoglossum. In 
man the sub-lingua is reduced to a transverse fold, the plica 
pmbriata, readily seen if the mouth be opened and the tongue 
elevated. If this organ be taken as the homologue of the 
sauropsidan tongue, as its structure and position seem to indi- 
cate, the fleshy tongue of mammals is a new structure, de- 
veloped upon the dorsal side of the former, and gradually 
usurping its function. An opposing opinion rejects the claim 
of the sub-lingua as homologue of the sauropsidan tongue, and 
finds the rudiment of the os entoglossum in the septum linguae, 
a band of connective tissue running lengthwise through the 
tongue and serving as an attachment for the muscles, or more 
definitely in the lyssa, a vermiform structure composed of 
connective tissue, fat, cartilage, and muscle fibers, which oc- 
curs in the tongue of certain mammals, associated with the 
septum. 
Distinctive glands in the mouth cavity other than simple 
mucous cells are not found in fishes or wholly aquatic am- 
phibians and appear only as an adaptation to the terrestrial 
life for the primary purpose of keeping the surface moist. 
Such glands are thus constantly present in terrestrial verte- 
brates and become secondarily reduced in those that become 
readapted to the water, as in the case of the Cetacea. These 
glands are often voluminous, occur in all available positions, 
and are named accordingly, buccal, labial, lingual, sub-lingual, 
sub-maxillary (more properly sub-mandibular), etc. Al- 
though the primary function of all these was undoubtedly that 
given above, they have in numerous instances assumed more 
special functions and have altered the chemical nature of their 
secretion accordingly. Thus the intermaxillary glands of frogs 
and toads, which open into the roof of the mouth, secrete a 
viscid fluid by means of which the tongue is rendered adhesive 
for the apprehension of insects, the lingual glands of many 
salamanders and lizards have a similar function, and certain 
buccal glands in poisonous serpents become the elaborators of THE DIGESTIVE AND RESPIRATORY SYSTEM 
315 
the venom, which is inoculated into the victim by means of 
the poison fangs. These teeth are provided either with a 
groove along the external surface or possess a minute lumen 
through the center as in the case of a hypodermic needle. 
Another secondary function extensively employed is that of 
lubricating dry food to render it more easily swallowed, and 
in association with this the secretion often develops ferments, 
of use in the digestion of starch, and forms the saliva. The 
glands specialized to produce this fluid may be termed salivary, 
without reference to their position, and consist in mammals 
of the parotid, sub-mandibular [sub-maxillary BNA~\, sub- 
lingual, and retro-lingual, the last associated with the sub- 
mandibular and not found in all mammals. There also occur 
other voluminous glands, with a structure similar to the rest, 
that secrete a clear fluid without salivary attribute. Such 
are the molar glands of ungulates, or the voluminous orbital 
gland of the dog family (Canidse), which opens by a duct into 
the mouth cavity in the region of the last upper molar. 
Associated in origin with the pharyngeal region are cer- 
tain organs of more or less uncertain significance and varied 
destiny, the most prominent of which are the thymus and 
thyreoid glands. These, although not connected in function 
with the digestive system, may be treated here because of 
their origin. In the cyclostomes, which furnish the first, or 
most generalized stage in this history, there develop seven 
pharyngeal pockets on each side, each with a dorsal and a 
ventral recess. About each of these there develops a prolifera- 
tion of epithelial cells, forming an organ or organ-anlage. 
These anlagen appear alike in structure, but in the higher 
forms the differentiations from the dorsal series become col- 
lectively known as the thymus, those from the ventral as epi- 
thelial corpuscles. 
The later history of the thymus anlagen shows three ten- 
dencies, (i) to become early separated from their layer of 
origin, (2) to fuse upon each side to a single long organ, 
often showing its segmented origin, and (3) to become re- 
stricted in number, the more anterior ones being the first to 316 
THE HISTORY OF THE HUMAN BODY 
Fig. 107. Embryonal anlagen of the glands associated with the pharynx 
and pharyngeal pockets, [a-d, after Maurer; e-h, after Verdun.] 
Thymus anlagen are indicated by dots, thyreoid by diagonal lines, epithelial cor- 
puscles by radiating lines, and post-branchial (supra-pericardial) bodies in black. The 
pharyngeal pockets are designated by Roman numerals. 
(a) Cyclostome. (b) Teleost. (c) Urodele. (d) Lizard (Lacerta). (e) Chick, 
(f) Cat. (g) Man, (h) Mole. THE DIGESTIVE AND RESPIRATORY SYSTEM 
317 
disappear. These are all seen in the accompanying diagram 
(Fig. 107), which shows the phylogenetic steps in the process. 
Thus, beginning with the seven anlagen of the cyclostomes, 
the teleosts, which possess six pharyngeal pockets, show but 
jour anlagen, and these the most posterior. The first pocket 
has none, that of the second is represented by a rudiment, 
which early disappears. In the urodeles, in which five pockets 
appear, only the last three possess definite thymus-anlagen, 
while those of the first two are transitory rudiments. In the 
lizard, with four embryonic pharyngeal pockets, the first has a 
rudimentary thymus-anlage, and the second and third func- 
tional ones. The fourth is without one. In mammals the 
definite thymus is formed either wholly from the anlage asso- 
ciated with the third pocket, or else mainly from this with the 
addition of a small contribution from the fourth. 
The fate of the ventral anlagen, that form the epithelial 
corpuscles, is known only in part, but they form at best only 
small nodules of a more or less glandular nature, and never 
attain the bulk of a thymus. In amphibians that of the second 
pocket becomes associated with the carotid artery and forms 
the so-called “ carotid gland,” especially conspicuous in frogs 
and toads. Those of the third and first pockets are developed 
as epithelial corpuscles. In the lizard the only ventral an- 
lage that persists is that of the third pocket, which forms a 
carotid gland. In Echidna epithelial bodies appear, associated 
with pockets two and four, that of the second becoming a 
carotid gland as in amphibians. In other mammals pockets 
three and four, or pocket four alone, are provided with such 
anlagen. These may remain associated with their respective 
thymus anlagen, or may become detached, but possess little if 
any physiological significance. 
A third system of organs associated with this region ap- 
pears typically as a single pair of evaginations, which arise 
from the floor of the pharynx, in all cases immediately behind 
the last gill-slit. From their secondary relation to the peri- 
cardium in selachians they have received the name of supra- 
pericardial bodies, but they are better termed the post-bran- 318 
THE HISTORY OF THE HUMAN BODY 
chial bodies, a name which expresses a fundamental and uni- 
versal relationship. In selachians both members of the single 
pair of these organs develop, but in urodeles and in lizards 
the left alone completes its development, while the right one 
remains in a rudimentary condition, and eventually disap- 
pears. The occurrence of post-branchial bodies in birds and 
mammals is uncertain, but some identify with these a pair of 
evaginations that in the latter class arise from the same region 
and become eventually lost in the lobes of the thyreoid gland, 
the so-called parathyreoid bodies. Comparing the significance 
of the post-branchial bodies, they are looked upon by some as 
the rudiments of a pair of pharyngeal pockets posterior to the 
last definite ones, a point of view which has suggested for them 
the name of ultimo-branchial (rather than post-branchial) 
bodies; but the probability that the last apparent pocket in the 
various vertebrate Classes is not always the same one intro- 
duces a valid objection to this hypothesis. 
Still another organ associated with the pharynx as primarily 
an evagination from its walls is the thyreoid gland. This, 
in its first appearance, is constant from the selachians to the 
mammals, and arises as a median outpushing from the floor 
of the pharynx at a level corresponding to the interval be- 
tween the first and second pockets. In its later development 
it becomes, like the thymus, a compact glandular organ, with- 
out a duct, and equally uncertain and indefinite in its function, 
but in the larva of one of the cyclostomes (Petromyzon), we 
catch a glimpse of its past history, for here for a time it ap- 
pears as a trough, lined with cilia, in open communica- 
tion with the pharynx. In this condition it corresponds closely 
in structure and position with the hypo-branchial groove, or 
endostyle of Amphioxus, an organ which furthers the passage 
of the food down the pharynx by producing a slimy secretion 
and by furnishing a ciliated track along which the food may 
be propelled. The thyreoid gland was thus primarily a di- 
gestive organ and deserves a place in this chapter for reasons 
more fundamental than a mere topographical relation. In 
the true vertebrates, however, its function, with its, structure, THE DIGESTIVE AND RESPIRATORY SYSTEM 
319 
becomes completely changed, and it no longer assists in diges- 
tion, but presumably possesses some regulating effect upon the 
blood, especially that supplying the brain. 
The pre-vertebrate history of the thymus-anlagen is less 
certain, but attempts have been made to homologize them with 
the “ tongue bars ” of Amphioxus, gelatinous rods used to 
support the gill-clefts. If this be true the change of function 
seems even greater than in that of the thyreoid, for there is 
more similarity between a gland-lined groove and a compact 
glandular organ, than between the latter and a skeletal rod. 
In its development among the vertebrates the thymus is still 
more problematical than is the thyreoid; it is usually volu- 
minous, and in many mammals extends along the anterior 
mediastinal space between sternum and heart as far as the 
diaphragm. In the human species it is large in childhood, but 
suffers regressive changes; during middle life it is inconspicu- 
ous, and in old age is reduced to a few rudiments. 
The thymus and thyreoid, together with certain other gland- 
ular organs, like the hypophysis and the supra-renal [adrenal] 
bodies, are now spoken of as the endocrine organs, the secre- 
tions of which, transmitted by the blood, constitute the essential 
hormones which regulate certain physiological activities of the 
body. Their functions are thus of the greatest importance, 
and the endocrine organs are destined to become of the high- 
est value in medicine. 
Beyond the pharynx the alimentary canal becomes nar- 
rowed and forms the oesophagus, below which it again en- 
larges to form the great expansion known as the stomach, 
usually the most conspicuous portion of the entire tract. The 
oesophagus varies in length in proportion to the length of 
the neck region, one extreme being represented by frogs and 
toads, in which it is reduced to a simple constriction like the 
neck of a bag, the other by such cases as a long-necked bird. 
In birds that are graminivorous, or those in general which sub- 
sist upon hard or dry food, the middle portion of the oesoph- 
agus expands to form a crop (ingluvies), into which the 
food is first collected. Aside from its use as a receptacle in 320 
THE HISTORY OF THE HUMAN BODY 
which food may become softened by soaking, the crop is 
probably in part a provision for the safety of the birds, allow- 
ing them to greatly shorten the period of feeding, a time dur- 
ing which they are preoccupied and thus in especial danger 
from their enemies. 
The muscular fibers of the alimentary canal usually change 
from voluntary to involuntary in the upper part of the oesoph- 
agus, but in some mammals striated fibers, probably partly 
under the control of the will, extend farther down, and in 
ruminants, where the food is voluntarily disgorged in the 
form of cuds for a second chewing, the entire oesophagus is 
thus equipped. 
The stomach is originally a simple, spindle-shaped enlarge- 
ment of the canal, extended lengthwise and indefinitely sepa- 
rated from the oesophagus, although more completely limited 
below by a restriction, the pylorus, which forms a valve for 
the purpose of temporarily converting it into a closed sac. 
This typical form is seen in fishes and tailed amphibians, but 
an attempt to increase its efficiency both in digestive surface 
and in capacity causes the formation of a curvature extending 
to the left, so that there is a longer outline upon its left side 
and a shorter one upon its right, the greater and lesser curva- 
tures respectively. As the same tendency continues the 
stomach turns, still to the left, in such a way that ultimately 
its longitudinal axis lies across the body, which places the 
upper or cardiac end on the left, the lower or pyloric end, on 
the right, the lesser curvature above and the greater curvature 
below. Below the cardiac orifice the left end of the stomach 
usually bulges out laterally to form the fundus, which in some 
cases becomes a more or less distinct receptacle for the food 
when first received; and beyond approximately the middle the 
stomach tapers toward the pyloric end (right) and forms an 
upward curve at the culmination of which is placed the pylo- 
rus. This may be considered the typical form of mammalian 
stomach and is seen in Primates, Carnivora, Insectivora, and 
Edentata, these being in other respects also the most primitive 
of placental mammals (Fig. 108, a). THE DIGESTIVE AND RESPIRATORY SYSTEM 
321 
Modifications of this primary form are due, first, to an at- 
tempt to localize and define the different portions of the stom- 
ach and specialize their functions, and, secondly, to various 
attempts to increase the general surface and thus develop a 
greater physiological efficiency, usually in connection with in- 
nutritious food or with the necessity of taking in a large 
amount in a short space of time. 
Progress in the first of these directions is shown by such a 
stomach as that of the mouse (Fig. 108, b), in which the car- 
diac and pyloric halves are separated by a marked restriction, 
Fig. 108. Stomachs of various mammals, 
(a) Man; (b) mouse; (c) pig; (d) seal; (e) vampire bat; (f) manatee; (g) 
sloth; (h) sheep. 
and this tendency reaches its extreme in the ruminants (Fig. 
108, h), where each of these two primary sub-divisions is again 
divided, forming a stomach of four compartments, in two 
pairs. The cardiac portion is divided into a voluminous paunch 
(rumen) which receives the food when first taken in, and a 
small, round honey-comb stomach (reticulum), in which the 
food from the paunch is made up into cuds. The pyloric por- 
tion is divided into an omasus and an abomasus, into which 322 
THE HISTORY OF THE HUMAN BODY 
the food passes in succession when swallowed a second time. 
Local enlargements of surface frequently appear in the form 
of a prolongation of the fundus into one or more diverticula 
(Fig. 108, e, f and g). There are two of these in the hippo- 
potamus; three in Tarsipes, a small, insect-eating marsupial; 
and in the vampire bat, Desmodus (Fig. 108, e), the fundus 
is elongated to form a coecum of twice the length of the body, 
used as a reservoir for blood. 
It is of interest in this connection to trace the changes in the 
mesentery of this region, more precisely termed the mesogas- 
Fig. 109. Development of the peritoneal folds and of the windings of 
the alimentary canal in the human embryo. [From Hertwig, Figure I 
after His; Figure II after Toldt.] 
Figure I shows the spindle-shaped stomach, the lung anlagen, and the beginning 
of the liver in the form of a median diverticulum; in Figure II the peritoneum is 
shown, with pancreas and spleen; figures III and IV show the development of the 
omentum and the lesser peritoneal cavity. 
trium, as they appear in successive stages in the mammalian 
embryo, which are undoubtedly of historical significance (Fig. 
109). The formation of the initial curvature to the left natu- 
rally broadens the corresponding part of the mesogastrium, an 
effect still further increased by the lateral torsion of the entire 
stomach. At this point the widened mesentery comes under THE DIGESTIVE AND RESPIRATORY SYSTEM 
323 
the influence of the spleen, which develops within it and by its 
weight produces a fullness which sags down behind (dorsal 
to) the lesser curvature, while attached to the greater; and 
the continuation of this tendency causes the free lower fold 
of the bag-like extension to hang down behind the contour 
of the stomach. 
This fold is the greater omentum (omentum majus), which, 
as all mesenteries are essentially double, consists of four layers 
of serous membranes, applied two and two, each pair holding 
between them the blood and absorbent vessels naturally be- 
longing to a mesentery. The cavity of the bag is the lesser 
peritoneal cavity of human anatomy, and its mouth, opening 
into it behind the stomach, is the foramen epiploicum [foramen 
of Winslow]. In most mammals the bag is widely open, but 
in man the foramen is much reduced in size and the layers 
forming the pendulous fold are fused together, and form a 
four-layered apron that hangs below the stomach and covers 
the intestinal folds. 
The remainder of the canal below the pylorus forms the in- 
testine, and although this has been divided for convenience 
into several more or less definite regions, they are for the most 
part artificial in character. The most definite of these are 
the cloaca of Amphibia and Sauropsida, and the large in- 
testine of mammals [intestinum eras sum], both enlargements 
of the posterior portion of the intestinal tract, but probably not 
equivalent to one another; in distinction from this the re- 
mainder is termed the small intestine [intestinum tenue]. In 
this latter the most definite subdivision is defined by the en- 
trance of the biliary and pancreatic ducts; and the space be- 
tween the pylorus and this point is designated the duodenum. 
This portion often forms a conspicuous loop, consisting of 
ascending and descending limbs, which enclose the pancreas 
between them. 
The liver and the pancreas, the two digestive glands asso- 
ciated with this region, are derived from the mucosa of the 
intestines, from which they arise as evaginations, the former 
ventral, the latter dorsal. As they increase in size they pro- 324 
THE HISTORY OF THE HUMAN BODY 
trude beyond the intestinal walls and force their way between 
the two layers of their respective mesenteries, as a result of 
which relation they become invested with a serous membrane 
continuous with that covering the intestines (peritoneum), and 
remain attached by ligaments both to the latter and to the 
body wall. These relations are shown in the accompanying 
diagrams (Fig. no), which show the origin of these organs 
from the intestine, their serous investment and their dorsal and 
Fig. no. Diagrams showing the relation of the liver and pancreas to 
the peritoneum. [After Hertwig.] 
(a) Lateral view with ventral surface towards the left. The organs are seen lying within 
the peritoneum, which is represented in a vertical plane stretched across from mid-dorsal to mid- 
ventral lines, (b) A cross-section of the same. The place through which it is taken is indicated 
approximately in (a) by the arrows. 
Organs: s, stomach; n, spleen; l, liver; p, pancreas; i, intestine. Ligaments: x, 
ligamentum suspensorium hepatis; y, ligamentum hepatogastrium ( = lesser omentum); 
a and b, parts of the mesogastrium which form the pancreatic ligaments similar to 
those of the liver. 
ventral mesenteries. In spite of the great increase of size 
these typical relations remain in the case of the liver, and its 
two suspensory mesenteries become the ligamentum hepato- 
gastricum [y], and the ligamentum suspensorium hepatis [x]. 
While primarily the entire length of the alimentary canal that 
passes through the coelomic region becomes attached by both 
a dorsal and a ventral mesentery, the ventral one becomes THE DIGESTIVE AND RESPIRATORY SYSTEM 
325 
lost below the region of the liver, thus leaving a sharp ventral 
edge to the two hepatic ligaments. 
The gall-bladder is formed as an enlargement of the hepatic 
duct and is by no means of universal occurrence; it develops 
rather in response to certain conditions, much as in the case 
of the crop, and its slight physiological importance is shown 
by its occurrence in one of two allied animals and its ab- 
sence in the other. A good example of this is that of the 
pigeon and the common fowl, in the latter of which a well- 
developed gall-bladder occurs while it is absent in the former. 
The pancreatic duct is normally without such a reservoir, but 
a pancreatic bladder has occasionally been observed as an 
abnormality in the common cat, existing side by side with a 
normal gall-bladder, the two exhibiting about the same size 
and proportions. 
The terminal portion of the alimentary canal in Amphibia 
and Sauropsida, and in some fishes {e.g., selachians), enlarges 
into a cloacal chamber which bears within its walls the out- 
lets of the urinary and reproductive organs and receives their 
products as well as that of the intestines. In the Sauropsida 
and in monotremes the terminal portion of this serves as the 
functional cloaca and receives also the urinary and reproduc- 
tive products, but in all mammals except these last the urino- 
genital outlets are emancipated from the alimentary canal, 
which thus terminates in a rectum instead of a cloaca, and its 
external opening is a true anus and not a cloacal orifice. 
At the junction of the small intestine with the large, there 
is a strong tendency to form one or more coeca, or blind sacs, 
which often become digestive organs of great physiological 
efficiency. The characteristic form in reptiles is that of a single 
rather short and wide ccecum, symmetrically placed. In birds 
there are usually two symmetrical ones, which attain great 
length in scratching birds {e.g., the common fowl), and in 
ducks and geese, but are quite rudimentary in certain others 
(woodpeckers, parrots, etc.). Ostriches possess a single ccecum 
of great length (7 to 8 meters) and furnished with an internal 
spiral partition which greatly increases its effective surface. 326 
THE HISTORY OF THE HUMAN BODY 
In mammals a single coecum is developed,1 which varies 
greatly in size and functional importance. Rudimentary in 
edentates, most insectivores and bats, it frequently attains an 
enormous size in herbivorous or graminivorous forms. In 
certain rodents (e.gmuskrat, woodchuck) its total capacity 
equals or exceeds that of the remainder of the alimentary 
canal, and in the marsupial Phascoloarctus it is three times 
the length of the body. In the rabbit it is provided with an 
internal spiral valve; in certain other rodents and in the higher 
apes and man, the free end becomes rudimentary, restricts its 
lumen and forms a worm-like process, the processus (appen- 
dix) vermiformis, which like all rudimentary organs is sub- 
ject to a large amount of individual variation. 
Thus in the human subject the appendix varies in length 
between the limits of 2-23 cm., the average for an adult being 
8-9 cm. It is longest proportionally during fetal life, its 
length relative to that of the large intestine being 1:10, while 
in adult life it is 1:20. It is longest absolutely between the 
ages of ten and twenty, after which it shows a slight reduc- 
tion.2 Its status as a rudiment of slight functional value is 
suggested by the tendency towards the obliteration of its lumen, 
a tendency which increases steadily with age.3 Furthermore, 
1 There are two very short coeca in the arboreal ant-eater, and in the 
manatee a single coecum bears two supplementary diverticula. In Hyrax, 
in addition to a moderately large coecum, there are two smaller diverticula 
situated farther down on the colon. 
2 Zuckerkandl tabulated the length of the appendix in 161 bodies, with the 
following result: 
mm 
17 — 20 2 cases 
30 — 40 8 “ 
40 — 50 6 “ 
50 — 60 28 “ 
60 — 70 26 “ 
70 — 80 29 “ 
80 — 90 23 “ 
mm 
90 — 100 15 cases 
100 — no 4 “ 
no — 120 5 “ 
120 — 130 2 “ 
130 — 140 1 “ 
140—150 1 “ 
150 — i5o x “ 
3 Wilhelm Muller, from data obtained from 1,005 bodies dissected at Jena 
between 1895 and 1897, found the amount of obliteration, partial and total, to 
be as follows: THE DIGESTIVE AND RESPIRATORY SYSTEM 
327 
these two characters, reduction in length and obliteration of 
the lumen, go hand in hand, short appendices being usually 
solid while large ones are apt to possess a lumen. 
The position and arrangement of the colon varies consider- 
ably among various mammals. In man it begins low down 
on the right side from which there proceed in order an as- 
cending, transverse, and descending portion connected with 
the rectum by a sigmoid flexure, through which the tube attains 
the median line; a similar disposal is seen in many other an- 
thropoids, in lemurs and rodents, the majority of carnivores, 
and a few others. A more complex condition than this is pro- 
duced by the formation of long, narrow loops along the course 
of either the ascending or transverse colons, or both, and these 
loops may remain simple or roll into spirals. Such colon 
labyrinths are seen in ruminants, in certain rodents as the 
lemmings and jumping mice, and in a few lemurs (Figs, in 
and 112). 
Males 
Females 
No. of 
No. of 
Age in 
No. of In- 
Cases of 
Per- 
Age in 
No. of In- 
Cases of 
Per- 
Years 
dividuals 
Obliter- 
ation 
centage 
Years 
dividuals 
Obliter- 
ation 
centage 
o 
48 
0 
0 
0 
46 
0 
O 
i 
78 
0 
0 
1 
58 
0 
0 
2 — IO 
Si 
1 
2.0 
2 — 10 
41 
0 
0 
II 20 
39 
2 
5-i 
11 — 20 
19 
1 
5-4 
21 — 30 
47 
3 
6.4 
21 — 30 
23 
2 
8.7 
31—40 
SS 
7 
12.7 
31 —40 
38 
9 
23.8 
41 — 50 
84 
22 
26. 2 
4i — 5° 
46 
l6 
34-8 
51 60 
73 
15 
20.5 
Si — 60 
60 
18 
30.0 
6l 70 
58 
17 
29-3 
61 — 70 
48 
24 
50.0 
71 80 
3i 
12 
38.7 
71 — 80 
30 
8 
26.6 
8l 90 
iS 
8 
S3-3 
81 — 90 
17 
9 
52-9 
579 
426 
Comparison of the percentage columns shows that in women there exists a 
greater tendency towards obliteration than in men. The few discrepancies in 
the table, for example, the smaller average given for men between 50 and 60, 
and in women between 70 and 80, are doubtless due to the small number of indi- 
viduals examined. 328 
THE HISTORY OF THE HUMAN BODY 
From this brief review of the alimentary canal and its modi- 
fications the impression is gained that in this array of enlarge- 
ments, elongations, diverticula, spiral valves, and other de- 
vices, we have to do, not with a consecutive anatomical history, 
but with numerous special cases of physiological adaptations, 
developed in response to need; and that a similarity in one of 
these particulars implies not genetic relationship necessarily, 
but a similar demand responded to in a similar way. The 
main object to be achieved in all cases is to regulate the amount 
Fig. hi. Colon labyrinth of Cervus canadensis. [After Weber.] 
of digestive surface to the demands offered by the various 
kinds of food, and as there is but a limited number of me- 
chanical or architectural devices possible, the same ones are 
employed in unrelated groups of animals, having arisen in- 
dependently in response to a similar physiological need. This 
phenomenon of parallel development (or “ analogical resem- 
blance,” as Darwin calls it), may appear in any system or part 
and has been a frequent source of error in the estimation of 
the interrelationship of animals. THE DIGESTIVE AND RESPIRATORY SYSTEM 
329 
The relation of the total length of the intestine to the kind 
of food has been frequently emphasized, the idea prevailing 
that it is short in carnivores and long in herbivorous forms, in 
accordance with the difference in nutrient qualities and the 
Fig. i i 2. Colon labyrinth of Propithecus diadema. [[From Weber, 
after van Loghem.] 
ease of digestion in the two sorts of food, but this statement 
is to be accepted only in a general way, as it is subject to 
modifications through the compensation furnished by other 
factors, such as the special devices just considered. Thus in 
the ox the length of the entire intestine, small and large, in 
proportion to the length of the body taken as unity is 20: i? 330 
THE HISTORY OF THE HUMAN BODY 
while in the horse, which eats similar foods, it is but 12:1, but 
in this latter animal an enormously developed ccecum furnishes 
a compensation for the reduction in length of the main canal. 
Perhaps the greatest extremes of variation within the same 
Order are shown within the limits of the Cetacea, where the 
proportionate length varies between 4:1 and 32:1. This last, 
probably the largest among mammals, is reported to be that of 
Pontoporia, a South American dolphin, while the shortest mam- 
malian intestine is that found in certain insectivorous bats, 
the proportion of which to the body length is 2:1. A change 
in the length or volume of an organ is, however, so easily 
effected even during an animal’s lifetime, that it is probable 
that members of the same species may show considerable 
difference in length of intestine, especially if a comparison be 
made between specimens from quite different localities where 
the diet is different. Thus in man, the intestinal canal of the 
Japanese, whose food is largely vegetable, exceeds in average 
length by one-fifth that of Europeans; and in whites of me- 
dium stature the length of the intestine proper from pylorus 
to anus, averages 960 cm., while the average length of the 
same in nine negroes was but 866.7 cm. 
This whole subject, therefore, gives but little indication of 
phylogeny and is valuable in the present inquiry mainly as an 
example of the complete correlation between environment and 
structure. In the examples given mammals have been pur- 
posely emphasized and instances of adaptations in the other 
groups of vertebrates have been omitted as far as possible, 
since their inclusion would convey the subject far beyond the 
proper limits of this work. 
The junction of respiration is the simplest of the major 
functions, since it consists primarily of an interchange of gases 
through osmosis, and involves in itself nothing save a moist 
membrane, with air or aerated water, on one side and blood 
on the other. The blood must be constantly renewed through 
some form of circulation, and there is usually some auxiliary 
mechanism to create a current in the respiratory medium also. 
It is also imperative that the osmotic membrane be kept moist, THE DIGESTIVE AND RESPIRATORY SYSTEM 
331 
a matter of no difficulty in an aquatic animal, but one involving 
some little additional apparatus, usually an interior chamber 
with a regulated outlet, in terrestrial forms. Thus in aquatic 
invertebrates the respiratory membrane is usually external, 
often a modified portion of the integument. In many minute 
forms in which the integument is thin, respiration takes place 
through the general surface without the formation of localized 
organs for the purpose, and in larger forms effective organs 
of respiration are produced by the formation of external folds 
or other outpushings of the integument. These are formed 
in the embryo when the skin is still soft and thin and remain 
in the adult state unaffected by the process of chitin formation 
which often involves the surrounding integument. Such organs 
are called gills, a general term for all aquatic respiratory 
organs. These present numerous mechanical devices for in- 
creasing the surface; they may be in the form of single plates, 
sets of plates placed parallel to one another, dendritic structures 
formed by the repeated branching of simple diverticula, sets 
of parallel tubes for the blood with interspaces for the water, 
and so on, and are in most cases provided with accessory struc- 
tures, some for protection and others for producing a current 
of water. 
In a terrestrial animal, on the other hand, the respiratory 
system must be internal in order to secure the proper con- 
ditions of moisture, and as all’terrestrial animals are the de- 
scendants of aquatic ones that succeeded in adapting them- 
selves to the difficult environment of land, with its many dis- 
advantages, it forms an interesting study in adaptation to 
compare the respiratory system in each terrestrial group of 
animals with that of the animals which most nearly represent 
their aquatic ancestors. In some cases the old respiratory or- 
gans are retained by sinking them into deep recesses kept moist 
by glands, in others they are discarded in favor of new ones, 
formed, perhaps, by the transformation of some ready-to-hand 
cavity, which is lined with blood vessels and made to com- 
municate with the exterior through some regulated outlet. Still 
another principle is seen in the tracheal tubes of insects, which 332 
THE HISTORY OF THE HUMAN BODY 
are branching tubes lined with chitin and leading from a series 
of external openings into the interior, ramifying all the inter- 
nal organs. These, as shown by their development, are in 
origin integumental folds, like the plate-like gills of their an- 
cestors, which, as they develop, turn in instead of out, thus 
satisfying the conditions of aerial respiration. 
Essentially different in principle from all of the respiratory 
methods thus far mentioned is that which utilizes for the 
purpose some portion of the wall of the alimentary canal, a 
method employed sporadically among invertebrates, especially 
the echinoderms, and forming the essential system in verte- 
brates and allied forms. In this, which, by using the term in 
its most comprehensive sense, may be called intestinal respi- 
ration, the junction is usually located near one end of the ali- 
mentary canal, for the purpose of obtaining the respiratory 
medium, and the wall at this place is richly supplied with 
capillaries, through which the interchange of gases takes 
place. The respiratory medium, which may be either'air or 
water, is kept in motion by a system of involuntary or semi- 
voluntary muscles, and the motion thus generated is usually 
rhythmic in character. 
In the vertebrates, as well as in those invertebrates that 
probably represent their ancestors, the respiratory function is 
located in the pharynx and the respiratory current is primarily 
taken at the mouth and driven out through a series of lateral 
openings, the gill-slits.* 
These latter, as seen in the worm-like Balanoglossus, and 
in Amphioxus, as well as in the embryos of true vertebrates, 
are seen to be metameric in character, a pair for each somite, 
and to be arranged in a single row along each side; but in the 
more specialized group of Tunicata, these rows of slits which 
appear in the larva become secondarily modified by the forma- 
* Aside from the respiration at the anterior end of the canal there are 
a few isolated instances of .respiratory action in other parts of its extent. 
Thus in the teleost Cobitis, an Eastern Hemisphere carp, some respiratory 
function is possessed by the intestine; and in turtles, two lateral bladders, 
opening from the cloaca in association with the median, or allantoic bladder., 
are used for aquatic breathing. THE DIGESTIVE AND RESPIRATORY SYSTEM 
333 
tion of numerous cross-bars, so that ultimately the entire 
pharynx comes to resemble a grating or a loosely woven 
basket. 
The number of these slits is very large in both Balanoglos- 
sus and Amphioxus, but has suffered a considerable reduction 
in true vertebrates, so that even in the lowest of the fishes no 
more than nine pairs are ever indicated, and this number 
suffers a constant reduction in higher forms, the loss being 
progressively from behind forwards. In the lower vertebrates 
also the effectiveness of the system is increased by the forma- 
tion, in the endoderm lining the pharynx, of soft structures, 
richly supplied with blood vessels, which border the gill-slits 
and form the true respiratory organs, the definite gills or bran- 
chiae,. When, in the history of the race, vertebrates came out of 
the water and adapted themselves to a terrestrial element, they 
substituted for this branchial system a pulmonary one, em- 
ploying as lungs a pair of sacs which open into the floor of the 
pharynx a little behind the last gill-slits, and which were un- 
doubtedly in existence at the time of the change, employed as 
air bladders. In the gradual perfection of this second respira- 
tory system many of the parts of the old one obtained employ- 
ment, and were one after another selected and modified to add 
to its efficiency. 
This history of the sudden replacement of one system by 
another, and of the gradual perfection of the second by making 
over to its own use the material of the first, forms one of the 
most interesting although most complex bits of anatomical 
history, and one of which the record has been especially well 
preserved. As it involves, however, the entire region and in- 
cludes skeletal parts, muscles, nerves and other elements aside 
from those which may be strictly termed respiratory organs, 
much of the history will be found in the chapters devoted to 
those other parts. Here an attempt will be made to outline 
the history of the parts as a whole, with special reference to 
the function of respiration. 
The fish type of respiratory apparatus is presented in its 
most primitive form in the sharks and dog-fish, since numerous 334 
THE HISTORY OF THE HUMAN BODY 
modifications which have been acquired in the more specialized 
fish are absent. It is a type that looks both ways, and, while 
in many respects similar to that of Amphioxus, from it may be 
clearly derived the branchial respiratory system of higher 
forms. Like all special respiratory organs of vertebrates, it 
is essentially pharyngeal and consists primarily of a series of 
lateral pockets in the walls of the pharynx, which break 
through to the exterior and form slits. These openings are 
Fig. i 13. Aquatic Respiratory organs. 
(a) Cyclostome. (b) Selachian, (c) Teleost. (d) Selachian embryo in which the internal 
gills are pushed through the slits and appear on the outside, (e) Amphiuma larva, (f) Cryptobranchus 
larva [from a Japanese print]. 
n, nostril, s, spiracle, g, gill-slits. 
metameric in arrangement and are paired, each pair corre- 
sponding to a single metamere as expressed in the associated 
organs, but show considerable reduction in number from those 
found in Amphioxus, the functional slits in most cases being 
limited to five pairs (Fig. 113, b). In two especially primi- 
tive genera, however, Hexanchus and Heptanchus, these are 
respectively six and seven, facts which suggest that the 
number at present represents a reduction from a pre- 
viously more extensive series, the reduction being from THE DIGESTIVE AND RESPIRATORY SYSTEM 
335 
behind forwards. Upon the pharyngeal side of these 
slits there develops a series of soft organs in the form 
of fringes or tubes, which consist of localized elabora- 
tions of the pharyngeal wall, the gills or branchiae. These 
are profusely vascular and are supplied with a rich capillary 
network developed between two sets of branchial arteries, the 
afferent branchials, which bring the blood directly from the 
heart to the gills, and the efferent branchials, which collect the 
blood from the gill capillaries and unite to form the main 
aorta. Since the blood is aerated in the capillary network of 
the gills it follows that the blood coming from the heart 
through the afferent vessels is impure, while that in the effer- 
ent vessels is pure; and since these latter unite to form the main 
aorta, this vessel, the branches of which supply the entire body, 
contains only aerated blood, while the heart is employed merely 
to receive the venous blood which returns from all parts, and 
send it to the gills. This is the primitive type of vertebrate 
circulation, and obtains not only in all fishes, but reappears as 
the early form in all vertebrate embryos, thus proving its 
fundamental character as an historic stage. 
The essential respiratory cavity is thus the entire pharynx, 
through which a current of water is kept in constant flow by 
being taken in at the mouth and exhaled through the gill- 
slits; and while in earlier forms, as suggested by Amphioxus, 
the capillaries lie in the unmodified pharyngeal wall in the 
vicinity of the gill-slits, the selachians show a considerable ad- 
vance by the formation of definitely localized organs, with a 
large increase of surface, thus physiologically more efficient. 
In other fishes this gill-system, which is essentially similar to 
the foregoing, exhibits several secondary modifications, the 
most apparent of which is the formation of a large gill-flap 
(operculum), which arises in front of the most anterior slit 
and extends backwards, and as the slits become closely ap- 
proximated and are reduced in number to four, the operculum 
becomes capable of closing entirely over them, meeting a ridge 
of integument behind the last slit (Fig. 113, c). The current 
of water is directed by rhythmic respiratory movements, which 336 
THE HISTORY OF THE HUMAN BODY 
consist of opening and closing the mouth and operculum, the 
motions of the two alternating with each other. 
With the fishes true internal (endodermic) gills pass away, 
but in the permanently aquatic salamanders and in all larval 
amphibians one or more slits break through, usually two to 
three, in connection with which certain integumental struc- 
tures arise which are gills physiologically, but are unrelated to 
the former. The most widely distributed form of these is 
that of the external branchiae, three in number upon each side, 
and attached to the cartilaginous gill-arches. In structure 
they are usually plumose or dendritic (Fig. 113, e and f), 
although in a few cases they are thin and leaf-like. The slits 
appear between these, with occasionally an additional one in 
front of the first, and the animals obtain fresh water for res- 
piration in part by forcing a current through these slits in the 
manner of fishes, and in part by waving the branchiae up and 
down by means of special muscles with which these organs 
are furnished. As stated above, external branchiae are char- 
acteristic of the larvae of all amphibians, and are found per- 
manently in a few aquatic salamanders, which are either more 
primitive than the rest, or are paedogenetic, that is, they retain 
the larval form while becoming sexually mature. These sala- 
manders are called perennibranchiate in distinction from those 
in which the branchiae become lost, the caducibranchiate sala- 
manders. A second form of gills which are external in origin 
but become internal in position, occurs in frog larvae, where 
they replace the former, which appear at first. As these are 
plate-like and are attached to the gill-arches, they have often 
been considered exactly homologous with the gills of fishes, 
but their ectodermic origin renders such a conclusion impos- 
sible. 
Aside from the two sets of branchiae most amphibians pos- 
sess definite lungs, which arise in the larvae and exist for a 
time side by side with the external branchiae, usually replacing 
them in later life. These are often in the form of simple 
sacs, without any formation of internal partitions, and even 
when in their highest development, as in frogs, are far from THE DIGESTIVE AND RESPIRATORY SYSTEM 
337 
complex. It thus seems probable that, although they are true 
lungs anatomically, they play a subordinate role in respiration, 
and are perhaps primarily used either for regulating the spe- 
cific gravity of the animal in the water, or in the production 
of the voice, since the larynx is often very large and curiously 
specialized, and is of considerable importance in the produc- 
tion of sexual calls. The slight respiratory importance of the 
lungs in amphibians is further emphasized by the fact that in 
a large number of species of salamanders, both lungs, trachea 
and larynx are entirely wanting, although in a few cases rudi- 
ments of these parts attest the former presence of these organs. 
This reduction or loss of such typical organs as the lungs 
seems to be correlated with a mountain brook habitat, and 
to have been the experience of several groups of sala- 
manders, not closely related to one another. In the most of 
these instances there has been an entire loss of both lungs, 
trachea, and larynx, although in a few cases the lungs are 
present, although reduced in size, and in one (Salamandrina) 
although there are neither lungs nor trachea, the larynx is 
represented by a pair of rudimentary nodules, representing 
the arytenoids. 
The reason why mountain brook salamanders should have 
thrown away their lungs is plainly to avoid being carried down 
stream in the strong currents that are frequent in such locali- 
ties, and thus these animals offer a fine illustration of an 
unusual adaptation to an unusual environment. 
The question will naturally occur at this point: what are 
the means of respiration in adult amphibians if they have lost 
their branchiae and yet possess either no lungs at all or those 
of slight functional importance? The answer to this lies in 
the fact that amphibians have developed two other systems, 
neither branchial nor pulmonary, the assumption of which 
shows how great may be the systematic response to a physi- 
ological need, and suggests also the trying period of transition 
when vertebrates first essayed the terrestrial environment, and 
when attempts were made in all possible directions to adapt 
themselves to the new respiratory medium. These two sys- 338 
THE HISTORY OF THE HUMAN BODY 
terns are the integumental and the pharyngo-oesophageal, and 
as both of these demand for their highest efficiency a moist 
environment with an occasional submersion in water, they are 
successful in amphibians with their semi-aquatic mode of life, 
but in higher forms have been discarded in favor of the pul- 
monary system, which enables its possessor to leave the 
marshes and inhabit the dry land. 
The origin of the assumption of a respiratory function by 
the amphibian skin may be traced to the abundance of integu- 
mental glands, inherited from the fishes and used to protect 
the surface from the action of the water. The presence of 
these glands necessitates the formation of a superficial net- 
work of capillaries to supply them with nourishment, and the 
integument becomes thus transformed into an organ that pos- 
sesses the qualities necessary for a respiratory organ, that is, 
a moist surface bathed by the respiratory medium and sup- 
plied with a rich capillary net-work. Thus apparently by ac- 
cident, as in all morphological changes, an organ which be- 
comes modified for a certain function shows a capability of 
assuming a second one, not intended in the original plan, and 
in this instance the moist, glandular skin becomes an effective 
respiratory organ. 
The pharyngo-cesophageal system appears to be a special 
compensation for the loss of the lungs, and is present in only 
those salamanders in which the pulmonary system has been 
lost (Fig. 114). Here again the incentive towards the for- 
mation is a moist membrane richly supplied with capillaries, 
in this case, the mucous lining of the pharynx and oesophagus. 
The natural vascularity of this structure has been considerably 
increased, while the capillaries themselves have become more 
superficial and even invade the external epithelial layer, the 
only case known. The muscles of the lost pulmonary system 
have been in part retained, and through their aid, together with 
that of others which are developed for that purpose, the 
pharynx and oesophagus are dilated and contracted in asso- 
ciation with the usual respiratory movements of nostrils and 
floor of the mouth. The anterior part of the alimentary THE DIGESTIVE AND RESPIRATORY SYSTEM 
339 
canal thus becomes a junctional lung with the power of in- 
spiration and expiration, forming doubtless a more efficient 
organ than the simple air-sacs which these salamanders allowed 
to atrophy. 
Pharyngeal 
Muscle 
Art.Pharyngea 
Portion of 
PulmonaryAr chi 
Oesophageal 
Muscle 
'Art. Gastric a 
I anastomoses 
1 here with the 
PulmoiiaryArch 
Art.Gastrica 
V. oesotphagea 
Fig. i 14. Pharyngo-oesophageal lung of Desmognathus, showing pharyngeal 
and oesophageal muscles, and the net-work of blood-vessels in the walls of the 
pharynx and oesophagus. 
Above the amphibians, which, with their numerous methods 
of respiration, suggest the experimentation of our early an- 
cestors in their attempts to occupy what must have been at 
first an unnatural environment, the pulmonary system becomes 
supreme, and its further development is shown principally in 
the increased efficiency of its two main organs, the lungs and 
the larynx. The later history of this system is quite well 
known, especially that of its development in terrestrial verte- 
brates, but the origin of the system is still in part obscure, 
and rests upon surmises rather .than upon actual proof. 340 
THE HISTORY OF THE HUMAN BODY 
The history begins with the period represented by fishes, 
during which the pharynx exhibits a tendency to throw off 
median diverticula, sometimes dorsal and sometimes ventral, 
for the purpose of forming pneumatic cysts or air-bladders 
to add to the buoyancy and thus aid in swimming. In many 
cases these become closed and depend upon the adjacent 
blood vessels for the gases with which they become distended, 
but in others the original connection with the pharyngeal cavity 
is retained and the two are kept in communication through a 
small duct. In this latter case the cyst is filled with air, which 
is expelled and renewed through the mouth when the fish is 
at the surface of the water, a proceeding that demands some 
sort of regulator at the orifice of the duct, an opening to which, 
by an extension of meaning, the term glottis may be applied. 
Such an apparatus, which consists of muscles and fibro-carti- 
lage, is a functional larynx, of which there must be two distinct 
organs, a larynx dorsalis, and a larynx ventralis, in accordance 
with the postion of the pneumatic cyst. That cysts in these 
two positions cannot be homologous is evident; indeed, those 
in the same position in fish not closely related are not neces- 
sarily the same, yet until the subject has been thoroughly in- 
vestigated, the latter may be assumed to be the case. 
In several ganoids either one or the other of these pneu- 
matic systems becomes complex in character and serves as an 
accessory respiratory organ. The air-bladder functions as a 
lung; it becomes honeycombed with connective tissue parti- 
tions, and becomes profusely vascular, thus forming an organ 
of far greater functional activity than the definite lungs of 
many amphibians; corresponding to this its larynx, the regu- 
lator of the air supply develops an extensive set of muscles and 
masses of fibro-cartilage. That such a structure, when dorsal 
in position, as in Lepisosteus, cannot be the precursor of the 
final pulmonary system of higher forms is self-evident, but 
when such an organ is ventrally placed, thus corresponding 
exactly to the embryonic stages of the latter, as in Polypterus, 
such an homology, although not definitely proven, is very THE DIGESTIVE AND RESPIRATORY SYSTEM 
341 
Fig. 115. Diagrams of air bladders of various fishes. 
i, Sturgeon and many Teleosts; 2, Lepidosteus and Amia; 3, Erythrinus; 4, Ceratodus; 5. Polyp- 
terus and Calamoichlhys; 6, Lepidosiren and Protopterus; 7, Reptiles, birds and mammals. (Lungs.) 
[From Dean after B. G. Wilder.] 342 
THE HISTORY OF THE HUMAN BODY 
likely. As for the dorsal system, there is no indication that 
it is represented in any way above the fishes. 
If, however, the ventral air-bladder of Polypterus is identi- 
cal with the paired lungs of higher forms (which begin as a 
single median diverticulum that divides later into two 
branches), the larynx can be the same only in respect to its 
opening, the glottis, since the accessory parts, that form the 
functional organ, are derived from two totally different sources 
in the two cases. In the fish larynx the hard parts are derived 
from the adjacent connective tissue, and are composed of 
fibro-cartilage, which represents as it were the first stage in 
cartilage formation and differs but little from a compact form 
of simple connective tissue. The muscles are evidently slips 
differentiated from the muscular walls of the pharynx. That 
this forms a very effective organ cannot be denied and, had 
no better material for a laryngeal mechanism been furnished, 
that of the ganoid with its fibro-cartilage and slips of pharyn- 
geal muscle would have undoubtedly developed to fill all the 
needs of a pulmonary system, even including the functions of 
voice and speech. It chanced, however, that at this period, 
the fifth branchial arches with their accompanying muscles, 
emancipated from all respiratory function, and employed in a 
desultory way as tooth-bearing structures or as parts assisting 
in deglutition, were lying in the immediate neighborhood, one 
on each side of the glottis and but little anterior to it, and 
equipped with well-differentiated muscles; and it may well have 
happened that little by little these parts may have usurped the 
function of the other apparatus, being better fitted for the 
purpose. 
Be that as it may, when, after a succession of forms that 
have become lost, the curtain rises upon the lowest of the 
amphibians, this very pair of arches is seen lying, one upon 
each side of the glottis, and forming with its muscles a primi- 
tive though very effective larynx. These cartilages are proven 
to be the actual 5th pair of gill-arches through the identity of 
their nerve supply, and the weak point in the story is the 
identity of the two pulmonary systems, that of the ganoids and THE DIGESTIVE AND RESPIRATORY SYSTEM 
343 
the definite one found in terrestrial vertebrates, a point not yet 
proven; but, granting this, a theory which seems extremely 
probable, the rest must follow. In all events the history of 
both lungs and larynx from the amphibians on is a continuous 
one, and the latter organ, equipped at the start with the 5th 
branchial cartilages and their associated parts, becomes more 
complex by the gradual addition of other arches, proceeding 
from behind forwards, each accommodating itself in shape 
and position to the especial function desired in each case. 
The simplest amphibian larynx is that of the perenni- 
branchiate salamander Necturus, where the two cartilages 
in question are in the form of flattened triangular pieces, the 
(a) Necturus (mud-puppy), (b) Proteus, (cj Amphiuma. (d) Triton (Newt), (e) Rana (frog) 
Fig. 116. Laryngeal cartilages of various Amphibians, 
lateral cartilages, placed one upon each side of the glottis 
(Fig. 116, a). A short membranous trachea, entirely without 
cartilaginous support, leads to the bag-like lungs. In an 
allied form, Proteus (Fig. 116, b), a slight advance is seen in 
the fact that the posterior angles of the lateral cartilages are 
more prolonged and appear as slender processes which are 
applied along the sides of the entire trachea as far as the 
bronchi. These in adult animals show a tendency to separate 
from the main mass. This differentiates the cartilaginous 
pieces into an anterior pair of arytcenoids, upon either side 
of the glottis, and a posterior pair of tracheal pieces. Within 
the Class of Amphibia there are no new pieces formed beyond 
these, but they exhibit a great variety of forms, and become 344 
THE HISTORY OF THE HUMAN BODY 
especially complex in the Anura, where they are employed in 
the production of various sorts of notes used as sexual calls. 
The muscles associated with these skeletal elements consist 
originally of a pair of dilatators, which are attached to the 
outer edges of the cartilages and serve to draw them apart, 
and a double pair of adductors, the laryngei, which stretch 
across from one to another and serve to approximate 
them. These give rise in many of the more complicated 
cases to an entire system of muscles mainly connected with 
the arytaenoid cartilages, which form the essential skeletal or- 
gan of the larynx, and, to which the vocal cords in the form 
of mucous folds become attached. 
In the Sauropsida there are two conspicuous points of ad- 
vance; the one concerns the larynx; the other the trachea. The 
first consists of the addition of the 4th pair of branchial car- 
tilages, which become reduced in size, unite in the middle and 
form a triangular flap, the epiglottis; this, during passive 
breathing, stands erect above the glottis but shuts down over 
the latter during the act of swallowing, thus preventing the 
entrance of solid food into the trachea. The second advance 
consists of the presence of a series of rings of approximately 
equal size, which embrace the trachea and protect it from 
collapse. These are deficient behind, where the trachea comes 
in contact with the oesophagus, as a provision to allow the 
passage of a large mouthful, but are strongly developed in 
front and serve to keep the trachea distended. The most an- 
terior of these rings is much heavier than any of the others 
and is probably formed by the fusion of several of them. It 
is known as the cricoid cartilage and is, topographically con- 
sidered, a part of the larynx. The tracheal rings must have 
been developed in some way from the tracheal pieces that 
segmented off from the lateral cartilages, but the manner of 
their formation is not known. Similar rings occur in the 
trachea of the Gymnophiona (Coecilia) the rare Order of sub- 
terranean amphibians, but whether these are homologous with 
those of the reptiles and birds has not yet been determined. 
There is but little variation in laryngeal form among the THE DIGESTIVE AND RESPIRATORY SYSTEM 
345 
representatives of the Sauropsida, and this in spite of the great 
differentiation of voice in the case of the birds, since in these 
the voice is produced, not by the larynx, but by a special organ, 
the syrinx, or lower larynx, situated at the forking of the 
bronchi and not found outside of the group of birds. 
In mammals a conspicuous addition is seen in the thyreoid 
cartilage. The origin of this piece is not apparent in pla- 
cental mammals, in which it appears as an extensive shield, 
covering the ventral surface of the entire organ, but in the 
more primitive monotremes, instead of the single shield-like 
piece, there are two pairs of narrow bars which from their 
origin and their similarity to the more anterior ones, as well 
as from their mode of development, are clearly seen to be 
branchial arches, evidently the 2nd and 3rd (Fig. 117). This 
leaves only the first arch, which in this Order unites with the 
true hyoid arch to form the hyoid complex (“ hyoid bone ” 
of human anatomy), to which it contributes the posterior 
cornua, the thyreo-hyals. The cricoid cartilage is much as in 
Sauropsida and is manifestly the result of the consolidation 
of certain of the upper tracheal rings. 
The development of the lungs is mainly along the lines of 
physiological efficiency through a repeated subdivision of the 
interior. This results in the production of small chambers, 
commonly known as “ air-cells,” more properly alveoli, which 
are connected with the bronchi by means of numerous smaller 
branches. The walls of these alveoli are covered with a net- 
work of capillaries, thus making them the ultimate organs 
of respiration to which all other parts are accessory. Pri- 
marily there are no cartilages in the lungs themselves, but 
in reptiles they may be seen to develop along the course of 
the bronchi and invade the lungs; in mammals the smaller 
bronchial tubes are similarly equipped, almost as far as the 
ultimate branches, although in the course downwards the rings 
become less complete and are finally reduced to irregular 
pieces lying in the sides of the tubes. The smallest tubules, 
which are without cartilaginous pieces, are termed bronchioli. 
In birds and in many mammals the lungs are subdivided 346 
THE HISTORY OF THE HUMAN BODY 
by grooves into lobes, but in other cases the grooves are shal- 
low, and the lobes become hardly more than slight protuber- 
ances. Although quite constant in number and arrangement 
in a given mammal there is the greatest variation of the lobes 
in forms not closely related; and that these parts are of slight 
physiological importance is shown by their complete absence 
in mammals quite unlike structurally and occupying different 
(a) Ventral, (b) Luteral. 
St. H, stylo-hyal; EH, epi-hyal; CH, cerato-hyal; BI1, basi-hyal; Th. H, thyreo-hyal; Thy, I, 
first thyreoid cartilage; Thy, 2, second thyreoid cartilage. 
Fig. 117. Larynx of Echidna (monotreme). [After GceppertJ 
environments. Thus the lungs are without lobes in the Cetacea, 
Sirenia, and some seals, suggesting a modification due to an 
aquatic life, but on the other hand the lungs are similarly 
undivided in sloths and ant-eaters, and in certain rodents, 
as mice and squirrels. The left lung in the elephant is also 
without lobes. 
The development of a diaphragm in mammals separates the 
general body-cavity into thoracic and abdominal portions and 
cuts off the pleura, which invests the lungs and lines the tho- 
racic cavity, from the peritoneum, which stands in similar 
relationship to the abdominal viscera. These changes cause 
some variation in the mechanism of breathing, in which the 
diaphragm becomes a powerful accessory organ. CHAPTER VIII 
THE VASCULAR SYSTEM 
“ However, if we consider that all the characteristics 
which have been cited are only differences in degree of 
structure, may we not suppose that this special condi- 
tion of organization of man has been gradually acquired 
at the close of a long period of time, with the aid of 
circumstances which have proved favorable? What a 
subject for reflection for those who have the courage to 
enter into it! ” 
Lamarck in Recherches sur l’Organization 
des corps vivans. 1802. Transl. Packard, 
1901. 
A vascular system of some sort occurs in all coelomate 
animals, except in some reduced parasitic forms, and consists 
essentially of a cavity, or series of connected cavities, in which 
a fluid circulates, containing detached cells of one or more 
kinds. Both fluid and cells are concerned in metabolism 
and act as carriers of material both to and from the various 
tissues. In many Metazoa, especially the smaller and less 
highly organized ones, the system is lacunar, and the circu- 
lating medium, here often termed the perivisceral fluid, occu- 
pies everywhere the irregular spaces between the organs, and 
its circulation is furthered by the movements of these latter 
and of the entire body; in other cases the lacunar system be- 
comes reinforced, or largely replaced, by the formation of 
definite channels in the form of branching tubes, through 
which the fluid circulates. Such a circulation is said to be 
closed, as distinguished from the lacunar or open type first 
mentioned, and in such a system, deprived as it is of the pro- 
pelling power insured by the movement of external parts, de- 
pendence must be placed upon some intrinsic force within the 
vascular system itself, and thus there arise pulsating vessels, 
certain localized portions of the system of tubes, the walls of 
347 348 
THE HISTORY OF THE HUMAN BODY 
which are caused to dilate and contract rhythmically through 
the development of a layer of involuntary muscles. 
Vertebrates possess the tubular or closed type of vascular 
system, reinforced by a few definitely localized lacunae, and 
indirectly aided by the various serous cavities of the body like 
the coelom and the capsules of the joints. Both anatomically 
and physiologically this system is divided into two subordi- 
nate systems, haemal and lymphatic, of which the first is the 
one principally emphasized, while the other bears to it the 
relationship of an important auxiliary. The tubules of the 
first system are divided into the heart, a localized pulsating 
vessel with enormously hypertrophied muscular walls; arteries, 
in which the current flows from the heart; veins, in which the 
current flows toward the heart; and lastly the capillaries, the 
minute vessels which connect the arteries and veins and supply 
every tissue of the body. To these, which collectively bear 
the name of blood-vessels, there are associated a few definitely 
bounded lacunae, spaces limited by membranes, and mainly 
differing from the rest of the system, with which they are con- 
tinuous, by the absence of walls of their own. The circulatory 
medium contained in this system is termed blood, and consists 
of two main types of cells, the erythrocytes or “ red blood 
corpuscles,” and the leucocytes or “ white blood corpuscles,” 
suspended in a liquid plasma. The leucocytes are further sub- 
divided into three distinct types; monocytes (or histiocytes), 
granulocytes, and lymphocytes. 
The auxiliary system consists primarily of lymphatic vessels, 
which in distinction from the veins and arteries are small and 
thin-walled, and of lymph glands, which are not glands at all, 
but factories for the manufacture of lymphocytes. There are 
also lymph capillaries which are distinguished from blood 
capillaries by their larger diameter, and extremely thin walls. 
With the lymphatic system are associated the serous cavities 
of the body (coelom, capsules of joints, bursae about the larger 
tendons, etc.), with which numerous lymphatic vessels com- 
municate so that by a physiological though not a morphological 
right, these cavities have been considered as expanded lym- THE VASCULAR SYSTEM 
349 
phatic vessels. In the lower vertebrates a number of definitely 
located pulsating organs, or lymph hearts, further the circula- 
tion of the liquid medium. This medium is termed lymph, and 
consists of plasma containing a large number of lymphocytes, 
together with a few of the other types of blood cells. 
These two latter elements constantly escape from the blood 
through the walls of the capillaries during the process of 
feeding the tissues, and it is one of the functions of the lym- 
phatic system to collect these by means of its smaller vessels 
and eventually to return them to the blood. The other main 
function of the lymphatic system is to aid in the extraction 
of digested foods from the alimentary canal and convey them 
also to the circulatory system. 
In no system of the body does the embryonic record tell 
the story of the race development so completely as in the 
case of the circulatory system, and although the change in 
vertebrate history from water to air, replacing one set of 
respiratory organs by another, has profoundly modified the 
blood-vessels, yet even this change is repeated with great 
fidelity in the individual life of each of the higher vertebrates. 
This might be expected of the amphibians, in the most of 
which the actual change of external environment is individually 
experienced, yet a similar metamorphosis in the circulatory 
system takes place in Sauropsida and Mammalia, although 
it is confined to embryonic life. 
Since this is so, the best introduction to the history of the 
circulatory system is that furnished by embryology, the early 
part of which may be here given in the form of a general 
sketch. This, although not intended to represent the details 
of development in any one animal, or even of any one group, 
yet is based rather more upon the development of the higher 
vertebrates, since in these alone is the story complete. In 
beginning this sketch a certain characteristic of nearly all 
vertebrate embryos must be emphasized, since it is closely 
connected with the circulatory system, especially in its earlier 
stages, and that is, the extra-embryonal yolk-sac, which de- 
velops a set of blood-vessels for the purpose of feeding the 
embryo. 350 
THE HISTORY OF THE HUMAN BODY 
In this is seen a probable reason why the history of this 
system is retained in so much more perfect condition than 
are most of the others, and that is, because this system is 
actively junctional almost from the beginning of embryonic 
life, while in other cases the parts lie passive and let themselves 
gradually assume the final shape without contributing anything 
to the functional life of the organism, a condition most favor- 
able to the suppression of intermediate stages. 
The early vertebrate embryo, during its cleavage stages, ap- 
pears most frequently as a circular disc of rapidly proliferating 
cells floating on the surface of a spherical or spheroidal yolk- 
mass; and although at first these cells possess sufficient energy 
within themselves to continue development, there soon comes 
a time at which they become dependent upon the nutriment 
stored in the yolk, and it is thus one of the earliest cares of 
the organism to develop blood-vessels for the purpose of carry- 
ing yolk granules to the embryonic area. These blood-vessels 
first appear as irregularly branching spaces on the surface of 
the yolk beyond the limit of the definite embryo; these spaces 
soon form themselves into a capillary net-work and unite upon 
each side of the embryo into a vitelline vein. 
Within the embryo a similar process lays down the first 
blood-vessels and the entire system appears as in Fig. 118, A. 
The two vitelline veins unite into a median vessel, the future 
heart, situated ventrally with reference to the embryo, and 
immediately back of the future gill region. Still further an- 
teriorly the median vessel divides and forms two lateral loops, 
the first arterial gill-arches, which continue around the pharynx 
until they come in contact with one another upon the ventral 
side of the notochord, from which point they run posteriorly, 
forming two aortce. 
At a point a little posterior to the entrance into the embryo 
of the vitelline veins, the aortae pass mainly into the forma- 
tion of two vitelline arteries, which spread out over the yolk, 
but the small vessels which continue into the posterior end 
of the embryo form morphologically their real continuation. 
During later development the posterior aortce fuse into one THE VASCULAR SYSTEM 
351 
and increase greatly in size so that the proportions between 
them and the vitelline arteries become reversed; and as this 
part of the embryo expands and develops legs, tail, and pelvic 
organs, these latter become supplied by secondary branches 
from thei main trunk. A similar arterial supply to the head 
region is furnished by the artery which develops anteriorly 
from the arterial gill-arch. In the figure it appears as a mere 
Fig. 118. Early circulation of vertebrate embryo. 
(A) First appearance of definite vessels. (B) Later stage, during the formation of gill 
arteries. 
AC, carotid artery; A, dorsal aorta; AV, vitelline artery; VV, vitelline vein; H, heart. 
stump, but is destined to become the carotid artery, which in- 
creases in size and the complexity of its branches in exact 
proportion to the development of the part which it supplies. 
As these last two vessels, carotid artery and posterior aorta, 
distribute the blood outside of the main channel, a new set of 
vessels must be developed to bring it back again and thus 
complete the circuit. Those appear in the form of the four 352 
THE HISTORY OF THE HUMAN BODY 
cardinal veins, two anterior and two posterior (not shown in 
the figure), which collect the blood sent to the growing tis- 
sues of the embryo by the arteries and return it into the main 
channel. The anterior and posterior cardinals of each side 
unite opposite the heart and form a lateral vessel, the duct of 
Cuvier (ductus Cuvieri), which enters the heart from the side 
immediately after its formation through the union of the 
two vitelline veins. In this system of vessels is seen the first 
systemic venous system, the function of which is to collect 
from the body the blood supplied it by the arteries and return 
it to the heart. 
A considerable advance is seen in Fig. 118, B, which repre- 
sents a somewhat older embryo. The heart has increased both 
in caliber and in length, which has caused it to assume a 
contorted attitude, the prelude to those later changes which 
will result in the formation of a compact organ with 
definite compartments. To the single arterial loop which 
forms the first arterial arch in the gill region others have been 
added in a posterior direction, the general method of forma- 
tion being shown by the last one, in this figure the 5th. The 
appearance of limbs has caused the development of arteries to 
supply them, subclavians for the anterior, and iliacs for the 
posterior; these are duplicated by veins associated with the 
cardinal system. 
At about this stage a striking change, but one the signifi- 
cance of which is mainly embryonic, consists in the develop- 
ment of the bag-like allantois with its accompanying blood- 
vessels, the allantoic (or umbilical) veins and arteries (see 
Fig. 18). This appears indeed in amphibians as an evagina- 
tion from the ventral wall of the cloaca, where it functions 
as a urinary bladder, but here it never surpasses the limits of 
the body; in Sauropsida and Mammalia, however, it develops 
into an enormous extra-embryonal organ of great functional 
importance. It begins in the form of a small sac that pushes 
its way out from the embryo, and is supplied with two arteries 
from the posterior aorta, and two veins which enter the heart 
in association with the vitelline vein, but it soon increases THE VASCULAR SYSTEM 
353 
greatly in size, and its blood-vessels increase proportionately. 
Ultimately, in Sauropsida, animals with very large eggs en- 
cased in a porous shell, the allantois comes to line the entire 
shell and serves as the embryonal respiratory organ; in mam- 
mals it forms the main part of the placenta and umbilical cord, 
and functionally replaces the yolk sac, which is here a useless 
rudiment, although equipped with its full complement of 
blood-vessels. In both cases the allantois is cast away from 
the embryo at birth, haemorrhage being prevented by an atro- 
phy of the blood-vessels at the point at which they leave the 
body. 
Further important modifications of the circulatory system 
are caused by the development of liver and kidneys and by 
the increase in bulk of the intestine. Owing to an original 
continuity between the yolk sac and the intestine, the veins 
from this latter organ empty into the vitelline veins, forming 
a compound vein, composed of intestinal and vitelline branches, 
the omphalo-mesenteric. Of these the right one does not de- 
velop beyond a certain point, and the main, and ultimately 
the entire, duty falls upon the left. About this the develop- 
ing liver grows, and in such a way as ultimately to include it 
within its substance, and as a result of this that part of the 
vein which runs through the liver becomes divided into a 
system of capillaries. The result of this is that the blood 
coming from both yolk and intestine has no longer any way 
of getting directly into the heart through a large vessel, but 
must first pass through the capillary system of the liver, and 
be re-collected upon the other side. From this stage on the 
single omphalo-mesenteric vein, that originally of the left side, 
becomes known as the portal vein, and the collecting vein upon 
the other side of the liver, which brings the blood from that 
organ into the heart, forms the hepatic. Throughout this 
portion both of the original vitelline veins are preserved, and 
it thus happens that there are two hepatic veins, but only one 
portal. 
A similar change is that inaugurated by the development of 
the embryonic kidney. The blood comes back from the tail 354 
THE HISTORY OF THE HUMAN BODY 
in a median caudal vein, which, posterior to the cloaca, divides 
into two branches. These pass along the outer sides of the 
kidneys and are resolved entirely into a set of small branches, 
the venae renales advehentes, which enter these organs and 
break up into capillaries. From these the blood is re-collected 
by veins which emerge from their inner edges, the venae renales 
revehentes, which unite to form the posterior cardinals. 
There is thus formed a portal system similar to that of 
the liver, and called the renal portal, in distinction from the 
latter, the hepatic portal. This relationship is a permanent 
one in fishes and amphibians, but in the Sauropsida and Mam- 
malia the kidneys in which this portal system is developed 
function as such only in the embryo, and become eventually 
replaced as kidneys by a new organ in connection with which 
no such portal system becomes developed. 
Thus far in the development of the circulatory system all 
Classes of vertebrates agree, allowing for slight differences 
in the relations of the extra-embryonal parts, such as the rela- 
tive development of the allantois, or the amount of yolk; one 
Class, the lowest, that of fishes, remains permanently at this 
stage, while the others progressively modify the fundamental 
plan. It may be well, then, to consider the condition in the 
adult selachian, which is practically that of the foregoing 
sketch, after which the later development of the various parts 
of the system may be taken up one after another. 
The circulatory system of the selachians is represented in 
the accompanying diagram (Fig. 119), in considering which 
the reader may begin at the heart and trace the vessels an- 
teriorly. The heart is situated very far forward, immedi- 
ately behind the gills, its embryonic position in higher animals, 
and consists of four chambers arranged in a single longitudinal 
row along the median line. The most posterior of these, the 
sinus venosus, is the receptacle into which is brought the im- 
pure blood from all parts of the body. Next in order, into 
which the blood passes in succession, are the atrium, the 
ventricle and the conus arteriosus. This last and most an- 
terior compartment is prolonged into an arterial trunk (truncus THE VASCULAR SYSTEM 
355 
arteriosus), which breaks up into paired lateral branches, the 
afferent branchial arteries. These pass along the cartilaginous 
Fig. i 19. Diagram of primitive vertebrate circulation, based on the 
condition found in selachians. 
s, sinus venosus; t, atrium; v, ventricle; r, conus arteriosus; br, branchial arteries; ad, carotid 
artery; aoa, aortic arch; aod, dorsal aorta; ce, coeliac axis, consisting of (m) mesenteric, and 
hepatic and splenic (unmarked) branches; cau, caudal arteries and veins; il, iliac arteries and 
veins; sb, subclavian arteries and veins; ra, renales advehentes; rr, renales revehentes; l, lateral 
vein; cp, posterior cardinal vein; ca, anterior cardinal vein; p, hepatic portal vein; h, hepatic vein. 
gill-arches and supply the gills, dividing into very fine branches 
for the purpose. Thus far the blood is impure, in the state in 356 
THE HISTORY OF THE HUMAN BODY 
which it was received from the body, but at this point there 
intervenes a system of capillaries, in which the exchange of 
respiratory gases takes place, and when it is re-collected 
into the efferent branchial arteries, one pair less than the 
afferent branchials, the blood has become aerated. These 
latter arteries converge to the median line, where they unite 
to form a median aorta, which lies upon the ventral side of the 
vertebral centra, and gives off the main arteries of the body. 
Before the arches of the two sides unite they give off the 
carotid arteries, which supply the head and brain; and then, 
not far from the point of union, the subclavians, to the an- 
terior paired limbs (pectoral fins). Lower down appear 
branches that supply the body walls and the viscera; and the 
posterior paired limbs (ventral fins) are supplied by the 
iliacs. As these branches are given off, the aorta diminishes 
in size and terminates at the end of the tail as a mere thread, 
protected throughout the caudal region by the haemal arches 
of the vertebrae. 
The entire body is thus supplied with aerated blood from 
a single main channel with its branches, but on its return its 
course is not so simple, and involves three distinct venous 
systems connected with one another by capillaries. The first 
of these consists of four great longitudinal veins, the two 
anterior and the two posterior cardinals, which collect the 
blood from the head, the anterior fins, and the walls of the 
trunk. As in the embryological sketch, the anterior and pos- 
terior cardinal veins of each side unite into a ductus Cuvieri, 
which enters the sinus venosus. Associated with the posterior 
cardinals are the two large lateral veins which lie in the body 
wall and were perhaps originally situated along the bases of 
the lateral fin-folds, from which the paired limbs have been 
derived. They arise as very small vessels along the sides of 
the tail and enlarge rapidly as they proceed anteriorly through 
the assumption of tributary branches from each somite. In the 
cloacal region the two lateral veins communicate by numerous 
anastomosing branches, forming a cloacal plexus (represented 
in the diagram by a single vein), and receive the iliac veins THE VASCULAR SYSTEM 
357 
from the posterior fins. Anterior to this they still receive meta- 
meric contributions from each somite and finally empty into 
the posterior cardinals near their fusion with the anterior 
ones to form the ductus Cuvieri. The subclavian vein from 
the anterior fin enters either the lateral vein or the posterior 
cardinal near the entrance of the latter. In the former case, 
which may be considered the more primitive, we have the 
suggestion of the early relation of the lateral vein to the 
fin-fold, for this condition suggests strongly a primitive one 
in which the lateral vein received a branch from each meta- 
meric element of the fin-fold. When the definite limbs were 
established by the hypertrophy of an anterior and posterior re- 
gion and the loss of the intermediate portion, the veins corre- 
sponding to the regions retained became large and important, 
while the rest were somewhat reduced. To account for the 
retention of a single vein for each appendage, rather than one 
from each somite represented, one may suppose either the re- 
tention of one and the loss of the others, or the fusion of 
several. Since, in the pelvic fin of the skate, there are, in 
addition to the principal iliac vein, one or two small vessels 
which open independently into the lateral vein, the former 
alternative is the more probable. 
The second system begins by a median caudal vein, which 
starts at the tip of the tail and runs within the haemal arches, 
upon the ventral side of the aorta. When near the cloaca 
this vein divides into two lateral branches, which run along 
the lateral margins of the long and narrow kidneys, and give 
off to these organs numerous lateral branches, the venae renales 
advehentes. These break up into a capillary system within the 
substance of the kidneys and form the renal portal system. 
From this capillary net-work the blood is collected along the 
medial margin of the kidneys by numerous venae renales 
revehentes, the union of which into a common trunk forms 
the origin of each posterior cardinal. 
The third, or hepatic portal system, is exactly as given in the 
embryological sketch. It collects the blood from the intestines 
and stomach into a common trunk, the portal vein, which 358 
THE HISTORY OF THE HUMAN BODY 
enters the liver upon its dorsal side and becomes resolved into 
capillaries, as in the former case. From this organ the blood 
is re-collected by one or more hepatic veins, lying more on the 
ventral side of the liver, and is emptied into the sinus venosus. 
Thus all the impure blood, through one channel or another, 
finds its way into this most posterior chamber of the heart, 
from which it passes in succession through atrium, ventricle, 
and conus arteriosus, and finally into the gills, where it be- 
comes aerated. 
It thus happens that the heart contains only impure or 
venous blood, since it is not purified until it reaches the gills, 
which suggests that the terms “ arterial ” and “ venous,” as 
applied to pure and impure blood respectively, are not applica- 
ble in the case of the lower vertebrates, and are much better 
dropped, since they are often misleading. Furthermore, in 
Amphibia and Reptilia these two kinds of blood are not sharply 
defined, since both sorts are often allowed to mingle, forming 
a mixed blood of varying degrees of purity. All confusion on 
this point, however, may be avoided if the terms artery and 
vein and their corresponding adjectives are used in their an- 
atomical sense only, arteries being, as previously defined, those 
vessels in which the blood flows from the heart, and veins 
those in which the blood flows towards the heart. The physio- 
logical distinction which designates pure blood as arterial and 
impure blood as venous is taken from its condition in the two 
sets of vessels in birds and mammals, and even here in the case 
of the pulmonary system the conditions are reversed and 
physiologically arterial blood flows in the veins, and physio- 
logically venous blood in the arteries. 
This history of the arterial arches is shown in synoptical 
form by the accompanying series of diagrams (Fig. 120), 
which present the facts as deduced from the combined study 
of both the adult anatomy and embryological development of 
representatives of each Class of vertebrates. The diagrams 
represent the adult conditions in each case, the relationship 
being morphologically interpreted by the help of the develop- 
ment. THE VASCULAR SYSTEM 
359 
There are typically six 
pairs of arterial arches, 
which lie along the sides 
of the pharynx and extend 
from a ventral vessel that 
proceeds directly from the 
heart to a dorsal one that 
collects the blood from 
the arches and conveys it 
to all parts of the body, 
the ventral and dorsal 
aortse respectively (Fig. 
120, a). In all the dia- 
grams the parts of both 
sides are shown, viewed 
ventrally and flattened 
out so that the ventral 
aorta lies in the middle 
and the dorsal aortae con- 
verge from the outer 
sides. In selachians (Fig. 
120, b) five of these 
arches are present and 
functional; each arch is 
divided into an afferent 
and an efferent branch, 
between which respiration 
is effected by means of 
capillaries spread out over 
soft endodermic gills. 
From the anterior portion 
of the efferent system the 
carotids are given off, ves- 
sels which include the 
only remnants of the first 
arterial arches. 
In the urodelous am- 
Fig. 120. Diagrams showing modifi- 
cations in the arterial arches of Ver- 
tebrates. 
(a) Typical, embryonic, (b) Fishes, (c) 
Amphibians. (d) Reptiles. (e) Birds, (f) 
Mammals. 
1, 11, III, IV, V, Vl, arterial arches; cd, 
Art. carotis dextra; cs, Art. carotis sinistra; 
sd, Art. subclavia dextra; ss, Art. subclavia 
sinistra; ad, Aorta dextra; as, Aorta sinistra; 
bd. Ductus Botalli dexter; bs, Ductus Botalli 
sinister; pd, Art. pulmonalis dextra; ps, 
Art. pulmonalis sinistra. 360 
THE HISTORY OF THE HUMAN BODY 
phibians (Fig. 120, c) the first two arterial arches disappear 
in the embryo, leaving four functional arches. Of these arch 
III unites with remnants of I and II to form the carotids, IV 
and V on both sides form complete arches and unite to form 
the dorsal aorta, and VI becomes the pulmonary (here the 
pulmo-cutaneous). Of the two aortic arches IV is the prin- 
cipal one and V is a subordinate, and is of such slight func- 
tional importance that in the higher Classes it is destined to 
disappear altogether. These arches are usually continuous, 
and are not as a rule interrupted in the midst by the inter- 
position of respiratory capillaries as in fishes; in larval uro- 
deles, however, and in a few adult forms, the perennibranchs, 
a branch from the ventral side of the arch supplies the ex- 
ternal gill-bushes with capillaries, from which a collecting 
branch returns the blood to the arch at its dorsal end. When 
such a gill-bush is of much functional importance these lateral 
branches are large, and in extreme cases it is possible that 
practically all the blood of a given arch may pass through 
these indirect channels. In most cases the external gills, and 
with them their supplying branches, disappear at the expira- 
tion of larval life, and the arches form continuous vessels, as 
in higher forms. The sixth arch is in the larva a complete one, 
and joins the dorsal aorta, as do the two preceding; with the 
development of the lungs and the integumental respiration a 
small branch, which arises from this arch near the middle, 
becomes engaged in supplying the lungs and skin, and in- 
creases so much in size that it ultimately transmits all of the 
blood that enters the arch, leaving the distal half of the arch 
without employment. This part then closes its lumen and is 
retained as a connecting band, the ligamentum arteriosum 
[Botalli], extending along its old path between the pulmonary 
artery and the dorsal aorta. A similar ligament, or in many 
cases a small pervious artery, is also retained between the 
carotid arch and the main aortic arch (III and IV). 
In reptiles (Fig. 120, d) the metamorphosis of the arterial 
system is pushed back into embryonic life, and, from this point 
on, no longer appears after birth. In other words, the transi- THE VASCULAR SYSTEM 
361 
tion from water to land, an historic scene actually enacted 
during the post-natal existence of amphibians in the form of 
the metamorphosis, with all the changes involved, not only in 
the circulatory, but in other systems as well, is pushed back 
among the stages that are recapitulated in the embryo; there 
is a metamorphosis in reptiles and mammals just as truly as 
in the case of amphibians, but it is embryonic. Here, as in 
amphibians, arch III, with rudiments of I and II, forms the 
carotids and its connection with arch IV disappears. This 
latter becomes the aortic arch and is retained on each side, 
as right and left arches, the two uniting dorsally and back of 
the heart to form the main aorta. Arch V, which in am- 
phibians is practically superfluous, is given up in most reptiles, 
and from this point on is seen no more, save in the embryo, 
where it oftens appears as a rudiment. A pulmonary artery 
develops from the sixth arch of each side, as in amphibians, 
leaving a right and left ligamcntum arteriosum [Botalli\. The 
subclavian arteries, that supply the fore-limbs, and which in 
most fishes and amphibians arise from the dorsal aorta after 
the union of the two lateral arches, possess a more anterior 
origin and arise from the right aortic arch. Crocodiles and 
turtles present an exception to this, and in these, as in birds, 
the subclavians arise from the base of the carotids, an origin 
so radically different as to lead morphologists to believe that 
these vessels are not the subclavians at all, but are secondarily 
developed arteries (subclavioe secundarice) which have func- 
tionally replaced the true subclavians. 
In birds and mammals (Fig. 120, e and f) but a single 
aortic arch comes to development; in birds this is the one on 
the right side, in mammals the one on the left, a convincing 
proof, if proof were wanting, of the independent development 
of these two Classes. There is thus, in each case but one 
ligamentum arteriosum, connecting the pulmonary and aortic 
arches. In the mammalian fetus, in which pulmonary respira- 
tion is not assumed till the moment of birth, this vessel is 
functional and is known as the ductus arteriosus [Botalli]. 
It is still pervious at birth, but the lumen closes within a few 362 
THE HISTORY OF THE HUMAN BODY 
days by the rapid thickening of the wall of the vessel. There 
is here to be noted an important difference also in the sub- 
clavians; in birds, as in the turtles, these vessels are repre- 
sented by subclavian secundarice, branches of the carotids, but 
in mammals they are the primary vessels, homologous with 
those of typical reptiles. There is, indeed, a new morpho- 
logical distinction between those of the two sides, for while 
the left one, that of the side which furnishes the aortic arch, 
arises from this arch, that of the right is the equivalent, not 
only of that of the left side, but of the fourth arch as well. 
It is this relationship which causes the intimate association 
upon the right side between the subclavian and carotid, and 
the short common trunk, arteria anonyma \innominata], is 
thus the region once common to arches III and IV. Thus, 
while these vessels on the right side are superficially similar 
in both birds and mammals they are morphologically totally 
different. In the former the “ subclavian ” is a secondarily 
formed branch of the carotid, with the true subclavian prob- 
ably suppressed; in the latter the “ subclavian ” is the fourth 
arch plus the true subclavian that once branched from the 
point where this arch joined an aorta, as on the other side. 
In this relationship is seen also the explanation of the curious 
asymmetry in the origin of the human carotids and subclavians, 
a condition which is undoubtedly a primitive one. This be- 
comes more complicated in many other more specialized mam- 
mals by various secondary approximations and fusions. Thus, 
in the Carnivora the left carotid fuses near its base with the 
other and produces the phenomenon of three of the four ar- 
teries in question arising from a common stump, while the left 
subclavian is alone distinct: in ruminants this latter also shifts 
its position forward and fuses with the others, so that a single 
median trunk arises from the crest of the arch. This gives 
off first the two subclavians, and then, after continuing for- 
ward a little, divides into the two carotids. 
Ontogenetically the most anterior of the six arterial arches 
is the first to appear, and this, with the ventral and dorsal 
aortae, forms a lateral loop directed forwards. The other THE VASCULAR SYSTEM 
363 
arches are successively added through the formation along the 
course of the aortse of buds or sprouts that meet and join. In 
fishes all but the first of the arches are retained, but in higher 
forms, where the first two become lost, these begin to degen- 
erate before the more posterior ones appear. In the mam- 
malian embryo there are in the appearance of these arches 
two important points to notice; first, the successive supremacy 
in size and function of each arch down to the fourth, and, 
second, the extremely rudimentary condition of arch V, 
amounting in some cases to a complete suppression. This is 
shown in the rabbit embryo in the four states given in Fig. 121. 
In a arch I is the principal, or, in fact, the only functional 
one, and II and III are forming from approximated dorsal 
and ventral buds. In b arch I has become disintegrated, 
while the chief function is assumed by arch II: the ventral 
bud of arch IV is also seen. In c both first and second arches 
are lost and their remnants appear in part as continuations of 
dorsal and ventral aortae and in part as stumps of vessels from 
which important branches of the carotid system are to be de- 
veloped. Arch V appears at about its maximum here and in 
the next figure, and the ventral bud of arch VI has become 
well developed. In d arch IV, to be later the permanent 
aortic arch, is assuming good proportions, and arch VI, the 
future pulmonary arch, is completed. From the dorsal side 
of the dorsal aorta appear the beginnings of certain transitory 
arteries that correspond to the head somites. 
The complete ontogenetic history of the carotid system in 
mammals shows a curious shifting of branches from one source 
to another, and the development and decay of transitory ele- 
ments. In this the remnants of arches I and II, long sup- 
posed to be lost, play a prominent part, and unite with the 
main carotid arch (III) in the formation of the system. This 
history together with that of the other arterial arches, is seen 
in the accompanying set of diagrams (Figs. 122 and 123), 
based, with the exception of the last one, upon the embryology 
of the rat. In a is seen the first arch [Cf. Fig. 118, a], which 
starts from the conus arteriosus (ca), proceeds forward, and 364 
THE HISTORY OF THE HUMAN BODY 
Fig. i2i, Development of the arterial arches in Rabbit. [After Miss Lehmann.] 
(a) Ninth day. (b) Tenth day. (c) Eleventh day. (d) Eleven and a half days. 
I. II, III, IV, V, VI, branchial arches; tr, truncus arteriosus; ad, dorsal aorta. THE VASCULAR SYSTEM 
365 
returns as the dorsal aorta {ad), giving forth a carotid, the 
arteria carotis cerebralis {ccb), just before returning. The 
ventral bud of arch II has also appeared. In b arch II has 
become completed and arch III is forming from ventral and 
dorsal buds and in c, arches I, II, and III are complete, with 
a ventral bud forming for arch IV. Arches IV and VI are 
both formed in d, with several “ islands ” in the former, which 
probably have no especial significance. Arch I has broken, 
leaving dorsal and ventral stumps. 
From the dorsal aorta appear segmental arteries (5) which 
soon disappear; of these the lowest is the hypoglossal {hy). 
Below this are other segmental arteries, of which the first 
cervical {ec) is figured here. Stage e shows but little change 
save in proportions and the loss of segmental arteries. A 
pulmonary artery appears, arising from arch VI. The human 
embryo at about this stage shows a well-developed arch V. 
The segmental arteries of the head have disappeared. In / 
arch II has also broken through, leaving stumps, and of these 
the ventral becomes closely associated with that of arch I, and 
both are borne by the anterior end of the ventral aorta (av), 
a part destined to play an important role later on. Arch III 
has become large; arch IV is very large, and a rudiment of 
arch V has appeared. The dorsal stump of I has divided 
into two branches, the maxillaris {ms), which goes to the de- 
veloping upper jaw, and the mandibularis {md), which be- 
comes distributed to the lower jaw. 
In stage g the maxillary artery just mentioned has divided 
again into a supra-orbital {0) and an infra-orbital {i), thus 
giving three terminal branches of the dorsal stump of arch I. 
From the free end of the ventral aorta {av) appears a branch 
that goes to the tongue-anlage, the lingualis (/). The point 
especially to be noted here is that of the two buds, (x) and 
(y), which arise from the dorsal stumps of arches I and II, 
respectively, and grow toward one another. The formation of 
the arteria vertebralis cerebralis {vc) by the union of the 
hypoglossal and first cervical arteries with one from much 
further forward is also to be noticed, but is without special in- 366 
THE HISTORY OF THE HUMAN BODY 
Fig. 122. Development of arterial arches in Rat embryo. [After 
Tandler.] 
I, II, III, IV, V, VI, represent the respective arches or their rudiments; a, 
conus arteriosus; ad, dorsal aorta; av, ventral aorta (truncus arteriosus); ccb, art. 
carotis cerebralis; p, arteria pulmonalis; ec, first cervical artery; s, segmental arteries, 
hy, hypoglossal artery; ms, maxillary artery; md, mandibular artery; o, supra- 
orbital artery; i, infra-orbital artery; l, lingual artery; vc, art. vertebralis cere- 
bralis; x, y, stumps which ultimately join, and form the stapedial artery, (st.) THE VASCULAR SYSTEM 
367 
terest. In stage h the buds (x) and (y) have united the dorsal 
buds of I and II, and the significance of this step is seen by 
comparing this with stage i, for here the portion connecting the 
common origin of the supra- and infra-orbital and mandibular 
arteries has become lost and their source of supply has become 
Fig. 123. (h)-(l), Continuation of the series given in Fig. 122. (m), 
Ultimate condition in Man, for comparison with (1). [All figures after 
Tandler.] 
xy, the artery formed by the union of x, and y, in the previous figure; cc, 
common carotid artery; ce, external carotid artery; ci, internal carotid artery; st, 
stapedial artery; n, anastomosing branch between the external carotid and mandib- 
ular arteries. The other abbreviations are given under Fig. 122 or are explained 
in the text. 
transferred to the dorsal stump of II. The artery thus formed 
penetrates the mass of cells destined to become the stapes and 
forms the foramen characteristic of this bone in the higher 
Mammalia. In the monotremes, where this action does not 
take place, the bone is columnar, and without a foramen. 
From now on the artery formed by the dorsal stump of arch 368 
THE HISTORY OF THE HUMAN BODY 
II, the anastomosing branch (xy), and a bit of the dorsal 
stump of arch I, becomes called the stapedialis (st), through 
its relationship to the stapes. How it comes to bear the three 
important branches of the later external carotid has been 
already seen. 
Between stages h and i a second important change has 
been inaugurated in the reduction of that part of the dorsal 
aorta which connects arches III and IV. This finally effects 
a complete separation of the two arches in this place, and 
causes the third arch to become a common carotid artery (cc) 
which divides into two branches, an external carotid {ce) 
which was formerly the anterior part of the ventral aorta 
plus the ventral stumps of arches I and II, and an internal 
carotid (ci), the main third arch plus the original arteria 
carotis cerebralis. 
One more change in relationship is to be effected, and that 
is inaugurated through the growth of another anastomotic 
branch (n in stage i) which enters the side of the mandibularis 
(or, perhaps, the continuation of the stapedialis) and forms a 
complete circuit, as in stage k. From this point on the history 
differs in the rat and in Man, as is indicated by the two arrows, 
with their respective designations. In the rat the circuit 
breaks at the point between the infra-orbital and the man- 
dibular, and in Man at a point above the supra-orbital. The 
two results of these are seen in diagrams l and n, which rep- 
resent the adult condition of this detail in the rat and in Man, 
respectively. In the former (/) the stapedial artery, a branch 
of the internal carotid, bears both supra- and infra-orbital 
arteries, while the external carotid becomes continued mainly 
into the mandibular. In the latter the external carotid bears 
all three of the branches in question, while the stapedial ar- 
tery, being of no further use, disappears, and leaves in the 
stapes the hole through which it formerly ran, thus account- 
ing for the particularly curious shape of this little bone, which 
attracted the attention of the early anatomists, but for which 
they had no explanation. In considering the details of the de- 
velopment of any part of the circulatory system, the process THE VASCULAR SYSTEM 
369 
is seen to be a metamorphosis, correlated with the changes 
in the parts supplied by the blood-vessels under consideration. 
Such a metamorphosis is like the changes in the roads and 
paths of a given district, due to a shifting of the centers of 
population, and the development or decay of any points of 
human interest. Changes like these set the traffic now over 
one, now over another, series of roads, which increase or de- 
crease in width and degree of development in exact propor- 
tion to this use, certain ones becoming highways and others 
lanes, solely through the functional importance of the locali- 
ties which they connect. Even the atrophied rudiments have 
their counterpart in the ancient roads, entirely overgrown and 
lost to all save the antiquary. 
The branches of the aorta posterior to the arterial gill- 
arches and their derivatives are sufficiently similar in all ver- 
tebrates to be easily recognized, but it may be said in general 
that, as is the case with other systems, these branches show 
many more indications of metameric arrangement in the lower 
vertebrates and are accordingly more numerous. Instances 
of this are seen in the numerous lateral and dorsal branches 
which supply the muscles of the body wall and are segmen- 
tally arranged in fishes and amphibians, while in higher forms 
their number is much reduced, forming the intercostal and 
lumbar arteries. It is again strikingly shown in the mesen- 
teric arteries which, in lower forms, are very numerous and 
suggest a metameric series, while in higher forms they are 
collected at their origin into a common trunk (Fig 124). 
The relative size of the various branches varies directly 
with that of the parts which they supply, a fact especially 
noticeable in the case of the subclavian and iliac arteries, 
which are small and unimportant in fishes, which possess small 
lateral fins, but which become excessively developed in connec- 
tion with the hypertrophy of one or both pairs of limbs. The 
caudal aorta, like the other elements of the tail, retains its 
primitive character and gives off metamerically arranged 
branches in the case of well-developed tails, in which the 
other parts are sufficiently emphasized to allow it. In Man the 370 
THE HISTORY OF THE HUMAN BODY 
caudal artery becomes reduced to the insignificant arteria 
sacralis media, in which the earlier anatomists failed to see 
the continuation of the aorta. This is in part due, however, 
to the enormous development of the legs correlated with the 
erect position, which has developed the iliac arteries out of 
all proportion, giving the erroneous but inevitable impression 
that these latter arteries form the real continuation of the 
aorta, which becomes bifurcated, and that the arteria sacralis 
media is an unimportant median branch arising from the point 
of bifurcation and supplying the coccygeal region. 
In the adult selachians, which in their venous system rep- 
resent practically the starting point of the history so far as 
Fig. 124. Metamerism in the mesenteric arteries of Amphibia. [After 
Klaatsch.] 
(a) Siren, (b) Necturus. (c) Cryptobranclius. (d) Cryptobranchus (a second 
specimen), (e) Anura. 
vertebrates are concerned, the two sets of cardinal veins, an- 
terior and posterior, are in control of the venous blood, except 
that from the alimentary canal, and return it from all parts 
of the body to the sinus venosus. However, during the em- 
bryological development of these animals one catches glimpses 
of a still earlier systemic vein, the sub-intestinal, which, here 
embryonic and transitory, must have preceded the cardinal 
system historically and have been totally replaced by the latter THE VASCULAR SYSTEM 
371 
before the advent of true vertebrates as we now know them. 
It appears soon after the establishment of the two yolk veins, 
always for practical reasons the first to appear in vertebrate 
embryos, and extends from the left yolk vein, from which it 
arises, to the tip of the tail, lying in the median line just ven- 
tral to the intestine. At the very first it consists of a pair of 
fine vessels running very near one another, but these soon 
coalesce into a single median vessel, much as in the case of the 
aorta. At the level of the cloacal opening the two original 
elements remain distinct, and run along the sides of the in- 
testine, but fuse again posterior to it, forming a loop or ring. 
Previous to this the cardinal system has begun its develop- 
ment in the form of minute vessels which grow out from the 
sides of the sinus venosus, and as they extend farther and be- 
come of larger size the free ends of the posterior cardinals 
form several anastomoses with the subintestinal vein anterior 
to the cloacal ring and at the place about which the kidneys 
(mesonephros) are to develop. This connection furnishes two 
large lateral channels for the blood from the subintestinal 
system, a change which has two direct results, first, the gradual 
usurpation of function of that part of the subintestinal vein 
which lies anterior to the anastomoses, a relationship that leads 
to its ultimate disappearance, and second, the retention of the 
part posterior to the connection as the caudal vein, now become 
a part of the cardinal system. At the point where the original 
anastomoses occur, the development of the kidneys causes the 
formation of a rich capillary net-work, a process which ends in 
the establishment of the renal portal system with caudal veins 
for conveying the blood to the kidneys and posterior cardinals 
for re-collecting it and conveying it to the heart. 
Although we know that stages like those just described 
no longer exist in living adult animals it is quite certain that 
in these embryonic changes an early phylogenetic history is 
recapitulated; that in some past group of animals, dimly 
foreshadowing the vertebrate type, a well-developed subin- 
testinal vein existed, and that the usurpation of its function 
by the cardinal system, repeated with great faithfulness to 372 
THE HISTORY OF THE HUMAN BODY 
detail in selachian embryos, was once actually experienced 
and slowly worked out in adult animals through the action 
of natural selection or whatever other forces are and have 
been in operation for the gradual improvement of organisms. 
As has been shown above, the Class of fishes comprises 
forms which have remained at the stage last described, the 
one in which the cardinal system holds the supremacy; but 
by the time the amphibians are reached there has been another 
usurpation in that part of the body posterior to the heart, 
and the posterior cardinal system has, in its turn, become 
subordinated to a third system, that of the vena cava posterior, 
or, more briefly, the postcava. How this appears in full func- 
tional activity is seen in the diagram representing the main 
venous channels of a urodele (Fig. 125, pc), where it has 
secured nine-tenths of the traffic between kidneys and the 
heart and allows but a small part to be conveyed by the pos- 
terior cardinals, formerly completely in charge of this terri- 
tory. Still another rival of the cardinal system has appeared 
in the abdominal vein (abd), which begins as two lateral veins 
that issue from the iliacs, run along the ventral abdominal 
wall until they meet in the median line, and continue as a 
single vessel until opposite the liver, when the vessel leaves 
the body wall, and enters this later organ, forming a part of 
the hepatic portal system. 
The origin of the vena cava historically cannot be now 
learned from adult anatomy since it undoubtedly took place 
in those forms which successfully achieved the transition from 
an aquatic to a terrestrial life, or to, at least, a paludic one, 
and, having left for their descendants this new world with 
its opportunities, perished and left no trace save in the per- 
fected parts which render a terrestrial life possible. 
Here again, however, embryology furnishes us with some 
information concerning at least the place and mode of origin 
of this new vein, as may be seen by a comparison of the dia- 
grams given in Fig. 126, d and e, where is shown the develop- 
ment of the postcava in the lizard. During the early stages in 
the development of the liver and its extensive system of capil- THE VASCULAR SYSTEM 
373 
laries, developed in association with the portal system to be 
considered later, the postcava appears as a partially distinct 
element in this capillary system, and becomes gradually more 
definite. This vein grows posteriorly and ultimately reaches 
Fig. 125. Venous system of urodeles, based on that of Desmognathus. 
[In part after (Mrs.) Anne Barrows Seelye.] 
ji, internal jugular; je, external jugular; sc, subclavian; pc, postcava; h, hepatic; 
pt, portal; g, gastric; si, splenic; abd, abdominal; ves, vesical; ec, epigastric; ms, 
mesenteric; il, iliac; c, caudal; z, anastomotic branch between the two caudals; 
ra, venae renales advehentes; cl, lateral cutaneous; cdp, cardinalis posterior; «, 
anastomotic branch between postcardinal and renal. The systemic veins are given 
in black; the portal system is in outline. 374 
THE HISTORY OF THE HUMAN BODY 
the renal portal system and the anastomoses between the 
caudal vein and the posterior cardinals. Here it is united 
with the posterior ends of these latter vessels and annexes 
them as well as the caudal vein to itself, thus establishing a 
single path from the end of the tail, running between the kid- 
neys, to the heart. The remainder of the posterior cardinals, 
Fig. 126. Development of the postcava and the hepatic portal system 
in the lizard (Lacerta). [After Hochstetter.] 
The figures (a) to (e) represent consecutive stages of development, s, sinus 
venosus; c, c, ductus Cuvieri; ud, right umbilical vein; us, left umbilical vein; omd, 
right omphalo-mesenteric vein; oms, left omphalo-mesenteric vein; pc, postcava; i, 
intestine; xv xv xv commissures between the veins of the two sides. 
anterior to the connection with the vena cava, becomes re- 
duced in proportion to the loss of function and the two remain 
either as small but continuous vessels, as in the Amphibia, or as 
the azygos veins, which continue to play a subordinate role by 
collecting the blood from the sides of the trunk, especially 
from the intercostal spaces, a function which they exercise in 
sauropsids and mammals. THE VASCULAR SYSTEM 
375 
In some details the developmental history of reptiles, birds, 
and mammals differs a little. There are, in fact, within the 
limits of each group, differences due mainly to the temporary 
formation of veins subsidiary to the main posterior cardinal 
channels, which come to enter in varying degrees into the 
definitive venous system of the adult. The general features 
of the mammal are, however, those shown in Figure 127 in 
which it may be seen that the posteriorly developing hepatic 
region of the postcava, by forming first an anastomosis with 
two small subcardinal veins medial to the posterior cardinals, 
Fig. 127. Development of the postcava in mammals. [After Hoch- 
Stetter.] (a)-(d), Rabbit; (e), Man. 
j, anterior cardinal (jugular); y, z, the two parts of the posterior cardinal, 
divided by the anastomotic vessel x; pc, postcava; zd, zs, right and left posterior 
cardinals, between the commissure x and their union posteriorly; in (e) the left 
one of these atrophies in part, the remainder becoming a portion of the spermatic 
vein st; k, kidney; s, suprarenal body. 
establishes eventually upon each side a direct connection with 
the posterior cardinals (stages a to c). This subcardinal an- 
astomosis, x, divides the original posterior cardinal into two 
parts, y and z. The former becomes reduced and enters into 
the formation of the azygos, while the latter develops as a 
part of the postcaval system. 
The later stages as shown in d and e, show first a fusion 
of the two lateral elements in the posterior region and finally 
the attainment of functional monopoly on the part of the 
right channel with the consequent atrophy of the left. This 376 
THE HISTORY OF THE HUMAN BODY 
involves, at least in the case of the cat and Man,* a pair of 
dorsally located anastomosing supracardinals (not shown in 
the figure), parallel with and medial to the posterior cardinals, 
which develop as the drainage system from the dorsal region 
of the thoracic and abdominal walls. Anteriorly these enter 
into the azygos system through fusion with the y portion of 
the posterior cardinals, while posteriorly they anastomose 
with the posterior cardinals in the iliac region thus bringing 
about a median fusion of these elements. The supracardinals 
also enter into a more anterior anastomosis upon each side 
with the subcardinal anastomosis, x, thus forming temporarily 
a venous ring encircling the aorta at the level of the perma- 
nent kidneys. This ring is known as the renal collar, and it 
is into this that the renal veins, from the kidneys, lead. Sub- 
sequently the renal collar is broken by the atrophy of the 
supracardinal-subcardinal anastomosis upon the left side, the 
subcardinal persisting upon the left side only as the connection 
between the left renal vein and the postcava. Posteriorly the 
supracardinals form, through their fusion with each other, an 
almost median channel bringing the fused iliac regions of the 
posterior cardinals into direct communication with the post- 
cava through the right subcardinal anastomosis. This median 
channel comes gradually to supplant the two lateral posterior 
cardinal channels, only such remnants of the latter remain- 
ing as are needed to continue the drainage from the gonads 
as the spermatic (or ovarian) veins. These, owing to the 
atrophy of the venous elements upon the left side, are pecu- 
liarly unsymmetrical, the left one draining into the postcava 
through the left renal, while the right one enters the postcava 
directly. 
This embryological history explains the composite structure 
of the postcava as seen in the adult. Anteriorly a sprout 
from the liver capillaries, it is composed more posteriorly of 
a vein connected with the embryonic kidney and a portion of 
the right posterior cardinal, and to this is added, still more 
* Cf. Huntington and McClure, Anatomical Record, Vol. 20, No. 1. THE VASCULAR SYSTEM 
377 
posteriorly, the caudal vein, primarily a portion of the sub- 
intestinal. 
Concerning the abdominal vein, which seems to appear in 
the amphibians as suddenly as does the postcava, the em- 
bryology of urodeles shows it first in the form of paired lateral 
vessels lying in the body wall and emptying into the ductus 
Cuvieri without connection with the liver. Its embryonic 
position and relationships thus render it probable that this 
vein is the same as the lateral vein of fishes, which likewise 
runs in the body wall and empties into the ductus Cuvieri 
or near it. The connections of this vein with the iliacs pos- 
teriorly and with the liver anteriorly appear later on in em- 
bryonic development and are thus shown to be secondary 
modifications, and not features of the original vein. 
Above the amphibians there is nothing which at first sight 
resembles an abdominal vein, but the two lateral elements 
of which it is composed are probably identical with the simi- 
larly related umbilical veins, which in the embryo supply 
the allantois. This membrane is itself the amphibian urinary 
bladder extended beyond the limits of the embryo, and there 
is little doubt that the two veins which lie along its sides and 
enter the liver, are primary lateral elements which in adult 
amphibians fuse to form the median abdominal vein. In the 
later history of the umbilical veins, the right one becomes 
early reduced, and in advanced embryos the left one alone re- 
mains; this collects all the blood from the entire allantois, 
enters the body at the umbilicus, and conveys the blood from 
that point to the liver. 
At birth in the case of the mammal, and upon hatching, 
in reptiles and birds, the extra-embryonal portion of the al- 
lantois together with its blood-vessels, becomes pinched off 
at the umbilicus, but the umbilical vein, extending from the 
anterior body wall to the liver, is retained as a ligament (lig. 
teres s. hepato-umbilicale). 
The portal vein, previously described, which conveys the 
blood from the intestine to the liver, is a constant factor in 
vertebrate circulation from cyclostomes to mammals, and as 378 
THE HISTORY OF THE HUMAN BODY 
it is essentially similar in all cases there is but little history 
shown by the comparison of adult forms. The early em- 
bryonic development shows, however, the method by which 
this portal system becomes established, and is thus valuable 
in explaining the relation between the original morphological 
elements and the adult structures. The formation of the 
hepatic-portal system occurs always in connection with the 
two first veins that appear, the vitelline or omphalo-mesenteric, 
that lead in from the yolk and unite just posterior to the 
heart. The intestinal canal runs between them, and from its 
ventral aspect the liver buds out in the form of a connected 
group of diverticula, which surround the veins in question and 
cause them to develop a capillary net-work. In fishes and 
amphibians the process is a fairly simple one, but in Saurop- 
sida and Mammalia the matter becomes somewhat more com- 
plicated by the addition to this very region of the two um- 
bilical veins, which come in from the allantois. This develop- 
ment, in its more complex form, is shown in Figs. 126 and 
128, which are taken from the lizard and mammal, re- 
spectively. In a of either figure are seen the primary elements 
which enter into the process, namely the two omphalo-mesen- 
teric or yolk veins, the two umbilical veins, and the alimentary 
canal. The ducts of Cuvier, the cardinal veins and the sinus 
venosus are also shown, but they are not directly concerned 
here. 
The initiative is taken by the two omphalo-mesenteric veins 
which form successively three connecting bands that unite 
them to each other (x1} x2, and x3 in Fig. 128). Of these, 
x3 is the most posterior, and lies ventral to the intestine; the 
next, x2, is dorsal; and the third, xlt also the most anterior, is 
again ventral. The result of this is the formation of two rings, 
forming a figure 8, through which the intestine is threaded 
(Fig. 126, c, and Fig. 128, c). The subsequent suppression of 
the left side of the anterior ring and the right side of the 
posterior ring, as indicated in Fig. 128, c, produces a single 
large trunk, eventually the portal, which twists in a spiral 
about the intestine (Fig. 126, d and e). Meanwhile, the liver THE VASCULAR SYSTEM 
379 
has formed about the two omphalo-mesenteric veins anterior 
to the rings, and the necessity thus thrust upon them of sup- 
plying it with blood-vessels results as seen in Fig. 128, b and 
c. Each sends off lateral branches from the posterior side of 
the liver and gathers them up from the anterior side until 
they are resolved into a mass of capillaries, which permeate 
the liver substance in all directions. As a temporary necessity, 
to be removed at the end of embryonic life, there develops 
a vessel running diagonally through the liver and extending 
from the left omphalo-mesenteric vein posteriorly to the right 
one anteriorly, the ductus venosus Arantii. Through this the 
blood passes while the liver tissue is still embryonic, and be- 
fore the capillary system within it is fully established, but with 
the approach of birth the duct atrophies and the hepatic portal 
system becomes fully established. 
The two umbilical veins, which appear in all these figures, 
take no part in the formation of the hepatic system and may 
rank thus as structures wholly embryonic. It is remarkable, 
however, that, as in the case of the omphalo-mesenteric veins, 
the two become reduced to a single one, not through so com- 
plicated a process as in the former case, but through the com- 
plete suppression of the right vein. The anterior portion 
of the left, also, shares the same fate, and the umbilical vein 
finally establishes a direct connection with the heart through 
the ductus venosus Arantii (Fig. 128, c). 
The fate of the veins anterior to the heart, when compared 
with that of those posterior to it, is a very simple one, for 
while in the latter region three veins in succession have held 
the supremacy, the sub-intestinal, the posterior cardinal, and 
the postcava, anteriorly the first to appear are the anterior 
cardinals, and it is these very vessels which in the higher mam- 
mals, under the name of jugulars, continue to serve in the 
same capacity as at first. The changes in these parts are com- 
paratively slight, and consist mainly in the establishment of 
two definite branches on either side, the external and internal 
jugulars, and in the greater development of the subclavians, 
correlated with that of the fore-limbs, which gains for them an 380 
THE HISTORY OF THE HUMAN BODY 
anatomical rank equal to that of the anterior cardinal itself, 
and suggests the name vena anonyma for that part of the orig- 
inal anterior cardinal proximal to the entrance of the sub- 
clavian, since it appears to be formed by the union of two 
equal veins. 
Fig. 128. Development of the hepatic portal system in mammals. [After 
Hochstetter.] 
j, anterior cardinal (jugular); s, posterior cardinal; c, ductus Cuvieri; omd, oms, 
right and left omphalo-mesenteric veins; ud, us, right and left umbilical veins; 
sv, sinus venosus; xv x2, x3, commissures between the omphalo-mesenteric veins of 
the two sides; y, y, and v, v, beginnings of the hepatic and portal capillaries in 
the liver; a, ductus venosus Arantii. 
These changes of nomenclature, it will be seen, are purely 
anatomical, and express merely the apparent differences due 
to change in the caliber or the relative position of the separate 
portions. There are, however, a few genuine morphological 
changes in the higher Classes, doubtless rendered necessary 
by modifications in related parts. The most striking of these 
occurs in Man and some other mammals and consists of the THE VASCULAR SYSTEM 
381 
formation of a connection 
across the middle line be- 
tween the right and left 
jugulars, and the more or 
less complete atrophy of 
the right vena anonyma. 
The left innominate vein 
thus has to convey all the 
blood from both sides of 
head and neck and from 
both anterior limbs, and as 
it is greatly increased in 
size through the assump- 
tion of this double task, it 
early received the name of 
superior vena cava (an- 
terior or precava), in com- 
parison with the inferior 
vena cava (posterior or 
postcava), which enters 
the right atrium near it. 
A similar modification 
takes place in the mam- 
malian posterior cardinals. 
The formation of a trans- 
verse connection between 
the two allows one of them 
to assume the function of 
conveying to the heart the 
blood collected by the met- 
americ branches, and per- 
mits the other to sever its 
anterior connection with 
the heart. Although there 
is much variation in this, 
the more common condi- 
tion in Man consists of the 
Fig. 129. Diagram showing the 
history of the venous system in Man. 
[Modified from Thane, in Quain’s 
Anatomy.] 
Primitive vessels that become atrophied 
are marked by cross lines; those secon- 
darily established are marked by rows 
of dots. 382 
THE HISTORY OF THE HUMAN BODY 
preservation of the posterior cardinal on the right side, into 
which the other empties by means of the transverse connect- 
ing vein, and is deprived of all direct connection with the 
heart. The first, or complete vein, was called the azygos, 
or unpaired vein, from a mistaken early notion that it had 
no mate on the other side; the other, the incomplete one, 
was named the hemiazygos. What little applicability these 
names may possess, however, is confined to Man and allied 
forms, since in many other mammals quite different results 
obtain. Thus, in rabbits, the main trunk of the hemiazygos 
entirely disappears, and the azygos receives the intercostal 
veins from both sides, while in the pig the reverse is the 
case and it is the hemiazygos which persists. These relations 
are extremely variable, even in Man, where the occasional 
conditions classed as anomalies receive their complete ex- 
planation through the morphological history of the region. 
As a review of the venous system of Man, with the morpho- 
logical significance of the principal parts, there may here be 
presented Fig. 129, which shows the course of the main trunks 
in the adult, the older parts which have atrophied and the new 
connections which have been added. In the background may 
be seen the primitive cardinal system of fishes, and perhaps 
in the minute caudal vein, even a trace of the still earlier sub- 
intestinal system. Here, as elsewhere, we receive the distinct 
impression of the constant modification of old relations to fit 
new conditions, and we see the numerous mechanical difficul- 
ties which are the inevitable result of such a process. Here 
and there, where a difficulty is sufficiently great to interfere 
seriously with the preservation of the race, it is overcome, 
if possible; if not, the race dies out; but generally the adapta- 
tion is fairly complete, and, while we may never know of the 
countless forms which were lost in the sifting process, those 
that survive are not seriously handicapped by the circuitous 
paths through which their organs have arrived at their final 
condition, and the atrophied rudiments form no serious dis- 
advantage to the organism. 
As one example of the slowness of the adaptation where THE VASCULAR SYSTEM 
383 
the disadvantage is inconsiderable, we have the case of the 
vena anonyma of the left side. Although the plan by which 
all the venous blood is received upon the right side of the 
heart is inaugurated by the amphibians, there is, in the an- 
terior cardinal system, no anatomical recognition of this 
throughout amphibians, reptiles, and birds, all of which still 
possess symmetrical vence anonymee, and the left one is forced 
to bring its blood over to the right side. First among the 
mammals comes the forma- 
tion of an obliquely placed 
transverse connective, which 
allows the establishment of 
a true vena cava anterior, 
and rectifies the slight me- 
chanical disadvantage. The 
mere fact that this condition 
continues so long without 
readjustment suggests that 
the disadvantage must 
be exceedingly slight, far 
too inconsequent to come 
under the direct control of 
Natural Selection; and the 
bettered condition in these 
mammals cannot fail to sug- 
gest the result of mechan- 
ical causes, operating continually for a long time, and always 
in the same direction. 
The heart is in origin nothing but a localized portion of 
a large blood-vessel, the walls of which develop a thickened 
muscular layer, transforming it into a pulsating engine to 
promote the flow of blood. Similar pulsating vessels occur in 
all animals furnished with a closed circulation, usually a single 
one, but in some cases several in number. Thus, in arthro- 
pods and molluscs there is a single median heart, located 
dorsally upon the main blood channel in that region, but in 
annelids several of the lateral commissures are enlarged and 
Fig. 130. Four stages in the de- 
velopment of the amniote heart. 384 
THE HISTORY OF THE HUMAN BODY 
function as hearts. In vertebrates the hypertrophied region 
which forms the heart is located along the median ventral 
blood-vessel formed by the union of the two vitelline veins, and 
involves its posterior portion. This brings it topographically 
very far anterior, just back of the gill region, and this primary 
position is, indeed, that permanently retained in fishes and 
amphibians; but in reptiles, birds, and mammals it suffers a 
considerable change of location in a posterior direction and 
comes to lie in a thoracic cavity, formed by the ribs and ster- 
num, with some participation of parts of the shoulder-girdle. 
In its first stage, as shown by Amphioxus and in early em- 
bryos, the heart is still a straight tube, formed posteriorly 
by the joining of the hepatic veins, and, in true vertebrates, 
the two ducts of Cuvier and the two vitelline veins also, the 
last being embryonic and transitory. The chamber into which 
these vessels empty soon differentiates off from the rest as 
the sinus venosus, and, in like manner, by the formation of 
transversely placed constrictions, there are added successively 
an atrium, a ventricle, and a conus arteriosus, the latter being 
continued into the median artery that supplies the gills. 
The next advance is seen in a flexion of the axis of the 
heart into the form of an S-shaped tube, the bending being 
in such a way that the sinus venosus is dorsal and the conus 
arteriosus ventral, the atrium anterior and the ventricle pos- 
terior, a stage represented in adult fishes and in the embryos 
of higher forms. The atrium increases in width more than the 
other parts and forms two lateral recesses or broad diverticula, 
which, from the ventral aspect, appear on either side of the 
conus arteriosus, suggesting the division into two separate 
compartments, which is, in point of fact, the next advance 
(Fig. 130, c and d). Furthermore, the several parts brought 
into contact by the flexion become permanently adherent to 
one another and the heart becomes molded into a more com- 
pact organ. 
In the tailed amphibians a new physiological moment is 
introduced by the reception for the first time of arterial blood, 
which comes from the skin and lungs through the great pul- THE VASCULAR SYSTEM 
385 
mo-cutaneous veins and enters the left side of the atrium (Fig. 
131, B). The anatomical response to this consists of the di- 
vision of the atrium into right and left portions, the former 
for the reception of impure, and the latter for pure blood.* 
From now on the sinus venosus plays a subordinate role and 
consists merely of a vestibule of entrance for the systemic 
veins, applied to the dorsal side of the right atrium. In birds 
and mammals it is no longer distinct. The ventricle is still 
undivided in the urodeles, but in the tailless forms, probably 
as a further response to the new physiological condition, a 
partial septum appears in this also, which suggests a division 
into right and left ventricles, but contains a large opening 
through which the two kinds of blood still mingle (Fig. 131, C). 
In these animals also, the complete differentiation of the fourth 
and sixth arterial arches as aorta and pulmonary artery, re- 
spectively, leads to a longitudinal division of the conus arterio- 
sus by means of two longitudinal folds placed opposite to one 
another which grow from the inner walls, meet, and fuse. This 
causes a complete separation of these main vessels as far back 
as the ventricle, and their exits from this chamber are placed 
in such a way that the blood from the right side, which is 
mainly impure, passes into the pulmonary artery, a condition 
which continues as a permanent one from this point on. The 
relation of the two aortic arches, however, is not so perfect, 
for they cross in such a way that while the right one contains 
mainly pure blood, the left one, in company with the pulmo- 
nary artery, collects, in part, impure blood from the right side. 
This causes a mixture of blood in this arch as well as in the 
main aorta, and the result is that the blood is never wholly 
aerated, a condition which does not allow the establishment 
of a constant body temperature, but compels the animal to 
be “ cold-blooded,” or more correctly, poikilothermous, that is, 
changeable in temperature in more or less accordance with its 
surroundings. , 
* In the urodeles the septum atriorum is not complete, but possesses a 
few secondary perforations. In the lungless salamanders the left atrium, 
into which the pulmonary veins would empty under other circumstances, is 
suppressed. 386 
THE HISTORY OF THE HUMAN BODY 
The heart of reptiles (Fig. 131, D) is similar to the last 
save that the ventricular partition is more extensive, though 
still incomplete, therefore the relation of these animals to ex- 
ternal temperature is the same as in amphibians; in birds 
Frc. 131. Diagrams of the heart and its compartments in the different 
vertebrate Classes. [In part after Wiedersheim.] 
(A) Fish; (B) Urodele; (C) Anuran; (D) Reptile; (E) Bird; (F) Mammal. THE VASCULAR SYSTEM 
387 
(Fig. 131, E), however, the opening between the ventricles 
closes, thus, for the first time, completely separating the two 
kinds of blood. Probably correlated with this is the complete 
atrophy of the left aortic arch, leaving the right one as the 
only connection between heart and median aorta. 
Mammals (Fig. 131, F), although but indirectly related to 
the birds, have accomplished the same separation of pure and 
impure blood, corresponding to the left and right halves of the 
heart, respectively, a relation which is still further emphasized 
by the main blood-vessels, the systemic venous trunks as- 
suming a position on the right, in connection with the atrium 
of that side, the main aorta being on the left, instead of 
the right. In both birds and mammals, then, the tissues are 
supplied with pure blood and are thus enabled to maintain a 
constant body temperature, which fluctuates but a few degrees, 
and is usually higher than the external temperature. There 
thus comes to be developed in the two most highly specialized 
Classes of vertebrates, birds and mammals, a complete and 
almost symmetrical double heart, of which the right half is 
associated with the venous, the left with the arterial, blood. 
In both the symmetry is not a primary, but a secondary one; 
the heart begins and ends with four chambers, it is true, but 
they do not all correspond, the latter form resulting from 
the suppression of two of the first and the subdivision of the 
remaining two.* 
The lymphatic function, that which cares for the blood 
components that become infiltrated into the tissues, and re- 
* During fetal life there is, in the mammalian heart, an opening in the 
interatrial septum, through which the blood of the two atria freely mixes. 
This is, however, a secondary condition, developed in adaptation to the 
fetal circulation, as is shown by the fact that the septum is first com- 
pleted before the opening is formed. In the embryos of the monotremes 
and marsupials there are several small foramina instead of one big one; 
which is similar to the condition found in urodeles, where the perforations 
are also secondary. The large foramen in placental mammals, the foramen 
ovale, persists normally in the human infant for a few days after birth; but is 
occasionally permanent, producing the condition known as cyanosis, in which 
the individual suffers continually from the presence of venous blood in the 
arterial system. 388 
THE HISTORY OF THE HUMAN BODY 
turns them to the general circulation, is performed, not only 
by spaces and vessels primarily formed for that purpose, but, 
in the lower forms at least, by the most of the spaces and 
lacunae of the body that form parts of other systems. In 
fishes and amphibians a system of lymph channels becomes 
developed in the loose connective tissue that forms an external 
sheath for the larger blood-vessels, and here the lymph, not 
confined within special walls, is intercellular and circulates 
freely within the meshes formed by the branching con- 
nective-tissue cells. One of the largest of these lymph channels 
is the subvertebral space, which enfolds the aorta. The 
lymphatics that collect the digested food (chyle) from the in- 
testine communicate, either directly or indirectly, with this 
channel, which thus forms a very primitive thoracic duct. 
A second series of lymphatic spaces is found immediately 
beneath the membranes lining the great serous cavities of the 
body; such are the sub-peritoneal spaces in the walls of the 
coelom, the sub-dural and inter-dural spaces connected with 
the membranes that invest the central nervous system, and the 
peri- and endo-lymphatic spaces of the inner ear. Still others 
are intermuscular or sub-fascial, their locations being desig- 
nated by their names, and in tailless amphibians there is found 
an extensive series of sub-cutaneous lymph-sacs, some of great 
extent. 
These various spaces are in communication with one another 
and usually communicate with the venous system in four 
places: anteriorly with the two jugulars at or near their union 
with the subclavians; and posteriorly with either the caudal 
vein or the posterior cardinals near the entrance of the iliacs. 
It is to be noted that these four points at which the lymphatic 
and circulatory systems communicate are associated with the 
four limbs, and although the number of these points of com- 
munication is decreased in the higher vertebrates through a 
suppression in the adult of certain of these, there are no new 
ones formed, and the lymphatic system, even in its most spe- 
cialized form, still follows in this particular the lines laid down 
for it from the first. THE VASCULAR SYSTEM 
389 
Fig. 132. Venous system of frog, to show the two pairs of pulsatile 
lymph-hearts; an anterior pair opening into the vertebral veins, and a 
posterior pair opening into a cross vein between the ischiadic and femoral 
veins. 
ji,' internal jugular; je, external jugular; ubsc, subscapular; sc, subclavian; 
cu, cutaneous; pul, pulmonary; h, hepatic; card, cardiac; po, hepatic portal; 
abd, abdominal; r. adv, afferent renal; is, ischiadic; fe, femoral. 390 
THE HISTORY OF THE HUMAN BODY 
In fishes the lymphatic vessels, near their entrance into the 
veins, enlarge into thin-walled sinuses, organs which in tail- 
less amphibians develop into pulsating sacs with muscular 
walls, the so-called lymph-hearts, the action of which furthers 
the flow of the lymph (Fig. 132). Each lymph-heart pos- 
sesses a single venous ostium, by which the sac communicates 
with the vein, and several lymphatic ostia, through which the 
sac receives the fluid from as many lymphatic vessels. The 
former opening is equipped with two semilunar valves to pre- 
vent filling the sac with blood during its expansion, but the 
lymphatic ostia are without special valves. The tailed am- 
phibians seem to lack the anterior pair of lymph-hearts, but 
here, in addition to the posterior pair, a series of small pul- 
sating sacs occur along the lateral line. 
Progress in the history of the lymphatic system among the 
higher vertebrates is shown along two directions: first, in 
the formation of more and more vessels with walls of their 
own, the definite lymphatics, and, second, by the reduction of 
the lymph-hearts. Thus in reptiles only the posterior lymph- 
hearts persist in the adult, and the same appear in birds dur- 
ing development, but are here transitory structures and do 
not survive embryonic life. The division of the subvertebral 
space into two lateral thoracic ducts is inaugurated in croco- 
diles and turtles, and becomes definite in birds; in these the 
chyle from the intestines is collected and emptied into the 
veins at the junction of jugular and subclavian. There are 
also in all Sauropsida posterior connections with the venous 
system, but only in reptiles do pulsating hearts persist at these 
points. 
The above phylogenetic history of the lymphatic system is 
well recapitulated during the embryonic development of mam- 
mals, and the adult condition is best understood by tracing 
the steps in this development (Fig. 133). As in the case of 
the blood circulatory system, the preservation of so many 
phylogenetic steps in this developmental history is doubtless 
due to the continual functional activity of this system from an 
early embryonic period, its usefulness at all stages preventing THE VASCULAR SYSTEM 
391 
Fig. 133. Development of the lymphatic system in mam- 
mals (pig). [After Miss Sabin.] 392 
THE HISTORY OF THE HUMAN BODY 
the customary degeneration of transitory structures.* The lym- 
phatic system first appears in the form of a pair of tiny diver- 
tivula which bud out from the venous system at the angle 
formed by the meeting of the jugular and subclavian veins. 
From these develops an anterior pair of lymph-hearts, in lo- 
cation similar to those of lower forms, but without muscular 
walls; and from these chambers as centers, definite lymphatic 
vessels begin to develop, growing from their free ends and 
gradually invading the surrounding tissues. At a slightly 
later period a pair of posterior hearts appears, and from these 
in the same way there grow out branching lymphatics. 
It is at this period that the thoracic ducts make their ap- 
pearance, starting from the vessels connecting the lymph- 
hearts with the veins and growing posteriorly, following the 
aorta. There are two of these, which at first are about equal 
in size, but soon the left one gains the superiority, and branches 
to supply each side, while the right one remains small. The 
left duct thus comes to furnish two long lateral vessels, one 
on each side of the aorta, which extend posteriorly until they 
enter the posterior system near the lymph-hearts, thus uniting 
the two systems; and the severance of the original connection 
between the posterior lymph-hearts and the posterior cardinals, 
now the postcava, renders the posterior system dependent upon 
the anterior connections. During later development several 
important changes take place. The lymphatics gradually 
spread from the four primary centers over the entire body, 
the original anterior and posterior systems communicating 
at every point until all distinction between the two becomes 
lost; the lymph-hearts lose their identity; two posterior en- 
largements, the cisternce [receptacula] chyli, appear in the 
posterior parts of the lateral thoracic ducts; and the growing 
intestine becomes supplied with lymphatics from the left lateral 
thoracic duct. During the growth of the lymphatic vessels 
numerous secondary centers are formed, from which several 
* An embryo in which the lymphatic system is not in full functional 
activity becomes oedematous, and in extreme cases the result is a spherical 
ball, without indication of the normal shape. THE VASCULAR SYSTEM 
393 
vessels radiate in various directions, and about these, through 
participation of the connective tissue and the blood-vessels, 
there develop the characteristic lymphatic nodes or “ glands .” 
Similar centers, developed in the course of the left lymphatic 
duct as it branches within the mesentery, form the mesenteric 
glands. In these glands the physiological unit seems to be 
a tuft of blood (or blood and lymph) capillaries surrounded by 
lymphatic vessels, the whole packed in a loose connective 
tissue, the lymphoid or adenoid tissue. Such a structure is 
called a lymph follicle, and a node may consist of a large 
number of such structures. The interstices of such lymphoid 
tissue are filled with lymphocytes, which are identical with 
those of the blood (as determined by staining reactions) and 
are evidently formed from the reticular cells. Thus the lym- 
phatic nodes constitute one of the localities in which these cells 
are formed. 
Concerning the method by which the gradual growth of the 
lymphatic network takes place during its invasion of the 
entire embryo from the four original centers, there are at 
present two strongly contrasted schools. According to one 
the free ends of the growing network push forward, invading 
the new tissues by the continuous growth of their blind termini, 
while their lumina remain always in connection with the rest 
of the system. Thus an injection of the system from any 
point in it would fill the entire set of vessels, and reveal the 
exact distance to which the invasion has taken place at any 
given moment. 
The other school sees everywhere in the invaded tissues, 
beyond the continuous system, rows of discontinuous cavities, 
the walls of which break through from time to time, and add 
to the continuous system along the free ends. Thus, the part 
of the lymphatic system revealed by an injection includes 
only those spaces which have already been added to the rest, 
while the separate spaces which still lie isolated in the tissues 
beyond cannot be reached in that way, and are revealed solely 
by the examination of serial sections. The reply to this is 
that the appearance of separate spaces, as shown in serial 394 
THE HISTORY OF THE HUMAN BODY 
sections, is given by collapsed portions of the walls of the 
smaller tubes, where the lumen cannot be easily followed, a 
condition due to insufficient injection. Much of the decision 
rests upon the structure of the endothelium of the lymphatic 
sinuses, and the claim is made that “ the endothelium of the 
lymphatic sinuses reacts like vascular endothelium which is 
in line with the theory that the lymphatic system is a part of 
the venous system, its lining cells having the same fundamental 
origin from the angioblast stem as a differentiated cell, rather 
than coming from the flattening out of mesenchyme cells to 
line spaces.” * 
At present a definite decision between these two schools 
seems impossible to make. 
In the walls of the colon of mammals appear aggregations 
of nodules, similar to those connected with the lymphatic sys- 
tem, and forming large areas known as noduli lymphaticl 
aggregati [Payer’s patches]. Many attempts have been made 
to connect these with the lymphatic nodes, the extreme theory 
being that here is the center of origin for all the latter, and 
that they migrate from these patches during development, first 
invading the mesentery and forming the mesenteric glands; 
thence passing from these along the lymphatic channels to 
all parts of the body. If the nodules of Peyer’s patches are 
endodermic in origin, as some think, it would follow that, with 
such an origin, all of the lymphatic nodes, wherever found, 
must be also endodermic, and it was thus held that we have 
here an example of elements, originally endodermic, wander- 
ing over the entire body and invading practically all the tissues. 
The more recent exposition of the development of the lym- 
phatic system, as given above, renders such theories no longer 
tenable and shows the lymphatic system, at least in mammals, 
to be a definite system, budding out from that of the circula- 
tion, and, like it, probably mesenchymatous in origin. 
Whether this is the case in the lower Classes of vertebrates 
and whether the various spaces utilized by the lymph can be 
thus derived cannot be ascertained until the development of 
* Florence R. Sabin; Physiol. Reviews, II, i, 1922, p. 59, THE VASCULAR SYSTEM 
395 
the lymphatics in these forms is as well known as it is in 
mammals; but with our present knowledge it seems probable 
that certain definite channels that possess walls of their own, 
like the sub-vertebral space, are produced as outgrowths from 
the blood-vessels, and that these enter into secondary com- 
munication with numerous irregular spaces which can well 
be utilized as adjuncts of the lymphatic system until their 
function can be supplied by definite lymphatic vessels. 
Aside from the solitary and aggregated nodules, both of 
which appear to be centers of origin of lymphocytes, there 
are numerous other places in which the cellular constituents 
of the blood are developed. Many of these, as in the case of 
the aggregated nodules of the intestines, are developed within 
the wall of the alimentary canal and are therefore endodermic 
in origin. These include the tonsils, the thymus, and thyreoid 
glands, the associated epithelial bodies, and, perhaps, the 
spleen. The marrow of the bones is especially important in 
this respect, and develops large quantities of the blood-cells, 
especially erythrocytes (red blood corpuscles). Although 
lymphatic vessels secondarily invade these organs, and are 
hence found in the adult in close association with them, they 
are not to be considered parts of the lymphatic system any 
more than one would consider the kidneys a part of the circu- 
latory system because their tissues are invaded by special 
forms of blood-vessels. 
In their function as formative nidi for the cellular ele- 
ments of the blood and lymph these organs form physiologi- 
cally important auxiliaries to the vascular system as a whole, 
but belong elsewhere in their anatomical and developmental 
affinities. CHAPTER XI 
THE URINOGENITAL SYSTEM 
“ We do not draw conclusions with our eyes, but with 
our reasoning powers, and if the whole of the rest 
of living nature proclaims with one accord from all 
sides the evolution of the world of organisms, we can- 
not assume that the process stopped short of Man. 
But it follows also that the factors which brought 
about the development of Man from his Simian an- 
cestry must be the same as those which have brought 
about the whole of evolution.” 
August Weismann, The Evolution Theory, 
Authorized translation. Vol. II, p. 393. 
The two systems included under this compound name are 
those concerned with the very diverse functions of the elimi- 
nation of liquid waste and the formation of new individuals. 
They are, however, closely associated topographically and 
usually possess certain parts in common, so that they belong 
together anatomically, though not physiologically. They pos- 
sess also important relations to the body cavity or coelom 
(more strictly, metacoele), and as the latter is in by no means 
a primitive condition in either Amphioxus or the cyclostomes, 
recourse must be had to early embryonic stages and also to 
invertebrates in order to reconstruct the early period of the 
history of these organs, a knowledge necessary for the ex- 
planation of many of the existing relationships. To begin 
with, let us suppose an animal built on the plan of a gastrula, 
but with a space left between the endoderm and the ectoderm 
(Fig. 134, A; also Plate I). Such an animal consists of two 
tubes, one inside the other, and two cavities. The two tubes 
are, of course, alimentary canal and body wall, and the two 
cavities are the digestive cavity (gastrococle) and the primary 
body cavity (protocode). Within this protocoele are contained 
two sets of organs, each opening to the exterior, excretory tu~ 
396 THE URINOGENITAL SYSTEM 
397 
bules (nephridia) and germ glands (gonads). (Fig. 134, 
B.) If the animal is unsegmented, i. e., consists of a single 
segment, there is a single pair of each; if it is multisegmented, 
there is a pair of each for each segment. 
Each nephridium consists of a free tubule whose function 
is to extract from the protocoele certain waste products in 
liquid form, a function which it performs in part by a ciliated 
funnel-shaped opening, the nephrostome, and in part by the 
physiological action of the cells of which its walls are com- 
posed. In its simplest form it is straight or slightly curved, 
but it is more usually coiled in order to increase its length, 
Fig. 134. Diagrams to illustrate the prevertebrate history of the nephri- 
dia, the gonads and the coelom. 
(A) Stage of gastrula, with two germ layers, a gastrocoele (g), and a primary 
body cavity (p). (B) Here a third layer has appeared in the form of paired 
gonadic sacs (wi), and paired nephridial tubules (t); with external openings at 
o and x respectively. The nephridia are furnished with an inner opening, the 
nephrostom («). (C) In this the gonadic sacs (m) have expanded and form the 
definite ccelom, limiting the primary body cavity to a series of small spaces in all 
parts of the body. The nephridia open internally into these sacs, and their outer 
ends open into a longitudinal duct (*). 
and hence its functional efficiency within the prescribed limits. 
Nephridia of this type are frequent among invertebrates. 
The other sort of organ, the gonad, has the form of a 
simple epithelial sac, with a narrow duct. Its walls are con- 
stantly proliferating and furnish cells which project into the 
interior and finally become free, passing out through the 
duct. These are the germ-cells, and may be either ova or 
spermatozoa, the product respectively of female and male 
parent individuals. Gonads of this character are frequently 
found among invertebrates, often in quite typical form. 398 
THE HISTORY OF THE HUMAN BODY 
Taking now an animal of the type shown in Fig. 134, B, 
with a continuous protocoele and with a metamerism marked 
by several successive pairs of associated nephridia and gonads, 
imagine the result of a gradual and equal expansion of the 
latter until they attain the furthest possible limits (Fig. 134, 
C; also Plate II). The protocoele becomes suppressed and in 
its place exists a series of paired chambers, metacoeles, each 
pair in contact with the previous and succeeding ones and en- 
closing between them the alimentary canal. This latter part is 
thus hung between dorsal and ventral partitions, the mesenter- 
ies, each double and composed of the walls of the gonadic sacs; 
also each pair of cavities is separated from the next by similar 
double partitions which form intersegmental diaphragms or 
dissepiments. Each lateral metacoele opens to the exterior 
by the opening which was once that of the gonadic duct. Thus 
far no provision has been made for the nephridia, which, with 
the suppression of the protocoele, find themselves deprived of 
their original function. Their fate is, however, simple and 
obvious, for they receive an investment of the gonadic wall, 
and although lying between this and the outer body wall, 
project into the metaccelic cavity, with which they communi- 
cate through the ciliated nephrostome at their free end. But 
one further modification is necessary to adjust matters to 
the new conditions, and that concerns the walls of the meta- 
coeles. When in their original condition as the walls of 
small sacs employed for the production of germ cells, every 
portion of their surface is needed for the production of these 
latter elements, but when expanded to their final dimensions, 
they become mesenteries, dissepiments and the lining mem- 
brane of body walls, and form a thin and firm membrane, the 
peritoneum, while their original reproductive function is con- 
fined to certain restricted areas, situated near the dorsal me- 
senteries. These, by a slight evagination, produce rounded 
elevations that project into the lumen of the cavity and form 
specialized germ glands, the ovaries and the testes. Their prod- 
ucts, when mature, separate from their place of origin and 
wander freely about in the metacoelic cavity. From this they THE URINOGENITAL SYSTEM 
399 
have two avenues of escape, the nephridia and the original 
openings of the gonadic sacs, and while so far as is known 
no animal exists that utilizes both methods, either one may- 
become specialized to subserve this function. Furthermore, 
in an animal with many somites it is not necessary that ovaries 
or testes should develop in each pair of metacoelic sacs, but 
these may be confined to a few pairs or even a single pair, 
in which either the nephridia or the gonadic openings develop 
into special excurrent ducts for the liberation of the germ 
cells. 
The conditions of this second hypothetical historic stage 
are realized in almost every detail by the annelid worms, 
allowing for a few modifications. [Cf. Fig. 181.] To some 
this indicates the conclusion arrived at independently by the 
consideration of other systems, that these latter animals lie 
very near the main stem of vertebrate ancestry, but to others 
this is no more than a case of parallel development, in no way 
more remarkable than countless other adaptive resemblances, 
such as the instance of the eye in cephalopods and vertebrates. 
However this may be, the example of the annelid is most use- 
ful in showing us that animals can exist in precisely the con- 
dition of the hypothetical form indicated by the study of 
vertebrate embryology and constructed from the data thus 
furnished. 
From this second stage, which must be very near the actual 
condition in the ancestor of modern vertebrates, the final 
type may be reached by the introduction of a few slight and 
very natural modifications. The first of these concerns the 
metacoelic sacs and consists, first, of the breaking down of the 
dissepiments between the body segments, thus throwing all 
the sacs of each side into one, and secondly, a similar loss, at 
least in part, of the ventral mesentery, making the two lateral 
sacs confluent ventral to the intestine and allowing this latter 
to swing free in the cavity, suspended dorsally. 
Thus, for the first time is reached a single secondary body 
cavity or metacoele (the definite “coelom”), lined with peri- 
toneum, which is reflected along the mid-dorsal line and 400 
THE HISTORY OF THE HUMAN BODY 
furnishes an investment and suspensory ligament for the in- 
testine. The coelom is formed, as has been shown, by the ex- 
pansion and later fusion of numerous pairs of gonadic sacs, 
and is thus an expanded gonadic cavity, while the peritoneum 
is identical with the walls of a great compound gonadic 
sac. The many pairs of gonadic openings are lost and appear 
either as a single pair (the pori abdominales of cyclostomes 
and selachians) or are entirely lost (all higher vertebrates). 
Owing to this reduction either the second method for the 
liberation of germ-cells is employed, the utilization of ne- 
phridia, or else secondary ducts are developed to serve the 
purpose. The proliferating masses of germ-cells project from 
the peritoneal wall and become suspended in band-like liga- 
ments like the mesenteries of the intestine, the mesovarium or 
mesorchium, and may either remain in their original loca- 
tion or become displaced and assume a secondary position. 
Finally the nephridia become confined to a certain region of 
the body, where they may form a pair of single definite masses, 
the kidneys, the units of which no longer open externally by 
independent openings, but become attached to a common ex- 
current duct, which opens, either independently, or, more 
usually, into the cloaca, the terminal portion of the alimentary 
canal. 
Turning now from theory to fact, we may take up the uro- 
genital organs as they actually exist in the various vertebrate 
groups, thus tracing a history by means of which the con- 
dition found in Man may receive at least a partial explanation. 
The condition in the cyclostomes is difficult to interpret; the 
teleosts seem not to come into line with the rest and represent 
a side branch, which, perhaps, presents an independent so- 
lution of the problem; but from the selachians and certain ga- 
noids directly to the amphibians, and from them to the Am- 
niota the history is a fairly continuous one. For the sake of 
clearness it will be best to consider separately the two systems 
involved, beginning with the urinary. 
We have already learned that organs performing the same 
function in the different groups of vertebrates are not neces- 
sarily homologous, and are familiar with such phenomena as THE URINOGENITAL SYSTEM 
401 
the two tongues, the two sternums, the two sets of ribs and 
the possibility of two mouths, but here we enter into a 
greater complexity, for the history of the urinary organs in- 
volves three kidneys, pronephros, mesonephros, and metcinc- 
phros, each with its associated parts, which represent as many 
successive dynasties of organs that have replaced one another. 
In cases like that of the two respiratory systems, where the 
branchial system becomes replaced by the pulmonary, many of 
the parts of the first become employed by the second, often 
in quite a new capacity; but in the present case an added 
element is introduced on the part of the neighboring repro- 
ductive system, which not only employs at times portions of 
one of the urinary systems, but retains them in its service long 
after the system of which it formed a part has disappeared. 
The first, or pronephric, system, appears in the embryo of 
all vertebrates; it functions during the larval life of some 
fishes and amphibians (Fig. 135), and in a few teleosts per- 
sists as a functional organ in the adult, but in other fishes and 
in all higher forms it becomes reduced to a few rudiments. 
It thus strongly suggests the assumption that it once formed 
the functional kidney in some vertebrate ancestors, from which 
it has been inherited. It consists of a few nephridial tubules, 
strictly metameric in arrangement, that is, a pair for each 
of several successive somites, situated very far anteriorly, 
often involving the first of the trunk somites. The nephridia 
of each side become associated together to form a single 
kidney, the pronephros, and enter a common pronephric 
duct, laterally placed, and opening either directly to the ex- 
terior in the vicinity of the cloacal opening or, more usually, 
within the cloaca itself by means of a papilla which projects 
from its dorsal wall. 
This duct is, for the most part, like the nephridia them- 
selves, mesodermic in origin, although in some of the lower 
forms the posterior portion arises from the ectoderm, giving 
to the entire duct a double origin. This strange condition 
may be in part accounted for if we consider that originally 
.there was a larger number of nephridia and that each opened 402 
THE HISTORY OF THE HUMAN BODY 
by itself directly to the exterior. It may then be supposed 
that for the better disposal of the excretory fluid the separate 
openings became connected by a groove which continued to 
the side of the cloacal orifice and deepened posteriorly into a 
trough, from which by a further continuation of the process 
an internal tube would be formed, 
opening either at the margin of the 
cloaca or just within it. The meso- 
dermic anterior portion may be the 
result of the fusion of the outer 
ends of the successive nephridia, 
each one contributing that portion 
belonging to its own somite. 
Typical pronephridia (Fig. 136, 
A), the units of the pronephros, 
closely resemble the one given in 
the theoretical description above. 
They possess at the inner end 
ciliated nephrostomes and show a 
greater or less tendency to coil, 
suggesting a former condition of 
considerable physiological effi- 
ciency. Aside from this, they show 
the beginning of a relationship es- 
sentially vertebrate, and carried 
out in greater detail in the 
meso- and meta-nephric systems, 
namely, an association with capil- 
lary blood-vessels, enabling the 
nephridia to extract waste mate- 
rial directly from the blood. This 
association is here very slight, 
and consists of segmentally arranged tufts of capillaries, glo- 
meruli, which protrude into the ccelomic cavity and form 
rounded elevations covered by the peritoneum. These are 
located opposite the nephrostomes, and the excretory fluid, 
which passes from the glomeruli to the ccelomic cavity, is 
Fig. 135. Frog tadpole with 
functional pronephros and de- 
veloping mesonephros. [After 
Marshall.] 
v, ventricle of heart; t, truncus; 
g, gill arteries; ph, pharynx; a, 
aorta; h, anlage of hind limbs; 
an, anus; I, pronephros; II, me- 
sonephros; gl, glomerulus; w, 
Wolffian duct. THE URINOGENITAL SYSTEM 
403 
Fig. 136. Diagrams illustrating the two forms of nephridia character- 
istic of pro- and meso-nephros, respectively. 
(A) Stage of the Pronephros. (B) Stage of the Mesonephros. Farther explanation in text. 404 
THE HISTORY OF THE HUMAN BODY 
taken up by these latter organs. There is no direct connec- 
tion between glomerulus and nephridium, although in several 
instances both the elevation containing the former and the 
nephrostome become included within a recess of the coelom, 
an arrangement which furthers the mutual action of these 
parts. The number of pairs of nephridia involved is usually 
small (3-4), but in the Gymnophiona, in which the prone- 
phros functions for a considerable period, there may be as 
many as 10-13. Naturally the pronephric system is seen 
in its most complete state among the lower vertebrates; in 
Amniota it is often quite rudimentary and variously modified. 
The pronephros, even when best developed, possesses but 
a temporary existence and becomes supplanted by the mesone- 
phros, the kidney of the second or mesonephric system. This 
organ is formed from nephridia which are, like the first, seg- 
mental in origin and arise from somites posterior to those 
associated with the previous system. It forms the perma- 
nent kidney of fishes and amphibians, and in the embryo of 
Sauropsida and Mammalia it is large and prominent and has 
been known as the “ Wolffian body,” named in honor of its 
discoverer. 
The separate units of this system, the mesonephridia (Fig. 
136, B), differ in one essential particular from those of the 
pronephros, namely, in their closer association with the ar- 
terial glomeruli. 
In the case of the pronephridia these capillary tufts were 
merely brought into close relation to the nephrostomes, but 
each mesonephridium surrounds a glomerulus with a thin- 
walled evagination from its side, which fits about it like a 
double cup and forms what is known as a Bowman’s capsule. 
The entire structure thus formed, including both the capsule 
and its glomerulus, forms a renal [Malpighian] corpuscle. 
Otherwise the mesonephridia are like those of the former 
system, and possess nephrostomes and coils. They develop 
no duct of their own but utilize the pronephric duct, be- 
coming secondarily connected with it posterior to its con- 
nection with the pronephridia. Later on both pronephridia THE URINOGENITAL SYSTEM 
405 
and that portion of the duct anterior to the connection with 
the mesonephridia become atrophied, and the duct thus be- 
comes the mesonephric, the “Wolffian duct” of an earlier 
nomenclature. On account of this utilization of the prone- 
phric duct by the mesonephric tubules it has been held by 
some that both belong to one system and that the latter are 
merely later appearing elements of the pronephric series, 
but this is discredited by others on the ground of the differ- 
ence in the time of functional activity of the two systems and 
also on account of the several somites without nephridia of 
either kind that intervene between the two. The structural 
difference between the two types of nephridia, one with a 
Bowman’s capsule, the other without, may be also employed 
as an argument in favor of the distinctness of the two systems, 
but this argument is weakened by a consideration of the man- 
ner of formation of this new part, and by the assumption of the 
existence of almost every grade of transition between the two. 
Thus in its more usual form a pronephric tubule is related 
to the accompanying glomerulus much as in Fig. 137, A, but 
in some cases the projection bearing the glomerulus becomes 
partly enclosed in a recess of the coelom, and the nephrostome 
opens into this instead of into the main cavity (Fig. 137, 
B). The development of cilia at the narrow passage which 
leads into the recess, and the loss of them around the mar- 
gin of the original nephrostome, would convert the entire 
apparatus into a mesonephric tubule, in which the added 
portion, including the new nephrostome and the Bowman’s 
capsule, is a contribution from the peritoneum. These dia- 
grams seem absolutely persuasive, but unfortunately do not 
correspond with the actual facts, since, although in special 
cases the glomeruli of the pronephric system are related 
much as in the second diagram, the Bowman’s capsules of the 
mesonephric tubules develop directly as evaginations from 
their walls, and there is thus no indication of either the par- 
ticipation of the coelom or of the formation of a new nephro- 
stome. 
The mesonephric system may be found in full functional 406 
THE HISTORY OF THE HUMAN BODY 
activity in any adult fish or amphibian. Originally involving 
a large number of somites the mesonephros extends, usually as 
a pair of long, narrow organs, along a large portion of the 
trunk, lying close up against the vertebrae and ribs. The 
original nephridia become greatly multiplied and lose more 
or less of their original segmental arrangement, the nephro- 
stomes appearing irregularly along the ventral surface, that is, 
the surface turned toward the coelomic cavity. The mesone- 
Fig. 137. Diagrams to illustrate a theory of the relationship of the 
pro- and meso-nephric tubules to each other. [After Gegenbaur.1 
(A) Stage of the pronephros. Its nephrostome (/>) is placed opposite the 
glomerulus, but with a small portion of coelom interposed between them. (B) Stage 
of the mesonephros. Here by the formation of a new nephrostome at j the inter- 
posed piece of the ccelom has become included in the nephridium, forming a Bow- 
man’s capsule. 
phric (Wolffian) duct lies along its outer side and sustains 
that relationship to the cloaca noted above under the pro- 
nephric system, usually opening by a urinary papilla into 
the dorsal wall of the cloaca. 
The third urinary system, the metanephric, arises directly 
from the second, and thus the relation between the two is 
closer than that between the second and the first. It is the 
definitive urinary system of the amniotes, and in all reptiles, 
birds and mammals ultimately replaces the mesonephric, al- 
though this latter system is well developed during embry- THE URINOGENITAL SYSTEM 
407 
onic life. The metanephric system is not laid down at first 
in the form of nephridia as in the other cases, but arises as a 
blind canal or evagination from the mesonephric duct near 
its lower end. This evagination, medial at its origin, comes 
to lie dorsal to the mesonephric duct and develops anteriorly 
until it comes in contact with the dorsal wall of the ccelomic 
cavity, where it meets a mass of indifferent cells proliferating 
from it. From the differentiation of this cell mass develop 
numerous nephridia of a type similar to but in certain charac- 
ters distinct from either of the other types, and from the re- 
peated branching of the anteriorly growing canal there de- 
velop collecting tubules with which the nephridia unite. The 
expanded bases of the terminal branches of the tube form a 
pocket or pelvis, which collects the fluid from the tubules. The 
nephridia and collecting tubules form together the definitive 
kidney, the metanephros, while the main tube, beginning with 
its expanded pelvis, becomes the ureter. 
Each elementary unit of the metanephros, a metanephri- 
dium, is like that of the previous system without the nephro- 
stome. The connection with the circulatory system through 
the glomeruli, which when first introduced was clearly a sec- 
ondary function of the nephridia, becomes in the mesone- 
phros of primary importance through the development of a 
Bowman’s capsule, and in the metanephros all direct connec- 
tion with the coelom is given up. Here, in addition to the 
association with the circulatory system through the Bowman’s 
capsules, the tubules themselves become very long and at- 
tenuated, and, as they are accompanied by a rich network of 
capillaries, they are enabled to extract the waste products 
through their entire length as well as at the localized renal 
corpuscles. 
This history of the development of the metanephric sys- 
tem, so different in origin from that of the other two, and yet 
so similar in its results, has led to much speculation. It can- 
not be supposed for a moment that nephridia so nearly alike 
as those of the meso- and meta-nephros can have developed 
independently, for that would involve also an independent 408 
THE HISTORY OF THE HUMAN BODY 
origin in the two cases of such complicated structures as the 
renal corpuscles. The primary location of the metane- 
phros, posterior to that of the mesonephros, or at least to that 
of its functional portion, leads to the idea that the nephridia 
of this system were originally a part of the mesonephric 
series, belonging to its more posterior somites, and that their 
development from a structureless mass is a case of shortened 
development, in which the primary segmental arrangement has 
become lost. The necessity for the development of a new 
ureter is easily seen in the employment of the older one (the 
mesonephric or Wolffian duct), as a ductus [z/cw] deferens, 
a point to be brought out later in connection with the re- 
productive system. 
The external form of the metanephros varies considerably. 
This in the Sauropsida is in accordance with the form of the 
dorsal skeletal wall, to which it is closely applied. In struc- 
ture it is usually distinctly divided into lobes that correspond 
to the terminal branches of the ureter. This is characteristic 
of the kidney of most mammals, and the compact form found 
in Man is attained considerably after birth, and is met with in 
only a few cases. 
As may be followed from the development, the ureters ter- 
minate posteriorly in the mesonephric ducts and may be ex- 
pected to share the common outlet into the cloaca. This is 
actually the case in snakes, crocodiles and birds, which con- 
sequently never perform urination as a distinct act, but in 
other reptiles and in mammals there is found a terminal 
resevoir, the urinary bladder, with which the ureters become 
secondarily connected. This opens at first directly into the 
cloaca, but its narrowed neck develops in the higher mammals 
into a distinct canal, the urethra, which in the male comes into 
direct association with the ductus deferens. The urinary blad- 
der is no new formation, but is the remnant of the inner end 
of the allantois, an extensive embryonal membrane, which 
passes out of the body at the umbilicus and becomes in the 
Sauropsida an external respiratory organ, and in mammals 
furnishes the essential parts of the umbilical cord and placenta. THE URINOGENITAL SYSTEM 
409 
After birth a portion of this becomes shut within the body by 
the closure of the umbilical connection, and as this portion is 
in the form of an open bag leading out from the cloaca, it is 
easily converted into a reservoir for urine, the greatest change 
necessary being a slight shifting of the terminal portions of 
the ureters. Only the lower portion is actually utilized for this 
purpose, and the remainder atrophies into a ligament, which 
extends from the apex of the bladder to the umbilicus. Ap- 
proaching the cloaca the bladder becomes narrowed to a 
small neck which is continued as a median duct or canal, the 
urethra, and opens, in common with the genital ducts, into 
the urogenital sinus. 
A structure called a urinary bladder is present in amphib- 
ians. This is in the form of a collapsed bag leading out from 
the ventral wall of the cloaca and is without direct connection 
with the urinary system. This seems to represent morpholog- 
ically an undeveloped allantois, and is thus really homologous 
with the bladder of the Amniotes. Its function is not wholly 
understood, but the occasional presence within it of excretory 
salts suggests a subordinate use in connection with the urinary 
system. 
The second of the two associated systems to be considered 
is that of reproduction (generation), and consists primarily 
of the germ glands, in vertebrates a single pair, together with 
some definite avenue of escape for the mature germ cells. To 
these may be added secondarily external parts to insure the 
union of the two sorts of germ cells. 
The germ glands, the essential organs of reproduction, 
develop as localized areas on the peritoneal wall of the coelom, 
and are primarily located dorsally, one on either side of the 
vertebral column, in about the middle of the trunk region. 
This similar origin, from a layer which otherwise forms noth- 
ing but investing membranes and suspensory ligaments, is 
easily explained by the theory given above, which considers 
the entire coelom as the result of the fusion of a series of ex- 
panded gonads, a theory perfectly in harmony with all the 
related facts. The germ glands, primarily patches of germinal 410 
THE HISTORY OF THE HUMAN BODY 
epithelium, become reinforced from behind by the prolifera- 
tion of connective tissue, containing nerves and blood-vessels; 
and thus are formed mounds projecting into the coelom, cov- 
ered with germ cells. This association becomes more intimate 
through the intrusion of the germinal epithelium into the in- 
terior in many places, where the cells receive the nourishment 
necessary for their complete development. 
The germ cells themselves are of two sorts, ova and sper- 
matozoa, and their differences in form and size necessitate a 
more or less apparent difference in the organs that produce 
them, the ovaries and testes respectively. In certain cases 
among cyclostomes the same germ gland produces both sorts 
of germ cells, although at different times, but with this ex- 
ception the sexes in all vertebrates are normally separate. 
The occasional occurrence of a few cells of one sort in a gland 
which normally produces the other, as the development of a 
few ova on the side of a testis, or vice versa, occurs as an 
anomaly among many of the lower vertebrates, and this 
phenomenon, taken in connection with the cases among the 
cyclostomes just cited, has led to the possible theory that 
the ancestors of vertebrates were hermaphroditic, as is the 
case in many invertebrates, but there is little else to indicate 
this. Reported hermaphrodites among higher vertebrates are 
usually apparent rather than real and are in fact malformations 
due to some error in development affecting mainly the ex- 
ternal parts. 
All that is essential for the production of a new organism is 
the complete and intimate union of the two germ cells, one 
of each sort, but the varied environment of the parents often 
makes it a problem to arrange the means by which this may 
be accomplished. It offers the least difficulty in the case of 
aquatic forms, for all that is here necessary is to liberate the 
cells of both sorts into the water, in which the union can be 
easily effected, since the water furnishes the fluid medium nec- 
essary for the locomotion of the spermatozoa. Often, too, 
such animals associate in pairs and develop elaborate instincts 
which insure the discharge of the two products in close prox- 
imity to one another. THE URINOGENITAL SYSTEM 
411 
This absolute necessity of a fluid medium causes the de- 
velopment in land forms of a number of accessory parts. 
Thus there develop in the male special glands to supply a ve- 
hicle for the spermatozoa, forming a spermatic fluid; and as 
this cannot be allowed to dry up, it must be conveyed directly 
to the female by an internal copulation, necessitating again 
certain modifications of the cloacal margin, from which de- 
velop the various external organs. Although it is evident that 
the development of the process of copulation is here due solely 
to the terrestrial life, there are sometimes other conditions 
that develop it, for although, on the one hand, it is universal 
among terrestrial forms, invertebrates as well as vertebrates, 
it is occasionally found among aquatic animals, notably- in this 
connection the selachians; that, however, it is here an inde- 
pendent development is shown by the source from which the 
copulatory organs are derived, namely, from the inner margin 
of the ventral fins, and not from the rim of the cloaca, as in 
higher vertebrates. 
There are thus two groups of accessory reproductive organs 
to be considered, (i) those which furnish an outlet for the 
germ cells, and (2) those which are concerned in internal 
copulation. These may be taken up in order. 
The conception of the peritoneal cavity as an expanded go- 
nadic sac demands that the germ cells generated in its wall 
should break loose and float about within the coelom, until 
finally expelled either through some direct channel of com- 
munication with the exterior, the original gonadic ducts, or 
else by utilization of some part of the nephridial system; and 
as a matter of fact all conditions found in vertebrates, with 
the possible exception of that of teleosts, may be directly re- 
ferred to one of these two methods. 
The most primitive condition is that seen in cyclostomes, in 
which the peritoneal cavity communicates directly with the 
exterior by means of a pair of pori abdominales, canals which 
begin at the posterior part of the coelom and open along the 
sides of the cloacal orifice. The germ cells, when matured, 
become freed from their place of origin and float about in the 412 
THE HISTORY OF THE HUMAN BODY 
peritoneal cavity until discharged through these abdominal 
pores. The urinary organs have no direct connection with 
this system other than through the nephrostomes which open 
into the peritoneal cavity, and these are not specialized to re- 
ceive the free germ cells. 
There is thus shown the original condition of gonads and 
their excurrent ducts, slightly modified by the fusion of all the 
gonads into one and the reduction of the gonadic ducts to a 
single pair. Otherwise the primitive physiological functions 
are carried on as they were before the gonadic cavities became 
converted into a metacocle. 
It will be noticed that in the above description the ne- 
phrostomes open directly into the coelomic cavity and thus 
suggest the possibility of the use of nephridia for the exit of 
the germ cells. Such is actually the next stage in the history 
of these organs, for in the selachians certain of the nephridia 
are so employed while the pori abdominales, although they 
still exist, are no longer used for their original purpose. In 
the male the testes lie in close proximity to the anterior por- 
tion of the kidneys, and enter into direct connection with the 
nephridia of this region through the development of a series 
of tubes, the vasa efferentia, which extend from the testes and 
enter the nephridia a little beyond the nephrostomes. The 
original function of this part of the kidneys is not impaired, 
and during the greater part of the time it exercises the urinary 
function alone; but during the periods of sexual activity the 
nephridia involved become filled with the spermatic fluid and 
deliver it directly from the testes to the mesonephric duct 
and thence to the cloaca. From there it is received into a 
channel formed by the approximation of the inner modified 
portions of the ventral fins, and delivered within the cloaca 
of the female by an internal copulation, an unusual method 
among aquatic animals. That there is no genetic connection 
between this act and that developed among terrestrial verte- 
brates may be seen from the employment of very different 
organs for the purpose in the two cases and from the fact of 
the interposition, in the direct line of descent, of forms that THE URINOGENITAL SYSTEM 
413 
do not develop any such method. Through this close con- 
nection between the originally distinct reproductive and uri- 
nary systems it results that both the anterior part of the meso- 
nephros and the mesonephric duct become, apart from their 
urinary function, accessory reproductive organs, the former 
serving as a “ sexual kidney,” and the latter as a ductus 
deferens, or excurrent seminal duct. 
In the female selachian a different modification takes place, 
seemingly not due to association with the urinary system, but 
proven to be so by the developmental history of the parts. 
The ovary of the adult occupies about the same position as do 
the testes of the male, but shows no direct connection with the 
anterior part of the kidney. In place of this there appears on 
each side a long tube running along the side of the mesone- 
phrotic duct and opening posteriorly into the cloaca beside 
that of its associate. This is the oviduct, orce Muller’s duct” 
of many writers. At the free anterior end, which extends to 
almost the forward limits of the coelom, it opens by an ex- 
panded mouth, ostium tubce, directly into this latter cavity 
and receives into this the mature ova which become released 
from the ovary and are shifted about in the coelom in the 
primitive fashion. The oviduct arises in the embryo as a tube 
segmented off longitudinally from the mesonephric duct by the 
common method of the development of two longitudinal folds 
opposite one another, and thus points to a period at which 
the ova as well as the spermatozoa were conveyed to the cloaca 
through the mesonephric duct. The ostium is probably an 
enlarged and specialized nephrostome, associated with a single 
nephridium,* and it is thus easily imagined that the primary 
conditions in the female corresponded closely to that of the 
male, but, that owing to the greater size of the products to be 
* The not infrequent occurrence, even in the human subject, of two 
ostia upon one side may possibly be the result of the retention of two 
nephrostomes instead of a single one, or it may be simply an anomaly 
like the multiplication of digits or other parts. If it be the first it con- 
cerns a very ancient bit of history. 414 
THE HISTORY OF THE HUMAN BODY 
transmitted, a single nephridium with its nephrostome became 
differentiated for this purpose, and that later on there came a 
longitudinal splitting of the primary mesonephric duct, be- 
ginning above and progressing gradually, for the better ac- 
commodation of the sexual products. The employment of a 
nephrostome instead of vasa efferentia is quite a fundamental 
difference, but rudiments of these latter vessels are to be de- 
tected in association with the ovaries, and thus the use of the 
former may have been a later adaptation. 
The urinogenital relations of the selachians seem to have 
been inherited directly by the amphibians (Fig. 138, a and c), 
for the two correspond closely; in the male there is the same 
relationship between testes and sexual kidney, and the mesone- 
phric duct is a common ureter and ductus deferens. A 
rudimentary oviduct tapering anteriorly to a blind end, is usu- 
ally found attached to the side of this latter tube. In the fe- 
male the oviduct is often very long and convoluted, and its 
walls are often glandular and furnish membranous and gelat- 
inous encasements for the eggs. In a few instances the lower 
part of the tube is expanded into a uterus for the retention 
of the larva. 
Corresponding to the lack of internal copulation there are 
no external organs, but there are various instincts developed 
which have for their purpose the mingling of the sexual 
products. Thus the males of some aquatic salamanders pro- 
duce conical spermatophores, which rest upon the sand at the 
bottom of the pond and are taken up by the cloaca of the fe- 
male; a similar purpose is seen in the amplexation of frogs and 
toads, in which the males embrace the females during ovi- 
position and void the seminal fluid over the egg masses as soon 
as laid. 
A fundamental change of relationship is seen in the Am- 
niota, caused by the appearance of the third kidney, the meta- 
nephros. This organ, which possesses a separate ureter and 
is thus a complete urinary system in itself, assumes the entire 
control of this function, and leaves to its predecessor, the 
mesonephric system, nothing but reproductive functions. THE URINOGENITAL SYSTEM 
415 
Fig. 138. Comparison of the urinogenital system in Anamnia and Amniota 
For further description see text, p. 416. 416 
THE HISTORY OF THE HUMAN BODY 
As a result of this, those parts of this latter system which have 
been previously employed for reproductive purposes are re- 
tained and even become more highly specialized, while the 
parts that were wholly urinary disappear, with the exception 
of a few vestiges. 
In this the two sexes are affected differently, as may be made 
clear by a reference to Fig. 138, in which A and B show the 
changes produced in the male, C and D those in the female. 
In the male amphibian the sexual parts are the testes (t), the 
vasa efferentia and the mesonephric duct (w); in the male 
amniote these parts are retained while the remainder of the 
mesonephric system has disappeared, being replaced by the 
metanephric (k). The mesonephric duct, released from all 
urinary function, becomes the definite ductus deferens (w), 
and the remaining portion of this system, including vasa effer- 
entia, sexual kidney and collecting efferent tubules, becomes 
closely associated with the testes under the name of the epidid- 
ymis (e). In the female amphibian the reproductive system 
has become practically independent of the urinary through the 
development of a separate excurrent duct, the oviduct (m), 
and thus, with the rise of the metanephric system, that of the 
mesonephros becomes reduced to a few functionless vestiges 
(e); yet the more conservative embryonic history records the 
fact that both oviduct and ostium were originally portions of 
the mesonephric system, and, although with a different his- 
tory, both sexes are in reality about equally indebted to it 
for their accessory organs. 
Although the reproductive organs, as given in the above 
sketch, are the common heritage of all amniotes, the separate 
groups of reptiles, birds, and mammals have been left to work 
out the details in accordance with their own necessities. In 
each there is a metanephric urinary system, with kidneys 
and ureters distinct from the reproductive system except for 
intimate topographical relationships at their outlets; in the 
male the testes are accompanied by an epididymis and a ductus 
deferens, respectively the anterior portion of the mesonephros THE URINOGENITAL SYSTEM 
417 
and the mesonephric duct; and in the female there is an ovi- 
duct with an enlarged ostium, into which the wandering ova 
are received. In the present treatise the details of these parts 
in reptiles and birds cannot be considered further, but the 
history that is shown in mammals is of much importance, as 
it includes the human conditions. 
In the mammalian embryo the mesonephric system attains 
a high degree of development, and the mesonephros, under 
the name of the “ Wolffian body,” is large and conspicuous. 
In the marsupial young of monotremes and marsupials it 
forms the functional kidney, and as this is but one of several 
organs that become profoundly modified or replaced during 
later life, the development may be rightly considered a true 
metamorphosis in which the marsupial young represent a 
larval stage. In placental mammals a similar replacement of 
urinary systems takes place, but as the intra-uterine life is 
here much longer than in former cases and includes also ap- 
proximately the period passed by lower mammals in the mar- 
supial pouch, there is no free larva, and the changes are con- 
sidered a part of the embryonic development. 
As both the stage of the functional mesonephros and its 
later reduction are of importance in understanding the adult 
conditions, they may be first studied by the aid of the accom- 
panying Plate. During the period designated as that of sexual 
indifference, which includes all the early development and 
continues until the embryo is quite well matured in many 
other particulars (up to 70 or 80 days in the human species), 
the sexes, although definitely determined, show absolutely no 
difference in the general appearance of the urinogenital organs. 
[Plate III, a]. The mesonephros is large and functional and 
stands out freely from the dorsal wall of the abdomen, held 
in place by a suspensory fold of peritoneum, the mesonephric 
ligament. This fold becomes prolonged posteriorly beyond 
the limits of the Wolffian body and forms the inguinal liga- 
ment, a part of great importance in subsequent relationships. 
Upon the inner side of each mesonephros appears a longitudi- 418 
THE HISTORY OF THE HUMAN BODY 
nal fold of its peritoneal investment, along the free edge of 
which the cells become proliferated and form the germ gland; 
the remaining, or basal portion of this fold is later to form 
the mesorchium or mesovarium, the suspensory ligament of 
the mature testis or ovary respectively. The Wolffian (meso- 
nephric) duct lies along the free edge of the mesonephros, 
and not far from this is Muller’s duct, suspended in a fold 
which projects from the ventral surface of the mesonephros. 
These two pairs of ducts are brought together at their distal 
ends and form a common chamber, into which all four empty, 
the urinogenital sinus. 
From this stage the conditions found in the female are 
readily developed. [Plate III, c.] Its most important organs 
are the germ gland, which becomes the ovary, and Muller’s 
duct, the upper part of which becomes the oviduct [uterine 
(.Fallopian) tube], and the lower part, the uterus. The en- 
tire mesonephric system, since it is in no wise concerned in 
the reproductive function and since the urinary function is 
wholly assumed by the metanephric system, disappears ex- 
cept for a few useless vestiges; its loss allows the mesone- 
phric ligament to become continuous with that of Muller’s 
duct, and thus to extend as the broad ligament of oviduct and 
uterus from the dorsal body wall to the latter organs. The 
round ligament is formed from the posterior extension of the 
ligamentum inguinale. 
Muller’s duct gives rise to the oviduct, uterus and vagina, 
which are thus seen to be nothing more than differentiations 
of the various regions of a single tube. The ostium is much 
nearer approximated to the ovary than in Sauropsida, and as 
it opens and partly surrounds the latter during ovulation, the 
entrance of the ova into the oviduct is practically assured. 
In some mammals there is a special arrangement in the form 
of a recess or pocket of peritoneum, the bursa ovarica, in which 
the ovary lies, covered by the ostium, and in a few cases the 
fusion of the edges of ostium and bursa convert the latter into 
a capsule which may either open to the coelom through a small Plate ill. Development of uro-genital system in Amniotes from stage of sexual indifference (a) to male 
(b), and to female (c). In part after Gegenbaur. THE URINOGENITAL SYSTEM 
419 
foramen or may be absolutely closed. In the primates, in- 
cluding Man, the connection is not as intimate as this, and the 
ova occasionally escape into the coelomic cavity, as is normally 
the case among lower forms. Here, however, they usually 
disintegrate and become lost, but in rare cases a fertilized 
egg escapes in this way and may even attain considerable de- 
velopment through the formation of a sort of placenta, at- 
tached to the coelomic wall. 
A uterus is more or less an adaptive organ, related to 
Muller’s duct much as the crop is to the oesophagus; it is 
primarily nothing but a localized enlargement and develops 
whenever needed, in some cases appearing in a given form, 
while absent in a closely related one. Thus in vivaparous 
sharks (e.g., Squalus), the expanded lower portion of each 
Muller’s duct becomes enlarged and forms a uterus in which 
the embryos are retained until they reach practically the adult 
form; and the same is true in the case of certain salamanders. 
In none of these cases, however, is the organ more than 
a container or brood cavity, and there is no placenta 
formation or other direct connection between embryo and 
uterine wall. The same is true of the lower mammals, 
the marsupials and monotremes, in which there is no placenta, 
and the young are produced in a very immature state; but in 
the higher, or placental, mammals, the wall of the uterus be- 
comes differentiated for the purpose of the nutrition of the 
embryo, and thus becomes a definite physiological organ. 
There are numerous types of uterus among the mammals, 
depending on the degree of fusion between the Muller’s ducts 
of the two opposite sides; and these types consequently pre- 
sent a regular graded series between two distinct lateral uteri 
and a single median one (Fig. 139). 
In the first type of this series, that seen in monotremes, the 
two ducts are entirely distinct from one another. They are 
short, thick walled, and of rather large caliber, and may be 
termed oviducts or uteri, according to the taste of the writer, 
although the former term is more usually applied to them. 420 
THE HISTORY OF THE HUMAN BODY 
They open separately into a common urinogenital sinus in close 
association with the openings of the ureters. There are no 
vaginae, although the urinogenital sinus serves functionally as 
one. In the oviducts the small oval eggs, 3.5 x 4.0 mm. in 
diameter when they leave the ovary, are retained for some 
time and increase in size to about 12x15 mm• through the ab- 
sorption of nutrient fluids secreted by the oviducal walls. 
The next stage is that of the marsupials, in which the two 
ducts are still distinct, but each shows a differentiation into 
DUPLEX 
BIPARTITUS 
BlCORNIS 
SIMPLEX 
Fig. 139. Different types of mammalian uterus: explanation in text. 
oviduct, uterus and vagina. The two vaginal orifices open 
into a common urogenital sinus, which is here prolonged into 
a canal, and opens to the exterior separately from the rectum. 
Here for the first time the complete separation of these two 
canals, urinogenital and alimentary, is met with, since in mono- 
tremes, although internally distinct, there is externally a com- 
mon cloacal orifice. The partition separating the two becomes 
thicker and of more importance in the higher mammals, and 
forms the perinceum. THE URINOGENITAL SYSTEM 
421 
Above the marsupials the two ducts fuse into one at their 
terminal portions, and form a single vagina, though usually 
with two lateral uteri, various stages of the fusion of which 
form the successive steps known as uterus duplex, uterus bipar- 
tite, and uterus bicornis. The first of these types possesses 
two distinct openings for the uteri (ora uteri) which open 
into a common vagina; in the others there is a single os, but 
two uterine compartments. In all cases the vagina is single, 
but an indication of its former duplicity is observed in a few 
animals (e.g., Equus), in the form of a median longitudinal 
fold. 
The duplex type occurs in rabbits, and in Procavia (Hyrax), 
an isolated group of small mammals found in Western Asia 
and in Africa; the bicornis is widely distributed and occurs in 
ungulates, cetaceans, most bats, and other forms; the biparti- 
te occurs in carnivores, the pig, and a few bats. 
When several embryos are developed simultaneously, as in 
most small mammals, the two uterine halves become drawn 
out into long tubes, and the embryos are fixed at approximately 
equal intervals, each half containing about the same number. 
As the embryos develop, the portions of the tube in which they 
lie become greatly enlarged, while the intervening parts are 
restricted, giving the whole the appearance of a necklace or 
a string of sausages. 
The uterus simplex, characteristic of man and the apes, 
represents the extreme degree of fusion of the two parts. In 
this nearly all signs of its double origin are lost and the 
uterus assumes the form of a balloon-shaped or piriform or- 
gan, somewhat flattened dorso-ventrally, and possessing two 
oviducts, which open at the slightly prolonged antero-lateral 
angles. 
Uterus and oviducts are supported in all mammals by two 
principal suspensory ligaments, the broad and the round (ligg. 
latum et teres), which are easily explained by comparison with 
the indifferent condition. [Plate III, a.] Here the two layers 
of peritoneum that form the mesonephric ligament contain 
between them the mesonephros and its duct, and become con- 422 
THE HISTORY OF THE HUMAN BODY 
tiniied along the ventral surface of the mesonephros as a low 
longitudinal fold which contains in its margin the Mullerian 
duct. The germ gland (here the ovary) is attached to this 
along the inner side of the mesonephros by means of a narrow 
mesovarium. Imagine now the two important changes which 
actually occur, namely, the complete reduction of the meso- 
nephric system and the development of the Mullerian duct 
into oviduct and uterus, and we have as a result the broad 
ligament, arising from the body wall and extending to the 
oviduct, which it enwraps along its free edge. The round 
ligament is formed from the posterior slip of the original meso- 
nephric ligament, the ligamentum inguinale. 
The vestiges of the mesonephric system are found exactly 
where they would be expected, lying in the broad ligament 
not far from the oviduct, between it and the insertion of the 
mesovarium. They consist of three portions, epoophoron, its 
longitudinal duct, and paroophoron. [Plate III, c.] The first 
of these (“ organ of Rosenmiiller ”) consists of a series of 
blind tubules attached to a common duct, and plainly repre- 
sents the vasa efferentia and the upper portion of the Wolffian 
duct, in other words, the “ sexual kidney.” Below this are a 
few scattered tubules, forming the paroophoron and represent- 
ing the lower or urinary portion of the mesonephros. The 
longitudinal duct of the epoophoron (“ Gartner’s duct ”) is 
the remnant of the main part of the Wolffian duct, and lies 
imbedded in the wall of the uterus; it occasionally joins its 
upper part and thus completes the representation of the meso- 
nephric system. This occurs quite regularly in the pig, the 
horse and in ruminants, but is only occasional in Man. 
The original direction of the mesonephric ligament, that 
is, the direction which it has in the embryo, and that which is 
retained in adult Sauropsida, becomes changed in mammals 
and comes to lie transversally across the dorsal wall as though 
laid over laterally from above, the lower part remaining as at 
first. The principal effect of this is to remove the ovaries, 
and with them the oviducts, from their primary position in 
the lumbar region and locate them near or within the brim THE URINOGENITAL SYSTEM 
423 
of the pelvis, not far from the inguinal region. This occa- 
sions several compensatory changes, such as the lengthening 
of the ovarian nerves and blood-vessels, which are contained 
in the mesovarium. The significance of this process, known 
from its principal feature as the decensus ovariorum, is un- 
known, but it seems to correspond in part to a somewhat 
similar but more extensive descensus of the testis, found in 
the male. 
To comprehend the relationships of the male organs in mam- 
mals, it is best to begin again at the indifferent stage [Plate 
III, a], which is thus seen to furnish the starting-point for 
the explanation of the reproductive organs in both sexes. 
While the accessory organs in the female are mainly the pro- 
duct of differentiation in the Mullerian duct, the Wolffian duct 
becoming vestigial, in the male it is the Wolffian duct that is 
emphasized, together with the upper portion of the mesone- 
phros, while the Mullerian duct is reduced to a few rudiments. 
[Plate III, b.] As in male selachians and amphibians, the 
anterior tubules of the mesonephros serve as vasa efferentia 
for the conduction of the spermatozoa, but here they are used 
exclusively for this purpose, while the nephrostomes, Bow- 
man’s capsules, and all parts of those tubules once associated 
with the urinary function, are no longer developed. The 
tubules are much convoluted and form a compact mass, closely 
associated with the testis, the epididymis. The remaining 
mesonephric tubules, those of the posterior, or exclusively 
urinary, portion in lower forms, never develop into functional 
organs, but one or two of them, with blind free ends, may 
retain their connection with the Wolffian duct, and form the 
so-called vasa aberrantia, while the remainder, without con- 
nection at either end, form a rudiment termed the paradidymis 
(“Organ of Giraldes”), the homologue of the paroophoron, 
of the female. The Wolffian duct, freed from all association 
from urinary functions, becomes the exclusive spermatic duct, 
the ductus deferens (vas deferens). The Mullerian duct is 
lost along the greater portion of its extent, but leaves rudi- 
ments at either end. The posterior end is represented by the 424 
THE HISTORY OF THE HUMAN BODY 
appendix testis [hydatid of Morgagni], a knobbed body at- 
tached to the epididymis; the posterior by a median vesicle 
which leads from the dorsal wall of the urethra and lies im- 
bedded in the prostate gland, the prostatic vesicle, sometimes 
referred to as the uterus masculinus. 
The history of the urogenital organs, as thus far consid- 
ered, with their correspondence in the two sexes, may be con- 
veniently shown in a table, in which the first column gives each 
part in its primary morphological significance, while the second 
and third state their ultimate fate in the male and female 
Amniota respectively. Vestigial parts are given in italics. 
This table may be studied in connection with the one at the 
end of the chapter, in which the external parts are considered 
in the same way. 
In the monotremes the ductus deferentes [vasa deferentia] 
open into a urinogenital sinus, the ventral recess of a common 
cloaca, in common with the urethra or excurrent duct of the 
urinary bladder; in all higher mammals, however, with the 
formation of a perinaeum or division between this urinogenital 
sinus and the rectum, the ductus deferentes are received by 
the much prolonged urethra so that the distal portion of this 
is a common duct for both urinary and reproductive products, 
a resumption of early conditions under another form. 
While in the monotremes and in certain placental mammals 
the testes remain throughout life in or near the original posi- 
tion, in others they experience a more or less marked change 
of location. This is termed the descensus testiculorum, and 
is more or less comparable to a similar descent on the part of 
the ovaries, although the procedure involves different parts, 
and is quite likely of a different historical significance. Aside 
from the monotremes, no appreciable descent takes place in 
elephants and in certain insectivores, while in sloths and ant- 
eaters the testes descend considerably and take up a final posi- 
tion in peritoneal folds between bladder and rectum, but still 
within the pelvic cavity. The only other placental mammals 
in which there is no external manifestation of this process are 
the armadillos, related to these last, and the two aquatic or- THE URINOGENITAL SYSTEM 
425 
ders of Cetacea and Sirenia, in which the condition is plainly a 
secondary modification due to the needs oj an aquatic life. In 
all remaining mammals the process is connected with the for- 
mation of an inguinal canal, a subcutaneous evagination of 
the body wall involving muscles and peritoneum, and the 
testes pass into this either periodically, in association with 
sexual activity, or permanently. The former condition occurs 
Morphological Designation 
(Embryonic or phylogenetic) 
Male 
Amniote 
Female Amniote 
Germ gland 
Testis 
Ovary 
Mesonephros (upper portion) 
Epididymis 
Epobphoron (in part) 
Mesonephros (lower portion) 
Ductuli 
aberrantes 
Paradidymis 
Paroophoron 
Mesonephric (Wolffian) duct 
Ductus deferens 
Epobphoron (in part) 
Longitudinal duct of 
epobphoron = Gartner's 
duct 
Muller’s duct 
Appendix 
testis 
Vesicula 
prostatica 
Oviduct 
Uterus 
Vagina 
Urogenital sinus 
Morphological 
urethra, i.e. the 
portion between 
the bladder and 
entrance of the 
ductus deferentia 
Urethra (entire) 
among many insectivores and rodents, and in the bats; the 
latter is characteristic of the land carnivora, ungulates, most 
lemurs and the primates. 
In the majority of animals coming under this latter head, 
that of permanent descent, the testes lie in a special integumen- 
tal sac, the scrotum, but in some cases, as tapirs, rhinoceros, 
etc., there is no definite scrotum, and the testes lie beneath the 
integument of either the inguinal or perinseal regions. The 
scrotum is originally double, furnishing a separate sac for each 
testis, but usually the two are fused into a single median sac in 426 
THE HISTORY OF THE HUMAN BODY 
which the suture of union is usually apparent. In relation to 
the penis, the scrotum is originally anterior to it, prepenial, as 
in all marsupials that possess one, but in placental mammals 
these relations are reversed and the scrotum becomes postpenial 
through the migration of the penis in an anterior direction. 
No definite cause for the descensus is known, either phylo- 
genetic or physiological, and the phenomenon has gained rather 
than lost in complexity through recent researches which show 
the cooperation of several distinct elements previously not 
taken into consideration. Formerly a mechanical explanation 
was found in the gradual contraction of the band of perito- 
neum which extends from the testis to the inguinal region 
(Plate III, b), and termed the gubernaculum in reference to 
its supposed function, but the matter is not as simple as that, 
since this band itself is composed of several originally dis- 
tinct elements, and, furthermore, can hardly be considered to 
exert the tension ascribed to it. The initiative in the process 
seems to be a slight invagination of the abdominal wall at the 
point of insertion of the inguinal ligament. Through a sub- 
sequent evagination followed by a second invagination a coni- 
cal body is formed, the conus inguinalis (Fig. 140, A), which 
involves the muscular layers, and by a final outpushing of 
this and the surrounding structures a subcutaneous muscular 
pouch is formed, the bursa inguinalis, in the bottom of which 
lies the conus, which serves as a point of insertion of the in- 
guinal ligament. The bursa is lined by a pocket of perito- 
neum, the processus vaginalis, which is reflected up over the 
conus. The inward development of the conus absorbs and 
shortens the inguinal ligament, and eventually the testis comes 
to lie in the bursa, covered internally by the reflected peri- 
toneum. As shown above, the bursa may or may not become 
placed in a scrotal sac, but when it does, a scrotal ligament 
(1chorda gubernaculi) extends from the bottom of this sac to 
the base of the conus. 
In a complete, or typical descensus, in which the bursa is 
contained in a scrotal sac, the parts are related as in Fig. 140, 
B. The processus vaginalis of the peritoneum, continuous THE URINOGENITAL SYSTEM 
427 
beyond the sac with that which lines the abdominal wall, 
wraps itself partly around testis and epididymis, thus forming 
a membrane, the tunica vaginalis propria, with a parietal and 
visceral layer, and a serous cavity included between them. 
This serous cavity is naturally continuous with the main ab- 
dominal cavity, the coelom, and the passage between them re- 
mains open in those mammals in which the external appearance 
of the testes is periodic; in those, however, in which the 
descent is final and definite, it closes up during late, often 
post-natal, development, and all communication between the 
two cavities is lost. The vessels and nerves of the testis, to 
which is added the ductus deferens, become united by con- 
nective tissue into a single structure, the spermatic cord, which 
escapes from the testis along the side not invested by perito- 
neum, becomes recurved and enters the abdominal cavity by 
running along the wall of the pouch, covered by the parietal 
layers of peritoneum, i.e., the tunica vaginalis propria. 
Outside of this come three layers which represent the ab- 
dominal muscles and their fascia; in order, beginning from 
within: i. the tunica vaginalis communis, i.e., common to both 
<J # 
testis and spermatic cord, a continuation of the fascia trans- 
versa; 2, the cremaster muscle, a continuation of the trans- 
versalis and internal oblique, and 3, the fascia cremasterica 
[Cooper’s], which represents the external oblique, but is with- 
out muscular fibers. 
Beyond this comes the integument, although this is often 
differentiated into two layer through the development of its 
involuntary muscular fibers into a layer of integumental mus- 
cle, the tunica dartos, which occasions a wrinkling of the 
surface in response to slight stimuli. 
The external reproductive organs associated with the 
cloaca have arisen as one of the adaptations required by 
the assumption of a terrestrial existence, the ultimate 
cause being found in the non-suitability of the air as a 
medium for the transmission of the spermatozoa. These 
delicate motile cells can exist only in a liquid medium, 
and from this cause alone arise in all terrestrial animals the 428 
THE HISTORY OF THE HUMAN BODY 
necessities, first, of secreting a liquid to serve as a vehicle for 
the male germ cells, and second, of developing organs through 
which this liquid may be directly transmitted to the cavities 
of the female organs, without suffering from the drying action 
of the air. That this necessity was not immediately apparent 
is due to the semi-aquatic habits of most amphibians, even the 
most terrestrial of which resort to the water at the breeding 
Fig. 140. Diagrams illustrating the descent of the testes in mammals 
[After Weber.] 
t, testis; p, epididymis; s, spermatic cord; m, mesorchium; It, ligamentum testis; 
li, ligamentum inguinale; t, bursa inguinalis; x, conus inguinalis; y, chorda guber- 
naculi (=ligamentum scroti); a, tunica vaginalis propria, visceral layer; b, the same, 
parietal layer; c, tunica vaginalis communis, continuous with the fascia trans- 
versa; d, cremaster ( = transversalis-obliq. int. abdom.); e, fascia cremasterica Cooperi 
(=obliq. ext. abdom.); f, integument, including the tunica dartos and involuntary 
muscular layer. 
season and are thus able to dispense with any external mech- 
anism; yet here, notwithstanding the absence of external or- 
gans, there have arisen numerous habits, such as the love 
antics of salamanders and the amplexation of frogs and toads, 
which are designed to secure a greater likelihood of fertiliza- 
tion and thus form the prelude to the development of a gen- 
uine copulation. THE URINOGENITAL SYSTEM 
429 
It is evident, however, that with the complete relinquish- 
ment of an aquatic life, and the subsequent impossibility of 
employing an external vehicle for the conveyance of the sper- 
matozoa, some method must be found by means of which the 
seminal fluid may be conveyed direct from the male to the 
female; and this process, beginning with the most natural 
stage of the approximation of the two unmodified cloacae, 
would develop first a temporary evagination of a portion of the 
inner cloacal wall, and then a permanent modification of this 
evaginating portion; a development which would naturally 
take place in the male alone, as the producer of the fluid to be 
transferred. There thus arises for the first time in vertebrates 
a cloacal intromittent organ or penis, three distinct types of 
which are found; these appear to have arisen independently, 
although in all cases by a modification of the cloacal wall. 
The first is seen in those highly specialized burrowing am- 
phibians, the Gymnophiona, and consists of a protrusible tube 
worked by muscles; the second is that of lizards and snakes, 
and is in the form of two lateral protrusible sacs, the walls 
of which are often cornified, and possess a spiral groove for 
the conveyance of the spermatic fluid; the third occurs in its 
simplest form in turtles and crocodiles and suggests a terrestrial 
origin for both groups. This latter is the type from which the 
penis of mammals is derived, and may be described more 
at length. Owing to the imperfectly understood law of 
sexual homology which obtains among vertebrates, this organ, 
sometimes termed the phallus to distinguish it from the 
other types, exists also in the female in a much reduced 
form, and is termed the clitoris. Although useless as an 
intromittent organ, it reflects the peculiarities of the male 
organ and in the various groups often shows in a reduced 
form the characteristics developed by the phallus. 
The phallus develops from the ventral wall of the cloaca 
and consists of a longitudinal thickening of fibrous tissue, the 
corpus pbrosum, upon which rests a mass of cavernous (erec- 
tile) tissue in the form of two lateral ridges, the corpora 
cavernosa, with a median groove between them, the seminal 430 
THE HISTORY OF THE HUMAN BODY 
groove. This entire organ is somewhat tongue-shaped and 
free at the tip, and is capable of considerable protrusion be- 
yond the cloacal orifice. The urogenital sinus, bearing the 
openings of the ductus deferentes, opens into the seminal 
groove near its proximal end. 
Although the phallus of these reptilian forms seems at first 
sight quite distinct from that of mammals, and although there 
exist at present no transition forms among adult animals, the 
development of these parts in mammals supplies the missing 
portions of the history and substantiates the homology. The 
essential change is that of the conversion oj the spermatic 
groove into a complete tube, which is accomplished by the 
increase in size of the lateral ridges and their subsequent 
fusion, a process repeated biogenetically during early develop- 
ment. The failure to complete this produces the condition 
known as hypospadias, and is thus seen to be a case of arrested 
development, the retention of the reptilian stage. 
The relative position of the penis changes completely dur- 
ing its mammalian history from a post-scrotal one with the 
free end directed posteriorly to one that is pre-scrotal and 
directed anteriorly. The first of these positions is similar to 
that of the turtles and crocodiles and is seen in the mono- 
tremes, and to a lesser extent in marsupials; the latter posi- 
tion is characteristic of placental mammals. This change may 
be made clear by the accompanying diagrams (Fig. 141). 
In the monotremes the conditions are still essentially rep- 
tilian. There is a common cloaca and the penis projects a 
little from its ventral wall. The ureters, ductus deferentes and 
urinary bladder form a common duct which under normal con- 
ditions serves merely as a passage for the urine. This duct 
is morphologically the urethra as far as the entrance of the 
ductus deferentes and ureters; beyond this point it is morpho- 
logically the urinogenital sinus. The erection of the penis, 
through the slight lengthening of its inner end, closes the 
entrance into the cloaca, but continues the urogenital canal 
into its own lumen, thus forming a direct outlet from the 
ductus deferentes to the exterior. At the same time the free THE URINOGENITAL SYSTEM 
431 
end becomes protruded from the cloacal orifice, and the organs, 
usually wholly subservient to the urinary function, become 
for the time being wholly reproductive. 
Fig. 141. Relationships of the male urogenital organs in mammals 
[After Weber.] 
(a) Monotremes. (b) Marsupials. (c) Placental mammals. 
s, symphysis pubis; in, intestine; v, urinary bladder; u, ureter; t, testis; w, ductus 
deferens; p, prostate; g, vesicular gland; c, bulbo-urethral gland; cp, corpus 
cavernosum penis; cu, corpus cavernosum urethras spongiosum). 432 
THE HISTORY OF THE HUMAN BODY 
The marsupials show an intermediate condition by which 
the transition to the placental mammals can be explained. 
The cloaca has been divided by a perinseum and the alimentary 
and urogenital outlets have become entirely separated. The 
testes show a marked descensus and usually come to lie in a 
scrotal sac, which is prepenial in position. The penis is 
posterior to the testes and is still directed backwards as in 
monotremes and sauropsids, but becomes attached at its proxi- 
mal end to the posterior margin of the os pubis. 
The true urethra is very short, as the ductus deferentes enter 
the tube soon after its origin, but the urogenital tube thus 
formed is permanently continuous with the lumen of the penis, 
forming a long urogenital canal. This condition is essentially 
that found in placental mammals except for the relative posi- 
tion of the penis, which in the latter animals, retaining its 
proximal attachment to the lower margin of the os pubis, 
turns about and becomes directed anteriorly, thus changing its 
apparent relations with the testes, which are now post-penial. 
Connected with the penis are various sorts of glands, em- 
ployed mainly for the purpose of furnishing a liquid vehicle 
for the spermatozoa. They are thus the most widely developed 
in mammals of marked fertility, like rodents and insectivores, 
and may be arranged in five groups, each associated with a 
definitive part of the spermatic tract (Fig. 142). The glandules 
ductus deferentis are thickenings of the wall of the ductus de- 
ferens, and are situated near its entrance into the urinogenital 
canal. The glandules vesicales are large and evident glands, 
which open near the latter. These have often been considered 
as receptacles for the spermatic fluid, and are hence usually 
called seminal vesicles, but they are clearly glandular in their 
nature, and their cavities contain spermatozoa only by acci- 
dent. Of the remaining three, which open into the urinogenital 
canal, the primitive condition is seen in the urethral glands, 
tubular glands occurring in the walls of the above canal, es- 
pecially along its proximal portion. From such elementary 
structures are derived the two other sets, the prostate and the THE URINOGENITAL SYSTEM 
433 
bulbo-urethral [Cowper’s]. Of these the former are more 
proximal in position, the latter more distal. 
The function of these five sets of glands seems in all cases 
that given above, and their occurrence in the various mam- 
Fig. 142. Penis of placental mammals. 
(A) Mouse (Mus musculus). [Combined from Rauther and Oppel.] (B) 
Hedgehog (Erinaceus curopaeus.) [From Oppel, after Seubert.] 
k, kidney; u, ureter; b, bladder; t, testis; e, epididymis; v. d., ductus deferens; 
cc, corpora cavernosa; v, vesicular glands; pr, prostate glands; c, bulbo-urethral 
(Cowper’s) glands; p, preputial glands. 
mals is such that the large development of one is compensatory 
for the small size of another. 
Thus in monotremes and marsupials there is no prostate 
gland, but the urethral glands are very abundant; the vesicular 
glands are wanting in carnivores, but large and well developed 
in primates. In Man the most important of these is the pros- 
tate, but the vesicular are also well developed. The bulbo- 
urethral glands are evident but not voluminous. In addition 
to the above glands, the function of which is to furnish a 434 
THE HISTORY OF THE HUMAN BODY 
liquid vehicle for the spermatozoa, occur certain modified in- 
tegumental glands, like the preputial, the function of which 
is to lubricate the parts. 
The external organs of the female are but slightly developed 
and appear to represent the various elements found in the 
male, though retained permanently in a reduced and almost 
embryonic condition. This is best shown by a comparison of 
the two as they appear in development, differentiating from 
an indifferent condition common to both, as in the case of the 
internal parts (Fig. 143). As this history begins with a 
simple cloaca and develops the external parts from its walls 
and margin, the history recapitulates also, in a very complete 
fashion, the stages shown phylogenetically in the preceding 
pages. 
In an early human embryo the cloacal orifice is approxi- 
mately circular in shape and is surrounded by a rounded and 
somewhat elevated margin, the genital ridge. From within 
its anterior wall, projecting a little beyond the cloacal ori- 
fice, rises a conical papilla, the genital tubercle [g], which is 
really in the form of an inverted trough, enclosing the uro- 
genital sinus and freely open along its ventral aspect, thus 
forming the genital cleft [r\. At a later stage the cloacal 
orifice becomes more prolonged dorso-ventrally, and the genital 
ridge has become more pronounced along the edges, forming 
two lateral ridges [/?], instead of a circular lip. The genital 
tubercle has also developed and projects conspicuously from 
the ventral margin of the orifice; its groove is still conspicu- 
ous, but not so widely open, and its lateral lips take on the 
aspect of rounded folds [c]. The terminal end of the rectum 
has become visible and forms an anus, distinct from the gen- 
ital parts, but almost continuous with them. 
Thus far the conditions in the two sexes are precisely alike 
and the stages are termed indifferent although we have reason 
to believe that the sex determination is made at a far earlier 
period than the first one considered here, probably even at 
the time that the egg is fertilized. 
At about the point we have now reached, however, sexual THE URINOGENITAL SYSTEM 
435 
differences begin to appear, as may be seen by a comparison 
of the remaining figures. The female organs, which remain 
nearer the embryonal condition, are not essentially different, 
save in proportions, from the last stage common to both. 
The genital cleft remains open, forming the introitus vaginae, 
into which empty the united Mullerian ducts (uterus) and 
Fig. 143. Development of the external genitals in Man, 
(a) and (b) Indifferent stages, (c) Early stage of the male organs, (d) Early 
stage of the female organs. 
g, genital turbercle; c, genital folds; h, genital ridges; r, genital cleft; ra, anus; 
p, perinasum. 
the two ureters. The genital folds form the corpora cavernosa 
(labia minora or nymphos) and the free tubercle itself forms 
the clitoris. The external lips of the cloaca, the lateral genital 
ridges, form the greater labia (labia majora). 
In the male the genital tubercle develops into the glans and 
corpus cavernosum urethrae [corpus spongiosum], and the 
genital folds become the corpora cavernosa penis. By the 
fusion of these latter the groove becomes converted into the 
urinogenital canal, which becomes continuous with the urethra, 436 
THE HISTORY OF THE HUMAN BODY 
and into which the ductus deferentes empty. The lateral gen- 
ital ridges unite to form the scrotal sac, and the point of union 
between these is marked by a raphe. A median line of dark 
pigment lies along the under side of the penis continuous with 
the latter and marks the fusion of the lips of the genital 
groove. Special muscles, also in part sexually homologous, 
develop in connection with the external organs of both sexes. 
These are composed of striated fibers and are more or less 
under the control of the will. 
The elements of the indifferent stage and their differentia- 
tion in the two sexes may be expressed in the following table, 
which shows the sexual homologies of the external parts. This 
table, taken in connection with the one given above, for the 
internal parts (p. 425) will form a brief synopsis of the 
entire subject. 
Embryonal Part 
Male 
Female 
Genital ridges 
Scrotum 
Labia majora 
Genital tubercle 
Corpus cavernosum 
urethrae 
Gians penis 
Clitoris 
Genital cleft 
Pigmented line 
Introitus vaginae 
Genital folds 
Corpora cavernosa 
penis 
Labia minora CHAPTER XII 
THE NERVOUS SYSTEM 
“ Indeed, while Nature is wonderfully inventive of 
new structures, her conservatism in holding on to 
old ones is still more remarkable. In the ascending 
line of development she tries an experiment once 
exceedingly thorough, and then the question is solved 
for all time. For she always takes time enough to 
try the experiment exhaustively. It took ages to 
find how to build a spinal column or brain, but 
when the experiment was finished she had reason to 
be, and was, satisfied.” 
John Tyler, The Whence and Whither of 
Man, p. 173. 
Any treatment of the Nervous System, even the morpho- 
logical one such as is to be considered here, involves certain 
fundamental principles of histology and physiology which 
must be kept in mind at all points. They are therefore to 
be considered first. 
Nervous tissue is ectodermal in origin and is thus ultimately 
derived from the external surface of the body, naturally from 
that part which first comes into contact with the external 
world. It thus differentiates in response to the countless stim- 
uli which assail it from outside. 
The nerve cells, which form the distinctive structural ele- 
ments of the nervous system, are peculiar in that they have 
departed far from the typical cuboidal form of embryonal 
ectoderm and have developed extensive branched cytoplasmic 
outgrowths or processes from the nucleated cell body (Fig. 
144). In the diffuse type of primitive nervous system such 
as is found in certain of the lower invertebrates, and, indeed, 
in certain limited regions of the vertebrate nervous system, 
such as the plexuses within the walls of certain of the viscera, 
these branched cells form a true nervous syncytium through 
437 438 
THE HISTORY OF THE HUMAN BODY 
Fig. 144. Varieties of neurones in Man. 
(A) From spinal ganglion; (B) from ventral horn of spinal cord; (C) Pyramidal cell from cerebral cortex; (D) Purkinje cell 
from cerebellar cortex; (E) Golgi cell from spinal cord ; (F) Fusiform cell from cerebral cortex; (G) Sympathetic. 
a. Axone; d. Dendrites. [From Santee.] THE NERVOUS SYSTEM 
439 
the protoplasmic continuity of the ends of their branches. 
In the centralized types of nervous system such as have de- 
veloped in the higher invertebrate phyla and in the vertebrates, 
the nerve cells are for the most part merely in contact with 
each other through the fine terminal branches of their processes 
(the end arborizations or telodendria). Such a connection 
constitutes a synapsis. To a nerve cell of this sort the term 
neurone is given; and since, through their series of synapses, 
they form paths for the conduction of energy in the form 
of nerve impulses, neurones constitute not only the structural 
but also the physiological units of the nervous system. 
As functional parts of the nervous system, serially related 
neurones form the conducting portions of mechanisms known 
Fig. 145. A diagram of a reflex arc. 
Showing the simplest type of reflex arc in which the adjustor (Ar) consists of only two neurones, a 
sensory (S.N.) and a motor (M.N.); and a more complicated type in which the adjustor includes (A2) 
association neurones (A.N.) interposed between the sensory and the motor neurone. R, Receptor 
E, Effector. 
as reflex arcs, each of which is a device for bringing about 
such activity as will constitute the suitable response of the 
organism to a stimulus which has been applied to it. A reflex 
arc (Fig. 145) comprises in addition to the synaptic series of 
neurones which forms its conducting mechanism or adjustor: 
a receptor in the form of some device (i.e., a sense organ) 
through which the stimulus is applied to the first neurone of 
the series, known as the sensory neurone; and an effector in 
the form of muscle fibers or gland cells to which the last neu- 
rone of the series, known as the motor neurone, transmits the 440 
THE HISTORY OF THE HUMAN BODY 
stimulus which brings about the muscular or glandular re- 
sponse. Reduced to its lowest terms the adjustor may consist 
of a sensory and a motor neurone, but there are usually inter- 
posed between these two a variable number of association 
neurones, and it is thus possible for a reflex arc to involve 
extensive regions of the nervous system. 
The synaptic relationship of neurones involves a polarity 
which forms the basis for the classification of their processes. 
Each neurone of whatever type has a single process, known 
as the neurite or axone, which is so placed that it conducts 
impulses away from the cell body. The other processes, 
which are known as dendrites, bring impulses to the cell body. 
Sensory neurones have each a single dendrite which like the 
neurite is usually greatly prolonged. Its end arborizations are 
thus located usually at a considerable distance from the cell 
body among the cells of other tissues which are so modi- 
fied as to form the sense organ or receptor. Sensory neu- 
rones have thus typically the form known as bipolar, with a 
spindle-shaped cell body, one end of which is continuous with 
the dendrite while the other end is continuous with the neu- 
rite. In the higher vertebrates, however, the majority of bipolar 
neurones have become secondarily modified in form so that 
the cell body is pear shaped or flask shaped and thus ap- 
parently unipolar. In reality it is structurally and function- 
ally bipolar, however, since the dendrite and the neurite have 
simply united at a short distance from the body of the cell. 
Association neurones and motor neurones have more than one 
dendrite and are thus known as multipolar neurones. The 
dendrites of such neurones present usually the extensively 
branched tree-like form to which the name dendrite was origi- 
nally appropriately applied. 
The cell bodies of neurones are grouped together in masses 
which constitute the gray substance of the nervous system, 
while those cytoplasmic processes which are so greatly pro- 
longed that they pass out beyond the limits of the gray sub- 
stance become the axis cylinders of nerve fibers, which are 
collected into bundles or fasciculi having the same gen- THE NERVOUS SYSTEM 
441 
eral course, and constitute the white substance of the nervous 
system, so named because of the white appearance due to 
the presence of a non-cellular fatty sheath, the myelin sheath 
or medullary sheath, which such prolonged processes usually 
acquire. In the case of bipolar (sensory) neurones both the 
dendrite and the neurite are generally thus prolonged to form 
axis cylinders of nerve fibers. In the case of multipolar neu- 
rones, on the other hand, it is the neurite alone which may 
be prolonged. Motor neurones always have the neurite 
prolonged in this way, and it is true of many association 
neurones as well, but there are also large numbers of the latter 
in which the neurite, as well as the dendrites, is so short as 
to remain within the limits of the gray substance in which 
the cell body is located. 
Masses of gray substance constitute the various nerve 
centers such as (i) ganglia which are somewhat isolated 
nodules, and (2) nuclei which are masses, more or less con- 
tinuous, located within the boundaries of the brain or spinal 
cord. In the higher vertebrates, notably in the mammals, 
there are extensive superficial layers of gray substance form- 
ing the cerebral and cerebellar cortices of the brain. The 
white substance, when made up of bundles of fibers which 
proceed from one nerve center to form synapses with neurones 
of another nerve center, is known as a nerve tract. When, on 
the other hand, the bundles of fibers connect a nerve center 
with structures outside of the nervous system they are known 
as nerves. It is obvious that nerves may comprise two types 
of nerve fibers, (1) sensory fibers, which are the dendrites 
of sensory neurones and bring impulses into the nerve center 
from the receptor, and (2) motor fibers which are the neurites 
of motor neurones and carry impulses from the nerve center 
to the effector. 
The central nervous system begins its history as a straight 
tube lying along the mid-dorsal line just beneath the integ- 
ument. Anteriorly this tube ends blindly and exhibits a series 
of three vesicular enlargements, the beginnings of the brain; 
posteriorly it ends blindly also and tapers to a point, although 442 
THE HISTORY OF THE HUMAN BODY 
there are certain mysterious indications in the embryonic 
record of a former connection with the lumen of the alimentary 
canal, indications which have not as yet received any satis- 
factory explanation, and which may be, after all, merely de- 
velopmental necessities, without historic significance. Through 
modification of this simple neural tube without the addition of 
extraneous elements save as auxiliary to this, there arise in all 
vertebrates the brain and spinal cord, which, even in their 
highest and most complicated form, appear to the morpholo- 
gist as still tubular; the walls, enormously thickened in places 
and often folded, give rise to such massive structures as the 
cerebellum and the cerebral hemispheres; the lumen persists 
as the ventricles of the brain, and their continuation through 
the spinal cord as the canalis centralis. 
All nervous systems have arisen in the beginning in response 
to stimuli from without, and hence developed originally upon 
the surface of the body, especially upon that portion which, 
through the customary position of the body, is exposed to 
such stimuli. Such superficial systems are still found among 
lower invertebrates; in sessile radiate forms equally developed 
on all sides of the projecting rim or upon the tentacles, in free- 
swimming forms as an apical plate located upon the point 
which first comes in contact with external objects. That such 
was also the case with the unknown ancestor of vertebrates 
is suggested by the embryonic history of the neural tube, for 
it is formed here by the rolling in of the external dorsal sur- 
face of the early embryo. This process is inaugurated by the 
formation of two longitudinal neural folds, one upon either 
side of the middle line, and as these are united around the 
anterior end and diverge posteriorly, they form for a time a 
figure shaped not unlike a hair-pin. The elongated area or 
neural plate enclosed between the neural folds becomes some- 
what sunken, and as the two folds, beginning anteriorly, ap- 
proach one another and finally unite, the area becomes the 
bottom of a trough and eventually the inner surface of a 
tube. 
The complete coalescence of the folds and the pinching off THE NERVOUS SYSTEM 
443 
of the trough are the final steps in the process, which results 
in the formation of the neural tube as described above, the 
anlage of the central nervous system. If we may take this 
process as a recapitulation of pre-vertebrate conditions, a view 
sustained by its universality, it suggests that the primitive 
ancestor of vertebrates was exposed to external stimuli mainly 
over its dorsal surface, a supposition which in turn suggests 
a slightly flattened, worm-like form, with the ventral side 
resting upon the ground, here undoubtedly the ocean bottom. 
The greater development of the anterior portion of this tube, 
even from the first, suggests a locomotive habit, which would 
thus favor the anterior end in this regard. As this superficial 
nervous system became more highly developed, and hence 
more sensitive, it was protected in the most natural way for 
such a system, by the formation of elevated ridges along its 
lateral borders, thus forming a dell or trough, in the bottom 
of which lay the sensitive surface. Such method of protection, 
once inaugurated, Gould have but one logical outcome, the 
gradual formation of a tube through the increase in height and 
the approximation of the protecting lateral folds, until in this 
way the form was attained with which all the present-day verte- 
brates are equipped. We must here not lose sight of the fact 
that the original external, and hence the sensitive, surface is 
that which becomes the inner surface of the tube, since this ex- 
plains the primitive location of the gray substance along the 
lumen of the tube rather than along its external surface. 
A central nervous system by thus sinking into the interior 
and becoming entirely covered up by a much less sensitive 
surface, gains the protection which it seeks, but in order to 
be still accessible to external stimuli, a condition upon which 
its very existence as a nervous system depends, it retains its 
connection with the exterior through sets of secondary cells 
which become interposed between the external surface and 
the central organ. These are the sensory cells, the vast majority 
of which eventually develop into true sensory neurones. In 
a few instances, however, the sensory cells retain not only 
their original position upon the external surface, but some- 444 
THE HISTORY OF THE HUMAN BODY 
thing of their epithelial character, so that it is difficult to 
decide whether they are to be considered as nerve cells or 
epithelial cells. 
Fig. 146. Developmental stages of neural tube and spinal ganglia in a salamander, 
Ambystoma punctatum. Cross sections. 
(A) one side of neural plate, showing early stage of neural fold; (B) neural tube, just after closure, 
cells of neural crest indicated by shading ; (C) interstage, showing beginning migration of spinal 
ganglion cells; (D) still later stage, cells of neural crest have migrated iatero-ventrally to form spinal 
ganglion. [From Child, after JohnstonJ THE NERVOUS SYSTEM 
445 
The sensory cells take their origin from the ectoderm by 
two general methods. The most extensive origin is from 
structures known as the neural crests (Fig. 146), the differ- 
entiation of which begins simultaneously with that of the 
neural plate itself since along the line of the neural folds 
there is proliferated upon each side a plate of ectodermic cells 
which lie along the line of junction of the neural plate and 
the more lateral integumental ectoderm, and as the neural 
tube closes in by the approach and coalescence of the neural 
folds, these plates intrude themselves as neural crests be- 
tween the integumental ectoderm and the neural tube, with 
which they retain at first their connection along the mid- 
dorsal line. The crests are practically co-extensive with the 
neural tube, breaks in the continuity occurring only in the 
anterior region. In certain of the lower vertebrates the crest 
cells appear at first to be incorporated into the dorsal wall 
of the neural tube, to migrate out as distinct overhanging 
wings later. In fishes it is thought that some of these cells 
may become permanently incorporated into the neural tube 
and form the “ giant cells ” true sensory neurones within the 
spinal cord. 
As the neural crests assume a more lateral position, they 
become divided up into segmentally arranged, paired masses, 
certain cells of which send out two cytoplasmic outgrowths, 
the dendrite growing peripherally toward the external sur- 
face of the body, the neurite growing centrally to enter the 
latero-dorsal region of the neural tube as component fibers 
of the dorsal nerve root. These cells are the sensory neurones 
and their cell bodies are located in the segmentally arranged, 
paired spinal ganglia upon the dorsal roots of the nerves. 
The second method by which sensory cells arise from the 
ectoderm is by the development of a series of structures known 
as placodes. These are ectodermal thickenings in certain re- 
gions where the developing neural crest comes into contact 
with the inner surface of the integumental ectoderm. Some 
of the cells from certain of these placodes detach themselves 
and become an intrinsic part of the ganglia which develop from 446 
THE HISTORY OF THE HUMAN BODY 
the crest. Others of the placode cells may remain in a more 
primitive position as sense cells in the ectodermal layer, but 
connected with the neural tube either directly through true 
neurites which grow into the central organ, or through the 
intermedium of the sensory neurones of a ganglion. 
But not alone are the sensory neurones undergoing differ- 
entiation during this early developmental stage. Within the 
neural tube those cells which are to become the functional 
nerve cells of the brain and spinal cord are sending out cyto- 
plasmic outgrowths, first the neurites, then in rapid succession 
the several dendritic processes, since the neurones of the neural 
tube are multipolar neurones of the motor and association 
types. The processes of the association neurones either enter 
into the formation of a rapidly increasing synaptic maze within 
the limited region in which the cell bodies are located, form- 
ing the type of structure known as the reticular formation, or 
they may bring more extensive regions of the central nervous 
system into the reflex arcs of which they form a part, by send- 
ing out greatly prolonged neurites which pass to other parts 
of the spinal cord or brain as component fibers of nerve tracts. 
The prolonged neurites of the motor neurones extend out in 
their growth from the latero-ventral region of the neural tube 
as component fibers of the ventral nerve root and reach the 
adjacent myotomes, thus providing motor nerve fibers to the 
developing musculature. 
There are certain cells, moreover, which during this early 
development of the nervous system, bodily migrate out from 
the neural tube, following either the course of the neural crest 
or the ventral nerve root. Among these there are cells which 
develop into the neurones of the sympathetic system, the 
ganglia of which are located outside the limits of brain and 
spinal cord. 
Thus simultaneously with the formation of the central 
nervous system there arises also the peripheral nervous system 
by means of which every part of the living organism is brought 
into communication with the central organs, the spinal cord 
and brain. THE NERVOUS SYSTEM 
447 
While the secondary system of sensory cells, like the system 
of scouts and outposts of a modern army, meets the external 
world with its hazards, the central nervous system, and more 
especially the brain, like the general staff, remains in safety, 
though in constant communication with the front. The eyes 
see, the ears hear, the outer surface receives constant evidence 
of the external world, while the brain, immured within a dense 
wall of bone sits in utter darkness and silence. It neither 
hears nor sees; no ray of light ever penetrates its obscurity, 
and even when exposed through injury or operation it is found 
to have no power of direct perception or even of sensation; 
and yet it directs the entire mechanism with the utmost in- 
telligence, sending its messages to the motor system, and 
causing the entire body to act in the strictest harmony with 
the external conditions. In the performance of this function 
it has developed a complexity immeasurably in excess of that 
of any other organ, and even far beyond that of its own sense- 
organs, since these latter attain a high degree of development 
among fishes, while the brain continues its development 
through amphibians and reptiles, becomes larger and more 
complex among the mammals, especially along the line leading 
to the anthropoids, and attains its highest point in the human 
species, a member of the latter Order not otherwise to be 
especially distinguished from the remainder of the group. It 
is thus to be concluded that the remarkable development of 
brain characteristic of mammals in general, and the Anthro- 
poidea in particular, has not been brought about through a 
greater perfection of the sense-organs, but rather by increasing 
its own power of receiving the sensory impressions and of 
recording and interpreting them through the formation of as- 
sociation paths; and this, like all other structural advances, 
has been gradually brought about through the working of 
natural law, as a more perfect adaptation to environment. 
The material history of this advance appears to the mor- 
phologist as the gradual modification of the simple neural tube 
described above, a development which is traceable alike in the 
comparison of adult animals, Class by Class, and in the em- 448 
THE HISTORY OF THE HUMAN BODY 
bryological record of a single animal, the lower forms preserv- 
ing in greater detail the early stages of the history, the higher 
forms recording the later stages. In this way from the num- 
berless fragmentary records presented to the investigator, the 
history of the neural tube in its progressive modifications 
may be reconstructed. 
In all except the head region the neural tube retains its 
general tubular form and develops into the spinal cord, the 
more conservative as well as the more extensive region of the 
central nervous system. Its lumen, the canalis centralis, re- 
mains minute in caliber while its walls become enormously 
thickened, except in the mid-dorsal and mid-ventral lines 
where deep longitudinal furrows thus develop. In relative 
weight, however, as compared with the more highly modified 
region, the brain, the cord may be said in a general way to 
decrease as we ascend the scale of vertebrates, but this is 
due rather to the increase in the size of the brain than to a 
decrease in that of the cord. There is, however, another prin- 
ciple, that of progressive cephalization, which tends to shorten 
the cord and concentrate it at the anterior end, and it is 
through this that the changes may be best explained. This 
principle appears equally well among many groups of inverte- 
brates and is shown in (i) a tendency to shorten the body 
axis, and (2) to concentrate and hence shorten the longitudinal 
nerve axis. The result of this process may be especially well 
followed among such a group of animals as insects, in which 
the central nervous system originally consists of a pair of 
small ganglia for each somite, this condition extending through 
the entire body. 
Thus in the myriapod (Fig. 147, A), an ancestral form, the 
primitive condition is still realized; in such a low form of insect 
as the dragon fly or grasshopper the concentration of ganglia 
has commenced, and in the fly the highest cephalization is 
reached. That these stages are passed through during the de- 
velopment is shown by a comparison of the nervous system of 
the fly in its various stages, that of the larva still showing 
a quite primitive condition (Fig. 147, cf. B and C). THE NERVOUS SYSTEM 
449 
This principle is shown in vertebrates by the progressive 
shortening of the spinal cord in a series of gradually cephaliz- 
ing forms, but can be used as a criterion of development only 
within the limits of a single group. Thus the frog with 
Fig. 147. Nervous systems of invertebrates, showing the principle of 
concentration. [A, from Lang, after Oudemans; B and C after Lowne.] 
(A) Nervous system of the myriapod Lithobius, showing a connected chain of 
approximately equal ganglia. (B) Nervous system of the larval Chironomus, the 
“harlequin fly,” showing a long chain of ganglia as in A. (C) Nervous system 
of the adult Chironomus, with all the ganglia concentrated into two, cephalic and 
mid-thoracic. 450 
THE HISTORY OF THE HUMAN BODY 
its extreme shortening of the cord, exhibiting but ten 
pairs of spinal nerves, is not to be compared with Man, 
in which there are more than thirty, but with the long- 
Sp.cor 
Brach i 
Hjjgl. 
Brach 
Thor ahfl. 
HIo? 
Cru 
Sciat. 
II.lv>' 
Cru. 
Sciat. 
Fig. 148. Spinal nerves of two amphibians, showing differences in the 
degree of concentration. 
(a) Frog. [After Gaupp.] (b) Necturus. [Combined from drawings by Waite.] 
Hpgl, hypoglossal nerve; Brack, brachial; Thor, abd, thoraco-abdominal; Sp. cor, 
supracoracoid; 11. hy, ilio-hypogastric; Cru. crural; Sciat, sciatic. The vertebrae 
are numbered by arabic numerals, the spinal nerves by roman. THE NERVOUS SYSTEM 
451 
bodied salamanders, animals in its own Class, in which 
the cord is nearly coterminous with the tail (Fig. 148). In- 
deed, an almost absurd re- 
suit of this is shown in a 
certain teleost, Orthagoriscus 
(Fig. 149, b), in which the 
entire cord is perhaps a little 
shorter than the brain. In 
all cases in which a shorten- 
ing has occurred two con- 
nected phenomena may be 
observed at the posterior 
end of the cord: first, the 
thick functional portion ter- 
minates more or less abrupt- 
ly and the cord is continued 
as a tapering thread known 
as the filum terminale, with- 
out function as a nervous 
organ, and secondly, the 
shortening is usually so great 
that the posterior portion of 
the cord is drawn up con- 
siderably ahead of the parts 
which it supplies, compelling 
the involved to turn 
at a progressively sharper 
angle until the most poste- 
rior ones run in a longitu- 
dinal direction parallel with 
the filum terminale. This 
bundle of approximately lon- 
gitudinal pairs of nerves 
which appears thus to ter- 
minate the cord, is known 
collectively as the cauda 
equina and is often a notice- 
Fig. 149. Spinal cords, show- 
ing the intumescentia, also a 
marked length variation. 
(a) Turtle. [After Bojanus.] (b) 
Orthagoriscus (telecost). [From Gegen- 
baur after B. Haller.] (c) Human 
spinal cord without the brain. [After 
WlEDERSHEIM.] 452 
THE HISTORY OF THE HUMAN BODY 
able object, as in the frog and in Man (Fig. 149, c). In the 
higher vertebrates, especially in mammals, the relation of spinal 
cord to tail becomes quite different from that of the lower 
forms, a change that is correlated in an interesting way with 
changes in the musculature of that part. In such forms as 
fishes and salamanders, the metameric musculature is not dis- 
continued at the cloaca, which marks the posterior limit of the 
body cavity, but is continued in a gradually reducing series to 
the extreme tip of the tail. Each of these caudal metameres is 
supplied with a pair of spinal nerves, to furnish which the cord 
must of necessity be continued quite or nearly to the end. In 
mammals, although in some cases the tail is long and extensive, 
its metameric muscles have been given up with the exception of 
those of its most anterior somites, and the tail is moved by a 
complex system of tendons proceeding from these latter. The 
only nerves necessary for the tail, then, are those of its ante- 
rior metameres, which are easily supplied from the cauda 
equina, thus obviating all necessity on the part of the cord for 
extending very far posteriorly. Indeed, with the exception of 
the primitive Ornithorhynchus and a few rodents, the spinal 
cord of mammals fails to reach even the sacrum. 
In much the same way as the development of the caudal 
muscles conditions the point and manner of termination of the 
cord, so is its caliber modified in other places through the rela- 
tive amount of muscular development in the various body 
regions, especially the limbs. In such a form as Amphioxus, 
where the successive metameres and spinal nerves are prac- 
tically alike, the cord is of about an equal caliber throughout 
the body, gradually tapering to the end of the tail; and for 
the same reason in forms like eels and snakes which have 
secondarily lost their metameric differentiation, the cord is 
correspondingly simple; where, however, a certain metamere, 
or a series of successive ones, becomes greatly developed, the 
nerves which supply this part are necessarily increased in 
caliber, and this causes a corresponding increase in the cord 
at or near the point from which they originate. 
The most conspicuous example of this principle is, of course, THE NERVOUS SYSTEM 
453 
that of the limbs, which are often excessively developed in 
the higher forms and cause a corresponding increase of size 
in the metameric nerves from which they receive their supply, 
as well as in the region of the cord from which these nerves 
proceed. It will be remembered that the musculature of each 
vertebrate limb above those of the fishes is a development of 
a few (not more than 5-6) metameres, and thus involves pri- 
marily a corresponding number of nerves. There are thus in 
the spinal cord two swellings (intumescentice), cervical and 
lumbar, corresponding respectively to the anterior and pos- 
terior limbs. These swellings are directly proportionate to the 
amount of development in each pair and are markedly un- 
equal in such forms as bats, with their exaggerated fore limbs 
and reduced hinder pair, and in the ostrich, in which the de- 
velopment shows the reverse tendency. 
In snakes, in which both pairs of limbs have been lost, the 
intumescentise are absent; on the other hand, in turtles, mem- 
bers of the same class, the disappearance of the most of the 
trunk muscles has caused a considerable reduction in the size 
of the cord between the intumescentise, making the latter, 
which are well developed, seem still greater by contrast (Fig. 
149, a). 
The most exaggerated development of these spinal intu- 
mescentiae seems to have been among the extinct dinosaurs, in 
which the excessive development of the hind limbs, on which 
they supported their enormous weight, caused a proportionate 
exaggeration of the corresponding swelling, the intumescentia 
lumbalis. As shown by the cavities in the vertebrae (neural 
canal) this intumescence was often considerably larger than 
the entire brain, exceeding that organ some twelve times in 
Stegosaurus. 
In Amphioxus the thickened lateral walls do not push 
dorsally and ventrally sufficiently to result in the formation of 
definite mid-dorsal and mid-ventral furrows. In cyclostomes 
the cross-section appears strongly flattened, convex dorsally 
and concave ventrally. In amphibians it assumes an oval or 
elliptical form and a narrow dorsal as well as ventral furrow 454 
THE HISTORY OF THE HUMAN BODY 
may be seen. In mammals by the deepening of these furrows 
to form the ventral fissure and dorsal sulcus respectively, and 
by the further addition of two dorso-lateral and two ventral- 
lateral sulci, all longitudinal and parallel, the well-known form 
of a fluted column is produced, superficially divided upon each 
side into dorsal, lateral, and ventral regions (posterior, lateral, 
anterior, BNA). 
In all except some of the more primitive forms (e.g. 
Petromyzon) in which the fibers of the cord have not ac- 
Somatic sehsory column 
Visceral sensory column 
Visceral motor column 
Dorsal funiculus 
Dorsal column 
Somatic motor column 
Lateral column 
Ventral column 
Fig. 150A. Diagrammatic transverse section through the spinal cord of a fish, 
(Menidia) to illustrate the relation of the four functional columns of the gray 
substance to the nerve roots. [From Herrick.] 
Somatic sensory column 
Visceral sensory column 
Visceral motor column, 
Somatic motor column 
Fig. 150B. Diagrammatic transverse section through human spinal cord. [From 
Herrick.] 
quired myelin sheaths, the cross-section of the cord shows 
a clear differentiation of gray and white nerve substance, 
the former retaining always its primitive inner position sur- 
rounding the lumen of the tube, the latter more superficially 
located and thus enclosing the gray area. There is never a 
sharp dividing line between the gray and white substance, 
however, since the latter includes fibers which come out from THE NERVOUS SYSTEM 
455 
the gray area where their cell bodies are located, as well 
as fibers which enter the gray area to form synapses with other 
neurones. The shape and the proportionate size of the gray 
area varies in different vertebrates and somewhat, also, in 
different levels of the same cord. It is larger in the regions 
of the cervical and lumbar enlargements, since at these levels 
are located the nuclei or groups of cell bodies most directly 
concerned in the innervation of the appendages, in addition to 
the axial structures of these segments. In the lower forms 
it appears as a symmetrical triangle or oval (see Fig. 150), 
in higher forms and especially in mammals the area is that 
of two crescents, their concavities turned laterally and their 
convex borders connected across the middle of the cord by 
a narrower bar in which the lumen of the central canal is 
located. The whole figure is thus something like that of a 
capital H, and may be topographically divided on the basis 
of the position with relation to the cross bar of the H, into 
regions, each of which is the cross section of a continuous 
longitudinal column of gray substance, known respectively as 
the dorsal, lateral, and ventral columns. 
The superficial topographical divisions of the cord together 
with the projecting dorsal and ventral columns of gray sub- 
stance serve to mark off the white substance of the cord into 
three funiculi upon each side, designated as the dorsal, lateral 
and ventral funiculi respectively. These funiculi consist largely 
of bundles of fibers (fasciculi) which extend longitudinally. 
In the lower forms in which there is less cephalization, the 
bulk of these fasciculi are short course tracts known as proper 
fascicidi in the sense that they are tracts which connect the 
nuclei of different levels of the gray substance of the cord with 
each other and thus belong to the cord itself. Such fasciculi 
are in general adjacent to the gray substance. Some of the 
fasciculi are, however, even in the lower forms, long course 
tracts, which connect various levels of the spinal cord with 
the centers located in the brain or vice versa and thus as 
capitalization advances with the evolution of the higher forms, 
these long course tracts come to constitute a larger and larger 456 
THE HISTORY OF THE HUMAN BODY 
proportion of the white substance of the cord (Figs. 150 A and 
B). It is obvious that in such a cord as this the proportionate 
amount of white substance will rapidly decrease from anterior 
to posterior end, since each segment of the body has its own 
connection with the brain through the long course tracts. 
This explains the fact that cross-sections of the human spinal 
cord show proportionately larger gray areas as we follow 
them from the anterior toward the posterior end, until at the 
extreme posterior end the white substance is practically 
negligible. 
Through experimental work upon the vertebrate nervous 
system, as well as from the embryological record of its de- 
velopment during the stage when the processes of the neu- 
rones are establishing their connections with the parts with 
which they are to be functionally associated, we are enabled 
to locate with considerable definiteness the junctional divi- 
sions of the cord. The consideration of functional divisions 
is also the basis for much of the discussion of the comparative 
morphology of the brain, and it is well to explain briefly the 
general principles underlying this purely physiological as- 
pect of morphology. The activities of all animals fall into 
two general groups which are designated as somatic and vis- 
ceral on the basis of the nature of the results attained. Somatic 
activities are those as a result of which the animal adjusts 
itself to its surroundings, and they take the usual form, as 
the name indicates, of a bodily movement, such as a change 
of location or position of the body as a whole, or of major 
parts of the body. Such activities take place primarily in 
response to external stimuli. The visceral activities, on the 
other hand, are those which are concerned with the nutritive 
functions, and they result in the maintenance of those in- 
ternal conditions which are necessary to the life of the or- 
ganism. The stimuli which give rise to such responses pro- 
ceed, primarily, from the internal chemical states of the body, 
such as might arise from a deficiency of food or oxygen, or 
an accumulation of carbon dioxide or other waste products. 
One must recognize, however, at the outset, that the somatic THE NERVOUS SYSTEM 
457 
and visceral activities cannot be separated by a hard and fast 
line, since they are not only closely coordinated but actually 
overlap in many instances as is obvious, for example, when 
we consider how much of the bodily or somatic activity of an 
animal is concerned in the search for and capture of food, a 
process which is in itself directly concerned with the visceral 
function. However highly organized the animal may become, 
therefore, and to whatever extent the control of the somatic 
and visceral functions may become localized in the nervous 
system, there is a corresponding tendency toward the develop- 
ment of correlation and coordination mechanisms which pre- 
serve the integration of that same nervous system, so that the 
various somatic and visceral activities are always component 
parts, only, of the activity of the animal as a whole. 
Underlying the structure of the nervous systems of all 
vertebrates, however, there is the same fundamental ground 
plan of arrangement of parts on the basis of their relationship 
to the somatic and visceral activities of the organism, and 
thus these parts may be traced in the evolutionary history of 
the higher forms. The primary functional divisions of the 
nervous system, the somatic and visceral, are each further 
subdivided into (i) that part which has to do with the in- 
coming (afferent) nerve impulses conveyed to the nerve centers 
from the receptors where they originate, and (2) that part 
which has to do with the outgoing (efferent) nerve impulses, 
conveyed from the nerve centers to the effectors which actu- 
ally make the response. Upon this basis the following four 
functional divisions are established:— 
Somatic Sensory (afferent). 
Somatic Motor (efferent). 
Visceral Sensory (afferent). 
Visceral Motor (efferent). 
A comparison of the cross-sections of the spinal cord of 
the fish and of Man (Figs. 150 A and B) emphasizes the con- 
stancy of the relationships in location of the four functional 
divisions of the gray areas of the vertebrate spinal cord, even 458 
THE HISTORY OF THE HUMAN BODY 
although the shapes of the gray areas are characteristically 
different in the two classes. The dorsal column is somatic 
sensory, the ventral column somatic motor; the dorsal re- 
gion of the lateral column, adjacent to the somatic sensory, 
is visceral sensory, and the ventral region of the lateral column 
adjacent to the somatic is visceral motor. 
The white substance of the cord belongs largely to the so- 
matic divisions, the long course tracts practically wholly so, 
so that in the proper fasciculi alone are there to be found 
visceral tracts. Here, moreover, the visceral and somatic are 
so intermingled that they do not form distinct topographical 
divisions of the white substance of the cord. Comparison of 
the white substance of the fish and the mammalian cord brings 
out the interesting fact of the small relative size of the dorsal 
funiculi in the lower vertebrates due to the absence from this 
region of the long course tracts which form a large and im- 
portant part of the somatic sensory division in the higher 
forms. These are the proprioceptive tracts which convey to 
the higher correlation and coordination centers of the brain, 
impulses arising from various forms of pressure stimuli in 
all regions of the body, and their absence in the fishes is not 
due, as it would seem at first sight, to a lack of these important 
functions but rather to the fact that a system of organs known 
as neuromasts or “ lateral-line ” organs constitutes a highly 
developed system of pressure receptors in these aquatic forms 
and that the paths from these to the coordinating centers 
consist of branches of the cranial nerves which therefore do 
not at all involve the cord (Cf. p. 501). The disappearance 
of this whole aquatic pressure apparatus in forms which have 
attained a terrestrial life has been accompanied by the develop- 
ment of a new proprioceptive apparatus involving the cord. 
We may now turn our attention to the history of the more 
highly specialized anterior region of the central nervous sys- 
tem, the brain. 
It begins, so far as the records go, with a form in which the 
anterior part of the neural tube is somewhat enlarged and 
divided by two transverse constrictions into three successive THE NERVOUS SYSTEM 
459 
vesicles (Fig. 151, A), the fore-, mid-, and hind-brain, or, more 
technically, prosencephalon, mesencephalon, and rhombence- 
phalon, each with its cavity. Of these the first seems to rep- 
resent the very rudimentary cerebral vesicle found in Am- 
phioxus and may be termed the archencephalon, or primordial 
brain, while the others may be considered to represent the an- 
Fig. 151. Diagrams of the primary and secondary cerebral vesicles. 
(A) The primary vesicles. (B) The typical form of brain of vertebrates as de- 
rived from A. 
The correspondence between the two is indicated by the horizontal dotted lines, 
which mark off the areas of the primary vesicles, I, II, and III. 
terior end of the spinal cord, which becomes added to the 
brain at some point between Amphioxus and the cyclostomes. 
Simultaneously with the closure of the neural tube a lateral 
outpushing appears upon each side in the dorso-lateral wall 
of the forebrain, and as development continues, these two 
outpushings become more and more constricted off from the 
main axis of the tube, each retaining, however, its connection 460 
THE HISTORY OF THE HUMAN BODY 
with it through a hollow stalk. These lateral evaginations are 
the optic vesicles and are destined to become the retinas of 
the eyes. 
Of all vertebrates the cyclostomes alone possess a brain 
which may be interpreted as still consisting of three primary 
vesicles; in all others several modifications take place (Fig. 
151, B). The prosencephalon becomes subdivided transversely 
into a more anterior telencephalon and a diencephalon. The 
former becomes further modified by the formation of two 
diverticula, which are thrown out from the sides and grow 
anteriorly, often reaching a point considerably beyond the 
anterior limits of the primary tube. These are the two lobes 
of the telencephalon (the cerebral hemispheres of the higher 
vertebrates). 
Internally the cavity of the prosencephalon becomes divided 
into the two lateral ventricles of the telencephalon and the me- 
dian unpaired third ventricle located mainly in the diencepha- 
lon; the latter communicates with the two first through a pas- 
sage which is inclined to become narrow, the joramen inter- 
ventricular e (foramen of Monro). 
The mesencephalon is the most conservative of the primary 
vesicles, and other than a lateral expansion which sometimes 
forms a pair of prominent optic lobes, suffers no marked 
change. Its cavity is often large and obvious, but is not 
counted as one of the ventricles of the adult brain. 
The third primary vesicle, the rhombencephalon, shows a 
greater or less differentiation of its anterior portion, which 
forms the metencephalon (the cerebellum of higher forms), 
while the posterior portion, which tapers indefinitely into the 
spinal cord, is designated as the myelencephalon or me- 
dulla.* The ventricle of the third primary vesicle, or more 
* The ease with which the German anatomists have translated into the 
vernacular the somewhat ponderous Greek terms for the parts of the brain 
(V order him, Zwischenhirn, Mittelhirn, etc.) has led to various attempts on 
the part of English-speaking scholars to emulate their example, but with 
varied success. Thus “ fore-brain ” and “ mid-brain ” for prosencephalon 
and mesencephalon respectively are convenient although somewhat mediaeval, 
and these, together with the inelegant “ hind-brain,” are now in general use. THE NERVOUS SYSTEM 
461 
especially that of the myelencephalon, is a large and conspicu- 
ous cavity in lower vertebrates and in the embryos of the higher 
ones, and is known as the fourth ventricle or rhomboid jossa. 
That part of the lumen which lies between this and the third 
ventricle, including the cavity of the mid-brain, forms in Man 
a small tube or duct, and has consequently received the name 
of aqueductus cerebri [Sylvii], or the “iter a tertio ad quar- 
tum ventriculum.” 
The original three primary cerebral vesicles, by a secondary 
subdivision of the first and third, thus become increased to 
jive, the telencephalon, diencephalon, mesencephalon, meten- 
cephalon and myelencephalon, and form a fundamental plan 
to which the brain of all higher vertebrates may be referred. 
In the development of the many forms of adult brains from 
this ground plan certain mechanical principles are involved 
which it is well to consider separately before continuing the 
special history. These mechanical principles are as follows: 
1. Evaginations or out pushings of more or less extensive 
portions of the wall of the vesicles. This is a mechanical 
device for increasing the amount of the nerve substance with- 
out a development of undue thickness or massiveness in any 
one part. It is an important principle to keep in mind since 
it lies at the basis of the very extensive development which 
has taken place in the telencephalon leading to the formation 
of such elaborately evaginated cerebral hemispheres as have 
developed in the higher mammals. 
2. Foldings of the walls of the vesicles. This device serves 
here as elsewhere, as a means for increasing the area of the 
The forms “after-brain” for myelencephalon (Ger. Nachhirn) and “twixt- 
brain,” or “tween-brain” for diencephalon (Ger. Zwischenhirn) are less 
happy, and it is doubtful if they will ever receive general favor. The Greek 
terms are, on the whole, the most satisfactory, and are more in accordance 
with our usage than are their rather crude Anglo-Saxon equivalents. 
The numbering of the cerebral ventricles is that of an old enumeration 
and does not at all correspond with the morphological value of the parts. 
They would be more appropriately named in accordance with the vesicles of 
which they form the cavities, thus: telocceles, diaccele, mesocosle, and rhombocoele 
(metaccele and myeloccele). 462 
THE HISTORY OF THE HUMAN BODY 
surface and hence the physiological efficiency, without a corre- 
sponding increase in bulk. The surface foldings or convolu- 
tions so extensively developed in the mammalian cerebral 
hemispheres, afford an excellent example of this. So also do 
the foldings shown by the roof of the metencephalon in its 
development into the cerebellum. 
3. Increase in the thickness of the wall of certain regions. 
This is seen in all the vesicles, but the extent of the develop- 
ment in thickness varies much. It is well shown by the 
thickening of the lateral wall of the telencephalic lobes, form- 
ing the corpora striata, or by that of the evaginated roof and 
sides of the same parts in the higher vertebrates, forming the 
cerebral hemispheres. 
4. The retention of the embryonal thinness over a definite 
area, forming a place where the lumen is separated from the 
exterior by merely a thin, often transparent, membrane. Such 
places are extremely puzzling, and misled anatomists until 
within a generation. They serve the physiological purpose of 
allowing the blood to communicate with the lymph of the 
ventricles and to nourish the inner surfaces without violating 
the integrity of the original neural tube. A plexus of blood- 
vessels is thus usually the accompaniment of such a thin re- 
gion, and the relations often become still more complicated 
by the sinking into the cavity of the entire structure, although 
each loop of capillaries is covered and veiled by the membra- 
nous wall, and thus the integrity of the tube is never violated. 
In extreme cases almost the entire thin area, covering a net- 
work of capillary loops and following its intricacies, may come 
to lie within the cavity of a ventricle and form a chorioid 
plexus. In other cases localized thin areas push out instead 
of in and thus constitute evaginations of more or less impor- 
tance in the formation of various organs supplementary to 
the brain. In this way, for example, several problematic 
structures arise from the roof and floor of the diencephalon. 
5. Flexure, or bending upon itself of the longitudinal axis 
of the neural tube. Such bendings are apparently due to the 
mechanics of growth (Fig. 152). The first appears very early, THE NERVOUS SYSTEM 
463 
usually before the closure of the tube is completed, and con- 
sists in the bending ventrally of the forebrain. The apex of 
the angle which the forebrain thus makes with the remainder 
of the neural tube lies in the roof of the mid-brain which thus 
temporarily comes to be the most anterior region of the head. 
This flexure may be designated the apical or the parietal 
Angle between axes ab and cd — apical flexure. Angle between axes cd and 
ef = flexure of the pons. Angle between ef and gh — cervical flexure. 
Fig. 152. Diagram of the cerebral flexures, 
flexure. The second flexure has its apex in the floor of the 
anterior part of the rhombencephalon and thus results in the 
whole region anterior to this point bending dorsally. As the 
region of this flexure becomes later the pons, this flexure is 
known as the pontile flexure. The third flexure has its apex 
at about the junction of the brain and spinal cord and results 464 
THE HISTORY OF THE HUMAN BODY 
in the bending of the whole brain ventrally again. It is known 
as the cervical flexure. 
In the lower vertebrates the flexures subsequently straighten 
out and tend to disappear. In the mammals, however, they 
are in part retained, and in Man, particularly, they serve the 
purpose of making possible the development of a very large 
brain within the length limits of a relatively short skull. The 
upright position in Man has conspicuously preserved the 
cervical flexure. 
With the above principles in mind the further history of 
the development of the five vesicles of the brain, as shown in 
the different vertebrate Classes, may be studied by the help 
of the accompanying diagrams [Plates IV and V], which rep- 
resent the adult brains of a dog-fish, a teleost, an amphibian, a 
reptile, a bird and a mammal, sectioned sagittally in the median 
plane and viewed from the inner aspect. These, it should 
be kept constantly in mind, are wholly diagrammatic and are 
to be used only for general proportions of the successive ves- 
icles, rather than for the details of their parts and relationships. 
The comprehension of these diagrams will be facilitated by 
comparing them with Fig. 153 which shows the dorsal aspect 
of the same series. 
That the telencephalon of the primitive vertebrate was 
wholly given over to the olfactory function is evidenced by the 
fact that in all of the Ichthyopsida the olfactory function still 
dominates this region! In fact in the most primitive living 
vertebrates, the cyclostomes, the entire forebrain is simply a 
part of the olfactory apparatus. 
The earliest evidences of the development of telencephalic 
lobes, the precursors of the cerebral hemispheres, are there- 
fore seen in the pair of anterior outpushings which are known 
as the olfactory bulbs. In fishes in general the olfactory 
bulbs, which usually project forward some little distance be- 
yond the lamina terminalis or anterior extremity of the median 
axis of the brain, form practically the whole of the telence- 
phalic lobes. In the selachians there is a thickening of the 
lamina terminalis which thus pushes posteriorly into the un- Plate IV. Longitudinal median sections of Vertebrate 
brains corresponding to the first half of the series in Fig. 117 in 
the text, [(b) and (c) after Edinger.] 
(a). Selachian; (b). Teleost; (c). Amphibian. 
Color Scheme : yellow, telencephalon; blue. diencephalon; red, mesenceph- 
alon; green, metencephalon; brown, myelcncephalon and cord. Plate V. Longitudinal median sections of Vertebrate brains 
corresponding to the second half of the series in Fig. 117. [After 
Edinger.] 
(d). Reptile; (e). Bird; (f). Mammal. 
Color Scheme: yellow, telencephalon; blue, diencephalon; red, mesenceph- 
alon; green, metencephalon; brown, myelencephalon and cord. THE NERVOUS SYSTEM 
465 
paired part of the ventricle of the telencephalon and gives a 
somewhat misleading bilobed appearance to this middle un- 
evaginated region (Fig. 153 a and Fig. 154 a and b). The 
Fig. 153. Dorsal views of vertebrate brains, corresponding to the lon- 
gitudinal sections given in plates IV and V. 
(a) Selachian (dog-fish), (b) Teleost (sculpin). (c) Amphibian (frog.) (d) Rep- 
tile (turtle), (e) Bird (sparrow), (f) Mammal (cat). 
I, telencephalon; II, diencephalon; III, mesencephalon; IV, metencephalon; 
V, myelencephalon. In (f) cerebrum and cerebellum have been drawn apart to 
expose the mid-brain.  THE HISTORY OF THE HUMAN BODY 
466 
middle region possesses throughout a slightly thickened wall, 
except in the posterior part of the roof, which retains its em- 
bryonal thinness and becomes invaginated as a chorioid plexus. 
Quite a different pattern characterizes the telencephalic de- 
velopment of the teleosts and ganoids. In these forms, also, 
the olfactory bulbs constitute the only part of the telen- 
cephalon which has become bilobed in the true sense of paired 
evaginations. Moreover the lamina terminalis remains thin in 
Showing in the case of each example a horizontal and a transverse section, the location of the 
latter with reference to the former being indicated by a dotted line. The evaginated region of the 
telencephalon is shown by horizontal lines; the median, unevaginated region by oblique lines; the dien- 
cephalon by vertical lines; the mesencephalon by plain white areas. /, lamina terminalis; m, median 
region of cavity of telencephalon; of, olfactory lobes; A and A', Selachian, Squalus acanthias; B and 
B', Ganoid, Acipenser; C and C', Teleost; D and D’, Dipnoan, Polypterus bichir; E and E', Am- 
phibian. [From Herrick.] 
Fig. 154. Series of diagrams of telencephalon of Ichthyopsida, 
these forms so that it does not push in as in the selachians 
to simulate a lobing of the unevaginated median part of the 
telencephalon. On the other hand, in the teleosts and ganoids 
there occurs an excessive thickening in each latero-ventral wall 
of this unevaginated region (Fig. 154 c-f) and, as the entire 
roof remains thin and transparent and is generally lost or 
ignored in dissection, this conspicuous pair of elevations, when 
viewed from the dorsal aspect, gives the erroneous impression 
of a pair of telencephalic lobes or cerebral hemispheres, and 
have thus frequently been described. 
Among the Dipnoi, Protopterus and Lepidosiren show a THE NERVOUS SYSTEM 
467 
still different development. In these forms the olfactory 
bulbs are not extended out to so great a distance as in the case 
of the other groups of fishes in which the bulbs are usually 
connected with the median part of the telencephalon by slender 
hollow stalks (formerly erroneously considered to be the ol- 
factory nerves). There is on the other hand an evagination 
of the whole lateral wall of the telencephalon to form a pair 
of elongated cerebral hemispheres, the anterior region of each 
of which is formed by the olfactory bulb itself. The cerebral 
hemisphere has a wall of nearly uniform thickness, and the 
median part of the telencephalon is reduced to almost neg- 
ligible proportions. 
When we turn to the amphibians we see in the condition of 
the telencephalon a great similarity to that of the dipnoans, 
but with a still more extensive evagination, since it involves 
a posterior as well as lateral and anterior outpushing. The 
cerebral hemispheres thus formed, although slender in their 
proportions, represent true telencephalic lobes, and each termi- 
nates anteriorly as in the dipnoans, in the olfactory bulb. It is 
evident that some such line of evolution as that shown by the 
dipnoans and the amphibians led to the type of cerebral hemi- 
sphere formation which characterizes the sauropsidans and 
mammals. 
It is probable that only a vertebrate possessing such a 
telencephalon would have been able to make the transitions 
from an aquatic to a terrestrial mode of life, the apparent 
outcome of subjection to periodic drought. The relatively thin 
walled, extensively evaginated telencephalic lobes of such 
forms are far better equipped for obtaining an adequate supply 
of oxygen from the surrounding plexus of blood vessels than 
would be the case with a form of telencephalon like that of the 
teleost or ganoid in which the chief development takes the 
form of greatly thickened regions. 
Moreover, when the transition wTas once made and the 
animal found itself in the more varying terrestrial environ- 
ment with its ever changing demands for adaptation to new 
conditions through the establishment of new correlations, such 468 
THE HISTORY OF THE HUMAN BODY 
a uniformly evaginated telencephalic wall gave the scope 
necessary for these adjustments. Already in the amphibian 
brain non-olfactory tracts have advanced from the more 
posterior regions of the nervous system, and, entering the 
cerebral hemispheres, have formed correlations with the still 
dominant olfactory mechanism. In the case of the Sauropsida 
this process has gone still further, but with divergent results 
in the different groups. The birds show the highest degree of 
specialization in that these invasions have become definitely 
located in the thickened latero-ventral walls of the telen- 
cephalon which form the bodies known as the corpora striata 
while the remainder of the wall has undergone little thicken- 
ing. In many of the reptiles a similar condition has been 
reached, but has not gone so far as in the birds. The more 
generalized reptiles show a great similarity to the amphibian 
on the one hand and to the primitive mammals on the other 
in the more uniform development of the evaginated telence- 
phalic walls and in the utilization of these regions for the 
invading non-olfactory functions. 
It is in the mammal that the cerebral hemispheres which 
have thus evolved come into their full glory in the develop- 
ment of an extensive cortical layer, or pallium of gray nerve 
substance which spreads itself over the entire surface. Here 
are located the multitude of higher nerve centers connected 
with the lower centers through extensive association tracts 
which have in the process of evolution pushed on farther and 
farther, until through the channel of the corpora striata they 
have invaded the cerebral hemispheres, at first forming correla- 
tions with the olfactory centers, then developing more and 
more independent non-olfactory centers and finally in the 
higher mammals assuming the dominance over all other parts 
of the body, even the olfactory centers themselves. The orig- 
inal olfactory mechanism becomes thus completely over- 
shadowed by these extensive developments of the mammalian 
cerebral cortex. In lower mammals, such as the marsupials 
and rodents, the outer surface of the hemispheres remains 
;smooth, but in the higher Orders, such as the ungulates, the THE NERVOUS SYSTEM 
469 
carnivores, and especially the primates, it becomes folded 
up into irregular rounded elevations, the gyri or convolutions, 
separated from one another by grooves, the deeper of which 
are termed fissures, e.g., the lateral cerebral fissure (fissure 
of Sylvius); and the others, sulci, e.g., the sulcus centralis 
(fissure of Rolando). 
Formatio pallialis 
Mesial edge of 
hippocampus 
Corpus para* 
terminate 
Corpus striatum 
Tuberculum olfactorium 
Fig. 155. A coronal section through the left cerebral hemisphere of an adult 
Lepidosiren paradoxus, a short distance in front of the lamina terminalis. [From 
G. Elliot Smith after Graham Kerr.] 
This folding of the surface has the evident effect of still 
further increasing the physiological efficiency of the cerebral 
cortex by extending its surface area within the same mass 
limits. 
The history of this important formation, the cerebral cortex, 
or pallium, is interesting. It makes its appearance even in 470 
THE HISTORY OF THE HUMAN BODY 
the dipnoans, in the form of a segregated layer of nerve cells 
in the dorsal region of the evaginated telencephalic wall (Fig. 
155). Here, as also in the amphibians where the pallium is 
not so definitely segregated, it is unquestionably given over 
H»PP. 
PARAHiPR 
PRIMNCOPALUI 
PRIM. j 
oentataI 
AREA 
PYRiFQRMkS 
LAT STAIATC 
AR.TE Rf 
FAR AH • PP. 
Pallium 
- FtSS. RRINALIS 
HIPk. 
FASCIA f 
DENTATAf 
- CL AuSTRUM 
lOBuS 
PYRlFORMlS 
PLAT STRIATE ARTERY 
Fig. 156. Diagrams of transverse sections of the right cerebral hemisphere. 
Tu&ER. OLF. 
To indicate how the primitive reptilian arrangement (left; above) becomes transformed in the 
evolution of the mammal, and especially the way in which the hypopallium of the reptile (II. i, 2, 3, 
4, 5) becomes converted into the upper part of the nucleus caudatus (N.C.), putamen (P) of the nucleus 
lentiformis, and claustrum. [From G. Elliot Smith.] 
mainly to olfactory functions, although it probably serves as a 
region in which connections are made with invading non-ol- 
factory tracts or from which olfactory tracts pass to form 
associations with the more posterior nerve centers. In the THE NERVOUS SYSTEM 
471 
reptiles the pallial formation is far more extensive and defi- 
nite but is still mainly olfactory, the pyriform area in the 
lateral wall and the hippocampal and adjacent areas in the 
medial and dorso-lateral walls being particularly well differ- 
entiated. There is, however, a limited area, located in the 
antero-lateral region and intruding itself between these, which 
has become the seat of wholly non-olfactory centers, and is 
the beginning of the development of the neopallium as dis- 
tinguished from the primitive archipallium of the olfactory 
mechanism. It is a significant fact that the neopallium lies 
in a region adjacent to the corpus striatum and that it is thus 
directly in line with association tracts which grow up into 
it through the striate body. In fact even in the reptiles a 
layer of pallial nerve cells adjacent to the corpus striatum 
seems to be drawn inward toward the source of stimulation in 
accordance with the principle of neurobiotaxis, and come 
actually to form a cap of pallial formation, the hypopallium, 
over the surface of the striate body. The accompanying dia- 
grams (Fig. 156 A, B, and C) suggest the method by which 
the transition is made in the process of evolution from the 
reptilian relationships to those found in the primitive mam- 
mal, and thus offer a very plausible explanation of the mor- 
phology of the mammalian corpus striatum with its nucleus 
caudatus and the larger part of its nucleus lentiformis and 
adjacent claustrum derived from the hypopallium while the 
bundles of fibers which constitute the internal capsule pass 
between these hypopallial masses and spread out into the 
neopallium. 
Figure 157 shows the further evolutionary history of the 
neopallium and archipallium and it will be seen from the set 
of examples there given how the neopallium has gradually 
increased in extent involving, as it has done so, a more and 
more extensive folding of the external surface, thus enormously 
increasing the neopallial cortical area. Meanwhile both the 
hippocampal and the pyriform archipallial regions have be- 
come relatively less important and crowded out of their orig- 
inal locations. The hippocampus is pushed into the medial 472 
THE HISTORY OF THE HUMAN BODY 
wall and rolled in upon itself, and as the cerebral hemispheres 
become more extensively evaginated, this primitive structure, 
still of great functional importance, curves posteriorly and 
ventrally in the medial wall of the hemisphere and finally 
Fig. 157. Diagrammatic transverse sections through the cerebral hemispheres of 
(A) Python, (B) Perameles, (C) Hypsiprimnus, and (D) Canis, to show the evolution of the 
mammalian, neopallium and archipallium from the reptilian type. Neopallium in solid black; Hypo- 
campus in heavy dots. [After Edinger.] 
bends so far around on the ventral side as to appear there in 
the transverse section, while by the same process the pyriform 
area or lobe has been pushed out of the plane of the section. 
One further development of importance has occurred, in the 
form of ever increasing subcortical white nerve substance made 
up of the intricate sets of tracts which connect the various 
parts of the cortex with each other and with the more posterior THE NERVOUS SYSTEM 
473 
parts of the nervous system. Among these are certain com- 
missural tracts which connect corresponding regions of the 
two hemispheres and so must pass across the midline to reach 
the opposite side. In the primitive brain, such as that of the 
python, these tracts are wholly connected with the archipal- 
lium. In the mammals the commissures of the neopallium 
become increasingly important and in the higher mammals 
they actually form an extensive commissural sheet, the corpus 
callosum, which bends forward above the original free sur- 
face of the middle region of the telencephalon, and constitutes 
the largest commissure of the whole nervous system. 
It is important here to call attention to the fact that the 
popular idea that the amount of “ gray matter ” in the cerebral 
cortex is indicative of the intellectual status of an individual 
is largely a misconception, and a far more reliable criterion 
may be found in the amount of sub-cortical white nerve sub- 
stance. It is not the mere presence of the nerve cells but 
the extent and nature of the connections which they make 
through their association and commissural tracts which stands 
for intellectual achievement. The relative massiveness of the 
corpus callosum as a sharply delimited and readily compa- 
rable structure of this nature is therefore a far more definite 
and accessible indication of mental development than any 
region of the gray cortex. 
That which particularly characterizes the type of telen- 
cephalon which has evolved in the mammals and has reached 
its highest expression in Man, is the fact that in its very 
configuration it has, from the time of its appearance, pre- 
sented an unlimited field for modification and adaptation to 
changing conditions. 
“ From a general survey of what is known regarding the 
correlation of forebrain patterns with behavior patterns, it 
appears that solid masses of cerebral tissue may be structur- 
ally well adapted for the performance of the most complex 
types of reflex and instinctive activity whose patterns are 
inherited and relatively stable, as illustrated in teleosts and 
birds. High specialization in this direction, however, seems 474 
THE HISTORY OF THE HUMAN BODY 
to have precluded the possibility of any great development 
of the more labile individually modifiable sorts of behavior 
and especially of the culmination of this kind of behavior 
as manifested by capacity for rapid learning by individual 
experience and intelligence in general. 
Conversely, the development of the labile functional type 
goes hand in hand with the extensive elaboration of thin sheets 
of correlation tissue, as exemplified in the cerebral cortex, in 
which numerous functionally distinct fields are well separated 
in space and are at the same time in free communication 
through systems of association fibers of unlimited complexity, 
which find ample room in the subcortical white matter. There 
is no assignable limit to which structural specialization of this 
sort can extend.” 1 
Concerning the portion of the telencephalon which does not 
become involved in the cerebral hemisphere formation there 
is little further to be said. Incidentally the formation of the 
corpus callosum has resulted in the inclusion beneath it of 
a narrow space between the two delicate vertical plates which 
form the medial walls of the two hemispheres in this region. 
These two plates constitute the septum pellucidum which thus 
separates the two lateral ventricles; the space inclosed be- 
tween them is the cavum septi pellucidi to which the errone- 
ous name, the “ fifth ventricle,” was formerly applied. It is 
obvious that this is not a true ventricle and has no connection 
with the lumen of the neural tube. The true anterior bound- 
ary of the lumen of the tube in the mid-line is always the 
lamina terminalis, although the cerebral hemispheres may ex- 
tend far anterior to this. 
Behind the lamina terminalis is the cavity of the third ven- 
tricle, which is thus located in part in the telencephalon, al- 
though its major part is in the diencephalon. So also the roof 
of the middle part of the telencephalon is formed by the 
thin epithelial chorioid lamina which is continued posteriorly 
as the diencephalic roof. With the development of the cere- 
1 From C. Judson Herrick’s paper on the Origin of the Cerebral Hemis 
pheres, in the Journal of Comparative Neurology, 1921. THE NERVOUS SYSTEM 
475 
bral hemispheres, the lateral edges of this lamina have be- 
come folded in to form a voluminous chorioid plexus lying 
within each of the extensive lateral ventricles. Other struc- 
tures, more or less problematic, develop in connection with 
the thin roof of the third ventricle, but as these are in the 
debatable ground between telencephalon and diencephalon, 
they will be discussed in connection with the latter. 
In the evolution of the diencephalon, the chief develop- 
ment of nerve centers occurs within lateral thickenings of 
the walls known as the thalami. In the lower vertebrates 
these are, like the telencephalic centers, dominated by the 
olfactory mechanism, but in the Amphibia and possibly earlier 
they become invaded by the visual tracts, and they are des- 
tined to become, in the higher vertebrates, important way 
stations in the general somatic paths to the corpora striata 
and the cerebral cortex. 
The diencephalon, never an extensive element in the verte- 
brate brain, becomes nearly or wholly covered dorsally and 
laterally in the higher forms by the excessive development 
of other parts, but though small and of subordinate interest 
in itself, it is especially characterized by the formation of 
secondary organs, either as in- or out-pushings. Some of 
these latter become of fundamental importance, notably, of 
course, the optic vesicles which early push out to form the 
retinas of the eyes, while others appear to be more or less 
vestigial, presumably inherited from prevertebrate ancestors 
and of problematic significance. 
Several of these formations occur along the dorsal aspect, 
where in addition to the chorioid plexus of the third ventricle, 
there appear in the thin roof typically three median diverticula, 
which owing to the varying grades of development which they 
show in different animals, have been more or less confounded 
with each other (Fig. 158). The most anterior of these is 
the paraphysis, an evagination of the telencephalic roof as is 
shown from its location anterior to the transverse infolding, 
the velum transversum, which in all vertebrate embryos and 
in the adults of lower vertebrates, forms the boundary be- 476 
THE HISTORY OF THE HUMAN BODY 
tween telencephalon and diencephalon. The paraphysis at- 
tains a conspicuous development in the amphibia. Posterior 
to the velum transversum there are two possible diverticula 
in the diencephalic roof, both correctly termed epiphyses, 
parietal organs, or pineal organs. Both show a tendency to 
pass through the skull and attain a position directly beneath 
the skin in the middle line, developing there a rudimentary 
Epiphysis 2 
.Epiphysis 1 
Tectum 
Comm. 
post. 
Paraphysis 
Velurrv 
Nuc. haben 
Fig. 158. A sketch showing in median sagittal section the typical relationships 
of the paraphysis and two epiphyses of the vertebrate brain. [From Johnston.] 
sense organ of uncertain nature but probably an eye in each 
case. The identity of the structure may always be determined 
from its central connections. 
In the cyclostomes, fishes, and reptiles both epiphyses are 
usually present and one or the other may attain a high de- 
gree of development, in some cases reaching its height dur- 
ing embryonic life and in others appearing as a well-formed 
organ in the adult. Thus in the cyclostome Petromyzon, the 
optical nature is indicated by the presence of pigment in what 
may be a vestigial retina. In the selachians the epiphysis 
passes through a minute foramen in the skull and reaches the 
surface, where its terminal organ is visible. The highest 
development is reached among certain lizards where the ter- 
minal organ lies in a socket, the parietal foramen, formed in 
the interparietal suture, and represents a fairly good eye with 
pigmented retina, a more or less makeshift lens, and a well- 
developed nerve connecting the terminal organ with the brain. 
Above this, on the surface, is situated a transparent scale sur- 
rounded by a ring of smaller opaque ones, making a conspicu- 
ous object on the heads of these forms. In birds and mam- 
mals the epiphysis is reduced to the form of the so-called THE NERVOUS SYSTEM 
477 
“ pineal gland ” pushed backwards from its original position 
by the growth of other parts. In man it lies so hidden that 
the early anatomists, finding it as it were in the innermost 
penetralia of the organ of life and individuality, deemed it 
the seat of the soul, a view from which the morphologists 
of the present day have escaped only by substituting one 
mystery for another. 
The sporadic occurrence of these vestigial sense organs, 
which, save perhaps in the case of the parietal eye of the 
lizard, cannot be of the slightest use, points definitely to the 
presence of similar organs in a functional condition in some 
remote ancestor. That these parts were organs of vision there 
can be but little doubt, and there are certain indications which 
lead us to think that they were once paired, although always 
close together. Beyond this, investigation has as yet shown 
nothing, and the whole subject remains at present one of those 
half-completed histories, of which the record consists of a 
few poorly preserved fragments. 
Another diverticulum arises from the diencephalic floor and 
is directed downwards. It does not form a complete organ 
in itself but unites with a similar diverticulum which develops 
upward from the roof of the mouth and together they form 
an organ of much importance as a part of the endocrine sys- 
tem. It is probable, nevertheless, that we have here a vestigial 
organ like those developing dorsally from the same region, 
and that it, like them, represents the remnant of an organ of 
considerable importance in some unknown ancestral group. 
This organ is a noticeable feature of the ventral aspect of 
all vertebrate brains, and bears the noncommittal name of 
hypophysis, literally that which grows beneath, in allusion to 
its position. In most skulls, especially in the more completely 
ossified one of the amniotes, there is a distinct depression for 
its lodgment (the sella turcica of human anatomy), and, as 
the hypophysis is often connected with the brain by a narrow 
stalk around which the bone may fit quite tightly, it is seldom 
removed in its entirety with the brain, and hence its true re- 
lations are apt not to be wholly understood. 478 
THE HISTORY OF THE HUMAN BODY 
The portion contributed by the diencephalon is in the form 
of a hollow cone or funnel, the infundibulum. About this the 
evagination from the mouth cavity, which is glandular in its 
nature, and termed pituitary body, becomes developed, and by 
the secondary loss of the original connection between this 
latter and the roof of the mouth, the hypophysis appears like 
a single organ, attached to the brain. 
Although there is no feeling of certainty among morpholo- 
gists concerning the original form of this organ, the opening 
of the pituitary portion into, or rather from, the exterior 
in the more primitive forms, suggests that this part may repre- 
sent the rudiment of an earlier mouth, the paloeostoma, with 
which, as shown by other data, the prevertebrate ancestors 
seem to have been equipped prior to the development of the 
definite vertebrate mouth, the neostoma* 
The mesencephalon is the most conservative of the elements 
of the brain; it develops very little that is new throughout 
its entire history, and in Man and the other mammals, al- 
though suffering no actual diminution in size, it becomes pro- 
portionately small through the excessive growth of the sur- 
rounding parts. This is made clear by the diagrams, in 
which the mesencephalon may be followed through fishes, 
amphibians and reptiles with but little change. As the telen- 
cephalon is primitively a part of the olfactory mechanism, so 
the mesencephalon is originally and primarily the center for 
the reception of visual impulses since, although the optic 
vesicles are formed from the diencephalic walls, when the 
nerve fibers from the retinas later grow back into the brain 
to form the optic nerves, they establish their associations in 
the mesencephalon. Its roof and outer sides are moderately 
thickened and usually divided along the mid-dorsal line by 
a longitudinal groove, thus forming a bilobate organ, the cor- 
* The pituitary diverticulum arises in gnathostomes from the ectoderm 
of the stomatodaeal invagination, but in cyclostomes is beyond the limits 
of the mouth and pushes in from the external surface of the head in close 
association with the medial nasal invagination. For further details and theories 
concerning this part see Chapters XIII and XIV; also Fig. 171. THE NERVOUS SYSTEM 
479 
pora bigemina or lobi optici. In many fishes they form a 
conspicuous part of the brain which, so long as the cerebral 
lobes remain but slightly developed, must be of great func- 
tional importance for visual reflexes. In some teleosts, for 
example, they constitute at least two-thirds of the dorsal sur- 
face. In amphibians and reptiles the gradual establishment 
of visual centers in the thalami, and later in the cerebral 
hemispheres, reduces the relative importance of the optic 
lobes, although in birds, forms not in the direct line of human 
ancestry, they again reach a certain prominence; thus when 
the enormously developed corpora striata and the small and 
thin walls of the hemispheres are taken into consideration, 
birds are seen to be unique in their brain development as they 
are in their skeleton and their general form. 
In mammals the mesencephalon is to be looked for between 
the two greatly hypertrophied elements, telencephalic lobes 
and metencephalon (cerebral hemispheres and cerebellum), 
and here the bilobed organ has become transformed into one 
with four lobes, the corpora quadrigemina. The beginning 
of this change may be found in some reptiles, where the de- 
velopment of a pair of small subordinate lobes posterior to 
the main ones makes it clear that the four-lobed form in 
mammals is due to the development of a new pair of lobes 
posterior to the others, and not merely to the formation of 
a cross-furrow. Subordinate lobes like those of reptiles are 
found also in birds. 
During the process of phylogenetic development the roof 
and sides of the metencephalon become selected as a region 
where a large part of the coordinating work of the central 
nervous system is accomplished. This part, the cerebellum, 
is thus almost always large and voluminous, and often, even 
in fishes, becomes folded up into several plicae, thus em- 
phasizing its early functional importance. In the adult dog- 
fish these folds, only three or four in number, are seen with 
the clearness of a diagram; in others, as in birds and mammals, 
the surface folding becomes very complicated and extensive, 
with a well-defined cortex of gray substance, and the under- 480 
THE HISTORY OF THE HUMAN BODY 
lying white substance takes on a tree-like form (the arbor 
vitae) as seen in median sagittal section, with secondary and 
tertiary branches forming the cores of the corresponding 
systems of foldings. The floor of the metencephalon is utilized 
in part for the location of commissural fibers between the two 
lateral halves of the cerebellum, and in mammals, correspond- 
ing to the increase in size of this organ, this commissural 
bundle becomes large and conspicuous, forming a broad loop 
around the base of the medulla, the pons (Varolii), each end 
of which disappears into the cerebellum as its middle peduncle 
(the brachium pontis). Thus although there are occasional 
exceptions to the progressively increasing importance of the 
cerebellum as seen, for example, in the singularly small cere- 
bellum of the frog, the phylogenetic history of the part is more 
comparable to that of the cerebral hemispheres than to any 
other part of the brain. It forms, together with the cerebral 
hemispheres, h sort of “ super-brain,” sometimes referred to 
as the neencephalon as distinguished from the primitive verte- 
brate brain, the paleencephalon, to which only that portion of 
the higher vertebrate brain which is functionally and struc- 
turally a continuation of the spinal cord, and is known as 
the brain stem, corresponds. Thus while certain regions of 
the brain stem, such as the optic lobes, and the thalami, have 
varied in their relative importance in different classes of verte- 
brates, there has occurred a progressive increase in the assump- 
tion of dominance over all the lower centers by the cerebral and 
cerebellar cortices, the former becoming the seat of modifiable 
reactions leading to increasing mentality; the latter, itself 
dominated by the cerebral cortex, becoming a more and more 
perfect mechanism for the coordination of muscular responses. 
The remainder of the rhombencephalon, in reality the 
myelencephalon with the metencephalon surrounding it like 
a ring, is obviously and definitely an anterior continuation of 
the spinal cord. In fact, the line of division between the cord 
and the medulla as this part of the brain is called, is a wholly 
artificial one, the latter being considered as coterminous with 
the skull in all cases, and since the skull itself in its evolution THE NERVOUS SYSTEM 
481 
has absorbed one or more vertebrae, the arbitrary nature of 
this distinction from a morphological standpoint is apparent. 
The cavity of the rhombencephalon, the jourth ventricle or 
rhomboid jossa, is conspicuously widened laterally and has, 
with the exception of the thickened region formed by the 
cerebellum, a distinctly thin roof. That part of the roof which 
is posterior to the cerebellum, the posterior medullary velum 
as distinguished from the anterior, is developed into a chorioid 
plexus. Its removal enables one to look into the floor of the 
rhomboid fossa, the lateral borders of which converge pos- 
teriorly and become the dorsal funiculi of the spinal cord. 
The brain stem, as the anterior continuation of the spinal 
cord, shows the same general arrangement of the functional 
divisions of the gray substance as the cord itself. The gray 
substance does not, however, form continuous columns as 
in the cord, since the nuclei of which it is made up are some- 
what isolated, being, in fact, more or less definitely recogniz- 
able in their relation to the succession of cranial nerve roots. 
The medulla, in which, as the less specialized region of the 
brain stem, the resemblance to the cord is most striking, 
differs from the cord mainly in the lateral expansion of its 
cavity to form the fourth ventricle, and in the corresponding 
extent of its thin chorioid roof. Consequently the gray sub- 
stance, as shown in cross-section (Fig. 159), also spreads 
laterally, still retaining, however, the typical arrangement 
of its functional divisions, the most dorsal and lateral being 
the somatic sensory, then the visceral sensory, then visceral 
motor, and finally in the most ventral and medial position, the 
somatic motor. Anterior to the myelencephalon, as may be 
seen in the primitive vertebrate type of brain (palaeenceph- 
alon (Fig. 160), the typical arrangement becomes rapidly 
modified, the visceral motor being the first to be discontinued. 
In describing the evolution of the cerebral hemispheres, 
reference was made to the ever-increasing number of tracts 
from the more posterior regions of the central nervous sys- 
tem, which grow forward into the telencephalon, beginning 
even in the amphibians with the visual tracts. Naturally, 482 
THE HISTORY OF THE HUMAN BODY 
then, as vertebrate evolution advanced, the number and rela- 
tive importance of such tracts increased, until, in the higher 
mammals, we find a very large proportion of the mass of the 
brain stem formed by these ascending tracts together with 
the descending ones which proceed from the dominant cere- 
bral cortex to the parts of the body over which it has gained 
-Acoustic area 
Somatic sensory column 
—Spinal V tract 
_Vagal lobe 
Visceral sensory column- 
.Nucleus ambiguus 
•Cutaneous root X 
■Visceral sensory root X 
•Visceral motor root X 
Reticular formation 
-Ventral column 
Visceral motor column. 
Somatic motor column. 
-Spinal nerve (XII) 
Fig. 159A. Diagrammatic transverse section through the medulla of a teleost 
at the level of the vagus nerve, showing the arrangement of the four functional 
columns of gray substance. [From Herrick.] 
Nuc. dorsalis vagi^ 
Ala cinerea 
Trigonum hypoglossi 
Nuc. fasc. solitarius^ 
Fasc. solitarius^ 
Nuc. vestibularis spinalis 
Fasc. long. med. 
Nuc. fasciculus 
cuneatus" 
Nuc. XII- 
Lemniscus V 
Spinal V nuc.. 
Spinal V tr- 
Corpus resti- 
forme 
Nuc. sal. inf.- 
Tr. spino-cereb. 
dorsalis 
Nuc. ambiguus- 
X root' 
- Tr. rubrospinalis 
Reticular' 
formation 
Tr. spino-cereb. 
ventral is 
Lemniscus 
spinalis 
Inferior olive' 
Tr. tectospinalis 
Lemniscus 
medialis 
XII root 
Pyramidal tract 
Fig. 159B. Diagrammatic transverse section through a similar region of the adult 
human medulla. [From Herrick.] 
control. The cerebellum also acquires large tracts reaching it 
both from the cerebral cortex and from the spinal cord, con- 
stituting its superior and inferior peduncles respectively 
(brachia conjunctiva and corpora restiformia). The com- 
parison of the proportionate areas of white substance THE NERVOUS SYSTEM 
483 
shown by the transverse sections through a corresponding 
region of the teleost and the human medulla (Fig. 159 A and 
B), illustrates well this evolutionary change. The massive- 
ness of the tracts is best seen, however, in the structures 
known as the cerebral peduncles which lie in the floor of the 
mesencephalon and comprise practically the whole system of 
tracts, both ascending and descending, by which the cerebral 
hemispheres are connected with the more posterior parts of 
the body. These tracts are almost without exception somatic 
tracts, since the somatic activities are particularly the ones 
which come into the realm of consciousness. The ascending 
ones, whether from the spinal cord or from the medulla, are 
known as the lemnisci, and they form synapses with relays 
of neurones in the thalami, from which the paths are con- 
tinued through the internal capsules of the corpora striata to 
the various sensory projection areas of the cerebral cortex. 
Tuberc. acust 
Commiss infima 
Somatic sensory 
Somatic motor 
Visceral sensory 
Visceral motor 
Fig. 160. Diagrams to represent the extent and arrangement of the functional 
divisions of the brain of a Selachian. 
(A) Projection upon a median plane. Band (C) transverse sections at the levels indicated by the 
arrows. [From Johnston ] 
The descending tracts proceed directly from the motor pro- 
jection areas of the cortex, through the internal capsules and 
into cerebral peduncles, and either descend the medulla and 
cord as the pyramidal tracts or enter the cerebellum as the 
superior peduncles. 484 
THE HISTORY OF THE HUMAN BODY 
The embryonic origin of the parts of the peripheral nervous 
system simultaneously with the formation of the central nerv- 
ous system has already been described. The connection of 
the peripheral with the central system lies in the roots of the 
cranial and spinal nerves which have a general metameric 
arrangement, a pair of nerves corresponding to each body 
somite. This relationship is, of course, particularly well shown 
in the case of the spinal nerves, since they supply regions in 
which the original metamerism is still expressed in the muscles 
and skeleton. The cranial nerves, although showing indica- 
tions of a former metameric arrangement, cannot be so satis- 
factorily resolved into their separate elements. 
According to their use, nerve components are sharply di- 
vided into two groups, sensory and motor. The first are 
distributed chiefly to the external surface and are the media 
by which the central organ receives intelligence concerning ex- 
ternal stimuli. These terminate in many cases in special 
sense organs arranged to receive certain definite stimuli, but 
are distributed also over the general surface, where they re- 
spond to simple contact. Other sensory nerves supply certain 
internal parts, as the muscles and viscera. In all sensory 
nerves the impulse necessarily travels from the terminus to 
the central organ, and these nerves are consequently desig- 
nated as centripetal or afferent. The other type of nerve, 
the motor, supplies the muscles and furnishes them with the 
impulse to contract. In these nerves the impulse travels from 
the center to the terminus, and they are thus centrifugal, or 
efferent. The nerves that regulate the action of other organs, 
such as the secretion of glands, are also included in the motor 
class. 
In accordance with the nature of the activities with which 
they are concerned, nerve components, both sensory and 
motor, are further classified as somatic and visceral, and thus 
a typical nerve has all four functional divisions represented, 
the somatic sensory and motor and the visceral sensory and 
motor. The visceral connections are made chiefly through the 
relays of neurones which constitute the sympathetic system THE NERVOUS SYSTEM 
485 
and thus each nerve is typically connected with a sympathetic 
ganglion by means of a branch known as the ramus commu- 
nicans. 
The somatic receptors to which the somatic sensory division 
is distributed comprise not only the sense organs, or receptors, 
through which external stimuli are received (exteroceptors) 
but also receptors (proprioceptors), such as those located in 
voluntary muscles and in joint structures, which are stimu- 
lated by the very changes of pressure and tension involved 
in making the somatic response to the external stimuli. The 
proprioceptive system thus furnishes a channel whereby the 
nerve centers may be kept informed of the nature and ex- 
tent of the response. 
The somatic motor division is distributed in general to the 
voluntary muscles, the somatic effectors. Strictly speaking 
only the metameric voluntary muscles are somatic effectors, 
since the branchiomeric musculature was in its original func- 
tion distinctly concerned with visceral activities (respiratory 
movements) and these muscles, whatever their later adapta- 
tion to other functions in the higher vertebrates, still receive 
their innervation from the visceral rather than the somatic 
motor division of the brain and spinal cord. 
The visceral sensory division of the peripheral nervous 
system is distributed to the receptors (interoceptors) which 
are stimulated by internal conditions, primarily of a chemical 
nature. They are located, in general, in the viscera, but if 
so placed that they receive stimuli from outside the body, as 
is the case of the highly specialized olfactory and gustatory 
organs, these stimuli are of a chemical nature and call forth 
primarily distinctly visceral responses. 
The visceral effectors to which the visceral motor distribu- 
tion is made are mainly the involuntary muscle coats and 
glandular structures wherever these may occur in the body. 
As explained above, the branchiomeric muscles are also to 
be classed as visceral effectors. 
The accompanying figure (Fig. 161) shows the relation 486 
THE HISTORY OF THE HUMAN BODY 
of the various components of a typical spinal nerve as it thus 
exists in the higher vertebrates. From this it will be seen that 
in the nerve roots there is a segregation of the sensory and 
the motor components, the former constituting the dorsal 
Fig. 161. A diagram showing the typical functional components of a spinal nerve 
in their relation to the spinal cord. 
Somatic sensory paths shown by dotted lines; somatic motor by solid lines; visceral sensory by 
short dashes; visceral motor, preganglionic, by long dashes; visceral motor, postganglionic, by wavy 
lines. C. Rm, communicating ramus; D. Rt, dorsal root; D, Rm, dorsal ramus; S.M., somatic motor, 
S.S., somatic sensory; Sm. G., sympathetic nerve; Sp. G., spinal ganglion; F.S., visceral sensory; 
V.M., visceral motor; V. Rm., ventral ramus; V. Rt., ventral root. 
nerve root upon which the spinal ganglion is located, and the 
latter the ventral nerve root. It must be kept in mind that 
the sensory fibers of the dorsal root are the neurites of 
sensory neurones the cell bodies of which are located in the 
spinal ganglion. This root may, however, particularly in the 
lower vertebrates, contain a small admixture of motor fibers 
which in all cases proceed from the cord itself. 
The made-up nerve, as distinguished from the nerve roots, 
comprises all the functional components, not merely those THE NERVOUS SYSTEM 
487 
connected with the cord but those (visceral motor) which have 
their source in the sympathetic ganglion and reach the main 
nerve by way of the ramus communicans. These are on their 
way to innervate those effectors which are located in the out- 
lying districts, namely, the blood vessels and glands of the soma 
or body as a whole. The ramus communicans is thus com- 
posed of two parts, the gray ramus made up of these postgan- 
glionic visceral motor components, and the white ramus made 
up of both visceral sensory and preganglionic visceral motor 
components. The former are on their way from the coelomic 
viscera to the spinal cord via the spinal ganglion and sensory 
nerve root, while the latter proceed from the cord via the 
motor root to be distributed to the sympathetic ganglia, from 
which the final distribution to the visceral effectors is made. 
The made-up nerve, with its various components, sensory 
and motor, separates into dorsal and ventral divisions or rami, 
each of which is a similar mixture of components and as the 
rami further divide up into the various nerve branches it is 
not until they approach their ultimate distribution that the 
functional components are finally sorted out. 
This typical arrangment of nerve roots and their association 
in the formation of single pairs of metameric nerves is a con- 
stant one in all vertebrates and is already suggested by the 
somewhat more primitive condition in Amphioxus. In the 
lower phylogenetic stages, however, the plan is a little less 
precise, and there is sufficient indication to show that here, too, 
as elsewhere, the final arrangement has been attained by a 
natural development from a less definite one. 
Thus in Amphioxus the motor roots consist of a series of 
fibers distinct from one another; the sensory roots are more 
definite and are placed in the intervals between the first, in 
such a manner that the motor roots correspond to the myo- 
tomes, the sensory roots to the myocommata. Moreover, 
since in this singular animal the body somites on the two sides 
do not match but alternate with one another, the nerve roots 
do the same, and a sensory root of one side will lie in the 488 
THE HISTORY OF THE HUMAN BODY 
same transverse plane as a motor root of the other. The 
sensory and motor roots do not unite, and the former becomes 
associated with a subcutaneus ganglion. 
This alternate arrangement of the roots, excepting the non- 
correspondence between the two sides, is continued in most 
fishes. In the selachians, for example, the motor root passes 
through a foramen in the side of the vertebra, the sensory root 
through a similar foramen in the intercalary piece; the latter 
is thus inter-, the former intra-vertebral. 
As we ascend the series the tendency is more and more 
towards an intervertebral exit jor both roots, but even in 
birds and mammals there are cases of an exit through a verte- 
bra. Thus in the pre-sacral vertebrae of birds there are two 
foramina on each side in the bodies of vertebrae for the sepa- 
rate exit of the two roots; there are also many instances 
among mammals of the piercing of a vertebra for nerve exits; 
for example, the majority of the cervical and thoracic verte- 
brae in pigs, or the thoracic and lumbar vertebrae among 
ruminants. 
Regarding the union of dorsal and ventral roots, in the cy- 
clostome Petromyzon the two remain separate, although in 
other cyclostomes (e.g., Myxine) they unite. In true fishes 
the union of the two roots takes place outside of the vertebral 
canal; in the higher forms the union is within it and the united 
nerves pass through the inter- (or intra-) vertebral foramen. 
The spinal ganglia, which are associated with the sensory 
roots, may possibly be homologous with the subcutaneous 
ganglia found on the sensory nerves of Amphioxus. 
In studying the distribution of the peripheral nervous sys- 
tem there are two fundamental principles to be first considered, 
(i) that of the exact relation between the size of a nerve and 
the amount of development of the part to which it is dis- 
tributed, and (2) that of the permanence of nerve distribution. 
The first follows from the fact that every cell or related group 
of cells in a given organ has each its own nerve fiber, and 
there are thus as many fibers in the nerve bundle supplying THE NERVOUS SYSTEM 
489 
the organ as there are such units in the organ itself. If, then, 
a part reduces or increases its total number of cells, the 
change is directly indicated in a corresponding reduction or 
addition of nerve fibers; and, furthermore, as the separate 
nerve fibers must each reach a central cell in the brain or 
cord, there are changes there also. These latter are some- 
times sufficient to become easily noticeable, as in the case of 
the intumescentise of the spinal cord, which are correlated 
with the development of the limbs. 
The second principle, that of the permanence of nerve dis- 
tribution, has already been referred to in several places, and 
many of our safest and surest conclusions concerning homolo- 
gies are based upon it. This principle, more fully expressed, 
affirms that a part never changes its nerve supply, and that a 
given nerve, once associated with a certain organ or complex 
of organs, will follow it through all its subsequent transforma- 
tions and even migrations. A good illustration of this is seen 
in the history of the stapedius muscle of the middle ear, which 
is supplied by a branch of the facial nerve. This supply is by 
no means the most convenient, and is reached only through 
overcoming a series of mechanical disadvantages, yet it is 
rendered necessary by the fact that the muscle in question was 
once a part of the digastricus (the posterior belly of the mam- 
malian muscle of the same name), and as such was supplied 
by the Facialis. Through the application of this inviolable 
principle numerous homologies have been established, and 
others, long believed in, have been disproven. 
Of undoubted connection with this close correspondence be- 
tween peripheral nerves and the organs to which they are dis- 
tributed, as enunciated in the above principles, is the singular 
phenomenon of plexus formation, seen in the nerves which 
supply the limbs. These plexuses consist of a more or less 
intricate set of intercommunications between the spinal nerves 
that are distributed to the limbs, and are hence two in number 
upon each side, plexus brachialis and plexus lumbo-sacralis, 
involving the nerves which supply the anterior and posterior 490 
THE HISTORY OF THE HUMAN BODY 
limbs respectively. The number of nerves involved in each 
plexus differs considerably, and reaches a large number in 
certain fishes, in which the fins are associated with a large 
number of myotomes, but in animals with the hand form of 
limb (chiridia) the number varies between two and seven. 
Of these series one or two, usually the central ones of each 
series, perform the greater part of the task of supplying the 
limb, and are consequently the largest; the others grade off 
above and below to those of normal size. The number and 
complexity of the intercommunications also reach their ex- 
tremes in the center of the plexus in connection with these 
larger nerves, above and below which the nerves become grad- 
ually less involved, until those are reached which have so 
slight a connection with the plexus that they are included 
within it by some authors and not by others. There is, in fact, 
considerable individual variation in a given plexus, and a de- 
batable nerve may furnish a communicating branch in one 
specimen which may be absent in another. 
The organization of a plexus may be best learned by actual 
examples, for which the brachial plexuses of two amphibians, 
two birds, and two mammals may be selected (Fig. 162). 
From these it will be seen that not only is the number of 
nerves involved a different one, but that the nerves themselves 
are not the same, counting from the first. This latter fact is 
but another way of saying that the girdles shift along the 
columns in different animals, locating in all cases at the point 
where the support will be the most effective, a fact brought 
out in previous chapters in relation to the bones and muscles. 
It shows clearly that homologies cannot rest upon definite 
body metameres, since there is great variation, both in the 
total number of metameres and in the relative length of each 
subdivision of the body; neck, trunk and tail. Thus in a 
frog, with a total of but ten pairs of spinal nerves, the brachial 
plexus involves the first three and the lumbo-sacral plexus the 
last four, leaving but three pairs of spinal nerves not involved 
in plexus formation; yet the metameres thus represented can- THE NERVOUS SYSTEM 
491 
not be taken, metamere for metamere, as the homologues of 
the first ten of other animals, which, in some cases, as in most 
birds, for example, would be included entirely in the neck; it 
Fig. 162. Brachial plexus of various vertebrates. 
(a) Frog. [After Gaupp.] (b) Axolotl (urodele). [After Fu'rbringer.] (c) 
Cassowary. [After Furbringer.] (d) Domestic fowl. [Fu'rbringer.] (e) Dog. 
[After Ellenberger and Baum.] (f) Man. [Gegenbaur.] 
The roman numerals indicate the spinal nerves. The other abbreviations art 
sufficiently complete to designate the nerves coming from the plexuses, 492 
THE HISTORY OF THE HUMAN BODY 
may rather be said that the ten of the frog are homologous 
in a general way with the total number of other animals, the 
two plexuses serving as fixed points for comparison. 
In comparing the various forms of plexus with one another, 
there are, in spite of the great diversity of combinations, cer- 
tain points of similarity. In the first place, both plexuses are 
always formed entirely of the ventral divisions of the spinal 
nerves, the dorsal branches being in all respects similar to 
those of adjacent nerves; secondly, the final outcome of the 
branching results in the formation of distinct dorsal and ven- 
tral branches, each one or two in number, the former dis- 
tributed along the extensor, the latter along the flexor, aspect. 
In the figures given, which are drawn from the ventral side, 
the dorsal elements are represented as forming a deeper layer, 
and are shaded for the purpose of rendering them more dis- 
tinct. It will be seen, also, that each of these final elements 
involves more than one root, and also that the same roots 
furnish fibers for more than one nerve. Furthermore, owing 
to the embryonal relation of the chiridium to the body, the 
first digit being anterior in both cases, the nerves supplying the 
inner (radial or tibial) side of each limb are derived more from 
the anterior portion of the plexus; those supplying the outer 
side from the posterior. Based upon the principles given 
above, the formation of a plexus possesses great morphological 
significance; for its intercommunications and its branching 
are, in part at least, records of the past history of the limbs, 
records which are so complicated that but little progress has 
as yet been made in their interpretation. It may be supposed, 
however, that if any two parts, two muscles, for example, 
each supplied by its own nerve, should coalesce, their nerves 
would also fuse into a single bundle, at least distally, and 
even that the extent of this fusion, that is, the distance from 
the origin at which these two nerves come together, would 
measure the relative length of time the parts have remained 
fused. Similarly, if a single part should differentiate into two, 
a phenomenon constantly occurring, among limb muscles, the THE NERVOUS SYSTEM 
493 
nerve would branch; and, furthermore, the increase of the 
differentiation between them, that is, a gain in the independ- 
ence of action, would tend to separate the nerve still more 
and cause the point of bifurcation to move proximally. 
It is thus probable that the plexuses have a meaning for 
him who is able to read it, the well-known conservatism of 
nerves in regard to their course assisting greatly in the preser- 
vation of these records. This conservatism is well shown in 
the case of snakes, in which the limbs have been lost, but 
where there are still traces of the plexuses, a fact attesting the 
former presence of the limbs. In certain other cases, as in 
the Ccecilia and in the lost hind limbs of the urodele Siren, 
not only have the limbs and their girdles utterly vanished, 
but there is also no trace of a plexus, showing that since the 
reduction of the limbs a much longer time has elapsed than 
in the former case, a conclusion in full accord with the relative 
place of these animals in the system and in their geological 
appearance. 
It is not probable, however, that all the changes in a plexus 
have a historic significance, since another factor must be taken 
into consideration, one that is the cause of certain changes, 
especially those of an individual character. This factor is 
found in the evident tendency, of certain forms at least, to 
shift the position of their girdles. This tendency is shown in 
individual cases by an increase in the size of certain of the 
nerves involved and a corresponding diminution in that of 
those either anterior or posterior to them; and, in certain 
species, by making careful counts of the separate fibers of 
the main nerves in a large number of individuals, the 
direction in which the girdle is migrating has been defi- 
nitely established. Thus in the common toad there is 
shown a tendency to push the shoulder girdle still further 
anteriorly, and as its present position is extremely cephalic, 
the continued tendency must be an instance of the inertia 
of variation through which a line of development, once started, 
is often carried far beyond the point of greatest efficiency. 494 
THE HISTORY OF THE HUMAN BODY 
This procedure involves more generally the posterior than the 
anterior girdle, and hence the lumbo-sacral plexus is more apt 
to vary individually. This migratory tendency may result 
in the establishment in a given species of two or three types of 
plexus, to which all individual variations may be referred, 
as has been established in the case of the urodele Necturus, 
well known also for its variability in pelvic attachment. 
The early anatomists, by a careful count of the nerve roots 
as they were found proceeding from the brain in the human 
subject, enumerated the following twelve pairs of cranial 
nerves, that is, of nerves which originate within the cranial 
cavity and escape through foramina in its floor: — 
Name 
Function 
I. 01 factories 
special sense, smell. 
II. Opticus 
special sense, sight. 
III. Oculomotor 
motor. 
IV. Trochlearis [Patheticus] 
motor. 
mainly sensory, with a small 
j motor root. 
V. Trigeminus [Trifacial] 
VI. Abducens 
motor. 
VII. Facialis 
mixed 
VIII. Acusticus [Auditorius] 
special sense, hearing. 
IX. Glossopharyngeus 
mixed. 
X. Vagus [Pneumogastricus] 
mixed. 
XI. Accessorius (Willisii) 
mixed, mainly motor. 
XII. Hypoglossus 
mixed. 
Of these the first two arise in association with the primary 
fore-brain, the telencephalon and diencephalon respectively; 
the remaining ten take their origin from the more posterior 
parts of the brain. It would thus seem that the former may be 
nerves of the archencephalon or primary brain, laid down 
in Amphioxus, while the latter belong to the secondary addi- 
tion from the anterior end of the original spinal cord. The 
last ten were thus at first spinal nerves, in which, in spite of 
their extreme specialization, it might be possible to recognize 
the original elements, each with its sensory and motor roots, THE NERVOUS SYSTEM 
495 
its accompanying ganglion, and so on. That the original 
elements have in some cases become modified is evidenced 
by several facts, first, the existence among them of wholly 
motor nerves without sensory fibers and lacking a ganglion; 
and, again, the fact that some of the nerves in the above list 
are shown to be composed of several primary nerves by their 
origin from multiple roots, or from the presence of several as- 
sociated ganglia. The twelfth nerve is outside of the cranium 
in fishes, and becomes later included within it, probably by 
the fusion with the skull of the vertebra with which it is 
associated. The eleventh is closely associated with the vagus 
and appears as a distinct cranial nerve only in mammals. 
Aside from the elements found in the above there are traces 
of several other spinal elements originally belonging to the 
primary anterior end of the cord, which do not survive in 
the higher forms as definite cranial nerves. These are desig- 
nated as the spino-occipital nerves, and are first met with in 
the selachians, where they appear as 1-5 pairs, placed very 
far back, along the medulla. They are spinal in character 
and not associated with the other cranial nerves, although 
they are all included within the skull. As this latter part 
ends abruptly with the otic region in cyclostomes and is im- 
mediately followed by the successive pairs of true spinal 
nerves, it seems reasonable to suppose that when, in the sela- 
chians, the cranial cavity became enlarged by an addition at 
the posterior end, several of the original spinal nerves were 
included, forming the nerves in question. In the higher car- 
tilaginous fish (Holocephali), and in ganoids, this set of nerves 
becomes reduced to two pairs, yet a second set, also of 1-5 
pairs, has been taken in, presumably in the same way. To 
distinguish between these two sets of spino-occipital nerves, 
the first are termed occipital, the second occipito-spinal. Rep- 
resentatives of both sets occur in varying proportions in other 
fishes, but in the amphibians they seem to have wholly dis- 
appeared, and are never seen again as distinct nerves. Al- 
though nothing has as yet been definitely proven in the matter, 496 
THE HISTORY OF THE HUMAN BODY 
it is probable from other evidence that above the fishes the 
occipital region suffers considerable reduction, during which 
many of these elements may have become lost, while others 
may have become established among the root elements of the 
twelfth nerve, the hypoglossal, since this nerve appears first 
as a cranial element in the reptiles and continues throughout 
Sauropsida and Mammalia. 
It is evident from this account that the cranial nerves are 
for the most part fragmentary examples of the typical meta- 
meric nerve with its four functional components, and further- 
more that the components which are present have undergone 
extreme differentiation involving in many cases the loss of 
metameric identity. Some have become excessively large and 
of great importance; others are reduced to mere vestiges. 
In the description which follows, the attempt is made to point 
out the functional components which are present. These are 
represented in an extremely diagrammatic form in Fig. 168. 
The cranial nerves fall naturally into groups which are best 
considered separately. 
I. The anterior group (Oljactorius, Opticus). 
These, the first two pairs, are nerves of special sense, the 
olfactory primarily a visceral sensory and the optic a somatic 
sensory nerve. They are, however, peculiar as sensory nerves 
in that their fibers do not develop as outgrowths from sensory 
ganglia which have themselves arisen from the neural crest 
cells. They are, indeed, associated with a region of the neural 
tube in which there is no typical neural crest formation. In 
their make-up each is unique in character. The sensory 
neurones from which the olfactory nerve fibers proceed are 
located in the sense organ itself, the epithelial lining of certain 
regions of the nasal cavity. The component fibers of the 
nerve are therefore the neurites of these cells, and are really 
comparable to the nerve roots of a typical sensory nerve com- 
ponent, rather than to the whole nerve. These fibers enter the 
olfactory bulbs, where they make their first central connec- 
tions. THE NERVOUS SYSTEM 
497 
The optic nerves are still less typical than the olfactory. 
They are composed of neurites of typical association neurones 
which are located in the so-called “ ganglion cell layer ” of 
the retina, itself originally an evagination of the diencephalic 
wall. They are, therefore, as truly nerve tracts as any which 
are found in the brain itself. In the lower vertebrates these 
fibers as they grow back into the brain along the course of the 
optic stalk, pass over the ventral surface of the diencephalon 
and make their first central connections in the mesencephalon. 
In the higher vertebrates a part of the fibers enter the thala- 
mus and make central connections there. 
Attempts have been made to bring the peculiar conditions 
of the olfactory and optic nerves into accord with those found 
in Amphioxus, for here the archencephalon, the possible 
equivalent of the forebrain of vertebrates, bears two rudi- 
mentary sense-organs, the first an olfactory pit and the second 
a pigment speck. Of these the first is connected with the 
brain by a short diverticulum, while the second is embedded 
within the brain wall. Although similarity of function of the 
two sets of organs in the two cases tempts one to believe in 
an homology between them, the decision really hinges upon 
the homology of these sense-organ rudiments with the per- 
fected organs of the higher vertebrates; for if the olfactory 
groove and the pigment speck are historically the anlagen of 
the nasal pit and eye, a point not definitely established, then 
the identity of the nerves with the corresponding parts of the 
archencephalon naturally follows. In favor of this latter as- 
sumption is the fact of the origin of these two nerves from the 
primary fore-brain, while none of the others arise anterior 
to the mesencephalon. 
An entirely problematical element belonging to this region 
is that of a definite pair of nerves, Nervus terminalis, which 
occur in all selachians and in the embryos, at least, of many 
higher forms, including man. They extend from the anterior 
aspect of the olfactory stalks. As they have been but recently 
discovered they have escaped enumeration with the classical 498 
THE HISTORY OF THE HUMAN BODY 
twelve pairs. They seem to represent a somatic sensory com- 
ponent for the most anterior segment of the head, thus making, 
together with the far larger and more important visceral 
Fig. 163. The Nervus terminalis of the selachians. [After Locy.] 
(a) Dorsal view of the brain of the dog-fish, Squalus acanthias. (b) Horizontal 
section through the anterior part of the same, showing the origin of Nervus ter- 
tninalis. 
Nas, nasal capsule; Olf, olfactory lobe; N. ter, Nervus terminalis; gl, its ganglion; 
Tel., telencephalon; Di, diencephalon; Mes, mesencephalon; Met, metencephalon: 
Myel, myelencephalon. 
sensory element, the olfactory nerve, a complete sensory quota 
for this segment of the body. 
II. The motor nerves of the eyeball. (Oculomotorius, 
Trochlearis, Abducens.) 
These three pairs are small and very special nerves, having 
no other distribution than the six muscles of the eyeball. 
The oculomotor has its nucleus of origin in the mesen- 
cephalon, the trochlear in the extreme posterior region of the 
mesencephalon and in part from the anterior region of the 
rhombencephalon, and the abducent in the more posterior 
region of the rhombencephalon, all in the somatic motor di- 
vision of the gray substance. THE NERVOUS SYSTEM 
499 
They are thus primarily somatic motor nerves and on the 
theory that the cranial nerves, excepting, perhaps, those of 
the preceding groups, represent modified spinal nerves, seem 
to correspond to the motor roots of three original nerves, the 
sensory roots of which are either lost, or, more probably, 
contained in the sensory elements of adjacent cranial nerves 
such as Trigeminus or Facialis. Each has, however, a small 
somatic sensory component which is proprioceptive from the 
sensory nerve endings in the muscles themselves. The strictly 
metameric character of their field of distribution, namely, the 
eye muscles, as is shown by their developmental history, bears 
out this hypothesis as to their original relationships. In 
selachian embryos, which have preserved this early his- 
tory more completely than have the higher forms, there de- 
velops in the head a series of myotomes or head somites, simi- 
lar to and continuous with those of the trunk. Some of them 
soon atrophy, but the first three fold about the developing 
eyeball and furnish it with muscles. From the first arise 
three of the straight muscles and one oblique, from the second 
the other oblique, and from the third the remaining straight 
muscle. These three myotomes are innerved by the three 
nerves under consideration, and in their natural order of suc- 
cession, as follows: 
Somite 
Muscles Developed 
Nerve 
No. 
Myotome I 
Rectus superior 
Rectus medialis 
Rectus inferior 
Obliquus inferior 
Oculomotor 
Ill 
Myotome II 
Obliquus superior 
Trochlearis 
IV 
Myotome III 
Rectus lateralis 
Abducens 
VI 
These relationships are practically constant throughout all 
vertebrates, although there seems to be a slight modification 
of the plan in the case of Squalus acanthias which may pos- 
sibly be found to hold true in other vertebrates. This is shown 500 
THE HISTORY OF THE HUMAN BODY 
diagrammatically in Fig. 164 and consists in the fact of the 
derivation of the obliquus superior from only a dorsal moiety 
of the second myotome, while its ventral moiety joins the 
third myotome and thus contributes to the formation of the 
rectus lateralis. This whole history corroborates the idea that 
we have here the enumeration of some very primitive mor- 
Fig. 164. A diagram of the left lateral aspect of the Squalus embryo showing the 
hypothetical primitive relations of the eye muscles to the lateral trunk musculature. 
Myotomic divisions not functional in Squalus are indicated by broken lines. Only those pre- 
otic myotomes which have become functional as eye muscles have persisted. Neuromeres indicated 
by Roman numerals, myotomes by Arabic numerals, with the addition of d for the dorsal and v for 
the ventral subdivisions. [From Neal.3 
phology. In certain Orders, in response to special needs, 
other special muscles appear in connection with the eyeball, 
but these are seen to be differentiations of certain of the above, 
and retain the same innervation; thus the retractor bulbi1 
arises from the rectus lateralis, and, like it, is innerved by the 
sixth nerve. 
The relation of these three nerves to adjacent sensory ele- 
ments and their right to be considered ventral roots are matters 
concerning which few definite conclusions may be drawn as 
yet, although much work has been done upon them. The 
oculomotor, although mainly somatic motor, yet becomes con- 
nected with the small ciliary ganglion of the sympathetic sys- 
tem, through which its fibers innerve the ciliary muscles and 
the iris, and thus has a small visceral component. This gan- 
1 This muscle, is rudimentary or wanting in the Anthropoidea. THE NERVOUS SYSTEM 
501 
glion may have the morphological value of the one belonging 
to a sensory root now lost, a conclusion which would make 
this nerve comparable to an entire spinal element with a reduc- 
tion of the sensory root. Other views associate with 'it as its 
sensory element a portion of the Trigeminus, the ophthalmicus 
projundus, while still another theory finds in the transitory ap- 
pearance of a neural crest in the diencephalic region of the 
selachian embryo, an indication of a hypothetical sensory 
component which has been named the Nervus thalamicus, and 
is regarded as the sensory component corresponding to the 
oculomotor. The Trochlearis, although essentially a motor 
nerve, possesses in fishes and amphibians a few sensory fibers, 
yet, in spite of this, all are agreed that the sensory element 
originally associated with this has become incorporated with 
the Trigeminus. The sensory portion of the Abducens is prob- 
ably also a part of the Trigeminus, although certain facts 
indicate an association with the Facialis. 
III. The trigeminus-facialis group (Trigeminus, Faci- 
alis, Acusticus). 
This group and the next are by far the most extensive, and 
together constitute the main bulk of the nerves of the head. 
Their relationships differ considerably in fishes and aquatic 
amphibians on the one hand (Plate VI), and in terrestrial (and 
secondarily aquatic) vertebrates on the other (Plate VII), a 
difference largely due to the presence in the one and the sup- 
pression in the other of an extensive system of external sense- 
organs for the reception of pressure stimuli and hence of 
great importance in aquatic life. These organs, variously 
termed “ integumental sense-organs,” “ dermal canal system,” 
“ lateral line organs,” or, more technically, neuromasts, are 
visible externally and are arranged in definite lines running 
about the head and continued in a single (or double) longitu- 
dinal row, the lateral line, down the sides of the body. The 
somatic sensory nerves which supply these (PL VI) are in 
their origin connected with the early development of the 
Acusticus, also, a somatic sensory nerve. This whole sys- 
tem which is spoken of as the Acustico-lateral-line system, is 502 
THE HISTORY OF THE HUMAN BODY 
somewhat different from the typical sensory nerve in its de- 
velopment. Early in the embryonic history a pair of dorso- 
lateral ectodermic thickenings appear on either side in the 
region where an interruption in the neural crest occurs pos- 
terior to the Facialis crest. This thickening is called the 
dorso-lateral placode and a part of it gives rise to a hollow 
vesicle, the otic vesicle, which sinks in below the surface and 
eventually develops into the organ of hearing, the ear. A cer- 
tain portion of it, however, with which the Facialis crest comes 
in contact, not only gives rise to the Acusticus with its 
two ganglia {vestibulare and spirale) but also by growing 
anteriorly and posteriorly, forms the entire system of neuro- 
masts, together with the ganglia and nerves which connect 
these with the brain. The posterior branch of this Acustico- 
lateral-line system, which supplies the lateral line organs 
proper, and extends the whole length of the body, becomes 
secondarily associated with the sensory root of the Vagus 
nerve, but the anteriorly directed division, consisting of three 
main branches, retains its original relationship as a part of 
the Facialis. Each of these four branches possesses typically 
a special ganglion, placed near its origin; but these are dis- 
tinct in only a few forms (selachians, dipnoans, a few aquatic 
amphibians). In all others the three anterior ones become 
completely fused with the ganglion semilunare of the Trigem- 
inus and are demonstrable only in the embryo. To the 
compound ganglion thus resulting, which is found in most 
amphibians and in all the amniotes, may be applied the term 
Gasserian, long in use in human anatomy for this organ. 
The most dorsal of the three Facialis branches of this sys- 
tem is the superficial ophthalmic (ramus ophthalmicus super- 
ficialis septiml), and is accompanied by a like-named branch 
of the Trigeminus {ramus ophthalmicus super ficialis quinti), 
which supplies general sensation to this region. The second 
branch is the buccal {ramus buccalis), accompanied in its turn 
by the maxillary branch of the Trigeminus. The third is the 
external mandibular, divided into anterior and posterior 
branches. The companion branch from the Trigeminus, Plate VI. Diagram of cranial nerves in Anamnia. [After Wiedersheim.] The eye muscle nerves are omitted. 
Roman numerals indicate the cranial nerves. Branches of these are designated as follows: 
r. o. s, ramus ophthalmicus r. o.p, ramus ophthalmicus profundus; r. bucc, ramus buccalis; r. pal, ramus palatinus; r.md.ex, 
ramus mandibularis externus; r. ('»/, ramus mandibularis interims; r, hy. tnd, ramus hyomandibularis; r. comm, ramus cominunicans (in 
Dipnoi); r. lat, ramus 1 .teralis; r. intes, ramus intestinalis. 
Color Scheme: uncolored, the entire supply to the lateral line organs, also the Acusticus; black, Trigeminus; grey, ophthalmicus profundus; 
rreen, Facialis, sensory elements; blue, Facialis, motor elements; yellow, Glosso-pharyngeus and Vagus. Plate VII. Diagram of cranial nerves in Amniota. [After Wiedersheim.] The eye muscle nerves are omitted and 
there are a few other slight changes. Roman numerals indicate the cranial nerves. 
sfi. i, and sp 2, are first and second spinal nerves respectively. Branches are designated as follows . r. oph, ramus ophthalmicus; r. mx, ramus 
maxillaris; r.pal, ramus palatinus; ch. tymp, chord 1 tympani (ramus mandibularis internus); r. ling, ramus lingualis; r. tnd, ramus mandibularis; 
mim, mimetic nerve; r. hy md, ramus hyomandibularis; r. ventr, ramus ventralis; pet. sup. ;naj, petrosus superficialis major; pet. sup. min, petrosus 
mino’; n.Jac, Jacobson's nerve; j and m, denote sensory and motor branches respectively; exponents d and v denote dorsal and ven- 
tral roots respectively; a. ciliary ganglion; b. sub-maxillary ganglion; r, spheno-palatine ganglion; d. otic ganglion. 
Color Scheme; uncolored, motor branch of Trigeminus, Acusticus, Accessorius, Hypoglossus, Spinal nerves; black, remainder of Trigeminus; 
green. Facialis, sensory elements; blue. Facialis, motor elements;yellow. Glosso-pharyngeus and Vagus. THE NERVOUS SYSTEM 
503 
associated with the anterior of the two subdivisions, is the man- 
dibular of that nerve (ramus mandibularis quinti). In the 
Dipnoi there is a communicating branch between this part of 
the system and that belonging to the Vagus, but this seems 
to be wanting in other cases. 
The remaining branches of the facial nerve are divisible 
into two portions, sensory and motor, each of which is pri- 
marily visceral, as will be seen from its distribution to the 
pharyngeal walls and their derivatives. The sensory portion 
arises from the neural crest and possesses at its origin the 
large genicular ganglion, from which proceed (i) a large 
palatine branch and (2) a small internal mandibular. The 
motor portion becomes incorporated with the external man- 
dibular branch of the lateral line system given above, and this 
forms the mixed hyo-mandibular branch which supplies the 
region of the lower jaw and the hyoid arch. 
Much of the Trigeminus has already been described in 
association with the Facialis. Like the Facialis, its motor 
component is wholly visceral. It is very small, however, and 
found only in the mandibular branch. The sensory compo- 
nent, on the other hand, is very large and is wholly somatic, 
with distribution to the skin and interior of the mouth. It 
arises in the typical manner from a large neural crest which 
lies anterior to that of the Facialis and is separated from it 
by a distinct interruption. Its ganglion is the large semilunar 
ganglion (often fused with the sensory ganglia of the lateral 
line nerves of Facialis) which lies at the base of the three 
branches already described. The deep ophthalmic branch 
(ramus ophthalmicus profundus) of the Trigeminus possesses 
a ganglion of its own and issues from the skull by a separate 
foramen. It is thus semi-distinct from the remainder and 
may be considered a separate nerve, originally anterior to the 
Trigeminus, and secondarily associated with it. There are 
some indications to show that it may have once been associated 
with the Trochlearis, as sensory and motor roots, respectively, 
of the same elementary nerve. 
The later history of the parts above considered may be 504 
THE HISTORY OF THE HUMAN BODY 
followed from the second diagram (Plate VII), which repre- 
sents in a general way the terrestrial type, but which in its 
proportions and certain other details suggests more especially 
the mammalian condition. The first and most striking change 
is the loss of the special system supplying the “ lateral line ” 
sense-organs, the entire equipment for which vanishes in com- 
pletely terrestrial vertebrates. This causes at once, among 
other changes to be considered later on, a loss of three branches 
of the Facialis nerve, the superficial ophthalmic, the buccal, 
and the external mandibular. There remain the palatine, 
the internal mandibular and the motor element of the hyo- 
mandibular, of which the first two become reduced in size and 
fuse with Trigeminus elements, while the third loses in one 
direction but more than compensates for it in another. The 
palatine, under the name of N. petrosus superficialis major, 
passes through the pterygoid \Vidian\ canal (in Mammals) 
and enters the spheno-palatine ganglion of the sympathetic 
system, where it meets with fibers of the trigeminal nerve, and 
continues under the name of palatinus major, usually consid- 
ered as a part of the Trigeminus. The internal mandibular, 
now known as the chorda tympani, transverses the tympanic 
cavity, running between malleus and incus. It leaves the 
middle ear through the Glaserian fissure (in mammals) and 
blends with the lingual branch of ramus mandibularis V., en- 
countering in its course the sub-maxillary ganglion (better, 
sub-mandibular) of the sympathetic system. It becomes the 
gustatory nerve of the anterior part of the tongue. 
The explanation of these complicated relationships becomes 
clear when we consider the history of the related parts. Mal- 
leus and incus are primarily the condyle of the jaw and the 
quadrate bone, respectively, and the branch in question runs 
along the articulation between them. In the Mammalia these 
osseous elements become drawn within the cavity of the 
middle ear, where they undergo a transformation into auditory 
ossicles; and the nerve, in order to preserve its original rela- 
tionships, must follow them, thus producing a complicated 
condition, easily explained by their morphological history, but THE NERVOUS SYSTEM 
505 
wholly inexplicable otherwise. Furthermore, with the in- 
creased importance of the tongue, and more especially with 
the development of the fleshy part of it in mammals, the orig- 
inal branch of the Trigeminus gains in importance and the 
two become secondarily associated. 
With the reduction in bulk of the hyo-branchial musculature 
the motor element of the hyo-mandibular, the only portion 
now remaining, tends to decrease in size, but this is more than 
compensated for in mammals by the development of the mi- 
metic muscles. These have been shown to originate from the 
integumental muscular layer of the neck region, innerved by 
the branch under consideration, and, as this layer spreads up 
over the neck and differentiates into specialized slips, the in- 
nervation increases also and spreads eventually over the entire 
face, thus gaining its right to the name “ Facialis,” a right 
which, curiously enough, it possessed originally, in connection 
with the lateral line organs, but which it afterwards lost until 
it regained through its motor elements what it had lost in 
its sensory. The branch to the stapedius muscle of the middle 
ear, N. stapedialis, proceeds also from the hyo-mandibular, 
and comes originally from the branch supplying the digastric 
muscle. 
The Trigeminus of the second, or terrestrial, type, suffers 
no reduction through the loss of the lateral line organs, since 
it has nothing directly to do with them, but the jour original 
branches become reduced to three through the loss of the deep 
ophthalmic element, which seems, in part at least, to fuse with 
the superficial branch of the same nerve and form the “ first 
branch ” of human anatomy, the ophthalmicus. The maxil- 
laris and mandibularis show but little change and form the 
second and third branches, respectively, thus giving the reason 
for the name “ Trigeminus,” first applied in Man. 
The great increase in the size of the lingual branch of the 
mandibularis has already been noticed; otherwise the most 
important innovation is found in the new relations of the 
Trigeminus with the Facialis, the Glossopharyngeus, and the 
sympathetic ganglia. The first of these has already been 506 
THE HISTORY OF THE HUMAN BODY 
treated in detail. The Glossopharyngeus sends a communi- 
cating branch (tympanic [Jacobson’s] nerve) to the otic gan- 
glion, which rests upon the base of ramus mandibularis; the 
two nerves also come into indirect relation in the tongue, where 
the fibers of the gustatory and lingual branches of the two 
nerves interlace. Four ganglia of the sympathetic system, the 
entire cephalic group, become associated with the Trigeminus, 
the ciliary with the first branch, the spheno-palatine with the 
second, and the otic and submaxillary with the third. The 
Acusticus, originally a part of the Facialis element which 
supplies the “ lateral line ” system, is consequently nothing 
more than a branch of the superficial sensory system of the 
Facialis, the region of distribution of which chanced to develop 
a high degree of complexity as an organ of special sense. 
IV. The vagus group. (Glossopharyngeus, Vagus, Ac- 
cessorius.) 
This group includes in all vertebrates the Glossopharyngeus 
and Vagus nerves, to which is added in mammals the Acces- 
sorius, secondarily derived from the Vagus, and existing in 
the Sauropsida as a semi-independent slip. This group is pri- 
marily associated with the gill-region, but secondarily sends 
branches backwards and forwards which may even reach the 
extreme ends of the body, thus having a more extensive dis- 
tribution than that of any other cranial nerves. In the lower 
forms this group is extremely regular and possesses a well- 
pronounced metamerism, thus strongly suggesting its origin 
from spinal nerves, similar to those which form a direct con- 
tinuation of the series. Taken in connection with the much 
greater differentiation of the other cranial nerves it seems 
evident that the acquisition of the anterior end of the primor- 
dial spinal cord by the cranium has been a gradual one, and 
that the Vagus group is less modified than the nerves anterior 
to it, because it has been annexed later. The primitive condi- 
tion, that found in fishes, may be first considered by the help 
of the diagram previously referred to [Plate VI]. The most 
superficial, and at the same time, the most aberrant, is the 
ramus lateralis, which belongs to the system of lateral line THE NERVOUS SYSTEM 
507 
nerves and is thus only secondarily annexed to the vagus. It 
supplies the lateral line itself, and extends typically to the 
end of the tail. It is thus the longest nerve in the body, co- 
extensive with the spinal cord itself. The lateral nerves of 
the two sides are connected with one another by the supra- 
rx-x-xi gang crest 
Opthal div 
Sup max piy'.- 
A/ masticatorius 
Inf. max.divd 
Gang petros.. 
Gang nodos. 
N. taryg sup. 
Arm. 
Leg. 
Fig. 165. Reconstruction of peripheral nerves in human embryo. 
[After Streeter.] Four weeks human embryo, 6.9mm long. 
N. tymp, tympanic nerve; N. laryg. sup, superior laryngeal nerve; Gang, petros, 
ganglion petrosum; Gang nodos, ganglion nodosum; Fronep, Froriep’s ganglion. 
The cranial nerves are designated by roman numerals, the spinal by arabic. The 
other designations are evident. 
temporal branch, which forms a connecting loop over the top 
of the head; in the Dipnoi, though not in other fish, lateral 
communicating branches connect it with the superficial oph- 
thalmic nerve of the Facialis, thus uniting the two parts of 
the system. The ramus lateralis has at its proximal end a 508 
THE HISTORY OF THE HUMAN BODY 
ganglion of its own (ganglion later ale), although it arises in 
close association with the combined ganglionic mass of the 
Vagus. 
With the exception of a small somatic sensory component 
of the Glossopharyngeus and Vagus (each with its own gan- 
glion, the ganglion superius and the ganglion jugulare respec- 
tively), which have a cutaneous distribution, the remaining 
elements of this group are wholly visceral nerves. They form 
for the most part five serially similar elements located ven- 
tral to the ramus lateralis, each associated with a gill slit and 
each typically possessing a ganglion upon its sensory compo- 
nent. In the origin of their sensory components they involve 
not only the typical neural crest formation but also placodes 
which appear, in the lowest forms, segmentally above the gill 
slits (epibranchial placodes) and contribute to the correspond- 
ing ganglion formation, although, unlike the dorso-lateral 
placode, they do not give rise to sense organs. 
This extremely primitive condition, a segmental arrange- 
ment of ganglia, is seen in a few forms only (e.g., the rays and 
skates), but these animals are in other respects so primitive, 
and the condition is so exactly what one would expect as an 
early one, that it may be taken as undoubtedly the starting 
point. The first of these elements with its ganglion petrosum 
is more distinct than the others and belongs to the Glosso- 
pharyngeus; the remaining four are Vagus elements and in all 
but very primitive forms possess a single ganglionic mass, the 
ganglion nodosum, formed of a fusion of the four primary 
ones. It is with this that the ganglion laterale of the ramus 
lateralis is associated. Each of these five elements (Glosso- 
pharyngeus, and the four Vagi) possesses an identical mode 
of distribution. From the ganglion the main stem passes down- 
wards, and forks into two branches including a gill-slit in the 
fork. The two branches, one in front of, and one behind the 
slit, are known respectively as rami prce- and post-trematici. 
The former is wholly sensory, the latter mixed sensory and 
motor. In this connection it is interesting to see that the 
spiracular opening, which probably represents the gill-slit next THE NERVOUS SYSTEM 
509 
in order anterior to the regular series, is similarly included 
between the internal mandibular (chorda tympani) and the 
hyo-mandibular oj the Facialis, which thus become, respec- 
tively, the prce- and post-trematic branches oj that nerve, 
It is even possible in like manner to consider the maxillary 
and mandibular branches oj the Trigeminus as similarly re- 
lated to the mouth opening, resting upon the probability oj 
the identification of the mouth with an original gill-slit an- 
terior to the spiraculum. We have thus a character of great 
value in the resolution of the cranial nerves into their original 
metameric elements, one which will be considered later in the 
treatment of this difficult and unsolved problem. 
Of these five branchial nerves, the Glossopharyngeus, as 
the most anterior and consequently the most modified (earliest 
absorbed by the cranium), possesses additional branches not 
represented in the others, and these have run forward and 
supply parts anterior to it. One of these is a communicating 
branch between this nerve and the Facialis and passes from its 
ganglion (ganglion petrosumj to that of the Facialis (ganglion 
genicularej. This nerve possesses no special name in lower 
forms, but in the higher forms it becomes the tympanic (nerve 
oj Jacobson), and forms an intimate means of communication 
between these nerves and the Trigeminus. Below this is the 
palatine, lying near the Facialis branch of the same name, and 
developing a few connections with it. A small lingual branch is 
present in the Dipnoi. 
There remains but one further element to be considered, 
but this is an extensive visceral motor one, the ramus intesti- 
nalis Vagi. This is the branch which, even more than the 
lateralis, has earned for the nerve to which it belongs the title 
of Vagus (wandering), since it becomes distributed to the 
oesophagus and stomach, the heart, and, in higher forms, the 
lungs also. In spite of its great length and extensive distri- 
bution, however, this is not to be considered a compound nerve, 
but its length is due rather to the extension posteriorly oj parts 
once placed jar forward and thus within the legitimate prov- 
ince oj the original nerve. Thus, the heart has primarily a 510 
THE HISTORY OF THE HUMAN BODY 
very anterior position; the oesophagus and stomach were prob- 
ably once very far forward, and the lungs are diverticula of 
the primary oesophagus. The wide distibution of this branch 
is a striking illustration of the principle of conservatism 
of nerve distribution enunciated above, and belongs in the 
same category as the case of the stapedial nerve or that of the 
innervation of the mimetic musculature. 
In the transition to terrestrial life the Vagus group suffers 
naturally the loss of the branchial elements in which metamer- 
ism was so clearly displayed, but has gained by the greater de- 
velopment of the tongue and the sense of taste, and has differ- 
entiated the Accessorius element. The intestinal branch also, 
corresponding to the higher development of its field of distribu- 
tion, is still more extensive and complex. 
The extreme differentiation of this entire group may be 
learned from Plate VII, which here especially, in the separation 
of the Accessorius and in other points, suggests the mammalian 
conditions. The communicating branch, the tympanic nerve, 
becomes somewhat more complicated and forms connections 
between four ganglia; starting from the ganglion petrosum of 
the Glossopharyngeus nerve it runs forward and sends branches 
to the ganglion geniculare of the Facialis, and to the otic and 
sphenopalatine ganglia of the sympathetic system. It is also 
involved in a small plexus of sympathetic nerves which sur- 
round the carotid artery. The relationships of this nerve, 
which are so complex in mammals, become especially so in 
Man, owing to the shortening of the longitudinal axis of the 
skull and the formation of the cervical flexure, both of which 
tend to the shortening of the distance between the nerve roots. 
The relations found in Man are shown in the accompanying 
figure (Plate VIII), which is to be carefully compared with 
Plate VII. As special names are often given in human anat- 
omy to parts spoken of by the morphologists under more 
general terms, a list of equivalent terms is here added, for the 
better comparison of the figures alluded to. Plate VIII. Inter-relation of Trigeminus, Facialis, Giossopharyngeus, and Vagus, together with the sympathetic 
ganglia in man. Based upon diagrams by several anatomists (Arnold, Gray, Gegenbaur). 
For explanation of designations, reference to the two preceding plates and to the text will be sufficient. 
Color Scheme; uncolored, Motor oculi; black,Trigeminus; grey, sympathetic ganglia, lacrimal gland, ahd arteries; green, Facialis; yellow, 
Glosso-pharyngeus and Vagus. THE NERVOUS SYSTEM 
511 
Terms used in human anatomy. 
Morphological equivalents. 
Ganglion ophthalmicum superfi- 
cial VII. 
Ganglion buccale VII. 
Ganglion mandibulare VII. 
Ganglion semilunare V. 
Ganglion ophthalmicum profun- 
dum. 
Ganglion Gasseri, composed of 
Sensory ganglion of VII, excepting 
the parts belonging to the lateral 
line system. 
Ganglion nodosum. 
Ganglion petrosum 
Ganglion suoerius. 
Together form the sensory gan- 
glion of IX. 
Compound sensory ganglion of X, 
formed by the fusion of the 
ganglia of all of the original 
sensory elements with the excep- 
tion of the ganglion laterale of 
the lateral line system. 
Ganglion nodosum 
Ganglion jugulare. 
N. petrosus superficialis major Ramus palatinus VII. 
Continuation of the ramus pala- 
tinus VII beyond the spheno- 
palatine ganglion, plus some 
fibers from the Trigeminus. 
N. palatinus major V 
Chorda tympani N. mandibularis internus VII. 
N. tympanicus 
N. petrosus superficialis minor 
Together form Jacobson’s nerve 
which is, morphologically, ra- 
mus communicans IX. 
Both included in the branch of the 
above to the spheno-palatine 
ganglion. 
N. petrosus profundus major. 
N. petrosus profundus minor. 
The lingual branch of the Glossopharyngeus, which appears 
first in the Dipnoi, becomes in the Amniota, and especially in 
the mammals, a large and important nerve, and specializes as 512 
THE HISTORY OF THE HUMAN BODY 
the main gustatory nerve. The remaining branch is motor and 
is distributed to those muscles which are derived from those of 
the first branchial arch. In the same way motor branches of 
the Vagus supply the muscles of the larynx and trachea, and 
the walls of the pharynx. 
The ramus lateralis disappears with the advent of terrestrial 
life, but, on the other hand, the increase in size and complexity 
of heart, lungs, oesophagus, and stomach so increase the im- 
portance of ramus intestinalis that it alone comes to be con- 
sidered the main nerve (hence the name “ Pneumo-gastric ”), 
of which the other elements are considered branches. 
The circumstance which leads in Sauropsida to the partial, 
and in mammals to the complete, separation of the Accessorius 
is clearly found in the greater development and higher degree 
of independence of the parts to which it is supplied, the tra- 
pezius and sterno-cleido-mastoid muscles, a development due 
in its turn to the increased importance of the neck. This 
nerve is still a part of the Vagus in the human embryo and 
shows the steps of its gradual emancipation during develop- 
ment (Figs. 165 and 166). 
V. Hypoglossus. 
The hypoglossal nerve, which appears in the higher verte- 
brates as the last of the cranial nerves, is plainly one or more 
adopted spinal nerves, found still in their original office in 
fishes and amphibians. It is mainly a somatic motor nerve 
and arises from several roots which belong in the ventral series; 
this is rendered more certain by the appearance, usually 
transitory and embryonal, of corresponding dorsal roots, 
equipped with ganglia, which thus complete the elements nec- 
essary for genuine spinal nerves. In Plate VII these latter are 
indicated by dotted lines; and the two spinal nerves which are 
shown in Plate VI may be considered to represent the poten- 
tial hypoglossal still in an indifferent condition. Some of the 
spino-occipital elements may also enter into the formation of 
the Hypoglossus, but this has not yet been definitely shown. 
The Hypoglossus enters into connection with one or two of 
the first cervical nerves, forming the ansa hypoglossi, a union THE NERVOUS SYSTEM 
513 
Vagus root gang. 
Accessory root gang. 
IX root gang.- 
Gang petros. 
N tymp 
Froriep 
Br to 
iCarotid plexus 
N. laryg. sup. 
Gang nodos 
Vagus 
i 
Sympathetic 
Fig. 166. Reconstruction of peripheral nerves in human embryo. 
[After Streeter.1 Six weeks human embryo, 17.5mm long. 
For abbreviations see Fig. 125, a.  THE HISTORY OF THE HUMAN BODY 
514 
from which proceed motor nerves to certain of the hyoid and 
extrinsic laryngeal muscles. 
Before leaving the description of the cranial nerves, at- 
tention should be called to the fact that the Trigeminus, 
Facialis, Glossopharyngeus, Vagus, and Accessorius, are all 
similar in that their motor components are wholly visceral. 
These, moreover, are not wholly distributed to the usual types 
of visceral effectors (glands and involuntary muscles), but 
have a large distribution to that portion of the voluntary 
musculature which is designated as branchiomeric (Cf. Ch. 
VIII). The reason for this lies in the fact that in the primi- 
tive forms these were important visceral effectors, since, as 
their name implies, they constituted the musculature of the 
gill arches. The muscles derived from these in higher forms 
retain, of course, the same innervation. 
In the relation of the visceral motor nerve roots to the brain 
stem it is very evident that they form a series arising from 
the more laterally located visceral motor division of gray 
substance of the cord, while the somatic motor roots, the Ocu- 
lomotor, Trochlear, Abducens, and Hypoglossus arise like 
typical spinal motor nerve roots from the ventrally located 
somatic motor division. 
It is now time, after this review of the cranial nerves and 
their morphological history, to take up the question of the 
original segmentation of the vertebrate head, and consider 
what light the nerves throw upon this obscure and much-dis- 
puted subject. 
The question rests upon the assumption that the precursors 
of the vertebrates were like Amphioxus, headless but com- 
pletely segmented, and that the head was first formed by the 
union of a certain definite, though probably rather small, num- 
ber of somites, each with its similar set of organs, into a single 
body unit or complex. A similar process has been postulated 
in the case of the head of insects, and the two cases have much 
that is analogous, especially the gradual addition of primarily 
trunk somites in proceeding from lower to higher forms. In 
a myriapod, for example, or, still better, in an annelid like the THE NERVOUS SYSTEM 
515 
earth-worm, the somites are practically alike, each containing 
one pair of nerve ganglia, one pair of segmental organs (in the 
latter case), one set of metameric muscles, and one pair of ex- 
ternal appendages, and it is by comparing the number of gan- 
glia, of appendages, or other metameric parts, that morpholo- 
gists attempt to resolve the head complex into its primary 
somites. In much the same way the body somites of Amphi- 
oxus, or, to a lesser degree, of a fish, are also similar, and the 
pairs of nerves, the gills, the nephridia, and especially the myo- 
tonies, are metameric, at least in the embryo. By thus ascer- 
taining the original number of each of these metameric ele- 
ments that exist in the head, as shown in the embryological 
record, morphologists have sought here also to reconstruct the 
early conditions and translate the head of modern vertebrates 
into a definite series of somites, each with its metameric parts. 
Thus far the views of investigators are widely apart, and the 
suggested number of primary somites varies from three or 
four to eighteen, or even more; the most usual results agree, 
however, in placing the number between the limits of nine and 
eleven. 
The posterior portion of the head in fishes, and especially 
in certain primitive selachians, shows a definite metamerism, 
marked in the cartilaginous gill-arches, the arterial arches, 
and the cranial nerves (Vagus group), but anterior to the 
otic region this becomes effaced, and it is extremely difficult 
to see here any suggestions of segmentation even in the em- 
bryo. Some have, indeed, asserted that the proechordal and 
parachordal portions of the head, divided at about the hypo- 
physis, are distinctly different in this respect, and that while 
the latter forms the original anterior end of the primary seg- 
mented ancestor and may thus be expected to show traces of 
metamerism, the former or praechordal portion represents a 
later addition, gained somewhere between Amphioxus and 
cyclostomes, and is thus primarily unsegmented. It is more 
probable, however, that this prsechordal portion is primarily 
metameric as well as the others, and that the indications of 
this have become more completely effaced, first, because it 516 
THE HISTORY OF THE HUMAN BODY 
began to be modified much earlier than the other part, and 
secondly, that, because of its position, it became naturally the 
seat of important organs of special sense and became more 
modified through their influence. 
The advocates of a larger number of head segments base 
their claim upon a primitive segmentation of the neural tube 
itself, a matter which is open to some question. It is true 
that in the developing vertebrate embryo, a series of segments 
or neuromeres appears in the spinal cord and in the rhomben- 
cephalon. Similar appearances may also be observed in the 
more anterior region of the brain, but these are very variable 
and are probably attributable to mechanical pressure involved 
in growth rather than to an inherent segmentation. In fact, 
the well marked neuromeres of the spinal cord seem to be 
distinctly due to the pressure of the adjacent myotomes. In 
the rhombencephalon, however, such an explanation does not 
hold true and there is certainly found here evidence of true 
segmentation or neuromere formation, corresponding to the 
segmentation of other structures in this region. The absence 
of all suggestion of neuromeres in Amphioxus, however, while 
the mesodermic somites have a typical development, indicates 
that the neuromeres which appear in the vertebrates do not 
stand for a primitive condition but may be looked upon as 
ccenogenetic developments. 
The most reliable evidence as to segmentation of the verte- 
brate head lies, therefore, in the number of mesodermic somites 
which appear and in the nerve distribution to these. Even in 
cyclostomes and selachians in which a more complete quota 
of mesodermic somites appears in the head region than is the 
case with higher vertebrates, we do not find a basis for a 
larger number of head segments than eleven. 
In the consideration of this problem, the cranial nerves offer 
an especially hopeful material, as the various sensory and mo- 
tor elements, sensory ganglia and other parts, suggest that 
they have differentiated from an original series of typical 
spinal nerves. Thus, as primary sensory roots, each with a 
ganglion, we may suggest the Trigeminus, Facialis, Glosso- THE NERVOUS SYSTEM 
517 
pharyngeus, and Vagus, the latter a compound nerve, capable 
of resolution into four, or perhaps, five elements. If to these 
the ramus ophthalmicus profundus be added with its ganglion 
as an originally separate element, we have the sensory roots 
of eight original pairs. The three nerves of the eye muscles 
are, both in origin and function, motor roots, and in some cases, 
as in the relation of Abducens to Facialis, they seem to belong 
with certain definite sensory elements. The tracing out of 
prse- and post-trematic branches which include a gill-slit, as 
above mentioned, assists in locating seven elements, if the 
mouth opening and spiraculum be included. 
The above relations are summarized in the accompanying 
diagram, which, although not claimed as the ultimate solution 
of the problem is, at least, suggestive (Fig. 167, A). The 
head is here represented as composed of nine somites on 
the basis of nine pairs of “ head cavities ” (muscle somites or 
myotomes) found in dog-fish embryos. These are represented 
by the heavy black rings numbered from I-IX. For the first 
somite the Ophthalmicus profundus represents the sensory, 
and the Oculomotorius the motor root. To the second, the re- 
maining portion of the Trigeminus and the Trochlearis are 
similarly related, the former with prae- and post-trematic 
branches about the mouth. The third is supplied by the Fa- 
cialis for a sensory and by the Abducens for a motor root, but 
for the fourth we have the Acusticus alone, with the motor 
root wanting. The remaining five possess the five elements 
shown in the Glossopharyngeus and Vagus in fishes, while 
two Hypoglossus roots supply motor elements for the last 
three somites. Fig. 167, B, shows a suggestion made by 
another investigator, and based mainly upon the relation of 
prae- and post-trematic branches to their corresponding gill- 
slits. 
Later researches have modified these diagrams somewhat, as, 
for example, the discovery of transitory sensory roots for the 
hypoglossal elements, and indications of other nerves anterior 
to the Ophthalmicus profundus; the spino-occipital nerves, al- 
though of unknown value, seem indicative of still other somites 518 
THE HISTORY OF THE HUMAN BODY 
in the occipital region and need to be thoroughly explained 
before the problem of the segmentation of the head can re- 
ceive its final solution. Unquestionably one somite at least 
should be added at the anterior end on the basis of the pres- 
ence in the selachians of an additional transitory mesodermic 
head somite (the anterior somite) in front of the preman- 
Ram 
intes 
Fig. 167. Two suggestions for the solution of the problem of verte- 
brate cephalogenesis. 
(A) According to van Wijhe. (B) According to Beard. 
The roman numerals enclosed in the ovals in A designate the head somites, 
all other roman numerals refer to the cranial nerves, m, mouth; n, nasal opening; 
sp, spiracular cleft; Glt Gv etc., gill-slits; Olf, olfactory nerve; Cil, ciliary nerve. 
dibular. This anterior somite from which there are now ap- 
parently no muscular derivatives, must stand for at least a 
hypothetical motor nerve component while the Olfactorius 
and the Nervus terminalis represent respectively the 
visceral sensory and somatic sensory components, the latter, THE NERVOUS SYSTEM 
519 
however, reduced to a mere vestige. The accompanying dia- 
gram (Fig. 168) represents the segmentation of the verte- 
brate head so far as it can be worked out in a generalized 
form on the basis of a combination of real and hypothetical 
functional nerve components. The comparison of this with 
Somatic 
sensory 
division 
Visceral 
sensory 
division 
Visceral 
motor 
division 
Somatic 
motor 
division 
A, 1, 2, 3, 4, etc., mesodermic somites; i, ii, Hi, iv, neuromeres; M, II, Bi, B2, etc., mandibular, 
hyoid and branchial arches; m, h, bl, b2, etc., mandibular, hyoid and gill clefts; ei, e2, epiphyses; 
L, nervus terminalis; op. v., optic vesicle; au. v., auditory vesicle; cil. g., ciliary ganglion; sy. g., 
sympathetic ganglion; Ir. n., trunk nerve. [From Johnston.] 
Fig. 168. A diagram of the segmentation in a generalized vertebrate head. 
the simpler diagrams of Fig. 167 may best be made from 
the starting point of the facial nerve (VII) which has the 
same relationship in both schemes to the hyoid slit (spirac- 
ulum), while in the regions both anterior and posterior to 
this, hypothetical components are involved which are capable 
of a difference in interpretation. 
There remains to be mentioned that important part of the 
visceral division of the peripheral nervous system which con- 
sists of the auxiliary system of ganglia and nerves known as 
the sympathetic system, often erroneously treated as originally 
a distinct nervous system coordinate with the cerebro-spinal. 
As a matter of fact, however, the sympathetic system is an 
integral portion of the latter, as is shown by its development 
from cells which migrate out from the neural tube, and its 
differentiation from this may be followed both in the race 
history as well as during individual development. It attains 
its greatest degree of individuality only among the higher 520 
THE HISTORY OF THE HUMAN BODY 
forms. It consists primarily of a series of paired ganglia, the 
chain ganglia, which are connected with the pairs of spinal 
and cranial nerves through communicating branches (rami 
communicantes). The ganglia of the two sides, which come 
to lie ventral to the spinal nerves on either side of the verte- 
bral column, are also connected with one another by longitu- 
dinal connectives, thus .forming two lateral sympathetic 
trunks. They thus form a series of paired ganglia which par- 
take primarily of the metamerism of the body. In the more 
highly specialized anterior region, however, they are reduced 
in number, so that in the head there are only four pairs 
(ciliary, spheno-palatine, otic, and submaxillary), and in the 
neck only three pairs. 
This appearance of metameric ganglia connected in two 
longitudinal series, and especially their ventral positions, has 
caused the sympathetic system to be compared to the ventral 
chain of ganglia found in articulates (e.g., insects, crusta- 
ceans), a suggestion of homology that is sufficiently disproved 
by the mode of origin and the fact that the similarity is most 
perfect in the higher forms. Indications suggest that the 
separation of sympathetic ganglia began historically in the 
head, since in fishes the system is better developed in this 
region than in the trunk. In the embryo also the cephalic 
portion develops before the rest. 
The system appears well developed in Amphibia, with its 
two lateral trunks. In Sauropsida a pair of subsidiary trunks 
in the neck region accompanies the vertebral arteries, the only 
instance of a distinct dorsal position for any part of this 
system. 
From the ganglia as centers numerous nerve fibers proceed, 
supplying not only the larger “ viscera ” which are located in 
the coelomic cavity, but also joining the main rami of the 
spinal nerves to reach the widely distributed but less con- 
spicuous viscera (smaller blood vessels and glands) through- 
out the body. The favorite mode of distribution is an intri- 
cate plexus, which spreads over the broader surfaces and en- 
wraps the smaller parts, and may well be seen, for example, 
in the case of the alimentary canal and larger arteries. CHAPTER XIII 
THE SENSE-ORGANS 
“ Die wunderbare und wirklich uberraschende Ahn- 
lichkeit in der inneren Organisation, in den ana- 
tomischen Structurverhaltnissen, und did noch- 
merkwiirdigere Ubereinstimmung in der embryonalen 
Entwickelung bei alien Thieren, welche zu einem 
und demselben Typus, z. B., zu dem Zweige der 
Wirbelthiere, gehoren, erklart sich in der einfachsten 
Weise durch die Annahme einer gemeinsamen Ab- 
stammung derselben von einer einzigen Stammform. 
Entschliesst man sich nicht zu dieser Annahme so 
bleibt jene durchgriefende Ubereinstimmung der 
verschiedensten Wirbelthiere im inneren Bau und 
in der Entwickelungsweise vollkommen unerklarlich.” 
Ernst Haeckel, Schopfungsgeschichte, Kap. III. 
It will be remembered that the nervous system is primarily 
external, developed in response to stimuli from without, and 
that, as this system becomes more specialized, and hence of 
greater importance to the organism, it withdraws in great part 
into the interior, retaining its connection with the surface 
through secondary sense cells, also ectodermal in origin, which 
constitute the distinctive cells of sense-organs as well as true 
sensory neurones. 
With the development of such highly centralized nervous 
systems as the vertebrates possess there have been formed, 
in addition, extensively distributed sensory nerve endings not 
only in connection with the epithelial structures which form 
the mucous membranes of various organs, but also in skeletal 
and muscle tissues. All of the receptors thus formed whether 
concerned in the somatic or visceral activities of the organism, 
may, in the broadest application of this term, be considered 
as sense-organs. Those with which we are particularly con- 
cerned in this chapter are, however, the more or less highly 
differentiated structures which, with the exception of the ol- 
521 522 
THE HISTORY OF THE HUMAN BODY 
factory and gustatory sense-organs, belong clearly to the 
somatic division. 
All of these organs under discussion are capable of receiving 
impressions and transmitting them to the central organ. Tak- 
ing into consideration the intimate connection between these 
two mechanisms, external and internal, it might be supposed 
that the enormous development in size and complexity shown 
by the brain would be the result of a corresponding degree of 
differentiation of the external parts, yet such is by no means 
the case. The sense-organs of vertebrates are early brought 
to a high state of efficiency and develop but little during the 
entire vertebrate history. The eye of the fish is almost as good 
an optical instrument as is that of the mammal, and, save for 
a few external parts, is as complex; the sense of hearing, 
although not as early in development as the eye, is very acute 
in reptiles, and gains in birds and mammals very little except, 
perhaps, the recognition of musical tones; indeed, the early 
aquatic forms possess in the lateral line organs an entire sys- 
tem, no trace of which seems to have survived the transition 
to land, and yet, with no special progress on the part of the 
sense-organs, the central nervous system, and especially the 
brain, the receiving organ of the special senses, has increased 
from a simple condition to one showing a marvelous degree of 
complexity. The cause of this extreme development must be 
laid, then, not to the sense-organs, but to the direct and cumu- 
lative influence of the impressions received. The motor cen- 
ters, which have contributed not a little to the complexity of 
the central nervous system, have also developed in response 
to the external environment, though rather more indirectly, 
perhaps, through the necessity of controlling the more special- 
ized limbs and other parts, which, in their turn, were directly 
influenced by external conditions. 
The morphological history of the sense-organs does not, 
therefore, show the extensive progress exhibited in the case 
of most of the other systems; but as certain definite changes 
were necessitated by the transition from water to land, this THE SENSE-ORGANS 
523 
history is divided into two great stages, (i) that of the aquatic, 
and (2) that of the terrestrial life. 
Throughout the animal kingdom, the elementary type of 
sense-organ is a single epithelial cell, which either through a 
cytoplasmic outgrowth of its own, or through connection with 
an outgrowth reaching it from another cell, is able to 
mit the stimulus which it receives to cells capable of making 
the response. From this as a starting point, higher efficiency 
is gained in three ways: (1) by the association of a number 
of these elementary units to form a larger sensory area, (2) 
by the specialization of the cell itself, and (3) by the develop- 
ment of accessory parts. 
The area over which a given form of sense cell may occur 
may be a general surface of indefinite limits, or it may be 
restricted and form a definite sense-organ. Most generally 
the epithelial cells composing such an area are not homogene- 
ous, but are differentiated among themselves into two sorts, 
sense cells and supporting cells; the first are the receptive 
units of the nervous system in that they are either the sensory 
neurones themselves or are in contact with the end arboriza- 
tions of dendrites of sensory neurones; the latter are non- 
sensitive, and are grouped about the sense cells in such a way 
as to form a support and protection for the essential elements 
of the sense-organ. 
Regarding the differentiation of the sense cells themselves, 
they may present at their free end a ciliated or simple sur- 
face, or may bear one or more flagella. In the most special- 
ized types the flagella themselves may be modified for the 
better reception of certain definite forms of impressions, as in 
the case of the rods and cones of the retina, or the acoustic 
hairs of the inner ear; these types, however, owing to the ex- 
treme delicacy of the projecting parts, can exist only upon an 
external surface bathed by water, as in many aquatic inverte- 
brates, or upon a surface that faces some internal cavity filled 
with fluid or semi-fluid. 
Accessory organs for the reception and intensification of the 
external stimuli or for the protection and care of the essential  THE HISTORY OF THE HUMAN BODY 
524 
parts are the rule in the case of the more specialized and com- 
plex organs, but are not employed to assist in the reception of 
general tactile impressions save in certain invertebrates with 
a thick and hard exo-skeleton which would naturally prevent 
such impressions from registering. In this latter case 
the sensations are transmitted by sensory hairs which are 
protruded through pores in the external armor, and communi- 
cate with underlying sense-organs. In those vertebrates in 
which the integument is covered by non-sensitive parts, such 
as scales, feathers, or hairs, often necessary to protect the ani- 
mal from serious injury, the sense-organs are developed be- 
tween them or may be situated about their bases, in which 
case they are stimulated indirectly through the movements of 
the insensitive outer parts, much as in the previous case. 
Concerning the actual sensations produced by the different 
kinds of sense-organs found among vertebrates we know very 
little, and inferences must be made with extreme caution. Al- 
though something can be deduced from the mechanical struc- 
ture of a terminal organ, especially from that of its accessory 
parts, and although from the physiological side something can 
be learned from the behavior and responses of an animal 
under observation, the only certainty concerns our own sense- 
organs and those formed like them. Thus, we are positive con- 
cerning the sense furnished by the eye, since the variations 
from the human structure are very slight in any case, even in 
fishes; the ear, however, is somewhat more variable, and pre- 
sents several problems, since the lower types of ear lack 
certain parts, like the cochlea, which in Man are essential to 
the completeness of the sense of hearing as we understand it. 
In this case we have two alternatives, either that in the differ- 
ent forms the same function is subserved by different parts, 
as has been shown to be true in the case of the brain, or else 
that there are elements in the human sense of hearing not 
perceived by ears belonging to other types. 
Aside from the above, the sense of smell seems to be a com- 
mon possession, and in terrestrial forms the sense of taste 
also seems general if we are to judge from the similarity in THE SENSE-ORGANS 
525 
the location and structure of certain specialized sense-organs 
and the identity of their nerve supply. After these are ex- 
cepted, however, there remains a large number of types of 
sense-organs, more or less localized in different areas, the spe- 
cial functions of which are practically unknown, but are in- 
cluded within the comprehensive terms of touch or feeling. 
That many distinct impressions are involved in this is shown 
by this very dissimilarity in the structure of the terminal or- 
gans, the complexity of which, in certain cases, suggests the 
possibility of definite senses, such as temperature perception, 
and the perception of the distance between two points of stimu- 
lation, at least as distinct as those of smell or taste; but as 
few of these types occur in Man, and as even here the ele- 
mentary sensations have not been wholly coordinated with 
the various forms of nerve terminations, but little can yet be 
stated on the subject, and the psychology of the tactile sensa- 
tions of the lower vertebrates remains an unexplored field. 
Probably the lowest form of vertebrate sense-organs, and 
one that is universally distinguished among them, is that of 
simple sensation, the contact sense, which resides in the epi- 
dermis and is thus generally met with over the entire surface 
constituting the general cutaneous division of somatic recep- 
tors. The vertebrate epidermis, which, in contrast to that of 
invertebrates, is many cells thick, is supplied everywhere by 
sensory nerves which branch repeatedly and with their ulti- 
mate fibers form a delicate net-work which permeates the 
entire layer, stopping only at the most external cells. This 
type is characteristic of all vertebrate integument, including 
both cyclostomes and mammals, and furnishes them all with 
a surface capable of receiving general tactile impressions. 
Aside from this general cutaneous tactile sense, vertebrates 
are equipped with a system of special cutaneous sense-organs 
which were undoubtedly adapted to the reception of pressure 
stimuli reaching the animal through an aquatic medium. The 
most extensive of these, and the only one to develop into a 
definitely organized system, is that of the neuromasts or 
“ lateral line ” organs, referred to above in connection with 526 
THE HISTORY OF THE HUMAN BODY 
Canals are indicated by parallel outlines, ampullae by clustered circles, pit organs by isolated circles. Roman numerals 
indicate cranial nerves, the branches of which are indicated as follows: r. o. s. superficial ophthalmic branch r. o. p. deep 
ophthalmic branch; r. buc, buccal branch; r. mx, maxillary branch; r. pal, palatine branch; ch. tymp, chorda tympani; 
r. hy. m, hyo-mandibular branch; mot, motor branch r. lat, lateral branch; r. intes, intestinal branch; cil, ciliary ganglion. 
The supra-orbital canal is innerved by r. o. s. VII; the infra-orbital canal by r. buc. VII; the hyo-mandibular canal by 
r. hy. m. VII; the lateral canal by IX and X. 
Fig. 169. The sensory canal system in the head of Laemargus. [After Ewart.] THE SENSE-ORGANS 
527 
the cranial nerves and possessed by the primarily aquatic 
vertebrates (fishes and amphibians). This system, as such, 
together with its nerves, disappears utterly with the assump- 
tion of a wholly terrestrial life; it is yet of importance in this 
connection because of at least one closely related structure in 
higher forms, the ear; while merely as interesting suggestions, 
since there is little basis for the claim, such other structures as 
the mammalian hair and the taste buds have been mentioned 
as possible derivatives. 
In its simplest form a neuromast consists of a small group 
of sensory cells, slightly convex in form and protected by a 
wall of non-sensitive supporting cells. This organ gains its 
simplest form of protection by sinking slightly beneath the 
surface, its supporting cells remaining at the general level, 
or even projecting a little above it. By continuing this process 
the sense-organ comes to lie at the bottom of a flask-shaped 
cavity, communicating with the surface by a narrow neck. 
From this point on, greater complexity may be gained by 
development in one of two directions, the flask-shaped cavities 
may either become associated in rows and break down their 
adjacent walls, forming the slime canals, or else each separate 
flask may become elongated, bearing the sense-organs at its 
very bottom, as in the canals of Lorenzini. In the first of 
these, the coalescence may result in the formation of either a 
deep trough or, more usually, an enclosed tube, running just 
below the surface; and in this latter case, each organ may open 
by its own pore placed directly above it, or an entire tube 
may open by a single common pore at one end. These canals 
are usually filled with a clear mucous or gelatinous material, 
secreted by the walls, and destined to protect the sense cells. 
In the case of the second form of development, there will be 
produced local groups of associated, though distinct, canals, 
each beginning superficially at an external pore and running 
obliquely beneath the surface to terminate proximally in a 
bulb or ampulla, in which the sense-organ is located. Such 
organs occur in localized masses in the heads of selachians, 
associated topographically with the mucous canal type, the 528 
THE HISTORY OF THE HUMAN BODY 
ampullae with their nerves clustered in such a way as .to re- 
semble bunches of grapes. 
That a system so highly developed and so extensive in its 
distribution as that of the neuromasts should have wholly 
disappeared together with its nerves in terrestrial vertebrates, 
is a phenomenon of so unusual a nature that numerous at- 
tempts have been made by morphologists to find its direct 
continuation among the parts of higher vertebrates. Thus, 
one well-known theory associates these organs with the taste- 
buds, a view arising naturally from the extreme similarity 
between the two structures. There seem, however, to be no 
definite data to form the logical steps between the two, and 
the fact that the nerve supply to the taste-buds does not come 
from any part of the extensive sensory system associated with 
the lateral line organs speaks strongly against this homology. 
As an added evidence in the same direction there are found 
in the nasal mucous membrane of many fishes and of certain 
of the lowest urodeles {Siren) groups of cells forming “ smell- 
buds,” extremely similar to taste-buds, and yet by no pos- 
sibility connected with the lateral line system. 
A second possible derivative of the neuromasts is seen by 
some in the hair of mammals. This theory is based upon a 
certain similarity in the early stages of development of the 
two structures, that is, the initial procedure in both cases 
concerns the epidermis alone and consists of a concentric ar- 
rangement of a small group of cells. If this homology be a 
true one we must also consider the amphibians as the direct 
ancestors of mammals, since neuromasts do not occur in 
reptiles. In this comparison the original sense-organ is, of 
course, the equivalent of the convex hair papilla, which lies at 
the root, covering the corium papilla, and from which prolifer- 
ate the cornified cells of the hair shaft. Something analogous 
to this exists in the so-called “ pearl-organs,” horny bodies 
which develop from certain of the lateral line organs in some 
fishes, showing that there is present in these organs a tendency 
to produce cornified structures. 
Again, the sensitiveness of the hair root, and its abundant THE SENSE-ORGANS 
529 
nerve supply, especially in cases like that of the vibrissoe 
(whiskers) of the upper lip in many mammals, speaks in favor 
of such a derivation. The great multiplicity of the hairs, con- 
sidering that each represents an original sense-organ, and also 
their almost universal distribution, is paralleled by the adap- 
tive multiplication of other parts, such as the mammae in some 
forms or the vertebrae in elongated animals. On the other 
hand, the test of nerve supply fails to even suggest this hypoth- 
esis, since in the earliest terrestrial vertebrates the extensive 
system of superficial nerves associated with the lateral line 
organ becomes entirely lost (unless the Acusticus may be 
looked upon as derived from it); furthermore, the close asso- 
ciation between hairs, scales, and integumental glands turns 
the argument in a totally different direction. (Cf. Chap. IV.) 
The inner ear as the only possible representative of the 
neuromast system to be found in terrestrial vertebrates can 
hardly be regarded as derived from the neuromasts since it 
must have become differentiated from the rest of the system 
very early while the neuromasts themselves were fully func- 
tional organs. 
There is much evidence that the ear which is undoubtedly a 
highly differentiated part of the special cutaneous system, 
was actually in its origin a portion of the lateral line system 
which early developed an adaptation in its structure for the 
reception of a higher rate of vibratory pressure stimuli, and 
thus came to somewhat precede the rest of the system in its 
appearance in the embryo. Its general structure as a system 
of fluid-filled canals, the similarity of its sense cells (hair cells) 
to those of a lateral line organ in their adaptation to receiv- 
ing vibrations transmitted through a fluid medium, the close 
association of the auditory vesicle and its ganglion with the 
lateral line organs in origin, and finally the nature of the 
secondary connections formed by these nerves within the 
brain, all definitely support the theory. With the attainment 
of the terrestrial mode of life, only that part of the whole 
system which had become specialized for the ready reception 
of stimuli arising from pressure changes due to slight varia- 530 
THE HISTORY OF THE HUMAN BODY 
tions in the positions of the cephalic region persisted and thus 
the semicircular canals of the ear become the most highly 
specialized proprioceptive sense-organ. 
Another theory concerning the otic vesicle is that it forms 
one of a series of very early organs, to which belong also the 
lens of the eye and possibly the nasal sacs, as well as a few 
transitory structures (the epibranchial placodes) associated 
with other cranial nerves and usually interpreted as lost sense- 
organs of unknown function. Since, however, the neuromasts 
also have their origin in placodes, these two theories are not 
inconsistent. 
In our present state of knowledge we may at least assert 
that the entire functional neuromast system of the Ichthyop- 
sida, which is associated with an aquatic habitat, disappears 
completely when the assumption of a terrestrial life renders 
it no longer necessary. 
In connection with the description of the general tactile 
sense, located in the so-called “ free nerve endings ” of the 
skin, certain more specialized forms of nerve termini were 
referred to, which in our present lack of precise knowledge 
are classed also under the general head of organs of touch. 
The most elementary of these are the tactile cells (Fig. 170, 
b), which are scarcely more than isolated units of the general 
type, somewhat more specialized and thus rendered conspicu- 
ous. They are first seen in tailless amphibians, where they 
are associated in groups, forming small areas known as tactile 
spots. In other cases the ending has a tendency to form a 
bulb or sphere, composed of many cells, and often of ap- 
preciable size; these are termed collectively tactile corpuscles, 
each different type being designated by the name of the in- 
vestigator who first made an accurate description of it. These 
tactile corpuscles show various types of structure and make 
use of very different mechanical principles. The great major- 
ity of them do not involve the epidermic cells at all but lie 
in the connective tissue of the corium beneath the epidermis, 
the deeper surface of which often shapes itself to the pro- 
jecting papilla in which the sense organ is located. Thus, in THE SENSE-ORGANS 
531 
Meissner’s corpuscles (Fig. 170, c) the terminal cells form an 
oval core, upon which the terminal branches of the nerve 
fibers are wound in an irregular branching spiral. Certain 
types, on the other hand, seem to possess no epidermal ele- 
ments, as in Krause’s corpuscles (Fig. 170, d), which consist 
of a globular snarl of nerve termini like a capillary glomerulus, 
enclosed within a thin covering of connective tissue. In still 
(a) Free nerve ending. (b) Merkel’s corpuscles. (c) Meissner’s corpuscle, 
(d) Krause’s corpuscle. (e) Grandry’s corpuscle. (f) Pacini’s corpuscle. 
Fig. 170. Various endings of sensory nerves. 
another type, shown by Grandry’s corpuscles (Fig. 128, e), the 
nerve terminus is enclosed between two large cells which seem 
to produce the stimulus by transmitting any pressure to which 
they are subjected. This last principle is employed in a more 
elaborate manner in Pacini’s corpuscles, perhaps the largest 
and most complex of the series, where the nerve terminus is 
enclosed first by a series of consecutive lamellae of connective 
tissue, something like the coats of an onion, and finally by an 532 
THE HISTORY OF THE HUMAN BODY 
external connective tissue wrapping, the continuation of the 
nerve sheath or neurilemma. 
All of the above types of tactile corpuscles occur in man 
and other mammals with the exception of Grandry’s corpuscles, 
which are found only in the beaks of various birds. They do 
not seem to occur over the general hair-covered surface, but 
are found, often in association with one another, upon such 
modified hairless surfaces as the palms and soles, the tips of 
the digits, the lips, the nipples, the external genitals, and, in 
many mammals, on the end of the nose or snout. The Pacin- 
ian corpuscles, or very similar sense-organs, also occur in 
such various deeper regions as in the joint capsules and the 
mesentery. This internal distribution has led to the interpre- 
tation of their function as that of the reception of pressure 
stimuli and they thus undoubtedly form a part of the general 
proprioceptive system of receptors. 
The organs of taste, cut off from all association with the 
sense of smell, are restricted in function to the perception of 
certain elementary qualities of liquid substances, as sweet, 
sour, bitter, and salt, qualities which are chemical in their 
action. 
The ultimate organs of the sense of taste, the taste-buds 
or taste-beakers, are found, from the amphibians on, only 
within the cavity of the mouth and pharynx, especially upon 
the tongue and palate; but in fishes they are far more general 
in their distribution, and have been found in some species 
(e.g., bull-heads) scattered over the skin of the external sur- 
face. Such fishes are thus probably enabled to taste the water 
through which they pass. A taste-bud consists of a group of 
long, spindle-shaped cells, surrounded by a rampart of sup- 
porting cells of shape similar to the others, but longer, and thus 
greatly resemble in form the terminal organs of the lateral 
line system. At their free ends the sense cells usually possess 
one or more modified flagella, which project into a small space 
that is formed about them by the supporting cells, and com- 
municate with the exterior through a small opening, the 
taste pore. In mammals the taste-buds are associated to- THE SENSE-ORGANS 
533 
gether in groups in connection with several sorts of papillae, 
especially the circumvallate, and the joliate, the latter not 
occurring in Man. 
The organ cf smell is located in a pair of ectodermic 
cavities, the nasal cavities, situated anterior to the eyes, thus 
forming the most anterior of the sense organs. They are thus 
in the most favored position for organs of sense, and although 
the data are too insufficient for theories, this fact suggests 
that they were the earliest to develop and that the primeval 
habitat was either in mud or in the deep sea where the ol- 
factory sense was of primary importance. The embryological 
evidence however shows their antiquity to be exceeded by that 
of the organs of sight. In Amphioxus the organ of smell is 
located in a median ciliated pit at the anterior end and pushed 
a little to the left side by the development of the median fin. 
Although an unpaired structure, its similarity in location and 
in relationships to the olfactory sense-organ in cyclostomes 
strongly suggests that it is the same organ. The early stages 
of the cyclostomes furnish much material for speculation, but 
unfortunately there is no certainty felt as yet concerning the 
meaning of the details presented. Here (Fig. 171) there ap- 
pear at the anterior end two median invaginations, the more 
posterior of which is the cavity of the mouth (stomatodoeum). 
The anterior depression is that of a median nasal cavity, which 
suggests a primitive condition and a kinship with the ciliated 
pit of Amphioxus in spite of the fact that it is connected by 
two olfactory nerves with as many olfactory lobes, showing 
that the single or monorrhine condition has here been second- 
arily attained from a previous paired (amphirrhine) one. 
From the posterior wall of this depression there develops a 
tubular process, which, in Myxine, connects ultimately with 
the pharynx and thus forms a direct communication between 
nose and throat, but in the other cyclostomes ends blindly 
and soon disappears. This passage is of interest as a prophecy 
of the similar connection to develop later in air-breathing 
vertebrates, and is of still greater interest for its direct con- 
nection with the hypophysis, which develops from it. If in 534 
THE HISTORY OF THE HUMAN BODY 
this may be seen the remains of an earlier entrance into the 
alimentary canal, then its associated mouth (palceostoma) must 
have been the later nasal cavity, and its unpaired condition in 
Fig. i 71. Median sagittal sections through the head of two stages of 
the Ammocaetes embryo of Petromyzon. [After von Kupffer.] 
I, II /// the three primary cerebral vesicles; d, intestine (mesodaeumV 
m mcuth cavity (stomatodsum); nc, notochord; ep, epiphysis; hy, hypophvTis’ 
Ay portion detached from distal end of hypophysis; rg, olfactory pk; lo medial' 
olfactory lobe; ch, optic chiasma. v ’ ’ meaiar THE SENSE-ORGANS 
535 
cyclostomes, in spite of the contradictory testimony of the 
olfactory nerves, would then be primitive and not secondary. 
In all other vertebrates the nose is strictly amphirrhine, that 
is, it consists of two lateral cavities, symmetrically placed. 
These cavities begin as localized thickenings of the ectoderm, 
in close proximity to the neural plate. The thickenings in- 
vaginate and form nasal sacs, a condition that persists in fishes. 
In the Dipnoi, the first air-breathers, these sacs become pro- 
longed posteriorly and break through into the mouth cavity, 
forming the choance [posterior nares], formations which are 
present also in amphibians and all higher vertebrates. When 
these are present, the lower part of the cavity is used more 
or less exclusively for respiration, and the olfactory sense be- 
comes limited to the more dorsal portion, thus dividing the 
cavity into a pars respiratoria and a pars olfactoria. 
Within possible limits the greatest diversity exists in the 
location of the nasal cavities, and especially their openings, 
the anterior and posterior nares. The former may be placed 
ventrally, as in dog-fish, and may occupy all intermediate 
positions between that and an extreme dorsal one. Marked 
adaptations in this respect are seen in those air-breathers which 
have become secondarily aquatic, enabling them to breathe at 
the top of the water without being seen. Thus in the whales 
and porpoises the nostrils seem to be moved to the top of the 
head, the deception being due to very short frontal and nasal 
bones and to an excessive anterior prolongation of the maxil- 
laries and premaxillaries. In some birds, like the albatrosses 
and petrels, the nostrils become prolonged into tubes, formed 
by the beak, so that they open near the tip of that organ in- 
stead of at its base. An extreme case is seen in the Dipnoi, in 
adaptation to their annual hibernation within a cocoon of dry 
clay; for in these animals the openings of the anterior nares 
lie within the mouth cavity, and the mouth is connected with 
the exterior during hibernation by means of a long tube com- 
posed of slime secreted by the animal. 
As regards the choanse, their original position is shown by 
amphibians to be very far forward, a position retained, with 536 
THE HISTORY OF THE HUMAN BODY 
some variation, by Sauropsida. In mammals this becomes 
greatly modified by the formation of the hard palate, which 
develops from the fusion of two lateral shelves, beginning 
anteriorly. This shuts off from the mouth cavity its own 
primary roof, including the openings of the choanae, and there- 
fore pushes back their communication with the mouth cavity 
to its posterior limit. A trace of the former communication 
is retained, however, in many mammals in the form of the 
naso-palatine canal (Stenson’s canal), which opens into the 
roof of the mouth behind the incisor teeth. A rudiment of 
this duct occasionally occurs in man, lodged in the incisive 
canal of the maxillaries. 
For greater efficiency the olfactory surface may be increased 
in three ways: (i) by folding the nasal mucous membrane in 
an oval or otherwise simple cavity, (2) by complicating the 
walls of the cavity itself, usually by means of ridges or shelves 
which may themselves become rolled or variously convoluted, 
or (3) by the addition of accessory cavities within the ad- 
jacent bones. The first of these devices is seen in fishes, where 
the folds are variously disposed, either transverse or longi- 
tudinal. This folding of the mucous surface may become 
extremely complex and thus furnish an organ of considerable 
efficiency. The second and third methods reach their highest 
development in mammals and are best treated separately. 
The interior of the mammalian nasal cavity, which is usu- 
ally very large, is by no means a simple space, but is well 
filled up by projecting folds, composed of thin lamellae of bone 
covered by mucous membrane. These are termed turbinalia 
and come under three categories, in accordance with their 
relationships to other parts: (1) a naso-turbinal, (2) several 
ethmo-turbinals, and (3) a maxillo-turbinal. 
The maxillo-turbinal lies ventral and usually anterior to 
the others, in the pars respiratoria, and in mammals has lost 
all olfactory function, but is often very complicated and forms 
a filter or screen to intercept the foreign matter in the air 
taken in, or to temper it if cold. This is the homologue of the 
single turbinal found in certain groups of reptiles, where its THE SENSE-ORGANS 
537 
function is wholly olfactory. The bone which forms the 
framework of this part is usually distinct from the ethmoid, 
and forms the “ inferior turbinated bone ” of human anatomy. 
The name “ maxillo-turbinal ” is to be preferred, as it better 
expresses its relationship. 
Fig. 172. Diagrams of ethmoturbinals in Mammals. [After Paulli.] 
(A) Type showing endoturbinalia alone. (B) Type with endoturbinalia (heavy 
lines) and two ranks of ectoturbinalia. (C) Diagram of turbinals and pneumatic 
cavities in the ox. (D), (E), (F) Diagrams of three actual cases in man, showing 
individual variation. 
The remaining turbinalia, all of which are olfactory, form 
a set of parallel projecting ridges, arising from the lateral wall 
of the cavity and arranged in series from above downwards; 538 
THE HISTORY OF THE HUMAN BODY 
the most dorsal of these is borne, at least in part, by the nasal 
bone and is termed the naso-turbinal; the others are ethmo- 
turbinalia, that is, they arise from the ethmoid. The total 
number of turbinalia, not counting the maxillo-turbinal, is 
most usually five, but larger numbers are met with up to 
eleven, the number found in certain edentates. Besides these 
primary turbinalia which, although arising from the outer 
wall, yet nearly reach the inner one, there is often present a 
variable number of secondary and even tertiary turbinalia fill- 
ing the spaces left free by the first. These relations are shown 
in Fig. 172, A and B, which represent diagrammatic cross sec- 
tions of nasal cavities; A, with primary turbinalia alone, and 
B, with secondary and tertiary ones. The primary ones are 
termed endoturbinalia from their position, to which all the 
rest are contrasted as ectoturbinalia. The various possibili- 
ties which arise from rolling the edges of the laminae are also 
shown; the edge may be rolled inwards (B, I), or outwards 
(B, IV), or again there may be two free edges either rolled 
in different ways (B, III), or in the same way (B, IT and 
II")- This latter possibility, when viewed from the free inner 
surface, appears like two separate turbinalia, but is morpho- 
logically a single one. 
Such involved forms of turbinalia are the rule rather than 
the exception among the quadrupedal mammals, the greatest 
degree of complexity being reached by the ungulates (Fig. 172, 
C), rodents and carnivores, a structure which gives them a 
high degree of power in the sense of smell. These structures 
become much reduced in the anthropoids; and in Man (Fig. 
172, D-F), but three ethmo-turbinalia are usually represented, 
and these are of the simplest character. Of these the two 
largest, II and III, form the “ upper and middle conchoe ” of 
human anatomy. The first, I, is a rudiment, and the last, 
IV, is usually absent. The “ lower concha ” is a distinct bone, 
the maxillo-turbinal, and is not shown in the diagrams. In the 
human embryo a larger number of ethmo-turbinals occurs, 
showing that man’s immediate predecessors possessed a much 
more highly developed olfactory sense than appears at present 
(Fig. 173). THE SENSE-ORGANS 
539 
Still another method for increasing the olfactory surface and 
thus sharpening the scent seems to be found in the system of 
accessory cavities, hollowed out in the surrounding bones and 
communicating with one another and with the primary cavity. 
These occur in the maxillary, sphenoid and frontal bones, and 
in animals with the most elaborate nasal equipments are lined 
with olfactory mucous membrane and may even develop tur- 
binalia themselves. Some of these cavities are retained after 
Fig. 173. Lateral wall of human nasal cavity, showing the turbinals. 
(A) Embryo, after Killian. (B) Adult, in part after Wiedersheim. 
I mx, maxilloturbinal; II-VI, ethmoturbinals. 
the loss of their olfactory function and are lined by simple 
mucous membrane. The largest of these accessory cavities 
in Man is the sinus maxillaris (antrum of Highmore) in the 
maxillary bone; the frontal and sphenoid sinuses also belong 
to the same system [Cf. Fig. 172, C-F.] 
This extraordinary development of the organ of smell in 
mammals is an illustration of the late perfection of a part that 
has existed as a functional organ for a very long time, yet 
without the necessity of a high degree of specialization. The 
need of a turbinal is first felt in reptiles, but here a single one, 
and that of the simplest pattern, is found to suffice. That the 
human nose during its own past history once reached a much 
higher state from which it has since failed through degenera- 
tion and loss of the parts once gained is shown by the anlagen 
in the embryo of turbinalia that never develop, and is indicated 540 
THE HISTORY OF THE HUMAN BODY 
also by the simple condition of the cavities, and the lack of 
complexity in the turbinalia. 
An interesting bit of morphology in connection with the 
history of the nose is that of Jacobson’s organ (vomero-nasal 
organ), at first a sinus or pocket leading out of the main nasal 
cavity, later an independent organ, and finally a rudiment. 
This organ is first seen in urodeles where it appears upon the 
medial side of the nasal cavity and gradually migrates along 
the floor, attaining ultimately a lateral position, though still 
included within the nasal capsule. This migration is seen by 
comparing the lower with the higher members of the order 
and actually takes place during the embryological development 
of such a form as Triton, where it may be followed step by 
step. When its lateral position is fully established, it gradually 
restricts its communication with the main cavity until it is 
connected by a small duct in the region of the posterior nares, 
as in Ccecilia. In lizards and snakes, where it reaches its 
highest degree of development, it forms upon each side a 
tubular, somewhat contorted organ, with a blind anterior end, 
opening posteriorly into the roof of the mouth by an inde- 
pendent opening, yet still near the posterior nares. Its ventral 
wall is rolled up into its thickened and strongly convex dorsal 
one, and this latter possesses olfactory sense-cells. The posi- 
tion of the organ has again changed, owing doubtless to the 
development of related parts, and it lies almost directly be- 
neath the primary nasal cavity, between it and the hard palate, 
and thus more nearly in its original position near the median 
line. 
In turtles, crocodiles, and birds, Jacobson’s organ exists 
only in the form of embryonic vestiges, but, on the other hand, 
it is very evident in mammals, appearing in the embryo as 
a definite organ persisting throughout life in many cases. It 
is located upon either side of the cartilaginous nasal septum, 
and is protected by a cartilage of its own, the paraseptal, vo- 
mero-nasal, or Jacobson’s, cartilage. When well developed the 
organ is in the form of a short tube, which opens anteriorly 
into the naso-palatine canal, thus retaining its early relation- THE SENSE-ORGANS 
541 
ship to the primitive choanse, but in anthropoids and some 
other mammals it is quite vestigial and appears only in em- 
bryonic life. In the monotremes it reaches the highest de- 
velopment attained among mammals, and is here entirely en- 
cased in its cartilage, through which passes a small branch of 
the olfactory nerve to supply the organ. From the lateral 
wall of the cartilage a turbinal process develops, similar to 
the turbinalia of the main cavity, but very simple in form and 
covered with indifferent non-olfactory epithelium. Remains of 
this process are seen in marsupials and even in rodents, forms 
in which the entire organ is well developed. 
If we except such special adaptations as the tubular pro- 
longations of the nostrils which exist in certain fishes and a 
few aquatic birds, an external nose as a separate organ is a 
mammalian characteristic. It possesses a cartilaginous frame- 
work derived from the primordial skeleton and thus, in part, 
homologous with the cartilaginous capsule of amphibians, and 
it is supplied with superficial muscles from the mimetic group. 
It shows great power of adaptation in the various mammals, 
sometimes forming a flexible snout or trunk, as in swine, ele- 
phants, and moles, and sometimes developing a moist, sensitive 
surface, as in carnivores and ruminants. In the anthropoids 
it is reduced in size, corresponding to the lessened importance 
of the olfactory sense, although its muscles are very mobile 
and assist greatly in the expression of emotions. These latter 
powers seem to be undergoing degeneration in Man in spite 
of the fact that the external nose is more prominent than in 
most primates. 
The ultimate organ of smell is the olfactory membrane, 
which in fishes and amphibians is distributed over the entire 
nasal cavity, but which, with the establishment of air-breathing 
and the setting apart of a respiratory portion, tends to confine 
itself to the more dorsal region. It consists of a highly differ- 
entiated form of epithelium in which occur the olfactory sense 
cells surrounded by supporting cells, some or all of which 
may be ciliated. In certain fishes and in the lower urodeles 
there occur in the nasal mucous membrane definite groupings 542 
THE HISTORY OF THE HUMAN BODY 
of the olfactory cells, surrounded by protecting cells, thus 
forming olfactory buds, almost identical in form with the 
taste-buds which in their turn resemble the lateral line organs. 
Some have seen in this, as well as in the condition of the pri- 
mary olfactory pits, which, in the embryo, form the anlage of 
the nasal cavities, a possible genetic connection with the lateral 
line organs, but this homology is rendered improbable from 
other reasons. Of these the most fundamental is the singu- 
lar fact that the olfactory sense cells are true nerve cells, 
sensory neurones, the cell bodies of which have a unique loca- 
tion among the epithelial cells of the sense organs, while each 
sends out a neurite which enters the olfactory bulb, and thus 
constitutes a component fiber of the olfactory nerve. This 
continuity of fiber with terminal cells is a very primitive 
relationship, characteristic of the sense cells of the lower in- 
vertebrates and suggests that the sense of smell, or at least 
the primary olfactory membrane, has been inherited from 
some far-away invertebrate ancestor, and is thus much older 
than any of the other sense organs. Another possible com- 
parison with certain other parts will be taken up below in 
connection with the lens of the eye. 
The essential organ of hearing is the labyrinth or inner ear, 
a series of membranous tubes or sacs, the complicated struc- 
ture of which has suggested its name. Although generally 
thought of as organs of hearing, it must be borne in mind that 
in all of the lower vertebrates the ear is adapted to the recep- 
tion of such slow vibratory stimuli that they can scarcely be 
classed as sound waves. Moreover, a very large part of the 
ear even in the mammal is, as has already been explained, a 
proprioceptive rather than a true exteroceptive organ, since it 
is concerned with pressure stimuli arising from changes of po- 
sition. In fishes the ear is placed immediately beneath the 
dermal bones of the head and in its comparatively superficial 
position needs no accessory apparatus, but in the higher verte- 
brates it is located deep in the interior and associates with 
itself a number of auxiliary parts to aid in the collection and 
transmission of sound vibrations. Probably the chief reason THE SENSE-ORGANS 
543 
for this difference lies in the change from water to air, since 
the denser medium transmits the sound waves with so much 
more intensity than does the air that the apparatus which de- 
velops in adaptation to the former requires an intensifying 
mechanism when placed in the latter. 
The anlage of the labyrinth appears in the early embryo as 
a slight thickening of the ectoderm over a small lateral area 
at about the level of the metencephalon. As the cells of this 
Fig. 174. Development of human otic capsule. Drawn from models 
by F. Ziegler, after Wm. His. 
area proliferate more rapidly than those of the surrounding 
ectoderm, they gradually fold in and form a deep pit, which, 
as the process continues, pushes further into the interior, where 
it expands into an otic vesicle, retaining its connection with 
the exterior, however, through a narrow tube, the ductus 
endolymphaticus. In selachians this connection is retained 
throughout life, and a minute but evident external pore is 
found near the top of the head which communicates through 
a small duct with the interior of the labyrinth; but in all 
other cases the connection with the exterior becomes severed 
and the endolymphatic duct ends in a somewhat expanded 
blind sac, the saccus endolymphaticus. In mammals the endo- 
lymphatic duct is lodged in a canal of the petrosal bone, the 544 
THE HISTORY OF THE HUMAN BODY 
aqueductus vestibuli, and enters the cranial cavity, where its 
terminal sac lies just beneath (i.e., outside of) the dura mater.* 
The otic vesicle itself develops, mainly by the unequal 
growth of the different regions, into the labyrinth. This de- 
velops first into two expanded portions, utriculus and sacculus, 
with a restricted portion between them, the utriculosaccular 
canal. [Cf. Figs. 175 and 176.] From the utriculus, which 
lies dorsally, develop three flattened folds, which by the adhe- 
sion and subsequent atrophy of the middle portion of their 
wall, develop their marginal portions into tubes. Thus are 
formed the three semicircular canals, which are constant in 
all vertebrates above the cyclostomes, and vary but little in 
general appearance or relationships. They are set approxi- 
mately at right angles to one another in such a way that one 
lies horizontally and the other two vertically, each at an angle 
of about 450 with the bilateral plane of the body. They con- 
nect at either end with the utriculus, one end of each being 
expanded into a flask-shaped ampulla. These are so placed 
that the ampullae of the anterior vertical and horizontal canals 
open together into a pocket of the utriculus (recessus utriculi), 
while that of the posterior vertical canal opens by itself into a 
* In some cases the ductus endolymphaticus and its terminal sac attain 
a high degree of development and come into association with organs re- 
mote from the place of origin. Thus in certain teleosts the two ducts unite 
into a median sinus which is connected with the air bladder by a chain of 
four ossicles (Weber’s apparatus), developed from the ribs of the first four 
vertebrae. By this means the degree of fullness of the air-bladder may be 
perceived. 
In the Anura the endolymphatic ducts form a common sinus, which ex- 
tends posteriorly, dorsal to the spinal cord and outside of the dura mater, 
as far as the tail rudiment (urostyle). This extensive sinus sends to the 
roots of the spinal nerves a series of diverticula which wrap themselves about 
the spinal ganglia and expand into sacs containing granules of calcium 
carbonate. In many reptiles the duct reaches the top of the skull and even 
escapes, its terminal sac being subcutaneous. In snakes this sac contains 
calcareous crystal, which in some cases may be seen through the skin of 
the living animal. This apparatus reaches its highest development, so far 
as reptiles are concerned, in the lacertilian family of the geckos (Ascalabotae), 
where the sac escapes from the skull through the parieto-occipital suture 
and pushes its way between the muscles of the neck and shoulder as far as 
the pharynx. It is filled with a soft calcareous mass. THE SENSE-ORGANS 
545 
similar pocket on the other side (sinus utriculi posterior). 
The two vertical canals unite at their expanded end and form 
the sinus utriculi superior, a diverticulum which extends di- 
rectly upward; the unexpanded end of the horizontal canal 
enters the main body of the utriculus unassociated. 
The parts of the utriculus with its derivatives, the semicir- 
cular canals, appear first in the form described above in the 
selachians and are retained with very little deviation by all 
higher vertebrates. The sacculus, however, exists in a simple 
form in fishes and shows considerable advance in the higher 
forms. This advance consists of the gradual development of 
a lateral sac, the lagena, which is situated upon the inner side 
and which in fishes and amphibians is barely indicated. In 
reptiles and birds the lagena becomes considerably elongated 
and curved, and in mammals it becomes spirally wound and, 
associating certain outside elements with itself, forms the 
cochlea, here attaining the highest degree of complexity of 
any part of the labyrinth. In the higher forms also the direct 
connection between utriculus and sacculus becomes replaced 
by an indirect one through the ductus endolymphaticus, which 
arises by two branches, one from each of the two parts [Cf. 
Fig. 176]. These unite and thus indirectly retain the con- 
nection in question. 
The labyrinth of the cyclostomes stops at a lower point of 
development than is represented by any of the gnathostomes 
and may well represent the permanence of what is, in the 
higher forms, an early embryonic stage. It consists of a simple 
oval sac, not yet differentiated into utriculus and sacculus, 
and possessed of either one (Myxine) or two (Petromyzon) 
semicircular canals. Its endolymphatic duct, however, is 
short and loses its connection with the exterior (Fig. 175, a). 
The walls of the embryonic labyrinth are composed of a 
single layer of epithelial cells of appreciable thickness and all 
alike; as development proceeds, however, the greater part of 
the cells become flattened and form a transparent membrane, 
while over certain definite areas, 6 to 8 in all, the cells are 
thickened and columnar, forming a sensory epithelium. Cer- 546 
THE HISTORY OF THE HUMAN BODY 
tain of these cells form the ultimate receptors and are pro- 
vided with various sorts of terminal flagella and other similar 
structures (auditory hairs), which project into the lumen of 
the labyrinth and are bathed in the endolymph, i.e., the fluid 
Mac. , 
com 
Maculir 
PapJia 
f 'Pap lag 
Mac) 
sac. 
Mack 
utr. 
Tuplag 
Mac. 
utn 
Mae 
sac 
Taplag 
Mac. sac. 
Mac 
sac 
, Mac. 
ali- 
aa 
fac 
Macutiij 
Tap lag 
Pap lag (-Cod^ 
Fig. 175. Ear labyrinths of various Vertebrates. [After Retzius.] 
Mac sac 
(a) Cyclostome; Myxine glutinosa (hag fish), (b) Selachian; Chimaera monstrosa, 
(c) Teleost; Anarrhichas lupus (wolf-fish), (d) Amphibian; Rana esculenta (frog), 
(e) Bird; Bubo ignavus (horned owl), (f) Mammal; Sus scrofa domestica (pig). 
aa, ampulla anterior; ae, ampulla externa; ap, ampulla posterior; e, ductus 
endolymphaticus; fac, facial nerve; mac, macula communis; mac. utr, macula 
utriculi; mac. sac, macula sacculi; n, macula neglecta; pap. lag, papilla lagenae; 
pap, bas, papilla basilaris. 
filling the interior. These are supplied with nerve fibers from 
the auditory nerve. About these are placed various sorts of 
supporting cells, which are without auditory function and serve 
to hold and protect the others. These spots, which thus show THE SENSE-ORGANS 
547 
a higher degree of cellular differentiation, are the seat of the 
auditory sense, and appear from their greater thickness as 
white patches in the otherwise transparent wall. They are 
further indicated by their association with the nerve. 
The largest occurring number of such auditory areas is 
eight, although all do not occur simultaneously. Of these the 
three which are associated with the semicircular canals are 
somewhat different from the rest and are absolutely constant. 
They are situated in the ampullae and are in the form of ridges 
which encircle the ampullae and project into the lumen. They 
are thus distinguished from the others as cristce acusticce. The 
remaining five are evidently formed by the successive breaking 
up of a single large area, due to the differentiation of parts of 
the labyrinth, and the history of this segmentation and later 
migration is represented in the phylogenetic series (Fig. 175). 
Thus, in the cyclostomes the acoustic region, aside from the 
one or two semicircular canals, is represented by a single area, 
macula acustica communis, covering the bottom of what is 
here a simple sac (Fig. 175, a). The differentiation of utricu- 
lus and sacculus gives a separate area to each, respectively, 
the macula acustica recessus utriculi and macula acustica 
sacculi (Fig. 175, b). With the gradual development of the 
lagena in fishes there appears an outgrowth of this latter area 
which finally separates from its place of origin and establishes 
itself as the auditory area for the newly developed part, under 
the name of papilla acustica lagence. This gradual separation 
of both lagena and its acoustic area is accompanied by a similar 
separation of the nerve, which splits off a supply branch for 
the new area. (Fig. 175, cf. b and c.) A small macula is 
also formed near the utricular macula, the macula acustica 
neglecta, evidently an offshoot from the latter, although no 
phylogenetic proof of this appears as in the former case. The 
fifth and last of the auditory areas, not counting the cristae 
acusticse of the ampullae, the papilla acustica basilaris, also be- 
longs in the lagenar region, and appears in the higher urodeles 
as an offshoot of the papilla lagenae (Fig. 175, d). 
To put these points into the form of a phylogenetic history 548 
THE HISTORY OF THE HUMAN BODY 
we may take as a starting point the macula communis of 
cyclostomes, which may be considered to hold all the later 
elements within itself. In fishes we find a primary macula 
for each of the two parts into which the labyrinth has become 
divided, also a macula neglecta, which has presumably sepa- 
rated from the one belonging to the utriculus at some point 
below the fishes. Within the group of the selachians the papilla 
lagense appears, sometimes as a separate area, sometimes as 
a lobe of the macula sacculi, thus giving four acoustic maculae 
for most fish. In amphibians the papilla basilaris appears 
while the others are retained, although there is a slight trans- 
position of the macula neglecta. This condition, with five 
acoustic areas, is retained by the Sauropsida with some varia- 
tion, such as the division of the papilla basilaris in certain 
lizards and a great reduction of the macula sacculi in turtles, 
points referable to special adaptation and of no general signifi- 
cance. In mammals there are important changes. The macula 
neglecta has entirely disappeared and the papilla lagena is 
found only in Ornithorhynchus (monotreme), leaving in this 
Class but three acoustic areas aside from the three cristae 
acusticae of the semicircular canals. 
The most important difference in the mammalian labyrinth 
is the great development of the lagena. The tendency, already 
shown in crocodiles and birds, to prolong this part and to 
curve its axis, results here in an excessive elongation which be- 
comes wound into a close spiral, the nerve forming the central 
axis. The number of complete coils in man is about 3, but 
varies among the mammals between the limits of and 5.* 
This coiled lagena becomes complicated by the addition of 
parts of the outer bony labyrinth, to be explained later, which 
* Examples are as follows: 
Erinaceus (Hedgehog) ij 
Whales and porpoises ii 
Rabbit 2j 
Cat 3 
Ox 3i 
Swine 3i 
Ccelogenys (South American rodent) 5 THE SENSE-ORGANS 
549 
form two additional coiled passages, scala vestibuli and scala 
tympani, that receive between them the lagena under the 
anatomical name of ductus cochlearis [scala media]. This 
entire organ, including both this part of the labyrinth and its 
accessory organs, is called the cochlea. 
The papilla basilaris lies along the floor of the coiled lagena 
(scala media) and becomes highly differentiated into a num- 
ber of kinds of histological elements, which change their pro- 
portionate size along the course, being the smallest at the base 
and the largest at the apex. The most important of these 
canalis cochlearis. 
/canalis 
treuniens. 
Fig. 176. Diagram of membranous labyrinth of human ear. [From 
Gegenbaur, after Retzius.] 
A, A, A, ampullae; U, utriculus; S, sacculus; E, ductus endolymphaticus; 
ant, ext, post, the three semicircular canals. 
cellular elements are certain elongated cells associated in pairs, 
the rods of Corti, and two groups of cells with specialized 
terminal organs, the outer and inner hair cells. The ventral 
portion of the lagena, that is, the floor of the scala media be- 
yond the auditory area, is termed the basilar membrane, and 
the dorsal portion (roof) is Reissner’s membrane; these terms 
are, however, purely anatomical ones, expressing certain re- 
lationships to the surrounding parts and are without morpho- 
logical significance. 
The membranous labyrinth as above described, that is, the 
higher development of the auditory vesicle of the embryo, be- 
comes surrounded while still embryonic by a gelatinous con- 
nective tissue, its first accessory organ. Later on in develop- 
ment this tissue becomes converted either to cartilage or bone, 550 
THE HISTORY OF THE HUMAN BODY 
leaving, however, a nearly uniform layer of the original tissue 
between it and the membrane. There is thus formed a mold 
which reproduces the membranous labyrinth in its details, the 
bony (or cartilaginous) labyrinth. The gelatinous tissue be- 
comes soon converted into a serous fluid, called the perilymph, 
in distinction from the endolymph of the interior, and the cav- 
ities involved are conveniently distinguished as the perilym- 
phatic and endolymphatic cavities respectively. The mem- 
branous labyrinth is held in place by scattered strings of the 
original connective tissue, which connects it with the bony wall, 
aside from which the two come into close contact at the places 
of entrance of the various branches of the auditory nerve. In 
the lower vertebrates the outer labyrinth remains as a mold 
imbedded in the petrosal bone, but in higher forms, especially 
in mammals, the bony labyrinth appears over certain regions, 
especially the semicircular canals and the lagena, as a thin 
but very hard wall, with a space between its outer surface and 
the main mass, thus reproducing from without, also, the main 
details of the membranous labyrinth. 
The transition from water to air, undoubtedly the greatest 
change which vertebrates have ever experienced, and one 
which demanded modifications affecting every part, affected the 
organ of hearing directly, for a change was made from a 
denser medium, which readily transmitted sound vibration, to 
a lighter one in which transmission was more difficult. This 
disadvantage was undoubtedly felt by the urodeles, which ex- 
hibit a new organ, evidently destined to assist in the recep- 
tion of less powerful vibrations. In the cartilaginous otic 
capsule surrounding the labyrinth, that which partly corre- 
sponds to the “ bony labyrinth ” of higher forms, there exists 
an oval opening with a reinforced rim, closed by an ossicle in 
the form of a lid, usually with a process projecting from its 
center [cf. Fig. 40, op]. The opening, which persists in all 
higher vertebrates, is the fenestra ovalis, and the osseous lid, 
which is, in origin, a portion cut off from the wall of the 
capsule, is the operculum. This latter is fitted to the rim of 
the fenestra ovalis by a membrane, and, as it is nearly sub- THE SENSE-ORGANS 
551 
cutaneous, it is set in motion by the impact of sound waves, 
and thus serves to slightly intensify the vibrations. 
This apparatus proves sufficient for urodeles, which are 
much in the water, but in the tailless forms (Anura), far more 
terrestrial than the salamanders, the sound-receiving appa- 
ratus is much improved by an important addition, the tym- 
panum, or cavity of the middle ear. This is developed from 
the gill pouch of the spiracular opening, the one associated, 
as will be remembered, with the hyoid arch. The inner por- 
tion of this cavity communicates with the pharynx and forms 
the auditory or Eustachian tube, but direct communication 
with the outside is prevented by the presence of a circular 
tympanic membrane at the outer end, just beneath the skin, 
and usually very evident from the outside. 
This membrane, which is covered outwardly by integument 
and on its inner side by mucous membrane, is a separate 
formation, usually of connective tissue, but in a few cases it 
is cartilaginous. It has been doubtfully homologized with the 
spiracular cartilage of selachians, but this is too uncertain to 
be definitely asserted. In many cases there exists a second 
opening in the wall of the otic capsule, the fenestra cochleae 
[rotunda], covered by a thin membrane, the inner tympanic 
membrane. This part is present in all higher verte- 
brates, thus giving the tympanum the characteristic from 
which it derives its name, i.e., two drum heads, outer and 
inner. To complete the likeness of the middle ear to a drum 
the Eustachian tube represents the opening always present in 
the cylinder of a drum and employed in both purposes for 
equalizing the air pressure on either side of the drum heads. 
A mechanism, however, which is lacking in a drum, is that 
formed by the columella, a delicate spindle of bone or carti- 
lage, which extends from the center of the outer tympanic 
membrane to the operculum. By this means the sound vibra- 
tions that impinge upon the former are transmitted directly to 
the latter, and through it to the perilymph within the otic 
capsule. Another channel for the transmission of sound waves 
is furnished by the air enclosed in the tympanic cavity, the 552 
THE HISTORY OF THE HUMAN BODY 
vibrations striking the inner tympanic membrane. The ap- 
parently new skeletal element, the columella, is perhaps 
nothing more than a process of the operculum, but it has been 
considered by some to be a distinct element and to represent 
the hyo-mandibular of fishes, employed there as a suspensory 
piece for the mandible and originally the dorsal segment of 
the second visceral arch (hyoid). 
In the Sauropsida there is but little change in the tympanic 
cavity from that of the Anura. The two Eustachian tubes 
often form by their union a median duct which opens into the 
pharynx in the mid-dorsal line. Such is the case in birds and 
in crocodiles, and in the latter the tubes under consideration 
form a complicated system of cavities, many of which are 
lodged in the bones of the cranium. In other cases similar 
systems extend out from the main tympanic cavity, and in 
certain instances the two communicate with another across 
the median line. 
A characteristic and important addition is gained in mam- 
mals by the appearance within the tympanic cavity of the artic- 
ular and quadrate bones, hitherto employed in forming the 
mandibular articulation. These form respectively the malleus 
and incus, and become added to the columella to form a chain 
of ossicles which reaches across the cavity from the outer 
drum head to the fenestra ovalis, thus assuming the function 
formerly sustained by the columella alone. This latter ap- 
parently becomes reduced in size and forms the stapes. (Cf. 
Chap. V.) The singular and characteristic foramen in this 
bone, to which it owes its similarity to a stirrup, is caused by 
the development of a small artery, which perforates the 
columella. This relation appears only in the embryo in most 
mammals, including Man, but in some (certain rodents and 
insectivores) it persists throughout life. (Cf. Chap. IX.) In 
others still, mainly monotremes and marsupials, the perfora- 
tion does not take place, but the bone remains in the primitive 
cylindrical form. 
The stapes is supplied by a tiny muscle, the stapedius, which 
is shown by its embryology to be a slip separated from the THE SENSE-ORGANS 
553 
digastric muscle, an element primarily associated with the 
hyoid arch. To the malleus is attached a second small muscle, 
the tensor tympani. This was originally a portion of the com- 
mon mass from which the masticatory muscles of the jaw have 
differentiated, the abductor mandibuli of the selachians. This 
little slip arises from that portion which forms the pterygoid 
muscles, and is innerved by a branch from Trigeminus, the 
nerve associated with the first or mandibular arch. These two 
tympanic muscles have thus had a history as old as the parts 
to which they are attached, and form here, together with 
their associated nerves and ossicles, groups of parts which 
have retained their original relationships through all their mi- 
grations and changes of form and function. 
Another characteristic mammalian element, not directly 
within the tympanic cavity but closely associated with it, is 
the tympanic bone (os tympanicum). This, when in its full 
development, forms a complete bony ring or frame about the 
outer tympanic membrane, and often develops in addition a 
concave osseous shell or tympanic bulla, which forms a con- 
spicuous object at the base of the skull and aids in protecting 
the delicate parts of the middle ear. Occasionally, too, the 
bone extends outwards to form an osseous wall for the ex- 
ternal meatus. This bone remains distinct throughout life in 
monotremes, marsupials and a few others, but in the majority 
of cases, as in Man, it fuses with the petrous elements and 
becomes eventually lost in the complex designated as the 
“ temporal bone.” The homologies of this bone are uncertain, 
although some consider it the same as sauropsidan quadrato- 
jugal. It can hardly be a new osseous element, but that it 
appears here in a new role and is thus a new bone physio- 
logically is evident. 
The external ear, characteristic of the mammalia, is mainly 
a cartilaginous structure covered by integument, and consists 
of a round tube, the external auditory meatus, and an external 
flap, the auricula [pinna]. The first of these, the meatus, 
allows the outer tympanic membrane to sink below the surface 
and still retain connection with the exterior and its curve 554 
THE HISTORY OF THE HUMAN BODY 
affords the membrane a more or less complete concealment. 
In cases where the tympanic bone furnishes a prolonged tube 
for this purpose, as in Man, the external cartilaginous element 
is less extensive, and the two together form the wall of the 
canal. 
The auricula shows a large degree of adaptation, being very 
large and mobile in cases where acute hearing is desired, for 
example, in bats, and in most ungulates; and is reduced or 
entirely wanting in many burrowing or aquatic forms. The 
characteristic anthropoid ear is shaped at the base much as in 
Man, but there is no lobule and little or no recurving to the jree 
edge. This latter peculiarity, which starts at the upper part oj 
the base, is distinctively human, but extends over a varying 
distance in different individuals, and is often hardly begun in 
the new-born infant. A rudimentary point, tuberculum auriculi 
[Darwini], is often retained at the free edge, and is brought 
over by the rolling process so as to point forward instead of 
backward, its primary position. This is occasionally a con- 
spicuous feature, and in all cases its place can be determined 
by feeling, being indicated by a thicker, harder area on the 
outer rim a little below the top of the curve. A lobule is 
usually present, but is rudimentary or absent in certain races. 
Concerning the origin of the cartilaginous elements of the 
external ear, it becomes evident from the condition found in 
monotremes that it is largely or wholly derived from the upper 
end of the hyoid arch, which curves about the tympanic mem- 
brane and forms the tubular meatus together with a rudimen- 
tary pinna. This leaves unaccounted for a series of protuber- 
ances in the integument surrounding the opening of the meatus, 
which are seen to form in the human embryo and fuse to- 
gether to build up the external portion (pinna). These pro- 
tuberances are considered by some to be elements furnished 
by the first four visceral arches, i.e., mandibular, hyoid and 
the first two branchial, but this is rendered very improbable 
by the innervation of the pinna, which is wholly from the 
Facialis. It may thus prove to be a modification of devel- 
opment, and portions which were originally hyoid elements may THE SENSE-ORGANS 
555 
here appear in this form. In the Sauropsida the outer tym- 
panic membrane is frequently depressed a little below the sur- 
face and provided with small protuberances or flaps which as- 
sist in its protection, but these are evidently incidental adapta- 
tions and can have nothing to do with the external ear of 
mammals. 
The early developmental history of the eye shows that the 
essential part of this sense-organ, the retina, differs radically 
Fig. 177. Diagrams of the retina. 
(a) Section including the fovea, showing the separate elements. [From 
Gegenbaur, after Ramon y Cajal.] (b) More conventionalized representation of 
retinal layers. [After Gegenbaur.] 
fov, fovea; I, membrana limitans interna; II, nerve fiber layer; III, nerve cell 
layer; IV, inner granular layer; V, inner nuclear layer; VI, outer granular layer; 
VII, outer nuclear layer; VIII, membrana limitans externa; IX, rod and cone 
layer; X, tapetum. 
from all the others in being originally a portion of the brain 
wall evaginated from the dorso-lateral region of the forebrain 
in the form of the optic vesicles, which soon become spherical, 
and connected with the more ventral region of the diencephalon 
by hollow stalks made narrow by the development of a deep 
constriction from above. The optic vesicles come almost into 
contact at their outer surface with the inner surface of the 
integumental ectoderm. By an invagination of this outer 
surface the vesicle is transformed into a double layered cup, 
the optic cup (See Fig. 178), while the cavity of the vesicle 
thereby becomes practically obliterated. 556 
THE HISTORY OF THE HUMAN BODY 
The invaginated layer, now lining the cup, becomes the 
retina, while the other layer, now forming the covering of the 
cup, develops pigment and becomes the tapetum nigrum, 
which is applied to the inner surface of the vascular chorioid 
coat which eventually develops outside of the whole structure. 
Fig. 178. Development of the optic cup 
(a) Plastic representation. [After Hertwig.] (b) Median longitudinal section of 
(a), (c) Cross section in plane indicated by the line xy. 
The optic stalk, although not directly transformed into the 
optic nerve, forms the path along which it develops and thus 
marks its final position. 
During the time at which the optic cup has been forming 
by a turning in of the outer part of the vesicle, an associated 
process takes place in the ectoderm directly opposite the cup. 
This process consists of an inpushing from without on the 
part of this ectoderm, the inpushing going rapidly through the 
stages of a simple depression, a depression with a narrowed 
neck, and finally that of a spherical vesicle entirely cut off 
from its layer of origin. This vesicle develops into an auxil- 
iary though essential organ of the eye, the crystalline lens. 
This is accomplished by an enormous thickening of the inner- 
wall of the vesicle, the cells of which become extremely tall 
while remaining in a single layer. They finally fill up the 
entire lumen, leaving the outer wall to fit over it in the form 
of a protecting epithelium. The functional lens is formed by 
the cornification of these cells, and the mass thus formed is 
covered anteriorly with a thin epithelium, the original anterior 
wall of the vesicle. THE SENSE-ORGANS 
557 
It will be noticed that there is in this development of the 
lens a striking similarity with the early stages of both the 
nasal pit and the ear, and if there be taken in connection with 
these the epibranchial placodes which appear and vanish again 
during the embryonic life, the idea comes at once to mind that 
we have here the record of a series of ancient sense-organs 
laterally placed, perhaps a pair for each metamere, some of 
which have specialized in various ways while others have be- 
come lost. If this be true, the lens was originally, not a re- 
fracting medium, but a sense-organ itself, which has given up 
its primary function entirely and entered the service of another 
sense-organ, different in origin from that of any other in ver- 
tebrate history, namely, a specialization of a portion of brain 
surface. 
The idea of this ancient series of sense-organs suggests 
many questions. What was the primary function of this 
series? Did these sense-organs sustain any relation to the 
lateral line organs? To these questions, belonging themselves 
to the realm of pure suggestion, we can give but speculative 
answers. Both the nasal pit and the ear, as we have already 
seen, have in their structure and development something to 
suggest a kinship with the lateral line organs; this is espe- 
cially true of the latter, with its nerve appearing in connection 
with that element of the facial nerve that supplies these organs 
in fishes, and with its semicircular canals that resemble the 
canals of Lorenzini. The eye itself cannot, of course, be in- 
cluded in any series of sense-organs of integumental origin, 
but the lens can, and there seems no intrinsic difference, up to 
a certain stage, between the lens capsule and that of the inner 
ear. Moreover, in the association of the optic vesicle with the 
lens formation we have a somewhat similar phenomenon to the 
typical relation of neural crest to placode, as seen, for ex- 
ample, in the case of the Facialis crest and the dorso-lateral 
placode (otic vesicle). When we consider that the optic ves- 
icles, although assuming eventually the relationship of lateral 
evaginations of the neural tube, are in reality formed from 
its dorsal region which before the neural tube closed was im- 558 
THE HISTORY OF THE HUMAN BODY 
mediately adjacent to the neural fold at the lateral border of 
the plate, the similarity of optic vesicles to neural crests be- 
comes apparent, and further emphasizes the possibility of 
homology between the lens and placode. The nasal sacs, again, 
are capsules similar to the otic vesicles and lenses, that do 
not lose their connection with the exterior, and it must be 
remembered that in the endolymphatic duct of the selachians 
we see the same retention of the original connection. We seem 
here almost able to reproduce an important bit of lost history, 
but the proofs are not forthcoming and may always be want- 
ing, since the early phylogenetic stages were probably passed 
in those lost forms between Amphioxus and the selachians, and 
concern soft parts, no trace of which is likely to be found in 
fossil remains. 
To understand the addition of the accessory organs and the 
formation of the eyeball it is necessary to examine more 
thoroughly the early stages in the formation of the optic cup. 
The study of a few actual sections will show that the invagina- 
tion of the primary outpushing is not a symmetrical one, but is 
so effected that the cup is deficient for a little space on the 
ventral aspect, and that this deficiency is continued as a groove 
along the lower side of the optic stalk. It thus happens that 
when the lens, which at this time is added to the optic cup, be- 
comes closely applied to its rim, a fissure or oblong aperture, 
the chorioid fissure, is left, through which communication may 
be made with the interior of the cup behind the lens. Through 
this inlet migrate embryonal connective tissue cells (mesen- 
chyma) and form a gelatinous tissue, the basis of the vitreous 
humor. From similar mesenchymatous elements added ex- 
ternally is formed the vascular network of the chorioid coat, 
and outside of this is formed the sclera (sclerotic coat). The 
anterior portion of the chorioid forms the iris and the corre- 
sponding portion of the sclera forms the cornea. This latter 
stands out from the lens in front and thus forms an anterior 
chamber, filled with the aqueous humor, a colorless lymph, 
which serves as a refracting medium. The corresponding 
chamber of the structurally similar eye of the cephalopod THE SENSE-ORGANS 
559 
mollusc (squid, devil-fish, etc.) is perforated by a foramen 
communicating with the exterior, and through this it is filled 
with sea water which serves the same purpose. This expe- 
dient is comparable with that of the internal ear of selachians, 
with its direct communication with the exterior through the 
ductus endolymphaticus. 
In its histological structure the vertebrate retina shows close 
similarity to other well-developed portions of the brain wall 
and exhibits several layers of cells, the innermost of which, 
that is, the ones upon the convex surface of the invaginated 
layer of the optic cup, become the characteristic receptive 
cells of the retina known as the rod and cone cells. The rods 
and cones are in reality the receptive processes of these cells, 
and are imbedded in the pigment of the tapetum. 
The retina comprises in addition to the rod and cone cells, 
two other layers of neurones which together with the rod and 
cone cells form a synaptic series. Thus from the rank of 
neurones which lies next the inner chamber of the eyeball 
the neurites grow into the brain, as the component fibers of 
the optic “ nerves ” which are in reality as truly optic tracts 
as any which are located in the brain itself. At the exact 
focal center of the lens all but the terminal sense-cells dis- 
appear, and produce a small depressed area, the area cen- 
tralis. This is often in the form of a circular pit, fovea cen- 
tralis, but may be oval, or in the form of a broad band or 
streak. It is, however, not always depressed, and may be 
entirely wanting. These variations seem to bear little or no 
relation to phylogeny, since a fovea is present in some fishes 
and in most Sauropsida, while the area seems entirely lack- 
ing in many mammals (Insectivora and some rodents). In 
the anthropoids it is very pronounced, and in man it is desig- 
nated by a yellow color (hence “macula lutea”). Certain 
birds possess two such areas, medial and lateral. 
If we follow the in- and out-pushings of the retina from the 
beginning, it is clear that the original external surface lines the 
lumen of the neural tube and eventually forms the receptive 
retinal surface, that is, the surface turned toward the pig- 560 
THE HISTORY OF THE HUMAN BODY 
merited tapetum. Now in all cases of sense organs it is the 
primarily external surface that becomes specialized to receive 
external stimuli, and it is also the original outer or superficial 
end of the sense cells that develop the specially modified 
flagella and other parts. It thus happens that the receptive 
cells of the sense of vision are not only those of the outermost 
layer of the retina, which is several cell-layers in thickness, 
but also that their free ends, bearing the terminal rods 
and cones, point towards the interior of the head and 
away from the source of light. Thus in order that the 
image received through the pupil and focused by the lens upon 
the inner surface of the retina may reach the terminal rods 
and cones, all the intervening parts, the nerve fibers and the 
various layers of retinal cells, have to be perfectly transparent; 
and, furthermore, the terminal rods and cones must needs be 
buried in the pigment of the tapetum in order to stop possible 
extraneous light impressions coming from without the eyeball 
otherwise than through the lens. 
Moreover in order to transmit the stimuli thus received, to 
the brain, the impulses must travel back through the thickness 
of the retinal layers and, finally reaching the nerve fibers 
which converge from the innermost layer to form the optic 
nerve, must again penetrate the thickness of the retina as the 
optic nerve makes its exit from the eyeball. 
The necessity for this arrangement becomes clear to anyone 
who has followed the foldings of the embryonic layers, yet 
there is scarcely anything in vertebrate construction that seems 
a greater mechanical mistake, although there are others, like 
the inguinal canal in Man, where a lesser error involves far 
more serious consequences. This error in the arrangement of 
the retina, however, becomes still more apparent when a com- 
parison is made with the structurally similar cephalopod eye, 
in which the retina is developed directly from the surface 
ectoderm and is placed in the natural way, with the terminal 
cells lining its interior and the optic nerve leaving it from be- 
hind. Notwithstanding the fundamental differences in de- 
velopment between this eye and that of vertebrates, the final THE SENSE-ORGANS 
561 
results, when compared part by part, are marvelously similar, 
and the adult eye of each is furnished with retina and crystal- 
line lens, iris and cornea. This case is thus one of the best 
examples of what Darwin termed “ analogical resemblances ”; 
that is, the production of a similar organ in two unrelated 
forms and often from entirely different starting points, not 
through any genetic connection, but because of the same en- 
vironmental influences, which gives rise to the same necessities. 
The absolute size of the eyeball is very variable. In gen- 
eral it is somewhat in proportion to the size of the body, yet 
the eyeballs of the elephant or the whale, although large in 
both cases, are not proportionate to their enormous bulk when 
compared with those of Man, for instance. Again there is a 
certain proportion between the size of the eyeball and the 
sharpness of vision, as, for example, the enormous eyes of 
birds; but here, again, must be mentioned the small but ex- 
ceedingly acute eyes of rodents where the decrease of size 
seems to be due to the excessive development of the masseter 
muscles, and appears to have no direct influence upon the 
vision. The eyes are apt to be large in animals with nocturnal 
vision, like the lemurs, and it is possible that the relatively 
large eyes of Man, which have encroached upon the nasal 
cavities and thus reduced the power of smelling, may be the 
result of a nocturnal habit in some not very remote ancestors. 
Of the organs external to the eyeball which are accessory to 
the sense of sight the muscles have been treated in a preced- 
ing chapter (Chap. VIII). There thus remain for considera- 
tion only the eyelids and the glands, two sets of structures 
closely associated with one another. They are both employed 
in preventing the surface of the eyeball from becoming dry 
upon exposure to the air, and belong to that series of changes 
necessitated by the change of environment from water to 
air. They are consequently found only in the higher verte- 
brates, and are absent in fishes, and but poorly developed in 
aquatic urodeles. 
In fishes the integument fits smoothly over the region sur- 
rounding the eyeball, and is continuous over the latter as a 562 
THE HISTORY OF THE HUMAN BODY 
thin skin, usually transparent, but occasionally ornamented in 
places with pigmented areas, which continue the color scheme 
of the rest of the skin. The eyelids, which appear first in 
amphibians, form as dorsal and ventral folds of the integument, 
which may become stiffened, either by connective tissue or by 
cartilage, as in mammals (tarsal cartilages). That portion of 
integument which forms the inner face of the folds and is con- 
tinued over the front of the eyeball is very thin and sensitive, 
and forms the conjunctiva. A nictitating membrane is formed 
in some vertebrates by an inner fold of this last; it attains in 
birds and in some mammals the dignity of a third eyelid; in 
Man it is represented by the plica semilunaris, a delicate fold 
situated in the inner corner. 
The lubricating fluid, the “ tears,” is furnished by two 
groups of glands which arise as invaginations of the conjunc- 
tiva and retain their connection with that layer, supplying the 
pockets formed by the lids. These are (i) the Harderian 
glands, which are located about the anterior (inner) corner and 
are associated with the nictitating membrane, and (2) the 
lacrimal glands, located near the posterior (outer) corner. 
This differentiation is not found in the amphibians where the 
glands are all alike and are evenly distributed, but appears in 
reptiles, from which point the two groups are distinct, both in 
location and structure. The Harderian glands are well de- 
veloped in reptiles, birds and most mammals, but are rudi- 
mentary in the Anthropoidea. The lacrimal gland is asso- 
ciated in reptiles and birds with the lower eyelid, beneath 
which its ducts empty, but migrates in mammals to a more 
dorsal position and thus becomes almost exclusively associated 
with the upper lid. In some mammals a few ducts occur in 
the lower fold; indications of its former location. The lacri- 
mal fluid, supplied by both of these glands, is continually 
being secreted and is as constantly spread in an even layer 
over the outer surface of the eyeball by the movement of the 
lids. The excess is conveyed to the nasal cavities through the 
nasolacrimal duct, which appears in amphibian larvae as an 
integumental groove extending from the eye toward the nostril. THE SENSE-ORGANS 
563 
This eventually closes up, sinks into the interior, and gains 
its independence from the integument, thus forming an in- 
ternal canal connecting the conjunctival sac with the anterior 
end of the nasal cavity. 
Aside from these conjunctival glands there appear in mam- 
mals certain glands associated with the eyelashes. These are 
(i) the tarsal (Meibomian), which are modified sebaceous 
glands, and (2) the ciliary, modified perspiratory glands. 
These open along the edges of the lids and produce narrow 
lines of oil which repel the lacrimal fluid and assist in retaining 
it within the peripheral folds. As the eyelashes are modified 
hairs, the tarsal glands may be looked upon as the associated 
sebaceous glands, considerably hypertrophied, and changed 
somewhat in their relation to the hairs. 
This entire lacrimal apparatus, including the glands and the 
nasolacrimal duct, becomes much reduced in such aquatic 
mammals as the hippopotamus, seal and otter, and in the 
pelagic whales and porpoises is entirely rudimentary. In 
snakes there occurs a singular adaptation, which protects their 
eyes from the danger of the thick grasses and twigs by fusing 
the two eyelids together over the eyeball and then rendering 
them absolutely transparent. There is thus formed a plate, 
in shape like a watch glass, and serving as a second cornea. 
This is shed with each successive skin and forms a conspicuous 
feature of the exuviae, or “ snake-skins,” objects commonly 
met with in fields frequented by snakes. A lacrimal apparatus 
is wholly wanting. 
As special protective organs to the eye may be mentioned 
the long superciliary bristles, which in cats and a few other 
mammals project over the eye and when touched cause the 
automatic closing of the lids; also the eyebrows of the higher 
anthropoids, specially Man, the hairs of which point towards 
and curve downwards at the outer end to receive the perspira- 
tion of the forehead and convey it away from the eyes. CHAPTER XIV 
THE ANCESTRY OF THE VERTEBRATES 
“ Ainsi la plus ancienne couche fossilifere connue nous 
montre des representants de presque toutes les classes 
d’lnvertebres. Cela demontre l’existence d’une longue 
periode anterieure a celle sur laquelle la Paleontologie 
peut nous fournir des renseignements et dans laquelle 
ont pris naissance presque tous les types actuels. 
Parmi ces etres, dont les formes resteront toujours un 
mystere, devaient se trouver les ancetres sans squelette 
des Vertebres actuels.” 
Delage et Herouard, Les Procordes, 1898, p. 357, 
Previous to the establishment of the modern theory of evo- 
lution, which removed each animal and plant from an isolated 
position unrelated to the rest, and assigned to it a place in a 
connected chain of organic beings, morphological speculation 
was limited to ingenious comparisons with little or no logical 
basis, conjured up to explain real or fancied resemblances. 
Thus Lorenz Oken, having conceived the idea that the head 
must possess parts corresponding to those of the trunk, consid- 
ered the nasal cavity, the cephalic thorax; the mouth cavity, the 
cephalic abdomen; and the palate, the cephalic diaphragm; to 
him the halves of the upper and lower jaws represented re- 
spectively the anterior and posterior limbs, in which the teeth 
were the digits. Thus Geoffroy St. Hilaire compared insects 
with vertebrates, making the exoskeletal rings the equivalent of 
the vertebrae, and the jointed legs that of the ribs. The rela- 
tion of nerve cord, intestine, and main blood-vessels was made 
the same by placing the insect upon its back. 
Others, like Goethe and Cuvier, sought to base the compari- 
son between different forms upon the assumption of an arche- 
type (Goethe’s “ Urbild ”), of which a certain related group of 
animals might be considered as so many various modifications. 
If such an archetype had been considered to have or to have had 
564 THE ANCESTRY OF THE VERTEBRATES 
565 
a real existence, it would have been the ancestor of the group in 
the modern sense, but there is little to be found in the writings 
of these early morphologists to suggest such a relationship, 
and the archetype seems to have been considered a mere ab- 
straction, a working hypothesis in definite architectural form, 
employed for the purpose of facilitating comparison. There is 
often indeed the idea that the archetype, non-existent in its per- 
fection, forms a divinely conceived plan upon which the 
Creator has modeled each member of a group of organisms, and 
that Man is able to grasp and understand this plan through his 
spiritual insight, a faculty akin to that of the Deity himself. 
Says Goethe, “ Sollte es denn eben unmoglich sein, da wir 
einmal anerkennen, dass die schaffende Gewalt nach einem 
allgemeinen Schema die vollkommeneren organischen Naturen 
erzeugt und entwickelt, dieses Urbild, wo nicht den Sinnen, 
doch dem Geiste darzustellen, nach ihm als nach einer Norm 
unsere Beschreibungen auszuarbeiten und, indem solche von 
der Gestalt der verschiedenen Thiere abgezogen ware, die 
verschiedensten Gestalten wieder auf sie zuriickzufuhren ? ”* 
This employment of an hypothetical archetype for the com- 
parison of organisms reached its culmination in the marvelous 
structure reared by the English anatomist, Sir Richard Owen, 
who first established his great fundamental conception of a 
typical vertebra, and then described in terms of this all the 
skeletal parts of every known vertebrate, including here not the 
vertebral column alone but the skull and appendicular skeleton 
as well. 
His diagram of a typical vertebra, reproduced here (Fig. 
179), shows that he, too, as well as so many others, conceived 
of the typical or primordial form as a symmetrical and perfect 
* “ But should it then be impossible, when once we recognize that the 
Creative Power has produced and developed the more completely organ- 
ized natures after a general plan, for us to represent this Archetype, if 
not to the senses, at least to the mind, to elaborate our descriptions in ac- 
cordance with it as with a norm, and, since such archetypes were taken 
from the forms of different animals, to refer the most varied forms back 
to it again?” Johann Wolfgang Goethe, “Uber einen aufzustellenden 
Typus zu Erleichterung der vergleichenden Anatomie,” 1796. 566 
THE HISTORY OF THE HUMAN BODY 
one; the Golden Age idea appearing in Anatomy, but here, as 
everywhere else, the reverse of actual history. Yet this dia- 
gram, although erroneous as an explanation of early con- 
ditions, represents in a clear manner the parts that appear in 
actual cases, especially in the higher forms, and as such has 
been taken as the foundation of our modern nomenclature. 
This typical vertebra consists of a cylindrical centrum, fur- 
nished with a neural arch, a hcemal arch and several lateral ele- 
FiG.179. Owen’s original diagram of a typical vertebra, to illustrate 
his theory of the archetype. [After Owen.] 
HS, neural spine; z, zygapophysis; np, neurapophysis; d, diapophysis; pi, pleura- 
pophysis; p, parapophysis; hp, haemapophysis; hs, haemal spine; h, haemal canal. 
ments. The neural arch consists of a pair of neurapophyses 
and a neural spine, and bears a pair of articular processes, the 
zygapophyses; and in like manner the haemal arch is composed 
of a pair of hcemapophyses, 3 hcemal spine and a second pair of 
zygapophyses. Of the lateral pieces the central ones are the 
plenrapophyses or rib elements, sometimes forming free ribs, 
and dorsal and ventral to these lie respectively the diapophyses 
and parapophyses, more occasional elements. THE ANCESTRY OF THE VERTEBRATES 
567 
From this typical vertebra Owen was able to explain the 
skeletal elements in each segment of the body in every verte- 
brate, and was thus able to construct the skeleton of an Arche- 
Fig. 180. Owen’s interpretation of mammalian skulls. [After Owen.1 
(A) Generalized mammal. (B) Man. For explanation see accompanying 
table given in text. (p. 569) 
type, which consisted of a series of such vertebrae gradually 
tapering and losing their characteristic features in the caudal 568 
THE HISTORY OF THE HUMAN BODY 
region; somewhat modified also at the anterior end, through 
the development of the brain and the introduction of the sense- 
organs. 
The skull was formed by four of these typical vertebrae, 
called nasal, frontal, parietal and occipital. The centra are rep- 
presented by vomer, presphenoid, basisphenoid, and basi-occipi- 
tal, the ncurapophyses by prefrontals, orbitosphenoids, alisphe- 
noids and exoccipitals, and the neural spines, composed mainly 
of paired pieces, by the nasals, the frontals, the parietals and the 
supraoccipital. The postfrontal was the diapopliysis of the 
frontal vertebra, and the mastoid that of the parietal. Pleura- 
pophyses were represented by the palatine, which belonged to 
the nasal vertebra, the tympanic, which belonged to the fron- 
tal, the stylohyals, parts of the parietal vertebra, and the 
suprascapula and scapula, which were reckoned with the oc- 
cipital. The hcemopophyses were respectively represented by 
the maxillaries, the articularia, the ceratohyals and the cora- 
coids, and the hcemal spines by the premaxillaries, the dentaries, 
the basihyals and the episternum. Other parts, such as the 
squamosal, the thyreohyal, and the free limb, formed ele- 
ments called appendages. 
Never was there a more stupendous result of the labor 
of a single human life than this great work of Owen, and yet 
of the entire structure reared by his incessant toil all that re- 
mains is the large amount of accurate description and a great 
enrichment of osteological nomenclature. It was a house built 
upon the sand, and Owen’s “ typical vertebra ” may be placed 
alongside of Goethe’s “ Urbild ” as the noble attempt to picture 
the great truths which they felt in spirit and saw but dimly. 
The formulation by Charles Darwin in 1859 of the doctrine 
of animal descent, with the implied conception of actual blood 
relationship between the different groups, introduced into 
The theory may be further elucidated by the help of the following table 
and by the accompanying diagrams (Fig. 180, A and B). The small letters 
added to the names of the separate elements in the table correspond to 
those used in the diagrams, so that the former may be used to explain the 
latter. THE ANCESTRY OF THE VERTEBRATES 
569 
Name of Vertebra 
Neural Spine 
N EUR APOPHYSIS 
Diapophysis 
Centrum 
PleurAPOPHY- 
SIS 
Hjemapophysi 
IDemal Spine 
Appendage 
I. Nasal 
nasal 
P 
prefrontal 
1 
h 
vomer 
d 
palatine 
t 
maxillary 
X 
premaxillary 
z 
jugal 1 
pterygoid 2 
squamosal 8 
II. Frontal 
frontal 
o 
orbito- 
sphenoid 
k 
postfrontal 
g 
presphenoid 
c 
tympanic 
s 
articulare 
of mandible 
w 
dentary 
of mandible 
w 
(wanting in 
mammals) 
III. Parietal 
parietal 
n 
alisphenoid 
j 
mastoid 
f 
basisphenoid 
b 
stylohyal 
r 
epihyal 
ceratohyal 
V 
basihyal 
y 
thyreohyal 
4 
IV. Occipital 
supraoccipital 
m 
exoccipital 
i 
paroccipital 
process 
e 
basioccipital 
a 
suprascapula 
scapula 
q 
coracoid 
u 
bones of arm 
and hand 570 
THE HISTORY OF THE HUMAN BODY 
anatomical speculation that necessary principle which had been 
lacking- in the philosophy of pre-Darwinian anatomists, and 
from that time on actual ancestors took the place of theoretical 
archetypes. It became then of fundamental importance to 
establish the true interrelationships of the various animal 
groups, that the structure of a given form might be explained 
in terms of the ancestral structure of which it might be consid- 
ered a modification. 
Naturally the intensest interest centered about the establish- 
ment of the group from which the vertebrates were derived, 
and here for a long time the most of the speculation followed 
the lines laid down by St. Hilaire with his reversed insect. 
Certainly one of the most characteristic features of a verte- 
brate, and one of the earliest to appear in the embryo, is the 
division of the body into somites, and the search for a bilateral 
segmented ancestor must inevitably lead back to the articulates, 
which alone of the invertebrates emphasize this characteristic to 
an equal degree. It is true that in the two groups the arrange- 
ment of the internal organs is in the main reversed, for in the 
one the central nervous system is ventral and the main blood- 
vessel dorsal, and in the other the former is dorsal and the lat- 
ter ventral; but the device employed by St. Hilaire to explain 
this is by no means an absurd one, since what is called dorsal 
or ventral in a given animal is merely its constant physiological 
relation to the surface of the earth, and in several cases, like the 
flounder and the squid, is known to be quite at variance with 
the condition usual in related forms. 
Thus by postulating the occurrence of a change quite in ac- 
cord with several known instances the differences in the rela- 
tionships of the different systems in the simplest members of 
both groups (annelids and selachians) may be brought into 
almost complete harmony (Fig. 181, a and c). Even the noto- 
chord, perhaps the greatest problem of vertebrate structure, 
may be compared to the “ Faserstrang,” a bundle of fibers run- 
ning along the nerve chain and serving as a support. This 
and the notochord lie in a precisely similar position in relation 
to the other organs, and in both cases they are enclosed with THE ANCESTRY OF THE VERTEBRATES 
571 
Fig. 181. Figures illustrating the Annelid theory. [After Semper.] 
(a) Cross section of Annelid (reversed). (b) Longitudinal section of Annelid, 
(c) Cross section of Selachian. (d) Longitudinal section of Selachian in region of kidney. 
n, nerve cord; nc, notochord; g, “ faserstrang ” of Annelid; sh, sheath surrounding nerve 
cord and notochord; d and v, muscle masses; en, intestine; nph, nephridium; <} and h, 
longitudinal blood vessels. 572 
THE HISTORY OF THE HUMAN BODY 
the nerve cord in a common sheath of connective tissue. In the 
blood system there are equal points of resemblance, for in 
each case there are two median longitudinal vessels, one on 
either side of the intestine. In the vertebrate the dorsal one 
is the aorta, which sends the blood in a posterior direction, 
while the ventral one, with a current in the reverse direction, 
is represented by the embryonic subintestinal vein posteriorly 
and by the heart anteriorly. In an unreversed annelid it is 
true that the dorsal blood-vessel sends its blood from tail to 
head, while in the ventral one the blood flows from head to 
tail, but by reversing the animal the correspondence in the 
direction of the current becomes complete. The vertebrate 
aorta is then represented by the original ventral (now dorsal) 
vessel in which the current flows backwards, and the sub- 
intestinal vein and heart are represented by the original dorsal 
(now ventral) vessel, in both cases with the current directed 
forwards. 
The most convincing of the many correspondences, how- 
ever, lies in the nephridial system of annelids and selachians, 
which in both cases consists of segmentally arranged pairs of 
tubes that open into the coelom at their inner ends by ciliated 
nephrostomes. There are also close correspondences in the 
relation of these tubules to the germ glands and to the coelom. 
The more primitive type may be considered to be that found 
in annelids, in which each somite possesses a separate coelom, 
or rather a pair of coeloms, since those of adjacent somites are 
separated by tranverse dissepiments, and those of the two 
sides of the same somite by median sagittal partitions, which 
form dorsal and ventral mesenteries. Each of the compart- 
ments thus formed is supplied with a single nephrostome, 
the tubule from which pierces the dissepiment and enters 
the next posterior coelomic pocket, where it exhibits a con- 
voluted portion and a vesicular enlargement, and eventually 
opens directly into the exterior by a lateral opening. (Fig. 
181, a and b.) The germ glands develop on the anterior walls 
of the coelomic cavities, and the germ cells become free and 
float about in the coelomic fluid until they are taken up by the THE ANCESTRY OF THE VERTEBRATES 
573 
nephrostome and find their way to the exterior, through the 
nephridium, which thus serves as ductus deferens or oviduct. 
Typically, as in the diagram, a germ gland belongs with each 
lateral coelomic cavity, but in actual cases they develop in 
only a few segments, and the associated nephridia become 
especially modified for the conveyance of the germ cells from 
the body. 
If, now, the primitive vertebrate nephridia, germ-glands and 
coelom, as described in Chapter XI above, be compared with 
the annelid condition, the similarities are found to be remark- 
able. Here also the nephridia are at first strictly segmental, 
although they no longer open to the outside independently, 
but through the medium of a common Wolffian duct. Since, 
however, this develops in part from the ectoderm, it may have 
begun as a simple external trough-like depression which ran 
along the sides of the animal and connected the several indi- 
vidual openings for the better disposal of their secretion. The 
segmental subdivisions of the coelom are no longer continued 
in the higher vertebrates but the mesodermic diverticula, 
which appear clearly in Amphioxus, and in a more imperfect 
manner in the others, suggest the former presence of trans- 
verse dissepiments, and both dorsal and ventral mesenteries 
actually persist as far back as the posterior boundary of the 
liver, beyond which the ventral one disappears. Nor can it 
be said that the transverse dissepiments are wholly lacking, 
since, although they no longer divide the coelomic cavity, they 
are still represented in the body wall by the intermuscular 
septa (myocommata) with which the nephridia sustain in the 
embryo similar relationships as in annelids (Fig. 181, d). In 
both cases the germ glands arise as localized portions of the 
coelom (peritoneum), and the presence of a single pair in the 
true vertebrates may be correlated with the confluence of the 
several coelomic cavities into a single one. The larger number 
of gonads in Amphioxus suggests the former presence of a 
much larger number of coelomic cavities. That in vertebrates 
as in annelids the nephridial system is made to furnish chan- 
nels of exit for the germ cells has been already shown 574 
THE HISTORY OF THE HUMAN BODY 
(Chapter XI), and the opening into the oviduct has been 
homologized with a pronephridial nephrostome, while in the 
male the entire anterior portion of the mesonephros and its 
duct becomes utilized for the passage of the seminal fluid. 
One of the most fundamental characteristics of vertebrates 
is the presence of paired gill-slits, extending in two lateral 
rows along the pharyngeal region and forming direct com- 
munications between the pharynx and the exterior; these may 
be readily derived from nephridia by supposing, first, that the 
inner ends of these tubes become secondarily connected with 
the pharyngeal lumen, and secondly, that the tubules become 
reduced in length until ectoderm and endoderm come in con- 
tact. Only in some such way can one explain the embryonic 
development of gill-slits from a series of ectodermic inpush- 
ings that meet a similar series of endodermic outpushings, a 
mechanical process that necessitates some reason back of that 
which is apparent in order to account for the accuracy with 
which these several folds meet one another. In the embryo of 
the cyclostome Myxine, precisely the form where we would 
look for the retention of the earliest phases, there still appears 
at first a fairly long canal between each ectodermic inpush- 
ing and its endodermic associate, perhaps a remnant of the 
nephridial tube. It may also be more than a coincidence that, 
when genuine nephridia of the pronephric system appears im- 
mediately posterior to the gill region, none arise in the somites 
that develop the gill-slits. 
Important changes seem to have taken place in both out- 
lets of the alimentary canal, and indications show that verte- 
brates have acquired both a new mouth and a new anus, 
although they still retain in the embryo many traces of the 
older organs. That the mouth of the vertebrates is not the 
primitive one is shown by a variety of indications, one of the 
strongest being its exceptionally late appearance in embryonic 
life. A mouth is one of the most essential of organs, and in 
other animals, corresponding to its important function, is one 
of the first to appear. In vertebrates, however, the reverse is 
true; the nervous system is laid down, the brain is differenti- THE ANCESTRY OF THE VERTEBRATES 
575 
ated, the notochord is formed, even the special sense-organs 
appear, and still the alimentary canal remains a sealed cavity, 
without communciation with the exterior. At last a mouth 
appears, placed very far ventrally, in line with the gill-slits, 
and in certain fishes appears first as two lateral openings which 
eventually become confluent. All this seems a complete cor- 
roboration of the fact arrived at independently through adult 
anatomy that the vertebrate mouth has resulted from the con- 
fluence of a pair of gill-slits anterior to those now functioning, 
and still equipped with gill-arches which serve as jaws. There 
comes, then, the inevitable conclusion that previous to this 
conversion, and while the mandibular slits were still function- 
ing as gill-slits, the ancestral forms must have had another 
mouth, traces of which are to be looked for in the earlier em- 
bryo. Such a primary mouth is actually indicated in pre- 
cisely the place where it would be looked for in the annelid, 
taking the reversal of the body into account, and this indica- 
tion appears in the widely open rhomboid fossa of the nerve 
cord. 
In annelids, as in all Articulata, the mouth is upon the ven- 
tral side, and, since the alimentary canal is dorsal to the nerv- 
ous system, this position is reached by means of an oesophagus, 
which turns downwards almost at right angles to the re- 
mainder of the canal, and runs between the two nerve con- 
nectives that connect the first and second pairs of ganglia. 
The first ganglionic pair thus becomes the supra-cesophageal, 
the second, injra-oesophageal, and the connections between 
them form a circum-oesophageal ring through which the 
oesophagus passes. These relationships will be clearly seen 
by reversing the accompanying figure (Fig. 182), which will 
thus give the conditions as seen in annelids. In all true verte- 
brates the actual external opening of this early mouth has 
disappeared, but it may be identical with the neuropore in the 
embryo of Amphioxus, which forms in this place a direct com- 
munication between the lumen of the neural tube and the ex- 
terior and is otherwise unaccounted for. Aside from the in- 
dications of the early mouth and its oesophagus, furnished by 576 
THE HISTORY OF THE HUMAN BODY 
rhomboid fossa and neuropore, there is also the hypophysis, 
or rather, that portion of it that is pushed up from the ali- 
mentary canal, for which there is yet no satisfactory explana- 
tion. Its origin from the alimentary canal, its constant ap- 
pearance in all vertebrates, its relationship to the nervous 
system and its position, all suggest that it also is a remnant 
of an early oesophagus. 
The formation of a new, ventrally placed anus is due to a 
procedure similar to that which forms the new mouth, al- 
though there is here no suggestion of a gill-slit. The verte- 
brate anus arises as a mid-ventral inpushing of the ectoderm at 
some distance from the end of the tail, and thus reaches the 
OrrXMNV 
VERTEBRATE 
Reversible designations, applying to both forms: S, brain; X, nerve cord; 
H, alimentary canal. Designations applying to Annelid only: m, mouth; a, anus. 
Designations applying to Vertebrate only; st, stomatodaeum; pr, proctodaeum; 
nt, notochord. 
Fig. 182. Reversible diagram illustrating the Annelid theory. 
primary intestine along its course, leaving beyond it a piece of 
considerable length, the post-anal gut, which soon atrophies. 
This phenomenon, inexplicable by other means, is easily ex- 
plained by the postulate of an annelid ancestor, for in these 
animals the anus is at the posterior extremity of the body, and 
the formation of a new anus in the vertebrate position would 
actually leave just such a piece as the one in question. 
The gills of aquatic vertebrates receive also an adequate 
explanation through the annelid hypothesis. The annelid gills 
are external duplicatures of the integument, and occur upon 
the sides of every somite, attached to the parapodia, or short 
locomotor organs. In simple forms they are plates, but when 
specialized they become fringed or dendritic and somewhat 
resemble the external gill-bushes of amphibians. Although 
primarily distributed along the entire body, in certain special- 
ized forms they are confined to the anterior end. Gills of this THE ANCESTRY OF THE VERTEBRATES 
577 
sort are well adapted to slow-moving or crawling forms, but 
when there is a necessity for the development of rapid motion, 
as is indicated for the direct ancestor of the rapidly moving 
fishes, such gills, especially if long and fringed, would tend, 
to retard the motion. It would thus be natural to consider 
that they might wander within the openings of the nephridia, 
which in annelids lie close to these external gills, and this 
relationship gives, in its turn, the motive for the secondary 
connection of such nephridia with the alimentary canal, in 
order to supply the gills with a current of water. 
The increase in the size of the gills would tend to develop 
some firmer tissue at their base to support them, and in this 
way there may have been developed a series of cartilaginous 
arches, which, together with the gills themselves, may have 
been at first and for a long time coextensive with the body 
itself, or have extended at least as far as the anus. When at 
a later period the gills became restricted to a few anterior 
pairs while the rest atrophied, the arches accompanying the 
former would be the persistent gill-arches, and form the vis- 
ceral skeleton of vertebrates, while the remainder, freed from 
their gills, and repeating themselves metamerically, might be- 
come the ribs. It is even permissible to conceive of the limb 
skeletons as further derivatives of the metameric system of 
gill-arches; perhaps also the original elements of the pri- 
mordial skull, the trabeculae and parachordals, may be traced 
to the same source, thus accounting for all parts of the skele- 
ton save the dermal bones, which are integumental, and the 
notochord, which has already been accounted for. 
Convincing as these comparisions seem when taken by 
themselves, the influence of later investigations has tended 
rather away from the annelid hypothesis, and at present, al- 
though there are many investigators who seek the ancestors 
of vertebrates in some worm-like form, there are few who 
wish to definitely assert that this ancestor was an annelid. 
The annelid theory rests largely upon the definite body seg- 
mentation of both these animals and vertebrates, yet segmen- 
tation is not in itself as fundamental a character as would 578 
THE HISTORY OF THE HUMAN BODY 
appear at first, and may be easily acquired by an animal 
group in any one of several different ways. It is likely, for 
example, that such a segmentation as that possessed by verte- 
brates may have been gained through the muscular action 
of a previously unsegmented form, and the fact that in verte- 
brate embryos the segmentation first appears in the mesoderm, 
from which the muscles are derived, furnishes a strong support 
for this view. The oldest of the annelids, on the other hand, 
begins life as an unsegmented larva, upon which the somites 
become developed one after another through a sort of bud- 
ding, a process totally unlike that in which the vertebrate 
initiates its segmentation. 
A second group of vermian forms from which the verte- 
brates may have developed is that of the nemerteans, a group 
of mainly marine worms, of uncertain affinities, but probably 
allied to the platyhelminths (flat-worms). Here the nervous 
system is not a ventral one, but consists of two lateral cords 
imbedded in the body wall, and often a smaller mid-dorsal 
cord, the three being bound together by commissural nerves 
which run around the animal (Fig. 183, A). A branching 
intestinal nerve proceeds from one of these and is distributed 
to the sides of the intestines; and from the ventral portion 
of some of the anterior commissural nerves small nerve 
branches appear, also distributed to the intestinal wall. The 
anterior end of each lateral nerve is enlarged into a ganglion, 
from which a few nerves proceed anteriorly. 
The manner in which such a nervous system may become 
converted into that of a typical vertebrate may be readily 
seen by a comparison of A and B of Fig. 183, the first of 
which has already been referred to. Of the three longitudinal 
nerves the dorsal one may have become the central nervous 
system, and may have expanded its anterior end into a brain, 
while the two lateral nerves may have become subordinated 
to it, but persist in lower vertebrates as the lateral nerves of 
the vagus system, rami laterales X, with which the long in- 
testinal nerve is also associated. The original ganglion of the 
lateral nerves may break up into the various ganglia found THE ANCESTRY OF THE VERTEBRATES 
579 
in vertebrates, in association with the cranial nerves. As for 
the commissural nerves, they may become alternately sensory 
and motor in function and associate together in pairs, forming 
the metamerically arranged spinal nerves of vertebrates, the 
elements of which are in lower forms still separate and issue 
from the neural canal through separate foramina. Lastly a 
Fig. 183. Nemertean theory of the origin of Vertebrates. [After 
Hubrecht.] 
(A) Typical diagram of Nemertean. d, dorsal nerve cord; gl, ganglion; It, 
lateral nerve cord; v, intestinal nerve; sb, small intestinal branches. 
(B) Typical diagram of Vertebrate; db, brain; d, dorsal nerve cord; s, sensory, and 
m, - motor spinal nerves; gl, sympathetic ganglia; v, ramus intestinalis vagi; It, ramus 
lateralis vagi; sb, sympathetic branches. 
sympathetic system may be formed by collecting together the 
small intestinal branches that come from the ventral portions 
of the commissural nerves. 
Concerning the other systems it is only fair to say that their 
correspondence is by no means as close as is that of the 
nervous system, although a characteristic nemertean struc- 
ture, the proboscis, has been likened to the hypophysis, while 
its sheath, into which it may be retracted, has been cited 580 
THE HISTORY OF THE HUMAN BODY 
as possibly furnishing material for the notochord. Attention 
has also been called to the respiratory function of the anterior 
portion of the intestinal canal in nemerteans. 
Aside from all hypotheses which have at their basis the 
consideration of a worm-like ancestor may be briefly men- 
tioned a recent theory which finds the vertebrate ancestor 
among the more primitive arachnoids, now represented by 
such animals as the scorpion and the horse-shoe crab 
(Limulus) and formerly exhibited by the extinct group of 
Merostomata. To appreciate this one must at the outset 
dispose of the cyclostomes and other low forms like Amphioxus 
as degenerate and without special significance, and take as 
the starting point of vertebrates such forms as the ganoids, or 
more especially the placoderms, which lived in Devonian times 
and were contemporaries of certain aquatic arachnoids, allies 
of the horse-shoe crab. 
As the starting point in this theory there may be taken a 
certain series of resemblances between the brain and cranial 
nerves of vertebrates and the fused cephalo-thoracic ganglionic 
mass found in such arachnoids as the scorpion and the horse- 
shoe crab. In these forms this central mass is divisible into 
three distinct portions, comparable to fore-, middle- and hind- 
brains, with an accessory part corresponding to the medulla. 
The number of neuromeres, or primary nerve somites of which 
these parts are composed, i.e., 3-1-5 for the brain and 2 to 4 
for the medulla, also corresponds closely with the conclusion 
of many specialists concerning the segmental values of those 
parts of the head in vertebrates. A similarly suggestive re- 
semblance exists in the cranial nerves and the relations of 
the organs of sense. 
Although the anatomy of the soft parts of the Merostomata 
will never be known, they could not have been very different 
from the condition found in Limulus and the scorpion, and it 
may even be supposed that they and modern vertebrates have 
developed in distinctly different directions from these as com- 
mon ancestors, and that thus their condition may have been 
far more like that of the vertebrates than is that of any of THE ANCESTRY OF THE VERTEBRATES 
581 
the arachnoids now living. Aside from the nervous system, 
numerous other parts are more or less comparable. For ex- 
ample, the eyes of the Merostomata consisted of a pair of 
widely divergent lateral eyes, and a pair of closely approxi- 
mated median eyes, a peculiarity seen in the present-day 
Limulus; in the vertebrates there are the same widely diver- 
gent lateral eyes, and the median pair is well represented by 
Fig. 184. Comparison of heart and gill arches of (a) Arachnoid, and 
(b) Vertebrate. [After Patten.] 
the pineal eye, in the development of which there are many 
suggestions of its having been double at an earlier period. 
In Limulus the central nerve-complex is protected by an in- 
ternal skeletal piece, variously called “ sternum ” and “ endo- 
cranium,” which in general form and still more in its rela- 
tionships to other parts resembles the primordial skull of 
vertebrates. The arterial system shows in each case a tubular, 
somewhat contorted heart, an arterial trunk proceeding anteri- 
orly from this, and dividing into pairs of arterial arches, from 
which, after sending a branch to the head, the blood is re- 582 
THE HISTORY OF THE HUMAN BODY 
collected into two lateral channels, in the one case a sinus, 
in the other a definite vessel. This relationship suggests a cer- 
tain homology between the appendages of Limulus and the 
gill-arches of the vertebrates, and as a matter of fact it has 
been suggested that the second pair of arachnoid appendages, 
so largely developed in the scorpion, represent the mandibular 
arch, and that the next pair, or perhaps the next two, may 
Fig. 185. Comparison between extinct Crustacean and Vertebrate. 
(A) Gigantostracan, Eurypterus [after Niedzkowsky]. (B) Placoderm, Pterichthys 
[after Neumayer]. 
represent the hyoid. The vertebrate gills themselves seem, 
however, more nearly comparable in structure with the gill- 
plates or gill-books of modern arachnoids, and their more 
posterior position in those latter is accounted for through a 
backward migration, since the nerves supplying them are pre- 
cisely the ones which, for other reasons, have been compared 
with Vagus elements. 
Perhaps one of the strongest points in this erratic theory 
lies in the fact that the vertebrates are here derived, not from THE ANCESTRY OF THE VERTEBRATES 
583 
a primitive segmented form, like an annelid, in which the 
somites are much alike, but from one in which the differentia- 
tion of the head somites and their groupings to form complexes 
has already progressed quite far, and apparently along the 
same line. The external resemblance between the heavily 
armored placoderm fishes and their contemporaries among the 
arachnoids is certainly striking, and the similarity extends also 
in the head region to the plates of which the shell or cephalo- 
thorax is composed (Fig. 185). 
Without subjecting the arachnoid theory to further com- 
ment other than to say that it has received very little recog- 
nition, we may pass to that theory which places especial 
weight upon the notochord, the gill-slits and the dorsal posi- 
Fig 186. Amphioxus. 
o, oral hood with cirrhi; x, mouth; g, gonads; y, atriopore; z, anus; t, caudal fin: 
f, dorsal fin. 
tion of the central nervous system, and by means of these has 
traced the line of vertebrate ancestry through a series of 
invertebrate forms, externally very unlike one another, and 
each somewhat isolated in' its systematic position. The first 
of these in descending series, and representing in a way a 
simplified vertebrate, stripped of everything save the essentials, 
is the now famous Amphioxus [or Branchiostoma], first de- 
scribed in 1778 as a shell-less snail, or slug (Limax lance- 
olatus). It is a shore form, and, with a few specific differ- 
ences, occurs on almost all coasts. It is one or two inches 
in length, flattened laterally, and pointed at both ends; it is 
thus without a distinct head, but the mouth, which is situated 
a little ventrally at the anterior end, is equipped with a mem- 
branous expansion in the form of a hood. The adult burrows 
perpendicularly into the sand, leaving exposed only the an- 
terior end, and in this temporarily sessile condition it expands 584 
THE HISTORY OF THE HUMAN BODY 
the oral hood and collects the debris that drifts past it, much 
after the manner of non-locomotive forms. The larva is, 
however, actively free-swimming, and the adult often changes 
its locality and thus retains the power of rapid motion. 
A striking external feature of Amphioxus is the regular seg- 
mentation of its muscular system, which shows through the 
transparent skin and is marked by lines formed by the myo- 
commata, each bent in the form of a V, the point directed 
forwards. The reproductive organs or gonads, also, are often 
sufficiently developed to appear through the skin as a suc- 
cession of square or slightly rounded masses, each correspond- 
ing to a segment, yet with those of the two sides placed 
alternately, as is also the case with the myomeres and the 
other lateral parts. There is a slightly indicated median fin, 
supported by minute fin rays, and extending along the entire 
back, around the tail and upon the ventral side considerably 
past the anus, throwing this latter opening out of the median 
line, and dislocating it to the left. Anterior to this and at 
the termination of the fin is a large and conspicuous opening, 
the atriopore, through which the water that is continually 
taken in at the mouth is as continually expelled. The chamber 
from which the atriopore leads is termed peribranchial, and 
appears at first like an internal cavity; a little investigation 
shows that it is in reality external and develops in the larva 
from two longitudinal folds that arise along the sides and 
grow together ventrally. The region of the body which they 
enclose is perforated laterally by a very large number of 
obliquely placed gill-slits, communicating with the pharynx, 
so that the water taken in at the mouth passes through these 
slits and thus enters the peribranchial chamber, from which it 
is finally expelled through the atriopore. A similar device is 
found in frog and toad tadpoles, and the external outlet, here 
called the “branchipore,” varies in position in the different 
genera, but appears quite high up on the left side in the true 
frogs (Rana). As the peribranchial chamber develops during 
larval life, the gill-slits of young larvae open directly to the 
outside, as in true vertebrates, and suggest that, as in the THE ANCESTRY OF THE VERTEBRATES 
585 
frog tadpole, this chamber has been developed as a special 
adaptation to the needs of this particular animal, and is thus 
not ancestral in character. 
Where the two lateral folds that form the peribranchial 
chamber come together they unite in such a way as to leave 
their original edges in the form of a pair of parallel metapleural 
folds, and it is quite possible that either these, or more prob- 
ably the original folds before they extended far enough to 
unite ventrally, are identical with those folds from which, at 
some period in the long history between Amphioxus and the 
fishes, the two pairs of limbs were originally derived. [Cf. 
Chapter VII.] 
Of the internal organs the most conspicuous is the noto- 
chord. This is an elastic skeletal rod of gelatinous tissue, 
surrounded by a firm connective tissue sheath. It extends 
to the extreme ends of the animal and insures to it a certain 
grade of rigidity while allowing the body to be extremely 
flexible. Lying along the dorsal aspect of this rod, and en- 
closed in a sheath which is continuous with that of the noto- 
chord, is the dorsal nervous system, closely resembling that 
of fishes, but with no brain other than a slight club-shaped 
enlargement at the anterior end, the archencephalon. An 
olfactory pit on the left side, and a median pigment spot, are 
its only definite sense-organs. Beneath the notochord lies the 
alimentary canal, which expands beyond the mouth into a 
pharynx, that extends more than half the length of the body 
and passes into a straight intestine with no especial differ- 
entiation of parts. The pharynx is perforated by 60-80 pairs 
of narrow gill-slits, placed obliquely, and kept open by an 
elaborate system of skeletal rods, formed of a material re- 
sembling chitin, and thus more like an invertebrate than a 
vertebrate structure. Both the elaborateness of this skeletal 
system and the very large number of gill-slits are plainly 
secondary modifications, like the peribranchial chamber, 
since they are not found in the larva, and mark Amphioxus 
as a much modified form, probably that one out of a large 
group, which survived on account of these very modifications. 586 
THE HISTORY OF THE HUMAN BODY 
A characteristic structure runs along the floor of the 
pharynx in the form of a ventral groove. This is the endo- 
style, an organ that performs a very important service in the 
collection and retention of food. Its surface is covered with 
cilia, which produce a backward directed current, and it is 
furnished also with gland cells that secrete a viscous fluid. 
The nutrient particles that enter the pharynx in the water 
current are engaged by the viscous fluid, and the combined 
mass is conveyed to the intestine by the motion of the cilia. 
The main blood-vessels consist of a sub-intestinal vein, ven- 
tral to the alimentary canal, in which the current is directed 
forwards, and an aorta, situated between the intestine and 
the notochord, in which the blood flows posteriorly. Aside 
from these there is a series of branchial vessels, a pulsating 
heart, and other vessels, the relations of which are like that 
found in vertebrates, but simpler. 
The general impression given of the structure of Amphioxus 
is that of a diagrammatic vertebrate modified by several sec- 
ondary adaptations. Had it been known to Goethe, he would 
have almost denominated it the realization of the primordial 
vertebrate, an incarnate “ Urbild.” In the general arrange- 
ment of the essential organs, the dorsal nervous system in 
tubular form, the notochord, the dorsal aorta, the intestine 
with its pharynx perforated by gill-slits, it is essentially verte- 
brate: while in its peribranchial chamber, and its great multi- 
plication of gill-slits it suggests the successful attempt of an 
animal to survive through the power of acquiring secondary 
adaptations, the only one out of a large group that has come 
down to us. The segmentally arranged muscles are essentially 
vertebrate, and a series of rather complex nephridia in the 
gill region may be homologous with a pronephros. Even those 
mysterious organs of true vertebrates, the thymus, thyreoid, 
and epithelial bodies, the history of which begins in the 
cyclostomes with a series of segmental anlagen, are probably 
seen here in an earlier stage, for it has been suggested that 
the endostyle is the homologue of the thyreoid series, and the THE ANCESTRY OF THE VERTEBRATES 
587 
numerous thymus anlagen have been rather hesitatingly 
identified with certain elements of the gill skeleton. 
There seems thus no doubt that in Amphioxus we have a 
genuine, although somewhat modified, ancestor of the verte- 
brate group, the sole survivor of a lost race, the more typical 
members of which were probably simpler and more like the 
true vertebrates than is their present-day representative. This 
conclusion serves, however, only to put the question one 
stage further back, and we now ask it in this form: What is 
the relation of Amphioxus to vertebrates? In order to sum- 
marize the most fundamental characteristics of Amphioxus, 
those should be selected which it possesses in common with 
the vertebrates, since characters possessed by Amphioxus 
alone may, with great probability, be considered secondary 
modifications, and thus unrepresented in their ancestors. 
Thus reduced, search should be made for an animal with a 
notochord, a dorsal nervous system, and a pharynx perforated 
by gill-slits, and possessed of a mid-ventral endostyle. Some 
have included among the essentials a pronounced segmenta- 
tion, that shows itself in the muscular and nephridial systems, 
but a study of segmentation in general leads to the opinion 
that the segmentation of an animal, either expressed in the 
body-wall, or by the repetition of some of the organs, is not 
a fundamental character, but one easily acquired, and thus 
is not to be considered necessarily as an essential character- 
istic of the group from which Amphioxus and its allies have 
come. 
The direction of the search thus defined, one is led in- 
evitably to another isolated and problematic group of inverte- 
brates, which have been variously classified among molluscs, 
worms, and other comprehensive and noncommittal groups, 
which have had, in short, about the same treatment as that 
accorded to Amphioxus. This group differs from the latter, 
however, in that, although isolated, it is extremely rich in the 
species still extant, and is represented by forms so variously 
modified, and so widely different from one another, that they 
form several Orders and numerous Families, thus showing all 
possible modifications of their fundamental plan. 588 
THE HISTORY OF THE HUMAN BODY 
This group is that of the Tunicata, a Class of marine ani- 
mals, some of which are free-swimming throughout life, while 
others, although active as larvae, soon settle to the bottom and 
become sessile. Contrary to expectation, it is the latter 
which in general have retained the more primitive character 
istics, while the free-swimming ones are often greatly modi- 
fied. In a typical sessile tunicate, like the one shown in Fig. 
187, a, the delicate and rather complex organism is shut within 
(a) External view, (b) Diagram of internal anatomy. 
IN, incurrent; and EX, excurrent canals; Ph, pharynx; INT, intestine; An, 
anus; CL, cloacal chamber; Tun, tunic; Integ, integument; End, endostyle; GL, 
ganglion; G, gonadic gland; GD, gonadic duct; GO, gonadic opening; H, heart. 
Fig. 187. Typical Tunicate. 
a tough and often wrinkled external coat or tunic, in which 
there appear but two definite structures, an incurrent and an 
excurrent orifice, through which the water is continually driven, 
much as in sponges. The incurrent orifice, which is really 
the mouth, leads into a capacious pharynx, and this continues 
into an intestine which, as usual in sessile forms, becomes 
bent upon itself and opens by an anal orifice not far from the 
pharynx. The anus opens, not directly to the exterior, but into 
a cloacal chamber, which is really outside of the animal but THE ANCESTRY OF THE VERTEBRATES 
589 
enclosed within the outer tunic. This same chamber sur- 
rounds the pharynx and receives the water taken into the 
latter through a series of gill-slits, which, in larvae, are few in 
number and disposed in lateral rows, but which usually be- 
come almost indefinitely multiplied, and in many cases trans- 
form the entire pharyngeal wall into a structure resembling 
basket-work. Thus the cloacal chamber, except for the re- 
lation of the anal outlet, is precisely similar to the peribranchial 
chamber of Amphioxus, the excurrent orifice being the equiva- 
lent of the atriopore. The pharynx in the two animals is also 
similar in the presence and secondary multiplication of its 
gill-slits, and here also there is an endostyle, which lies along 
that side of the pharynx which in the free-swimming larva is 
ventral. The nervous system consists of a single ganglion, 
placed in the adult between the two arms of the U-shaped 
intestine, but dorsal to it in the larva, and the vascular sys- 
tem is represented by a ventrally placed heart. The repro- 
ductive gonads lie in the bend of the intestine, and open by 
ducts of their own into the cloacal chamber. 
These numerous suggestions of an organization akin to that 
of Amphioxus which are noticeable in the adult are far more 
apparent in the larva (Fig. 188, a). In this stage the ani- 
mal is tadpole-like and possesses a long tail, flattened lat- 
erally and provided with a typical notochord, similar in its 
development to that of Amphioxus. A series of segmentally 
arranged muscles, separated by myocommata, is also found 
here, especially developed posteriorly. The nervous system 
appears in the form of a prolonged neural tube and expands 
at its anterior end into a sensory vesicle (brain) which is pro- 
vided with a pigment speck that forms a primitive eye, and 
a somewhat problematic organ usually interpreted as an ear, 
although both organs are exceedingly simple in construction. 
In this larval condition the gill-slits are few in number and 
simple in arrangement, and the endostyle is in the proper po- 
sition for comparison with that of Amphioxus. The heart 
also lies ventrally and just posterior to the oesophagus. 
The changes that take place during the assumption of the 590 
THE HISTORY OF THE HUMAN BODY 
Fig. 188. Development of Tunicate from free-swimming larva. [After 
Seeliger.] 
p', p, adhesive suckers; Br, brain; C, nerve cord; 1NT, intestine; H, heart; End, 
endostyle; St, stolo; n, notochord; my, myotomes; t, tail. The orientation is in* 
dicated by arrows, which mark the incurrent and excurrent orifices. THE ANCESTRY OF THE VERTEBRATES 
591 
sessile position are shown in Fig. 188, b and c, and are ex- 
plicable by assuming a twist or rotation of the animal towards 
the left after fixation, by means of two papillae of attachment. 
During this metamorphosis, which is a regressive one, the 
tail, with its notochord and segmented muscles, becomes lost, 
and the central nervous system becomes much reduced. The 
posterior end of the intestine connects with the cloacal cham- 
ber and the adult relationships are gained. 
From this sketch of the organization and metamorphosis 
of the tunicates it is evident that the group is somewhat closely 
related to Amphioxus, and hence to the vertebrates, but that, 
since the time of the common ancestor, the Tunicata have fol- 
lowed for a long distance a divergent road of special adapta- 
tion, which, although it has allowed them to continue exist- 
ence, has been one of degeneracy and loss and has masked 
their true relationships. The ancestor that we here seek is 
better seen in the larva than in the adult, and we may believe 
that there once existed an adult animal with attributes like 
that of the tunicate larva of the present day, and that this ani- 
mal was the direct ancestor of that group of which Amphioxus 
is now the only living representative. 
Is there now any record of the history of vertebrate descent 
back of the tunicate ancestor? Is there any other invertebrate 
animal possessed of gill-slits, a notochord, and a dorsal nerv- 
ous system? And as an answer to this, a very incomplete 
and uncertain one at best, there is only a single animal form, 
though represented by several closely allied species, an animal 
as unpromising in its exterior, and here, perhaps, almost as 
much so in its interior also, as those hitherto considered. 
This form is Balanoglossus, a marine worm that lives in self- 
constructed tubes of sand between tide-waters, and, like 
Amphioxus, has a wide distribution. Externally the animal 
is worm-like and is possessed of four body regions, a conical 
proboscis, a collar with a free anterior edge, a flattened gill 
region and a cylindrical posterior part. The mouth is situated 
ventrally, immediately beneath the edge of the collar, and 
receives the sand mixed with nutrient material as it becomes 592 
THE HISTORY OF THE HUMAN BODY 
unearthed by the burrowing action of the proboscis. The 
pharynx is quite extended and communicates directly with the 
exterior through two lateral rows of paired gill-slits, which are 
supported by a branchial skeleton very much like that of 
Amphioxus. 
This characteristic of the possession of pharyngeal gill- 
slits is extremely significant, for nowhere else, except in ver- 
Fig. 189. Balanoglossus. [After Loos., in Leuckart charts.] 
tebrates and in their allies, above considered, do such organs 
occur, and investigators have naturally been led through these 
to seek for the other essential vertebrate characters of a noto- 
chord and a dorsal nervous system. In this search they have 
been to a qualified extent successful, although these characters 
are far from appearing with the same distinctness as in the 
case of the gill-slits. The nervous system consists in general THE ANCESTRY OF THE VERTEBRATES 
593 
of a diffuse net-work of nerve fibers lying in the depth of the 
surface epithelium and occurring everywhere, a very low type 
of nervous system. This net-work is, however, reinforced and 
formed into four longitudinal cords, slightly differentiated 
from the rest, a dorsal, a ventral, and two lateral, all of which 
run the entire length of the animal. Of these the dorsal re- 
ceives slightly more emphasis than the others, since it con- 
tinues forward to the base of the proboscis, where it divides 
into two diverging branches, which encircle it in the form of a 
ring. As for the notochord, this has been doubtfully identified 
with a small diverticulum, which arises from the dorsal wall 
Fig. 190. Harrimania maculosa. [After Ritter.] 
Schematic representation of dissection, including collar and small portion of 
the anterior pharyngeal region. The anterior and posterior aspects are designated 
as A and P, respectively, es, cesophagus; es. No-c, oesophageal notochord; d. n. c, 
dorsal nerve cord; SK. C3 skeletal crura; br. o, branchial orifices. 
of the pharynx, and extends some distance forward into the 
proboscis, and this supposition has been greatly strengthened 
through the discovery of an allied form belonging to the 
genus Harrimania in which the diverticulum is much larger. 
In its mode of origin this organ is strikingly similar to that 
of the true vertebrate notochord, and is thus without much 
doubt homologous with this organ. 
From the testimony afforded by the structure of Balanoglos- 
sus and its allied genera (the group Enteropneusta) it may be 
quite confidently asserted that these forms lie nearly in the 594 
THE HISTORY OF THE HUMAN BODY 
line of vertebrate descent, and represent an earlier stage than 
that of the tunicates. But here the chain seems to end, for 
Balanoglossus is itself unusually isolated and shows no close 
affinity to any other invertebrate types. There is, in such 
cases, but one possible way out, a single remaining clew, and 
that is, the embryology of the form in question, and even here 
the primary, historic features may be overlaid with secondary 
changes rendered necessary as an adaptation, and thus the 
true value of a given feature is often hard to estimate. In the 
case of Balanoglossus, however, it seems probable that the 
early development is in great part an actual repetition of the 
Fig. 191. Comparison of Tornaria larva with larval Echinoderms. 
[After O. Hamann.] Main ciliated bands in black, lesser systems cross- 
lined. 
(A) Tornaria, ventral view. (B) Tornaria, dorsal view. (C) Auricularia, ven- 
tral view. (D) Bipinnaria, ventral view. 
race-history, but if so, it leads us to surprising and not very 
satisfying results, for the animal begins life as a minute trans- 
parent floating larva, the Tornaria, furnished with bands of 
cilia, by which it moves, a larva strikingly like that of star- 
fish, sea-urchins, and other echinoderms, and one which an 
unprejudiced mind would not hesitate to classify with these 
latter. These larvae are all of about the same size, all bilateral 
in structure, all transparent and equipped with bands of cilia, 
and there is even a close correspondence in the manner of 
disposal of these bands. 
In the case of the echinoderms the universal occurrence of 
such larvae is taken everywhere as a proof that they represent THE ANCESTRY OF THE VERTEBRATES 
595 
an early stage in the history of this Class, and that the an- 
cestors of these radiate, crawling, or sessile bottom forms were 
bilateral and pelagic. Now it would be highly improbable 
that an unrelated form should, as an adaptation, so modify its 
early stages as to resemble these echinoderm larvae as closely 
as does the Tornaria, and the only alternative is to accept as 
a very ancient common ancestor of both echinoderms and 
vertebrates the form which all these larvae may be said to copy; 
a form having the characteristics common to all, including bi- 
laterality, minute size, transparency, locomotion by bands of 
cilia, and pelagic life. The lineal descendants of this hypo- 
thetical ancestor chose two paths, the one leading to the 
Fig. 192. Comparison of Tornaria and Echinoderm larvae, lateral 
views. [After Balfour.] 
(A) Tornaria. (B) Auricularia. (C) Bipinnaria. 
a, apical area; b, oral area; c, post-oral area; d, anal area. 
Echinodermata, the other to Balanoglossus, the Tunicata, 
Amphioxus, and eventually the Vertebrata. 
This theory, although incomplete and unsatisfactory in parts, 
is consistent with the most approved lines of biological 
thought; it rests upon development as well as adult structure, 
and bears the indorsement of the majority of investigators 
at the present time. The weakest part of the argument is that 
of the significance of the Tornaria larva; and while the ac- 
ceptance of this gives us very little enlightenment, to abandon 
it would be to sacrifice but little, and would render the gulf 596 
THE HISTORY OF THE HUMAN BODY 
between the adult Balanoglossus and other invertebrates only 
a little more profound. To summarize in the words of two 
recent authors,* “ The question of the descent of the Chordata 
is not solved by accepting their relationship to the Enterop- 
neusta, since this latter group holds an uncommonly isolated 
position. Only from the structure of the Balanoglossus larva 
can there be concluded a distant connection with the echino- 
derms. We must resign ourselves to the thought that at the 
present time we are not in a condition to assert from what 
ancestral form the Chordata, and with them Balanoglossus, 
are to be derived. The origin of the vertebrates is lost in the 
obscurity of forms unkown to us.” 
* Korschelt u. Heider. Entwickelungsgeschichte. Jena, 1893, p. 1465. 
“ Die Frage nach der Abstammung der Chordaten wird durch die Annahme 
von verwandtschaftlichen Beziehungen derselben zu den Enteropneusten nicht 
gelost, da die letzere Gruppe selbst ungemein isolirt dasteht. Nur aus dem 
Bau der Balanoglossuslarve lasst sich eine entferntere Zusammenhang mit 
den Echinodermen erschliessen. Wir miissen uns bei den Gedanken resigniren, 
dass wir vorlaufig nicht im Stande sind, anzugeben, von welchen Urformen 
die Chordaten und mit ihnen Balanoglossus herzuleiten sind. Der Ursprung 
der Wirbelthiere verliert sich in das Dunkel uns unbekannter Formen.” APPENDIX 
CLASSIFICATION OF THE VERTEBRATA. 
The following list of the larger subdivisions of the Vertebrata, 
arranged in synoptical form, may be of use in explaining the 
names of groups as used in the body of the work. Through 
the labors of palaeontologists so many forms have been unearthed 
and so many new groups established that it seems best to include 
in the list these latter as well as modern animals, especially since 
many of the extinct groups consist of generalized forms from 
which several living groups have differentiated. The names of all 
groups, of whatever rank, that contain living representatives are 
printed in bold-faced type; those groups in which these latter 
are represented by but one or two isolated forms are farther 
designated with an asterisk. In this way the amount of dam- 
age wrought in the phylogenetic record, as well as the relative 
position of all groups, may be seen at a glance. The synopsis 
follows:— 
VERTEBRATA (or Chordata). 
Division I. Cyclostomata.* 
Class I. Marsipobranchii.* 
Sub-Class I. Cyclostomi.* 
Order i. Myxinoidea * (Myxine, the hag-fish). 
Order 2. Petromyzontoidea * (Petromyzon, the 
lamprey eel). 
Sub-Class II. Ostracodermi. 
Order 1. Heterostraci (Pteraspis). 
Order 2. Osteostraci (Cephalaspis). 
Order 3. Antiarchi (Pterichthys), 
597 598 
THE HISTORY OF THE HUMAN BODY 
Division II. Gnathostomata. 
Super-Class I. Ichthyoidea. 
Class I. Pisces. 
Sub-Class I. Elasmobranchii. 
Order i. Pleuropterygii (Cladoselache). 
Order 2. Ichthyotomi (Pleuraeanthus). 
• Order 3. Acanthodii (Acanthodes, Diplacanthus). 
Order 4. Selachii. 
Sub-Order 1. Squali (sharks, dog-fish). 
Sub-Order 2. Raiae (skates). 
Sub-Class II. Holocephali.* 
Order 1. Chimaeroidei * (Chimcera). 
Sub-Class III. Dipnoi.* 
Order 1. Sirenoidei * (Protopterus, Ceratodus). 
Order 2. Arthrodira (Coccosteus, Dinichthys). 
Sub-Class IV. Teleostomi. 
Order 1. Crossopterygii.* 
Sub-Order 1. Haplistia (Tarrasius). 
Sub-Order 2. Rhipidistia (Holoptychius; Oste- 
olepis). 
Sub-Order 3. Actinistia (Ccclacantlius; Undina). 
Sub-Order 4. Polypteroidei * (Polypterus; Cala- 
moichthys). 
Order 2. Actinopterygii. 
Sub-Order 1. Chondrostei * (Palceoniscus; Aci- 
penser, sturgeon). 
Sub-Order 2. Protospondyli * (Lepidotus; Eu- 
gnathus; Amia, bow-fin). 
Sub-Order 3. iEtheospondyli * (Aspidorhynchus; 
Lcpisostcus, gar-pike). 
Sub-Order 4. Isospondyli (Leptolepis; herring; 
salmon; trout). 
Sub-Order 5. Eventognathi (carp; minnow; 
sucker). 
Sub-Order 6. Nematognathi (siluroids, e.g. bull- 
heads, cat-fish, etc.). 
Sub-Order 7. Haplomi (pike; killifish). 
Sub-Order 8. Apodes (eels). 
Sub-Order 9. Synentognathi (flying-fish). APPENDIX 
599 
Sub-Order io. Lophobranchii (sea-horse; pipe- 
fish) . 
Sub-Order n. Hemibranchii (stickleback). 
Sub-Order 12. Acanthopteri (mackerel; cod; 
perch; sculpin; flounder). 
Sub-Order 13. Pediculati (angler-fjsh; frog-fish). 
[In the Class of Pisces the process of extinction has left 
in our modern fauna five more or less isolated groups, 
usually treated as Orders. These are the Selachii, Holo- 
cephali, Dipnoi, Ganoidei and Teleostei. The first is the 
only remaining group of the elasmobranchs) of the second 
and third the only fossils known are much like those of 
the present day and their affinities have not been definitely 
traced; and the fourth and fifth are respectively the earlier 
and later types of teleostomes. 
The living ganoids are very few in number and are 
for the most part unrelated to one another. There are 
but two crossoptergyians, and but one or two living 
genera of each of the three groups Chondrostei, Protospon- 
dyli, and JEtheospondyli. At this point the “ ganoids ” 
are considered to end, and the remaining Sub-orders, be- 
ginning with the Isospondyli, are included with the tele- 
osts (i. e. Teleostei, to be carefully distinguished from 
Teleostomi, the larger group). 
Sub-orders 4-8 are sometimes grouped as the Physostomi 
and the remaining sub-orders, 5-13, as the Physoclysti. 
In the former of these the air-bladder retains its connec- 
tion with the alimentary canal; in the latter this becomes 
lost during development and the air-bladder is a closed 
sac.] 
Class II. Amphibia (Batrachia). 
Order 1. Stegocephali. 
Sub-Order 1. Phyllospondyli (Branchiosaurus). 
Sub-Order 2. Lepospondyli (Diplocaulus). 
Sub-Order 3. Temnospondyli (Archegosaurus, Ery- 
ops). 
Order 2. Urodela (Caudata). 
Sub-Order 1. Mutabilia (Desmognathus, Spelerpes). 
Sub-Order 2. Proteida (Necturus, Proteus). 
Sub-Order 3. Meantes {Siren). 
Order 3. Anura (Salientia). 
Sub-Order 1. Aglossa (Pipa, Xenopus). 
Sub-Order 2. Arcifera (Bufo, Hyla). 
Sub-Order 3. Firmisterna (Rana). 600 
THE HISTORY OF THE HUMAN BODY 
Order 4. Coecilia (Gymnophiona) {Coecilia, Epi- 
crium). 
[Of the four Orders of Amphibia, one, Stegocephali, is 
entirely extinct, and the other three essentially modern, with 
a few traces of older representatives. The Stegocephali, with 
a greatly increased knowledge of its representatives in the 
past few years, show such great differences in their funda- 
mental structure, especially of the skull and vertebrae, that 
they are frequently divided into sub-divisions of the value 
of Orders. The Stegocephali thus become a convenient term 
within which to include ancient Amphibians with many forms 
of structures, between the fishes and the reptiles. The 
Phyllospondyli like Branchiosmrus, were much like the 
modern Salamanders, and undoubtedly were their direct an- 
cestors. The Temnospondyli gave origin to reptiles, through 
the Cotylosaurs, and there is but little difference between 
such forms as Eryops, a Stegocephalian, and the Cotylosaur 
Seymouria.il 
Super-Class II. Sauropsida. 
Class III. Reptilia. 
Sub-Class I. Anapsida. 
Order 1. Cotylosauria (Seymouria; Pariosaurus; this 
order contains the most primitive of all 
reptiles). 
Order 2. Chelonia (Turtles). 
Order 3. Sauropterygia {Plesiosaurus). 
Sub-Class II. Parapsida. 
Order 1. Ichthyosauria {Ichthyosaurus). 
Order 2. Mosasauria {Mosasaurus). 
Order 3. Lacertilia {Lacerta; Sceloporus; Iguana). 
Order 4. Ophidia {Coluber; Lampropeltis; Crotalus). 
Sub-Class III. Diapsida. 
Order 1. Rhynchocephalia {Sphenodon). 
Order 2. Parasuchia {Belodon; Mystriosuchus). 
Order 3. Crocodilia {Crocodilus; Alligator). 
Order 4. Vteros?Mna.{Pterodactylus; Rhamporhynchus) ■ 
Order 5. Dinosauria. 
Sub-Order 1. Saurischia {Allosaurus; Brontosaurus; 
Diplodocus). 
Sub-Order 2. Ornithischia {Iguanodon; Stegosaurus; 
Triceratops). APPENDIX 
601 
Sub-Class IV. Synapsida. 
Order i. Theromorpha (Varanosaurus; Casea). 
Order 2. Therapsida (Lystrosaarus; Theriodont an- 
cestors of Mammals). 
[Of the four Sub-Classes all are represented in modern 
times. Turtles are Anapsida; Lizards and snakes are Parap- 
sida; and the Crocodiles are Diapsida. Mammals are de- 
scendants of the Synapsida, probably through the Therapsids, 
and the birds are now believed to be descendants of the 
Diapsida, through the Dinosaurs. 
As is indicated above by the difference in type the proc- 
ess of extinction in the group of Reptilia has gone very 
far, leaving but four isolated spots to be represented among 
the living forms; (1) the Chelonia, essentially a modern 
group, (2) the Rhynchocephalia, represented by a single 
living species, (3) the last two Orders of the Parapsida, 
the lizards and snakes, and (4) a very few modern representa- 
tives of the Crocodilia. In arrangements in which living forms 
are alone taken into consideration, these are often given as five 
Orders; the lizards and snakes count as two; in the earlier 
works Sphenodon was counted among the lizards, reducing 
the number to four. The living Orders, as thus arranged, are 
as follows: Chelonia, Rhynchocephalia, Lacertilia, Ophidia, 
Crocodilia.] 
Class IV. Aves. 
Order 1. Saururae (Archaeopteryx; Laopteryx). 
Order 2. Odontormse (Ichthyornis; Apatornis). 
Order 3. Odontoholcae (Hesperornis; Lestornis). 
Order 4. Eurhipidurae. 
Sub-Order 1. Dromaeognathi. 
Section I. Struthiones (ostrich, casuary). 
Section II. (Mpiornis). 
Section III. Apteryges * (Apteryx). 
Section IV. Crypturi (Crypturus). 
Section V. Gastornithes (Gastornis). 
Sub-Order 2. Impennes (penguins). 
Sub-Order 3. Euornithes [a recent group, begin- 
ning in the Eocene] 
Section I. Desmognathae (ducks; herons; eagles; 
hawks; owls; cuckoos; kingfisher; trogon; 
parrot). 602 
THE HISTORY OF THE HUMAN BODY 
Section II. Schizognathae (grebes; loons; gulls; 
snipes; grouse; quails; pigeons; humming- 
birds; woodpeckers). 
Section III. ZEgithognathae (Passeres, a group 
which includes over one-half of the species 
of living birds). 
Super-Class III. Mammalia. 
Class V. Mammalia. 
Sub-Class I. Prototheria * [Ornithodelphia]. 
Order i. Pantotheria (Dromotherium; Amphilestes). 
Order 2. Multituberculata (Ctenadon; Polymastodon). 
Order 3. Monotremata* (Ornithorhynchus; Echid- 
na). 
Sub-Class II. Eutheria. 
Super-Order 1. Didelphia [Marsupialia]. 
Order 1. Polyprotodontia (opossum: Thylacinus). 
Order 2. Paucituberculata * (Coenolestes). 
Order 3. Diprotodontia (Petaurus; wombat; kan- 
garoo). 
Super-Order 2. Monodelphia [Placentalia]. 
Order 1. Insectivora (moles; shrews; hedgehog). 
Order 2. Cheiroptera (bats). 
Order 3. Galeopithecidae * (flying lemur). 
Order 4. Edentata. 
Sub-Order 1. Tubulidentata * (Orycteropus, the 
earth-hog or “ aard-vark ”). 
Sub-Order 2. Pholidota* (Manis, a scaled animal 
of Asia and Africa). 
Sub-Order 3. Xenarthra (Glyptodon; Megatherium; 
Grypotherium; ant-eaters; armadilloes; 
sloths). 
Order 5. Rodentia. 
Sub-Order 1. Tillodontia (Tillotherium; Esthonyx). 
Sub-Order 2. Duplicidentata (hares; rabbits). 
Sub-Order 3. Simplicidentata (squirrels; beavers; 
mice). 
Order 6. Primates. 
Sub-Order 1. Mesodonta (Adapis; Anaptomorphus). 
Sub-Order 2. Lemuroidea (Lemurs; Chiromys; 
Tarsius). APPENDIX 
603 
Sub-Order 3. Anthropoidea. 
Division 1. Platyrrhini (Hapale; Midas; Cebus). 
Division 2. Catarrhini (Cercopithecus ; Semno- 
pithecus; Gorilla; Homo). 
Order 7. Creodonta (Arctocyon; Hycenodon). 
Order 8. Carnivora (cats; dogs; bear; weasel). 
Order 9. Pinnipedia (seals; walrus; sea-lion). 
Order 10. Cetacea (whales; porpoises). 
Order n. Condylarthra (Phenacodus). 
Order 12. Hyracoidea * (Procavia [Hyrax] ). 
Order 13. Amblypoda (Coryphodon). 
Order 14. Sirenia * (manatee; dugong). 
Order 15. Proboscidea * (Mastodon; elephants). 
Order 16. Ancylopoda (Homalodontotherium). 
Order 17. Typotheria (Typotherium). 
Order 18. Toxodontia (Toxodon). 
Order 19. Litopterna (Proterotherium). 
Order 20. Perissodactyla (Palcootherium; Titano- 
therium ; rhinoceros; horse). 
Order 21. Artiodactyla. 
Sub-Order 1. Suina (Elotherium pigs; peccaries; 
hippopotamus). 
Sub-Order 2. Tylopoda (Oreodon; camel; llama). 
Sub-Order 3. Anthracotherioidea {Anthracotherium). 
Sub-Order 4, Dichobunoidea (Dichobune; Anoplo- 
therium). 
Sub-Order 5. Traguloidea (Tragulus, several small 
species in E. Indies). 
Sub-Order 6. Pecora (deer; sheep; cattle; giraffe). 
[The arrangement of animal groups in the form of a 
list, in which they follow one another in a single series, 
is seldom more unsatisfactory than it is in the case of 
mammals. The inadequacy of this method in expressing 
the true relationships is seen if the list be compared with 
the phylogenetic trees given in Chapter II. (Figs. 7 and 8.) 
There are several distinct stems to be followed and the order 
in which they are taken in a list is largely a matter of 
preference. Here the attempt is made to proceed from 
the generalized to the more specialized ones, and thus the 
main stem of the Insectivora is taken first; then that of the 
Primates, and lastly the complex and highly specialized branch 604 
THE HISTORY OF THE HUMAN BODY 
leading to the carnivore and ungulate Orders. This arrange- 
ment has the advantage of emphasizing the primitive and 
rather generalized structure of the Primates as compared with 
the groups just mentioned, a comparison entirely lost sight 
of by the usual arrangement, which places the apes and 
Man at the top. If the arrangement be made solely on the 
basis of the development of the nervous system there can 
be no question of the rightfulness of this position; but if all 
the systems be taken into consideration, and especially the 
bones, muscles and teeth, which in other groups form the 
principal criteria for the purpose of classification, the Pri- 
mates are found, to have retained a larger number of primitive 
characters than any other placental group with the excep- 
tion of the Insectivom, Rodentia, and Edentata, and thus 
to stand far lower in the scale of specialization than the- 
manifold descendants of the Creodonta and Condylarthra. 
The arrangement of the subdivisions of the Primates given 
above is a conservative one, and will accord with the most 
of the literature on the subject. Certain important modifica- 
tions have, however, been recently proposed, based upon a 
more complete study of anatomical characters. Through 
these the extinct genus Anaptomorphus, which is probably 
very near the direct ancestral line leading to Man, has been 
placed in Sub-Order 3, Anthropoidea, and with it has been 
placed the living genus, Tarsius, a closely related form. 
For convenience in classification all the descendants of 
the Condylarthra, together with this latter, but excepting 
the aberrant Sirenia, are often grouped together under the 
single Order of Ungulata, or hoofed animals, connected both 
by descent and by the common peculiarities embodied in 
the name. This will include Orders 11-21 in the above list. 
In the same way the Creodonta may be included in the 
Carnivora, although the Cetacea are treated as a separate, 
though allied group. The GaXeopithecoidea are often included 
within the Insectivora. This reduces the Orders of placental 
mammals to nine, viz.: Insectivora, Cheiroptera, Edentata, 
Rodentia, Primates, Ungulata, Sirenia, Cetacea, Carnivora. 
There are, of course, as in all groups of animals, many 
other possible arrangements, the differences being based on 
the relative value of the various groups, their relationships 
to one another, and the comparative degree of specialization 
of each; points upon which there is much room for difference 
of opinion.] 
Owing to the extinction of so many of the groups, especially 
those forming the connection between two others, a classification 
that rests wholly upon living forms is far from complete and in 
some points differently arranged from one that includes all known APPENDIX 
605 
forms. Thus among the fishes the selachians alone are left of all 
the elasmobranchs; a few remnants remain of the first few Orders 
of teleostomes, isolated from one another and from the others; and 
of the Holocephali and Dipnoi only a few species occur. Among 
the amphibians, the Stegocephali, the most important Order of all, 
have disappeared entirely, and among the reptiles a still greater 
destruction has left but four isolated spots in a once continuous 
history. This loss has affected also all the transition forms between 
reptiles and the modern type of birds, and completely isolated 
the Aves from all related forms. The mammals are still rich in 
Orders, but the synthetic types that once united them have long 
since disappeared. Without going into the Sub-Orders this ab- 
breviated classification, in some respects different from the above 
synopsis, may be given here for convenience in comparison. Only 
the gnathostomes may be considered. 
SYNOPSIS OF VERTEBRATA (living forms alone). 
Class I. Pisces. 
Sub-Class I. Selachii. 
Sub-Class II. Holocephali (often considered with the pre- 
vious group). 
Sub-Class III. Ganoidei. 
Sub-Class IV. Teleostei. 
Sub-Class V. Dipnoi. 
Class II. Amphibia. 
Order i. Urodela. 
Order 2. Gymnophiona. 
Order 3. Anura. 
Class III. Reptilia. 
Order 1. Chelonia. 
Order 2. Lacertilia (including Sphenodon). 
Order 3. Ophidia. 
Order 4. Crocodilia. 
Class IV. Aves. 
Sub-Class I. Ratitae (running birds; with flat breast-bone, 
e.g., Ostrich). 
Sub-Class II. Carinatae (flying birds; with keeled breast- 
bone) . 
Class V. Mammalia. 606 
THE HISTORY OF THE HUMAN BODY 
Sub-Class I. Prototheria. 
Order i. Monotremata. 
Sub-Class II. Eutheria. 
Super-Order i. Didelphia (Marsupialia). 
Super-Order 2. Monodelphia (Placentalia). 
Order 1. Edentata. 
Order 2. Insectivora. 
Order 3. Rodentia. 
Order 4. Cetacea. 
Order 5. Sirenia. 
Order 6. Ungulata. 
Order 6a. Proboscidea (occasionally separated from 
the Ungulata). 
Order 6b. Hyracoidea (occasionally separated from 
the Ungulata). 
Order 7. Carnivora. 
Order 8. Cheiroptera. 
Order 9. Primates. 
In this arrangement there will be noticed especially the separa- 
tion of ganoids and teleosts, the small number of reptilian Orders, 
the complete isolation of the birds, and the arrangement of the 
mammalian Orders in such a way as to bring the Primates at the 
top. Certain of these faults, like the isolation of the birds, have 
been corrected for some time, the separation of ganoids and teleosts 
is a convenient one for purposes of comparative anatomy, and 
is employed for this purpose in the body of this work. The ar- 
rangement of the mammals to show the supremacy of Man is 
natural, and is based, of course, in part, on the high development 
of the brain, but much is due to natural human pride which 
recognizes man’s mental supremacy, and feels that a supremacy 
in physical structure must also be granted. As a matter of fact, 
in all other aspects save that of the brain, the apes and man are 
rather primitive in their structure and show a far less bodily 
specialization than almost any of the other living Orders of 
mammals, the Insectivora and a few others being alone excepted. INDEX 
Abducent nerve, 498, 499, 500, 501 
Acipenser, posterior limbs of, 190, 191 
Acoustic nerve, 501, 502 
Activities, somatic vs. visceral, 456, 457 
Acustico-lateral line system, 501, 502 
Adjustor, 439 
Air bladders, 299, 341 
Alimentary canal, comparative dia- 
grams of, 292 
Alimentary canal, comparative lengths 
of, 293 
Alimentary canal, formation of, 288 
Allantois, 72, 73, 352 
Allantois of mammals, 353 
Allantois of Sauropsida, 353 
Amnion, 72, 73 
Amniota, extraembryonal membranes 
of, 72 
Amniota, lungs of, 339-342 
Amniota, urinogenital system of, 415 
Amphibians, lungs of, 336, 337 
Amphibians, spinal cord of, 453 
Amphibians, uterus of, 4x4 
Amphicoelous vertebrae, 129 
Amphioxus, 26, 27 
Amphioxus, as vertebrate ancestor, 
583-587 
Amphioxus, brain of, 494 
Amphioxus, nerve roots of, 487, 488 
Amphioxus, rudimentary sense organs 
of, 497 
Amphioxus, spinal cord of, 452, 453 
Amphioxus, tongue bars of, 319 
Amplexation, 414, 428 
Ampullae of semicircular canals, 544 
Anamnia, urinogenital system of, 415 
Anapsida, 32 
Anlage, definition of, 64 f. 
Annelid theory of Semper, 570-578 
Anterior medullary velum, 481 
Anterior nares, in aquatic forms, 535 
Anterior somite, 518 
Anthropoidea, 39, 42 
Antitropists, 269, 270, 271 
Anura, 31 
Anura, endolymphatic ducts of, 544 f. 
Aortae, embryonal, 350 
Appendicular muscles, 243-272 
Appendix testis, 424 
Appendix vermiformis, 326, 327 
Aqueductus cerebri (Sylvii), 461 
Aqueductus vestibuli, 544 
Arachnoid theory of Patten, 580-583 
Arboreal environment, 14 
Arbor vitae, 479, 480 
Archegosaurus, skull of, 155 
Archencephalon, 459, 494 
Archeopteryx, 33 
Archetype, of Goethe, 564, 565 
Archetype, of Owen, 567, 568 
Archipallium, 471 
Archipterygium, 202 
Archisternum, 142 
Area centralis of eye, 559 
Arterial arches, development of, 364- 
368 
Artery, or arteries, 348 
allantoic, 352 
aorta, 356 
afferent branchial, 355 
carotid, 356 
carotid, ontogony of, 363-369 
ductus arteriosus, 361 
efferent branchial, 356 
external carotid, 368 
gill arch (arterial) first, 350 
gill arches (arterial), 358-364 
iliac, 352, 356 
infraorbital, 365 
internal carotid, 368 
intercostal, 369 
lingual, 365 
lumbar, 369 
mandibular, 365, 368 
maxillary, 365 
mesenteric, 370 
stapedial, 368 
subclavian, 356 
supraorbital, 365 
urqbilical (cf. allantoic) 
Artery and vein, distinction between, 
_ 3S8 
Artiodactyla, 40 
Arytasnoid cartilages, 343 
607 608 
INDEX 
Assimilation, 1, 2 
Association neurone, 439, 440 
Atlas, 135 
Auditory meatus, 553, 554 
Auditory ossicles, 159 
Auditory ossicles, homology of, 504, 552 
Auditory tube, 551 
Augurs, Roman, divination by, 294 f. 
Auricula, 180, 553, 554 
Axial muscles, 228-243 
Axis, 135 
Axone, 440 
Balanoglossus, 26 
Balanoglossus, as related to Amphioxus, 
591-593 
Balanoglossus, as related to Echino- 
derms, 594, 595 
Basipterygium, 201 
Biogenesis, 15 
Biogenetic law, 15 
Bipolar neurones, 440 
Bisexual individuals, 49 
Bladder, gall, 325 
Bladder, urinary 408 
Blastula, 60 
Blood, 348 
Bones (cf. Skeletal parts). 
Bones, dermal, 84, 85 
Brachium conjunctivum, 482 
Brachium pontis, 480 
Brain of Amphioxus, 459 
Brain, comparison of, in vertebrate 
classes, 464, 465 
Brain, Cyclostome, 460, 464 
Brain, early development of, 458-464 
Brain stem, 480 
Brain stem, functional divisions of, 
481, 482 
Brain stem, tracts of, 482, 483 
Brain vesicles, 459 
Brain vesicles, nomenclature of, 460, 
461 f. 
Branchial system, 289 
Branchiomeric musculature, 218, 272- 
275. 485 
Branchiomeric musculature, innerva- 
tion of, 514 
Branchiomeric muscles of Necturus, 222 
Branchiostoma (cf. Amphioxus) 
Broad ligament, 418 
Bronchi, 299 
Bulbo-urethral glands, 433 
Bursa inguinalis, 426 
Bursa ovarica, 418 
Caeca, cloacal, 295, 325 
Caeca, colic 295 
Caeca, pyloric, 295 
Caenogenetic characters, 16 
Canalis centralis, 442, 448 
Capillaries, 348 
Carapace of Armadillo, 87 
Carapace of turtles, 107 
Carnivora, 38 
Carpal bones, names of, 203, 205, 206 
Carpus, supernumerary elements of, 
206, 207 
Carpus, various forms of, 205 
Cartilage bones, 147 
Cartilages (see Skeletal parts) 
Cartilages, costal, 138 
Cauda equina, 451 
Cavum septi pellucidi, 474 
Central nervous system, origin of, 441- 
443 
Centrum of vertebra, 129, 566 
Cephalization, 448-451, 455 
Cephalogenesis, 518 
Cerebellum, 460, 479 
Cerebellum, peduncles of, 480, 482 
Cerebral cortex, 468 
Cerebral cortex, convolutions of, 469 
Cerebral cortex, fissures of, 469 
Cerebral cortex, sulci of, 469 
Cerebral flexures, 463, 484 
Cerebral hemispheres, 460 
Cerebral hemispheres, evolution of, 
464-474 
Cerebrum, peduncles of, 483 
Cervical fistula, 298 
Cetacea, 38 
Chiridium of mammals, 94 
Chiroptera, 38 
Chiropterygium, 187, 202, 213, 214 246 
Choanas, 298, 535 
Choanae, original position of, 535 
Chondrocranium, 146, 147 
Chorda gubernaculi, 426 
Chorda tympani, 504 
Chordata, descent of, 596 
Chorioid fissure, 558 
Chorioid plexus, 462, 475 
Chorion, 73 
Chorion frondosum, 73 
Chorion laeve, 73 INDEX 
609 
Chromatin, 54 
Chromosomes, 54, 55, 56, 57 
Chromosomes in heredity, 59 
Chromosomes, number of, 55, 57 
Ciliary ganglion, 500 
Ciliary glands, 563 
Circulation, in embryos of vertebrates, 
351 
Circulation, haemal, 348 
Circulation, lacunar, 347, 348 
Circulation, lymphatic, 348 
Circulation, open vs. closed, 347 
Circulation, in Selachians, 354, 355 
Circumvallate papillae, 533 
Claustrum, 471 
Claws, 108 
Cleavage, 60 
Clitoris, 429, 435 
Cloaca, 295 
Cochlea, 548, 549 
Ccecilia, 31 
Coelom, 66, 287, 397 
Colon, 327 
Colon labyrinths, 327, 328, 329 
Columella, 551 
Commissural tracts of brain, 473 
Conchae, upper and middle, 538 
Condylarthra, 38, 39, 40 
Conjugation, 6, 8, 50 
Conjunctiva, 562 
Contact sense, 525 
Continuity of life, 1, 2 
Conus inguinalis, 426 
Cooper’s fascia, 427 
Copulation, 411 
Copulation, necessity for internal form 
of, 411 
Corium, 80 
Cornea, 558 
Corpora bigemina, 478 
Corpora cavernosa, 429 
Corpora cavernosa penis, 433 
Corpora quadrigemina, 479 
Corpora striata, 468, 483 
Corpus callosum, 473 
Corpus cavernosum urethrae, 435 
Corpus fibrosum, 429 
Corpus restiforme, 482 
Cortices, cerebral and cerebellar, 441 
Corti, rods of, 549 
Costal cartilages, 138 
Cowper’s glands (cf. bulbo-urethral 
glands) 
Cranial nerve roots, 484 
Cranial nerves, anterior group of, 496- 
498 
Cranial nerves, enumeration of, 494 
Cranial nerves, of eyeball, 498-501 
Cranial nerves, functional components 
of, 496, 498, 499, 500, 501, 503, 
508, 509, 512, 514, 518, 519 
Cranial nerves, list of, 494 
Cranial nerves, morphological signifi- 
cance of, 516-519 
Cranial nerves, trigeminus-facialis 
group of, 501-506 
Cranial nerves, vagus group of, 506-512 
Creodonta, 40 
Cristae acusticae, 547 
Crop, 319 
Cryptobranchus, skull of, 149 
Crystalline lens, 556 
Curvatures of stomach, 320, 321 
Cuticula of invertebrates, 78 
Cuvier, morphological theories of, 564 
Cuvier, skull theory of, 151 
Cyanosis, 387 f. 
Cyclostoma, 296 
Cyclostomes, 28 
Cyclostomes, development of head of. 
534 
Cyclostomes, labyrinth of, 545 
Cyclostomes, nerve roots of, 488 
Cyclostomes, spinal cord of, 453 
Decidua, 74 
Deep muscles of the back, 235 
Dental formulae, 305-307 
Dendrites, 440 
Dentine, 81 
Dentition, diphyodont, 309, 311 
Dentition, heterodont, 304 
Dentition, homodont, 304 
Dentition, monophyodont, 309 
Dermal bones, 84, 85, 147 
Descensus ovariorum, 423 
Descensus testiculorum, 423, 424, 428 
Descent of the Chordata, summary con- 
cerning, 596 
Descent of the testes, diagram of, 428 
Desmognathus, skull of, 156 
Developmental history, 15 
Development of brain, mechanical 
principles of, 461-464 
Diaphragm, 346 
Diapophysis, 566 INDEX 
610 
Diapsida, 32 
Didactyl limb, 208, 209 
Didelphia, 36 
Diencephalon, 460 
Diencephalon, development of, 479 
Digits, number of, 208 
Diphyodont dentition, 309, 311 
Dipnoans, 30 
Dipnoans, hibernation of, 535 
Dipterus, 146, 148 
Distribution of peripheral nervous 
system, general principles of, 488, 
489 
Dorsal nerve root, 445, 486 
Double teeth, derivation of, 307-309 
Dryopithecus, 44 
Ductus deferens, 423, 424 
Ductus endolymphaticus, 543, 544 f. 
Duodenum, 295, 323 
Ear, external, 180, 553 
Ear as exteroceptor, 542 
Ear as proprioceptor, 529, 530, 542 
Ecdyses, 79 
Echidna, 34 
Ectoderm, 61 
Ecto-turbinals, 538 
Edentata, 38 
Effector, 439 
Eggs, 51, 52 
Embryonal elements, derivatives of, 
76, 77 
Enamel, 81 
Endoderm, 61 
Endolymph, 546 
Endolymphatic duct, 543, 544 f. 
Endolymphatic ducts in Anura, 544 f. 
Endolymphatic space, 388 
Endoskeleton, parts of, 124 
Endo-turbinals, 538 
Eoanthropus, 44 
Epidermis, derivatives of, 80, 81, 82 
Epididymis, 416, 417, 423 
Epimere, 65, 66, 67, 218 
Epiphyses, 476 
Episkeletal muscles, 230 
Epithelial bodies, 395 
Epithelial chorioid lamina, 474 
Epithelial corpuscles, 315, 316, 317 
Epitrichium, 97 
Eponychium, 97 
Epoophoron, 422 
Erythrocytes, 348 
Ethmo-turbinal, 536, 538 
Eustachian tube, 298, 551, 552 
Eustachius, diagrams of human muscles 
from, 250, 251 
Eutheria, 36 
External ear, 180, 553 
External genitalia, development of, 434, 
435 
table of homologies of, 436 
Exteroceptors, 485 
Extraembryonal membranes, 71-75 
Exuviae, 79 
Eye of cephalopod, 558, 559, 560 
Eye, early development of, 555 
Eye, muscles of, 241 
Eye muscles, morphology of, 499, 500 
Eye, vertebrate vs. cephalapod, 560 
Eyeball, accessory organs of, 558, 561- 
563 
Eyeball, size of, 561 
Eyelids, 562 
Eyelid, third, 562 
Facial muscles, 282, 283, 284, 285 
Facial nerve, 501, 502, 503, 506, 509, 
510, 5i4 
Fallopian tube, 418 
Fasciculi, 440 
Fasciculi of spinal cord, 455 
Female, external genitalia of, 434 
Fenestra cochleae (rotunda), 551 
Fenestra ovalis, 550 
Fertilization, 50, 55 
Fifth ventricle (cf. Cavum septi 
pellucidi). 
Filum terminale, 451 
Fin, or fins: 
anal, 184 
caudal, 184 
dorsal, 184 
median, 184, 185 
paired, 184, 185 
pectoral, 184 
ventral, 184 
Fin-fold theory, diagrams of, 183 
Fin-form as origin of hand-form, 213 
Fingers, names of, 203 
Fin skeleton, development of, 188 
Fin spines, 182 
Flexures of neural tube, 462-464 
Foliate papillae, 533 
Footprint of Thinopus, 209 
Foramen epiploicum, 323 INDEX 
611 
Foramen interventriculare, 460 
Foramen of Munro (cf. interventricu- 
lare) 
Foramen ovale, 387 f. 
Foramen of Winslow, 323 
Forebrain, 459 
Fore limb modifications, 210, 211 
Fourth ventricle, 461, 481 
Fovea centralis, of eye, 559 
Free nerve endings, 530 
Frontal sinus, 539 
Friction ridges, 90-93 
Friction skin of foot, 95 
Friction skin of hand, 96 
Functional components of cranial 
nerves, 496, 498, 499, 500, 501, 
S°3, S°8, 509, 512, 514, 518, 519 
Functional components of spinal nerves, 
484-486 
Functional divisions of brain, 481-483 
Functional divisions of nervous system, 
457 
Functional divisions of spinal cord, 
456, 457, 458 
Fundus, of stomach, 321 
Funiculi of spinal cord, 455 
Galea aponeurotica, 284 
Galeopithecus, 38 
Gall bladder, 325 
Gametes, 7, 48 
Gametogenesis, 57-59 
Ganglion, or ganglia: 
chain, 520 
ciliary, 506 
Gasserian, 502 
genicular, 503, 510 
jugulare, 508 
laterale, 508 
nodosum, 508 
otic, 506, 510, 520 
petrosum, 508, 510 
semilunar, 502, 503 
spheno-palatine, 506, 510, 520 
spinal, 445 
spirale, 502 
submaxillary (submandibular), 506, 
520 
superius, 508 
vestibulare, 502 
Ganoids, 29 
Gartner’s duct, 422 
Gastrocoele, 287 
Gastrula, 61, 63, 286 
Gastrular mouth, 287 
Genicular ganglion, 503 
Germ cells, 7-9, 57 
Germ glands, 397, 398, 409-412 
Germ plasm, continuity of, 59 
Germ and soma, 7-9 
Gill arches of Selachians, 168, 169, 172 
Gills, 331 
Gill slits, 297, 332, 333 
Geraldes, organ of, 423 
Gland, or glands: 
acinous, in, 114 
bulbo-urethral, 433 
ciliary, 563 
cloacal, 112 
integumental, 80 
lacrimal, 562 
lymphatic, 348, 392 
mammary, 116 
meibomian, 115, 563 
mesenteric, 392 
musk, 112 
perspiratory, 114 
pharyngeal, anlagen of, 316 
pineal, 476 
preputial, 115, 434 
prostate, 432 
salivary, 314, 315 
sebaceous, 115 
tarsal, 115, 563 
thymus, 315, 316, 395 
thyreoid, 315, 316, 318, 395 
tubular, m 
Tyson’s, 115 
urethral, 432 
uropygial, 113 
vesicular, 432 
Glands, morphology of, 109 
Glands, types of, no 
Glandular structures, 485 
Gians penis, 435 
Glossopharyngeal nerve, 505, 506 
Glottis, 299 
Gnathostoma, 296 
Gnathostomes, 29 
Goethe, Johann Wolfgang, morpho- 
logical theories of, 564 
Goethe’s Urbild, 564, 565 
Gonads, 49, 397 
Gorilla rib, 140 
Grandry’s corpuscles, 531, 532 
Granulocytes, 348 612 
INDEX 
Gray nerve substance, 440 
Gray vs. white nerve substance, 473 
Gubernaculum, 426 
Gustatory nerves, 504, 506, 512 
Gymnophiona (cf. Coecilia) 
Gynecomastism, 120 
Haemal arches, 128, 566 
Haemal circulation, 348 
Haemal spine, 566 
Hair arrangement, 88-90 
Hair, development of, 98, 99 
Hair direction, 103-106 
Hair as sense organs, 528 
Hair, typical, 113 
Hand-form, 187, 213 
Harderian glands, 562 
Hard palate, 536 
Harrimania, notochord of, 593 
Head cavities, 241, 242 
Head, parachordal and praschordal 
portions of, 515 
Head somites, 499 
Hearing, organ of, 542 
Heart, 348 
Heart, development of, 383 
Heart, diagrams of in different verte- 
brates, 386 
Heredity, basis for, 53 
Hermaphroditism, 49 
Hesperanthropus, 44 
Heterodont dentition, 304 
Hibernation of Dipnoi, 535 
Highmore, antrum of, 539 
Hindbrain, 459 
Hippocampal area, 471, 472 
Homodont dentition, 304 
Homo heidelbergensis, 44 
Homology of muscles, 225, 226 
Homology, serial, of limbs, 264 
Homo neandertalensis, 43 
Homo rhodesiensis, 43 
Homo sapiens, 43 
Hoofs, 108 
Horn, 81, 86 
Horns, 108 
Hubrecht, Nemertean theory of, 578— 
580 
Human ancestry, 45 
Huxley, skull theory of, 151 
Hyatid of Morgagni, 424 
Hyobranchial support of tongue, 313 
Hyomandibular, 552 
Hyperdactylism, 208 
Hypermastism, 119 
Hyperthelism, 119 
Hypertrichosis, 101 
Hypomere, 65, 66, 67, 218 
Hypopallium, 417 
Hypophysis, 477 
Hyposkeletal muscles, 228, 229 
Hypospadias, 430 
Hyracoidea, 39 
Ichtyopterygium, 187, 213, 214 
Ilia of Necturus, 192 
Immortality of Protozoa, 5 
Incus, 552 
Infundibulum, 478 
Ingluvies, 319 
Inguinal ligament, 417, 418, 422 
Inner ear, 542 
Insectivora, 36 
Integration, 457 
Integumental bones, 84 
Integumental muscles, 277 
Interdural spaces, 388 
Interoceptors, 485 
Intermuscular spaces, 388 
Intestinal canal, length of in Man, 330 
Intestine, large, 323 
Intestine, small, 323 
Intumescentias of spinal cord, 453 
Involuntary muscle, 485 
Ischia of Necturus, 192 
Jacobson’s organ, 540 
Jaws, origin of, 170 
Keratin, 86 
Krause’s corpuscles, 531 
Labia majora, 435 
Labia minora, 435 
Labyrinth, 542, 544, 546, 549 
Labyrinth, anlage of, 543 
Labyrinth, bony, 550 
Labyrinth, cartilaginous, ssD 
Labyrinth of cyclostomes, 545 
Labyrinth, phylogenetic history of, 
547, 548 
Labyrinths of colon, 327-329 
Lacerta, skull of, 160, 161 
Lacrimal glands, 562 
Lacunar circulation, 347, 348 
Lagena, 545 INDEX 
613 
Lamina terminalis, 464, 474 
Lanugo, 100 
Larynx, 299 
Larynx of Echidna, 346 
Larynx, origin of, 340, 342-346 
Laryngeal cartilages of Amphibians, 
343 
Lateral ganglion, 508 
Lateral line, 221 
Lateral line organs (cf. neuromasts) 
Lateral ventricles, 460 
Law, biogene tic, 15 
Laws, ontogenetic, 21-25 
Laws, phylogenetic, 16-20 
Lemnisci, 483 
Lemuroidea, 42 
Lens, crystalline, 556 
Leucocytes, 348 
Life, continuity of, 12 
Life cycle, diagram of, 9 
Ligamentum arteriosum, 360, 361 
Ligamentum Botalli (cf. ligamentum 
arteriosum) 
Ligamentum inguinale, 422 
Ligamentum nuchas, 136 
Limbs, failure of, 185, 186 
Limbs, modifications of, 185, 186, 210 
Limb muscles, development of, 243-245 
Limb muscles, serial homology of, 265- 
269 
Limbs, names of bones of, 203 
Limbs, serial homology of, 264 
Liver, 295, 323 
Liver, development of, 324 
Liver, ligaments of, 324 
Lizard, skull of, 160, 161 
Lobi optici, 478 
Lorenzini, canals of, 527 
Lungless amphibians, 337 
Lungs, 299 
Lungs, of amniotes, 339-342 
Lungs, of amphibians, 336, 337 
Lymph, 349 
Lymphatic circulation, 348 
Lymphatic function, 387 
Lymphatic glands, 348, 492 
Lymphatic nodes, 392 
Lymphatic system, development of, 
391-393 
Lymphatic vessels, 348 
Lymph follicle, 392 
Lymph hearts, 349, 388, 389 
Lymphocytes, 348 
Lymphoid tissue, 392 
Lyssa, 314 
Macrogametes, 7 
Macula lutea of eye, 559 
Maculae acusticae, 547 
Maculae, phylogenetic history of, 547, 
548 
Made-up nerve, 486 
Malleus, 552 
Mammalian nasal cavity, interior of, 
536 
Mammals, allantois of, 353 
Mammary glands, 116 
Man, line of descent of, 45 
Mandible, dermal elements of, 173-175 
Mandibular articulation, 552 
Manidae, 87 
Marsupials, 34, 35 
Maxillary sinus, 539 
Medulla, 480 
Meissner’s corpuscles, 531 
Membrane, olfactory, 541 
Meibomian glands, 563 
Mesencephalon, 459, 460 
Mesencephalon, development of, 478, 
479 
Mesenchyme, 61 
Mesenteric glands, 392 
Mesenteries, 290, 294, 398 
Mesodaeum, 288 
Mesoderm, 61, 62, 69 
Mesodermic diverticula, 64 
Mesodermic somites, 69, 70 
Mesodonta, 39, 40 
Meso-hypomeres, 67 
Mesomeres, 65, 66, 67 
Mesonephric system, 402, 403, 404- 
406 
Meso-pterygium, 201 
Mesorchium, 418 
Mesovarium, 418 
Metacoele, 62, 64 
Metameric musculature, 227-272, 485 
Metameric muscles of human embryo, 
219 
Metameric muscles of Nec turns, 220, 
221 
Metanephric system, 406-409 
Meta-pterygium, 201 
Metazoan development, early stages of, 
50, 60 
Metencephalon, 460 614 
INDEX 
Metencephalon, development of, 479, 
480 
Microgametes, 7 
Midbrain, 459 
Middle ear, 298 
Mimetic muscles, 282, 283, 284, 285 
Mimetic muscles, innervation of, 505 
Mitosis, 54 
Molar cusps, phylogeny of, 308 
Monocytes, 348 
Monodelphia, 36 
Monophyodont dentition, 309 
Monotremes, 34, 197, 198 
Morgagni, hydatid of, 424 
Morphological equivalents of cranial 
nerves and ganglia, 511 
Motor nerve fibers, 441 
Motor nerves of eyeball, 498-501 
Motor neurone, 439 
Mouth cavity, bones of, 301 
Mouth cavity, organs of, 300 
Mouth with jaws, formation of, 296 
Mucosa, 289 
Mucosa, elaboration of, 291 
Muller’s duct, 413, 418 
Multipolar neurones, 440 
Multituberculata, 36 
Muscle, or muscles: 
abductores breves, 260 
abductores caudi, 238 
abductor coccygis, 238 
abductors, of pollex and minimus, 
263 
adductores, 255 
adductores breves, 260 
adductor mandibulas, 273 
anconeus, 248, 253 
appendicular, 218, 243-272 
axial, 218, 228-243 
axillary arch, 278, 280 
of the back, 232, 235 
biceps brachii, 253 
biventer cervicis, 236 
brachialis, 253 
branchiomeric, 218, 219, 222, 272- 
275, 485 
caudal, 238-240 
cervicalis ascendens, 233 
coccygeus, 238, 240 
coraco-brachialis, 253 
coraco-brachialis brevis, 248 
coraco-brachialis longus, 248 
cremaster, 427 
crureus, 255 
cucullaris, 249 
deltoidens, 252 
of diaphragma pelvis, 240 
digastric, 274 
dorsalis antebrachii, 257 
dorsalis scapulae, 248, 252 
dorso-laryngeus, 274, 275 
extensores breves, 258 
extensor carpi radialis, longus et 
brevis, 263, 265 
extensor carpi ulnaris, 263, 265, 266 
extensor communis digitorum, 262 
extensores caudi, 238 
extensor quadriceps femoris, 255 
extensor radialis, 258 
extensor ulnaris, 258 
episkeletal, 230 
of the eyeball, 241, 499, 500 
facial, 282-285 
flexor brevis minimi digiti, 263 
flexores breves profundi, 260 
flexores breves superficiales, 260 
flexor carpi radialis, 263, 265, 266 
flexor carpi ulnaris, 265, 266 
flexor caudi, 238 
flexor digitorum profundus, 262 
flexor digitorum sublimis, 262 
flexor pollicis longus, 262 
flexor radialis, 260 
flexor ulnaris, 260 
genio-glossus, 243, 276 
gemelli, 255 
glutaei, 255 
glutaeo-cruralis, 256 
gracilis, 255 
humero-antebrachialis, 253 
hyo-glossus, 243, 276 
hyposkeletal, 228, 229 
ilio-coccygeus, 240 
ilio-costalis, 233 
ilio-extensorius, 255 
ilio-femoralis, 255 
ilio-fibularis, 255 
infra-spinatus, 252 
in tegumental, 277 
intercostales externi, 230 
intercostales interni, 230 
intermandibularis anterior, 274 
intermandibularis posterior, 274 
intermetacarpales, 261 
interossei, 263 
interspinales, 236 INDEX 
615 
intertransversarii, 236, 237 
intertransversarii caudi, 238 
intervertebral system of, 236 
ischio-cavernosus, 240 
laryngei, 274, 275 
latissimus dorsi, 230, 248, 249, 277 
levator ani, 240 
levatores arcuum, 274 
levator scapulae, 248 
lingualis, 243, 276 
longissimus dorsi, 233, 241 
longus colli, 230 
lumbricales, 263 
masse ter, 273, 274 
metameric, 219, 220, 221, 227-272, 
485 
mimetic, 282-285 
multifidus, 234 
mylo-hyoideus, 275 
obliquus capitis, 241 
obliquus capitis inferior, 237 
obliquus capitis superior, 237 
obliquus extemus abdominis, 230 
obliquus internus abdominis, 230 
obliquus inferior oculi, 242 
obliquus superior oculi, 242 
obturator externus, 255 
obturator internus, 255 
opponens hallucis, 269 
opponens pollicis, 263 
palmaris profundus, 260 
palmaris superficialis, 260 
panniculus carnosus, phylogenesis of, 
277, 278 
panniculus carnosus, rudiments of, 
279-281 
patagial, 277 
pectoralis, 248, 252, 277 
pectoralis abdominis, 279 
peronaeus brevis, 266 
peronaeus longus, 266 
peronaeus tertius, 268 
piriformis, 256 
of posterior cervical region, 236, 237 
procoraco-humeralis, 248, 253 
pronator, 261, 263, 266 
propatagialis, 277 
psoas magnus, 230 
psoas parvus, 230 
pterygoideus externus, 274 
pterygoideus internus, 274 
pubo-coccygeus, 240 
pubo-ischio-femoralis externus, 254 
pubo-ischio-femoralis interims, 254 
pubo-ischio-tibialis, 254 
pubo-thoracicus, 229 
pubo-tibialis, 255 
rectus abdominis, 229 
rectus capitis posterior major, 237 
rectus capitis posterior minor, 237 
rectus femoris, 255 
rhomboideus, 231, 252 
rotatores, 234 
sacro-coccygei, 238, 239 
sacro-lumbalis, 233 
sacro-transverso-transversalis system 
of, 232-234, 240 
sartorius, 255 
semi-membranosus, 255 
semi-spinalis, 234 
semi-tendinosus, 255 
serratus magnus, 248, 252 
serrati posteriores, 231 
sphincter ani, 240 
sphincter colli, 281, 282, 284 
sphincter cloacae, 277 
sphincter marsupii, 277 
spinalis cervicis, 234 
spinalis dorsi, 234 
spino-spinalis system of, 234, 240 
splenius capitis, 231 
splenius cervicis (colli), 231 
stapedius, 275, 552 
sternalis, 279, 281 
sterno-cleido-mastoideus, 249, 275 
sterno-hyoideus, 229 
sterno-thyreoideus, 229 
stylo-glossus, 243, 276 
stylo-hyoideus, 275 
subclavius, 252 
supinator, 257, 263, 266 
supra-coraCoideus, 248, 252 
supra-spinatus, 252 
of tail, 238-240 
temporalis, 273, 274 
tensor tympani, 274, 553 
tenuissimus, 256 
teres major, 249 
teres minor, 252 
thyreo-hyoideus, 229 
tibialis anterior, 266 
tibialis posterior, 266 
trachelo-mastoideus, 233 
transversalis colli, 233 
transverso-spinalis system of, 234, 
240 616 
INDEX 
Muscle, or muscles: 
trapezius, 230, 249, 275 
triceps, 253 
vastus externus, 255 
vastus internus, 255 
vastus medialis, 255 
Muscles, classification of, 217, 218, 227 
derivation of, 218 
homology of, 223-226 
origin and insertion of, 224, 226 
striated vs. unstriated, 217 
Musculature, branchiomeric, 218, 219, 
222, 272-275, 485 
Musculature, metameric, 219, 220, 221, 
227-272, 485 
Musculosa, 289 
Myelencephalon, 460 
Myelencephalon, development of, 480, 
481 
Myocommata, 221 
Myotomes, 221 
Myotomic buds, 243, 244, 245 
Nails, 108 
Nares, anterior, 535 
Nares, posterior, 298, 535 
Nasal cavity (cavities), 298, 539 
Nasal cavity, pars olfactoria of, 535 
Nasal cavity, pars respiratoria of, 535 
Naso-lacrimal duct, 562 
Naso-palatine canal, 536 
Neck, muscles of, diagrammatic cross 
section, 276 
Necturus, ilia of, 192 
Necturus, ischia of, 192 
Necturus, muscles of, 220, 221, 246- 
249, 252-264, 266 
Necturus, posterior limb of, 192 
Necturus, pubic bones of, 192 
Necturus, sacrum of, 130 
Neencephalon, 480 
Nemertean theory, of Hubrecht, 578— 
580 
Neopallium, 471 
Neosternum, 142 
Neostoma, 478 
Nephridia, 397 
Nephrostome, 397 
Nerve, or nerves: 
abducens, 498-501, 514 
accessorius, 510, 514 
acusticus, 501, 502 
cranial, 494-512 
facialis, 501-503, 506, 509, 510, 514 
buccal branch of, 502 
external mandibular branch of, 502 
internal mandibular branch of, 503 
palatine branch of, 503 
superficial ophthalmic branch of, 
502 
glossopharyngeus, 505, 506, 509, 514 
lingual branch of, 506, 509, 511, 512 
tympanic branch of, 506 
gustatory, 504, 506, 512 
hypoglossus, 512, 514 
Jacobson’s, 506, 509 
occipital, 495 
occipito-spinal, 495 
oculomotorius, 498-500, 514 
olfactorius, 496 
ophthalmicus profundus, 501, 503 
opticus, 496 
plexus, brachial, 489, 491 
lumbo-sacral, 489 
spinal, 484-487 
spinal (vestigial), 495 
spino-occipital, 495 
terminalis, 497, 498, 518 
trigeminus, 501-503, 5°5, 5°9, 5*4 
lingual branch of, 505, 506 
mandibular branch of, 503-505 
maxillary branch of, 502, 505 
ophthalmic branch of, 505 
superficial ophthalmic branch of, 
502 . 
trochlearis, 498-501, 503, 514 
vagus, 503, 506, 509, 512, 514 
intestinal branch of, 509, 510 
lateral branch of, 507, 508, 512 
Nerve cells, 437, 438 
Nerve centers, 441 
Nerve components, 484 
Nerve, definition of, 441 
Nerve distribution, conservatism of, 
5i° 
Nerve fibers, 440 
motor, 441 
sensory, 441 
Nerve root, cranial, 484 
dorsal, 445, 486 
ventral, 446, 486 
Nerve substance, gray, 440 
white, 441 
Nerve tract, 441 
Nervous system, centralized type of, 
439 INDEX 
617 
Nervous system, concentration of, 449, 
450 
Nervous system, diffuse type of, 437 
Nervous system, functional divisions of, 
457 
Nervous system, origin of, 441-447 
Neural arches, 128, 566 
Neural crests, 444, 445, 508 
Neural folds, 442 
Neural plates, 442 
Neural spine, 566 
Neural tube, 64, 442 
Neural tube, developmental stages of, 
444 
Neurapophyses, 566 
Neurenteric canal, 287 
Neurite, 440 
Neurobiotaxis, 471 
Neuromasts, 458, 501, 525, 528 
Neuromasts, possible derivative of, 
528-530 
Neuromeres, 516 
Neurone, 439 
Neurones, parts of, 440 
Neurones, types of, 438-440, 443-446 
Nictitating membrane, 562 
Nipples, 117-119 
Nose, external, 541 
Notochord, 64, 124, 287 
Nuclei (nerve), 441 
Nucleus, 3 
Nucleus caudatus, 471 
Nucleus lentiformis, 471 
Nymphae, 435 
Occipital condyles, 135 
Occipital region, reduction of, 496 
Oculomotor nerve, 498-500 
(Esophagus, 319 
Oken, Lorenz, morphological theories 
of, 564 
Olfactory buds, 542 
Olfactory bulbs, 464 
Olfactory membrane, 541 
Olfactory nerve, 496 
Olfactory sense cells, 541 
Omentum, greater, 323 
Omentum, lesser, 323 
Ontogenesis, 15 
Ontogenetic laws, 21-25 
Oogenesis, 57, 58 
Operculum, 156, 550 
Operculum of skull, 156 
Opisthocoelous vertebrae, 133 
Optic cup, 555, 556 
Optic lobes, 460 
Optic nerve, 496 
Optic nerve as nerve tract, 497 
Optic stalk, 556 
Optic vesicles, 555 
Optic vesicles, development of, 459, 
460 
Orbital ring, 155 
Organisms, interrelations of, 17 
Organisms, unicellular vs. multicellular, 
10, 11 
Organs, early differentiation of, 286, 287 
Ornithorynchus, 34 
Osteolepis, skull of, 150, 151 
Ostium tubae, 413 
Otic ganglion, 506 
Otic region, 155 
Otic vesicle, 502, 543 
Ovary, 410, 418 
Oviduct, 413, 416, 418, 543 
Oviduct, origin of, 418 
Ovum, 48, 50 
Owen, Sir Richard, morphological 
theories of, 565-569 
Owen, skull theory of, 151, 567 
Owen’s theory of skull, estimation of, 
568 
Pacini’s corpuscles, 531, 532 
Pads of mammals, 92 
Palaeostoma, 478, 543 
Palatine cleft, 300 
Paleencephalon, 480 
Palingenetic characters, 16 
Pallium, evolution of, 468, 469-472 
Pancreas, 295, 323 
Pancreas, development of, 324 
Pantotheria, 36 
Papilla acustica, 547 
Parachordal region of skull, 126 
Paradidymis, 423 
Paraphysis, 475, 476 
Parapophysis, 566 
Parapsida, 32 
Paraseptal cartilage, 540 
Parasternum, 143 
Parathyreoid bodies, 318 
Parietal organ, 476 
Parker, W. K., 46 
Paroophoron, 422 
Patagium, 277 618 
INDEX 
Patten, arachnoid theory of, 580-583 
Pearl organs, 528 
Pectoral fin, development of, 244 
Penis, 429 
Penis of marsupials, 432 
Penis of monotremes, 430 
Penis, morphology of, 431 
Penis, of placental mammals, 433 
Perilymph, 550 
Peripheral nervous system, 446, 484, 
488, 489, 507, 513 
Perissodactyla, 39, 40 
Peritoneal cavity as expanded gonad, 
397, 4ii 
Peritoneum, 67, 290 
Peritoneum, folds of, 322 
Peyer’s patches, 394 
Phallus, 429, 430 
Pharyngeal glands, anlagen of, 316 
Pharyngeal pockets, 297 
Pharyngeal pouches, 294, 295 
Pharyngo-oesophageal lungs of am- 
phibians, 338, 339 
Pharynx, 294, 295 
Philthrum, 300 
Phylogenesis, 15 
Phylogenetic laws, 16 
Phylogenetic tree of mammals, 37 
Phylogenetic tree of vertebrates, 28 
Pigment, 80, 120-123 
Piltdown man, 44 
Pineal gland, 476 
Pineal organ, 476 
Pinna, 180, 553, 554 
Pinnipedia, 39 
Pithecanthropus, 44 
Pituitary body, 478 
Placenta, 35, 73, 74 
Placental mammals, 35 
Placode, 445, 508 
Placode, dorso-lateral, 502 
Placoderms, 29 
Plasma, 348 
Plastron of turtles, 107 
Pleura, 67, 346 
Pleurapophyses, 566 
Plexus formation, significance of, 489- 
494 
Plexuses, brachial, 489 
Plexuses, degree of constancy of, 492 
Plexuses, lumbo-sacral, 489 
Plica semilunaris, 562 
Polar bodies, 58 
Polypterus, anterior limb of, 213, 214 
Polypterus, fins of, 216 
Polypterus, posterior limb of, 190, 191 
Pouches, pharyngeal, 294, 295 
Pons, 480 
Pori abdominales, 411, 412 
Post-branchial bodies, 317, 318 
Post-minimus, 208, 269 
Posterior limb, development of, 189- 
191 
Posterior medullary velum, 481 
Posterior nares, 535 
Praschordal region of skull, 126 
Prae-hallux, 208 
Prae-pollex, 208, 269 
Prse and post-trematic rami of bran- 
chial nerves, 508, 509 
Preputial glands, 434 
Prevertebrata, 26 
Primates, 38, 41, 42 
Primitive ancestor of vertebrates, 443 
Primordial skull, diagram of, 144 
Proboscidia, 39 
Processus vaginalis, 426 
Procoelous vertebrae, 133 
Proctodaeum, 288 
Pronephric system, 401-404 
Proprioceptors, 485 
Proprioceptive tracts of spinal cord, 
458 
Pro-pterygium, 201 
Projection areas, motor, 483 
Projection areas, sensory, 483 
Prosencephalon, 459 
Prostate gland, 432, 433 
Prostatic vesicle, 424 
Protocoele vs. metacoele, 398 
Protochordata, 26 
Protoplasm, 1, 2 
Prototheria, 36 
Pubic bones of Necturus, 192 
Pulmonary system, 289, 299 
Pylorus, 295, 320 
Race history, 15 
Ramus communicans, 485, 520 
Ramus communicans, gray, 487 
Ramus communicans, white, 487 
Ratio of surface to mass, 3, 290, 291 
Receptor, 439 
Receptors, somatic, 525 
Rectum, 325 
Reflex arc, 439 INDEX 
619 
Reissner’s membrane, 549 
Reproduction by fission, 4, 5 
Reproductive organs, female, 418-423 
Reproductive organs, male, 423-434 
Respiration, amphibian type of, 334, 
336, 337 
Respiration, fish type of, 333-335 
Respiration, function of, 330 
Respiratory organs, aquatic, 334 
Reticular formation in nerve tissue, 446 
Retina, 460, 555 
Rhombencephalon, 459, 460 
Rhomboid fossa, 461, 481 
Rhynchocephalia, 32 
Rib, cervical, 140 
Ribs, 137-139 
Ribs, abdominal, 143 
Rodentia, 38 
Rods and cones, 559 
Rosenmuller, organ of, 422 
Round ligament, 418 
Ruminants, stomach of, 321 
Sacculus, 544 
Sacrum of Man, 131 
Sacrum of Necturus, 130 
St. Hilaire, Geoffroy, morphological 
theories of, 564, 570 
Salivary glands, 314, 315 
Sauripterns, pectoral fin of, 215, 216 
Sauropsida, allantois of, 353 
Scales, of mammals, 87, 90, 91 
Scales, placoid, 82 
Scaphyrhynchus, posterior limb of, 190, 
191 
Sclera, 558 
Scrotal ligament, 426 
Scrotum, 425 
Selachians, 29 
Selachian skull, 144, 145 
Selachians, circulation of, 354, 355 
Selachians, gill arches of, 168, 169, 172 
Selachians, shoulder girdle of, 193 
Selachians, venous system of, 370, 371 
Sella turcica, 477 
Semicircular canals, 544 
Semilunar ganglion, 502, 503 
Seminal groove, 429, 430 
Seminal vesicles, 432 
Semper, annelid theory of, 570-578 
Sense cells, olfactory, 541 
Sense cells vs. supporting cells, 523 
Sense organ, 439 
Sense organs, early relations of, 557, 558 
Sense organs, general consideration of, 
521-524 
Sense organs, general cutaneous, 525 
Sense organs, special cutaneous, 525 
Sensory canal system, 526 
Sensory cells, 443 
Sensory cells, origin of, 443-446 
Sensory nerve endings, types of, 530- 
532 
Sensory nerve fibers, 441 
Sensory neurone, 439 
Sensory neurone, origin of, 443-446 
Septum linguae, 314 
Septum pellucidum, 474 
Serial homology of carpus and tarsus, 
206 
Serial homology of limb muscles, 265- 
269 
Serial homology of limbs, 203, 204, 264, 
270 
Serosa, 289 
Sesamoid bones, 204 
Sexual homology, of external parts, 
table of, 436 
Sexual homologies, table of, 425 
Sexual kidney, 413 
Shoulder girdle, of fishes, 194 
Shoulder girdle, of mammals, 195 
Shoulder girdle, primitive, 193 
Shoulder girdle of Selachians, 193 
Sigmoid flexure, 327 
Sinus maxillaris, 539 
Sirenia, 39 
Skeletal elements of skull, list of, 163, 
164 
Skeletal parts: 
alisphenoid, 160, 165 
angulare, 172 
articulare, 147, 159, 173 
arytaenoid, 179, 343 
auditory ossicles, 159 
basi-branchial, 172 
basi-occipital, 158, 160, 165 
basi-sphenoid, 158, 160, 165 
carpalia, 203 
centrale, 204 
cerato-branchial, 172 
cerato-hyal, 180 
circum-orbital, 148 
clavicle, 195, 196 
cleithrum, 195, 196 
columella auris, 159 620 
INDEX 
Skeletal parts: 
coracoid, 197, 198 
cricoid, 344 
dentary, 159, 172 
ecto-pterygoid, 161 
entoglossum, 312, 314 
ento-pterygoid, 161 
epi-branchial, 172 
epicoracoid (cf. procoracoid) 
epiglottis, 179, 344 
epi-hyal, 180 
epi-otic, 161 
epi-pterygoid, 161 
ethmoid, 162 
exoccipitals, 147, 156, 160, 165 
falciforme, 208 
femur, 202, 203 
fibula, 202, 203 
fibulare, 203 
frontal, 148, 154 
furcula, 195, 197 
humerus, 202, 203 
hyobranchial complex, 179 
hyomandibular, 158 
hypo-branchial', 172 
incus, 159 > 
inferior turbina! 4 162, 537 
interclavicle, 194, 196, 200 
intermedium, 204 
inter-operculum, 156 
inter-parietal, 155 
inter-temporal, 157 
jugal, 149, 155, 158 
labial cartilages, 172 
lacrimal, 149, 155 
malleus, 159 
maxillary, 149, 158 
maxillo-turbinal, 536, 537 
Meckel’s cartilage, 159 
metacoracoid (cf. coracoid) 
meta-pterygoid, 161 
nasal, 148, 154 
naso-turbinal, 536, 538 
operculum, bones of, 156 
opisth-otic, 147, 156 
orbital ring, 155 
orbito-sphenoid, 147, 160, 165 
otic region, bones of, 155 
palatine, 149, 159 
para-sphenoid, 160, 164 
parietals, 148, 154 
paroccipital, 156 
patella, 204 
petrosum, 156 
post-frontal, 149, 155 
post-orbital, 155 
pre-maxillary, 149, 158 
pre-operculum, 156 
pre-parietal, 155 
pro-otic, 147, 156 
pre-frontal, 149, 155 
pre-sphenoid, 158, 160, 165 
procoracoid, 197 
pter-otic, 157, 161 
pterygoid, 149, 159 
quadrate, 147, 158 
quadrato-jugal, 158 
radiale, 203 
radius, 202, 203 
ribs, 13 7-140 
sacrum, 131 
scapula, 197, 199 
septo-maxillary, 161, 162 
sesamoid, 204 
sphen-otic, 161 
spheno-turbinal, 162 
squamosum, 156 
stapes, 159, 552 
sternebrae, 142 
sternum, 140-142, 199, 200 
stylo-hyal, 180 
styloid process of skull, 180 
sub-operculum, 156 
supra-occipital, dermal, 154 
supra-temporal, 157 
tarsalia, 203 
temporal, 553 
thyreoid, 179, 345, 346 
tibia, 202, 203 
tibiale, 203 
tracheal pieces of larynx, 179 
transversum, 161 
tympanic, 553 
tympano-hyal, 180 
ulna, 202, 203 
ulnare, 203 
vertebrae, 132 
visceral arches, 172 
vomer, 159, 165 
wish-bone, (cf. furcula, 
Skeleton, in relation to soft parts, 
125 
Skull, attachment to vertebral column, 
157 
Skull fusion of elements, 166 
Skull, inner arcade of, 159 INDEX 
621 
Skull, list of skeletal elements of, 163, 
164 
Skull, outer arcade of, 158 
Skull, primordial, 144 
Skull, roof of, 154 
Skull, Selachian, 144, 145 
Skull, sides of, 157 
Slime canals, 527 
Smell buds, 528, 542 
Smell, organ of, 533 
Soma, 11, 48 
Soma and germ, 7-9 
Soft palate, 300 
Somatic effectors, 485 
Somatic receptors, 485 
Somatic receptors, general cutaneous 
division of, 525 
Somatic vs. visceral activities, 456, 457 
Spermatic cord, 427 
Spermatogenesis, 57, 58 
Spermatophores, 414 
Spermatozoon, 48, 50-52 
Sphenoid sinus, 539 
Spheno-palatine ganglion, 506, 510, 520 
Spinal cord, 448 
Spinal cord of amphibians, 453 
Spinal cord of Amphioxus, 452, 453 
Spinal cord of cyclostomes, 453 
Spinal cord, external form of, 451-454 
Spinal cord, fissures and sulci of, 454 
Spinal cord, functional divisions of, 454, 
456-458 
Spinal cord, funiculi of, 455 
Spinal cord, gray and white substance 
of, 454-456 
Spinal cord, intumescentiae of, 451, 453 
Spinal cord, long course tracts of, 455 
Spinal cord, proper fasciculi of, 455 
Spinal cord, proprioceptive tracts of, 
458 
Spinal cord, regions of, 454 
Spinal ganglion, 486 
Spinal nerve roots, 484 
Spinal nerves, functional components 
of, 484-486 
Spinal nerves, vestigial, 495 
Spiraculum, 298 
Spiral ganglion, 502 
Squalus, skull and branchial skeleton 
of, 169 
Stable vs. labile functional types, 473, 
474 
Stegocephali, 30 
Stegops, skull of, 153 
Stenson’s canal, 536 
Sternebrae, 142 
Sternum, 140-142, 200 
Stomach, 319 
Stomach, cardiac and pyloric ends of, 
320, 321 
Stomach, compound, of ruminants, 321 
Stomach, curvatures of, 320, 321 
Stomach, fundus of, 320 
Stomachs of mammals, 321 
Stomatodaeum, 288 
Stomato-pharyngeal cavity, 295 
Stratum corneum, 79 
Stratum germinativum, 79 
Stratum lucidum, 79 
Stylo-hyoid ligament, 180 
Subcutaneous lymph-sacs, 388 
Subdural space, 388 
Subfascial spaces, 388 
Submaxillary (submandibular) ganglion, 
506, 520 
Submucosa, 289 
Subperitoneal spaces, 388 
Subvertebral space, 387 
Superciliary bristles, 563 
Supernumerary elements of carpus and 
tarsus, 206, 207 
Suprapericardial bodies, 317 
Sympathetic system, 446,484, 485, 519, 
520 
Symphysis pubis, 193 
Synapsida, 32 
Syntropists, 269-271 
Syrinx, 345 
Tactile cells, 530 
Tactile corpuscles, 530 
Tactile sense, general, 530 
Tail, muscles of, 238-240 
Tapetum nigrum, 556 
Tarsal cartilages, 562 
Tarsal glands, 115, 563 
Tarsal bones, names of, 203 
Tarsus, supernumerary elements of, 
206, 207 
Taste buds, 532 
Taste, organs of, 532 
Teeth, 300-312 
Teeth, insertion of, 302 
Teeth, origin of, 82, 83 
Teeth, replacement of, 310 
Teeth, types of, 302, 304 622 
INDEX 
Telencephalic lobes of Ichthyopsida, 
464-467 
Telencephalon, 460 
Teleosts, 30 
Teleosts, skull of, 152 
Testes, 410 
Testes, descent of, 428 
Thalami, 475 
Therapsida, 33 
Theories of development of lymphatics, 
two schools of, 393 
Theromorpha, 33 
Thinopus, 216 
Thinopus, footprint of, 209 
Third ventricle, 460, 474 
Thoracic duct, 387, 390 
Thymus glands, 315, 316, 395 
Thyreoid cartilage, 179, 345, 346 
Thyreoid glands, 315, 316, 318, 395 
Toes, names of, 203 
Tongue, 312 
Tongue bars of Amphioxus, 319 
Tonsils, 395 
Tornaria larva of Balanoglossus, 594 
Tornaria, as related to Echinoderms, 
594 
Trachea, 299 
Trachea of amphibians, 343 
Tracts, ascending, 482 
Tracts, descending, 482 
Tracts, proprioceptive, 458 
Tracts, pyramidal, 483 
Trochlear nerve, 498-501, 503 
Trigeminal nerve, 501-503, 505 
Tritubercular theory, 307-309 
Tunicata, 26 
Tunicata, as ancestors of Amphioxus, 
588-591 
Tunica dartos, 427 
Tunica vaginalis communicans, 427 
Tunica vaginalis propria, 427 
Turbinalia, 536 
Turtle, skull of, 157, 158 
Tylopoda, 40 
Tympanic bulla, 553 
Tympanic membrane, 551 
Tympanum, 551 
Typical nerve, functional divisions of, 
484-487 
Typical spinal nerve, divisions of, 
487 
Typical spinal nerve, rami of, 487 
Typical vertebra, of Owen, 565, 566 
Umbilical cord, 35 
Uncinate processes, 138 
Ungulates, 38 
Urbild, of Goethe, 564, 565 
Ureter, 407 
Urethra, 408 
Urethral glands, 432 
Urinary bladder, 408 
Urino-genital organs, male, 431 
Urino-genital outlets, 325 
Urino-genital sinus, 418 
Urino-genital system in Anamnia and 
. Amniota, 415 
Urino-genital system, relation to 
coelom, 396, 397 
Urodeles, 31 
Urodeles, venous system of, 373 
Uterus, 418-420 
Uterus, of Amphibia, 414 
Uterus, masculinus, 424 
Uterus, origin of, 418 
Uterus, varieties of, 420, 421 
Utriculo-saccular canal, 544 
Utriculus, 544 
Vagina, origin of, 418 
Vagus nerve, 503, 506 
Vasa aberrantia, 423 
Vasa efferentia, 412 
Vas deferens (cf. ductus deferens) 
Veins, 348 
abdominal, 372, 376 
allantoic, 352 
anonyma, 380, 382 
azygos, 374, 375, 381 
cardinal, 352, 356, 357 
cardinal, in mammals, 375 
Cuvier, duct of, 352, 356, 357 
ductus venosus Arantii, 378, 379 
hemiazygos, 381 
hepatic, 353, 358 
hepatic portal system, 354, 357 
hepatic portal system, development 
of, 374, 379, 380 
iliac, 356 
jugular, 380 
lateral, 356 
omphalo-mesenteric, 353 
portal, 353, 357, 377 
post-cava, 372 
post-cava, development of, 374, 
375 
renal, 354, 357 INDEX 
623 
renal portal, 354, 357, 377 
sinus venosus, 356 
subclavian, 356, 380 
umbilical (cf. allantoic) 
vitelline, 350 
Velum palati, 300 
Velum transversum, 475 
Venous system of frog, 389 
Venous system of Man, history of, 
381 
Venous system of selachians, 370, 371 
Venous system of urodeles, 373 
Ventral nerve root, 446, 486 
Ventricles of brain, 442, 460 
Vemix caseosa, 97 
Vertebra, typical, of Owen, 565, 566 
Vertebrae, kinds of, 132 
Vertebrae, typical, of skull, 568, 569 
Vertebral column, development of, 126- 
129 
Vertebrate embryos, diagrams of, 65, 
69 
Vertebrate embryos, early stages of, 70 
Vertebrate head, segmentation of, 514- 
5i9 
Vertebrate skull, elements of, 154-163 
Vesicular glands, 432, 433 
Vestibular ganglion, 502 
Vibrissae, 529 
Visceral, arches, 172 
Visceral effectors, 485 
Visceral skeleton, morphology of, 178 
Vomero-nasal organ, 540 
Weber’s apparatus, 544 f. 
White nerve substance, 441 
Winslow, foramen of, 323 
Wolffian body, 417 
Wolffian duct, 418 
X-chromosomes, 56 
Yolk, influence of, 71 
Zygapophyses, 566 
Zygote, 7, 48