BIOLOGY
LIBRARY

'HISTORY

OF

THE HUMAN BODY

BY

HARRIS HAWTHORNE WILDER

ii

Professor of Zoology in Smith College

NEW YORK

HENRY HOLT AND COMPANY

1909

LtBRAHY
G

COPYRIGHT, 1909,

BY
HENRY HOLT AND COMPANY

Seinem Lehrer und Freunde
<£*lj*tm-Ijufrat Unhurt WwterBljrtm

wird dieses Buck
in Liebe und Dankbarkeit gewidmet.

DER VERFASSER.

257867

PREFACE

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-

VI

PREFACE

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 vii

1. In the case of the bones of the carpus and tarsus. For
these the BNA nomenclature employs the terms used on the
Continent, and especially Germany (e.g., triquetrum, multan-
guhim majuSj 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 vence cava 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 (i) 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
[vas] deferent, 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

VI 11

PREFACE

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 v

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 ... 76
V. THE ENDOSKELETON . . . . 122

VI. THE MUSCULAR SYSTEM 189

•

VII. THE DIGESTIVE AND RESPIRATORY SYSTEMS . . . 257

VIII. THE VASCULAR SYSTEM 317

IX. THE URO-GENITAL SYSTEM 365

X. THE NERVOUS SYSTEM 406

XL THE SENSE-ORGANS 465

XII. THE ANCESTRY OF THE VERTEBRATES 506

APPENDIX . . . . 539

PLATES

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 uro-genital system in Amniotes
from stage of sexual indifference (a) to male vb),
and to female (c). In part after GEGENBAUR. . 386

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]. 414

PLATE V. Longitudinal median sections of Vertebrate brains
corresponding to the second half of the series in
Fig. 117. [After EDINGER] . . . . 415

PLATE VI. Diagram of cranial nerves in Anamnia. [After

WIEDERSHEIM]. ...... 448

PLATE VII. Diagram of cranial nerves in Amniota. [After

WIEDERSHEIM]. ...... 449

PLATE VIII. Inter-relation of Trigeminus, Facialis, Glossopharyn-
geus, and Vagus, together with the sympathetic
ganglia in man. Based upon diagrams by several
anatomists (ARNOLD, GRAY, GEGENBAUR). . 456

"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 tmd wiederhole es, die Welt konntt
nicht bestehen, wenn sie nicht so einfach ware."

JOHANN WOLFGANG GOETHE, in Eckermann,
Gesprache mit Goethe, n Apr., 1827.

OXE 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^itxaj) constantly builds up its substance anew from
the surrounding medium, usually in excess of that lost by dis-
integration, and possessed of qualities identical writh 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

^^:;>; '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^ukimately 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
oxygen, and also the expulsion of all waste products, take place
on the external surface, 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.

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 (i) a lengthening of the nucleus;
(2) a contraction of its middle portion, producing a form

FIG. i. 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
one, 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, Ha-ma-
morba 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 phenomena of reproduction by fission, there
is another procedure which has been observed in many forms

6 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 Paramacium.
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. 5 (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 introduction 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 usually 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 so ma, 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

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 living 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-

A A

^ _

A A * A A A,J_A .A. A A

A ' " A A ~ ATTuA AMAj

• • • • s t yrfrfrtiTT"""8' «^sffff*«***«

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 th'e 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.

io 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 n

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 specialization, 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 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.

v 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
by the vast hordes of carnivorous enemies generated in the
depths 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 re-
tained in the cranial region, forming the dermal bones of the
skull (f rentals, 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 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 result 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 IVieder-
hohmg der Phylogenie (Stammesgeschichte)."

16 HISTORY OF THE HUMAN BODY

are called c&nogerietic, 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 vvay into neiv 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

15

FIG. 6. Hypothetical tree illustrating the interrelations of organisms.

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.

by the continuance of the line 29-35, m the same direction as
28-29, either remain exactly as their ancestors, or probably
become more highly specialized in the same direction. Con-

i8 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 inter-relationship 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 of 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
300 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 now distinct,
and they have thus been of the greatest value in tracing out
phylo genetic 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 having 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, a 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 Lingula 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 HISTORY OF THE HUMAN BODY

condition is indicated in the diagram by such forms as 30 or
300 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 dinosaurs 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 those of quite distant
ancestors, as 6 and 30, have become similarly modified, until
they may resemble one another so closely as to deceive the
casual observer. Forms 20 and 33, representing totally dis-
tinct 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 palseontological 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 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 the 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 iSb has passed through the
stages 1 8a, 18, 17, 6, 5, 4, 3, 2, and I 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 of the two developmental histories will
be coincident.

To illustrate those points: if the development of 16 and 34
be compared, only the early stages i, 2 and 3 will be seen to
coincide; if, however, the developmental histories of 21 and
1 6 be taken, they will be found coincident as far as their last
common ancestor, 5. In 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 specialised the animal, the more
changes its ancestors have passed through; and therefore so
much the more is to be recapitulated 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 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 palin genetic characters, or those
which are true repetitions of the past history, and cccnogenetic
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 sub-
stantial 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 tunlcates, and the
small and slender Amphioxus. Formerly classed at great
distances from one another among molluscs, worms and even
plants (e. g., sessile 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, a small marine
creature something like a headless fish, which is found in the

* For a detailed classification of vertebrates, to accompany this chap-
ter, the reader is referred to the Appendix

THE PHYLOGENESIS OF VERTEBRATES 27

shore water of the warmer seas, usually buried in the sand in
a perpendicular position, with the anterior end projecting into
the water, expanded into a sort of hood for the collection 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-
o.nis 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 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,

/PLACE.VTAL MAMMALS

XUS

FIG. 7. Phylogenetic tree of 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.

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 Amphio.rus
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-

phlOJCUS.

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 T:ew 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 a very close relationship to one
another. They are characterized by the tendency of the
scales to fuse into bony plates, a tendency which in the past
resulted in the development of a special group, -the placo-
dcrms, which were entirely covered by a suit of mail formed
in this way. Similar plates cover the head in all modern
ganoids and they occur in rows along the body in a few
forms (sturgeons). The skeleton is mainly cartilaginous in

30 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

Gymnophiona, which still possess scale rudiments, not visible
externally. The Stegocephali are of extreme importance, 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 priceless value to mor-
phologists. As it is, however, we are in possession of a large
number of fossil remains, many of them extremely well pre-
served, and representing four distinct orders ; and further dis-
covery along this line may well be expected at any time. Of
the soft parts the fossil imprints furnish 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 internal 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, also in many sections, unfortunately,
" 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 Cryptobranchus of
Japan, the largest of all living amphibians, attains the length
of a meter.

The Anura, 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
Gymnophiona 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.

Arising also from the Stegocephali come the reptiles, which
have apparently developed along two lines, the one leading to

32 HISTORY OF THE HUMAN BODY

the birds, the other to the mammals. Of the first of these, the
oldest group is that of the Rhyjiekocephalia, mainly fossils,
but with a single living species, which fate has preserved in
New Zealand, the Sphenodon (Hatteria). This represents
the ancestor of lizards and snakes, Lacertilia and Ophidia re-
spectively, and also a group of extinct reptilian giants, the
dinosaurs, whose nearest living allies are the crocodiles. Here
this line would have ended, so far as human knowledge is
concerned, had it not been for the chance discovery, about
the middle of the nineteenth century, of two specimens of one
of the most remarkable " missing links " ever found, the
Archceopteryx, a form midway between reptiles and birds, and
of undoubted affinity to the stem of the dinosaurs. This
creature was bird-like, possessed wings and a certain number
of contour feathers, but had a long vertebrated tail, several
free digits in the hand, furnished with curving claws, and a
heavy jaw containing conical teeth, reptilian in character. This
discovery, followed by that of the toothed birds, completed
the chain of evidence, and supplied one of the most isolated
groups of vertebrates with a definite line of ancestry.

The other line of reptiles, which may have arisen from the
Stegocephali more or less independently of the first, was that
beginning with the theromorphs, an extinct group, many of
which attained a gigantic size. Some members of this group
are so near the mammals in many particulars that it has been
only with the greatest care, and through the consideration of
all the available parts, that their reptilian nature has been de-
termined. In studying the remains of these forms, especially
those of the sub-group of theriodonts, the most of which were
small animals, like the earliest mammals, it seems impossible
not to assign them a close relationship to the latter, probably
that of actual ancestry. Indeed, there is at present but one
other claimant for that position, and that is the group of
Stegocephali, and as these were contemporary with the
theromorphs,. and at one time probably graded into them
by imperceptible transitions, the two views are not very wide
apart. All things considered, it seems that the gap between

THE PHYLOGENESIS OF VERTEBRATES 33

Stegocephali and the mammals requires some intermediate
link, and thus the addition of the theromorphs in this place
seems rather a completion than an opposition to the theory
of Stegocephalan ancestry.

The only living reptiles associated with the same branch as
the theromorphs are the turtles (Chelonia), which, although
highly specialized in the matter of trunk skeleton, are of the
greatest value in regard to their soft parts, which are un-
doubtedly similar to those of the extinct members of the
branch, and are thus the best living representatives of
the important stage between amphibians and the early
mammals.

The earliest mammalian remains are contemporary with
those of the theromorphs, 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 New Zealand, the Duck-bill Platypus (Ornith-
orhynchus) and the spiny ant-eater (Echidna). The latter
has no connection with the true ant-eaters (Myrmecophagida)
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

34 HISTORY OF THE HUMAN BODY

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 mono-
tremes 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 an-
cestors that were thus related; that is, that the ancestor of
modern marsupials was a direct descendant of the ancestor of
the monotremes.

Beyond the marsupials all the mammals are placenta!, 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 between
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 com-
paratively recent, geologically speaking, have specialized in all
directions, and now occupy almost every available environment,
not only of the land, but of the water. 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 PHYLOGENESIS OF VERTEBRATES 35

the manner of parasites. Many are specially adapted to the
capture of insects, either on or beneath the surface 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 dif-
ferentiated and become specially adapted to feed either upon
low herbage or the leaves of trees, roots, bark or 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 theromorphs, 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. The monotremes are the nearest living
descendants, and they have been derived from them through
an ancient and closely related group, the Multituberculata.
All three of these groups were reptilian in structure, and may
be classed together and in contrast to all the other mammals,
as the Sub-class Prototheria.

\Yhile 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-

HISTORY OF THE HUMAN BODY

came respectively the Didelphia, or marsupials, and the Mono-
delphia, or placentals. The first of these lines then differ-

PROTOTHERIA

EUTHERIA

FIG. 8. Phylogenetic tree of mammals.

A branch that terminates in an arrow point still possesses living representativesr,
one that ends in a short cross bar is extinct.

entiated into the marsupialian Orders of the present time, dis-
tinguished mainly by variation in the dentition, and the second,

THE PHYLOGENESIS OF VERTEBRATES 37

which resembled the present-day Insectivora, passed over into
that Order.

This insectivorous stem, in addition to perfecting its own
type along the narrow lines first laid down, developed several
lines of differentiation, and it was from these that all the
higher placental mammals have arisen. A very ' primitive
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 they are called the Xenar-
thra in contrast to those in which this joint is normal, the No-
marthra. This group has 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 Chiropfera or bats, and the Galeopithecus, a single species
found in Madagascar, but not nearly related to any of the
other stems.

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
mammais, 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-

38 HISTORY OF THE HUMAN BODY

rections and thus gave origin to the Primates, the Condy-
larthra, a generalized form of ungulate, 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,
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, 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

THE PHYLOGENESIS OF VERTEBRATES 39

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-
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 mammuth, 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 interrelationships 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

40 HISTORY OF THE HUMAN BODY

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
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 palseontological 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 presentation
which, although opposed to the prevailing opinion, is in strict
accord with the facts ; since in anatomical structure these ani-
mals show comparatively little deviation from the primitive
mammalian type and do not exhibit the extreme specialization
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; certain other
groups, the peculiarities of which are not so striking, 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 am-
phibians 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,

THE PHYLOGENESIS OF VERTEBRATES 41

however, are very simply constructed in these particulars
and remain close to the lower type as shown in the marsupials.
Primates are also primitive in their muscular system, pos-
sessing in many instances a single undifferentiated 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 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 im-
pressions, 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 far beyond the condition of
the other Primates that the world has long, and perhaps 'will-
ingly, been deceived in regard to their true relationship. In
spite of all prejudice, however, man is, anatomically speak-
ing, 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 development of the
cerebral hemispheres, is not a modification calculated to pro-
duce important correlated changes in the other parts. Of
the two living Sub-orders, the Lemur oidea and the Anthro-
poidea, 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 separates
the orbital and temporal fossae, Man is seen to be an Anthro-
poid; and in important characters, such as the reduction of
the premolars from three to two, he agrees with the Catarrhine
division of this Sub-order. If we employ the usual schedule
of values to be attached to points of structural difference, 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 be between them.

42 HISTORY OF THE HUMAN BODY

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
mammuth, and numerous carvings and incised drawings have
been discovered in which the latter animal has been por-
trayed 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.

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
a distinct species, Homo primigenius (H. neanderthal ensis).
Such remains have been found at Spy in Belgium, in the
Neanderthal near Diisseldorf, at Cannstadt in Prussia, in
the bed of the river Liane near Boulogne-sur-mer, and in
other localities, the specimens all closely corresponding to one
another and equally unlike the present living species. The
forehead and cranium of the " Neanderthal man," as seen
from these specimens, was extremely low and flat, and the
superciliary ridges above the eyes were so heavy and promi-
nent that they formed together a pair of projecting arches
hung over the deep-set eyes. There was almost no chin. The
height was that of a rather small man; the arms were not
excessively long, but the thigh-bones were permanently curved
and the tibiae were short, so that an absolutely erect position
was impossible. In spite of the general ape-like appearance
and the low character of the cranium, the actual capacity of
the latter was about that of a modern Australian, and the
presence of flint implements in association with the remains
show that this species could lay claim to being termed a man
although of a distinct type from the one that has survived.

The fossil remains of an animal, in many respects pre-

THE PHYLOGENESIS OF VERTEBRATES 43

cisely intermediate between Homo primigenius and the an-
thropoid apes, were discovered in Java in 1891 in deposits of
the late Tertiary period, and were named Pithecanthropus
erectus, the generic name, " ape-man," having been proposed
some years before for the then hypothetical transition form, the
" missing link " of popular fancy. These remains consist of
a cranium, a femur and three molar teeth, and although not
found in contact with one another, their relation to their
surroundings was such as to declare them the -fragments of a
single skeleton.

In the cranium the ape-like characters seen in Homo primi-
genius are here still more pronounced ; the cranial vault is still
lower, the superciliary ridges are still more prominent, closely
approximating those of a chimpanzee or gibbon. The pro-
portions of the teeth suggest a dental arcade intermediate be-
tween the flattened form seen in man and the elongated arch
of the living anthropoids; the probable shape of the tongue
and hard palate, as deduced from this, would seem to have
allowed the production of many of the more elementary sounds
occurring in human speech. An independent fact that corrob-
orates this conclusion was determined later when the con-
figuration of the brain surface was obtained by means of a
cast of the interior of the cranium, for here the center of
articulate speech (the left lower frontal convolution) was
found to have been more developed than in the highest apes
but considerably less so than in man.

The femur does not exhibit the transitional characters
which one would be led to expect from the nature of the
cranium, for it is essentially human in form and shows a
higher type than that of the European Homo primigenius.
Pithecanthropus must thus represent a parallel or collateral
form in which the development in the direction of an erect
position had reached a high plane while the cranium and
brain remained at a stage intermediate between the highest
apes and the Neanderthal man.

Concerning the ancestry of Pithecanthropus and its rela-
tionships to the apes the widest opinions still prevail, but the

44 HISTORY OF THE HUMAN BODY

trend of opinion leads to the rejection of the four living an-
thropoids (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 expect to
find the connection in some extinct type, as, for example, the
European Dryopithecus of the middle Miocene. As the case
stands at present, however, there are few animal species con-
cerning 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 theromorphs,
which would throw further light upon the reptilio-amphibian
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
morphologist, but collateral lines furnish many helpful sug-
gestions, 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.

5. Urodeles (as a substitute for the Stegocephali).

6. Reptiles (preferably the chelonians, as the nearest living

allies of the theromorphs).

7. Monotremes (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).

THE PHYLOGENESIS OF VERTEBRATES 45

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 anthro-
poids).

11. Cercopithecidoe (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
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 ge-
nera 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
features 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

46 HISTORY OF THE HUMAN BODY

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
what could just be read off as pertaining to the sea-turtle ;
whilst, underlying the whole, the Fish in its simplest Myxinoid
form could be traced in morphological hieroglyphics." *

In following out the historical development of the different
systems, as outline^ in the ensuing chapters, both embryonic
and phylogenetic records have been drawn upon as the primary
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 caenogenetic changes ; in the latter the record is far
1 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

* W. K. Parker, in Trans. Roy. Philos. Soc , 1869, pp. 803-804.

THE PHYLOGENESIS OF VERTEBRATES 47

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.

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.
Chap. VIII) suggests that the phenomenon may have been
widespread 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-
gametes 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

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

FIG. 9. Earliest stages of Metazoan development.

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.

into the ovum, i. e., to the union of the two gametes, and is
thus synonymous with conjugation, when 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 here amoeboid in form and possesses functional pseudo-
podia, being often impossible to distinguish from genuine
Amoebae, 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
attention in the ontogenetic recapitulation.

In size ova vary greatly, but the difference is due mainly
to the actual amount of food stuff, or yolk, which is 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 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 now, as has been shown
bv 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
c.ble 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 doii'n to each cell of the organism. Although this is
still the mystery of mysteries to the biologist, the careful study
of the past twenty or thirty years, directed upon this very
point, has revealed much, but in so doing has added more

* Until 1875 it was generally supposed that more than one sperma-
tozoon took part in the fertilization of an egg. The true facts in the
case were first determined by observation, and later proven by direct
experiment. Polyspermy, or the introduction into the egg of more than
one spermatozoon, has been experimentally brought about in the eggs of
various marine animals by such methods as the application of heat and
cold or the use of poisons, and in all cases the resulting development has
been abnormal.

54

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
extreme susceptibility to staining fluids when artifically treated
for purposes of microscopic examination, has been designated
by the non-committal term of chromatin. During functional

VII

VIII

FIG. 10. 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.

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 equal bodies, the
chromosomes. The number of these found in any somatic
cell of a given species of animal is always the same for that
species, but may be different in an allied species, and the num-
ber seems to bear no reference to the size or the degree of
complexity of the animal. For instance, the number four oc-
curs in Ascaris, the pin-worm, eight in certain nematode

THE OXTOGEXESIS OF VERTEBRATES 55

worms, twelve in the mole-cricket, and sixteen in a water
beetle, the rat and Man, as well as in the pine and the
onion. Cyclops, a minute crustacean, possesses twenty-four,
as do also the frog, mouse, snail, lily and a fern (Osmnnda).
The earth-worm has thirty-two chromosomes, the torpedo thir-
ty-six, and Artemia, a small shrimp, the unusual number of
1 68. Whenever a cell divides in a growing or proliferating
tissue, the maintenance of the same number of chromosomes
in each of the two resulting cells is effected by means of a
complex mechanism of minute threads, radiating from two
opposite centers, which results in the separation of each in-
dividual chromosome into equal halves, thus assuring for each
daughter cell, not merely the same number of chromosomes,
but halves of the same ones. This process is known as mito-
sis or karyokinesis.

To the general rule concerning the constancy in the num-
ber of the chromosomes, there is, however, one very important
exception, and that is, in the germ cells, that become the
gametes in a conjugation, the starting point of a new organ-
ism. Here, owing to a difference in their mode of formation,
the number of chromosomes in a given species is exactly one-
half of that characteristic of the somatic cells of the same
species, and it is only by the fusion of the two gametes, ovum
and spermatozoon, that the normal somatic number is re-
stored. This reduction of the number of chromosomes is
brought about through an extremely complex process, the es-
sentials of which are : first, the formation of certain germ-cells,
spermatogonium or oogonium, which develop twice the normal
number of chromosomes, and, secondly, two successive divi-
sions of the cells, and of the number of chromosomes also, by
means of which four cells are produced, each with one-half
the normal number. In the case of the male cells each of the
four is effective, and, through a metamorphosis in its form,
becomes a functional spermatozoon; but in the case of the
female, owing to the disadvantages which would arise from
the division of the yolk into four ova of equal size, one of
them retains it all and becomes a functional ovum while the

56 HISTORY OF THE HUMAN BODY

others become yolkless, abortive, eggs, attached to the ovum
and called polar globules. Owing to the enormous disparity
in size between the abortive and the functional ova the divi-
sions of the oogonium by which they are formed were for a
long time not recognized as true cell divisions, but the polar
globules were spoken of as extruded or cast off from the

VI VII

FIG. ii. Diagram of fertilization.

Stage I represents the egg just previous to maturation. The chromosomes, ar-
ranged in tetrads, are twice the number found in somatic cells, which, in this dia-
gram is assumed to be 12. At II a mitotic figure is formed, which, in III, results
in the formation of two cells; a little one, the first polar globule, a, and the egg,
each with a reduced number of chromosomes, in this case 12. In IV and V a
second mitotic figure is formed, which results in the expulsion of a second polar
globule, b, and the reduction of the chromosomes of the egg nucleus to six, one-
half the normal number. Meanwhile a spermatozoon head has entered the egg, com-
posed mainly of chromatin, the equivalent of the six chromosomes of the reduced egg
nucleus, and a new centrosome, to replace that of the egg which was destroyed during
the expulsion of the second polar globule. The spermatozoon head rotates through
180°, thus bringing the centrosome between the male and female germ nuclei, as
in VI. The first cleavage spindle is seen forming in VII and VIII, after which
the cell divides into two and development begins.

"egg/' terms which express the phenomena as observed, but
mask their true biological significance.

Usually, owing probably to the rudimentary condition of
the polar globules and their lack of function, the globule
formed by the first of the two reductive divisions, and hence
the equivalent of the definite ovum plus one abortive egg,
does not carry through its division into two, but remains as

THE ONTOGENESIS OF VERTEBRATES 57

a single mass, and is spoken of as the " first polar globule "
in distinction from that resulting from the second division,
which is termed the " second." Strictly speaking, the first
and second polar globules are not equivalent, but the first is
the equivalent of two abortive eggs and the second of but
one ; and corresponding to this the first polar globule pos-
sesses twice the number of chromosomes exhibited by
either the second globule or the functional egg. Furthermore,
the two polar globules are frequently not extruded until after
the entrance of the spermatozoon, the presence of which seems
to act as a stimulus for these cell divisions. In these cases,
the unfertilized " egg " is, strictly speaking, not the ovum,
but the oogonium, which requires the two reductive divisions
to become the equivalent of the spermatozoon.

To illustrate this by an actual example, let us suppose an
animal that possesses in each somatic cell sixteen chromosomes.
The spermatogonium twould thus possess thirty-two which, by
the reductive divisions, would result in the formation of four
spermatozoa, each with eight. Similarly the oogonium would
possess thirty-two chromosomes, a number which would be
reduced to sixteen by the expulsion of the first polar globule,
the latter body having the like number. The second reductive
division would result in the formation of a second polar body
with eight chromosomes, and would leave eight in the egg.
This number, when added to the same amount introduced by
the spermatozoon, restores the normal number, sixteen, and
thus forms the first cell of the new organism, equipped with
the regular somatic number, one-half from either parent.

In this is seen a provision for avoiding that enormous in-
crease in the number of chromosomes that otherwise must be
the inevitable result of each conjugation. Furthermore, when
taken in connection with the fundamental law of heredity that
in the long run the two parents are equally potent in trans-
mitting their characteristics to their offspring and that neither
sex has the preponderance of influence in this direction, it is
seen that the hereditary substance must lie in the chromosomes
alone, since these are the only elements in which both parents

58 HISTORY OF THE HUMAN BODY

are equally represented. Neither the preponderating bulk of
the ovum (macro-gamete) nor the flagellum and other loco-
motor 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 development of the new or-
ganism ; 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 generation.

In this " Continuity of the germ-plasm " is found the ma-
terial basis also for the recapitulation theory, the law of
biogenesis explained in the first chapter; for the continuously
living chroniatin, which pervades each cell of an organism,
has in its ozvn 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
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, fol-
low a general geometrical plan and result in the formation of a
mass of cells that shape themselves into a definite embryological
stage, that of the blastula. As the various geometrical forms
assumed by the cells during the cleavage stages are all rep-
resented 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

THE ONTOGENESIS OF VERTEBRATES

59

floats freely in the water, as in that of many of the inverte-
brates, each cell is provided with long vibratile flagella, 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 collapse of one portion of the blastula, as in the diagram,

in

vm

FIG. 12. Early Metazoan development; typical. [After models of Am-
phioxus by HATSCHEK.]

I, the egg. II, III, and IV, cleavage stages. V and VI, blastula f in VI, which
represents a somewhat older stage than V; one-half has been removed. VII repre-
sents the beginning of the gastrular invagation, and VIII is the completed gastrula,
both sectioned as in VI.

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 presence of the gastrula mouth
or protostome. This latter leads into the large central cavity,
the gastroccele, which has developed from the exterior at the
expense of the cavity of the blastula, the blastoccele. In some

60 HISTORY OF THE HUMAN BODY

gastrulae this latter cavity is completely obliterated by the com-
pletion of the process of imagination, 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 medusse 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 medusse 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
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 de-
velop, although through different means, the three following
attributes, which differentiate them from the lower Metazoa,
the Ccelenterata :

1. The formation of a third germ element, the mesodenn,
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-

* This is carefully to be distinguished from the mesoderm, or middle
layer, which appears first in animals above the coelenterates 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.

THE ONTOGENESIS OF VERTEBRATES 61

trula assumes the form and position shown in Fig. 13, b, in
which it becomes placed horizontally, with the apical pole for-

a

•ZtHHHHSSS

ant,

FIG. 13. Diagrams of gastrulae. [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, t. e., that of the gastrula; ab, secondary axis, that of the adult
Amphioxus; en, neural canal; cne, neurenteric canal; np, neuropore; ntc, notochord.

ward and the protostome posterior and dorsal and in the
median line. The plan of structure is a bilateral one, with

62 HISTORY OF THE HUMAN BODY

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 XII. ]

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-
resented as cut transversely across, the diagram representing
the posterior portion and showing the cross-section as well as
a portion of the length. The most superficial of these changes
involves a longitudinal mid-dorsal stripe, which becomes grad-
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

* The word Anlage is borrowed from the German to express a concep-
tion for which there is no 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.

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

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
intestinefand expand until they fill practically the entire space
between ectoderm and endoderm and lie in close contact to
one another. They thus form a series of paired cavities, the
metacccles, those of each side separated by transverse par-
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
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
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 protoccele, the original cavity included
between ectoderm and endoderm, and thus at the completion
of the process the protoccele has become reduced to a com-

64 HISTORY OF THE HUMAN BODY

plicated system of narrow spaces lying everywhere between
the other layers. The protoccele is thus called the primary
and the metaccele 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-

a

FIG. 14. Diagrammatic cross sections through vertebrate embryos, based
upon the conditions found in selachians. [Modified, after VAN WIJHE.]

(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 di-
verticula 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, hypomere; a, aorta; g, gonad; *, intestine; m, myo-
tome of epimere; me, metaceele (the definite crelom) ; n, nerve cord; nc, notochord;
nph, nephridium; sk, sklerotome, the anlage of the axial skeleton; w, protonephrotic
duct (Wolffian duct).

brates, the so-called dcelom or pleuro-peritoneal cavity. The
narrowed spaces of the protoccele become filled with embry-
onal connective tissue cells, the tnesenchyme -, 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 as well. Canals are left here and there which in time
are built up into a continuous system of vessels, with walls of

THE ONTOGENESIS OF VERTEBRATES 65

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. 14.] 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
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 mid-ventral 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 continues 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

66 HISTORY OF THE HUMAN BODY

definite ccelom, 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 ex-
cept mammals the membrane is a continuous one, but here,
through the formation of the diaphragm and the consequent
setting apart of a separate thoracic cavity, the portion thus
cut off is treated as a distinct membrane and called the pleura.

Although the above sketch represents the underlying plan
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 ccelom. 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

THE ONTOGENESIS OF VERTEBRATES

67

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

FIG. 15. Four cross-sections of vertebrate embryos showing develop-
ment of the mesoderm.

(a), Amphioxus [after HATSCHEK]; (&) Triton (a salamander) [after HERT-
WIG]; (c) bird, diagrammatic; (c?) 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.

with a definite lumen, which is here 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.

68

HISTORY OF THE HUMAN BODY

In it the region of the epimeres becomes easily distinguished
by a great increase in the thickness of the layer, and an indi-
cation of the separate diverticula appears 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-

FIG. 16. Three early vertebrate embryos, showing mesodermic somites.

(a) turtle [after MITSUKURI]. (&) chick [after DUVAL]. (c) pig [after KEIBEL].
nt, nerve cord (brain) ; nt', nerve cord (spinal cord) ; ms, mesodermic somites; e, ear;
yv, yolk veins.

sively both anteriorly and posteriorly until the full number is
reached. The meso-hypomeric region remains for a time as a
single undivided layer, but ultimately splits into two, outer and
inner, containing between them a single undivided space, the
future ccelom. This latter is here called a schizoccele, in re-
spect to its mode of origin.

There is thus attained in the higher vertebrates a much
shortened and greatly modified method of producing the ele-

THE ONTOGENESIS OF VERTEBRATES 69

ments for the later developments, hardly recognizable on com-
paring it with the more expanded and simple form found
among the lower types. Aside from such modifications as
those mentioned, which are explained through mechanical
exigencies, there appear to be differences in the origin of the
first mesoderm cells themselves, differences which tend to
shake our faith in the absolute homology of the germ layers.
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 caenoge-
netic adaptations 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 positions.
Beyond a certain proportion, in fact, no cell division is possi-
ble, 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-sac. These two areas are indicated in some eggs, as
in those of the frog, by a difference in color, the protoplasmic
area being deeply pigmented and the yolk area not. 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-sac, and as the former increases in size, the latter dimin-

70 HISTORY OF THE HUMAN BODY

iS&fc,

FIG. 17. Diagrams of Amniotes, showing the relation of the extra-
embryonal membranes.

(A) Sauropsidan, with functional yolk-sac and respiratory allantois. (B) Mam-
ma!, with functionless yolk-sac and with the allantois converted into an umbilical
cord and placenta.

THE ONTOGENESIS OF VERTEBRATES 71

ishes, so that by the time the animal assumes a free life the
yolk-sac 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 nourishment. The two membranes of the most extensive
occurrence are the amnion and the allantois, which are present
in reptiles, birds and mammals and absent in fishes and am-
phibians, 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 parie-
tal 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 mem-
brane. In this there develop two large allantoic (umbilical)
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, and this surface,
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

72 HISTORY OF THE HUMAN BODY

known as the chorion frondosum 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
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 lave).

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-

THE ONTOGENESIS OF VERTEBRATES 73

cialized organ, is termed detiduate, 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
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 sur-
face of the blastodermic vesicle, in which the form of placen-
tation 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 definite 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

74 HISTORY OF THE HUMAN BODY

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. 14],
but it may be also useful to introduce the chapters on the
several systems by a table which shows the derivation of each.
In studying this it must be borne in mind that the mesenchyme,
which is everywhere distributed and forms all of the connect-
ive tissues of the body, enters into the final structure of every
other part, and hence is not taken into consideration here.

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 thyroid
glands, larynx, trachea and lungs, liver and
pancreas.

Notochord; the anlage about which the ver-
tebrae (mesenchymatous structures) 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,

THE ONTOGENESIS OF VERTEBRATES 75

hyoid and branchial arches, including the
muscles of the larynx.

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 Morphologic
iiberhaupt gearbeitet und erreicht worden ist, so ist es,
meine ich, an der Zeit, den Blick wieder einmal ruck-
warts zu richten, das zu einem einheitlichen Ganzen
zusammenzufassen, was an vielen Orten zerstreut
liegt, tin 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 epidermis, which form the stratum cor-
neum, 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
confused with the non-cellular cuticula of invertebrates

76

THE INTEGUMENT AND THE EXOSKELETON 77

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 gcrm-
inativiim [sir. 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 cornenm. Some authorities dis-
tinguish 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 exuviat, 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

78 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 intppnm*u.tn.l gland* of all sorts, al-
though they usually dip down into the corium from which they
receive a fibrous investment. Pigment may_ba-.deriveH from
cither_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 79

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 hamjmd 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 may be composites formed from material furnished by
both layers

Thus exclusively horny structures, such as hairs or feathers,
are formed frorjuthe 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 VII, under Respiration.

8o

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

FIG. 18. 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 81

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 former have exactly the same embryonic
history as the latter and are composed of the same two layers,
it must be concluded that they 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 hard-
ness, which is correlated with the greater amount of work to
be accomplished. These teeth, seen here almost at their point
of departure from generalised placoid scales, are inherited by
all higher vertebrates, although in some cases, like turtles and
birds, they have become secondarily lost. Aside from the
correspondence in form, arrangement and structure, the ho-
mology is clearly shown by the development, which proceeds in
all cases from a fold, involving both corium and epidermis,
in which the tooth subsequently appears. These organs, when
once acquired, are subjected to great variations as an accom-
modation for the prehension and mastication of the innum-
erable 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 be-
come 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-

82

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

B

D

FIG. 19. Dorsal views of various skulls, showing the dermal bones.

(A) sturgeon (Acipenser). (B) salamander (Amllytfoma). (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, parieialj_
SQ, squamosal; PT, pterygoid; PO, pro-otic; OO, opisth-otic; SO, supra-occipital;
Oc. Lot., lateral occipital; OP, opercular; S. Cl., supra-clavicle.

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 83

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 / 'rentals, parietals, ma.nllaries
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 ccecil-

84 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 ventrally with well-developed, imbricated scales. These
facts together furnish sufficient proof that amphibians 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 85

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

86

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

a

FIG. 20. Hair arrangement in various mammals. Diagrammatic. [After
DE MEIJERE.]

(a) Myopotamus (South American rodent). Tail, with scales and hairs, (b) Mid:is
(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 87

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-

*e«

& ©

FIG. 21. Hair arrangement

£\ ,'.".. •'«;•; '& ,-v in various mammals.

\i? C • ; -- y <v v*3>

(a) Ursus arctos (brown

/TN ^-.. ,.£•-. ~£j, '';"• *{•$$ bear). Front of chest. Dia-

V^-5 C.*-; '-^ V*x' grammatic. [After DE MEI-

^ ,-;v .,... JERE.] (b) Cants familiaris

(T) V.~-. v*': ..^ - v***'? (d°g)« Four developmental

^' ,;.t ' •} stages. The adult arrange-

'£/ .^-. ment is like that of (a).

S (^ .-v. .'7 Vl' [After DE MEIJERE.] (c)

i.V> Homo. Scalp of negro. Cam-

— ,. ^^ era drawing from the actual

•"• -V> -^ «>*J1V ;%.' object.

fl v./ f O

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,

88

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. 22. Formation of friction ridges from single rows of epidermic
warts. [After Miss WHIPPLE.]

(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 89

FIG. 23. Formation of friction ridges in pairs from rings formed by
the confluence of epidermic warts. [After Miss WHIPPLE.]

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.

90 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-
rhynchus 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 utilise 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 91

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
without 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. 24.) 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. 24, 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. 24, 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 represented 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

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 spiral form

II

IV

FIG. 24. Ventral surface of anterior chiridium of an insectivore and of
a primate showing correspondence between relief and arrangement of
friction ridges. [After Miss WHIPPLE.]

(a) Crocidura caerulea (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

THE INTEGUMENT AND THE EXOSKELETON 93

species. The thenar pattern has here become entirely re-
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 modified at each point of the surface in ac-
cordance 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,
hypotheriar and apical pads tend to degenerate, while the in-
terdigitals 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 interdigital patterns
are usually lost or obscurely indicated. A hypothenar pattern
is frequent, especially in the white race, and occasionally oc-
curs as a whorl with three triradii ; but the thenar is of rare
occurrence, and then usually associated with the first interdigi-
tal. 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 preferential 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 balPof
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.

94

HISTORY OF THE HUMAN BODY

FIG. 25. 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 95

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), gre^
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,
greater part of which becomes lost by an embryonal ecd\sis.
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

96

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

Str.muc

Cutisf
Layerl

FIG. 26. Development of hair.

sir. c, stratum corneum; str. mite., stratum germinativum; seb, sebaceous gland;
fol, follicle; out, outer root sheath; in, inner root sheath; G, hair germ; d, 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 97

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 fol-
lowing figure (Fig. 26), 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 (/. e., the number of groups in

98 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. 20, c).

It may be said in general that arctic forms and those liv-
ing at high altitudes are the most plentifully supplied with
hair, while tropical and sub-tropical forms are sparsely cov-
ered. 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 ventral 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
Airius 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 99

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 vera, is due to an excessive growth of the permanent coat
which replaces the lanugo; the other, psendohypertrichosis, 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

ioo 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 Lophocomi, 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 ioo, 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 EXOSKELSTON i6i

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 groivth,
which varies in different parts of the 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
external 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 ascertained by shaving or clip-
ping 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; 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

IQ-2

HISTORY OF THE HUMAN BODY

FIG. 27. Hair direction in human fetus. [After VOIGT.]

The black lines designate the lines of parting, the arrows show the direction of
the hair currents. Rhomboids and vortices are also shown.

THE INTEGUMENT AND THE EXOSKELETON 103

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 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 depar-
tures 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.

104 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 and is directed ventro-
dorsally, as if in correlation with the customary inverted posi-
tion of the animal. In opposition to this, however, it may be
pointed out that in certain areas the direction 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 attempt 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 hence
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 105

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,

io6 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, 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. 28, a)
the parts to be noted are the convex dorsal plate (Krallen-
platte), the concave ventral plate (Sohlenhorn) and the apical
pads of the digit (Zehenballen). In the sauropsidan 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 well developed

THE INTEGUMENT AND THE EXOSKELETON 107

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 decreased in volume and
is indicated mainly by the friction-ridges, which are in the
form of a loop or whorl. The extreme of this line of develop-

FIG. 28. 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.

ment 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 represent-
ing 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

io8

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. 29. 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. Tubular 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 another 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 109

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 \yhere
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. 29 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 glo-
bules, 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

i io 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 prolifer-
ation 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 environ-
ment 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 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 of scales on

THE INTEGUMENT AND THE EXOSKELETON in

the inner aspect of the femora; at the time of pairing these
secrete a gummy fluid which hardens into short spines or teeth,
employed during copulation. In birds the sole integumental

Stratum corneum

Stratum lucidum }- Epidermis
Stratum mucosum j

- Corium

FIG. 30. Typical mammalian hair with its accessory parts. [After
WEBER.]

67. ac., acinous gland, usually sebaceous; GL tb., tubular gland, usually per-
spiratory in function; M, ar. piL, arrector pilarum muscle.

gland is the uropygial, a compound mass situated 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

H2 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. 30. 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, but it 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 fail entirely in the
two aquatic orders of Sirenia and Cetacea, also in Manis, in a
sloth (Chol&pus), and an insectivore (Chrysochloris). 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 im-
portance in an arboreal animal. When distributed over the
general surface and yielding the customary colorless watery

THE INTEGUMENT AND THE EXOSKELETON 113

fluid, these glands are usually called sweat glands (glandular
sudoriparc?), 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, a,t
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 tar sal [meibomian] glands of the
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 external genitals
(Tyson's glands, preputial glands, etc.). Corresponding to
their primary function as sebaceous glands, they are wanting
in Cetacea and in adult Sirenia ; the scale-covered Mqnis retains
only the modified orificial glands. They are, however, wholly
wanting in some sloths (Cholcepus) and in an African insec-
tivore (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

ii4 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 defense.
Other similar glands secrete odoriferous fluids employed 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 momo-
treme, Echidna, exhibits nearly their original condition. The
female of this animal possesses upon the ventral side an integu-
mental 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 tubu-
lar 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. 31, 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
extrusion of the nipples, and the mammary pocket is lost (f).

THE INTEGUMENT AND THE EXOSKELETON 115

This type of nipple, in reality a mammary pocket turned inside
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

FIG. 31. Morphology of nipples. [After WEBER.]

(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 th'e figures the area glandularis, *. e., the surface of the 'mammary pocket,
is represented by a dotted line; 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 gland*.

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 in
ruminants. In this the glandular area remains at the bottom
of the pocket, while the surrounding corium wall becomes ele-
vated more and more until a long pendulous nipple is formed
from that (Fig. 31, 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 lateral ridge,

n6 HISTORY OF THE HUMAN BODY

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 de-
pressions. 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. 31. The mammary ridge perhaps repre-
sents the wall of the marsupium or pouch, thus suggesting
that the placental mammals have been derived from ancestors
which possessed a marsupium. The conclusion is not neces-
sary, 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 monotremes, 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 appears 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 nursing. By the
suppression of the anterior end of this series inguinal mamma
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 developed to solve the prob-
lem 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 able to breathe through the
nostrils, which in these animals have migrated backward from

THE INTEGUMENT AND THE EXOSKELETON 117

their primary position. By a suppression of the posterior
portion of this series, pectoral mammce 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
mammae are pectoral. As they bear but a single young at
a time and nurse it by clasping it in the flippers while stand-
ing 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 many animals with a restricted number of mammae there
have been frequently observed cases of supernumerary nipples
or supernumerary mammae. These are termed respectively
hyperthelism and hypermastism, and are looked upon as
atavistic and indicative of the former development of a com-
plete 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 subject 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
correlation 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 sex of
parts that developed first in the female, and formed for a long

n8 HISTORY OF THE HUMAN BODY

time an exclusive characteristic. Since neither natural selec-
tion nor sexual selection could have a part in this transmis-
sion, it has been brought forward as a case of the direct trans-
mission 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 IX., 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 abnormality.

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
ccelom and invests the 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 those 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
integument of elephants and rhinoceroses, are pigmented, and
occasionally highly colored.

THE INTEGUMENT AND THE EXOSKELETON 119

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, or between
the American Indians in Canada and those in Mexico. A
similar reduction in pigment is found in people living at high
altitudes in comparison with the same race living in the bor-
dering lowlands. The importance of these correlations has
been repeatedly denied, 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
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

120 HISTORY OF THE HUMAN BODY

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
is still an obscure one, it seems probable that the presence of a
dense layer of pigment in the stratum germinativum effectually
prevents the direct action of the light upon the surface capil-

THE INTEGUMENT AND THE EXOSKELETON 121

laries, 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 glow-
ing 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 explana-
tion of them obscure; the problem is to be solved sometime,
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 ENDOSKELETON

". . . 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-

122

THE ENDOSKELETON 123

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. 32. 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

124 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 pra-
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 ENDOSKELETON

125

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

a

c

d

FIG. 33. 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 stiil 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 vertebrae, 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

126 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 hamal, 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, whicTTare 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 ENDOSKELETON 127

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 amphiccclous (=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 animalsy the
limbs were small and weak, and progress was attained trirough

128 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 iQth vertebra, counting from the head, but the
2Oth is occasionally employed instead, and two cases have been
reported in which the attachment was to the i8th. Cases are
also known in which the attachment is oblique, either to the

THE ENDOSKELETON

129

on one side and the 2Oth on the other, or to the i8th on
one side and the igth 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. 34. 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. 34, 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

130 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 ENDOSKELETON 131

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
precocious, while in the latter case the reverse condition is the
result, such vertebrae being designated as opisthoc&lous.

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 vertebrae become
accelous, 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 opisthoccelous

132 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-vertebrse 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 vertebrae 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 ENDOSKELETON 133

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
vertebrae. 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 vertebrae, occur-
ring in reptiles, birds and mammals, is modified in amphibians
through a secondary inclusion of the elements of the atlas

134 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
nuchce, 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 ENDOSKELETON 135

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 vertebras, 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
vertebras, 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

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

^ A

FIG. 35. 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-
tilages," characteristic 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 ENDOSKELETON 137

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-rib 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 5e>~
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-

138 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,3' 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 (f gorilla
rib." Variation in the sacral vertebrae has already -been
noticed [Cf. Fig. 34 and accompanying text].

The origin of the sternum is still a matter of controversy,
and it seems likely that there may have been two sternums,
of different origin, the one succeeding the other during his-
torical development, the archisternum and the neosternum.
Fishes lack the part entirely, but it is present in some form
in all other vertebrates save in a few aberrant cases, for ex-
ample, the snakes, where it is incompatible both with their
mode of locomotion by means of the ends of the very numer-
ous ribs, and with their habit of swallowing huge mouthfuls,
far too large to pass through the ring formed by the vertebrae,
ribs, and sternum, as is the usual arrangement. What is ap-
parently the first indication of a sternum is seen in the sala-
mander Nee turns ~, perhaps the lowest amphibian, in which
from three to five of the thoracic myocommata chondrify in
the ventral region, forming small V-shaped elements, indefi-

THE ENDOSKELETON

139

nite in shape and irregular in occurrence, as is usually found
in an organ at its beginning, before the type has become fixed.

d

in —

c

FIG. 36. 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.

If we seek the reason for their appearance we shall probably
find it in an attempt to lessen the pressure upon an important

140 HISTORY OF THE HUMAN BODY

point. One of the sternal elements, usually the largest, lies
in the fourth myocomma, in close connection with the over-
lapping coracoids, and as at the same point in higher salaman-
ders there is a definite sternal plate of a rhomboid shape, this
latter has evidently developed from the element in question,
while the others have been lost. This must 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, a characteristic that defi-
nitely distinguishes this archisternum from the neosternum
of the Amniota. This last organ, the second form of
sternum, is characteristic of reptiles, birds, and mammals,
and is not only always connected with several pairs of
thoracic ribs, but undoubtedly owes its origin to them, being
probably due to the fusion of the ribs in the mid-ventral
line. This fusion forms in reptiles and birds a flat plate,
especially extensive in the latter, where it serves as a place of
origin for the enormously developed muscles of flight, but in
the mammals the sternum, continuous with the ribs while in
the cartilaginous state, ossifies in the form of a series of sepa-
rate elements, the sternebra, one for each pair of ribs involved.
The original number of these elements may be retained
throughout life, as in most mammals, or may become reduced
by a secondary fusion to a smaller number.

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

THE ENDOSKELETON 141

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 Nectums,
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
archisternum in the salamanders by furnishing an exact phy-
siological parallel. Associated with the sternal region, both
in Amphibia and in the Amniota, there is a rather problematic
element, known as the episternum, of which no continuous
history is yet known, so that it is not even certain that the
various elements in different animals called by that name are
homologous. The typical episternum is a skeletal piece occur-
ring in lizards and consisting of a thin cross-shaped or T--
shaped piece lying, as its name denotes, upon (i. e., on the
ventral side of) the sternum, and a little anterior to it.

This part is not clearly present in other vertebrates, but
similar pieces occur in several cases, and are often designated
by the same name. Thus, in the shoulder-girdle-sternum com-
plex of the frog there is a piece extending anteriorly along the
mid-ventral line, between the clavicles, and closely resembling
the true sternum (archisternum) that extends posteriorly.
This has been often called the episternum, but is more likely
a portion of the archisternum, formed like the other, from
myocommata. Again, the well-known " wish-bone " of birds
is formed by a fusion of the two clavicles with a middle
piece, the inter clavicle, which forms the " head " and is es-
pecially well developed in the common fowl. This element also
has been identified with the episternum by some investigators,
as have also certain parts of the keel of the sternum, which
develop from separate centers of ossification.

142

HISTORY OF THE HUMAN BODY

Among the Mammalia the lowest Order, the Monotremata,
possess in this region a large T-shaped bone, the stem of
which, very broad and flat, articulates with the true sternum,
forming its anterior extension, while the lateral arms are ap-
plied along the sides of the clavicles. This piece has been
called by some an episternum and by others- an interclavicle,
but its precise homologies are not definitely determined. In
all other mammals the clavicles apparently articulate directly
with the most anterior of the sternal pieces, the manubrium;

FIG. 37. 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;
m, manubrium; stb, sternebrae; x, xiphisternum.

but in the embryo there are found definite disc-shaped
skeletal elements, interposed between the two, which develop
later into thin, interarticular discs. These, usually designated
omosternum, have been likened to the lateral arms of the
T-shaped bone of the Monotremata.
X"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

THE ENDOSKELETON 143

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 t\yo ancKa secqrKi, almost as
great, between the latter and the selacmans/sVUiat: it may well
be conceded that adult animals representing the stages indi-
cated by the embryonic history once existed in the places now
left vacant. Nothing could fit better into this ontogenetic
history at a later period than the selachian skull, as will be
shown further on, thus verifying the record at an important
point, and rendering it more probable that the earlier em-
bryonic 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 prachordal 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
embryonic life, such as the one placed 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

144

HISTORY OF THE HUMAN BODY

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-

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.

chord and the other anterior to .it, the parachordal and prce-
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 trabeculce (di-
minutive of trabs, trabis, 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

THE ENDOSKELETON 145

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 opening.
As these several cartilaginous elements, the para- and prae-
chordals and the sense capsules, increase in size, they fuse
together in about the following manner. The two trabeculse
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 otic capsules, which must
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-
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-

146 HISTORY OF THE HUMAN BODY

cations, for use during active adult life, and are not empha-
sized in the embryonic history of higher forms.

This stage of the chondro cranium, or the selachian stage,
as it may be called, is passed through with during the de-
velopment of all the higher vertebrates, and although in
the various forms the shape and proportion of the parts often
differ widely in anticipation of the various needs of the adult,
they all possess in common the origin in the same' way, from
the same elemental parts, and the characteristic regions may in
all cases be readily identified.

For the next stage in this history it will not be necessary
to have recourse to embryology save to verify the conclu-
sions, since it is represented with almost diagrammatic clear-
ness among the ganoids, a very few of which have been, by
a fortunate chance, saved from the general destruction of the
Order during an earlier geological period. This stage may be
thus conveniently denominated the ganoid stage, for the type
of which we may select the sturgeon. Although similar to the
selachians in many respects, this animal differs markedly
from them in its external covering, for while the former is
evenly and uniformly covered by small placoid scales arranged
in a regularly imbricated pattern, the sturgeon possesses a
series of large, bony plates, or scutes, as they are called,
which may be considered as having been formed originally
from the fusion of the basal pieces of many scales. These
scutes are arranged on the body in longitudinal rows, leaving
the intervening regions bare, but are continued over the head
as somewhat modified scutes, the edges of which are in con-
tact, thus forming an external armor, with sutures between the
different scutes (Fig. 19, A). Immediately beneath this lies
a cartilaginous skull, very similar to that of selachians, and
the dermal armor encases it like an external skull, which it
really is. These dermal plates are quite definite in their
arrangement, and the same general plan may be followed
throughout the Order of ganoids. The snout, or rostrum, is
covered by a series of small rostral plates, which extend back
as far as the nostrils ; back of these openings may be found a

THE ENDOSKELETON 147

pair of nasals; behind these again, and between the eyes, is a
pair of frontals, often accompanied by prce- and post-frontals.
Behind these is a pair of parietals, and one or more supra-
occipitals. On the sides of the head, at about the level of the
parietals, are the squamosals, and around the eye are several
orbitals, distinguished as pre-, supra-, post-orbitals, etc. The
operculum, or gill-flap, which is present in these fishes, is cov-
ered and augmented by supra-, sub-, and pre-operculars.

In short, to anticipate the history a little at this point, we
see in the dermal scutes the first appearance of the so-called
dermal bones of the skull which in later forms are to sink in
beneath the surface and become internal, thus coming into
close connection with the primordial skull and the osseous
elements derived from it. They are not all inherited by higher
forms exactly as they occur in the ganoid, the question of
their retention being based in each case upon their functional
importance. Thus, the opercular series, retained in the fish,
becomes lost with the reduction of the part which they cover ;
the orbital series is retained in part by reptiles, but becomes
lost in birds and mammals, with the single exception of one of
the prae-orbitals, which becomes the lacrimal; and the supra-
occipital series becomes reduced to a single piece. On the
other hand, certain ones are retained in all higher vertebrates,
and are recognizable throughout, although by secondary fu-
sions and divisions they are not always strictly homologous.
Thus, the frontals may or may not include the originally
separate prae- and post-frontals, and in a given case the ab-
sence of one of these latter elements as a distinct piece may
mean either that it has fused with one of the others or has
been gradually reduced in size until it has become lost. The
frontals in some form, however, are among the most constant
of dermal elements, and the same may be said of the parietals,
squamosals and nasals, which can be traced in all the verte-
brate classes (Fig. 19). The ventral side of the cranium
becomes also encased in a similar manner by dermal bones that
develop in the roof of the mouth, among which are the vomers,
the palatines, the pterygoids, and the extensive parabasal

i48 HISTORY OF THE I^UMAN BODY

*
* >

(parasphenoid*) , which forms almost the entire base of the
cranium in fishes and amphibians. Certain of these last named
do not develop, strictly speaking, iivassociation with the cra-
nium, but are formed about certain'elements of the visceral
skeleton, as will be explained below, but as these latter ele-
ments early lose their physiological independence and become
closely incorporated with the original chondrocranium, the
statement is in no way misleading.

'These dermal plates thus form an almost complete case
of bone, surrounding and protecting the internal cartilaginous
skull, and, by supplying the deficiencies of the latter, effect the
complete enclosure of the brain within the skeletal parts.
There are thus formed two skulls, one within the other, and in
the ganoids, where the relation between the two is not as yet
a very intimate one, the outer or bony skull may be easily
removed from the other.

The next step in advance is one shown also among the
ganoids, and consists of the strengthening of the chondrocra-
nium directly by the development of centers of ossification
within the cartilage itself (endochondral ossification) forming
definite osseous elements, called from their mode of origin
cartilage bones, in distinction from the other, the dermal
bones. Among these centers may be enumerated the exoc-
cipitals, the pro-oticsf epiotics, and opisthotics, which together
form the petrosals, the all-sphenoids, the orbito-sphenoids, and
the ethmoids, well-known elements in the skulls of higher
vertebrates, but here found at their inception, arising as iso-
lated areas of the chondrocranium and developing at the ex-
pense of the cartilage, clearly differing from the dermal bones
in origin.

We thus find in the skull of the ganoids the elements of
the vertebrate skull almost at their beginning, and can trace
the origin of parts familiar to us as they appear in the spe-
cialized skulls of mammals, where, under the cloak of an
exactly similar external appearance, their diverse origin has
become lost. The ganoids seem thus a vital link in the story
of the skull, yet even had they become entirely extinct, as they

THE ENDOSKELETON

149

came very near being, this portion of the history might have
been deciphered from the embryological records, since even in
the mammals the primordial skull develops from its primitive
elements, the cartilage bones appear as centers of ossification
within it, and the dermal bones, never preformed in cartilage,
appear as subcutaneous ossifications in the connective tissue.

A

FIG. 39. 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.

In that case, however, we could hardly have obtained an idea
of the appearance of the adult ganoids', since embryology, with
its distortion of the facts through an early assumption of the
proportions of the perfected animal, is an unsafe guide upon
which to base more than very general conclusions. Had the
ganoids been lost we would have believed in a general way
in the former existence of fish-like forms in which the dermal
bones were still in the form of an exoskeletal armature, but
their exact appearance and relationship would have given rise

150 HISTORY OF THE HUMAN BODY

to endless controversy, such as always occurs with regard to
places where the records are incomplete, and this vital period
in the history of the skull would have lost much of its reality.

As to the necessity which caused the appearance of these
endochondral ossifications in the primordial skull, there has
been pointed out a curious relationship between them and the
principal cranial nerves, namely, that the ossifications de-
velop about their places of exit from the brain cavity as though
to protect them. Thus we have the olfactory nerve surrounded
by the ethmoid, the optic nerve perforating the orbito-sphe-
noid, and similar relations existing between the trigeminus
and the alisphenoid, the facial nerve and the prootic, and the
ninth and tenth and the exoccipital. These are certainly the
topographical conditions, but whether a causal relation really
exists between them is not known.

In completing the history of the skull, it remains to no-
tice the ^amphibian stage, best exhibited by urodeles, and the
amniote stage., typically represented by reptiles and mammals.
In the first of these the dermal bones are no longer external at
any stage of their development and have become definitely
incorporated with the skull as physiological parts of the in-
ternal skeleton. Aside from this the characteristically piscine
elements, like the rostrals, the orbitals and those associated
with the operculum, have become lost, and the bones assume
more the number and relationships of the higher terrestrial
forms (Fig. 19, B, and Fig. 39).

In the Amniota one of the fundamental changes is the loss
of the parabasal as the main element of the cranial floor, and
its functional replacement by a series of median cartilage
bones, the basi-o capital, basi-sphenoid and pr<z-sphenoid.
The parabasal may be entirely lost, but in the light of recent
investigation it seems probable that it is continued as the
median vomer, which is thus not the same as the paired vo-
merSj which form a portion of the roof of the mouth of fishes
and amphibians, and which, if this view is the correct one,
probably disappear in amniotes.

Another characteristic is the secondary fusion of elements,

THE ENDOSKELETON 151

and as the purpose of this procedure is wholly physiological,
the object being to insure local strength or gain muscular
attachments, the process does not respect origin, but often-
times involves both dermal and cartilage bones, and may even
include, as well, elements of the visceral skeleton. The re-
sults are thus bone-complexes, each of which, in the adult, is
a morphological puzzle, to the history of which we have in
this stage no clew. 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 extreme 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.

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.

152 HISTORY OF THE HUMAN BODY

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 Amphloxus
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.

It 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 the
rudiments, 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 with the testi-
mony furnished by Amphioxus. The selachian condition,
showing all the pieces that may be accredited to the visceral
skeleton, together with the chondrocranium, is given in Fig.
40, somewhat diagrammatized from an actual preparation.
It may be safely assumed that at one time these visceral arches
were all similar to one another, associated with similar gill-
slits and all gill-bearing, although important modifications
have now taken place in certain ones of them. These modifica-
tions are of the highest importance in this history and may
now be considered, with constant reference to the figure.

THE ENDOSKELETON

153

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

IU

B

FIG. 40. 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. I. 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.

YII

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

154 HISTORY OF THE HUMAN BODY

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 ad-
mitting food, and that its manifest advantage over the other
mouth in its convenient skeletal equipment, caused the dis-
appearance of the old one and the perfection of the new;
there are, however, certain indications that point to the former
possession of a still older mouth, the pal&ostoma, homologous
with that of the tunicate, and with this both the hood-like
mouth of the cyclostomes and the slit form of other verte-
brates may be contrasted as the neostoma, or secondary mouth.
This last expression applies simply to the opening, which is
probably homologous in all vertebrates, but becomes com-
pletely metamorphosed by the addition of jaws. [Cf. Chapter
X., sub Hypophysis.]

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 thu°

THE ENDOSKELETON 155

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.

Associated with the first pair of visceral arches, the primi-

tive jaws, are the cartilages of the second pair, the
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
true jaws. To this is also attached the ventral piece, or hyoid
proper.

The remaining five arches, the genuine branchial arches,
are all much alike, and are gill-bearing, associated with gill-
slits. Each one consists of four pieces, two dorsal^and two
ventral, the two sets bent at an angle with each other. Along
the mid-ventral lines the pairs are united and held to one
another by median pieces, the basi-branchials, of which there
is typically one for each pair, but in living selachians the full
number is seldom represented. The additional gill arches in
the two primitive forms have been referred to above and are
represented in the diagram, Fig. 41, A, by dotted lines.

Aside from these definite and well-developed visceral arches,
as they may be called collectively, there are the rudiments
mentioned above, functionally of little importance, and 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;
another of these is the spiracular cartilage, crescentic in shape
and surrounding the spiracitlum or blow-hole, perhaps a modi-
fied gill-slit, anterior to the others. These, if admitted to the

156 HISTORY OF THE HUMAN BODY

series, will considerably increase the number of original vis-
ceral arches and render more probable their descent from a
form like Amphioxus.

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 chondro cranium. 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 angu-
lare, which covers the inner side.

Lying within these, as if bound in splints, is the cartilagi-
nous lower jaw, the original visceral arch, which is destined
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
articular et 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 embryo
human jaw, lying encased in the dermal bones, much as in the
adult ganoid or teleost.

The original upper jaw, however, loses still more prestige,
since its function as a jaw is entirely usurped by a set of
dermal bones, the prcemaxillary 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

THE ENDOSKELETON 157

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-
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 gilf-bearers, while the
fifth lies in the floor of the pharynx and is often covered
with sharp teeth arranged in several rows.

A notable modification, which, however, is mainly an ex-
ternal one, is found in the development of the opercuhim or
gill-flap, which appears as a fold of skin and becomes rein-
forced by special dermal bone. This ultimately develops pos-
teriorly 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 the case of 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 seems to disappear entirely, al-
though doubtfully identified by some morphologists with cer-
tain other elements in the otic region, and leaves to the quad-

158

HISTORY OF THE HUMAN BODY

FIG. 41. 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, Ib, 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 designations for the stapes
represent an interpretation at variance with the one given in the text, but formerly
widely accepted.

THE ENDOSKELETON 159

rate the responsible role of being the only suspensory piece
for the mandible. As a cartilage bone it persists in am-
phibians, reptiles, and birds. The several elements of the
mandible remain distinct in amphibians and reptiles, but con-
solidate 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 de-
veloping embryo, through which the facts first came to light.
Here both quadrate and articulare, external at first, as in am-
phibians and reptiles, become drawn into the tympanic cavity
(middle ear) where, still retaining approximately their original
shape, though proportionately reduced in size, they become the
incus and malleus, respectively, while the mandible, each half
consolidated into a single bone, forms a new articulation di-
rectly with the skull in the petrosal region. The old articula-
tion of the mandible, that between quadrate and articulare, 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, becomes 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 seems to have been originally de-
rived 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 oval base is an ossified
membrane, secondarily fused with the other piece. The fora-
men 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 Insectivora 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

160 HISTORY OF THE HUMAN BODY

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 trachea! 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
complex and furnish a skeletal equipment for the tongue, an
organ which often develops voluminously, fitted for very spe-
cial work. Thus, in the amphibians in general this complex
consists 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 ist gill-arch. In
most mammals the anterior cornua of this hyoid complex con-
sist of a chain of small bones, which, enumerated from the
basi-hyal ("body of the hyoid"), are: cerato-hyal, epi-hyalf

THE ENDOSKELETON 161

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 ff 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. Oc-
casionally a rudiment of this latter bone is found in the middle
of the ligament.

A recently described and very 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 trans-
formed 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 ! Beginning
as a set of similar arches, regulating the opening and closing
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 numer-
ous 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 laws and never originating or creating
new ones de novo. The results are thus often imperfect and

162 HISTORY OF THE HUMAN BODY

incomplete, and are frequently 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 mechanical disadvantage which the skill of a simple
human mechanic could have remedied, there is never the least
sign or indication of such an interference. Everything de-
velops as an inevitable 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.

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. 42, 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
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

THE ENDOSKELETON

163

needs of Amphioxus, and not transmitted to any of its de-
scendants.

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 localised func-

FIG. 42. 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.

tional activity, until certain portions became especially ^vell
developed, while the intermediate portions were entirely lost.
(Fig. 42, b). The significance of this is seen if this figure be
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

164 HISTORY OF THE HUMAN BODY

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. 42, 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 make-
shift arrangement to serve the purpose. Thus, aquatic am-
phibians develop a tail fin of integument without skeletal sup-
port, the tail of the crocodile is supplied with keels formed of
projecting scales, and the whale has manufactured perfect
dorsal and caudal fins out of whole cloth, as 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

THE ENDOSKELETON 165

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 iins, 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 extraordi-
nary development, and their high degree of success in occu-
pying so many sorts of environment, since by this means they
have been able to possess the surface of the earth, to occupy
the trees, to burrow in the ground, to return to the sea and
even to conquer the problem of aerial navigation. It is, more-
over, probable that the emancipation of the fore limbs from
the function of locomotion and the acquirement by them of
prehensible 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-
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 short 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

166 HISTORY OF THE HUMAN BODY

.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
numerous ribs, the loss of both pairs of limbs is usually
a total one, but in the boas the posterior limbs ap-
pear as spur-like rudiments, situated upon either side of the
cloacal orifice, and are of considerable use in climbing trees.
Aside from the snakes a similar form is assumed by several
groups 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-
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

* Cases of monstrosities with extra limbs, in which the total number ex-
ceeds four, are not violations of this principle, 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.

THE ENDOSKELETON 167

time for accomplishing this is now long past and, the number^
of limb-anlagen once established, no subsequent change is pos-f
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.
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 undoubtedly
due to the extinction of transition forms. This transition must
have taken place at the epoch at which the vertebrates first
attained the land, a transition which must have been a com-

i68

HISTORY OF THE HUMAN BODY

paratively 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 immediate descendants, which
were better fitted for the land, and with their immediate ances-
tors, which had never left the water, and their rapid extinction
was a necessary consequence. 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 at-

FIG. 43. 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.

tractive field for the anatomical philosopher, and the discus-
sion of some of the leading theories of this question will be
considered farther on in the present chapter.

Aside from this problem, however, the history of the de-
velopment of the limbs is by no means clear in other respects,
and although the faith in their complete 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
transitions 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 this it must be remembered that a his-
tory founded merely on a succession of adult forms, and re-

THE ENDOSKELETON 169

maining unsubstantiated by a parallel series of embryonic
stages, rests upon an insecure foundation and is liable to re-
ceive considerable modification through the discovery of ad-
ditional 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 corre-
spond quite closely, part for part, the girdles, although equally
a portion 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 ofj^once continuous fin-fold, its skeleton con-
sisted of a series of spines or rays, independent of one another,
and somewhat longer at their bases, tapering to their free
extremities (Fig. 43, 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; 43, 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
they meet and fuse across the mid-ventraMine^ thus forming
a very primitive girdleTwitrT which the free part would become
movably articulated (Fig. 43, 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. 44, d.)

170

HISTORY OF THE HUMAN BODY

The fault in the above theory is that, while offering- a direct
transition from the selachian to the amphibian form, it leaves

FIG. 44. 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 (b), 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 (g) 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.

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. 44, ^

THE ENDOSKELETON 171

although here the fault lies in the derivation of ganoid condi-
tions directly from the primitive form without accounting
for that of the selachians. The first of these figures is that
of Aclpenser, the sturgeon (}Fig. 44, 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. 44, 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
Polyptcnis, in many ways the highest of the living ganoids
and the one nearest the amphibians (Fig. 44, 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 at-
tached to a small mid-ventral piece, a rudimentary girdle,
which is divided by a suture into two portions, plainly the same
as the two small inner pieces of Scaphyrhynchus f here united
to form a rhomboid plate.

The derivation from this of the condition found in the
urodele Necturus becomes at once evident in a comparison
of the two (Fig. 44, c and d), in the latter of which the
steps in advance consist of the enlargement of the plate, its
connection with the vertebral column through two small
processes, the ilia, that extend dorsally, and the appearance
of two centers of ossification posteriorly. In this the real
transition 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

172 HISTORY OF THE HUMAN BODY

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 ele-
ments, the pubic bones f also appear in much the same con-
dition (Fig. 44, e).

From the latter form of girdle to that of a reptile (Fig.
44, 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. 44, d and
e). In some reptiles the obturator foramina become con-
fluent, 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 f ischiunL_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
a symphysis, although in man the symphysis involves the pubic
bones alone, the ischia being wide apart. This is doubtless 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 else-
where in nature.

The history of the gtinildfT-giV^10 differs considerably from
the foregoing in its later development as it becomes compli-
cated by the addition of membrane bones from without, as m
the case of the skull. Although in its first appearance, in
selachians, it differs somewhat in form from the hip-girdle,

THE ENDOSKELETON

it is probable that its early history is similar, and that it was
fomied_by a coalescence of basal portions nf thAfin-r^y^ as in

_

the ether case, its size and shape being modified in accordance
with differences in function. In the adult selachians it ex-
tends dorsal to the insertion of the fin into long points, and
the two halves become distinct, forming a symphysis at the
mid-ventral line, thus somewhat resembling a pair of man-
dibles. Each lateral piece is termed scapulo-coracoid, as a
suggestion of the two elements to which it is to give rise,

FIG. 45. 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.

but, as in the case of the primordial skull, there is no sug-
gestion of boundaries between the two portions, although it
may be vaguely indicated by the point of insertion of the fin,
the portion dorsal to it being the scapular portion, and the
other the coracoid.

In ganoids there becomes associated with this cartilaginous
girdle a series of dermal bones, formed, as elsewhere,^ the
fusion of scales. There are two of these bones investing each
lateral piece, the clayick^Jd&irig ventral and the^oitot lat-
eral to these. Of these the former are a little larger, and
form the symphysis in the mid-ventral line, excluding the

174 HISTORY OF THE HUMAN BODY

cartilage. In teleosts the cleithra become excessively developed
and function as the entire girdle, while the clavicles are want-
ing and the cartilaginous element is much reduced. There are
also other small dorsal pieces, the supra-cleithra (post-tem-
poral and supra-clavide of many authors), which connect the
girdle with the skull.

As the teleosts represent the perfection of the fish type, but
are not in the direct line of descent, their condition represents
a specialization not closely related to higher forms, and the
direct history is carried over, with a small interval, from the
ganoids to the amphibians. In these latter the dermal element
is but little in evidence, while the cartilaginous part is volu-
minous and shows centers of ossification. In urodeles each
half-girdle is in the form of a thin plate, wrapped about the
side of the body and incompletely divided into three portions,
a dorsally extended scapula, containing an ossified area, and
two ventral extensions separated by a notch, the coracoid and
procoracoid, both cartilaginous. There are here no dermal
elements. In the tailless amphibians the cartilaginous founda-
tion is much the same as in urodeles, but there is also an ossi-
fied area in the coracoid, and a dermal clavicle, in the form
of an inverted trough, which fits closely over the procoracoid,
forming a compound piece. The two lateral halves become
also more or less closely associated with median sternal (and
episternal) elements, and in the more specialized frogs the
whole forms a complicated skeletal armatuie protecting the
vital organs and forming a functional thorax, something like
that of higher forms, although without rib components and
not involved in the respiratory process.

In the Amniota there appear three parts to the girdle, scap-
ula, coracoid, and clavicle, corresponding in the main to those
of Amphibia. The two first are preformed in cartilage and
ossify later on in development ; but the clavicle of reptiles and
birds has no cartilaginous stage, and seems thus to represent
the dermal element alone. In birds the two clayiHps fnsp with
a median inter ^7^^ (possibly an episternum), to form the
furgiila or " wish-bone." In mammals the scapula receives
the most emphasis, while the coracoid is never present as a

I

THE ENDOSKELETON

175

distinct bone save in the monotremes. In others it ossifies
from a separate center, but soon fuses with the scapula to form
the coracoid process. The clavicle is large and well developed
in those forms in which strength of shoulder is especially re-
quired, as in most cases among the Rodentia, Insectivora and

FIG. 46. Anterior fins of fishes.

(A) Dog-fish (selachian). (B) Ceratodus (Ji/moan).

s, scapula; ss, suprascapula; c, coracoid; p, propterygium ; ms, mesopterygium;
In B there are radials on both sides of a central

mt, metapterygium; rod, radials.
axis.

Primates, but is rudimentary in Carnivora, and is entirely
wanting in hoofed mammals and in Cetacea. It is stated by
some investigators that the mammalian clavicle is in develop-
ment a compound piece, formed, as in amphibians, of a carti-
laginous core, overlaid by a dermal element. If this be so, the
former may be the procoracoid, and^the latter the true clavicle,
but there is some doubt concerning the actual conditions of
development, and the whole matter needs further investigation.
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,

176

HISTORY OF THE HUMAN BODY

named in order, beginning at the front, pro-, meso-, and meta-
pterygium (Fig. 46, 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-

Carpus
[Tarsus

Metacarpus //
[Metatarsus] fj i

II

V

IV

V
III III

FIG. 47. 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] ;
b, ulnare [fibulare]; x, intermedium; y, centrale; i — 5, carpalia [tarsalia].

In both diagrams the more constant sesamoids are indicated by dotted lines.

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. 46, B). The highly specialized Dipnoi, however, are
not the proper animals to which to look for primitive condi-

THE ENDOSKELETON 177

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,
while it appears in almost its typical condition in animals
widely separated. (Fig. 47.) "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 Femur

Ulna (outer) Fibula (outer)

Radius (inner) Tibia (inner)

Carpus Tarsus

Metacarpus Metatarsus

Phalanges Phalanges

The digits are designated either by number, I-V, or else
the Latin names for the fingers are used, pollex, index, medius,
annular is, 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. 47, 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-

I78 HISTORY OF THE HUMAN BODY

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 II, car pale V, 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 (treated here in Chapter VI), yet the practical advan-
tage that has already resulted from a partial uniform nomen-
clature in carpus and tarsus shows the possibility of com-
pleting such a scheme as a working hypothesis, without rais-
ing the question of serial homology. Such a scheme is shown
in Fig. 47, 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 scsamoid
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. 48), there are many

THE ENDOSKELETON

179

facts which forbid us from imagining that it is really a primi-
tive condition. Thus the animal in which it comes to its most
perfect realization is the turtle, the carpus of which is almost
diagrammatic, 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. 48, a), and that the separate bones were ar-

in iv

FIG. 48. 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; *, intermedium; c, centrale; 1-5, car-
pal ia; 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.

In the nomenclature of the carpal and tarsal bones em-
ployed in human anatomy we have an unusually good illustra-

i8o

HISTORY OF THE HUMAN BODY

tion of parts named with reference merely to a single animal
form, for the names given them, e.g., cuneiform, trapezium,
multangulum ma jus, 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

( os calcis (calcaneus)

J astragalus (talus)
( (value undetermined)

lunare (lunatum)

intermedium

cuboides (triquetrum)

ulnare

fibulare

anlage in embryo,
with occasional
persistence

centrale

naviculare

trapezium (mult-
angulum majus)

carpale I

tarsale I

entocuneiforme

trapezoides (mult-
angulum minus)

carpale 1 1

tarsale II

mesocuneiforme

os magnum (capi-
ta turn)

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.

THE ENDCSKELETON 181

Aside from the normal bones in carpus and tarsus there
occur occasionally supernumerary elements of greater or less
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
thousand human carpi, is shown in Fig. 49. Similar super-
numerary elements occur in the tarsus ; the most important of

FIG. 49. Diagrams of the supernumerary carpal bones. [After
PFITZNER.]

r, radiale externum (constant in apes); ce, centrale; I, (epilunatum) ; s, hypo-
lunatum; c, triquetrum secundarium; p, epipyramis; y, praetrapezium ; s, styloideum;
i, parastyloideum; e, metastyloideum; m, capitatum secondarium; g, ossiculum Gru-
beri; h, os hamuh 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.

which are the trigonum, associated with the astragulus, and
present in 8% of the cases studied; the tibiale externum (n-
12%) ; the peroneum (8-9%) ; and an intermetatarseum (8-

182 HISTORY OF THE HUMAN BODY

9%), a derivative, sometimes of the first, sometimes of the
second, metatarsal.

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 certain salamanders, or of the
last alone, as in the feet of birds, to the extreme case ex-
hibited by the horse, in which the middle digit is alone de-
veloped, 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 the
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. 48, e), have often been in-
terpreted as pointing to a previous condition with more than
five digits, a condition that would well accord with the de-
rivation 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 prce-
pollex or prce-hallux respectively; the one continuing the
series beyond the fifth is a post-minimus.

The chiridial appendage just 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
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-

THE ENDOSKELETON

183

tation of the free limb to locomotion in the two difficult ele-
ments of water and air (Fig. 50). The chiridial appendage
(cheiropterygium) is primarily a walking leg (Fig. 50, 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

FIG. 50. Modifications of the fore limb.

(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 (i), those modified for flight. Designations as in the
previous figure.

modified to subserve the needs of an aquatic life it needs to
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.
50; 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-

184 HISTORY OF THE HUMAN BODY

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. 50; (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
cheiropterygium, 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.

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

THE ENDOSKELETON 185

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, and the theories
of prse-pollex and post-minimus point to a previous larger
number. Thus in a way the derivation of hand or foot from
the fish-fin may be accounted for ; but an insuperable difficulty
lies in the presence of the two lengths of limb bones inter-
posed between the girdle and the distal complex, which seem
to correspond with nothing found in the fin. It is to be re-
membered, 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 received a satisfactory explanation.

An early suggestion along this line was based upon the
anterior fin of selachians with its three basal pieces (Fig.
51, 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 pJtfji*La*uei and the pro- and metafirerpquun. slipping
away from the girdle, and bearing the free rays, would become
respectively radius and ulna (Fig. 51, 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 Pleisiosaurus, 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
as the primitive type, and named in consequence the " archi-
pterygium," it turned thought in a new direction, and an effort
was made to seek in the more primitive cases of the hand-form
a central axis with lateral rays proceeding from it. In one
such attempt the central axis was formed by humerus, ulna,
ulnare, carpale V and the fifth digit, to which the remaining
bones served as lateral rays upon the inner or radial side,

i86

HISTORY OF THE HUMAN BODY

thus forming a uniserial form, as in the selachians ; in another
the hind limb of the salamander Ranodon was used as a basis,

Radius

Radius

Ulna

[Ulna]

[Radius]

FIG. 51- Two theories of derivation of the chiridittm (cheiroptery-
gium) from the fin (ichthyopterygium). The figures represent the ante-
rior limb in all cases, [a-c, after HUXLEY; d-f, after POLLARD.]

(a) Cestracion, a primitive selachian, (b) Ichthyosaurus, an extinct lizard. (c)
Necturus, a primitive salamander, (d) Chlamydoselachus, a primitive selachian, (e)
Polypterus, a ganoid, (f) Ranodon, a slamander.

In the three upper figures the ulna is dotted, the radius designated by a diagonal
striping; in the three lower the reverse is the case.

and femur, fibula, intermedium, the two centralia, carpale II
and the second digit were taken as the central axis, leaving one

THE ENDOSKELETON 187

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. 51, ft), 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. // now the long pro- and meta-ptery-\
gia are taken as radius and ulna respectively, ignoring tfie
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 1
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
allied to that form. A more serious objection lies in the fact
that the posterior fin cannot be as easily developed into a
cheiropterygium as can the anterior one ; still the anterior and
posterior limbs may not have had exactly the same origin or
_^ early history, since a later similarity of use \vould cause a
convergence in anatomical structure. Moreover, as a mat-
ter of fact, the pelvic fin of Polypterus exhibits proximally

188 HISTORY OF THE HUMAN BODY

a single long bone, showing at least a tendency in a similar
direction. Here the matter must rest for the present from lack
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 palseontological Rosetta stone, by the aid of which the
discrepant records written on fin and foot may be deciphered
and brought into harmony.*

* Within the past few years a number of fossil Polypteri have been dis-
covered 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 VI
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 produc-
tion of nature as one which has had a long his-
tory; 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, un-
striated, or involuntary, in the form of cells, and striated or
voluntary, in the form of long fibers usually formed from
cell-complexes. The cells of the involuntary type are mesen-
chymatous in origin and are usually associated together to
form layers that supply the walls of certain internal organs and
the larger blood vessels with the power of expansion and con-
traction. All voluntary muscles, on the other hand, are de-
rived directly from the mesoderm, and the ultimate contractile
organs are here not the cells themselves, but long fibrils of
contractile substance built up by the cells, or often by long
rows of cells, and organically connected with them. The mus-
culature of the heart, in some characteristics seemingly inter-
mediate between the two classes of muscular tissue, is a modi-
fication of the first or involuntary type, the cells of which
possess a peculiar shape and become marked with striae. This
derivation is rendered clear through the embryological develop-
ment of the heart, which is seen to be originally an expanded
blood vessel with an hypertrophied muscular coat.

189

HISTORY OF THE HUMAN BODY

The muscles of the second type are in general under the
control of the animal's will, and are thus termed voluntary,
although there are regions, as along the pharynx and oesopha-
gus, where genuine striated muscle may become involuntary
and depend for its action upon external stimuli.

The striated muscles, which form the subject of this chap-
ter, fall into three anatomical divisions corresponding to those
of the skeleton, the axial muscles, the appendicular muscles,
and the visceral muscles. In the embryo the jijDpendicular
muscles are derived directly from the axial, both arising from
the dorsal portion of the mesodermic somites, the epimeres;
while the visceral muscles, limited to the anterior part of the
body, arise from the ventral portion, the hypomeres, the ele-
ment which in the remainder of the body develops into the
pleuro-peritoneum, and furnishes no voluntary muscles. This
difference in origin sharply divides the voluntary musculature
into two fundamental groups, (i) the parietal, arising from
the epimeres and including the axial and appendicular mus-
culatures, and (2) the visceral, arising from the hypomeres,
and limited to the anterior part of the body.

Aside from these three primary divisions of striated mus-
cles, there occur in almost all vertebrates certain superficial
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-
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, whether parietal or visceral. However, in spite of
their secondary origin from other muscular groups, they have
differentiated so far structurally that it is far more convenient
to treat them as a distinct group, the integumental muscles,
rather than to consider 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 first to be considered will

THE MUSCULAR SYSTEM

191

FIG. 52. Necturus, with the integument removed, showing the primi-
tive myotomic muscles and the differentiation in the regions of the limbs
and the visceral skeleton.

192 HISTORY OF THE HUMAN BODY

be the parietal or epimeric muscles, consisting of (i) the
axial, and (2) the appendicular musculature. This will be
followed by a short sketch of the visceral or hypomeric mus-
cles; and, lastly, the integumental muscles, a secondary sys-
tem, will be considered.

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. 52). 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 myocommata. 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 impor-
tance, as these masses are innerved respectively from the dor-
sal and ventral branches of the spinal nerves. The groove
dividing them marks the place of the lateral line of fishes,
in which are 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 limbs, however, the skeletal girdle comes to lie imbedded
within the body muscles, and this, together with the cloaca,
causes 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

THE MUSCULAR SYSTEM 193

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 hips 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 visceral muscu-
lature is also large and well shown. This occupies the ven-
tral 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 tongue, 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 sub-
ject fraught with especial difficulties, and is one in which the
ground is still 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-
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
visceral muscular system of selachians, and carry out the

I94 HISTORY OF THE HUMAN BODY

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 conditions
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 undifferentiated
cells.

In attempting to establish a basis for 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-
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

THE MUSCULAR SYSTEM 195

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 criterions for homology than
position 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 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

196 • HISTORY OF THE HUMAN BODY

muscle and the nerve which supplies it, since the nerve which
originally supplies a given primitive element, such as a myo-
tome or a limb-element, never forsakes 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 question
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
each fiber is presumably constant in its 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

THE MUSCULAR SYSTEM 197

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 visceral system, especially in the
case of fishes and amphibians, but in such cases as the highly
differentiated limb muscles, this method is difficult of appli-
cation, 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.

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

198 HISTORY OF THE HUMAN BODY

through an active larval period, and hence exhibits the earlier
stages in functional activity and consequently in greater com-
pleteness.

From both this source and from the study of adult com-
parative anatomy certain principles may be deduced concern-
ing the formation of muscles, some of which may be noted
here.

I. Separation of an indifferent mass into several elements.
This may be done in several ways :

(a) By the growth of process from the surrounding skeletal
parts, thus furnishing separate points of origin for different
bundles of fibers.

(b) By the growth of a skeletal process across the fibers of
a long, band-like muscle, thus cutting it in two; a secondary
segmentation.

(c) By the higher specialization of the parts of insertion,
thus causing a splitting up of the muscle bundle.

Illustrations of the first of these may be seen in the develop-
ment of the processes on the vertebrae, which results in the
breaking up of the indifferent myotomes into the extremely
complex slips seen in the muscles of the vertebral column of
higher forms. The second method is rare, and occurs in the
case of one of the hyoid muscles of the frog (the fourth
petro-hyoideus) , and in two of the occipito-cervical muscles
in the mammals (obliqui capitis, cf. Fig. 55). The third case
finds a complete illustration in the differentiation of the digits
in mammals and the splitting up, both of the tendons and of
the bellies of the common flexors and extensors, in exact ac-
cordance with the degree of independent action of the separate
digits.

II. Fusion of separate elements.

This may occur in the case of the degeneration of a function,
but it is seen in a progressive instance in the case of long
muscles derived from elements taken from separate myotomes.
Thus the external oblique muscle of the abdomen is a meta-
meric muscle formed by contributions from a series of suc-
cessive myotomes. In salamanders this is barely differentiated

THE MUSCULAR SYSTEM 199

and is still crossed by the myocommata, that mark the original
elements, but as the higher forms are reached the myocommata
disappear, and the primary segmental nature of the muscular
sheet is shown only by the distribution of nerves and blood
vessels. In the rectus abdominis the same thing has taken
place, but here the effacement of the myocommata is not com-
plete, and a few of them are still present, even in mammals,
forming the " tendinous interscriptions " of human anatomy
(3-4 in man).

III. Extension over a much larger area of an element origi-
nally belonging to one or two myotomes.

This is seen especially well in the case of certain of the
muscles of the anterior limb, notably the pectoralis and the
latissimus dorsi. Small and not very extensive in the earliest
land animals, these muscles increase with the importance of the
limbs to which they belong, and may eventually extend over
a large number of myotomes. In their final condition such
muscles closely resemble such a sheet as that of the external
oblique of the abdomen, but their composition is very dif-
ferent, the one being the result of the fusion of elements de-
rived from several myotomes, the other the extension of an
element derived from one or a few.

IV. Further extension of a pair of muscles that have come
to meet in the median line through the formation of a median
skeletal crest or keel.

The two most conspicuous examples of this are the pector-
alis muscle on the ventral side of the thorax and the tem-
poralis of the mandible. Of these, one is originally an appen-
dicular, the other a visceral muscle, each probably a derivative
of a single myotome. Both extend their area of origin with
their increase of function and size, the one dorsally over the
sides and top of the skull, the other ventrally over the chest.
Further progress being stopped by the meeting of the two
muscles in the median line, they obtain an extended point of at-
tachment through the formation of a median ridge. The most
excessive instance of this is seen in the enormous keel of the
sternum in certain birds, notably the humming-bird.

200 HISTORY OF THE HUMAN BODY

Both phylogenetically and ontogenetically the axial muscles
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 meta-
merically. The appearance of these, in the form of meso-
dermic somites, is one of the earliest post-gastrular stages in
the 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 epi-
meres, as in the theoretical sketch given above [Chap. Ill],
but in the Sauropsida and Mammalia, through acceleration of
development, the epimeres.are separated from an indifferent
mesoderm in the form of approximately cubical blocks, ar-
ranged 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 mesenchyma of the intervals between the meso-
dermic somites becomes transformed into the myocommata,
to which, on either side, the muscle fibers are attached.

There is thus formed the primitive system of axial muscles,
a condition which is retained with but little modification in
the primarily aquatic vertebrates, that is, in Amphioxus, cy-
clostomes, fishes and tailed amphibians. In the first of these
the myocommata are not in the form of planes, but are bent in
the middle at an acute angle, the point directed forwards, and
are set into one another so that several consecutive myocom-
mata would be cut in a cross-section through the body at any
plane. In many fishes the myocommata become still more com-
plex and consist of several pairs of cones, those of successive
myocommata being set into one another like nested cups. This
explains at once the shortness of fiber of fish meat, and also its
curious division into sets of concentric circles, when cut in
cross section, two phenomena with which everyone is familiar.
In the urodelous amphibia the myocommata are almost planes,

THE MUSCULAR SYSTEM 201

and cut the body nearly at right angles to the longitudinal
axis, thus presenting a much more primitive appearance than
that found in most fishes, and one that corresponds closely to
the embryonic condition.

Above the urodeles the development of more massive limbs
and limb-girdles, the same element that produces such pro-
found regional differentiation in the vertebral column, is oper-
ative here also in modifying the simple succession of typical
myotomes, a condition which was unaffected by the delicate
fins and weak girdles sufficient for their primitive aquatic
environment. The hip-girdle produces the most direct change
through the intrusion of the ilium between two successive
myotomes, following the course of a myocomma. By this the
axial myotomes become divided into those of the trunk and
those of the tail, and the gradual increase in size of the ilium
and its extension along the vertebral column, as well as the
formation of an immovable sacrum, widens the space between
these two groups of muscles, while the differentiation of func-
tion between trunk and tail effects profound changes in the
muscles themselves. The principal effect of the shoulder-girdle
in this regard is an indirect one, for while it presents no in-
truding process, as in the case of the ilium, it establishes the
anterior point of support and causes the differentiation of the
region between it and the skull into a neck, which becomes in
some cases extremely mobile.

In most reptiles and in mammals, however, in spite of these
modifications, the succession of myotomes remains distinct,
especially dorsally, and through all the regional differentia-
tions there may still be seen the segmental character of the
musculature; but birds and turtles represent two types of ex-
treme specialization in which this continuity becomes broken
through local reduction. The cause of this in turtles is the
formation of the carapace, in which all free movement of the
vertebral elements and ribs is lost through a complete anchy-
losis, and in this region the trunk muscles, though laid down
in the embryo, are early atrophied and become lost. The only
axial muscles retained are those of the neck and tail, together

202 HISTORY OF THE HUMAN BODY

with a powerful retractor system, lying on the ventral side
of the vertebral centra and employed in drawing back the
head and neck. This is undoubtedly homologous with the
prevertebral muscles of the cervical and thoracic region in
mammals (longus colli, etc.), and belongs with the ventral
division, innerved by ventral branches of the spinal nerves.
In birds, correlated with the lack of mobility of the trunk
vertebrae, the corresponding muscles are greatly reduced, but
as this loss of motion is not a complete one as in the turtle, so
also is the reduction of the muscles not as extreme. The
muscles of neck and tail, on the other hand, are extremely well
developed, thus emphasizing by contrast the almost rudimen-
tary condition of the muscles of the back.

In taking up the differentiation of the axial muscles more
in detail, their division into dorsal and ventral masses must
be emphasized, for, although both are derived embryologically
from the epimeres, that is, the originally dorsal portions of
the mesodermic somites, yet the distinction is of fundamental
importance topographically and morphologically, because they
are innerved respectively by the dorsal and ventral branches of
the spinal nerves, a criterion which may always be relied upon
to help out the homology in doubtful cases, where a muscle
lies near the boundary between them or where an extreme de-
gree of differentiation has changed the primary location.
These divisions may be taken up in order, beginning with the
ventral, which are less complicated than the other.

Throughout the course of these ventral axial muscles four
distinct regions may be distinguished, and the same layer, when
it is possible to trace it from one of these regions to the other,
becomes modified to partake of the features of the muscula-
ture characteristic of each. These are the' cervical, thoracic,
abdominal and caudal, the three former continuous, the lat-
ter more or less separated by the interposition of the hip-
girdle and the cloaca. In the abdominal region, even in fishes,
there is seen a tendency for the musculature to break up into
layers, each with a definite direction of fibers, distinct from
that of the others. In amphibians these layers consist pri-

THE MUSCULAR SYSTEM 203

marily of an obliquus intermix, an obliquus externus derived
from the former, and a rectus abdominis, which runs along
each side of the midventral line and is differentiated from
fibers belonging to the other two. To these are added during
the metamorphosis, an obliquus externus superficialis, formed
from the primary externus, and a transver sails, differentiated
from the primary internus. These muscles, with the well-
known differences in the direction of their fibers, persist in all
higher vertebrates. The obliquus externus superncialis of the
amphibians is no longer present as an abdominal muscle, but
seems to be identical with the two serrati posterior es, superior
and inferior, usually treated as muscles of the back although
innerved by ventral nerves. They appear in rodents and in
some other mammals as a continuous sheet, but in man the
two muscular portions (superior and inferior) are separated
by a considerable interval, although occasionally connected
by a thin tendinous sheet, the rudiment of the intermediate
portion.

The rectus abdominis is the only one that retains the primi-
tive longitudinal direction of its fibers, and, undoubtedly cor-
related with this, is another prirnitive character, that of the
persistence of some of its original myocommata, the " ten-
dinous interscriptions " of human anatomy. Connected with
the pubic end of the rectus in mammals is a small muscle, of
uncertain occurrence in man, the pyramidalis. This muscle is
a rudiment of the pouch muscle of marsupials, in which animal
it is extremely well developed, extending from the marsupial
bones, upon which it arises, as far anteriorly as the sternum.
The cremaster muscle of the mammalian scrotum, noticed more
particularly in connection with the decensus testiculorum in
Chapter IX, is a derivative of the obliquus internus. The
qiiadratus himborum belongs also in this place, although its
derivation has not yet been worked out. , It appears first as a
distinct muscle in reptiles and birds, where it is represented
by a few fibers associated with the transversalis. In mammals
it is large and important.

The essential elements of the abdominal musculature may be

'

204 HISTORY OF THE HUMAN BODY

followed into the thoracic region, although here a new char-
acter is introduced by the presence of the ribs, which occupy
the place of the myocommata, of which they are direct deriva-
tives, and thus impart to the layers a segmental character.
The obliquus exiernus (profundus) and internus are thus con-
tinued forward as the external and internal intercostals .respec-
tively, and the transversalis is represented by the transversus
thoracis (triangularis sterni), in man very variable in extent.
The levatores costarum appear as a new element, peculiar to
the thorax, but they undoubtedly belong with the external in-
tercostals and the external oblique.

In the cervical region the reduction of both ribs and body
cavity necessitates another series of modifications of the origi-
nal elements. There belong here, as ventral muscles of the
axial system, the scaleni, and the muscles of the prevertebral
group. Of these the scalenus posterior is a continuation of
the levatores costarum, and the other two scaleni, anterior
and medius, belong to the system of the intercostals and are
attached to those portions of the transverse processes of the
cervical vertebrae which are in reality anchylosed ribs. In the
same category belong the short lateral muscle, rectus capitis
later alls, and the "anterior" (ventral) series of intertrans-
versarii, which are innerved from ventral branches and are
also attached to the rib elements of the cervical vertebrae.
The prevertebral muscles, lying on the latero-ventral side of
the vertebrae, consist of the longus colli, and its anterior ex-
tension, longus capitis (rectus capitis anticus major), also of
the little rectus capitis anterior (rectus capitis anticus minor),
which extends between the skull and the atlas, near the rectus
lateralis. The history and serial homology of these latter
muscles, other than the fact that they belong with the ventral
axial muscles, is not known.

Both ventral and dorsal axial muscles are continued beyond
sacrum and cloaca along the caudal vertebrae, where their de-
velopment varies as much as does the tail itself. As it is more
convenient to treat of the caudal musculature as a single sub-
ject, the discussion of the ventral muscles of this region will

THE MUSCULAR SYSTEM

205

be postponed until later when both dorsal and ventral caudal
muscles \yill be considered together.

The dorsal axial muscles are much more restricted in ex-
tent than are the ventral ones, and are mainly confined to the

lonpssimus capilis
[ trachelo -mastoid ]

Jon&issirous cervicis
[ tmnsversalis colli

ilio-costaJis cervicis
[cervical is ascendens]

iljo-costolis lumtorum
sacro- lumbalis ]

FIG. 53. 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.

206 HISTORY OF THE HUMAN BODY

triangular prismatic spaces located on either side of the mid-
dorsal line, between the spines of the vertebrae and the trans-
verse processes and ribs ; they are, however, far more complex
in structure, and are characterized by the presence of almost
numberless tendons that repeat one another metamerically and
are attached to corresponding parts of successive vertebrae or
ribs. The muscular elements are also less differentiated from
one another than in other parts of the system, and in many
cases the subdivision into separate muscles is an arbitrary one.

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. These belong to the
appendicular system and appear in the urodeles 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, likewise
belong to the appendicular group. Still deeper, beneath all
of the appendicular muscles, are the two serrati posterior es, the
remains 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, under
which head they have already received treatment.

The removal of all the above exposes the genuine dorsal
axial muscles, the derivatives of the trunk myotomes, the

THE MUSCULAR SYSTEM 207

system innerved by the dorsal branches of the spinal nerves.
They lie lodged in the space embraced by the spinous and trans-
verse processes of the vertebrae and run in the main in a longi-
tudinal direction. They are beset with metameric series of
tendons, which attach themselves to the corresponding portions
of successive vertebrae or ribs, and plainly suggest a segmental
origin. Proceeding inwards from the more superficial series
they may be divided as follows, although it must be remem-
bered that the 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 anterior
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, splenins cervicis [colli],
inserts into the transverse processes of cervical vertebrae, while
the remainder, splenius capitis, inserts into the base of the
skull.

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
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 ro\vs, 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 elements
designating the origin, the third, taken in the broad sense and
including the ribs, the insertion.

The outer series begins posteriorly as the sacro-Iumbalis, and
becomes continued in the region of the ribs by a series of mus-
cular slips arising from lower ribs and inserting in upper,
miiscuhts accessorius ad sacro-lumbalem. A still further con-

208 HISTORY OF THE HUMAN BODY

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 vertebrse, 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-costalis f the three portions being dis-
tinguished respectively as ilio-costalis lurnborum, ilio-costalis
dor si and ilio-costalis cervicis. [BNA.]

The inner series, posteriorly blended with the former, ascends
along the lumbar and lower thoracic regions under the name of
longissimus dor si, and is reinforced anteriorly by the trans-
versalis colli and the trachelo-mastoid, the latter inserting on
the mastoid process of the skull. In placing the nomenclature
on a morphological basis the series may retain the name of its
most important member, the longissimus, the parts of which
are longissimus dorsi, longissimus cervicis and longissimus
capitis. [BNA.]

In tabular form the parts of this system with their synonyms
are as follows :

Sacro-transverso-transversalis System.

OUTER SERIES.

ilio-costalis lumborum (sacro-lumbalis)
ilio-costalis dor si (accessorius ad sacro-lumbalem)
ilio-costalis cervicis (cervicalis ascendens}

INNER SERIES.

longissimus dorsi

longissimus cervicis (transversalis colli s, cervicis}
longissimus capitis (trachelo-mastoideus. s. complexus
minor)

III. SPINO-SPINALIS SYSTEM. — This is 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 preceding,
an interrupted one, and is divided into two constant portions,
spinalis dorsi and spinalis cervicis. A few origins from

THE MUSCULAR SYSTEM

209

spinous processes of cervical vertebrae and associated with the
semi-spinalis of the next system, are of occasional ocurrence,
and represent a spinalis capitis.

scmispinalis / complexus
capitis ^brventer cervicis

supraspinatus
infraspinatus
teres minor
teres major

pirifannis
geraellus superior
obturator interims
e") gemeuus inferior
quajiraros femoris

FIG. 54. Muscles of the human back; the muscles lying beneath those
shown in Fig. 53.
Certain of the deeper muscles of shoulder and hip are also shown.

210 HISTORY OF THE HUMAN BODY

IV. TRANSVERSO-SPINALIS SYSTEM. — This is a complex sys-
tem, 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 system, by
which it is covered, and may be imperfectly divided into three
series or layers, superficial, middle and deep, called respectively
semi-spinalis, niultifidus 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
a large and well-developed muscle, divided longitudinally into
a median and a lateral bundle ; the inner one of these is tra-
versed by a myocomma, dividing the muscle into two fleshy
bellies, from which comes the older name of biventer cervicis.
The outer portion forms the complexus (major) of the older
anatomists.

V. INTERVERTEBRAL SYSTEM.— Still beneath the multifidus
and rotatores are several series of short muscles, stretching be-
tween 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 -
spinales, 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
contractile fibers. The intertransversarii of the lumbar region
are divided into medial and lateral portfons, a division probably
without morphological significance ; on the other hand, those of

THE MUSCULAR SYSTEM

211

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 inter transversarii are in-
nerved by ventral branches, and are attached to the ventral or
rib element of the complex " transverse process " of the cervical
vertebne. They are thus in the same series as the inter-
costals, and have been already treated in that connection. The

FIG. 55. 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.

dorsal cer^ncal inter transversarii, 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-

212 HISTORY OF THE HUMAN BODY

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, a perfect example of the principle designated above as
I (&)., p. 198.

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-
sores caudcz upon each side of the mid-dorsal line ; an extensor
caudce medialis, which is a direct continuation of the multifidus
and an extensor cauda 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 fiexorcs (de-
pressores) cauda. Of the above, as shown by the innerva-
tion, the extensors and dorsal abductors belong to the dorsal

THE MUSCULAR SYSTEM

213

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.

FIG. 56. Human caudal muscles. [After LARTSCHNEIDER.]

(A) Ventral. (B) Dorsal.

pir, piriformis; acv, abductor caudae ventralis; acd, abductor caudae dorsalis; eel,
extensor caudae lateralis; sea, sacrococcygeus anterior (^rflexor caudae). The lumbar
vertebrae are designated by Roman numerals, the coccygeal by Arabic.

214 HISTORY OF THE HUMAN BODY

In the tailless 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 inter transversarii have been detected, and certain of
them are fairly constant. The sacro-coccygei posteriores (ex-
tensores 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 100 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 ftexores caudce, de-
scribed under the names of sacro-coccygei anteriores, s. curva-
tores coccygis, and of the ventral abductor, here called the
coccygeus or abductor coccygis. The former was found in
102 out of 1 10 cases, the latter appears to be constant, although
often tendinous.

Two further caudal muscles, pubo-coccygeus and ilio-coccy-
geus, 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 an-
thropoid 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 principal ele-
ment concerned in the construction of the so-called diaphragnia
pelvis. This is a muscular and tendinous floor or partition,
closing the posterior outlet of the pelvic cavity, and its forma-
tion is unquestionably ai\ adaptation to the erect position of
the anthropoids and man, thus strengthening what would
otherwise become from this position a point of weakness.

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 perinseum,
and the genital organs, such as the sphincter ani, ischio-cav-

THE MUSCULAR SYSTEM 215

crnosus, etc., which, like certain of the pharyngeal muscles, are
derivations of the muscular layer of the intestinal wall, and
belong primarily to the involuntary system.

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-
simns, may be referred to the lateral portion. Of these two
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
portion 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 longissimns, 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 longissimns 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 obliqmts 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

216 HISTORY OF THE HUMAN BODY

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-
ciated with the third, fourth 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 (motor oculi) 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
controversial, there are several other pairs of head cavities
which come to nothing, and although there is at present no
generally accepted theory concerning the primary number of
cephalic myotomes, or the exact relationship of those that
produce the eyeball muscles, it is evident that primarily the

THE MUSCULAR SYSTEM 217

head as well as the trunk possessed a series of successive my-
otomes, that certain of those furnished materials from which
the eyeball muscles were derived and that all other muscular
elements thus disappeared at a very early period. [Cf. Fig.
126, A, and the text accompanying it.]

The appendicular muscles are the direct derivatives of cer-
tain axial metameres, as is especially well shown by the
ontogenetic 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
endoskeleton. Into this mass there grows from the ventral
portions of the myotomes a series of what have been aptly
termed myoiornic 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 num-
ber 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. 57) 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

218 HISTORY OF THE HUMAN BODY

A

(pectoral> fin in the dog-fish.

- which the

THE MUSCULAR SYSTEM 219

belonging 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. 57,
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 size 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 development of
limb muscles is found only in fishes, which, in their adult state,
are scarcely beyond the last of the three stages shown here,
while in all higher vertebrates, from the urodeles on, these
early stages are dropped out completely, and in a developing
limb, in which for a time the cells seem exactly alike, and with-
out differentiation of any kind, the first indication of any defi-
nite arrangement is the collection of these apparently indif-
ferent 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 subject
and the technical cifficulties in the way of such investigation,

220 HISTORY OF THE HUMAN BODY

much remains to be accomplished. There has, indeed, been a
large amount of anatomical work done on the subject, but little
has as yet been attained in the study of the phylogenetic devel-
opment 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 distance between
these two groups of animals is much less than is commonly
supposed, and the intermediate stages do not include 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 Northeastern
States, and the extreme South. It may thus be assumed to
represent in the muscles of its free limbs the earliest condition
of chiropterygial musculature yet remaining to us, and is con-
sequently of the utmost importance in the present inquiry. It
will be remembered that in most vertebiates there exists a
certain close correspondence in the skeletal parts of anterior
and posterior limbs, a so-called serial homilogy, and in many

THE MUSCULAR SYSTEM 221

cases this correspondence is at least suggested in certain details
of the muscular system. Here, in Necturus, however, the
correspondence of the free limbs front 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 reall\ 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
girdles, and therefore for descriptive purposes the limb may be
divided into unlike proximal portions, to be treated separately,

FIG. 58. Lateral view cf shoulder muscles of Necturus.

Id, latissimus dorsi; ds, dorso-scapularis; t, trapezius; /, omohyoid; k, levator
anguli scapulae; la, levatores arcuum; d, dorso-trachealis; ph, procoraco-humeralis; as,
anconeus scapular is; al, anconeus lateralis.

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 anterior
limb in Necturus are shown in figures 52 and 58. Of these
certain are extrinsic and extend from trunk or head to the
appendicular skeleton ; others are intrinsic, both origin and in-
sertion being upon the latter part.

_ Conspicuous among the first is the latissimus dorsi, certain
parts of which are still seen to arise from myocommata in the
form of elements in the ^act of separating themselves from the

222 HISTORY OF THE HUMAN BODY

axial musculature, while other fibers, the anterior portion, no
longer show their segmental origin. Anterior to the girdle
lies the trapezius, now, like the anterior part of the latissimus,
showing no trace of myotomic origin, but undoubtedly from
that source originally. Ventral to these are seen two slips
clearly derived from the long superficial rectus, and still farther
ventral, covering the chest region, lies the voluminous pector-
aliSj still in part composed of slips attached to the myocom-
mata. Beneath these superficial layers are deeper muscles, like
the levator scapula and the serratus magnus, here plainly de-
rived 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 scapulce 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-antebrachialis, 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 con-
stant 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 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 location alone (a^xou, elbow, ulna), is much preferable.

THE MUSCULAR SYSTEM 223

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. 59, 60). Latissimus dor si 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.

The deeper layer of extrinsic muscles, levator scapula and
serratns 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. In man these
muscles are again separated into two by the failure of certain

224

HISTORY OF THE HUMAN BODY

FIG. 59. 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

225

Fie. 60. Diagram of human muscles, showing their relation to the
skeleton. [After EUSTACHIUS. See explanation of Fig. 59.]

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.

226 HISTORY OF THE HUMAN BODY

of the intermediate slips. 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 rhomboid-
ens system. This inserts into the scapula and consists pri-
marily 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 mammals,
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 scapula? 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. VThe sufera-
coracoideus is probably represented by the supra- and ivrfra-
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 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

THE MUSCULAR SYSTEM 227

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 those of like name in Necturus. This leaves the humero-
antebrachialis 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 writh 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
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 Necturus (Fig. 61), 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),
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-femoralis externus, which inserts into the
femur. Upon the inner side of the plate there is a single large

228

HISTORY OF THE HUMAN BODY

muscle, the pub o-ischio-femor alls internus, the fibers of which
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-femoralis, also to the femur, and

FIG. 61. Ventral view of the pelvic muscles of Necturus.

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 abdominus. Other designations:
gl, cl., cloacal gland; Pub., pubic portion of pubo-ischiadic plate; isch., ischiadic
portion of the same.

an ilio-nbularis, 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

THE MUSCULAR SYSTEM 229

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
occurring 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 adductor es, the gracilis, which -belongs
with the adductor group, and possibly the sartorius. Of these
the obturator is probably the homologue of the pubo-ischio-
femoralis externus, and the remainder may be derivatives of
the pubo-ischio-tibialis, in spite of the difference in respect to
insertion. The pubo-ischio-femoralis internus seems to give
rise to the obturator internus -with the two associated gemelli,
as well as to the ilio-psoas complex, which appears first as a
distinct muscle in reptiles. The ilio-extensorius is probably the
prototype of the great complex of the front of the thigh, quad-
riceps femorisf composed of the three vasti, externus, medialis
\_crur eus~\ and internus, and the rectus femoris. The glutai
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,
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 teniiissimus. 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,

230 HISTORY OF THE HUMAN BODY

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,Jaside 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 forms; and
in their differences show the modifications of a primary type
due to environment rather than the suggestions 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,
has been commented on above and offers strong support for
the doctrine 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 ele-
ments applied 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

THE MUSCULAR SYSTEM 231

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. 62, a and b) is
largely taken up superficially by a single muscular mass, M.
dgrsalis 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-
ter-digital 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 (s), a muscle
which 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) , \vhich arise from the distal row of carpalia and become
continued into tendons that lie along the dorsum of the sep-
arate digits and insert 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
with the dorsalis and insert, the one along the shaft of the

* 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 condition in these two salamanders is primitive, and not a second-
ary 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.

232

HISTORY OF THE HUMAN BODY

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 (//>), *° which many of the

IT

III

FIG. 62. Muscles of the fore-paw of Necturus ; dorsal aspect

(a) Superficial muscles. (b) Deeper muscles.

da, djrsalis antebrachii, its separate insertions into the metacarpalia are shown
in (b) ; er, radial extensor; eu, ulnar extensor; fit, ulnar flexor, showing from the
other side; xx, extensores breves; s, supinator; 22, intermetacarpales.

ventral muscles are attached. This aponeurosis is a continu-
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 is also divided into four bands which run along
the ventral surface of the separate digits and insert into the

THE MUSCULAR SYSTEM

233

terminal phalanges. These slips are functionally and probably
morphologically the long flexor tendons (ft.), and correspond
in a way to the long extensor tendons of the dorsal side, 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,

III

III

FIG. 63. 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.

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 inserts into its dorsal
(internal) side. The action of these muscles upon the aponeu-
rosis 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

234 HISTORY OF THE HUMAN BODY

of a system of flexors, there are two sets of short flexors, Mm.
flex ores breves super ficiales and Hex ores breves profundi, each
consisting of four muscles, one to each digit. The super-
ficiales 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 insert upon the sides of the
penultimate phalanges. The Hexores profundi lie close to the
bone, arise beneath the former, and insert 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 insert
along the shafts of radius and ulna and into the corresponding
sides of the carpals, serving as flexors of antebrachium and
manus as a whole. These are respectively the flexor radlalis
.and flexor ulnaris (fr and fu).

The ventral muscles thus far enumerated, act either directly
T>r 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 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 inserts by a tendon into the
radial side of the base of metacarpal II.

Deepest of all, beneath the short abductors and adductors,
and reached equally well from either dorsal or ventral aspect,
a set of three intermetacarpales stretch their fibers across the
interspaces between the separate metacarpals and act either as

THE MUSCULAR SYSTEM 235

abductors or adductors of the separate digits (Fig. 62, b,

«~<~ \

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 perfection of the
sideways movement of the digits in the most primitive chirid-
ium knowm, 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 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 dorsalis
antebrachii, which in Necturus serves as an abductor-adductor
system and terminates at the base of the metacarpals, is con-
tinued into four long tendons, which insert into the terminal
phalanges, and thus becomes the extensor communis digitorum,
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 system.
The short extensors become more complicated than in Nec-
turus, but insert along the proximal part of the digits and
are no longer continued into long tendons to the ter-

236 HISTORY OF THE HUMAN BODY

minal phalanges, as 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 digit-
orum sublimis and profundus. In the monotremes the bellies
form a common mass, flexor communis digitoruni, although
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 ten-
don, 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
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, flex ores breves profundi, 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

THE MUSCULAR SYSTEM 237

aspect, and furnish a ftexor 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 metacafpal, 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 f
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
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-

238 HISTORY OF THE HUMAN BODY

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
the previous 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 metameric-
ally, and appearing probably as skeletal rays supplied with
muscles from the trunk musculature in the form of " myotomic
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, there 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

THE MUSCULAR SYSTEM 239

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 endoskeleton the close
correspondence was pointed out between the bones of the arm
and hand and those of the leg and foot; here the similar corre-
spondence 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 radialis, extensor
carpi ulnaris and the two extcnsores carpi radiales, longus and
brevis.

Their homologues in the leg have naturally become modified
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-gastrochnemius
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
tibalis anterior may be the serial homologue of both radial
extensors, while the ulnar extensor is represented by the two
peroncci, longus and brevis. In tabular form the above homol-
ogies appear as follows :

ARM . LEG

Flexor carpi radialis Tibialis posterior

Flexor carpi ulnaris Soleus-gastrochnemius

Extensor carpi radialis longus )

Extensor carpi radialis brevis J TtbiallS anU™r

Extensor carpi ulnaris Peronaus \

longus
brevis

240 HISTORY OF THE HUMAN BODY

The set of supinators and pronators may next be considered,
and in Necturus, in which in each limb there is a single
supinator 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 dorsal 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 remaining muscles, those controlling the action of the
separate digits, are still more in accord, and that too in spite of
the great difference in use between the hand and foot, especially
in civilized man, suggesting the conservatism of these parts,
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 pre-glacial times, a period
given by conservative estimates at 50,000 to 100,000 years, and
has as yet undergone but little change along the line of reduc-

THE MUSCULAR SYSTEM

241

tion. A needful progressive change has indeed taken place,
namely, the development of the peroncuits tertius, a muscle for
lifting the outer edge of the foot and thus counteracting the

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tendency to walk exclusively upon this portion ; but the change
in the line of reduction of a needless complexity of parts has

242 HISTORY OF THE HUMAN BODY

been but slight, simply because there has been no need of such
a modification.

To review the complete 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. 64), which gives all the muscles
of the flexor surface, excepting the lumbricales and the intcr-
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
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 pra-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

THE MUSCULAR SYSTEM

243

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

D

FIG. 65. 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 (C4) 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 a.n-
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.

of the little toe, radius was compared with fibula, and ulna
with tibia (Fig. 65, B). That this theory, fantastic as it may
seem, is not merely a vague speculation, but one to the aid

244 HISTORY OF THE HUMAN BODY

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
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
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
in the diagram at 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 considers 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 D. This comparison of the digits in the reversed
direction, however, when carried to its conclusion, leads also
to the homologizing also 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
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 upper arm with the exten-
sors of the thigh, but allows in the distal portion a direct com-
parison of the flexors with flexors and extensors with ex-
tensors.

As already suggested, the theories just enumerated are not
mere vague surmises, but rest in most cases upon careful study

THE MUSCULAR SYSTEM 245

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-
tively the radial and tibial side 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 in-
vestigators.

The visceral musculature differs from the axial-appendicular,
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 writh which it is associated
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 system
has thus its most extensive though perhaps not its most special-
ized 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
tut little need of controlling the movements of the separate
arches so precisely, the visceral musculature appears in a

246

HISTORY OF THE HUMAN BODY

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

FIG. 66. Diagrams of primitive visceral muscles.

(A) Typical form, hypothetical. (B) Condition based upon that of the urodele
Siren, with a few details supplied from Necturus.

I-VII&, levators of the arches; I-VIIv, depressors of the arches; m, mandibular
arch; h, hyoid arch; b\ to b7, branchial arches; t, trigeminus; fa, facialis; gl, glosso-
pharyngeus; v, to v4 vagus (pneumogastric) elements; x, temporalis; y, masseter; z,
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; /, laryngei.

ancestor from which may have been derived, are given
in Fig. 66.

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-

THE MUSCULAR SYSTEM 247

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
Icvators, the ventral depressors. In the second figure, B, is
shown the actual condition in urodeles, the derivation of which
from the first is obvious.

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, having its primary connection 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-
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
V], it extends ventrally to meet it. On account of this rela-
tionship 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 intcrmandibulares, 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
lowest of the urodeles. The sixth, under the often inappro-
priate name of hyo-laryngeus, 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 (laryngcus 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.

248 HISTORY OF THE HUMAN BODY

The two masticatory muscles, temporalis and masseter, occur
in all higher forms and are homologous throughout, save that
two farther slips, the pterygoideus externus and interims, 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 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 and that
of the posterior from the faciaKs, 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 mus-
cles of these same first two segments are perpetuated, 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 complicated larynx of
higher forms, but whether the muscular elements primarily
\ associated with them assist in the formation of the musculature
^ of the final organ, or whether this musculature is derived
entirely from the muscles primarily belonging to the seventh
arch, that is, dorso-laryngeus and the laryngei, cannot yet be
definitely stated.

The musculature of the tongue, especially its extrinsic
muscles, such as hyo-glossus, genio-glossus, stylo- glossus, etc.,
is probably derived from the visceral muscles, but here another
element is introduced, and that is the muscular layer of the
anterior end of the alimentary canal, which, although of
mesenchymatous origin, and primarily composed of unstriated
cells, involuntary in their action, are yet capable of acquiring
striae and of becoming at least semi-voluntary. From this

THE MUSCULAR SYSTEM 249

layer are derived the pharyngeal constrictors, and it is probable
that the intrinsic muscular fibers which make up the mass of
the tongue and known as the lingualis, come from the same
source.

Superficial to the muscular systems already described, and
lying directly beneath the integument there are found in many
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. These integumental
muscular elements have arisen from whatever preexisting
muscles happen to be adjacent to the location where such a part
is needed, and thus they may be in their origin either axial,,
visceral or appendicular, or may represent a combination of
these systems. They usually possess at one end a firm attach-
ment to some skeletal part or at least to skeletal muscles, while
at the other end, or perhaps along 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
integumental in their relations, but derived from the various
muscles of shoulder and arm ; of these the most important
are M. propatagialis, derived from the anterior portion of the
pectoralis, and 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,.

250

HISTORY OF THE HUMAN BODY

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

D

FIG. 67. Phylogenesis of the panniculus carnosus. [After TOBLER.]

(A) Macropus bennett (kangaroo). (B) Cynocephalus hacmadryas. (C) Cerco-
pithecus sabaeus. (D) Cercopithecus cephus.

from its fibers certain more specialized slips to serve as
sphincters (sphincter marsupii and sphincter cloacce).

A panniculus carnosus, perhaps here mainly a contribution
from the latissimus, is also present in marsupials (Fig. 67, 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
and cattle when these surfaces are stimulated by the bite of
an insect. In the lower primates, the panniculus appears as

THE MUSCULAR SYSTEM 251

a broad sheet upon each side, much as in marsupials (Fig.
67, B), but within this Order it is seen to separate into axil-
lary and inguinal portions (Fig. 67, C and D) and in the an-
thropoids, the former alone remains, much reduced in size
(Fig. 68). In man there appear to be normally no traces of

FIG. 68. Anterior remnant of the panniculus carnosus, " achselbogen,"
in the gorilla. [After TOBLER.]

pmj, pectoralis major; fob, coraco-brachialis fascia; Id, latissimus dorsi; pa,
" pectoralis quartus," a part of the panniculus; x, tendinuous fibers from the
latter.

this muscle, but there occurs occasionally a system of slips in
the axillary region, the axillary arch ("Achselbogen") asso-
ciated with both latissimus and pectoralis and very variable in
appearance, a typical characteristic of a rudiment. In asso-
ciation with this, there occasionally develops a more posterior
pectoralis slip, the pectoralis abdominis, a relic from a remote
part. Still another rudiment of the panniculus system is seen

252 HISTORY OF THE HUMAN BODY

B

FIG. 69. Two cases of axillary panniculis rudiments in man.

;, GEHRY]- pmj' Pectoralis major; pmn, pec-

minor; d deltoid; Id, latissimus; pa, •« pectoralis quartus," a part of the
panmculus; x, the definite «« achselbogen »; st, the sternalis, a rare muscular
anomaly, also a part of the panniculus.

THE MUSCULAR SYSTEM 253

in the stcrnalis muscle, an element of very rare occurrence and
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. 69). The st emails
muscle has been recently shown to occur much more frequently
in the Japanese than in Europeans (13 per cent, against 4
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.

Another system of integumental muscles is derived from the
visceral musculature and appears in its simplest form in the
sphincter colll of amphibians, reptiles and birds, and is itself
a direct descendant of a selachian muscle, the superficial con-
strictor. The fibers of this sheet enwrap the neck region and
in turtles and birds the muscle is well developed and covers the
entire neck. In mammals this sheet differentiates into two
layers, a more extensive superficial layer, the platysma,
and a smaller and deeper layer, which retains the original
name of sphincter colll. 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 direction 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 t
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. 70).

The superficial sheet or platysma extends upwards across

254

HISTORY OF THE HUMAN BODY

Occ.

Tri.

AlLttt

Hat

Front

Plat.

FIG. 70. 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, I, s, auriculo-labialis superior; A. I. i., auriculo-labialis
inferior; Au. ant, auricularis anterior; Au. post, auricularis posterior; Antitr, anti-
tragicus; Plat, platysma myoides; Occ, occipitalis; Tri, triangularis; MX. lab, maxillo-
labialis; O. or, orbicularis oris; Bucc, buccinator.

THE MUSCULAR SYSTEM 255

the side of the neck, and, reaching the ear, divides into two
sheets, dorsal and ventral. The dorsal sheet, auriculo-occipit-
alls, 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 portion 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-
maticus [major], the orbicularis oculi [palpebrarum] and the
levators of the lip and side of the nose. Finally a third element,
the or bit o -auricularis, furnishes two of the extrinsic ear mus-
cles, auricularis anterior and superior [attrahens and attol-
leus 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 gale a 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, as found in reptiles and mono-
tremes, lies within the province of the seventh cranial nerve

256 HISTORY OF THE HUMAN BODY

and is wholly supplied from this source. Exhibiting a superb
example of the constancy of a muscular innervation, the
branches of this nerve expand and differentiate into the mus-
cle which it supplies, and with the migration of the latter
to the face there comes also the nerve; and 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 at-
tracted 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 pri-
mates suggests a totally distinct function, that of communi-
cation of the moods of the animal to its associates, that is, a
language. That some outward expression for the developing
power of thought should show itself pan passu with the de-
velopment 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 supple-
mented by the development of mimetic muscles, through the
medium of which far more subtle shades of meaning could be
expressed. For a time, therefore, in the anthropoid precur-
sors of man, both forms of intercommunication must have ex-
isted 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 consequent flattening of the dental arcade a
greater differentiation of articulate sounds became possible,
these latter became more and more employed as the better
medium of intercommunication, 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 VII
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 Anatomic 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 become 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-

258 HISTORY OF THE HUMAN BODY

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 cceloin,
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
gastrocoele through the so-called neur enteric canal (Fig. 71,
B), but this connection is only transitory and the entire struc-

THE DIGESTIVE AND RESPIRATORY SYSTEM 259

ture soon disappears, leaving the gastroccele 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

B

FIG. 71. 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, gastroccele (=cavity of alimentary canal); n, neurocosle (=<cavity of the
neural tube); ne, neurenteric canal; np, neuropore; b, blastopore; an, developing
proctodseum; 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 stomatodccum, a middle endodermic one, the
mesodcntin, and a posterior portion, also ectodermic, the
proctod&um. In the articulates (crustaceans, insects, and
spiders), in \vhich the alimentary canal is formed in much
the same way, the proportion of the functional digestive tract

260 HISTORY OF THE HUMAN BODY

formed by stomato- and proctodseum, and therefore ectoder-
, mic, is extremely large, and the mesodaeum, though volumi-
nous, is employed mainly in the formation of laterally placed
digestive glands; but in the vertebrate the canal is mainly
mesodaeal, 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. XII. ] 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, submncosa, musculosat 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

THE DIGESTIVE AND RESPIRATORY SYSTEM 261

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, through 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-

262 HISTORY OF THE HUMAN BODY

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 dependent upon the quality of the food habitu-
ally taken, an innutritions 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 in-
tegument. 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-

THE DIGESTIVE AND RESPIRATORY SYSTEM 263

ment of a complicated alimentary canal is shown by the com-
parison of various phylogenetic stages (Fig. 72). At first 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. 72. 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; in, thymus gland; I, bronchi, leading to the
lungs; h, liver; ;', pancreas; k, spleen; y, pyloric diverticula (in fishes); z, cloacal
diverticuia (in birds); x, intestinal diverticulum, the ccecum (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 tadpole of which possesses
a spiral, much coiled intestine, while that of the adult shows

264

HISTORY OF THE HUMAN BODY

but a few windings, the change taking place within a few
weeks or even days in correlation with a change of food (Fig.
73). Allowing for a few isolated cases, however, the canal,
of fishes and amphibians is short and becomes considerably
lengthened in the Sauropsida and Mammalia, where the in-

FIG. 73. Comparison of alimentary canal in (A), tadpole and (B),
adult frog. [After the LEUCKART wall charts.]

testine, the part mainly involved in the increase of length,
becomes disposed in complicated folds and windings. These,
although apparently wholly irregular in their disposition, may
be referred to a definite system, as may be made clear by a

THE DIGESTIVE AND RESPIRATORY SYSTEM ,265

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 medial 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 this typical form,
however, owing to the peristaltic movements of the museulosa
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-
ment 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 narro\ved portion between these forms the oesopha-
gus. At its lower end the stomach is bounded by a restricted
portion, the pylorus, which, by a specialization of the circular

* 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 to-day in the case of palmistry, a " science "
founded like the other upon the erroneous and utterly baseless assumption
of the causal relation of two unrelated sets of phenomena.

,
V/

x

266 HISTORY OF THE HUMAN BODY

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
portion. A little below the pylorus it develops from its mucosa
two enormous glands or gland-complexes, liver and pancreas,
which 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 duode-
num. A second early and better marked subdivision of the in-
testine is a terminal enlargement which generally receives 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 forms 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 sur-
face. Of those the most important are the lateral pharyngeal
pouches previously mentioned; the pyloric cceca, found in
fishes, and often very numerous; colic cceca, at the beginning
of the large intestine in mammals ; and cloacal caeca, 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. 72]. 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 incom-
plete separation between the two. This cavity opens to the
exterior 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 cydostoma, is that seen in the lam-

THE DIGESTIVE AND RESPIRATORY SYSTEM 267

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 Chap. III.), and it may be held either that the gnatho-
stoma 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 per-
missible it may be allowed that in early vertebrates there was
a circular jawless mouth, provided with a hood and situated
at the anterior end of the body; that in some form midway
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, entirely usurped
the function of the old one, which thus became reduced and
finally disappeared. In one point alone, that of 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.

268

HISTORY OF THE HUMAN BODY

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. 74). In fishes these
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

Fio. 74. Pharynx and visceral arches in human embryo. [After His.]

(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 im-
par, which, with the eminences immediately behind it, form the tongue. The thy-
reoid anlage and the glottis and epiglottis are seen in (c).

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,
but in reptiles, birds, and mammals, although the pharyngeal
pockets and their corresponding external depressions form
during embryonic life, but two or three ever break through and

THE DIGESTIVE AND RESPIRATORY SYSTEM 269

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 BNA~\ 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 uncom-
mon 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 for-
mation of slime glands capable of producing an abundant se-
cretion. 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-

* Cf. Chap. V under Visceral skeleton ; Chap. VI under Visceral mus-
cles, Chap. VIII under Arterial arches; and the present chapter farther
on under Thymus and Thyreoid.

270 HISTORY OF THE HUMAN BODY

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 open-
ing in the pharynx the glottis, which becomes regulated by a
series of cartilages and muscles derived from the visceral sys-
tem and forming the larynx.

/""The idea naturally suggests itself that this ventral pulmon-
/ary 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 the 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. The first of these theories
seems by far the more probable, especially since the air-blad-
der 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 can-
not be traced, since the lungs are always paired and an air-
bladder is always single.

The nasal cavities are separated from the pharynx by an
approximately flat plate of bone that forms the roof of the

THE DIGESTIVE AND RESPIRATORY SYSTEM 271

mouth, or hard palate. In fishes and amphibians this is formed
in great part by the anterior end of a single extensive median
bone, the parabasal, but in higher forms this element is reduced
and its function is assumed by the palatines and pterygoids
and by horizontal plates directed inward from the maxillaries
and premaxillaries. These latter elements are all lateral and
arise in the embryo upon the sides of the head and grow to-
wards the center, finally uniting in the median line. In some
groups of birds the two halves remain disassociated, thus
forming a palatine cleft by which a direct communication is
established between the nasal and pharyngeal cavities. This
failure to unite may occur as an abnormality in mammals, pro-
ducing 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, 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 con-
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, which are themselves probably derivatives
of the musculosa of the anterior portion of the alimentary
canaL

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 formation 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-
ered in no respect from those that cover the exterior of the

272 HISTORY OF THE HUMAN BODY

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.

r 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
separated 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 dental
origin. Thus are formed the Hat 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 jaws, as well as that of the splint-like
dentare, and perhaps the angulare, that enclose Meckel's carti-
lage and form the mandible. (See Chap. V.) 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
as in the fossil toothed birds, the two become detached, and the

THE DIGESTIVE AND RESPIRATORY SYSTEM 273

teeth are inserted by what is termed a thecodont articulation
into deep pits in the bone called thccce or alveoli (Fig. 75).

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.

a b c d

FIG. 75. 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,
while in the second or closed type the addition of new layers

274 HISTORY OF THE HUMAN BODY

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 constant 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 ele-
phants, and the chisel-like incisors of rodents; in the last in-
stance 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 irregularity 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 prop-
erly. Similar, although here perfectly normal, instances are
those of the ornamental 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
modifications, 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 differentiation in use between the front teeth, that are
in a position for grasping the food, and the back teeth, which

THE DIGESTIVE AND RESPIRATORY SYSTEM 275

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 can-
ines 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 func-
tion is superfluous. The remaining teeth may be classed to-
gether 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, consid-
ered below. Of premolars the greatest number that occurs
in mammals is four in each half-jaw, of definite molars, five.

276 HISTORY OF THE HUMAN BODY

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:

i-Vi-3

1-2-1-3

3-1-2-1

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 Bovidae, the family
to which cattle and sheep belong, a formula showing a total
loss of canines and of upper incisors :

003-3

3-03-3

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:

5-4-T-5

5-4-1-5

5-1-4-5

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,
the former being never more than three upon each side. Thus,
in the opossum (a marsupial) the dental formula is:

THE DIGESTIVE AND RESPIRATORY SYSTEM 277

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:

3-1-4-3

3-1-4-3

Certain cases, on the other hand, show an extraordinary re-
duction, as in the mouse-like rodent, Hydromys, where the
formula is :

1-0-02

1-O-O2

The greatest reduction is found among some Cetacea, and in
certain edentates (Myrmecophaga). In certain of the first
group tfre 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-
per odd on, there are but four teeth, all in the lower jaw, the
anterior ones of conical shape, behind which are two imbed-
ded 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
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 primates there are two incisors upon each side and
a well-developed canine. In the monkeys of the Western

278 HISTORY OF THE HUMAN BODY

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:

2-I-2-1*

= V2

2-1-2-3 6

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:

2-1-2-0

2-1-2-0

20

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 si.vf the first of
which to appear is the protocone, or primary cusp. Associated
with the protocone are two secondary ones, the paraconef
which is anterior to it, and the metacone, which is posterior
(Fig. 76, 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. 76, III). A tooth, or a dentition, in which
the three cusps, para-, proto-, and meta-cone, still lie in a

THE DIGESTIVE AND RESPIRATORY SYSTEM 279

straight line following
the line of the jaw,
is called triconodont,
one in which the bend-
ing has taken place is
trigonodont. This tri-
tubercular tooth, still
under the influences of
the opposing tooth sur-
faces, may develop a
lateral spur, the hypo-
cone, upon which 1-3
tertiary cups, the co-
nidi, may develop, or
else the hypocone may
form the point of a pro-
jecting spur, the talon.
The nomenclature given
here is that of the up-
per teeth, the corre-
sponding parts of the
lower jaw being distin-
guished by the addition
of the suffix -id, thus :
protoconid, hypoconid,
talonid, etc. In favor
of this theory may be
urged its complete ap-
plicability to all known
forms, both living and
fossil, as a system of
nomenclature, and its
correspondence in se-
quence of stages to that
shown by the extinct
forms in consecutive
geological periods. The

rt

FIG. 76. Phyletic history of the molar
cusps. [After H. E. 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; mi,
metaconid; hd, hypoconid; hid, hypoconulid; ed,
entoconid.

280 HISTORY OF THE HUMAN BODY

embryological record, also, with a few exceptions, which may
be explained in other ways, is in accord 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 concrescence 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
of the mammals, and careful embryological records which
show numerous cases of rudimentary tooth germs both pre-
ceding and succeeding definite teeth, has caused a revision
of the entire subject. The matter becomes more compre-
hensible by referring to the lower vertebrate Classes, espe-
cially reptiles, where the papilla of each physiologically active
tooth is associated with a succession of additional tooth germs
in different stages of maturity, and designed to replace the
functional tooth in case of injury (Fig. 77).

In a vitally important tooth, as in the poison fangs of ser-
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 jaws 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. 78, A), and

THE DIGESTIVE AND RESPIRATORY SYSTEM 281

such has been undoubtedly the origin of this phenomenon, save
that it is not necessary to assume that all the feeth 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

FIG. 77. Succession of teeth in reptiles.

(A) Section through jaw of Phyllodactylus (a lizard) showing functional tooth
(I) 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; str. muc.t stratum mucosum; /, II, etc., the succession
of tooth germs.

that precede the milk dentition, and others that succeed the
permanent set; they thus consider that mammals have in-
herited from their reptilian ancestors four tooth generations,
prelacteal, lacteal, permanent and post-permanent, of which
the second and third have become generally established.* The
first developed in the premammalian ancestors, the last may
come to development in the future.

Regarding the occurrence of the two dentitions there is

* It is asserted that in Nasodon. a Tertiary ungulate, a pair of prelacteal
incisors becomes developed.

282

HISTORY OF THE HUMAN BODY

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.

A

B

FIG. 78. Figures representing diphyodont dentition.

(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.]

They are thus almost monophyodont, with a permanent milk
dentition, and are in sharp contrast to other monophyodont

THE DIGESTIVE AND RESPIRATORY SYSTEM 283

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. 78, B).
Although fishes possess no functional tongue_ihe 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 visceral 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 cntoglossum, 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 disposition when retracted becomes a
problem variously solved in different cases. Thus in a cer-
tain salamander, Spelerpes fuscus, the sheaths of the cornua
run down the sides of the body and are attached to the ilia,
and in the woodpecker they come 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.

284 HISTORY OF THE HUMAN BODY

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 hypo-hyal is reduced to a rudiment which con-
nects with the styloid process by a ligament in which no trace
of the cerato-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 totally 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-
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
timbriata. 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-

THE DIGESTIVE AND RESPIRATORY SYSTEM 285

veloped upon the dorsal side of the former, and gradually
usurping its function. An opposing opinion rejects the claim
of the sub-lingua as homolog of the sauropsidan tongue, and
finds the rudiment of the os entoglossum in the septum lingua,
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, and is often asso-
ciated with the septum.

Distinctive glands in the mouth cavity other than simple
mucous cells are not found in fishes or wholly aquatic am-i
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 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

286 HISTORY OF THE HUMAN BODY

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
others, that secrete a clear fluid without salivary attribute.
Such are the molar glands of ungulates, or the voluminous
orbital gjand of the dog family (Canidae), 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
thyre oid glands. These, although not connectecTTn function
with the dTgestive 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
disappear. These are all seen in the accompanying diagram
(Fig. 79), 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
four, 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,

THE DIGESTIVE AND RESPIRATORY SYSTEM 287

FIG. 79. 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.

288 HISTORY OF THE HUMAN BODY

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 functional ones.
The fourth is without one. In mammals the definite thymus
is formed either wholly from the anlage associated 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 siipra-
pencardial bodies, but they are better termed the post-bran-
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

THE DIGESTIVE AND RESPIRATORY SYSTEM 289

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 (Petromyzori), we
catch a glimpse of its past history, for here for a time it ap-
pears as an open trough, lined with cilia, and in open com-
munication with the pharynx. In this condition it corresponds
closely in structure and position with the hypo-branchial
groove, or end 'o style 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 digestive organ and deserves a place in this chap-
ter for reasons more fundamental than a mere topographical
relation. In the true vertebrates, however, its function, with
its structure, becomes completely changed, and it no longer
assists in digestion, but presumably possesses some regulating
effect upon the blood, especially that supplying the brain.
There is yet much that is problematic in this, but that the
association between the thyreoid and the brain is an intimate
one is shown both by the occurrence of cretinism, a curious
developmental malformation associated with the thyreoid and

290 HISTORY OF THE HUMAN BODY

in which varying degrees of idiocy play a principal part, and
by the frequency with which insanity has followed the extir-
pation of the thyreoid.

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. • */r-
1 ' 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 bircf.
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 tne
food is first collected. Aside from its use as a receptacle in
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
oesophagus, but in some mammals striated fibers, probably

THE DIGESTIVE AND RESPIRATORY SYSTEM 291

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
thus equipped. \

The stomach is originally a simple, spindle^sliaped enlarge-
ment of the canal, extended lengtjnvise and indefinitely sepa-
rated from the oesophagus,-a4tii6ugh 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, Carnrvora, Insectivora, and
Edentata, these being in other respects also most primitive
of placental mammals (Fig. 80, a).

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

HISTORY OF THE HUMAN BODY

stomach as that of the mouse (Fig. 80, b), in which the con-
diac and pyloric halves are separated by a marked restriction,
and this tendency reaches its extreme in the ruminants (Fig.
80, 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-

FIG. 80. Stomachs of various mammals.

(a) Man; (b) mouse; (c) pig; (d) seal; (e) vampire bat; (f) manatee; (g)
sloth; (h) sheep.

tion is divided into an omasus and an abomasus, into which
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. 80, 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. 80, e), the fundus
is elongated to form a ccecum of twice the length of the body,
used as a reservoir for blood.

THE DIGESTIVE AND RESPIRATORY SYSTEM 293

It is of interest in this connection to trace the changes in the
mesentery of this region, more precisely termed the mesogas-
trium, as they appear in successive stages in the mammalian
embryo which are undoubtedly of historical significance (Fig.
81). 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

ir

IV

FIG. 81. 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.

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 mafus), which,
as all mesenteries are essentially double, consists of four layers

294 HISTORY OF THE HUMAN BODY

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 cpiploicnm [foramen
of Winslow]. In most mammals'The bag is Twfclely 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 crassuni], 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 bilary 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 ev aginations, the former
ventral, the latter dorsal. As they increase in size they pro-
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 clearly shown in the accompanying
diagrams (Fig. 82), which show the origin of these organs
from the intestine, their serous investment and their dorsal and

THE DIGESTIVE AND RESPIRATORY SYSTEM 295

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 ligamcntum hepato-
gastriciim [y~\ (sometimes termed the lesser omcntnm), and
the ligamentum suspensoriitm hepatis \_x}. While primarily
the entire length of the alimentary canal that passes through
the ccelomic region becomes attached by both a dorsal and a

A

FIG. 82. 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. The place through which
it is taken is indicated approximately in (a) by the arrows.

Organs: s, stomach; n, spleen; /, 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 mesentery, the ventral one becomes 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-

296 HISTORY OF THE HUMAN BODY

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 absent in the former. The
pancreatic duct is normally without such a resevoir, but a pan-
creatic bladder has occasionally been observed as an abnor-
mality in the common cat, existing side by side with a normal
gall-bladder, the two exhibiting about the same size and pro-
po^tions.

,/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 uro-
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 cceca, 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.f the common fowl), and in
ducks and geese, but are quite rudimentary in certain others
(woodpeckers, parrots, etc.). Ostriches possess a single
coecum of great length (7 to 8 meters) and furnished with
an internal spiral partition, which greatly increases its ef-
• fective surface.

In mammals a single coecum is developed,* which varies
greatly in size and functional importance. Rudimentary in

* 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.

THE DIGESTIVE AND RESPIRATORY SYSTEM 297

edentates, most insectivores, and bats, it frequently attains an
enormous size in herbivorous or graminivorous forms. In
certain rodents (e.g., muskrat, 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) vermif ormis , 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 i :io, while
in adult life it is 1 120. It is longest absolutely between the
ages of ten and twenty, after which it shows a slight reduc-
tion.* Its. status as a rudiment of slight functional value is
shown by the tendency towards the obliteration of its lumen,
a tendency which increases steadily with age.** Furthermore,
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

*Zuckerkandl tabulated the length of the appendix in 161 bodies,
with the following result :

mm mm

17 — 20 2 cases. 90 — 100 15 cases.

30 — 40 8 100 — no 4

40—50 6 no— 120 5

5O — 60 28 120 — I3O 2

60—70 26 " 130—140 I

70—80 29 " 140—150 I

80—90 23 " 150—160 I "

** Wilhelm Miiller, 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: (See table on p. 298.)

298

HISTORY OF THE HUMAN BODY

on the right side, from which there proceed in order an
ascending, 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.
83 and 84).

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

Continued from p. 297.

MALES

FEMALES

Age in
Years

No. of In-
dividuals

No. of
Cases of
Oblitera-

Per-
cent-

Age in
Years

No. of In-
dividuals

No. of
Cases of
Oblitera-

Per-
cent-

tion

age

tion

age

o

48

0

o

o

46

o

o

i

78

0

o

i

58

o

o

2 — 10

5i

i

2.0

2 — IO

4i

o

0

II — 20

39

2

5-1

II — 2O

19

I

5-4

21—30

47

3

6.4

21—30

23

2

8-7

31—40

55

7

12.7

31—40

38

9

23.8

41—50

84

22

26.2

41—50

46

16

34.8

5r_6o

73

15

20.5

51—60

60

18

300

6 1 — 70

58

17

2-9-3

61 — 70

48

24

50.0

71—80

3i

12

38.7

71-80

30

8

26.6

81—90

15

8

53-3

81—90

i?

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 dis-
crepancies 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 individuals examined.

THE DIGESTIVE AND RESPIRATORY SYSTEM 299

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
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-

\

FIG. 83. Colon labyrinth of Ceruus canadensis. [After WEBER.]

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 inter-relationship of animals.

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
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

300 HISTORY OF THE HUMAN BODY

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 20:1,

FIG. 84. Colon labyrinth of Propithecus diadema. [From WEBER,
after VAN LOGHEM.]

while in the horse, which eats similar foods, it is but 12:1, but
in this latter animal an enormously developed coecum 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,

THE DIGESTIVE AND RESPIRATORY SYSTEM 301

probably the largest among mammals, is reported to be that of
Pontoporia, a South American dolphin, while the shortest
mammalian 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 prob-
able 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 function 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,
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

302 HISTORY OF THE HUMAN BODY

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 chitinization
which involves the surrounding integument. Such organs are
called gills, a general term for all aquatic respiratory organs.
These present numerous mechanical devices for increasing 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 structures,
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
descendants 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 with
that of the animals which most nearly represent their aquatic
ancestors. In some cases the old respiratory organs are re-
tained by sinking them into deep recesses kept moist by glands,
in others they are discarded in favor of new ones, formed, per-
haps, by the transformation of some ready-to-hand cavity,
which is lined with blood vessels and made to communicate
with the exterior through some regulated outlet. Still another
principle is seen in the tracheal tubes of insects, which 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.

THE DIGESTIVE AND RESPIRATORY SYSTEM 303

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 function 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 in 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-
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

* 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 respira-
tory 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.

chics. When, in the history of the race, vertebrates came out of
the water and adapted themselves to a terrestrial element, they

304 HISTORY OF THE HUMAN BODY

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-

\

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
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

THE DIGESTIVE AND RESPIRATORY SYSTEM 305

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
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. In two especially primitive genera, how-

FIG. 85. Respiratory organs.

(a) Cyclostome. (b) Selachian, (c) Teleost. (d) Selachian embryo, (e) Am'
phiuma larva. (f) Cryptobranchus larva [from a Japanese print].
n, nostril; s, spiracle; g, gill-slits.

ever, Hexanchus and Heptanchus, there are respectively six
and seven, facts which suggest that the number at present rep-
resents a reduction from a previously more extensive series,
the reduction being from behind forwards. Upon the pharyn-
geal 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 branchice. These
are profusely vascular and are supplied with a rich capillary
network developed between two sets of branchial arteries, the

306 HISTORY OF THE HUMAN BODY

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
to 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 Ampkioxus,
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
Jarge increase of surface, and thus physiologically more effi-
cient.

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. 85, a). The current
of water is directed by rhythmic respiratory movements, which
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

THE DIGESTIVE AND RESPIRATORY SYSTEM 307

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 branchic?, three in number upon each side,
and attached to the cartilaginous gill-arches. In structure
they are usually plumose or dendritic (Fig. 85, 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- j
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
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

308 HISTORY OF THE HUMAN BODY

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.

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-
tems are the integumental and the pharyngo-cesophageal, 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

THE DIGESTIVE AND RESPIRATORY SYSTEM 309

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. 86). Here again the incentive towards the for-
mation is a moist membrane richly supplied with capillaries,

Pharyngeal,.
Muscle

Gesophageal
Muscle

Art.Pharyngea

, Portion, of
PulmonaiyArcK

/Art.Gastrica

\anasiomoses

[heremththe

\pulraoiiaryArch

Art.Gastrica
V. oesophagea

FIG. 86. Pharyngo-cesophageal lung of Desmognathus, showing pha-
ryngeal and oesophageal muscles, and the net-work of blood-vessels in the
walls of the pharynx and oesophagus.

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

310 HISTORY OF THE HUMAN BODY

pharynx and oesophagus are dilated and contracted in asso-
ciation with the usual respiratory movements of nostrils and
floor of the mouth, and the anterior part of the alimentary
canal thus becomes a functional 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.

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.

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 position 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-

THE DIGESTIVE AND RESPIRATORY SYSTEM 311

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 is profusely vascular, thus forming an organ of far
greater functional activity than the definite lungs of many
amphibians ; corresponding to this its larynx, the regulator 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
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,

312 HISTORY OF THE HUMAN BODY

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 but a 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 iden-
tity of their nerve supply, and the weak point in the story is the
identity of the two pulmonary systems, that of the ganoid and
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 $th
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
lateral cartilages, placed one upon each side of the glottis
(Fig. 87, a). A short membranous trachea, entirely without
cartilaginous support, leads to the bag-like lungs. In an
allied form, Proteus (Fig. 87, 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

THE DIGESTIVE AND RESPIRATORY SYSTEM 313

pieces into an anterior pair of arytanoids, upon either side
of the glottis, and a posterior pair of tracked pieces. Within
the Class of Amphibia there are no new pieces formed beyond
these, but they exhibit a great variety of forms, and become
especially complex in the Anura, where they are employed in
the production of various sorts of notes used as sexual calls
(Fig. 87, c-e). 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 approxi-
mate them. These give rise in many of the more complicated

A

a b c d e

FIG. 87. Laryngeal cartilages of various Amphibians.

(a) Necturus (mud-puppy), (b) Proteus. (c) Amphiuma. (d) Triton (Newt),
(e) Rana (frog).

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

314 HISTORY OF THE HUMAN BODY

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, the rare Order of subterranean
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
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 s
branchial arches, evidently the 2nd and 3rd (Fig. 8
leaves only the first arch, which in this Order unites
true hyoid arch to form the hyoid complex (" hyo
of human anatomy), to which it contributes its
cornua, the thyreo-hyals. The cricoid cartilage is n
Sauropsida and is manifestly the result of the con

en to be
;). This
with the
id bone "
posterior
uch as in
solidation
of certain of the upper tracheal rings.

The development of the lungs is mainly along the lines of

THE DIGESTIVE AND RESPIRATORY SYSTEM 315

physiological efficiency through a repeated subdivision of the
interior. This results in the production of smalft chambers,
commonly known as " air-cells," more properly alvboli, 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 ultimaie 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

a

FIG. 88. Larynx of Echidna (monotreme). [After GCEPPERT.]

(a) Ventral, (b) Lateral.

St. H, stylo-hyal; EH, epi-hyal; CH, cerato-hyal; BH, basi-hyal; Th. H, thyro-
hyal; Thy, I, first thyreoid cartilage; Thy, 2, second thyreoid cartilage.

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 irregu-
lar 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
by grooves into lobes, but in other cases the grooves are shal-
low, and the lobes become hardly more than slight protuber-

3i6 HISTORY OF THE HUMAN BODY

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 environments. Thus the lungs are without lobes in
the Cetacea, Sirenia, and some seals, thus suggesting a modi-
fication due to an aquatic life, but on the other hand the lungs
are similarly undivided in sloths and ant-eaters, and in cer-
tain 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
condition 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 V Organization
des corps vivans. 1802. Transl. Packard,
1901.

A VASCULAR system of some sort occurs in all ccelomate
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
dosed, in distinction 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,

317

318 HISTORY OF THE HUMAN BODY

certain localized portions of the system of tubes, the walls of
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 lacuna, and
indirectly aided by the various serous cavities of the body like
the ccelom and the capsules of the joints. Both anatomically
and physiologically this system is divided into two subordi-
nate systems, hccmal 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 capillaries and sinu-
soids, two forms of the minute vessels which extend between
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 lacuna, here spaces
limited by membranes, and mainly differing from the rest of
the system, into which they are continued, 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 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
in the usual sense, but storehouses for leucocytes. With the
lymphatic system are associated the serous cavities of the
body (ccelom, capsules of joints, bursse about the larger ten-
dons, etc.), with which numerous lymphatic vessels communi-
cate so that, by a physiological though not a morphological
right, these cavities have been considered as expanded
lymphatic vessels. In the lower vertebrates a number of
definitely located pulsating organs, or lymph hearts, further

THE VASCULAR SYSTEM 319

the circulation of the liquid medium, which is here termed
lymph and consists of plasma containing leucocytes alone.

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, which, 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
stage, and that is, the extra-embryonal yolk-sac, which de-
velops a set of blood-vessels for the purpose of feeding the
embryo.

In this is seen a probable reason why the history of this
system is retained in so much more perfect condition than

320 HISTORY OF THE HUMAN BODY

are most of the others, and that is, because this system is
actively functional 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. 89, 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. Further anteriorly
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 backwards,
forming two aorta.

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 aortcc fuse into one
and increase greatly in size so that the proportions between

1HE VASCULAR SYSTEM

321

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 this 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
stump, but is destined to become the carotid artery, which in-

FIG. 89. 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.

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 ouside 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
cardinal veins, two anterior and two posterior (not shown in

322 HISTORY OF THE HUMAN IODY

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
Citvier (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. 89, 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 some-
what 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. 17). 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
greatly in size, and its blood-vessels increase proportionately.

THE VASCULAR SYSTEM 323

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
atrophy 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
develop 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
in a median caudal vein, which, posterior to the cloaca, divides

324 HISTORY OF THE HUMAN BODY

into two branches. These pass along the outer sides of the
kidneys and are resolved entirely into a set of small branches,
the vena 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
vence 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. 90), 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 longi-
tudinal row along the median line. The most posterior
of these, the sinus venosus, is the receptacle into which is
brought the impure 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 anterior compartment is prolonged into an
arterial trunk (truncus arteriosus), which breaks up into

THE VASCULAR SYSTEM

325

paired lateral branches, the afferent branchial arteries. These
pass along the cartilaginous gill-arches and supply the gills,
dividing into very fine branches for the purpose. Thus far

FIG. 90. Diagram of primitive vertebrate circulation, based on the
condition found in selachians.

s, sinus venosus; t, atrium; v, ventricle; x, 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; can, caudal arteries
and veins; it, iliac arteries and veins; sb, subclavian arteries and veins; ra, renales
advehentes; rr, renales revehentes; I, lateral vein; cp, posterior cardinal vein; ca, an-
terior cardinal vein; p, hepatic portal vein; h, hepatic vein.

the blood is impure, in the state in which it was received from
the body, but at this point there intervenes a system of capil-

326 HISTORY OF THE HUMAN BODY

laries, in which the exchange of respiratory gases takes place,
and when it is re-collected into the efferent branchial arteries,
corresponding in number to 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 anterior 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
from the posterior fins. Anterior to this they still receive meta-
meric contributions from each somite and finally empty

THE VASCULAR SYSTEM 327

into the posterior cardinals near their fusion with the an-
terior ones to form the ductus Cuvieri. The subdavian 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 metameric element of the fin-fold. When the definite
limbs were established by the hypertrophy of an anterior and
posterior region and the loss of the intermediate portion, the
veins corresponding to the regions retained became large and
important, while the rest were somewhat reduced. To ac-
count for the retention of a single vein for each appendage,
rather than one from each somite represented, one may sup-
pose either the retention 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 vena 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 vena 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 enters
the liver upon its dorsal side and becomes resolved into capil-
laries, as in the former case. From this organ the blood is re-

328 HISTORY OF THE HUMAN BODY

collected by one or more hepatic veins, lying more on the ven-
tral 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 physiological
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 physio-
logically arterial blood flows in the veins, and physiologically
venous blood in the arteries.

The history of the arterial arches is shown in synoptical
form by the accompanying series of diagrams (Fig. 91 ), which
present the facts as deduced from the combined study of both
the adult anatomy and embryological development of repre-
sentatives of each Class of vertebrates. The diagrams repre-
sent the adult conditions in each case, the relationship being
morphologically interpreted by the help of the development.

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 col-

THE VASCULAR SYSTEM

lects the blood from the
arches and conveys it to
all parts of the body, the
ventral and dorsal aortse
respectively (Fig. 91, a), f
In all the diagrams the •
parts of both sides are
shown, viewed ventrally
and flattened out so that
the ventral aorta lies in
the middle and the dorsal
aortse converge from the
outer sides. In selach-
ians (Fig. 91, 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,
vessels which include the
only remnants of the first
arterial arches.
/fin the urodelous am-
phibians (Fig. 91, 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

FIG. 91. Diagrams showing modifi-
cations in the arterial arches of Ver-
tebrates.

(a) Typical, embryonic, (b) Fishes. (c)
Amphibians. (d) Reptiles. (e) Birds. (f)
Mammals.

/, //. ///, IV, V, VI, arterial arches; of,
Art. carotis dextra; cs. Art. carotis sinistra;
sd, Art. subclavia dextra; ss, Art. subclavia
sinistra; ad, Aorta dextra; ay, Aorta sinistra;
bd. Ductus Botalli dexter; bs, Ductus Botalli
sinister; pd, Art. pulmonalis dextra; pst
Art. pulmonalis sinistra.

330 HISTORY OF THE HUMAN BODY

both sides form complete arches and unite to form the dorsal
aorta, and VI becomes the pulmonary (here the puhno-cutane-
ous). Of the two aortic arches IV is the principal one and V
is a subordinate, and is of such slight functional 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 interposition of respiratory
capillaries as in fishes; in larval urodeles, however, and in a
few adult forms, the perennibranchs, a branch from the ven-
itral side of the arch supplies the external 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 expiration 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 increases 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 perviotis
artery, is also retained between the carotid arch and the main
aortic arch (III and IV).

In reptiles (Fig. 91, 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-
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 VASCULAR SYSTEM 331

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 reptiles,
and from this point on is seen no more, save in the em-
bryo, where it often appears as a rudiment. A pulmonary
artery develops from the sixth arch of each side, as in am-
phibians, leaving a right and left ligamentum arteriosum \_Bo-
talli]. The subclavian arteries, that supply the fore-limbs,
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. Croco-
diles 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 be-
lieve that these vessels are not the subclavians at all, but are
secondarily developed arteries (subclavice secundaric?) which
have functionally replaced the true subclavians.

In birds and mammals (Fig. 91, 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
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-

332 HISTORY OF THE HUMAN BODY

sented by sub-clavi<z 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
arches are successively added through the formation along the
course of the aortae of buds or sprouts that meet and join. In
fishes all but the first of the arches are retained, but in higher

THE VASCULAR SYSTEM 333

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. 92.
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. 93 and 94), based,
with the exception of the last one, upon the embryology of the
rat. In a is seen the first arch [Cf. Fig. 89, a]f which starts
from 'the conus arteriosus (ca), proceeds forward, and re-
turns 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

334

HISTORY OF THE HUMAN BODY

THE VASCULAR SYSTEM 335

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 (s) which
soon disappear; of these the lowest is the hypo glossal (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 f
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 (az>),
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 (o) and an infra-orbital (t), 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 hypo-
glossal and first cervical arteries with one from much further
forward is also to be noticed, but is without special interest.
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 compar-
ing this with stage t, for here the portion connecting the com-

336

HISTORY OF THE HUMAN BODY

FIG. 93. 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; /, lingual artery; vc, art. vertebralis cere-
bralis; x, y, stumps which ultimately join, and form the stapedial artery, (st.)

THE VASCULAR SYSTEM

337

mon origin of the supra- and infra-orbital and mandibular
arteries has become lost and their source of supply has become
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

FIG. 94. (h)-(l), Continuation of the series given in Fig. 93.
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 ?rtery; st,
stapedial artery; n, anastomosing branch between the external carotid and mandib-
ular arteries. The other abbreviations are given under FIG. 93 or are explained
in the text.

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 II, the
anastomosing branch (xy), and a bit of the dorsal stump of
arch I, becomes called the stapedialis (st)f through its relation-

338 HISTORY OF THE HUMAN BODY

ship 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
(a), 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 cir-
cuit 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 / 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
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

THE VASCULAR SYSTEM 339

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. 95).

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, with small lateral fins, but
which become excessively developed in connection with the
hypertrophy of one or both pairs of limbs. The caudal aorta,
like the other elements of the tail, retains its primitive charac-
ter 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 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

340

HISTORY OF THE HUMAN BODY

of the legs correlated with the erect position, which has de-
veloped the iliac arteries out of all proportion, giving the er-
roneous but inevitable impression that these latter arteries
form the real continuation of the aorta, which becomes bi-
furcated, and that the arteria sacralis media is an unimportant
median branch arising from the point of bifurcation and sup-
plying the coccygeal region.

In the adult selachians, which in their venous system rep-
resent practically the starting point of the history so far as
vertebrates are concerned, the two sets of cardinal veins, an-

a

FIG. 95. Metamerism in the mesenteric arteries of Amphibia. [After
KLAATSCH.]

(a) Siren, (b) Necturus. (c) Cryptobranchus. (d) Cryptobranchus (a second
specimen). (e) Anura.

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 sys-
tem historically, and have been totally replaced by the latter
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

THE VASCULAR SYSTEM 341

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-j
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
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.

342 HISTORY OF THE HUMAN BODY

As has been shown above, the Class of fishes comprises
forms which have remained at the stage last described, tire
one in which the cardinal system holds the supremacy; but
by the time the amphibians are reached there has been another
usurpation in that partTof the body posterior to the heart,
and the posterior cardinal ,systprn ^S i" ^ turn,
subordinated to a third svsternj that nf the

or, more briefly, the postcava. How this appears in full func-
tional activity is seen in the diagram representing the main
venous channels of the urbdele (Fig. 96, pc), where it has
secured nine-tenths of the traffic between the kidneys and the
heart, and allows but a small part to be conveyed by the pos-
terior cardinals, formerly completely in charge of this territory.
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 latter 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
diagrams given in Fig 97, 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 capilla-
ries, developed in association with the portal system to be con-
sidered 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 the renal

THE VASCULAR SYSTEM

343

portal system and the anastomoses between the caudal vein
arid the posterior cardinals. Here it is united with the pos-
terior ends of these latter vessels and annexes them as well
as the caudal vein to itself, thus establishing a single path

FIG. 96. Venous system of urodeles, based on that of Desmognathus.
[In part after (Mrs.) ANNE BARROWS SEEL YE.]

ji, internal jugular; je, external jugular; sc, subclavian; pc, postcava; h, hepatic;
pt, portal; g, gastric; si, splenic; abd, abdominal; res, vesical; ec, epigastric; msf
mesenteric; il, iliac; c, caudal; z, anastomotic branch between the two caudals;
ra, venae renaJes advehentes; cl, lateral cutaneous; cdp, cardinalis posterior; x,
anastomotic branch between postcardinal and renal. The systemic veins are given
in black; the portal system is in outline.

344

HISTORY OF THE HUMAN BODY

from the end of the tail, between the kidneys, to the heart.
The remainder of the posterior cardinals, anterior to the con-
nection with the vena cava, becomes reduced 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

a

FIG. 97. 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 xz, xv commissures between the veins of the two sides.

the blood from the sides of the trunk, especially from the
intrrrostnl sprvrf^ a function which they exercise in gauropsids
and mammals.

Fig. 98 shows the development of the postcava in a mam-
mal in which the part played by the posterior cardinals is
especially emphasized. In some details the developmental

THE VASCULAR SYSTEM

345

history in reptiles, birds, and mammals differs a little; there
may, in fact, be slight differences within the limits of each
group, but the essentials are in all cases as given above. In
this specific case the posteriorly developing postcava enters
the right of two small veins developed in the (here transitory)
renal portal system (stage a). Stage b is developed from
stage a through the formation of a transverse anastomosis
between this vein and the two posterior cardinals, with an ac-
companying increase of size in these parts. This anastomosis,
x, divides the original posterior cardinal into two parts, y

FIG. 98. Development of the postcava in mammals. [After HOCH-
STETTER.] (a)-(d), Rabbit;1 (e), ' Man.

j, anterior cardinal (jugular) ; y, z, the two £arts pi^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. .f '

and 2, of which the former becomes reduced and forms the
azygos, while the latter develops as part of the postcaval sys-
tem. In stage -c the permanent kidneys have formed, The
ureters from which run through a temporary ring in the part 2,
a relation without special significance.

In stage d an important change is effected, first by the fusion
of the two lateral elements, once the caiiHal pnds ^f the pos-
ter jorjcardinals. ^and, second, by the precedence in size and
function established by the right portion of the part 2 anterior
to this fusion. T<he kidneys have also moved anteriorly, and
have developed the renal veins. The final -condition is shown

346 HISTORY OF THE HUMAN BODY

in stage e, in which the left limb of the loop (ss of stage d),
has for the most part disappeared, while the right has become
nearly median, and has thus straightened the entire vessel.
The left spermatic (or ovarian) vein, which in stage d enters
the left limb of the loop, has caused the retention of that part
through which its connection with the main system was origin-
ally established, while the right spermatic vein enters the post-
cava directly, since this was originally the part to which it
was attached.

This last diagram (e) is taken from the human embryo,
since in man the relation of the iliacs differs considerably
from that in the rabbit, from which the other diagrams of
this series are taken. In other respects there is no appreciable
difference between the two forms.

This embryological history explains the composite structure
vof the postcava as seen in the adult. A^morjy_5__sprout
"irom the liver capillaries, it is composed more posteriorly of

vein connected with the embryonic kidney and a portion of
he right jDOsterior cardinal, and to this is added, still more
posteriorly, the caudal vein, primarily a portion of the sub-
ntestinal.

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 VASCULAR SYSTEM 347

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 the 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 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 it is essentially similar in all cases there is but little his-
tory shown by the comparison of adult forms. The early
embryonic 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 they
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 tjiis 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. 97 and 99,

348 HISTORY OF THE HUMAN BODY

which are taken from the lizard and mammal, respectively.
In a of either figure are seen the primary elements which enter
into the process, namely the two omphalo-mesenteric 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 (xlf x», and x3 in Fig. 99). Of these,
x* is the most posterior, and lies ventral to the intestine; the
next, xz, is dorsal ; and the third, xlf 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
f Fig. 97, c, and Fig. 99, c). The subsequent suppression of
the left side of the anterior ring and the right side of the
posterior ring, as indicated in Fig. 99, c, produces a single
large trunk, eventually the portal, which twists in a spiral
about the intestine (Fig. 97, d and e). Meanwhile, the liver
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. 99, 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
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 VASCULAR SYSTEM

349

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

ud orns us
omd

b

ud

2

\
S

s
\
\

us

ud ouid or

FIG. 99. Development of the hepatic portal system in mammals. [After

HOCHSTETTER.]

j, anterior cardinal (jugular); s, posterior cardinal; c, ductus Cuvieri; omd, cms,
right and left omphalo-mesenteric veins; ud, us, right and left umbilical veins;
sz', sinus venosus; xv xv xy 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.

finally establishes a direct connection with the heart through
the ductus venosus Arantii (Fig. 99, 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-

350 HISTORY OF THE HUMAN BODY

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 subdavians,
correlated with that of the fore-limbs, which gains for it an
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 below the entrance of the subclavian,
since it appears to be formed by the union of two equal veins.

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
formation of a connection across the middle line between 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 assumption of this double task, it early
received the name of superior vena cava (anterior or precava),
in comparison with the inferior vena cava (posterior or post-
cava), which enters the right atrium near it.

A simple modification takes place in the mammalian pos-
terior cardinals. The formation of a transverse connection
between the two allows one of them to assume the function
of conveying to the heart the blood collected by the metameric
branches, and permits the other to sever its anterior connection
with the heart. Although there is much variation in this,
the more common condition in Man consists of the preserva-
tion of the posterior cardinal on the right side in its entirety,
into which the other empties by means of the transverse con-
necting vein, and is deprived of all direct connection with the
heart. The first, or complete vein, was called the azygos,

THE VASCULAR SYSTEM

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 lit-
tle 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 re-
verse is the case and it is
the hemiazygos which per-
sists. These relations are
extremely variable, even
in Man, where the oc-
casional conditions classed
as anomalies receive their
complete explanation
through the morphologi-
cal history of the region.
As a review of the ven-
ous system of Man, with
the morphological signifi-
cance of the principal
parts, there may here be
presented Fig. 100, which
shows the course of the
main trunks in the adult,
the older parts which have

ILint

FIG. 100. 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.

352 HISTORY OF THE HUMAN BODY

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 difficulties which are the in-
evitable 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 adaptation is fairly com-
plete, 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 disadvantage to
the organism.

As one example of the slowness of the adaptation where
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 o-f this
throughout amphibians, reptiles, and birds, all of which still
possess symmetrical vena anonymce, and the left one is forced
to bring its blood over to the right side. First among the
mammals comes the formation of an obliquely placed trans-
verse connective, which allows the establishment of a true vena
cava anterior, and rectifies the slight mechanical disadvantage.
The mere fact that this condition continues so long without
readjustment suggests that the disadvantage must be ex-
ceedingly slight, far too inconsequent to come under the direct
control of Natural Selection; and the bettered condition in
these mammals cannot fail to suggest the result of mechanical
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

THE VASCULAR SYSTEM

353

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 arthropods
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 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 pos-
terior portion. This brings
it topographically very far
anterior, just back of the
gill region, and this primary
position is, indeed, that per-
manently retained in fishes
and amphibians; but in rep-
tiles, birds, and mammals it
suffers a considerable change
of location in a posterior di-
rection and comes to lie in
a thoracic cavity, formed by
the ribs and sternum, 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

FIG. 101. Four stages m the de-
velopment of the amniote heart.

354 HISTORY OF THE HUMAN BODY

heart into the form of an S-shaped tube, the bending being
in such a way that the sinus venosus is dorsal and the conns
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.
Furthermore, the several parts brought into contact by the
flexion become permanently adherent to one another and the
heart becomes molded into a more compact 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-
mo-cutaneous veins and enters the left side of the atrium
(Fig. 102, B). The anatomical response to this consists of
the division 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 con-
dition, a partial septum appears in this, which suggests a di-
vision into right and left ventricles, but contains a large
opening through which the two kinds of blood still mingle
(Fig. 102, C). In these animals also, the complete differen-
tiation of the third and .fourth arterial arches' as aorta and
pulmonary artery, respectively, leads to a longitudinal division
of the conus arteriosus by means of two longitudinal folds
placed opposite to one another which grow from the inner

* 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.

THE VASCULAR SYSTEM

355

walls, meet, and fuse. This causes a complete separa-
tion 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 imgure, passes

FIG. 102. 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.

356 HISTORY OF THE HUMAN BODY

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 pulmonary 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 con-
stant bodily temperature, but compels the animal to be " cold-
blooded," or more correctly, poikilothermous, that is, change-
able in temperature in more or less accordance with its sur-
roundings.

The heart of reptiles (Fig. 102, 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
(Fig. 1 02, 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. 102, 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-vessek, the sy^tejnjc__v^nous^ trunks as-
suming a position on the Ight, in connection with the atrium
of that side, and the mairmaorta being on the left, instead of
the right. In both birds arid mammals, then, the tissues are
supplied with pure blood and are thus enabled to maintain a
constant body temperature, which fluctuates butfa 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.

THE VASCULAR SYSTEM 357

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 at 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 which become infiltrated into the tissues, and re-
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 connective-
tissue cells. One of the largest of these lymph channels is the
subvertebral ^space, which enfolds the aorta. The lympatics
that collect the digested food (chyle) from the4 intestine com-
municate, either directly or indirectly, with this channel, which
thus forms a very primitive thoracic duct.

A second series of lympliatic^spaces Ts" 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

* 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 the mammals, the foramen
ovale, persists normally in the human infant for a few days after birth;
but is occasionally pemianent, producing the condition known as cyanosis,
in which the individual suffers continually from the presence of venous
blood in the arterial system.

358 HISTORY OF THE HUMAN BODY

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.

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. 163). Each lymph -heart pos-
sesses a single venous ostium, by which the sac communicates
^wlth the vein,~and several lymphatic ostiq, through which the
sac receives the fluid from as many lymphatic vessels. The
forrneY^)peningjs^ equipped with two semi-lunar 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
lyrriph-hearts. Thus in" reptiles only the posterior lymph-
hearts persist in the adult, and the same appear in birds dur-

THE VASCULAR SYSTEM

359

fe.

FIG. 103. 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 reoal; is, ischiadic; fe, femoral.

360 HISTORY OF THE HUMAN BODY

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 junctu7e~~6:T~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.

Th£ 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. 104). As in the case of
the circulatory system, the preservation of so many phyloge-
netic 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
customary degeneration of transitory structures.* The lym-
phatic system first appears in the form of a pair of tiny diver-
ticula 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

* An embryo in which the lymphatic system is not in full functional
activity becomes cedematous, and in extreme cases the result is a spheri-
cal ball, without indication of the normal shape.

THE VASCULAR SYSTEM

362 HISTORY OF THE HUMAN BODY

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 cisterns [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
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 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.
Within the interstices of the lymphoid tissue occur large quan-
tities of lymphocytes, or wandering cells that appear to be
identical with the leucocytes or white blood corpuscles, and
although proof is thus far wanting, it is probable that the

^ lymphatic nodes form one of the localities in which these cells
are formed.

In the walls of the colon of mammals appear aggregations
of nodules, similar to those connected with the lymphatic
system, and forming large areas known as noduli lymphatici

y dggvcgati \Peyer' s patches']. Many attempts have been made I

THE VASCULAR SYSTEM 363

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 chan-
nels to all parts of the body. As the nodules of Peyer's
patches are endodermic in origin, 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 had here
an example of elements, originally endodermic, wandering
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, 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 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-verte-
bral space, are produced as outgrowths from the blood-vessels,
and that these enter into secondary communication with nu-
merous 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 leucocytes, 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 intestine, 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 re-
spect, and develops large quantities of the blood-cells, espe-

364 HISTORY OF THE HUMAN BODY

dally erythrocytes (red blood corpuscles). Although lym-
phatic 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 circulatory
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 IX
THE URO-GENITAL 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 ccelom
(more strictly, metaccele), 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. 105, 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 (gastrocwle) and the primary
body cavity (protoccele) . Within this protoccele are contained
two sets of organs, each opening to the exterior, excretory tu-

365

366

HISTORY OF THE HUMAN BODY

bules (nephridia) and germ glands (gonads). (Fig. 105,
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 protoccele 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,

B

FIG. 105. Diagrams to illustrate the prevertebrate history of the nephri-
dia, the gonads and the ccelom.

(A) Stage of gastrula, with two germ layers, a gastrocoele (g), and a primary
body cavity (/>). (B) Here a third layer has appeared in the form of paired
gonadic sacs (m), and paired nephridial tubules (f) ; with external openings at
o and x respectively. The nephridia are furnished with an inner opening, the
nephrostom (n). (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.

THE URO-GENITAL SYSTEM 367

Taking now an animal of the type shown in Fig. 105, 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. 105,
C; also Plate II). The protoccele 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 metacoelic 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-
cceles. 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 metaccelic cavity. From this they

368 HISTORY OF THE HUMAN BODY

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 metaccelic 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. 139.] To some
this indicates the conclusion arrived at independently by the
consideration of other systems, that these 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
metaccelic 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 below 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 metaccele (the definite (( ccelom"), lined ivith peri-
toneum, which is re-fleeted along the mid-dorsal line and

THE URO-GEXITAL SYSTEM 369

furnishes an investment and suspensory ligament for the in-
testine. The ccclom 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 abdominalis 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 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.

\Ye 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

370 HISTORY OF THE HUMAN BODY

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 metanc-
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 pronephrotic, system, appears in the embryo
of all vertebrates; it functions during the larval life of some
fishes and amphibians (Fig. 106), 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 pr one phr otic
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

THE URO-GENITAL SYSTEM

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 jusf within it. The meso-
dermic anterior portion may be the
result of the fusion of the outer
ends of the succesive nephridia,
each one contributing that portion
belonging to its own somite.

Typical pronephridia (Fig. 107,
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- FIG. 106. Frog tadpole with

and meta-nephrotic systems, fu*ctional pronephros and de-
1 . . . ~ velopmg mesonephros. [After

namely, an association with capil- MARSHALL.]

lary blood-VeSSels, enabling the v, ventricle of heart; *, truncus;
.... g, gill arteries; ph, pharynx; a,

nephridia to extract waste mate- aorta; h, aniage of hind Hmbs;
rial directly from the blood. This ^nep\nru0ss;; *' £™£™^l\ "£
association is here very slight, Wolffian duc*-
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

372 HISTORY OF THE HUMAN BODY

A

FIG. 107. Diagrams illustrating the two forms of nephridia character-
istic of pro- and meso-nephros, respectively.

(A) Pronephros. (B) Mesonephros. Farther explanation in text.

THE URO-GENITAL SYSTEM 373

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 ccelom,
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 pronephrotic 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 mesonephrotlc 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 " Wolflian body'' named in honor of its
discoverer.

The separate units of this system, the mesonephridia (Fig.
107, 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 pronephrotic duct, be-
coming secondarily connected with it posterior to its con-
nection with the pronephridia. Later on both pronephridia

374 HISTORY OF THE HUMAN BODY

and that portion of the duct anterior to the connection with
the mesonephridia become atrophied, and the duct thus be-
comes the mesonephrotic, the " Wolffian duct " of an earlier
nomenclature. On account of this utilization of the prone-
phrotic duct by the mesonephrotic tubules it has been held by
some that both belong to one system and that the latter are
merely later appearing elements of the pronephrotic 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 pronephrotic tubule is related
to the accompanying glomerulus much as in Fig. 108, A, but
in some cases the projection bearing the glomerulus becomes
partly enclosed in a recess of the ccelom, and the nephrostome
opens into this instead of into the main cavity (Fig. 108,
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 mesonephrotic tubule, in which the added por-
tion, 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 pronephrotic system are related
much as in the second diagram, the Bowman's capsules of the
mesonephrotic tubules develop directly as evaginations from
their walls, and there is thus no indication of either the par-
ticipation of the ccelom or of the formation of a new nephro-
stome.

The mesonephrotic system may be found in full functional

THE URO-GENITAL SYSTEM

375

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 ccelomic cavity. The mesone-

FIG. 108. Diagrams to illustrate a theory of the relationship of the
pro- and meso-nephrotic tubules to each other. [After GEGENBAUR.]

(A) Stage of the pronephros. Its nephrostome (p) is placed opposite the
glomerulus, but with a small portion of ccelom interposed between them. (B) Stage
of the mesonephros. Here by the formation of a new nephrostome at s the inter-
posed piece of the ccelom has become included in the nephridium, forming a Bow-
man's capsule.

phrotic (Wolffian) duct lies along its outer side and sustains
that relationship to the cloaca noted above under the pro-
nephrotic system, usually opening by a urinary papilla into
the dorsal wall of the cloaca.

The third urinary system, the metane phrotic, arises di-
rectly 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 mesonephrotic,
although this latter system is well developed during embry-

376 HISTORY OF THE HUMAN BODY

onic life. The metanephrotic 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 mesonephrotic duct near
its lower end. This evagination, medial at its origin, comes
to lie dorsal to the mesonephrotic 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 ccclom 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 metanephrotic 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

THE URO-GEXITAL SYSTEM 377

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 mesonephrotic
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
mesonephrotic or Wolffian duct), as a ductus [vas] 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 mesonephrotic 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 newr 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.

378 HISTORY OF THE HUMAN BODY

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, as it never appears to contain liquid, but the occa-
sional presence within it of excretory salts suggests a sub-
ordinate 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
o£ 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 ccelom,
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 avlayer which otherwise forms noth-
ing but investing membranes and suspensory ligaments, is
easily explained by the theory given above, which considers
the entire ccelom 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

THE URO-GENITAL SYSTEM 379

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 ccelom, 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 exr
ception the sexes are normally separate. The occasional oc-
currence 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 testes, 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 hermaphro-
dites among higher vertebrates are usually if not always ap-
parent rather than real and are in fact malformations due to
some error in development affecting mainly the external 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 jmedium 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.

380 HISTORY OF THE HUMAN BODY

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 tliis
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 ccelom, 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 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 ccelom 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

THE URO-GENITAL SYSTEM • 381

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 metaca?le.

It will be noticed that in the above description the ne-
phrostomes open directly into the ccelomic 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 mesonephrotic 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

382 HISTORY OF THE HUMAN BODY

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-
nefrhros and the mesonephrotic 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, or " Midler's duct "
of many writers. At the free anterior end, which extends to
almost the forward limits of the ccelom, it opens by an ex-
panded mouth, ostium tubes, directly into this latter cavity
and receives into this the mature ova which become released
from the ovary and wander about in the ccelom in the primitive
fashion. The oviduct arises in the embryo as a tube seg-
mented off longitudinally from the mesonephrotic 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 mesonephrotic 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 on other parts. If it be the first it con-
cerns a very ancient bit of history, and suggests an extreme degree of
reversion.

THE URO-GENITAL SYSTEM 383

transmitted, a single nephridium with its nephrostome became
differentiated for this purpose, and that later on there came a
longitudinal splitting of the primary mesonephrotic 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 uro-genital relations of the selachians seem to have been
inherited directly by the amphibians (Fig. 109, a and c), for
the two correspond closely; in the male there is the same re-
lationship between testes and sexual kidney, and the meso-
nephrotic 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
mesonephrotic system, nothing but reproductive functions.

384

HISTORY OF THE HUMAN BODY

A

FIG. 109. Comparison of the urogenital system in Anamnia and
Ammota. (Continued at bottom of p. 385.)

THE URO-GENITAL SYSTEM 385

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 specialised, 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. 109, 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, the vasa
efferentia and the mesonephrotic duct; in the male amniote
these parts are retained while the remainder of the mesone-
phrotic system has disappeared, being replaced by the meta-
nephrotic. The mesonephrotic duct, released from all urinary
function, becomes the definite ductus deferens, and the re-
maining portion of this system, including vasa efferentia,
sexual kidney ana collecting efferent tubules, becomes closely
associated with the testes under the name of the epididymis. In
the female amphibian the reproductive system has become prac-
tically independent of the urinary through the development of a
separate excurrent duct, the oviduct, and thus, with the rise of
the metanephrotic system, that of the mesonephros becomes
reduced to a few functionless vestiges ; yet the more conserva-
tive embryonic history records the fact that both oviduct and
ostium were originally portions of the mesonephrotic system,
and, although with a different history, 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 metanephrotic urinary system, with kidneys
and ureters distinct from the reproductive system except for
intimate topographical relationships at their outlets; in the

(A) Male anamnian. (B) Male amniote. (C) Female anamnian. (D) Female
amniote.

t, testis; o, ovary; ms, mesonephros; c, that part of the mesonephros which is
associated with the germ gland, (in male amniotes this becomes the epididymis);
t», Wolffian duct (ductus deferens); in, Miiller's duct (oviduct; mu, uterus); k,
metanephros; b, bladder (of the Amniota) ; n, ureter; r, rectum; v, vagina.

386 . HISTORY OF THE HUMAN BODY

male the testes are accompanied by an epididymis and a ductus
deferens, respectively the anterior portion of the mesonephros
and the mesonephrotic duct; and in the female there is an
oviduct 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 mesonephrotic system at-
tains a high degree of development, and the mesonephros,
under the name of the " Wolffian body," is large and con-
spicuous. In the marsupial young of monotremes and mar-
supials 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
made longer than in former cases and includes also approxi-
mately the period passed by lower mammals in the marsupial
pouch, there is no free larva, and the changes are considered
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 daysJn the human species),
the sexes, although definitely determined, show absolutely no
difference in the general appearance of the uro-genital 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 mesonephrotic
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.

C rt

2 a

i£

o

-H* e

w o

THE URO-GENITAL SYSTEM 387

Upon the inner side of each mesonephros appears a longitudi-
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-
nephrotic) 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 uro-genital 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 mesonephrotic system, since it is in no wise concerned in
the reproductive function and since the urinary function is
wholly assumed by the metanephrotic system, disappears ex-
cept for a few useless vestiges; its loss allows the mesone-
phrotic 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 this
latter, 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 ccelom through a small

388 HISTORY OF THE HUMAN BODY

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 ccelomic 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 ccelomic wall.

A uterus is more or less an adaptive organ, related to
Miiller'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
Miiller'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 a certain salamander
(Salamandra atra). 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 placenta!, mammals, the wall of the uterus
becomes 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 Miiller'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. no).

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.
They open separately into a common urogenital sinus in close

THE URO-GENITAL SYSTEM

389

association with the openings of the ureters. There are no
vaginae, although the urogenital 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
absorption 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
oviduct, uterus and vagina. The two vaginal orifices open

DUPLEX

BIPARTITUS

SIMPLEX

FIG. no. Different types of mammalian uterus: explanation in text.

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, urogenital 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 perin&um.
Above the marsupials the two ducts fuse into one at their

390 HISTORY OF THE HUMAN BODY

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-
titus, 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 Procavia (Hyrax), an iso-
lated group of small mammals found in Western Asia and in
Africa; the bicornis is widely distributed and occurs in ungu-
lates, cetaceans, most bats, and other forms; the bipartitus
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 mesonephrotic ligament contain
between them the mesonephros and its duct, and become con-
tinued along the ventral surface of the mesonephros as a low

THE URO-GENITAL SYSTEM 391

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-
nephrotic 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-
nephrotic ligament, the ligamentum ingninale.

The vestiges of the mesonephrotic system are found ex-
actly where they would be expected, lying in the broad liga-
ment not far from the oviduct, bet\veen it and the insertion of
the mesovarium. They consist of three portions, epoophoron,
its longitudinal duct, and paroophoron. [Plate III, cJ] 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 efTerentia 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-
nephrotic system. It occurs quite regularly in the pig, the
horse and in ruminants, but is only occasional in Man.

The original direction of the mesonephrotic 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 located them near or within the brim
of the pelvis, not far from the inguinal region. This occa-

392 HISTORY OF THE HUMAN BODY

sions several compensatory changes, such as the lengthening
of the ovarian nerves and blood-vessels, which are con-
tained in the mesovarium. The significance of this process,
known from its principal feature as the decensus ovarioruin,
is unknown, 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
mammals, 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 Miillerian duct, the Wolffian duct
becoming vestigial, in the male it is the Wolffian duct that is
emphasized, together with the upper portion of the meso-
nephros, while the Miillerian duct is reduced to a few rudi-
ments. [Plate III, &.] 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
mesonephrotic 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 Miillerian duct is
lost along the greater portion of its extent, but leaves rudi-
ments at either end. The anterior end is represented by the
appendix testis [hydatid of Morgagni], a knobbed body at-

THE URO-GENITAL SYSTEM 393

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 the
part in its primary morphological significance, while the second
and third state their ultimate fate in the male and female
Amniota respectively. Vestigeal 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 urogenital 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 uro-genital
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-
ders of Cetacea and Sirenia, in which the condition is plainly a

394

HISTORY OF THE HUMAN BODY

secondary modification due to the needs of 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

Epo'dphoron (in part)

Ductuli

Mesonephros (lower portion)

aberrantes
Paradidymis

Paro'dphoron

Mesonephrotic (Wolffian) duct.

Ductus de-
ferens

Epo'dphoron (in part)
Longitudinal duct of
epoophoron

Mailer's duct

Appendix
testis
Vesicula
prostatica

Oviduct
Uterus
Vagina

Urogenital sinus

Morphologi-
cal urethra,
/'. e. the por-
t i on be-

Urethra (entire)

tween the

bladder and

entrance of

the ductus

deferentia

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

THE URO-GENITAL SYSTEM 395

integument of either the inguinal or perinaeal regions. The
scrotum is originally double, furnishing a separate sac for each
testis, but usually the two are fused into single median sac in
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 postfcnial
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. in, 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
(chorda gubernaculi) extends from the bottom of this sac to
the base of the conus.

396 HISTORY OF THE HUMAN BODY

In a complete, or typical descensus, in which the bursa is
contained in a scrotal sac, the parts are related as in Fig. in,
B. The processus vaginalis of the peritoneum, continuous
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 ccelom, 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
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 cremast erica
[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 layers through the development of its
involuntary muscular fibers into a layer of integumental mus-
cles, the tunica dartos, which occasions a wrinkling of the
surface in response to slight stimuli.

The external reproductive organs have arisen as one of the
adaptations required by the assumption of a terrestrial exist-
ence, the ultimate cause being found in the non-suitability of
the air as a medium for the transmission of the spermatozoa.

THE URO-GENITAL SYSTEM

397

These delicate motile cells can exist only in a liquid medium,
and from this cause alone arise in all terrestrial animals the
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

FIG. in. Diagrams illustrating the descent of the testes in mammals.
[After WEBER.]

t, testis; p, epididymis; s, spermatic cord; m, mesorchium; It, ligamentum testis;
/*, ligamentum inguinale; i, 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 Cooped
( — obliq. ext. abdom.); f, integument, including the tunica dartos and involuntary
muscular layer.

is due to the semi-aquatic habits of most amphibians, even the
most terrestrial of which resort to the water at the breeding
season and are thus able to dispense with any external mech-
anism; yet here, notwithstanding the absence of external
organs, 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-

398 HISTORY OF THE HUMAN BODY

tion and thus form the prelude to the development of a gen-
uine copulation.

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
an intromittent organ or penis, three distinct types of which
are found; these appear to have arisen independently, al-
though in all cases by a modification of the cloacal wall. The
first is seen in those highly specialized burrowing amphibians,
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 convey-
ance 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 both birds and 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 characteris-
tics developed by the latter.

The phallus develops from the ventral wall of the cloaca
and consists of a longitudinal thickening of fibrous tissue, the
corpus fibrosum, upon which rests a mass of cavernous (erec-

THE URO-GENITAL SYSTEM 399

tile) tissue in the form of two lateral ridges, the corpora
cavernosa, with a median groove between them, the seminal
groove. This entire organ is somewhat tongue-shaped and
free at the tip, and is capable of considerably 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 of 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 during early development. The
failure to complete this produces the condition known as
hypospadias, and is thus seen to be a case of arrested develop-
ment, the retention of the reptilian stage.

The relative position of the penis changes completely during
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 monotremes, and to
a lesser extent in marsupials ; the latter position is characteris-
tic of placental mammals. This change may be made clear
by the accompanying diagrams (Fig. 112).

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 urogenital 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

400

HISTORY OF THE HUMAN BODY

into its own lumen, thus forming a direct outlet from the
ductus deferentes to the exterior. At the same time the free
end becomes protruded from the cloacal orifice, and the organs,

a

FIG. 112. 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 urethrze (^corpus spongiosum).

THE URO-GENITAL SYSTEM 401

usually wholly subservient to the urinary function, become
for the time being wholly reproductive. %

The marsupials show an intermediate condition by which
the transition to the placental mammals can be explained. The
cloaca has been divided by a perinaeum 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 proximal 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 ap-
parent 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. 113). The
glandules ductus deferentis are thickenings of the wall of the
ductus deferens, and are situated near its entrance into the
urogenital canal. The glandule? vesicates are large and evi-
dent 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 glan-
dular in their nature, and their cavities contain spermatozoa
only by accident. Of the remaining three, which open into
the urogenital canal, the primitive condition is seen in the
urethral glands, tubular glands occurring in the walls of
the above canal, especially along its proximal portion. From

402

HISTORY OF THE HUMAN BODY

such elementary structures are derived the two other sets, the
prostate and the 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. 113. 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

THE URO-GENITAL SYSTEM 403

to the above glands, the function of which is to furnish a
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. 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 (Fig.
114).

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 ventral wall, and 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 [h~\, 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 in the
fertilised egg previous to segmentation.

404

HISTORY OF THE HUMAN BODY

At about this point, however, sexual differences begin to
appear, as may be seen by a comparison of the remaining
figures. The female organs, which remain nearer the embry-
onal condition, are not essentially different, save in propor-
tions, from the last stage common to both. The genital cleft
remains open, forming the introitus vagince, into which empty
the united Miillerian ducts (uterus) and the two ureters. The
genital folds form the corpora cavernosa (labia minor a or

a

h-\—

FIG. 114. 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;
pt perinaeum.

nympha) and the free tubercle itself forms the clitoris. The
external lips of the cloaca, the lateral genital ridges, form the
greater labia (labia major a).

In the male the genital tubercle develops into the glans and
corpus cavernosum urethra [corpus spongiosum], and the
genital folds become the corpora cavernosa penis. By the
fusion of these latter the groove becomes converted into the
uro-genital canal, which becomes continuous with the urethra,

THE URO-GENITAL SYSTEM

405

and into which the ductus deferentes empty. The lateral gen-
ital ridges unite to form the scrota! sac, and the point of union
between these is marked by a raphc. 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-
tions in the two sexes may be expressed in the following table,
which shows the sexual homologies. This table, taken in
connection with the one given above, for the internal parts, will
form a brief synopsis of the entire subject.

EMBRYONAL PART

MALE

FEMALE

Genital ridges

Scrotum

Labia majora

Genital tubercle

Corpus c'avernosum
urethrae
Glans penis

Clitoris

Genital cleft

Pigmented line

Introitus vaginae

Genital folds

Corpora cavernosa
penis

Labia minora

CHAPTER X
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 de-
velopment 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 experi-
ment 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.

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
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 solid masses as the cere-
bellum or 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

406

THE NERVOUS SYSTEM

the surface of the body, especially upon that /portion whichjr
through the customary position of the body, isjfl^€xposed 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 i.y 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 medullary 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 not unlike that of an ordinary hair-pin. The area en-
closed by these, which consists of a strip along the dorsal
surface, becomes somewhat sunken, and as the two medullary
folds, beginning anteriorly, approach 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
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 and the reasonableness of the con-
clusions, it suggests that the primitive ancestor of vertebrates
was exposed to external stimuli mainly over its dorsal surface,
a supposition which in its 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, sug-
gests 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 pro-
tected in the most natural way for such a system, by the for-
mation of elevated ridges along its lateral borders, thus form-

408 HISTORY OF THE HUMAN BODY

ing a dell or trough, in the bottom of which lay the sensitive
surface. Such a method of protection, once inaugurated, could
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 at-
tained with which all the present-day vertebrates are equipped.
We must here not lose sight of the fact that the original ex-
ternal, and hence the sensitive, surface is not that of the ex-
terior, but that of the lumen of the tube, which explains the
fact, to be developed later, that in all lower vertebrates the
central or ganglion cells, which form the " gray matter," are
situated along the lumen, and not along the external surface,
a condition retained throughout in the more conservative
spinal cord, although secondarily in the higher forms large
masses of gray matter develop also over the external surface
of parts of the brain.

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
retain its functicn as a receiver of external stimuli, a function
upon which its very existence as a nervous system depends,
it must retain its connection with the exterior through sets of
secondary cells which remain external and are yet continuous
with the central organ.

These are the sensory cells, which, when grouped over a
certain area and specialized to receive a certain form of
stimulus, become definite sense-organs. These are connected
with the central system by sensory nerves, in which the direc-
tion of the impulse is always from without inward, that is,
afferent or centripetal. As the sensory cells become themselves
more specialized and hence more sensitive as well as more
vital to the organism, they themselves need protection, which
they obtain either by the formation of an external non-sensi-
tive horny layer, the epidermis, which protects the sensory
cells scattered over the general surface while it still allows the
transmission of the stimuli ; or, in the case of such special
sense organs as the patches of sensory cells that form the es-

THE NERVOUS SYSTEM 409

sential organs of vision and hearing- (retina and acoustic mac-
ula) elaborate series of protective organs become developed,
while at the same time the special stimuli are intensified by
various accessory organs.

Thus, while this secondary system of sensory cells, like
the rank and file of a modern army, meets the external world
with its hazards, the central nervous system, and more espe-
cially 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 intelligence, 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 them through the formation of association paths;
and this, like all other structural advances, has been gradually
brought about through the wrorking of natural law, as a more
perfect adaptation to environment.

4io

HISTORY OF THE HUMAN BODY

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-
bryological record of a single animal, the lower forms preserv-
ing in greater detail the early stages of the history, the higher

FIG. 115. 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.

forms recording the later stages. Completed in this way
from the numberless fragmentary records presented to the
investigator, the history of the neural tube in its progressive
modifications is as follows :

It begins, so far as records go, with a form in which the an-
terior part of the tube, that corresponding to the head of the

THE NERVOUS SYSTEM 411

animal, is somewhat enlarged and divided by two transverse
constrictions into three successive vesicles (Fig. 115, A), the
fore-, middle-, and hind-brain, or, more technically, prosen-
cephalon, mesencephalon and metencephalon, each with its
cavity or primary ventricle. Of these the first two seem to
represent the very rudimentary cerebral vesicle found in Am-
phioxus and may be termed the archencephalon, or primordial
brain, while the third may be considered the anterior end of
the spinal cord, which becomes added to the brain at some
point between Amphioxus and the cyclostomes. This cerebral
vesicle of Amphioxus bears two rudimentary sense organs, an
olfactory groove and a pigment spot; it may be more than a
coincidence, then, that in the higher forms the first two original
vesicles furnish but two pairs of nerves, olfactory and optic,
while the other nerves are derived from the primary third
vesicle.

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.
115, B). The prosencephalon becomes modified by the forma-
tion of two diverticula, which are thrown out from the sides
and grow anteriorly, often reaching a point considerably be-
yond the anterior limits of the primary tube. These are the
two lobes of the telencephalon (the cerebral hemispheres of
the higher vertebrates), in distinction from which the un-
paired remainder is designated as the diencephalon.

Internally the primary first ventricle becomes divided into
the two lateral ventricles and the one naturally denominated
the third; the latter communicates with the two first through
a passage which is inclined to become narrow, the foramen in-
terventriculare [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 ventricle is often large and obvious, but has re-
ceived no special name or number.

The third primary vesicle, the metencephalon, shows a

412 HISTORY OF THE HUMAN BODY

greater or less differentiation of its anterior portion, which
forms the metencephalon in a restricted sense (the cerebellum
of higher forms), while the posterior portion, which tapers in-
definitely into the spinal cord, is distinguished as the myelen-
cephalon or medulla.* The ventricle of the third primary vesi-
cle, or more especially that of the myelencephalon, is a large
and conspicuous cavity in lower vertebrates and in the embryo
of the higher ones, and is known as the fourth ventricle. That
part of the lumen which lies between this and the third ven-
tricle, including the ventricle 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 ventriculurn."

The original three primary cerebral vesicles, by a secondary
subdivision of the first and third, thus become increased to five,
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 :

i. Increase in the thickness of the wall over a definite area.

*The ease with which the German anatomists have translated into the
vernacular the somewhat ponderous Greek terms for the parts of the
brain (Vorderhirn, Zwischenhirn, Mittelhirn, etc.) has led to various
attempts on the part of English-speaking scholars to emulate their ex-
ample, 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 forms " after-brain " for myelen-
cephalon (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 are most conveniently named in accordance with the vesicles of which
they form the cavities, thus: telocozles, diaccele, mesoccele, metaccele and
myeloccele.

THE NERVOUS SYSTEM 413

This is seen almost everywhere, but the extent of the develop-
ment in thickness varies much. It is well shown by the thick-
ening of the floor of the telencephalic lobes, forming the
corpora striata, or by that of the roof and sides of the same
parts in the higher vertebrates, forming the cerebral hemi-
spheres.

2. 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 a transparent, membrane. Such
places are extremely puzzling, and misled anatomists until
within a generation. The physiological purpose of such a thin
place is to allow the blood to communicate with the lymph of
the ventricles and to nourish the inner surfaces without violat-
ing the integrity of the original neural tube. A plexus of
blood-vessels is thus the constant accompaniment of such a
thin place, and the relations usually become still more compli-
cated by the sinking into the cavity of the entire structure,
although each loop of capillaries is covered and veiled by the
membranous wall, and thus the integrity of the tube is never
violated.* In extreme cases almost the entire thin area, cov-
ering a network of capillary loops and following its intricacies,
may come to lie within the cavity of a ventricle and form a so-
called chorioid plexus.

The most important of these organs are (i) those of the
lateral ventricles, formed by the imagination of a thin area in
the inner wall of each, (2) a similar one in the third ventricle,
invaginated from its roof, and (3). one formed from the roof
of the fourth ventricle immediately behind the cerebellum.

* As the only exception to this rule there have been described in Man
and in certain other mammals one or more small perforations in the roof
of ihe fourth ventricle, the foramina of Majendie, which form a direct
communication between the lumen of the neural tube and the sub-
arachnoid space. The existence of this communication, which violates
the morphological principle of the complete integrity of the walls of the
neural tube, has given rise to much discussion, but it seems now probable
that, while these foramina certainly do occur occasionally, it is an individual
peculiarity, like the epitrochlear foramen of the humerus, and of no
especial significance.

414 HISTORY OF THE HUMAN BODY

In other cases localized thin areas push out instead of in and
form evaginations of more or less importance in the formation
of various organs, which are supplementary to the actual brain.
In this way are formed the retina of the eyes, a portion of the
hypophysis, and one or two problematic structures arising dor-
sally from the diencephalon.

3. Folding or creasing of a certain part of the wall. This
mechanical device means here as elsewhere an increase of sur-
face, and hence of physiological efficiency, without a corre-
sponding increase in bulk. Its best manifestation is, perhaps,
that of the cerebellum, morphologically formed from roof and
sides of the metencephalon. In some forms, as in the adult
dog-fish, these folds, three or four in number, are seen with
the clearness of a diagram; in others, as in adult birds and
mammals, the original creases become shallow by a coalescence
of the applied surfaces of adjacent folds, but the structure is
still marked by the characteristic dendritic arrangement of the
white matter, which marks the core of each fold. In some
cases a secondary or even tertiary folding is thus marked.

4. Flexure, or the bending upon itself of the entire longi-
tudinal axis of the neural tube (Fig. 116). The possible
flexures are three in number, apical flexure, flexure of the pons,
and cervical, the two first appearing in birds and quadrupedal
mammals, the last found only in Man, caused directly by the
erect position and the consequent bending of the skull over an
angle of nearly 90°. The reason for the other flexures is un-
doubtedly the same as in the case of the foldings of the surface,
since by folding the parts on themselves a larger brain may be
accommodated within the length limits of a given skull. The
gradual formation of these flexures may be well seen during
the embryonic development of a bird or mammal, preferably,
however, in Man, in which alone the third or cervical flexure
is involved. They occur in the order of their position, begin-
ning anteriorly, the first being dorsal, the second ventral, and
the third dorsal again, in accordance with the natural law of
folded objects.

With the above principal in mind, the further history

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, myelencephalon and cord.

r~i£>

-\- "', /^N

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; blutt diencephalon ; red, mesenceph-
alon; green, metencephalon; brmvn, myelencephalon and cord.

THE NERVOUS SYSTEM

415

of the development of the five areas 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. The comprehension

FIG. 116. Diagram of the cerebral flexures.

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.

of these will be facilitated by comparing them with Fig. 117,
which shows the dorsal aspect of the same series.

The telencephalon in the dog-fish is inconspicuous in size but
of a considerable thickness, which is approximately uniform
save at the posterior part of the roof, where a considerable
area retains its membranous character and invaginates to form

416 HISTORY OF THE HUMAN BODY

three chorioid plexuses, one for each telencephalic lobe and one
for the diencephalon (third ventricle). The thickening in the
floor forms the area to be known later as the corpus striatum;
that of the roof and sides is the potential cerebrum. The two
telencephala thus represent the cerebral hemispheres together
with the corpora striata ; their cavities are the lateral ventricles
in which lie the two plexus chorioides, the entrance of which
into the ventricles is effected through the interventricular
foramina. The anterior portion of each telencephalon forms
an extensive olfactory lobe (rhinencephalon), which is here
voluminous and stalked. This latter portion is in reality noth-
ing less than the " olfactory nerve," which, when stalked as
here, and especially when prolonged, as in some lizards and in
birds, gives the appearance of a true cranial nerve. It is here
seen not to be a genuine cranial nerve, but an element of the
brain.

The teleosts and ganoids show a unique development of this
part; the entire roof and sides remain membranous, but the
corpora striata are enormously developed. Since the mem-
branous portion, which is here called the pallium, or mantle,
is absolutely transparent and extremely delicate, it is usually
lost in dissection, or if retained, seems of no importance ; and
as the corpora striata are very large and convex, they seem to
the casual observer to be the true cerebral hemispheres. The
rhinencephala are well developed and appear as the direct con-
tinuation of these latter parts.

The telencephalon of amphibians and reptiles is not unlike
that of the selachians (dog-fish), of which it seems a direct
descendant. The rhinencephalon is proportionately smaller,
although in many lizards it becomes greatly extended, in adap-
tation to the prolonged snout.

In birds there is again, as in teleosts, an enormous develop-
ment of the corpora striata, which makes up the bulk of the
cerebrum, although the roof and sides have some thickness and
are not reduced to the condition of a pallium. In the mammals
the telencephalon reaches its highest development, when it usu-
ally greatly exceeds in bulk the remainder of the brain. This

THE NERVOUS SYSTEM 417

excessive development is mainly that of the roof and outer side
of each of the telencephalic lobes, which form enormous hemi-
spheres that extend forward over the rhinencephalon and
backward over di- and mesencephalon, usually coming- almost
in contact with the cerebellum (metencephalon), from which
they are separated merely by a membranous or bony par-
tition, the tentorium. In addition to increase in bulk there
is also an important histological change, namely, the appear-
ance of large masses of ganglion cells over the outer surface,
arranged in definite layers and constituting the most important
nervous element, the seat of the highest faculties. This
ganglionic tissue forms a definite layer of gray matter of con-
siderable thickness, the cortex cerebri. 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 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.t lateral cerebral fis-
sure [fissure of Sylvius] ) ; and the others, sulci (e. g., sulcus
centralis [fissure of Rolando]).

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.

While the main mass of the hemispheres is derived from the
roof and the outer side of the telencephalic lobes, the inner side,
remaining thin at first, makes a contribution in the form of a
longitudinal imagination which thickens, and forms a ridge
that encroaches upon the lateral ventricle. This is the hippo-
campus [hippocampus major or Amman's horn}. It attains a
considerable development in Man, where it forms a conspicu-
ous elevation upon the inner side of the floor of the ventricle
and becomes prolonged posteriorly into a free rounded end,
terminating in digitations [pes hippocampi]. This intrudes
itself upon the thick outer portion and lies imbedded in it,
covered by the temporal lobe.

4i8 HISTORY OF THE HUMAN BODY

Still further down, ventral to the hippocampus, and partly
enclosed by the surrounding parts, the same inner walls of the
two hemispheres come in contact and form a thin double par-
tition known as the septum pellucidum. The two walls en-
close a small space to which the name "fifth ventricle" was
formerly given. It is unnecessary to state that this is not a
true ventricle and has no connection with the lumen of the
neural tube.

The telencephalon of all higher mammals is further distin-
guished by the formation of an extensive bridge or commissure
across the middle line between the two lobes. This lies dorsal,
and is easily seen by drawing the hemispheres a little apart
and looking down from above. It is called the corpus callo-
sum, and consists of fibers of white matter that form a me-
dium of intercommunication between corresponding parts of
the two hemispheres and insure harmony of action. Aside
from this extensive commissural system, which has evidently
arisen in mammals in connection with the added needs coming
from larger hemispheres, there are three smaller transverse
bundles, common also to the brain of lower forms, the ante-
rior, middle and posterior commissures. Of. these the first
alone comes within the province of the telencephalon, the others
are respectively di- and mesencephalic.

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 characterised by the formation of second-
ary organs, either as in- or out-pushings. Some of these latter
become of fundamental importance while others appear to be
more or less vestigial, presumably inherited from preverte-
brate ancestors and of problematic significance.

Several of these formations occur along the dorsal aspect^
where over a considerable area of debatable territory between
tel- and di-encephala the roof remains thin. The most an-
terior consists of an extensive invagination into the third
ventricle, which lies just beneath this region. This invagina-

THE NERVOUS SYSTEM

419

tion is accompanied by blood-vessels, and by division forms
three chorioid plexuses, a median one for the third ventricle
(the tela chorioidea of human anatomy) and the two lateral

d e f

FIG. 117. 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.

420 HISTORY OF THE HUMAN BODY

ones already mentioned (tcenice chorioides), which pass
through the interventricular foramina and supply the two
lateral ventricles of the telencephalon.

Behind the plexuses there appear in the mid-dorsal line typi-
cally two median diverticula, which, owing to the many grades
of development under which they appear, as well as to the fact
that they have long been treated as identical, have received a
large number of distinct designations. The more anterior of
these is best known as the paraphysis, the posterior one the
epiphysis, but the former is also correctly known as the parietal
organ, the latter as the pineal organ. 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
sense organ of uncertain nature, but probably an eye in each
case.

In the cyclostome Petromyzon, both structures attain consid-
erable development, and the optical structure of the epiphysial
organ is evident through the occurrence of pigment in what
may be well a vestigial retina. The paraphysial organ is
smaller, but of similar structure. In no other form are both
of these structures so well developed, but in several cases one
may attain an even higher development while the other is rudi-
mentary. In some instances the highest point in development
is reached during embryonic life, while in others it is exhibited
by the adult. Thus in the selachians, the epiphysis passes
through a minute foramen in the skull and reaches the surface ;
its terminal organ is visible externally, but the paraphysis is
not developed at all. In frogs and toads the paraphysis attains
a development similar to that of the epiphysis in the former
case, while this latter part has not been found. The para-
physial organ, here known as the " frontal organ," is plainly
visible externally, but in the adult is entirely separated from
the brain by the retrogression of its stalk. The highest de-
velopment of either organ is reached among certain lizards,
where it is the epiphysis that is thus favored. The terminal
organ here lies in a socket (parietal foramen) formed in the
interparietal suture and represents a fairly good eye, with pig-

THE NERVOUS SYSTEM

421

mented retina, a more or less makeshift lens, and a well-devel-
oped nerve connecting the terminal organ with the brain.
Above this, on the surface, is situated a transparent scale,
surrounded by a ring of smaller opaque ones, making a con-

FIG. 118. Lateral views of the developing human brain; drawn from
wax models by F. ZIEGLER after WM. His.

The cranial nerves are indicated by roman numerals; exponent letters m and
s denotes respectively motor and sensory branches; e, the otic vesicle

spicuous object on the heads of these forms. The paraphysis
appears to be associated with this epiphysial structure. In
birds and mammals there seems to be no trace of a paraphysis,
while the epiphysis is reduced to the form of the so-called
"pineal gland," pushed backwards from its original position

422 HISTORY OF THE HUMAN BODY

by the growth of other parts. In man it lies so hidden that the
early anatomists, finding- it as it were in the innermost pene-
tralia of the organ of life and individuality, deemed it the seat
of the soul, a view from which the morphologists of the pres-
ent day have escaped only by substituting one mystery for
another.

The sporadic occurrence of these vestigial sense organs,
paraphysis and epiphysis, which, save perhaps in the case of
the parietal (epiphysial) 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 histo-
ries, of which the record consists of a few poorly preserved
fragments.

Far more satisfactory is the history of the diverticula which
develop laterally from the sides of the part under consideration,
for, although we do not have adult animals which show the
steps in the development, they are yet traced in perfect agree-
ment during the embryological history of every vertebrate, a
procedure familiar to all students of embryology. These appear
at an extremely early age, often beginning before the com-
pletion of the telencephalic lobes, and soon assume the form of
spherical vesicles, connected with the brain by narrow stalks,
and almost in contact at their outer surface with the external
germ-layer, the surface ectoderm. By an invagination of this
outer surface the vesicle is transformed into a double-layered
cup, and in this one may recognize the fundamental elements of
the eye. The primary vesicle is hence called the optic vesicle,
the transformed cup-like figure, the optic cup. [See Fig. 136.]

Of this the invaginated layer, now lining the cup, becomes
the retina, certain cells of which give rise to the rods and cones,
the essential nervous elements of the organ ; the other layer,
now forming the covering of the cup, develops pigment and

THE NERVOUS SYSTEM 423

becomes the tapetiun nigrum, a layer which, together with the
blood capillaries later to be associated with it, will become the
chorioid coat. The stalk, although not directly transformed
into the optic nerve, forms the path along which it develops
and thus marks its final position. (For the details of this
cf. the last part of Chapter XL)

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. That this may once have been the essential
sense organ to supply the needs of which the diverticulum from
the brain may have originated, seems likely from the similarity
of its early development to that of certain actual sense organs,
especially the otic capsule, which develops into the inner ear.
This latter, as will be shown later, appears to have been at
first merely a single unit of the system known as the " lateral
line organs," and the lens, although no longer sensory in func-
tion, may with some probability be referred to the same source.
In all present-day vertebrates, however, it is no longer sensory,
but develops into an auxiliary though essential organ of the
eye, the crystalline lens. This is accomplished by an enormous
thickening of the inner wrall of the vesicle, which finally fills up
the entire lumen, leaving the outer wall to fit over it in the
form of a protecting epithelium. During later development
the eye receives its vitreous humor, its blood-vessels, its sclero-
tic coat and other essential parts from the surrounding tissue,
mainly the mesenchyme, and develops into the adult form.

But one other diverticulum arises from the diencephalon,
and that one is directed downwards from the middle of its
floor. Like the lateral eyes, 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 slight functional importance, in respect to which the

424 HISTORY OF THE HUMAN BODY

elaborate method of development, involving as it does two dis-
tinct elements, is disproportional. It is thus generally sup-
posed that we have a vestigial organ like those developing dor-
sally and laterally 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, es-
pecially 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 relations are apt not to be wholly
understood.

The portion contributed by the diencephalon is in the form
of a hollow cone or funnel, the infundibulum. About this the
invagination 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, through the development of the
palate, the hypophysis is made to appear like a simple 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 palceostoma, 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*

Aside from these diverticula and the organs found in asso-

*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 farther details and
theories concerning this part see Chapters VI, XI and XII; also Fig. 129.

THE NERVOUS SYSTEM 425

ciation with them the diencephalon develops in mammals a
pair of lateral ganglionic masses, the thalami optici, which
arise as thickenings of the sides of the vesicle beneath the optic
stalks. These are to be sharply distinguished from the lobi
optici (optic lobes), under which name the lateral halves of
the mesencephalon are usually described.

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, although
suffering little or no actual diminution in size, it becomes re-
duced proportionately to a very small portion of the brain
through the excessive growth of the surrounding parts. This
is made clear by the diagrams, in which the mesencephalon may
be followed through fishes, amphibians and reptiles with but
little change. Its roof and outer sides are moderately thick-
ened and usually divided along the mid-dorsal line by a longi-
tudinal groove, thus forming a bilobate organ, the corpora
bigemina or lobi optici. In many fishes they form a conspicu-
ous part of the brain which, so long as the cerebral lobes remain
but slightly developed, must be of great functional importance.
In some teleosts, for example, in which the cerebral hemi-
spheres are represented merely by a non-nervous membrane,
they furnish at least two-thirds of the dorsal surface, and thus
perhaps functionally replace the former. In amphibians and
reptiles the gradual development of the cerebral hemispheres
reduces the 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 as 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, telencephalon and
metencephalon (cerebrum and cerebellum), and here the
bilobed organ has become transformed into one with four
lobes, the corpora quadrigemina. The beginning of this

426 HISTORY OF THE HUMAN BODY

change may be found in some reptiles, where the develop-
ment 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.

The floor of the mesencephalon is thickened in all cases and
is of considerable functional importance. Through this region
pass the fibers of connection between the cerebral lobes and the
medulla, and as the hemispheres increase in size, these bundles
become greater and form the pedunculi cerebri [crura
cerebri}, especially conspicuous in mammals, as would be
expected.

During the process of phylogenetic development the roof
and sides of the metencephalon become selected as a region
where a large part of the work of the central nervous system
is accomplished. This part, the cerebellum, is thus almost al-
ways large and voluminous, and often, even in fishes, becomes
folded up into several plicae, thus emphasizing its great func-
tional importance.

It has already been shown how both the corpora striata and
the lobi optici, although of supreme importance in some fishes,
eventually become, except perhaps in birds, entirely subordi-
nated to the cerebral hemispheres ; the cerebellum, on the other
hand, has retained from the first an office of great importance,
and in mammals becomes subordinated to the hemispheres
alone. There are occasional exceptions to the general impor-
tance of this part, as in the case of the singularly small cere-
bellum of the frog, but such cases are very few. The floor of
the metacephalon is utilized in part for the location of com-
missural fibers between the two lateral halves of the cerebellum,
and in mammals, corresponding to the increase in size of this
organ, this commissural bundle becomes large and conspicu-
ous, forming a broad loop around the base of the medulla, the
pons [ Varoli\\ .

Although this region of the myelencephalon is perhaps the
most complex of any part of the brain, this complexity lies in

THE NERVOUS SYSTEM 427

the minute structure rather than in the external form, in which
latter respect it is singularly simple and uniform throughout all
vertebrates. Its sides and floor, which alone come into consid-
eration as a nervous organ, together with the crura cerebri and
the pons, form the central system of commissures for the entire
nervous system, receiving the fibers from all other parts and
forming the necessary connections of these with one another
and with the spinal cord. That these connections become
vastly more complex in the higher than in the lower vertebrates
is evidenced both by the gradual growth of the various parts
of the brain in size and complexity, and by the results as seen
in the behavior of living animals. A rhomboidal area which
includes the greater part of the roof of this part remains mem-
branous and forms an important chorioid plexus, that of the
fourth ventricle (tcenia ventriculi quarti). As this thin place
and its subjacent rhomboid cavity (fossa rhomb oidalis) are
extremely conspicuous objects in all embryos, this portion
of the brain is often conveniently termed the rhomben-
cephalon.

Morphologically the medulla is the anterior continuation of
the spinal cord, and the nerves that proceed from it resemble
the spinal nerves more than do those which arise farther for-
ward. In fact the line of division between medulla and cord
is an artificial one, the first being considered as coterminous
with the skull in all cases. Similarly those nerves in that
region which obtain their exit through a foramen in the skull
are termed cranial and are accorded to the medulla. The ar-
tificial character of this distinction involves confusion at one
point at least, namely, the varying limits of the skull between
amphibians and reptiles due to the absorption of a vertebra.
(See Chap. V.) In this way the hypoglossal nerve (Xllth),
a spinal nerve in amphibians, becomes added to the list of
cranial nerves in the Amniota, although this case involves
rather more than the simple addition of a single pair of spinal
nerves, and is still a somewhat obscure point.

Beyond the medulla the neural tube becomes the spinal cordf
which, although it often shows some little regional differentia-
tion, is far more conservative than the anterior portion and

428 HISTORY OF THE HUMAN BODY

consists essentially in all cases of a tube with a minute lumen
(canalis centralis) and extremely thick walls. It consists of
both ganglion cells (gray matter) and connecting fibers (white
matter) and, as the latter usually form the greater part of its
bulk, it is to be considered in the main a great central nerve
bundle proceeding from the brain and distributing its fibers to
all parts. This distribution takes place through the formation
of pairs of spinal nerves, which are arranged metamerically, a
pair for each body somite.

The proportion of the spinal cord in weight as compared
with that of the brain 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 principle, that of pro-
gressive cephalization, which tends to shorten the cord and con-
centrate the nervous system 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 results of this process may be especially well
followed among such a group of animals as that of insects, in
which the central nervous system originally consists of a pair
of small ganglia for each somite, this condition running
through the entire body.

Thus in the myriapod (Fig. 119, 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. 119, cf. B and C).

This principle is shown in vertebrates by the progres-
sive shortening of the spinal cord in a series of gradually ceph-
alizing forms, but can be used as a criterion of development
only within the limits of a single group. Thus the frog, with

THE NERVOUS SYSTEM

429

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-bodied salamanders,

B

FIG. 119. 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.

430

HISTORY OF THE HUMAN BODY

animals in its own class, in which the cord is nearly coterminous
with the tail (Fig. 120). Indeed, an almost absurd result of
this is shown in a certain teleost, Orthagoriscus (Fig. 121, b),

a

Brach

H

ll/hy

Cm.

Sciat

FIG. 120. 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; Brach. brachial; Thor. abd, thoraco-abdominal ; Sp. cor,

supracoracoid ; //. hy, ilio-hypogastric ; Cru. crural; Sciat, sciatic. The vertebrae

are numbered by arabic numerals, the spinal nerves by roman.

THE NERVOUS SYSTEM

in which the entire cord is perhaps a little shorter than the
brain. In all cases in which a shortening has occurred two con-
nected phenomena may be
observed at the posterior end
of the cord: first, the thick,
functional portion terminates
more or less abruptly and the
cord is continued as a taper-
ing thread known as the
filum terminate, without
function as a nervous organ,
and secondly, the shortening
is usually so great that the
posterior portion of the cord
is drawn up considerably
ahead of the parts which it
supplies, compelling the
nerves involved to turn
around at a progressively
sharper angle until the most
posterior ones run in a
longitudinal direction paral-
lel to the filum terminale.
This bundle of approximate-
ly longitudinal nerves which
appears thus to terminate
the cord, is known collective-
ly as the cauda equina, and is
often a noticeable object, as
in the frog and in Man
(Fig. 121, 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 inter-

FIG. 121. 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.]

432 HISTORY OF THE HUMAN BODY

esting way with changes in the musculature of that part. In
such forms as fishes and salamanders, the metameric mus-
culature is not discontinued at the cloaca, which marks the
posterior limit of the body cavity, but is continued in a grad-
ually 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 except 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 anterior 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 in the case of the limbs. In such a form as
Amphioxus, where the successive metameres are practically
alike, the spinal nerves, and consequently the cord, are 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 differenti-
ation, 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 neces-
sarily increased in caliber, and this causes a corresponding in-
crease in the cord at or near the point from which they
originate.

The most conspicuous example of this principle is, of course,
that of the limbs, which are often excessively developed in
the higher forms and cause a corresponding increase of size in

THE NERVOUS SYSTEM 433

the metameric nerves from which they receive their supply, as
well as in the cord of the region from which these nerves pro-
ceed. It will be remembered that each vertebrate limb above
those of the fishes is a development of a few (not more than
5-6) metameres, and thus involves primarily a corresponding
number of nerves. There are thus in the spinal cord two
swellings (inhtmescentitp) , cervical and lumbar, corresponding
respectively to the anterior and posterior limbs. These swel-
lings are directly proportionate to the amount of development
in each pair and are markedly unequal in such forms as bats,
with their exaggerated fore limbs and reduced hinder pair,
and in the ostrich, in which the development shows the re-
verse tendency.

In snakes, in which the limbs have been lost, the intu-
mescentise are also 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 intumescentiae, making the latter,
which are well developed, seem still greater by contrast (Fig.
121, a).

The most exaggerated development of these spinal intu-
mescentise 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
Stegosavrus.

In shape, as seen best by cross-sections, the spinal cord varies
somewhat in the different regions of the body, and consider-
ably more in the various vertebrates, especially the lower as
compared with the higher. In the cyclostomes it is strongly
flattened, convex dorsally and concave ventrally. In am-
phibians it is elliptical, flattened from above downwards, and
with a noticeable, though not very deep, ventral furrow. In
mammals, by the addition of a dorsal and two lateral furrows,

434 HISTORY OF THE HUMAN BODY

all longitudinal and parallel, the well-known form of a fluted
column is produced, with a dorsal, lateral and ventral column
upon each side [posterior, lateral, anterior, BNA], The
shape of the mass of gray matter as seen in section varies from
that of a symmetrical triangle in lower forms to that of a figure
like a double crescent in the higher; in all cases it retains
the primitive position, bordering the lumen of the tube, the
original external surface.

All parts of the body are in constant communication with the
central nervous system through the medium of the peripheral
nerves, which are in structure essentially the same as the white
matter of the brain and cord, as seen in the various commis-
sures of the former and in the columns of the latter, save that
here there is added a connective tissue element, which not only
forms an external sheath for each entire nerve (perineurium),
but also a delicate wrapping about each nerve fiber (neuri-
lemma). These nerves issue in pairs from both brain and
cord, and, although in form and character the transition from
one group of nerves to the other is a gradual one, the two
groups are distinguished for convenience as cranial and spinal.

The latter, which are the less modified, issue from the cord
at approximately equal intervals and are metameric in arrange-
ment, a pair corresponding to each metamere or body somite,
as expressed in the muscles or the skeletal parts. This meta-
meric arrangement, which is often expressed with great clear-
ness in the trunk, is not distinct in the head, and the cranial
nerves, although showing indications of a former metameric
order, cannot be satisfactorily resolved into their separate ele-
ments.

According to their use nerves are sharply divided 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 external 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 respond to simple contact.
Other sensory nerves supply certain internal parts, as the

THE NERVOUS SYSTEM 435

muscles, giving these parts some degree of sensation. In all
sensory nerves the impulse necessarily travels from the termi-
nus to the central organ, and these nerves are consequently
designated 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 current runs from
the center to the terminus, and they are thus centrifugal, or
efferent. The nerves that regulate the action of other organs,

Dorsal

VENTRAL

Ventral

FIG. 122. Diagram of a typical spinal nerve.

such as the secretion of glands, are a subdivision of the motor
class.

Nerves issue from the central organ, whether brain or cord,
in bundles called roots, each of which contains mainly one
type of nerve fiber. The roots are hence called either motor
or sensory, but since a given part must usually be supplied with
both motion and sensation, two roots, one of each sort, become
associated together, and blend their fibers within a single ex-
ternal sheath, thus forming a mixed nerve. This is true of all
of the spinal nerves and of some of the cranial pairs, but
others of this latter class arise from single roots and retain
their simple character. Each metamere possesses typically a
pair of sensory and a pair of motor roots, the sensory situated
dorsally, the motor ventrally, thus forming two longitudinal

436 HISTORY OF THE HUMAN BODY

rows on each side.* The two roots of the same side unite
soon after their exit from the cord into a single metameric
nerve, containing both sorts of fibers, but then divides again
almost immediately into dorsal and ventral branches, each con-
taining both sorts of fibers. These, like all subsequent di-
visions, are merely topographical, and not physiological, as in
the case of the roots. A spinal ganglion appears in association
with each pair of roots, usually associated with the sensory
root, but in lower forms often connected with both and situ-
ated at the point of union.

This typical arrangement 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 obtained 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-
meres, 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 same
transverse plane as a motor root of the other. The sensory
and motor roots do not unite, and the former becomes as-
sociated with a subcutaneous 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
* The sensory roots often contain motor fibers.

THE NERVOUS SYSTEM 437

towards an intervertebral exit for 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 separate
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 dorsal vertebrae in
pigs, or the dorsal 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, in the higher forms, are exclusively
associated with the sensory roots, are often in the lower con-
nected with both. They may possibly be homologous with the
subcutaneous ganglia found on the sensory nerves of Amphi-
v.vus, but their development from a ridge along the spinal cord
does not seem to support this idea.

In studying the distribution of the peripheral nervous system
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 distributed ',
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 organ
as there are such units in the organ itself. If, then, a part re-
duces or increases its total number of cells, the change is di-
rectly 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 sometimes sufficient to
become easily noticeable, as in the case of the intumescentias of
the spinal cord, which are correlated with the development of
the limbs.

438 HISTORY OF THE HUMAN BODY

The second principle, that of the permanence of nerve dis-
tribution, has already been referred to in several places, since
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,
plexus brachialis and plexus lumbo-sacralis, involving the
nerves which supply the anterior and posterior limbs respec-
tively. 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 num-
ber varies between two and seven. Of this series one or two,
usually the central ones, 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

THE NERVOUS SYSTEM 439

their extremes in the center of the plexus in connection with
these larger nerves, above and below which the nerves become
gradually 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
debatable 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. 123).
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 num-
ber 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 forma-
tion ; yet the metameres thus represented cannot be taken, meta-
mere 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 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
alzvays formed entirely of the ventral divisions of the spinal

440

HISTORY OF THE HUMAN BODY

nerves, the dorsal branches being in all respects similar to those
of adjacent nerves; secondly, the final outcome of the branch-
ing results in the formation of distinct dorsal and ventral

xvn

XVIII

jxix

XX

FIG. 123. Brachial plexus of various vertebrates.

(a) Frog. [After GAUPP.] (b) Axolotl (urodele). [After FURBRINGER.] (c)
Cassowary. [After FURBRINGER.] (d) Domestic fowl. [FURBRINGER.] (e) Dog.
[After ELLENBERGER and BA'UM.] (f) Man. [GEGENBAUR.]

The roman numerals indicate the spinal nerves. The other abbreviations are
sufficiently complete to designate the nerves coming from the plexuses.

THE NERVOUS SYSTEM 441

branches, each one or two in number, the former distributed
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 distinct. 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 signifi-
cance; for its intercommunications and its branchings are,
in part at least, records of the past history of the limbs, rec-
ords 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 meas-
ure 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
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
preservation 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

442 HISTORY OF THE HUMAN BODY

Gymnophiona 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 reduc-
tion 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 appear-
ance.

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 of a given species, the direc-
tion in which the girdle is migrating has been definitely estab-
lished. 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. 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 the bone:

THE NERVOUS SYSTEM 443

NAME FUNCTION

I. Olfactorius special sense, smell.

II. Opticus special .sense, sight.

III. Motor oculi motor.

IV. Trochlcaris [Patheticus] motor.

V. Trigetmnus [Trifacial] { mainly sensory, with a small motor

( root

VI. Abducens motor.

VII. Facialis mixed.

VIII. Acusticus [Auditprius] special sense, hearing.

IX. Glosso-pharyngeus mixed.

X. Vagus [Pheumogastricus] mixed.

XI. Accessorius [Willisii] mixed, mainly motor.

XII. Hypoglossus mixed.

Of these the first two arise from the primary fore-brain,
the tel- and di-encephalon respectively ; the remaining ten take
their origin from the met- and myelencephala, leaving the
mesencephalon without any. 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 second-
ary addition 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 spinal elements, each with its sensory
and motor roots, 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 gan-
glion ; 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 sev-
eral associated ganglia. The twelfth nerve is outside of the
cranium in fishes, and becomes later included within it, proba-
bly 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

444 HISTORY OF THE HUMAN BODY

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
disappeared, and are never seen again as distinct nerves. Al-
though nothing has as yet been definitely proven in the matter,
it is probable from other evidence that above the fish 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.

For purposes of description and with reference to their
morphology the cranial nerves fall naturally into groups which
are best considered separately. These may now be taken up
in detail.

i. THE ANTERIOR GROUP. (Qlfactorius and Options.)
These, the two first in the list, are nerves of special sense,
the fibers of which are distributed respectively to the nasal-

THE NERVOUS SYSTEM 445

mucous membrane and to the retina. The morphological
position is doubtful, for, while they are considered by some
to be the first true cranial nerves and to belong to a much
earlier period than any of the rest, others deny them the right
to be called nerves at all, and treat them as parts of the brain,
the olfactory lobes (rhinencephalon) and the optic stalks re-
spectively.

Attempts have been made to bring this condition into ac-
cord with that found in Amphioxus, for here the archen-
cephalon, a rudimentary brain formed by the enlargement of
the anterior end of the spinal cord and possibly the equiva-
lent of the telencephalon of vertebrates, bears two rudimentary
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 identity of these
sense-organ rudiments and the perfected organs of the higher
vertebrates ; for if the olfactory groove and the pigment speck
are historically the anlagen of the nose and eye, a point not
definitely established, then the identity of the nerves with
the corresponding parts of the archencephalon naturally fol-
lows. In favor of this latter assumption is the fact of the
origin of these tzvo nerves from the primary fore-brain, while
none of the others arise anterior to the metencephalon. An
entirely problematical element belonging to this region is that
of a definite pair of nerves, Nervus terminalis, which occur in
all selachians, and extend from the anterior part of the telence-
phalic lobes, where they originate, along the anterior aspect
of the olfactory stalks. As they have been but recently dis-
covered they have escaped enumeration with the classical
twelve pairs; their origin from telencephalon is also anoma-
lous. Nothing can as yet be predicted of their morphological
significance.

II. THE MOTOR NERVES OF THE EYEBALL. (Motor OCllH,

Trochlearis and Abducens.)

446

HISTORY OF THE HUMAN BODY

These three pairs are small and very special nerves, having
no other distribution than the six muscles of the eyeball.
They are thus exclusively motor, and on the theory that the
cranial nerves, excepting, perhaps, those of the preceding
group, represent modified spinal nerves, seem to correspond
to the ventral roots of three original nerves, the sensory roots
of which are either lost, or, more probably, contained in the

,**m

a

Mes

FIG. 124. 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-
minalis.

Nas, nasal capsule; Olf ', olfactory lobe; N. ter, Nervus terminalis; gl, its ganglion;
Tel., telencephalon; Di, diencephalon; Mes, mesencephalon; Met, metencephalon;
Myel, myelencephalon.

sensory elements of adjacent cranial nerves such as Trige-
minus or Facialis. The fact which suggests this hypothesis
most strongly is the strictly metameric character of their
field of distribution, namely, the eye muscles themselves, as
is shown by their developmental history. In selachian em-
bryos, which have preserved this early history more completely
than have the higher forms, there develops in the head a series
of myotomes, similar to and continuous with those of the
trunk. Some of them soon atrophy, but the first three fold

THE NERVOUS SYSTEM

447

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 succession, as follows :

SOMITE

MUSCLES DEVELOPED

NERVE

No.

Myotome I

Rectus superior
Rectus intemus A1**
Rectus inferior IT
Obliquus inferior

Motor oculi

Ill
IV

Myotome II

Obliquus superior

Trochlearis

Myotome III

Rectus externus fif fl

Abducens

VI

These relationships are constant throughout all vertebrates,
corroborating the idea that we have here the enumeration of
some very primitive morphology. In certain Orders, in re-
sponse to special needs, other special muscles appear in con-
nection with the eyeball, but these are seen to be differentia-
tions of certain of the above, and retain the same innervation ;
thus the retractor bulbi * arises from the external rectus, 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, although much has been done, few definite
conclusions may be drawn as yet. The Motor oculi, although
its fibers are purely motor, yet becomes connected with the
small ciliary ganglion, through which its fibers innerve the
ciliary muscles and the iris. This ganglion may have the
morphological value of the one belonging to a sensory root
now lost, a conclusion which would make this nerve an entire
spinal element with a reduction of the sensory root. Other
views associate with it as its sensory element a portion of the
Trigeminus. The Trochlearis, although essentially a motor
nerve, possesses in fishes and amphibians a few sensory fibers,

* This muscle is rudimentary or wanting in the Anthropoidea.

448 HISTORY OF THE HUMAN BODY

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
probably also a part of the Trigeminus, although certain
facts indicate an association with the Facialis.

III. THE TRIGEMINUS-FACIALIS GROUP. ( Trf^eminus' Fa-
cialis, 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 suppression in the other of an extensive system of ex-
ternal sense-organs of uncertain function but undoubtedly
of assistance in an aquatic life. These organs, variously
termed " integumental sense-organs " or " dermal canal sys-
tem'' are visible externally and are arranged in definite lines
running about the head and continued in a single (or double)
longitudinal row, the lateral line, down the sides of the body.
The system of nerves which supplies these is shown in the
first of the accompanying diagrams, and consists of three su-
perficial trunks directed forwards and a fourth one directed
backwards, the former referred to the Facialis (VII), the lat-
ter to the Vagus (X). To each trunk there belongs typi-
cally a special ganglionic swelling, placed near its origin ; but
these are distinct in only a few forms (selachians, dipnoans, a
few aquatic amphibians) and in all others become completely
fused with the ganglion semilunare of the Trigeminus and
are demonstrable only in the embryo. To the compound gan-
glion 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 ophthalmiais super-
ficialis Septimi), and is accompanied by a like-named branch
of the Trigeminus (ramus ophthalmiais superficialis Quinti),

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THE NERVOUS SYSTEM 449

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,
associated with the anterior of the two subdivisions, is the
mandibular 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. The sensory portion pos-
sesses at its origin the large genicular ganglion, from which
proceed (i) a large palatine branch and (2) a small internal
mandibular. The motor portion incorporates within itself the
external mandibular 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 as-
sociation with the Facialis. There remain to be mentioned
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, and the
deep ophthalmic branch (ramus ophthalmicus profundus).
This latter 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
mandibular branch of the Trigeminus possesses a few motor
elements, which prevent the nerve from being classed as
wholly sensory.

The later history of the parts above considered may be fol-
lowed from the second diagram [Plate VII], which represents
in a general way the terrestrial type, but which in its propor-

450 HISTORY OF THE HUMAN BODY

tions 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 amphibians
that become terrestrial and never reappears. This causes at
jfonce, among other changes to be considered later on, a loss
of three branches of the facial 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 Fifth nerve, and con-
tinues under the name of palatinus major, usually considered 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 Glasserian fissure (in mammals) and blends with
the lingual branch of ramus mandibularis V., encountering in
its course the sub-maxillary ganglion (better, sub-mandibular)
of the sympathetic system.

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
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-

THE NERVOUS SYSTEM 451

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 afterward 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 four 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 to 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 im-
portant innovation is found in the new relations of the Tri-
geminus with the Facialis, the Glosso-pharyngeus, and the
sympathetic ganglia. The first of these has already been
treated in detail. The Glosso-pharyngeus sends a communi-
cating branch (tympanic [Jacobson's~\ nerve) to the otic gan-
glion, which rests upon the base of ramus mandibularis; the

452 HISTORY OF THE HUMAN BODY

two nerves also come into indirect contact 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 (eighth nerve) is originally a part of that
Facialis element which supplies the lateral line, and as the
essential part of the ear, the labyrinth, closely resembles in
its early development the sense-organs of the lateral line, the
suggestion is strongly felt that we have here a case of the
local specialization of a single element out of a series of simi-
lar parts, and that the Eighth nerve 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. (Glosso-pharyngeus, Vagus, Ac-
cessorius. )

This group includes in all vertebrates the Glosso-pharyngeus
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 secondary 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

THE NERVOUS SYSTEM 453

superficial, and at the same time, the most aberrant, is the ra-
mus lateralis, which belongs to the system of lateral line nerves
and supplies the lateral line itself, which 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-tem-
poral branch, which forms a connecting loop over the top of the
head ; in the Dipnoi, though not in other fish, lateral communi-
cating branches connect it with the superficial ophthalmic nerve
of the Facialis, thus uniting the two parts of the system. The
ramus lateralis has at its proximal end a ganglion of its own
(ganglion laterale), although it arises in connection with the
combined ganglionic mass of the Vagus. In spite of this asso-
ciation, however, it is probable that the ramus lateralis did not
originally belong to the Vagus alone, but zvas built up as a
collecting trunk from branches supplied by each metameric
nerve of the body, beginning with the Vagus. The gradual \
loss of these metameric connections, beginning posteriorly,
would, in time, leave the most anterior one alonef the condition
found at present.

Ventral to the ramus lateralis appear five elements, similar
to one another, each associated with a gill-slit, and possessed of
its own ganglion. This extremely primitive condition is seen
in a few forms only (e.g., the rays and skates), but these ani-
mals 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 is more distinct than the others, and forms the Glosso-
pharyngeus, treated as a separate nerve; the remaining four
are Vagus elements and in all but very primitive forms arise
from a single ganglionic mass, the ganglion jugidare, 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 ( Glossus-pharyngeus, and the four Vagi)
possesses an identical distribution. From the ganglion the
main stem passes downwards, and forks into two branches,
including a gill-slit in the fork. The two branches, one in

454 HISTORY OF THE HUMAN BODY

front and one behind the slit, are known respectively as rami
prcz- and post-trematici. In this connection it is interesting to
see that the spiracular opening, which probably represents the
gill-slit next in order anteriorly to the regular series, is simi-
larly included between the internal mandibular (chorda tym-
pani) and the hyo-mandibular of the Facialis, which thus be-
come, respectively, the prcz- and post-trematic branches of that
nerve. It is even possible in like manner to consider the maxil-
lary and mandibular branches of the Trigeminus as similarly
related to the mouth opening, resting upon the probability of
the identification of the mouth with an original gill-slit anterior
to the spiraculum. We have thus a character of great value
in the resolution of the cranial nerves into their original meta-
meric elements, one which will be considered later in the treat-
ment of this difficult and unsolved problem.

Of these five branchial nerves, the Glosso-pharyngeus, 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 petrosum) to that of the Facialis (ganglion
geniculare). This nerve possesses no special name in lower
forms, other than the generic ramus communicans, but in the
higher forms it becomes the tympanic (nerve of 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 one, the ramus intestinalis Vagi. This
appears in primitive forms as a separate element, with its
own ganglion, but in all other cases it arises with the rest of
the Vagus and its ganglion becomes lost in the general mass,
the ganglion jugulare. This is the branch which, even more
than the lateralis, has earned for the nerve to which it belongs

THE NERVOUS SYSTEM 455

the title of Vagus (wandering), since it becomes distributed
to the oesophagus and stomach, the heart, and, in higher forms,
the lungs. In spite of its great length and extensive distribu-
tion, however, it is not to be considered,, like the ramus lateralis,
a compound nerve, but its length is due rather to the extension
posteriorly of parts once placed far forward and thus within
the legitimate province of the original nerve. Thus, the heart
has primarily a very anterior position ; the oesophagus and
stomach were probably once very far forward, and the lungs
are diverticula of the primary oesophagus. The wide distribu-
tion of this branch is thus a striking illustration of the prin-
ciple 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 zvas 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 alsof
corresponding to the higher development of its field of distribu-
tion, is still more extensive and complex.

The extreme differentiation of this group may be learned
from Plate VII, which here especially, in the separation of the
Accessorius and in other points, suggests the mammalian con-
ditions. The communicating branch, the tympanic nerve, be-
comes somewhat more complicated and forms connections be-
tween four ganglia ; starting from the ganglion petrosum of the
Ninth nerve it runs forward and sends branches to the gan-
glion geniculare of the Seventh, and to the otic and sphenopala-
tine ganglia of the sympathetic system. It is also involved in a
small plexus of sympathetic nerves which surround the carotid
artery. The relationships of this nerve, which are so complex
in mammals, become especially so in Man, owing to the short-
ening 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

456

HISTORY OF THE HUMAN BODY

VIII), which is to be carefully compared with Plate VII.
As special names are often given in human anatomy to parts
spoken of by the morphologists under more general terms, a
list of equivalent terms is here added, for the better compari-
son of the figures alluded to.

TERMS IN HUMAN ANATOMY.

Ganglion Gasseri, composed of.

Ganglion geniculare,

MORPHOLOGICAL EQUIVALENTS.

Ganglion ophthalmicum superfi-

ciale VII.

Ganglion buccale VII.
Ganglion mandibulare VII.
Ganglion semilunare V.
Ganglion ophthalmicum prof undum.

C Sensory ganglion of VII, excepting
J the parts belonging to the lateral
|^ line system.

Ganglion petrosum J Together form the sensory gan-

Ganglion jugulare IX

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.

N. petrosus superficial major Ramus palatinus VII.

Ganglion jugulare X

N. palatinus major V.

Continuation of the ramus pala-
tinus VII beyond the spheno-
palatine ganglion, plus some
fibers from the Trigeminus.

Chorda tympani N. mandibularis internus VII.

N. tympanicus f T°gether f orm Jacobson's nerve

N. petrosus superficial minor'. \ which is'. morphologically, ramus

(^ commnnicans IX.

N. petrosus profundus major. ( *oth included in the branch of the
N. petrosus prof undus minor.. ] abovet° ^ spheno-palatme

ganglion.

THE NERVOUS SYSTEM 457

The lingual branch of the Glosso-pharyngeus, which appears
first in the Dipnoi, becomes in the Amniota, and especially in
the mammals, a large and important nerve, and specializes as
the nerve of taste (gustatory). 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 trape-
zius 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 (Fig. 125).

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 motor nerve and arises
from several roots which belong in the ventral series; this
is rendered more certain by the appearance, usually transi-
tory and embryonal, of corresponding dorsal roots, equipped
with ganglia, which thus complete the elements necessary 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 potential
hypoglossal still in an indifferent condition. Some of the
spino-occipital elements may also enter into the formation of

458

HISTORY OF THE HUMAN BODY

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

gang crest

Opthal div —
Sup max &
N masticatcrius
Tnf max.d/V

FIG. 125, a. 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.

from which proceed motor nerves to certain of the hyoid and
extrinsic laryngeal muscles.

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-disputed
subject.

THE NERVOUS SYSTEM

459

V<pgu,s root gong.

Accessory root gang.

/X root gang

Gang, petros
N tymp

Br to
carotid plexus

Froriep

XI.

Sympathetic.

FIG. 125, b. Reconstruction of peripheral nerves in human embryo.
[After STREETER.] Six weeks human embryo, I7.smm long.
For abbreviations see Fig. 125, a.

46o HISTORY OF THE HUMAN BODY

The question rests upon the assumption that the precursors
of the vertebrates were, like Ampkioxus, headless but com-
pletely segmented, and that the head ivas 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
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-
tomes, are metameric, at least in the embryo. By thus ascer-
taining the original number of each of these metameric elements
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 sug-
gested 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
embryo. Some, have, indeed, asserted that the prachordal and
parachordal portions of the head, divided at about the hypo-

THE NERVOUS SYSTEM 461

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 prsechordal 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
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.

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-pharyngeus,
and Vagus, the latter a compound nerve, capable of resolution
into four, or perhaps, five elements. If to these the ramus oph-
thalmicus profundus be added with its ganglion as an origi-
nally separate element, we have the sensory roots of eight origi-
nal 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 cer-
tain definite sensory elements. The tracing out of prae- and
post-trematic branches which include a gill-slit as above men-
tioned assists in locating seven elements, if the mouth opening
and spiraculum be included.

The above relations are summarized in the following dia-
gram, which, although not claimed as the ultimate solution of
the problem is, at least, suggestive (Fig. 126, A). The head
is here represented as being composed of nine somites on the
basis of nine pairs of " head cavities " (muscle somites or myo-
tomes) found in dog-fish embryos. These are represented by
the heavy black rings numbered from I-IX. For the first

462

HISTORY OF THE HUMAN BODY

somite the Ophthalmicus profundus represents the sensory,
and the Motor oculi the motor root. To the second the remain-
ing 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 Facialis for a sensory

rx

Olf. CiL v vn vra ix

' B

G« G* Gj

FIG. 126. 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; Gv Gv etc., gill-slits; Olf, olfactory nerve; Cil, ciliary nerve.

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 Vagus
group in fishes, while two Hypoglossus roots supply motor ele-
ments for the last three somites. Fig. 126, B, shows a sug-
gestion made by another investigator, and based mainly upon

THE NERVOUS SYSTEM 463

the relation of prse- and post-trematic branches to their cor-
responding 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 an-
terior to the Ophthalmicus profundus; the spino-occipital
nerves, although of unknown value, seem indicative of still
other somites in the occipital region and need to be thoroughly
explained before the problem of the segmentation of the head
can receive its final solution.

There remains to be mentioned an auxiliary system of gan-
glia and nerve fibers, not directly under the control of the will,
but often of great importance in regulating the physiological
activities of certain of the internal organs. This is the sym-
pathetic system, often erroneously treated as originally a dis-
tinct nervous system coordinate with the cerebro-spinal, thus
far the subject of this chapter. As a matter of fact, however,
the sympathetic system is an integral portion of the latter, 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
forms. It consists primarily of a series of ganglia, segmented
off from the sensory ganglia of the spinal nerves, although re-
taining connection with their places of origin through commu-
nicating branches. The ganglia of the two sides, which come
to lie ventral to the spinal nerves on either side of the ver-
tebral column, may become secondarily connected with one an-
other by longitudinal connectives, thus forming two lateral
trunks.

This appearance of metameric ganglia connected in two lon-
gitudinal series, and especially their ventral position, has led
the sympathetic system to be compared to the ventral chain
of ganglia found in articulates (e.g., insects, crustaceans), 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

464 HISTORY OF THE HUMAN BODY

fishes the system is better developed in this region than in the
trunk. In the embryo also the cephalic portion develops be-
fore 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 many of the internal organs, especially the alimen-
tary canal and the arteries, the favorite mode of distribution
being an intricate plexus, which spreads over the broader sur-
faces and enwraps the smaller parts.

Owing to the origin of the sympathetic system from the
strictly metameric sensory ganglia of the cerebro-spinal nerves,
this system also shows at first a metameric character; this ap-
pearance becomes modified, however, in regions of the greatest
differentiation, as in the head and neck and the pelvic region.
The four cephalic ganglia, ciliary, spkeno-palatine, otic, and
submaxillary, which in mammals assist in forming connec-
tions between certain of the cranial nerves, and which have
been treated with these latter parts, belong to the sympathetic
system.

CHAPTER XI
THE SENSE-ORGANS

" Die wunderbare und wirklich iiberraschende
Ahnlichkeit in der inneren Organisation, in den
anatomischen Structurverhaltnissen, und die noch-
merkwiirdigere Ubereinstimmung in der embryonalen
Entwickelung bei alien Thieren, welche zu einem und
demselben Typus, z B., zu dem Zweige der Wirbel-
thiere, gehoren, erklart sich in der einfachsten Weise
durch die Annahme einer gemeinsamen Abstammung
derselben von einer einzigen Stammform. Entschliesst
man sich nicht zu dieser Annahme so bleibt jene durch-
griefende Ubereinstimmung der verschiedensten Wir-
belthiere im inneren Bau und in der Entwickelungs-
weise 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, leaving upon the surface a set of sense-organs,
capable of receiving the impressions and transmitting them to
the central organ. Taking into consideration the intimate con-
nection between these two portions, 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 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 mam-
mal, and, save for a few external parts, is as complex; the
sense of hearing, although not as early in development as the
eye, is yet very acute in reptiles, and, perhaps, in amphibians,
and gains in birds and mammals very little except, perhaps,

465

466 HISTORY OF THE HUMAN BODY

the recognition of musical tones; indeed, the early aquatic
forms possess in the lateral line organs an entire system, no
trace of which seems to have survived the transition to land,
and yet, with no especial 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 spe-
cialized 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
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, connected by a nerve
with some sensory center. From this as a starting point
higher efficiency is gained in three ways: (i) by the asso-
ciation 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 development of accessory parts.

The area over which a given form of sensory 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,
sensory cells and supporting cells; the first are the receptive
units of the nervous system ; the latter are non-sensitive, and
are grouped about the sensory cells in such a way as to form

THE SENSE-ORGANS 467

a support and protection for the essential elements of the sense-
organ, which may thus attain a high degree of sensitiveness.

Regarding the differentiation of the sensory 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 impression, 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 extreme
delicacy of the projecting parts, can exist only upon an external
surface bathed by water, as in many aquatic invertebrates, or
upon a surface that faces some internal cavity furnished with
an artificial fluid or semi-fluid.

Accessory organs for the reception and intensification of the
external stimuli or for the protection and care of the essential
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 reaching the interior. 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 sensory organs are developed
between them, or may be situated about their bases, when they
are stimulated indirectly through the movements of the insen-
sitive 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

468 HISTORY OF THE HUMAN BODY

and those formed like them. Thus, we are positive concerning
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 presents sev-
eral problems, since the lower types of ear lack certain parts,
like the cochlea, which in Man are essential to the complete-
ness of the sense of hearing as we understand it. In this
case we have two alternatives, either that in the different forms
the same function is subserved by different parts, as is possible
in the case of the brain (cf. the pallium of teleosts and the
cerebral hemispheres of mammals), or else that there are ele-
ments in the human sense of hearing not perceived by ears be-
longing 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
location and structure of certain specialized sense-organs and
the identity of their nerve supply. After these are excepted,
however, there remains a large number of types of terminal
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, at least as distinct as those of smell
or taste ; but as few of these types occur in Man, and as even
here the elementary sensations have not been wholly coordi-
nated with the various forms of nerve terminations, but little
can yet be stated on the subject, and the psychology of the
tactile sensations of the lower vertebrates remains an unex-
plored field.

Probably the lowest form of vertebrate sense-organ, and one
that is universally distinguished among them, is that of simple
sensation, the contact sense, which resides in the epidermis
and is thus generally met with over the entire surface. The
vertebrate epidermis, which, in contrast to that of invertebrates,
is many cells thick, is supplied everywhere by sensory nerves

THE SENSE-ORGANS 469

which branch repeatedly and with their ultimate 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 re-
sponding to general tactile impressions.

Aside from this general tactile sense vertebrates possess a
large number of more or less specialized sensory endings,
usually classed also as tactile. These are local in distribution,
often confined to a single group of animals, and usually oc-
cur upon prominent portions of the body or occasionally, as
in the case of certain fishes, upon special papillae or long fila-
ments. The most extensive of these, and the only one to
develop into a definitely organized system, is that of the lateral
line organs, referred to above in connection with the cranial
nerves and possessed by the primarily aquatic vertebrates
(fishes and amphibians). Although this system, as such, to-
gether with its nerves, disappears utterly with the- assumption
of a terrestrial life, it is yet of importance in this connection
because of the possible derivatives from it in higher forms,
among which have been mentioned, with more or less basis
for the claim, the taste-buds, the inner ear, and the mammalian
hair.

In its simplest form a lateral line organ 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 bottem 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-organ 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

470

HISTORY OF THE HUMAN BODY

THE SENSE-ORGANS 471

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 sensory 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. These organs
occur in localized masses in the heads of selachians, associated
topographically with the mucous canal type, the ampullae with
their nerves clustered in such a way as to resemble bunches of
grapes (Fig. 127).

That a system so highly developed and so extensive in its
distribution as that of the lateral line organs should have
wholly disappeared in terrestrial vertebrates, together with its
nerves, is a phenomenon of so unusual a nature that numerous
attempts 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, ho\vever, to be no definite
data to form the logical steps between the two, and the fact
that the nerve supply to the taste-buds comes from the Glosso-
pharyngeus and not 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 direc-
tion there are found in the nasal mucous membrane of many
fishes and of certain of the lowest urodeles (Siren) groups of
cells forming (f smell-buds/' extremely similar to taste-buds, and
yet by no possibility connected with the lateral line system.

A second possible survival of the lateral line organs 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

472 HISTORY OF THE HUMAN BODY

cases concerns the epidermis alone and consists of a concentric
arrangement 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 the lateral line organs 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
proliferate 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
nerve supply, especially in cases like that of the vibrisscz (whis-
kers) 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
organs 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 third possible derivative of the lateral line system, the
inner ear, does not come into the same category as the taste-
buds and the hair, since if it came from this system at all, it
must have separated from it very early, and thus could not in
any case be considered a survival of the system as it exists in
fishes and amphibians. This theory receives its strongest sup-
port from the developmental origin of the Eighth nerve, which
has been clearly proven to segment off from that part of the
Seventh which supplies the lateral line organs, certainly a
strong argument, since, if the Eighth nerve were once an ele-

THE SEXSE-ORGANS 473

ment of this system, the part to which it is distributed must
have been so also. A suggestive comparison has also been
made between the semicircular canals of the inner ear, each
with its own ampulla, and the canals of Lorenzini, the resem-
blance between which is apparent. The actual value of this
comparison is somewhat questionable, and the theory itself, al-
though far better supported by the facts than are either of
the others, is seriously opposed by the actual developmental
history of the labyrinth ; which arises as a vesicle invaginated
from the exterior long previous to and not associated with the
lateral line organs. The otic vesicle seems rather to form 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 transi-
tory structures associated with other cranial nerves and usually
interpreted as lost sense-organs of unknown function. In our
present state of knowledge it seems a surer course to believe
that the entire lateral line system of the Ichthyopsida, the
function of which is in some way associated with an aquatic
habitat, disappears completely where the assumption of a ter-
restrial life renders it no longer necessary.

In connection with the description of the general tactile
sense, that of the so-called " free nerve endings," certain more
specialized forms of nerve termini were referred to, which in
our present lack of precise knowledge are classed under the
general head of organs of touch. The most elementary of these
are the tactile cells (Fig. 128, b), which are scarcely more than
isolated units of the general type, somewhat more specialized
and thus rendered conspicuous. 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 appreciable size; these are termed collectively
tactile corpuscles, each different type being designated by the
name of the investigator who first made an accurate description
of it. These tactile corpuscles show various types of structure
and make use of very different mechanical principles. Thus,
in Meissner's corpuscles (Fig. 128, c) the terminal cells form

HISTORY OF THE HUMAN BODY

an oval core, upon which the nerve fibers are wound in an
irregular branching spiral. Certain types, on the other hand,
seem to possess no epidermal elements, as in Krause's cor-
puscles (Fig. 128, d), which consist of a globular snarl of
nerve fibers like a capillary glomerulus, enclosed within a thin
covering of connective tissue. In still another type, shown by
Grandry's corpuscles (Fig. 128, e), the nerve terminus is en-
closed between two large epidermal cells which seem to pro-

FIG. 128. Various endings of sensory nerves.

(a) Free nerve ending. (b) Merkel's corpuscles. (c) Meissner's corpuscle,
(d) Krause's corpuscle;, (e) Grandry's corpuscle. (f) Pacini's corpuscle.

duce 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 layer of epithelial cells, then by a series
of consecutive lamellae of connective tissue, something like the
coats of an onion, and finally by an external connective tissue
wrapping, the continuation of the nerve sheath or neurilemma.

THE SENSE-ORGANS 475

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 Pacinian
corpuscles also occur in such various internal organs as the
pancreas, the submandibular gland, and, in the cat, even in the
mesentery. This internal distribution has led to doubt con-
cerning the function of these corpuscles as tactile organs, but,
on the other hand, their profuse occurrence and large size in
such places as the balls of fingers and toes can be accounted
for in no other way.

The sense of taste, which in popular estimation is raised to
the value of one of the special senses, is, all things considered,
but little more than a tactile sensation, and the organs in which
it is located are but little differentiated from certain of the
foregoing. The organs of taste themselves, 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
mechanical or chemical in their action and, as such, can be also
perceived and, in part, distinguished, by the general tactile
sense. To prove this last it is only necessary to bring some
acid or astringent liquid in contact with a surface from which
the external layer of the epidermis has been removed.

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, 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) scat-
tered over the skin of the external surface. 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 supporting cells of shape

476 HISTORY OF THE HUMAN BODY

similar to the others, but longer, and thus greatly resemble
the terminal organs of the lateral line system. At their free
ends the sensory cells usually possess one or more modified
flagella, which project into a small space, that is formed about
them by the supporting cells, and communicates with the ex-
terior through a small opening. In mammals the taste-buds
are associated together in groups in connection with several
sorts of papillae, especially the circumvallate, and the foliate,
the latter not occurring in Man.

The sense of smell is located in a pair of ectodermic cavities,
situated anterior to the eyes, thus forming the most anterior
of the sense organs. They are thus in the most favored po-
sition for organs of sense, and although the data are too in-
sufficient for theories, this fact suggest^ that they were the
earliest to develop and that the primaeval habitat was either in
mud or in the deep sea where the olfactory sense was of pri-
mary importance. Amphioxus gives no clew to this, for here
the sense of smell is located in a median ciliated pit at the
anterior end and pushed a little to the left side by the develop-
ment of the median fin. 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. 129) there appear at the anterior
end two median invaginations, the more posterior of which is
the cavity of the mouth (stomatod&um). The anterior de-
pression is that of a median nasal cavity, which would suggest
a primitive condition and possibly a kinship with the ciliated
pit of Amphioxus were it not for the fact that it is supplied by
two olfactory nerves from as many olfactory lobes, showing
that the single or monorrhine condition has here been secondar-
ily 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,

THE SENSE-ORGANS

477

and is of still greater interest for its direct connection with the
hypophysis, which develops from it. // in this may be seen
the remains of an earlier entrance into the alimentary canal,

FIG. 129. Median sagittal sections through the head of two stages of
the Ammoccetes embryo of Petromyzon. [After von KUPFFER.]

I, II, III, the three primary cerebral vesicles; d, intestine (mesodaeum) ;
m, mouth cavity (stomatodaeum) ; nc, notochord; ep, epiphysis; hy, hypophysis;
hy', portion detached from distal end of hypophysis; rg, olfactory pit; lo, median
olfactory lobe; ch, optic chiasma.

478 HISTORY OF THE HUMAN BODY

then its associated mouth (palaostoma) must have been the
later nasal cavity, and its unpaired condition in 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,
which invaginate and form nasal sacs, a condition that per-
sists in fishes. In the Dipnoi, the first air-breathers, these sacs
become prolonged posteriorly and break through into the
mouth cavity, forming the choana [posterior nares'], forma-
tions which are present also in amphibians and all higher ver-
tebrates. When these are present, the lower part of the cavity
is used more or less exclusively for respiration, and the olfac-
tory sense becomes 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 to an extreme dorsal one. Marked adaptations in
this respect are seen in those air-breathers which have become
secondarily aquatic, enabling them to breath 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 maxillaries and
premaxillaries. In some birds, like the albatrosses and pe-
trels, the nostrils become prolonged into tubes formed by the
beak, so that they open near the tip of that organ instead of
at its base. An extreme case is seen in the Dipnoi, in adap-
tation 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 choanae, their original position is shown by

THE SENSE-ORGANS 479

amphibians to be very far forward, a position retained, with
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
therefore pushes back their communication with the mouth
cavity to its posterior limit. A trace of the former communi-
cation 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 turtiinalia
and come under three categories, in accordance with their
relationships to other parts: (i) a naso-turbinal, (2) several
ethtno-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

48o HISTORY OF THE HUMAN BODY

single turbinal found in certain groups of reptiles, where its
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.

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;
the most dorsal of these is borne, at least in part, by the nasal
bone and is termed the naso-turbinal; the others are ethnio-
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 filling the
spaces left free by the first. These relations are shown in
Fig. 130, 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 end o turbinalia from their position, to which all the
rest are contrasted as ecto turbinalia. 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, II' 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 quadrupedal mammals, the greatest de-
gree of complexity being reached by the ungulates (Fig. 130,
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.

THE SENSE-ORGANS

481

130, 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 concha " of
human anatomy. The first, I, is a rudiment, and the last.

FIG. 130. Diagrams of ethmoturbinals in Mammals. [After PAULLL]

(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.

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

482 HISTORY OF THE HUMAN BODY

more highly developed olfactory sense than appears at present
(Fig. 131).

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
the loss of their olfactory function and are lined by simple
mucous membrane. The largest of these accessory cavities

FIG. 131. 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.

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. 130, 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

THE SENSE-ORGANS 483

in the embryo of turbinalia that never develop, and is indicated
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 his-
tory 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 writhin 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 Gymnophiona. In Hzards 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
position of the organ has again changed owing doubtless to
the development of related parts, and it lies almost directly
beneath 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 exists1
only in the form of embryonic vestiges, but, on the other hand,
K i& luLdlxd upun cither skle^oMhc, carlrtegiTiOtts-iiasal septitm^
a definite organ and persisting throughout life in many cases.

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

484 HISTORY OF THE HUMAN BODY

into the naso-palatine canal, thus retaining its early relation-
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 terminal olfac-
tory cells surrounded by supporting cells, some or all of which
may be ciliated. In certain fishes and in the lower urodeles

THE SEXSE-ORGAXS 485

there occur in the nasal mucous membrane definite groupings
of the olfactory cells, surrounded by protecting cells, thus
forming olfactory buds, almost identical in form with the taste-
buds, which in turn resemble the lateral line organs. Some have
seen in this, as well as in the condition of the primary olfac-
tory pits, which, in the embryo, form the anlage of the nasal
cavities, a possible genetic connection with the lateral line or-
gans, but this homology is rendered improbable from other
reasons. Of these the most fundamental is the singular rela-
tionship between the individual olfactory cells and their nerve
fibers, the two being directly continuous, and not, as in all
other known cases of sensory cells among vertebrates, simply
in contact with one another. This continuity of fiber with ter-
minal cells is, however, characteristic of the sensory cells of
the lower invertebrates 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
relationship with certain other parts will be taken up below in
connection with the lens of the eye.

The essential orafan 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. In fishes it is placed
immediately beneath the bones of the head and in its compara-
tively superficial position needs no accessory apparatus, but
in the higher vertebrates 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 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 develops in adaptation to the former requires an inten-
sifying 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
area proliferate more rapidly than those of the surrounding

486

HISTORY OF THE HUMAN BODY

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 endolym-
phaticus. 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 connec-
tion with the exterior becomes severed and the endolymphatic

FIG. 132. Development of human otic capsule. Drawn from model'
by F. ZIEGLER, after WM. His.

duct ends in a somewhat expanded blind sac, the saccus endo-
lymphaticus. In mammals the endolymphatic duct is lodged
in a canal of the petrosal bone, the aqueductus vestibuli, and
enters the cranial cavity, where its terminal sac lies just be-
neath (i.e., outside of) the dura mater.*

* In some cases the ductus endolymphaticus and its terminal sac attain

mgh degree of development and come into association with organs re-

from its place of origin. Thus in certain teleosts the two ducts

unite into a median sinus which is connected with the air bladder by a

cham of four ossicles (Weber's apparatus), developed from the ribs of

irst 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

THE SENSE-ORGANS 487

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 utriculo-saccular
canal. [Cf. Figs. 133 and 134.] From the utriculus, which
lies dorsally, develop three flattened folds, which by the adhe-
sion and subsequent atrophy of the middle portion of their
walls, 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, but at an angle
of about 45° with the bilateral plane of the body. They
empty at either end into 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
similar pocket on the other side (sinus utriculi posterior).
The two vertical canals unite at their expanded end and enter
or form the sinus utriculi superior, a diverticulum which ex-
tends directly upward; the unexpanded end of the horizontal
canal enters the main body of the utriculus unassociated.

The parts of the utriculus with its derivations, the semicir-
cular canals, appear first in the form described above in the

extends 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 them-
selves 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 crystals, 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 (Ascalabota} , 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 cal-
careous mass.

488 HISTORY OF THE HUMAN BODY

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 lageyia, 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. 134]. 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 (Petromyzori) semi-
circular canals. Its endolymphatic duct, however, is short and
loses its connection with the exterior (Fig. 133, 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 neuro-epithelinm. Certain
of these cells form the ultimate organs of hearing and are
provided with various sorts of terminal flagella and other
similar structures (auditory hairs), which project into the lu-
men of the labyrinth and are bathed in the endolymph, i.e., the
fluid 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

THE SENSE-ORGANS

489

serve to hold and protect the others. These spots, which thus
show a higher degree of cellular differentiation, are the seat
of the auditory sense, and appear from their greater thickness

a

Mac

Hacsac Pap lagKoch)

FIG. 133. Ear labyrinths of various Vertebrates. [After RETZIUS.]

(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 extcrna; ap, ampulla posterior; e, ductus
endolymphaticus; fee, facial nerve; mac, macula communis; mac. ittr, macula
utriculi; mac. sac, macula sacculi; n, macula neglecta; pap. lag, papilla lagenae;
pap. bos, papilla basilaris.

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 some-

490 HISTORY OF THE HUMAN BODY

what different from the rest and are absolutely constant. They
are situated in the ampullae and are in the form of ridges which
encircle them and project into the lumen. They are thus dis-
tinguished from the others as cristcz acusticcz, The remaining
five are evidently formed by the successive breaking up of a
single large area, due to the differentiation of parts of the laby-
rinth, and the history of this segmentation and later migration
is represented in the phylogenetic series (Fig. 133). 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. 133, a). The differentiation of utriculus and
sacculus gives a separate area to each, respectively, the ma-
cula acustica recessus utriculi and macula acustica sacculi
(Fig. 133, 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 lagena. 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. 133, cf. b and c.) A small macula is
also formed near the utricular macula, the macula acustica neg-
lecta, evidently an offshoot from the latter, although no phylo-
genetic proof of this appears as in the former case. The fifth
and last of the auditory areas, not counting the cristse acus-
ticae 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. 133, d).

To put these points into the form of a phylogenetic
history 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

THE SENSE-ORGANS 491

papilla lagenae appears, sometimes as a separate area, some-
times as a lobe of the macula sacculi, thus giving four acoustic
maculae for most fish. In amphibians the papilla basilaris ap-
pears while the others are retained, although there is a slight
transposition of the macula neglecta. This condition, with
five acoustic areas, is retained by the Sauropsida with some
variation, 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
acusticse 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 becomes
wound into a close spiral, the nerve forming the central axis.
The number of complete coils in man is about 3, but varies
among mammals between the limits of ij and 5.* This
coiled lagena becomes complicated by the addition of parts of
the outer bony labyrinth, to be explained later, which form two
additional coiled passages, scala vestibuli and scali 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 or-
gans, is called the cochlea.

The papilla basilaris lies along the floor of the coiled lagena
(scala media) and becomes highly differentiated into a number

* Examples are as follows :

Erinaceus (Hedgehog) i^

Whales and porpoises 1^2

Rabbit 2^

Cat , 3

Ox 354

Swine 3l/2

Ccelogenys (South American rodent) 5

492

HISTORY OF THE HUMAN BODY

of kinds of histological elements, which change their propor-
tionate size along the course, being the smallest at the base and
the largest at the apex. The most important of these 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 beyond the
auditory area, is termed the basilar membrane, and the dorsal
portion (roof) is Reissner's membrane; these terms are, how-
ever, purely anatomical ones, expressing certain relationships

canalis cochlearis.

FIG. 134. 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.

to the surrounding parts and are without morphological
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 connec-
tive tissue, its first accessory organ. Later on in development
this tissue becomes converted either to cartilage or bone, leav-
ing, however, a nearly uniform layer of the original tissue be-
tween 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-

THE SENSE-ORGANS 493

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 Sony 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 de-
tails 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 reception
of less powerful vibrations. In the cartilaginous otic capsule
surrounding the labyrinth, that which partly corresponds to the
" bony labyrinth " of higher forms, there exists an oval opening
with a reinforced rim, closed byx an ossicle in the form of a lid,
usually with a process projecting from its center [cf. Fig. 39,
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 subcutaneous, it is set in mo-
tion 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 apparatus
is much improved by an important addition, the tympanum, or
cavity of the middle ear. This is developed from the gill

494 HISTORY OF THE HUMAN BODY

pouch of the spiracular opening, the one associated, as will be
remembered, with the hyoid arch. The inner portion 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 evi-
dent 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 cochlea?
[rotunda], filled with a thin membrane, also termed 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 cartilage,
which extends from the center of the outer tympanic mem-
brane to the operculum. By this means the sound vibrations
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 fur-
nished by the air enclosed in the tympanic cavity, the vibrations
striking the inner tympanic membrane. The apparently new
skeletal element, the columella, is probably 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 man-
dible and originally the dorsal segment of the second visceral
arch (hyoid).

THE SENSE-ORGANS 495

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 sys-
tems extend out from the main tympanic cavity, and in certain
instances the two latter communicate with one 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 other drum
head to the fenestra ovalis, thus assuming the function formerly
sustained by the columella alone. This latter apparently be-
comes 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 re-
lation appears only in the embryo in most mammals, includ-
ing Man, but in some (certain rodents and insectivores) it per-
sists throughout life. (Cf. Chap. IX.) In others still, mainly
monotremes and marsupials, the perforation 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
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

496 HISTORY OF THE HUMAN BODY

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
migrations 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 conspic-
uous 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 external
meatus. This bone remains distinct throughout life in mono-
tremes, 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 physiologically 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
affords the membrane a more or less complete concealment.
In cases where the tympanic bone furnishes a prolonged tube
for this purpose, the external cartilaginous element is less ex-
tensive, 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

THE SENSE-ORGANS 497

Man, but there is no lobule and little or no reclining to the free
edge. This latter peculiarity, which starts at the upper part of
the base, is distinctively human, but extends over a varying dis-
tance 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 a tubular meatus together with a rudimentary
pinna. This leaves unaccounted for a series of protuberances
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
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 mam-
mals.

The developmental history of the eye, as given in the pre-
vious chapter, shows that this sense-organ, that is, its essential
part, the retina, differs radically from all the others in being
originally a portion of the brain surface, the cells of which have

498 HISTORY OF THE HUMAN BODY

become specialised in form and function so that they respond
directly to the stimulus of light vibrations. To this essential
part, which, with a pigmented outer layer, are the only parts
derived from the brain, accessory organs are added from two
sources to complete the formation of the eyeball; the lens,
formed from the ectoderm of the outer surface; the chorioid
and sclerotic coats and the vitreous body from the surrounding
connective tissue. Aside from the eyeball itself there are
many external accessory parts, such as muscles and glands,
conjunctiva and eyelids, which come from several sources and
aid in the movement and protection of the sensitive organ.

To begin with the essential sense-organ, that is, the retina, if
we follow the in- and outpushings of its layer of origin from
the beginning, it is clear that the original external surface lines
the lumen of the neural tube and eventually forms the outer
retinal surface, that is, the surface turned toward the pigmented
tapetum. Now in all cases 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 sensory cells
that develop the specially modified flagella and other parts. It
thus happens that the terminal cells of the sense of vision are
not only the outer ones of the retina, which is several cell-
layers in thickness, but also that their free ends, bearing the
terminal rods and cones, point in the same direction, namely,
towards the interior of the head and away from the source of
light. Moreover, since a sensory nerve must approach its
terminal cells from their physiological inner side, this arrange-
ment compels the optic nerve first to penetrate the entire retina
and attain the interior of the eyeball, and there spread out its
separate fibers, which severally become recurved and pass back
again through the retina to supply the terminal cells. Finally,
in order that the image, received through the pupil and focused
by the lens upon the physiological outer side 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, furthemore, the terminal rods and
cones must needs be buried in the pigment of the tapetum in

THE SENSE-ORGANS 499

order to stop possible light impressions from coming from with-
out the eyeball, i.e., the natural direction for the receptive
cells.

The necessity of 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 many others,
like the appendix and 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 ap-
parent when a comparison is made with the structurally sim-
ilar eye of the cephalopod molluscs (squid, devil-fish, etc.), in
which the retina is developed directly from the surface ecto-
derm and is placed in the natural way, with the terminal cells
lining its interior and the optic nerve entering it from behind.
Notwithstanding the fundamental differences in developrhent
between this eye and that of vertebrates, the final results, when
compared part by part, are marvelously similar, and the adult
eye of each is furnished with retina and crystalline lens, iris
and cornea. This case is thus one of the best examples of what
Mr. 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 environmental influences,
which give rise to the same necessities.

In its histological structure the vertebrate retina shows some
similarity to other well-developed portions of the brain, and ex-
hibits several layers of cells, connected with one another by
branching processes which interlace and thus continue the com-
munication from one to another. At the exact focal center of
the lens all but the terminal sense-cells disappear, and produce
a small depressed area, the area centralis. This is often in the
form of a circular pit, fovea centralis, 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

500

HISTORY OF THE HUMAN BODY

seems entirely lacking in many mammals (Insectivora and
some rodents). In the anthropoids it is very pronounced, and
in man it is designated by a yellow color (hence "macula
lutea"). Certain birds possess two such areas, medial and
lateral.

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-

a h

FIG. 135. Diagrams of the retina.

^ (a) Section including the fovea, showing the separate elements. [From
GEGENBAUR, after RAMON v CAJAL.] (b) More conventionalized representation of
retinal layers. [After GEGENBAUR.]

fov fovea; /, 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.

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 ven-
tral 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-

THE SENSE-ORGANS

501

chyma) and form a gelatinous tissue, the basis of the vitreous
humor. From similar mesenchymatous elements added exter-
nally is formed the vascular network of the chorioid coat, and
outside of this is formed the solera [sclerotic coat]. The an-
terior 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 cephalopod eye 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-

FIG. 136. 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.

dient is comparable with that of the internal ear of selachians,
with its direct communication with the exterior through the
ductus endolymphaticus.

The crystalline lens, the formation of which has been alluded
to elsewhere, is a product of the ectoderm and appears first as a
thickening opposite the optic cup. It soon invaginates and
pinches off from its layer of origin, at first as a vesicle with a
nearly uniform wall and a central lumen. The posterior wall
soon thickens and restricts the lumen more and more until this
latter becomes entirely suppressed, while the wall itself, becom-
ing lenticular in shape, is covered by the anterior portion as
by a cap. This thickening is produced by an extreme elonga-
tion of the cells, which remain in the form of a single layer.

502 HISTORY OF THE HUMAN BODY

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.

It will be noticed that there is in this development of the
lens a striking similarity with the early stages of both the nose
and the ear, and if there be taken in connection with these cer-
tain temporarily thickened areas of the external ectoderm in
association with the Facialis and the Glosso-pharyngeus
nerves, which appear and vanish again during the embryonic
life of the lower vertebrates, 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 trie
lateral line organs ? To these questions, belonging themselves
to the realm of pure suggestion, we can give but speculative
answers. Both the nose 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 especially true of
the latter, with its nerve appearing in connection with that ele-
ment 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 included 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. The nasal
sacs, again, are similar capsules that do not lose their connec-
tion with the exterior, and it must be remembered that in the
endolymphatic duct of the selachians we see the same retention

THE SENSE-ORGANS 503

of the original connection. We seem here almost able to re-
produce an important bit of lost history, but the proofs are not
forthcoming and may always be wanting, since the early phy-
logenetic stages were probably passed in those lost forms be-
tween Amphioxus and the selachians, and concern soft parts,
no trace of which is likely to be found in fossil remains.

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 exceed-
ingly 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 preceding
chapter (Chap. VI). There thus remain for consideration
only the eyelids and the glands, two sets of structures closely
associated with one another. They are both employed in pre-
venting the surface of the eyeball from becoming dry upon
exposure to the air, and belong to that series of changes necessi-
tated by the change of environment from water to air. They
are consequently found only in the higher vertebrates, 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
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

5o4 HISTORY OF THE HUMAN BODY

amphibians, form as dorsal and ventral folds of the integument,
which may become stiffened, either by connective tissue or by
cartilage, as in mammals (tar sal 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 twc
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 harderiar,
glands, which are located about the anterior (inner) corner anc
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 ir
reptiles, from which point the two groups are distinct, both ir
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 whicr.
its ducts empty, but migrates in mammals to a more dorsad
position and thus becomes almost exclusively associated witl
the upper lid. In some mammals a few ducts occur in the
lower fold; indications of its former location. The lacrima.
fluid, supplied by both of these glands, is continually being se
creted and is as constantly spread in an even layer over the
outer surface of the eyeball by the movement of the lids. Th<
excess is conveyed to the nasal cavities through the nasolacrima
duct, which appears in amphibian larvae as an integumenta
groove extending from eye to nostril. This eventually close;
up, sinks into the interior, and gains its independence from the
integument, thus forming an internal canal connecting the con-
junctival sac with the anterior end of the nasal cavity.

THE SENSE-ORGANS 505

Aside from these conjunctival glands there appear in mam-
mals certain glands associated with the eyelashes. These are

1 i ) the tarsal [meibomian], which are modified sebaceous, 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 mam-
mals 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 ab-
solutely 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 mammals
project over the eye and when touched cause the automatic
closing of the lids ; also the eyebrows of the higher anthropoids,
especially Man, the hairs of which point outwards and curve
downwards at the outer end to receive the perspiration of the
forehead and convey it away from the eyes.

CHAPTER XII
THE ANCESTRY OF THE VERTEBRATES

" Ainsi la plus ancienne couche fossilifere connue nous
montre des representants de presque toutes les classes
d'Invertebres. Cela demontre 1'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 tou jours tin
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 Geoffrey 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.
// such an archetype had been considered to have or to have had

506

THE ANCESTRY OF THE VERTEBRATES 507

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 constructed 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 zuruckzufuhren ? "*

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.
137), 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 Anatomic," 1796.

508

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-

ns

FIG. 137. Owen's original diagram of a typical vertebra, to illustrate
his theory of the archetype. [After OWEN.]

ns, neural spine; s, 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 hamapophyses, a hcemal spine and a second pair of
zygapophyses. Of the lateral pieces the central ones are the
pleurapophyses or rib elements, sometimes forming free ribs,
and dorsal and ventral to these lie respectively the diapophyses
and parapophyses, more occasional elements.

THE ANCESTRY OP THE VERTEBRATES 509

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. 138. Owen's interpretation of mammalian skulls. [After OWEN.]

(A) Generalized mammal. (B) Man. For explanation see accompanying
table given in text.

type, which consisted of a series of such vertebrae gradually
tapering and losing their characteristic features in the caudal

5io HISTORY OF THE HUMAN BODY

region and 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 neurapophyses by prefrontals, orbitosphenoids, alisphe-
fioids and exoccipitals, and the neural spines, composed mainly
of paired pieces, by the nasals, the f rentals, the parietals and the
supraoccipital. The postfrontal was the diapophysis 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 haemal 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. 138, A and B). The small letters
added to the names of the separate elements in the table correspond to
those used in the diagram, so that the former may be used to explain the
latter.

THE ANCESTRY OF THE VERTEBRATES 511

s

«1

S|

S

ii

g

if

n

o *^

*o^

1

ill

II

1

11

M

S5

s

|

_«

"3

i

'x »
•

If"

73 2

~

a

O

1

E

I

Ii

•s!

12

g

a "

1

""§ ^

8 0

V

CO

S

•

§

I

s

«

g

1

t«

2: 2

"S **

s «

•§ -

8I»

1"

1

S

i

(i

04

8

2

1

"3

D

fc.

|

•a

1

i "°

1 °

1, *

i •

o

1

i

i

s

3

la

•

*s

"SI

2

1(3

M

i **

it*

Q

i

5

1

"3

1

i

3

i

1

jf ^

f

! ~

M

•

2

i

B

2

1

E

IT.

_

"3

3

|a

1 a

1 °

•- a

1 S

1

«

S

a

s

i

^

g

f

Eb

O

SB

j

j .

J

1

•J

H

1-1

&

512 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. 139, 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 513

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 repre-
sented 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
l he ventral one the blood flows from head to tail, but by re-
versing the animal the correspondence in the direction of the
current becomes complete. The vertebrate aorta is then repre-
sented by the original ventral (now dorsal) vessel in which the
current flows backwards, and the subintestinal 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, however,
lies in the nephridial system of annelids and selachians, which
in both cases consist of segmentally arranged pairs of tubes
that open into the ccelom at their inner ends by ciliated nephro-
stomes. There are also close correspondences in the rela-
tion of these tubules to the germ glands and to the ccelom.
The more primitive type may be considered to be that found
in annelids, in which each somite possesses a separate ccelom,
or rather a pair of cceloms, since those of adjacent somites are
separated by transverse dissepiments, and those of the two sides
of the same somite by median sagittal partitions, which form
dorsal and ventral mesenteries. Each of the compartments
thus formed is supplied with a single nephrostome, the tubule
from which pierces the posterior dissepiment and enters the
next posterior ccelomic pocket, where it exhibits a convoluted
portion and a vesicular enlargement, and eventually opens di-
rectly into the exterior by a lateral opening. (Fig. 139, a and
b.) The germ glands develop on the anterior walls of the
ccelomic cavities, and the germ cells become free and float
about in the ccelomic fluid until they are taken up by the
nephrostome and find their way to the exterior, through the

514 HISTORY OF THE HUMAN BODY

sh

uph—

eh

FIG. 139 Figures illustrating the Annelid theory. [After SEMPER.]
(a) Cross section of Annelid (reversed). (b) Longitudinal section of

THE ANCESTRY OF THE VERTEBRATES 515

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 fewy segments, and the associated nephridia become espe-
cially modified for the conveyance of the germ cells from the
body.

If, now, the primitive vertebrate nephridia, germ-glands and
ccelom, as described in Chapter IX 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, how-
ever, this develops in part from the ectoderm, it may have be-
gun 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 transverse dissepi-
ments, 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 trans-
verse dissipiments 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. 139, d) . In both cases the germ glands
arise as localized portions of the ccelom (peritoneum), and the
presence of a single pair in the true vertebrates may be corre-
lated with the confluence of the several coelomic cavities into
a single one. The larger number of gonads in Amphioxus
indicates the former presence of a much larger number of
coelomic cavities. That in vertebrates as in annelids the

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 sur-
rounding nerve cord and notochord; d and r, muscle masses; en, intestine; nph,
nephridium; a and b, longitudinal blood vessels.

516 HISTORY OF THE HUMAN BODY

nephridial system is made to furnish channels of exit for the
germ cells has been already shown (Chapter IX), and the open-
ing into the oviduct has been homologized with a prone-
phridial 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 communica-
tions 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 inpushings
that meet a similar series of endodermic outpushings, a mechan-
ical 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 cyclo-
stome 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 inpushing 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 pronephrotic system appear immediately
posterior to the gill region, none arise in the somites that de-
velop the gill-slits.

Important changes seem to have taken place in both outlets
of the alimentary canal, and indications show that vertebrates
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, cor-
responding to its important function, is one of the first to

THE ANCESTRY OF THE VERTEBRATES 517

appear. In vertebrates, however, the reverse is true; the
nervous system is laid down, the brain is differentiated, the
notochord is formed, even the special sense-organs ap-
pear, and still the alimentary canal remains a sealed cavity,
without communication 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 \vhich serve as jaws. There
comes, then, the inevitable conclusion that previous to this con-
version, and while the mandibular slits were still functioning as
gill-slits, the ancestral forms must have had another mouth,
traces of which are to be looked for in the earlier embryo.
Such a primary mouth is actually found indicated in precisely
the place where it would be looked for in the annelid, taking the
reversal of the body into account, and this indication appears
in the widely open " fourth ventricle " 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 remainder
of the canal, and runs between the two nerve connectives that
connect the first and second pairs of ganglia. The first gangli-
onic pair thus becomes the supra-cesophageal, the second,
infra-Ksophageal, and the connections between them form a
circum-ocsophageal ring through which the oesophagus passes.
These relationships will be clearly seen by reversing the accom-
panying figure (Fig. 140), which will thus give the conditions
as seen in annelids. In all true vertebrates 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 communication between the
lumen of the neural tube and the exterior and is otherwise un-
accounted for. Aside from the indications of the early mouth

5i8 HISTORY OF THE HUMAN BODY

and its oesophagus, furnished by fourth ventricle and neuro-
pore, there is also the hypophysis, or rather, that portion of it
that is pushed up from the alimentary canal, for which there is
yet no satisfactory explanation. Its origin from the alimentary
canal, its constant appearance 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, although
there is here no suggestion of a gilt-slit. The vertebrate anus
arises as a mid-ventral inpushing of the ectoderm at some dis-
tance from the end of the tail, and thus reaches the primary
intestine along its course, leaving beyond it a piece of consid-
erable length, the post-anal gut, which soon atrophies. This

HI

FIG. 140. Reversible diagram illustrating the Annelid theory.

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.

phenomenon, inexplicable by other means, is easily explained
by the postulate of an annelid ancestor, for in these animals the
anus is at the posterior extremity of the body, and the forma-
tion 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 ex-
planation 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 re-
semble the external gill-bushes of amphibians. Although pri-
marily distributed along the entire body, in certain specialized

THE ANCESTRY OF THE VERTEBRATES 519

forms they are confined to the anterior end. Gills of this 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 visceral skeleton
of vertebrates, while the remainder, freed from their gills,
and repeating themselves metamerically, would become the ribs.
It is even permissible to conceive of the limb skeletons as
further derivatives of the metameric system of gill-arches ; per-
haps also the original elements of the primordial skull, the
trabeculse and parachordals, may be traced to the same source,
thus accounting for all parts of the skeleton save the dermal
bones, which are integumental, and the notochord, which has
already been accounted for.

Convincing as these comparisons seem when taken by them-
selves, the influence of later investigation has tended rather
away from the annelid hypothesis, and at present, although
there are many investigators who seek the ancestor of verte-
brates in some worm-like form, there are few who wish to defi-
nitely 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 segmenta-

520 HISTORY OF THE HUMAN BODY

tion is not in itself as fundamental a character as would 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 vertebrates may
have been gained through the muscular action of a previously
unsegmented form, and the fact that in vertebrate 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 budding, a
process totally unlike that in which the vertebrate initiates its
segmentation.

A second group of vermian forms from which the vertebrates
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 im-
bedded 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. 141, A). A branching intesti-
nal nerve proceeds from one of these and is distributed to the
sides of the intestine; and from the ventraLportion 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. 141, the first of which
has already been referred to. Of the three longitudinal nerves
the dorsal one has become the central nervous system, and has
expanded its anterior end into a brain, while the two lateral
nerves 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 intestinal nerve is also asso-
ciated. The original ganglion of the lateral nerves breaks up

THE ANCESTRY OF - THE VERTEBRATES 521

into the various ganglia found in vertebrates, in association
with the cranial nerves. As for the commissural nerves, they
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 sympathetic system is formed by collect-

B

FIG. 141. 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.

ing together the small intestinal branches that come from the
ventral portions o'f 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 structure, the
proboscis, has been likened to the hypophysis, while its sheath,
into which it may be retracted, has been cited as possibly fur-

522 HISTORY OF THE HUMAN BODY

nishing 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 con-
sideration of a worm-like ancestor may be briefly mentioned 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 for-
merly exhibited by the extinct group of Merostomata. To ap-
preciate this one must at the outset dispose of the cyclostomes
and 'other low forms like Amphioxus as degenerate and with-
out special significance, and take as the starting point of verte-
brates such forms as the ganoids, or more especially the placo-
derms, 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 gangl ionic
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 tfie conclusion of
many specialists concerning the segmental values of those parts
of the head in vertebrates. A similarly suggestive resemblance
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
arachnoids now living. Aside from the nervous system,

THE ANCESTRY OF THE VERTEBRATES 523

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 ap-
proximated median eyes, a peculiarity seen in the present-
day Limnlits; in the vertebrates there are the same widely
divergent lateral eyes, and the median pair is well repre-
sented by 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

•V

FIG. 142. Comparison of heart and gill arches of (a) Arachnoid, and
<b) Vertebrate. [After PATTEN.]

an internal skeletal piece, variously called " sternum " and
" endocranium," which in general form and still more in its re-
lationships 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-
collected into two lateral channels, in the one case a sinus,

HISTORY OF THE HUMAN BODY

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, do actually represent the
mandibular arch, and that the next pair, or perhaps the next
two, may represent the hyoid. The vertebrate gills them-

FIG. 143. Comparison between extinct Crustacean and Vertebrate.

(A) Gigantostracan, Eurypterus [after NIEDZKOWSKY]. (B) Placoderm, Pterichthys
[after NEUMAYER].

selves 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
precisely the ones which, for other reasons, have been com-
pared with Vagus elements.

Perhaps one of the strong points in this erratic theory lies
in the fact that the vertebrates are here derived, not from a
primitive segmented form, like an annelid, in which the somites

THE ANCESTRY OF THE VERTEBRATES 525

are much alike, but from one in which the differentiation of
the head somites and their grouping 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 arach-
noids 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. 143).

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 position of the
central nervous system, and by means of these has traced the
line of vertebrate ancestry through a series of invertebrate'

"Y

FIG 144. Amphioxus.

o, oral hood with cirrhi; x, mouth; g, gonads; y, atriopore; 2, anus; t, caudal fin;
f, dorsal fin.

lorms, externally very unlike one another, and each some-
what 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 membranous
expansion in the form of a hood. The adult burrows perpen-
dicularly into the sand, leaving exposed only the anterior end,
and in this temporarily sessile condition it expands the oral
hood and collects the debris that drifts past it, much after

526 HISTORY OF THE HUMAN BODY

the manner of other non-locomotive forms. The larva is, how-
ever, 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 myocom-
mata, each bent in the form of a V, the point directed for-
wards. The reproductive organs or gonads, also, are often
sufficiently developed to appear through the skin as a succession
of square or slightly rounded masses, each corresponding 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 termina-
tion 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 atripore leads is termed peribranchial, and appears at first
like an internal cavity; a little investigation, however, 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 " branchi-
pore," 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 frog tadpole, this
chamber has been developed as a special adaptation to the

THE ANCESTRY OF THE VERTEBRATES 527

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 probably
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. Chap-
ter V.]

Of the internal organs the most conspicuous is the noto-
chord. This is an elastic skeletal rod of gelatinous tissue, sur-
rounded 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 enclosed in a
sheath which is continuous with that of the notochord, 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 differentiation of parts. The pharynx
is perforated by 60-80 pairs of narrow gill-slits, placed ob-
liquely, and kept open by an elaborate system of skeletal rods,
formed of a material resembling chitin, and thus more like
an invertebrate than a vertebrate structure. Both the elab-
orateness of this skeletal system and the very large number of
gill-slits are plainly secondary modifications, like the peri-
branchial chamber, since they are not found in the larva, and
mark Amphioxus as a much modified form, probably that one
out of a large class which survived on account of these very
modifications.

A characteristic structure runs along the floor of the

528 HISTORY OF THE HUMAN BODY

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 homolog of the thyreoid series, and the
numerous thymus anlagen have been rather hesitatingly identi-
fied with certain elements of the gill skeleton.

THE ANCESTRY OF THE VERTEBRATES 529

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 other invertebrates? 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 secondarv modifi-
cations, 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 segmentation,
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 characteristic
of the group from which Amphioxus and its allies have come.

The direction of the search thus defined, one is led inevitably
to another isolated and problematic group of invertebrates,
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 sev-
eral Orders and numerous Families, thus showing all possible
modifications of their fundamental plan.

This group is that of the Tunicata, a Class of marine ani-
mals, some of which are free-swimming throughout life, while

530

HISTORY OF THE HUMAN BODY

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 characteristics,
while the free-swimming ones are often greatly modified. In
a typical sessile tunicate, like the one shown in Fig. 145, a,
the delicate and rather complex organism is shut within a
tough and often wrinkled external coat or tunic, in which
there appear but two definite structures, an incurrent and ex-
current orifice, through which the water is continually driven,

a

FIG. 145. Typical Tunicate.

(a) External view, (b) Diagram of internal anatomy.

IN, incurrent; and EX, excurrent canals; Ph, pharynx; INT, intestine; Ant
anus; CL, cloacal chamber; Tun, tunic; Integ, integument; End, endostyle; GL,
ganglion; G, gonadic gland; CD, gonadic duct; GO, gonadic opening; H, heart.

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 phar-
ynx. The anus opens, not directly to the exterior, but into a
cloacal chamber, which is really outside of the animal but en-
closed within the outer tunic. This same chamber surrounds
the pharynx and receives the water taken into the latter
through a series of gill-slits, which, in larvse, are few in num-

THE ANCESTRY OF THE VERTEBRATES 531

ber and disposed in lateral rows, but which usually become al-
most indefinitely multiplied, and in many cases transform the
entire pharyngeal wall into a structure resembling basket-work.
Thus the cloacal chamber, except for the relation of the anal
outlet, is precisely similar to the peribranchial chamber of
Amphioxus, the excurrent orifice being the equivalent 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 system is represented by
a ventrally placed heart. The reproductive 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. 146, 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 de-
velopment 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
sessile position are shown in Fig. 146, 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.

532 HISTORY OF THE HUMAN BODY

a INT \ Br

Br.

FIG. 146. Development of Tunicate from free-swimming larva. [After
SEELIGER.]

p', p, adhesive suckers; Br, brain; C, nerve cord; INT, 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 533

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 nervous
system ? And as an answer to this, a very incomplete and un-
certain one at best, there is only a single animal form, though
represented by several closely allied species, an animal as un-
promising 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-con-
structed tubes of sand between tide-waters, and, like Amphi-
o.nts, 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 unearthed
by the burrowing action of the proboscis. The pharynx is
quite extended and communicates directly with the exterior

534

HISTORY OF THE HUMAN BODY

through two lateral rows of paired gill-slits, which are sup-
ported by a branchial skeleton very much like that of Am-
phwxus.

This characteristic of the possession of pharyngeal gill-
slits is extremely significant, for nowhere else, except in ver-
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-

FIG. 147. Balanoglossus. [After Loos, in LEUCKART charts.]

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
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

THE ANCESTRY OF >HE VERTEBRATES 535

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 continues
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
of the pharynx, and extends some distance forward into the
proboscis, and this supposition has been greatly strengthened
through the recent discovery of an allied form belonging to
a new genus (Harrimania) in which the diverticulum is much

es- Lro

FIG. 148. 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, oesophagus; es. No-c, cesophageal notochord; d. n. c,
dorsal nerve cord; SK. C, skeletal crura; br. o, branchial orifices.

larger, and in its mode of origin 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 line
of vertebrate descent, and represent an earlier stage than that
of the tunicates. But here the chain seems to end, for Balano-
glossus is itself unusually isolated and shows no close affinity
to any other invertebrate types. There is, in such cases, but

536

HISTORY OF THE HUMAN BODY

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
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 race-
history, but if so, it leads us to surprising and not very satis-
fying results, for the animal begins life as a minute transparent
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

A

B

FIG. 149. 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.

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
an early stage in the history of the Class, and that the ancestors
of these radiate, crawling, or sessile, bottom forms were bi-
lateral and pelagic. Now it would be highly improbable that

THE ANCESTRY OF THE VERTEBRATES 537

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 verte-
brates the form which all these larvce may be said to copy; a
form having the characteristics common to all, including bi-
laterally, minute size, transparency, locomotion by bands of
cilia, and pelagic life. The lineal descendants of this hypotheti-
cal ancestor chose two paths, the one leading to the Echino-
dermata, the other to Balanoglossus, the Tunicata, Amphi-
oxus, and eventually the Vertebrata.

This theory, although incomplete and unsatisfactory in

B

FIG. 150. 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.

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 acceptance
of this gives us very little enlightenment, to abandon it would
be to sacrifice but little, and would render the gulf between
the adult Balanoglossus and other invertebrates only a little
more profound. To summarize in the words of two recent

538 HISTORY OF THE HUMAN BODY

authors,* " The question of the descent of the Chordata is not
solved by accepting their relationship to the Enteropneusta,
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 echinoderms. 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
unknown to us."

* Korschelt u. Heider. Entwickelungsgeschichte. Jena, 1893, p. 1465.
"Die Frage nach der Abstammung der Chordaten wird durch die An-
nahme von verwandtschaftlichen Beziehungen derselben zu den Enterop-
neusten 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 Balan-
oglossus 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 the 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 i. Heterostraci (Pteraspis).
' Order 2. Osteostraci (Cephalaspis).

Order 3. Antiarchi (Pterichthys).

539

540 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 (P I eur acanthus).
Order 3. Acanthodii (A cant hod es, Diplacanthus).
Order 4. Selachii.

Sub-Order I. Squall (sharks, dog-fish).
Sub-Order 2. Raiae (skates).
Sub-Class II. Holocephali.*

Order i. Chimaeroidei * (Chimcera).
Sub-Class III. Dipnoi.*

Order i. Sirenoidei * (Protopterus, Ceratodus).
Order 2. Arthrodira (Coccosteus, Dinichthys).
Sub-Class IV. Teleostomi.
Order i. Crossopterygii.*

Sub-Order i. Haplistia (Tarrasius).

Sub-Order 2. Rhipidistia (Holoptychius; Oste-

olepis).

Sub-Order 3. Actinistia (Ccelacanthus ; Undina).
Sub-Order 4. Polypteroidei * (Polypterus; Cala-

moichthys).
Order 2. Actinopterygii.

Sub-Order i. Chondrostei * (Palaoniscus ; Aci-

penser, sturgeon).
Sub-Order 2. Protospondyli * (Lepidottts; Eu-

gnathus; Amia, bow-fin).
Sub-Order 3. ^Etheospondyli * (Aspidorhynchiis;

Lepisosteus, 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 541

Sub-Order 10. Lophobranchii (sea-horse; pipe-
fish).

Sub-Order n. Hemibranchii (stickleback).

Sub-Order 12. Acanthopteri (mackerel; cod;
perch; sculpin; flounder).

Sub-Order 13. Pediculati (angler-fish; 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 elasmobranchSj of the second
and third the only fossils known are much like those of
the present day and their affinities have thus 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 sEtheospondyli. 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 i. Urodela.

Sub-Order i. Perennibranchiata (Necturus; Si-
ren).

Sub-Order 2. Derotremata (Cryptobranchus).
Sub-Order 3. Salamandrida (newts; salaman-
ders).

Order 2. Gymnophiona (subterranean forms, with-
out limbs or eyes).
Order 3. Anura.

Sub-Order i. Aglossa * (Surinam toad).
Sub-Order 2. Arcifera (toads; tree-toads).
Sub-Order 3. Firmisternia (frogs).

542

HISTORY OF THE HUMAN BODY

Order 4. Stegocephali.

Sub-Order I. Branchiosauria (Branchiosaurus) .

Sub-Order 2. Aistopoda (snake-like forms, with-
out limbs).

Sub- Order 3. Microsauria (small forms, in shape
like salamanders).

Sub-Order 4. Labyrinthodontia (Archcegosaurus,
Mastodonsaurus) .

[Of the four Orders of Amphibia, one is entirely extinct
and the other three essentially modern, and with few
traces of older representatives. With regard to the ex-
tinct group, that of Stegocephali, it could be placed in
the list either at the first or the last, since it shows strong
affinities to both ganoids and reptiles and thus lies inter-
mediate between the two. The modern Orders seem to
have arisen directly from the Stegocephali, the Urodela
being the least altered and hence the most important
morphologically. This complex relationship between the
groups mentioned, the Ganoidei, Stegocephali, Urodela,
Reptilia, etc., cannot thus be represented in linear lines
but may be partly expressed in the form of a tree, as in
the diagram given in Chapter II. (p. 28). The three modern
Orders of Urodela, Gymnophiona, and Anura, are quite
distinct from one another, and, the first and third espe-
cially, are well represented in the living fauna.]

Super-Class II. Sauropsida.
Class III. Reptilia.

Order i. Theromorpha.

Sub-Order i. Pariasauria (Pariasaurus).
Sub-Order 2. Theriodontia (Cynognathus; Trity-

lodon).
Sub-Order 3. Dicynodontia (Dicynodon; Gordo-

nia).
Order 2. Sauropterygia (Pleisiosaurus; Cryptocli-

dus).
Order 3. Chelonia.

Sub-Order i. Cryptodira (the majority of living

turtles).

Sub-Order 2. Pleurodira (Miolania; Chelys).
Sub-Order 3. Trionychia (soft-shelled turtles).
Order 4. Ichthyopterygia (Ichthyosaurus).

APPENDIX 543

Order 5. Rhynchocephalia.*

Sub-Order i. Proterosauria (Proterosaurus ; Pa-

Iceohatteria) .

Sub-Order 2. Rhynchocephalia vera * (Sphe no-
don [Hatteria], the only living
representative of the Order).
Order 6. Squamata.

Sub-Order I. Dolichosauria (Dolichosaurus).
Sub-Order 2. Pythonomorpha (Mosasaurus).
Sub-Order 3. Lacertilia (lizards).
Sub-Order 4. Ophidia (snakes).
Order 7. Dinosauria.

Sub-Order I. Theropoda (Anchisaurus) .
Sub-Order 2. Sauropoda (Brontosaurus).
Sub-Order 3. Ornithopoda (Iguanodon; Stego-

saurus).
Order 8. Crocodilia.*

Sub-Order i. Parasuchia (Belodon).

Sub-Order 2. Mesosuchia (Pelagosaurus; Teleo-

saurus) .
Sub-Order 3. Eusuchia * (Thoracosaurus ; Croco-

dilus; Alligator).

Order 9. Pterosauria (Pterodactylus; Rhamporhyn-
chus).

[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; (i) the Chelonia, essentially a modern
group, (2) the Rhynchocephalia, represented by a single
living species, (3) the last two Sub-Orders of the Squamata,
the lizards and snakes, and (4) a very few modern representa-
tives or the Crocodilia. In arrangements in which living forms
are alone taken into consideration, these are given as five
Orders; the lizards and snakes count as two; in the earlier
works Sphenodon was counted among the lizards, redr.cing
the number to four. The Orders, as thus arranged, are as
follows : Chelonia, Rhynchocephalia, Lacertilia f Ophidia,
Crocodilia.}

Class IV. Aves.

Order i. Saururae (Arch&opteryx; Laopteryx).
Order 2. Odontormae (Ichthyornis; Apatornis).

544 HISTORY OF THE HUMAN BODY

Order 3. Odontoholcse (Hesperornis; Lestornis).
Order 4. Eurhipidurae.

Sub-Order i. Dromaeognathi.

Section I. Struthiones (ostrich, casuary).
Section II. Aepiornithes (^piornis).
Section III. Apteryges * (Apteryx).
Section IV. Crypturi (tinamoo, 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) .

Section II. Schizognathae (grebes; loons; gulls;
snipes; grouse; quails; pigeons;
humming-birds ; woodpeckers ) .
Section III. .^Egithognathae (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; Polymasto-

don).

Order 3. Monotremata * (Ornithorhynchus; Echid-
na).

Sub-Class II. Eutheria.
Super-Order i. Didelphia [Marsupialia].
Order I. Polyprotodontia (opossum; Thylacinus).
Order 2. Paucituberculata * (Ccenolestes).
Order 3. Diprotodontia (Petaurus; wombat; kan-
garoo).

Super-Order 2. Monodelphia [Placentalia].
Order i. Insectivora (moles; shrews; hedgehog).
Order 2. Cheiroptera (bats).
Order 3. Galeopithecidae * (flying lemur).
Order 4. Edentata.

APPENDIX 545

Sub-Order I. 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; Megathe-
rium; Grypotherium; ant-eaters;
armadilloes; sloths).
Order 5. Rodentia.

Sub-Order i. Tillodontia ( Tillotherium; Es-

thonyx) .

Sub-Order 2. Duplicidentata (hares; rabbits).
Sub-Order 3. Simplicidentata (squirrels; beavers;

mice).
Order 6. Primates.

Sub-Order i. Mesodonta (Adapis; Anaptomor-

phus).
Sub-Order 2. Lemuroidea (Lemurs; Chiromys;

Tar si us).

Sub-Order 3. Anthropoidea.
Division i. Platyrrhini (Hapale; Midas; Ce-

bus).
Division 2. Catarrhini (Cercopithecus; Semno-

pithecus; Gorilla; Homo).
Order 7. Creodonta (Arctocyon; Hyanodon).
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 (Palaotherium; Titano-

therium; rhinoceros; horse).
Order 21. Artiodactyla.

546 HISTORY OF THE HUMAN BODY

Sub-Order i. Suina (Elotherium; pigs; peccaries;
hippopotamus).

Sub-Order 2. Tylopoda (Oreodon; camel; llama).

Sub-Order 3. Anthracotherioidea (Anthrac other i-
um).

Sub-Order 4. Dichobunoidea (Dichobune; Ano-
plotherium).

Sub-Order 5. Traguloidea (Tragulus, several
small species in E. Indies).

Sub-Order 6. Pecora (deer; sheep; cattle; gi-
raffe).

[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 tree given in Chapter II. (p. 36). 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 Inscctivora is taken first ; then that
of the Primates, and lastly the complex and highly spe-
cialized branch leading- to the carnivore and ungulate
Orders. This arrangement has the advantage of empha-
sizing the primitive and rather generalized structure of
the Primates as compared with the groups just men-
tioned, 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 ques-
tion 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
Primates are found to have retained a larger number of
primitive characters than any other placenta! group with
the exception of the Insectivora, Rodentia, and Edentata,
and thus to stand far loivcr in the scale of specialisation
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 im-
portant modifications have, however, been recently pro-

APPENDIX

547

posed, based upon a more complete study of anatomical
characters. Through these the extinct genus Anaptomor-
phus, 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 sepa-
rate, though allied group. The Galeopithecoidea are often
included within the Insectivora. This reduces the Orders
of placenta! mammals to nine, viz : Insectivora, Cheiroptera,
Edentata, Rodentia, Primates, Ungulata, Sirenia, Cetacea,
Carnivora. There are, of course, as in all groups of ani-
mals, many other possible arrangements, the differences
being based on the relative value of the various groups,
their relationships to one another, and the comparative de-
gree 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 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 im-
portant 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 abbreviated classification, in some respects
different from the above synopsis, may be given here for con-
venience in comparison. Only the gnathostomes may be con-
sidered.

548 HISTORY OF THE HUMAN BODY

SYNOPSIS OF VERTEBRATA (living forms alone).

Class I. Pisces.

Sub-Class I. Selachii.

Sub-Class II. Holocephali (often considered with the

previous 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 i. Chelonia.

Order 2. Lacertilia (including Sphenodon).
Order 3. Ophidia.
Order 4. Crocodilia.
Class IV. Aves.

Sub-Class I. Ratitse (running birds; with flat breast-
bone, e.g. Ostrich).
Sub-Class II. Carinatse (flying birds; with keeled

breast-bone).
Class V. Mammalia.

Sub-Class I. Prototheria.
Order i. Monotremata.
Sub-Class II. Eutheria.
Super-Order i. Didelphia (Marsupialia).
Super-Order 2. Monodelphia (Placentalia).
Order i. 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).

APPENDIX 549

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 Or-
ders, 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 anat-
omy, and is employed for this purpose in the body of this work.
The arrangement of the mammals to show the supremacy of
Man is natural, and is based, of course, in part, on the high de-
velopment of the brain, but much is due to natural human pride
which recognizes man's mental supremacy, and feels that a su-
premacy 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

(In "using this index consult, for a given animal, both the sci-
entific and common names.)

abdominal ribs, 141
abomasus, 292
achselbogen, 251
acinous glands, 97, 112
Acipenser, 171
acoelous vertebrae, 131
acoustic hairs, 467
acoustic maculae, 409
adenoid tissue, 362
afferent nerves, 408, 435
air-bladder, 270, 310, 311
air-cells, 314
albatross, 478
alimentary canal, 258, 259,

261, 262, 263, 264, 265, 266,

299, 365

alisphenoids, 148
allantoic arteries, 70, 71
allantoic veins, 70, 71
allantois, 70, 71, 322, 323, 324,

377, 378

alveoli, of jaws, 273

alveoli, of lungs, 314

Amblypoda, 38

Ammocoetes, 477

Ammon's horn, 417

amnion, 70, 71

Amniota, 19, 70, 78, 174, 373,

378, 383, 384, 385, 393, 427,
amniotic fluid, 70, 71
amphibians, no, 148, 166, 170,

174, 178, 200, 202, 203, 220,
239, 245, 253, 271, 272, 283,
288, 291, 294, 296, 308, 310,
322, 328, 331, 344, 346, 347,
353, 356, 358, 369, 375, 383,
392, 398, 409, 415, 416, 419,
427, 439, 444, 456, 464, 465,

amphibians — (Continued)
472, 479, 484, 488, 489, 491, 504,
5i8

amphiccelous vertebrae, 127
Amphioxus, 24, 26, 27, 28, 29, 44,
59, 61, 63, 66, 67, 76, 123, 143,
152, 154, 162, 163, 200, 260, 267,
289, 290, 303, 305, 306, 353, 365,
411, 432, 436, 443, 445, 460, 461,
476, 503, 515, 517, 521, 525, 526,
527, 528, 529, 531, 533, 534, 537
amphirrhine condition, 476
Amphiuma, 305, 313
amplexation, 383, 397
ampullae, of semicircular canals, 487
ampullae, of slime canals, 471
260, anal fin, 164
298, anal sacs, 113

Anamnia, 70, 78, 384
Anarrhichas, 489
Ancylopoda, 38
angulare, 156
347, annelids, 368, 460, 512, 513, 514,

515, 517, 518, 519, 520, 524
annelid theory of vertebrate ances-
try, 513, 520
ansa hypoglossi, 458
ant-eater, 316, 393
anterior girdle, 130
anthropoids, 38, 41, 43, 45, 90, 214,
375, 226, 230, 251, 256, 298, 409, 447,
457 480, 496, 500, 504

antitropists, 244

172, antrnm of Highmore, 482
237, Anura, 31, 220, 313, 355, 486, 493,
285, 495

319, anus, 258, 259, 296
352, aorta, aortse, 320, 326, 339, 351, 354
385, apes, 45, 214, 223, 229, 253, 297,
425, 390
469, apical pads, 91

551

552

INDEX

appendages, 510

appendicular muscles, 190, 192, 193

appendictilar skeleton, 122, 162

appendix, 297, 298

appendix testis, 392, 394

aqueductus cerebri, 412

aqueductus Sylvii, 412

aqueductus vestibuli, 486

aqueous humor, of eye, 501

arachnoids, 522, "524

arachnoid theory of vertebrate an-
cestry, 522, 525

Archaeopteryx, 19, 32

archencephalon, 411, 443, 527

archetype, 500, 509

archipterygium, 176, 185

archisternum, 138, 141

area centralis, of retina, 499

armadillo, 85, 393

Artemia, 55

arterial arches, 320, 321, 328-339

artery, or arteries: — 318, 328; al-
lantoic, 322; anonyma, 332;
aorta, aortse, 320, 326, 339, 351,
354; aortic arch, 331, 356; affer-
ent branchials, 325, 329; arterial
arches, 320, 321, 328; branchials,
afferent, 325, 329; branchials,
efferent, 326; carotid, 321, 326,
332-339; carotis cerebralis, 333,
338; caudal, 339; common
carotid, 338; ductus arteriosus,
331 ; ductus botalli, 331 ; efferent
branchials, 326; external carotid,
338; hypoglossal, 335; iliac, 322,
326, 339; innominata, 332; in-
fra-orbital, 335; intercostal, 339;
internal carotid, 338; liga-
mentum arteriosum, 330; liga-
mentum botalli, 330, 331; lingu-
alis, 3355 lumbar, 339; man-
dibularis, 335, 338; maxillaris,
335; mesenteric, 339, 340; poste-
rior aorta, 320; pulmocuta-
neous, 330; pulmonary, 330; 354;
sacralis media, 339, 340; seg-
mental, 335; stapedialis, 159,
337, 338; subclaviae secundarise,
331; subclavian, 322, 326, 332,
339; supra-orbital, 335; umbili-
cal, 322; vertebralis cerebralis.^
335-

articulare, 159, 495

articulates, 258, 259, 463

articulation of jaw, 450

Artiodactyla, 39

arytaenoids, 160, 313

Ascalabotae, 487

Ascaris, 54

assimilation, 2

Ateles, 254

atlas, 133

atriopore, of Amphioxus, 526 »

atrium, 324, 353, 357

auditory hairs, 488

auditory ossicles, 159

auditory tube, 494

auricula, 496, 497

Auricularia, 536, 537

axial muscles, 190, 192, 200, 217

axial skeleton, 122

axillary arch, 251

axis, 133

axolotl, 440

B

babyroussa, 274

Balanoglossus, 26, 303, 533, 534,
535, 536, 537

basihyal, 160, 284

basioccipital, 150

basipterygium, 169, 175

basisphenoid, 150

bat, 183, 184, 292, 394, 433, 496

bats, insectivorous, 301

beak: — of birds, 84, 105; of tur-
tles, 84.

beaker cells, no

belly, of a muscle, 197

biogenesis, law of, 15, 58

Bipinnaria, 536, 537

birds, in, 172, 174, 182, 183, 184, 201,
202, 203, 220, 249, 253, 263, 268,
272, 290, 296, 315, 352, 353, 354,
355, 356, 360, 375, 377, 3§5, 386,
398, 414, 415, 416, 419, 426, 437,
439, 465, 475, 483, 488, 489, 491,
495, 500, 504

birds, toothed, 272

bisexual, 49

bladder, 377, 378

blastocoele, 59

blastodermic vesicle, 73

blastula, 58

INDEX

553

blood, 318, 356

blood vessels, 65, 79, 318

body axis, 60, 61

body, of a vertebra, 127

body wall, 365

bone complexes of skull, 151

bone, or bones (see skeletal ele-
ments)

bony labyrinth, 492, 493

Bovidae, 276

Bowman's capsule, 373, 374

brachial plexus, 438, 439, 440, 441

brain, communication with out-
side world, 409

brain, development of, 410, 427

branchial arches, 155

branchial system, 260

Branchiostoma (see Amphioxus)

broad ligament, of uterus, 387, 390

bronchi, origin of, 270

bronchioli, 315

.Bubo, 489

buccal glands, 285

bulbo-urethral glands, 403

bull-heads, 475

bursa inguinalis, 395

bursa ovarica, 387

caducibranchiate amphibians, 307
caenogenetic characters, 16
canalis centralis, 62
canalis centralis, of spinal cord,

406

canals of Lorenzini, 469, 473, 502
Canidae, 286
canines, 275
Canis, 87
capillaries, 318
carapace : — armadillo, 85 ; turtles,

105

Carchesium, 8

cardiac end of stomach, 291

Carnivora, 37, 38, 175, 239, 250,

291, 394, 402, 417, 480, 484
carotid gland, 288
carpus : — nomenclature of, I77>

178, 179, 180; various forms of,

179; primitive condition of, 179;

supernumerary elements, 181.
cartilage, or cartilages (see skeletal

elements)

cartilage bones, 148

cartilage lateralis, 160

cassowary, 440

cat, 271, 276, 287, 419, 491

Catarrhini, 41, 278

cauda equina, 431

caudal fin, 164

caudal vertebrae, 130

cavernous tissue, 398, 399, 404

cell membrane, 3

centers of ossification, 83, 148

centrifugal nerves, 435

centripetal nerves, 408, 435

centrum of a vertebra, 127, 508,

5io

Cephalophus, 112
cephalopod eye, 499
cephalopods, 499, 501
cerato-hyal, 160, 284
Cercopithecidae, 45
cerebellum, 412, 417, 425, 426
cerebral hemispheres, 411, 413, 416
cerebrum, 411, 415-418, 425
cervical fistula, 269
cervical intumescence, in cord, 433
cervical rib, 138
cervical vertebrae, 130
Cervus, 299
Cestracion, 186
Cetacea, 20, 38, 40, 98, 113, 116,

175, 184, 275, 277, 285, 300, 316,

393

Chelonia, 33, 44
Chimaera, 489
chimpanzee, 45
chiridium, derivation from fin, 186,

220

Chironomus, 429
Chiroptera, 37
chiropterygium, 167, 184
chiropterygium, typical form of,

177
chiropterygium vs. ichthyoptery-

gium, 184-188
Chlamydoselachus, 186
choanae, 269, 478
Choloepus, 112, 113
chondrocranium, 145, 146, 153, 156
chordo gubernaculi, 395
Chordata, 538

chorioid plexuses, 413, 419, 420
chorion, 70, 71, 72

554

INDEX

chorionic villi, 70, 71
chorioid coat of eye, 498, 501
chorioid fissure of eye, 500
chromatin, 54, 58
chromosomes 54, 58
chromosomes: — number of, 54, 55,

57

Chrysochloris, 1 12, 113
ciliary ganglion, 447
ciliary glands, 505
circulation, in selachians, 324, 325
circumvallate papillae, 476
cisternae chyli, 362
clavicle, 173, 174, 175
claws, 105, 106, 107
cleithrum, 173
clitoris, 398, 404
cloaca, 266, 296, 370, 375, 378, 381,

383, 403

cloacal cceca, 266
cloacal glands, no
closed type of circulation, 317, 318
Cobitis, 303
coccyx, 135
cochlea, 488, 491, 492
cceca, of intestine, 296, 297, 298
Coelenterata, 59, 60
Ccelogenys, 86, 491
ccelom, 64, 66, 258, 365, 368, 369,

37i, 373, 374, 375, 376, 378, 381,

382

colic cceca, 266
colon, 298

colon labyrinths, 298
columella auris, 494
columns, of spinal cord, 434
commissures, of brain, 418, 427
concha, of ear, 161
conchae, of nose, 480-482
concrescence theory, 280
Condylarthra, 38, 40
conjunctiva, 498, 504
contact sense, 468
continuity of germ plasm, 58
continuity of life, 12
conjugation, 6, 49, 50, 51
conus arteriosus, 324, 353
conus inguinalis, 395
Cooper's fascia, 396
convolutions, of brain, 417
copulation, 49, 380, 381, 398

coracoid, 174

coracoid process, 175

coral polyps, 60

corium, 76, 78

cornea, 501

corpora bigemina, 425

corpora cavernosa, 399, 404

corpora striata, 413, 416, 426

corpora quadrigemina, 425

corpus callosum, 418

corpus cavernosum urethrse, 404

corpus fibrosum, 398

corpus spongiosum, 404

cortex cerebri, 417

costal cartilages, 136

cotyledonal placenta, 72

cranial nerves, 427, 442-458

cremaster, 396

Creodonta, 37, 40

cricoid, 314

cristae acusticae, 490

Crocidura, 92

crocodile, 164, 272, 360, 377, 398,
399, 483, 491, 495

crop, 290

crura cerebri, 426, 427

crustaceans, 259, 463

Cryptobranchus, 31, 235, 305

crystalline lens, 423, 498, 501, 502

ctenoid scales, 83

cuticula of invertebrates, 76

cutis, 76

Cuvier, theories of types, 506, 507

cyanosis, 357

cycloid scales, 83

Cyclops, 55

cyclostoma, 266

cyclostomes, 28, 29, 44, 49, 200,
267, 286, 287, 289, 305, 347, 365,
369, 379, 38o, 411, 420, 433, 437,
444, 461, 487, 488, 489, 490, 516

D

Darwin, Charles : — theory of de-
scent, 510, 512
Darwin's point, on ear, 497
Dasyurus, 86
decidua, 72
deciduate placenta, 73
deer, 299
Delphinus, 277

INDEX

555

dental formulae, 276-277

dermal bones, 79, 82, 146, 147, 156

dermal scutes, 79, 83, 147

dentary, 156

dentine, 79

dermal canal system, 448

descensus ovariorum, 392

descenscs testiculorum, 203, 392,

393

Desmodus, 292
Desmognathns, 309, 343
developmental history, 15
development of hair, 96
devil-fish, 499
diaphragm, 316
diapophyses, 138, 508, 510
Didelphia, 36

diencephalon, 411, 418-425, 443
diffuse placenta, 72
digestive cavity, 258, 315
digestive glands, 262
digits: — names of, 177; number of,

182 ; reduction of, 182.
dinosaurs, 32
diphyodont dentition, 280
dipnoans, 30, 176, 449, 453, 454,

457, 478

discoidal placenta, 72
dissepiments, 367
diverticula of intestine, 266
dog, 271, 286
dog-fish, 414, 415, 419, 446, 461,

478

dorsal fin, 164
dorsal nerves, 436
dragon-fly, 428
drum of ear, 493-496
Dryopithecus, 44
duck, 296

duck-billed platypus, 33
duct of Cuvier, 322
ductns arteriosus, 331
ductus Botalli, 331
ductus Cuvieri, 322
ductus deferens, 377, 383, 385, 386,

392, 393, 394, 401
ductus endolymphaticus, 486
ductus venosus Arantii, 348
duodenum, 266, 294
duplex placenta, 72
Duplicidentata, 37

E

ear, 485-497

ear, of anthropoid, 496-497

ear, external, 161, 496, 497

earth-worm, 460

ecdyses, 77 ; of epitrichium, 95

Echidna, mammary pocket of, 114

Echidna, 33, 288

echinoderms, 26, 258, 536-538

ectoderm, 60, 63, 257

ectoturbinalia, 480

Edentata, 37, 291

eel, 432

efferent nerves, 435

eggs, 50, 51, 52, 53, 56, 57, 69, 73

elephant, 274, 484

enamel, 79

endochondral ossification, 148

endoderm, 60, 63, 257

endolymph, 488, 492

endolymphatic space, 357

endolymphatic cavity, 493

endolymphatic duct, 486

endoskeleton, 122

endoskeleton, parts of, 122

endostyle, 289, 528, 531

endoturbinalia, 480

Enteropneusta, 260, 535, 538

entocone, 279

epi-hyal, 160, 284

epidermic warts, 88, 89, 90

epidermis, 76, 408

epididymis, 385, 386, 394

epiglottis, 160, 313

epimere, 64, 65

epimeres, 190, 245

epimeric muscles, 190, 192

epi-otics, 148

epiphysis, 420, 421, 422

episternum, 141

epithelial bodies, 363

epithelial corpuscles, 286

epitrichium, 95

eponychium, 95

epoophoron, 391, 394

Equus, 390

erectile tissue, 398, 399

Erinaceus, 402, 491

erythrocytes, 318, 364

ethmoid, 148

ethmo-turbinal, 479, 480

556

INDEX

Eurypterus, 524

Eustachian tube, 269, 494

Eutheria, 35

excretory tubules, 365, 366

exoccipitals, 148

exoskeleton, 79

exoskeleton of reptiles and birds,

84

external ear, 161, 496, 497
external genitals : — male, 398-401 ;
female, 403 ; development of,

403-405.
exuviae, 77
external nose, 484
eye, 497-505
eyeball, muscles of, 503
eyeball, skeletal elements of, 144,

145, 151

eyeball, size of, 503
eyebrows, 505
eye-capsules, 145
eye, development of, 422, 423
eyelids, 498, 503, 504
eye of cephalopod, 499

Fallopian tube, 387

fascia cremasterica, 396

femoral glands, of lizard, no, III

fenestra cochleae, 494

fenestra ovalis, of ear, 493

fertilization, 50, 379, 397, 398

fetal circulation, 320-324, 332-339,
342-352, 357

field-mouse, 91

fifth ventricle, 418

filum terminale, 431

fins of fishes, 164, 167

fin, development of, 168, 218-219

fin-fold, 327

fin-fold theory, 163, 238

fin rays vs. digits, 184, 185

fin spines, 162

fin vs. hand, 184-188

fishes, 29, 30, no, 148, 163, 164,
165, 167, 175, 192, 200, 245, 247,
262, 263, 268, 271, 272, 285, 291,
296, 304, 306, 307, 310, 311, 324,
330, 339, 342, 347, 352, 353, 355,
358, 375, 409, 425, 436, 437, 452,
460, 465, 468, 469, 471, 484, 485,
488, 490, 491, 503, 517, 519, 527-

fission, 4, 5

fiss'ues, of brain, 417

fissue of Rolando, 417

fissue of Sylvins, 417

flat-worms, 520

flexures, in brain, 414

fly, 428

fly, nervous system of, 428

foliate papillae, 476

foramen epiploicum, 294

foramen interventriculare, 411, 416,
420

foramen of Monro, 411

foramen ovale (of heart), 357

foramina of Majendie, 413

forebrain, 411, 443

fossa rhomboidalis, 427

fourth ventricle, of brain, 412

fovea centralis, of retina, 499

free-limbs: — development of, 218,
219; early history of, 175, 176,
177 ; modifications of, 165, 182,
183, 184; nomenclature of, 178;
origin of, 165 ; serial homology
of, 178; typical skeleton of, 176.

free nerve endings, 473

friction ridges, 90, 107 ; formation
of, 88, 89, 90; ground plan of,
92; relation to pads, 90, 91.

friction ridge patterns, 92, 93

frog, 198, 263, 264, 285, 290, 307,
313, 359, 37i, 397, 420, 426, 428,
430, 439, 440, 489, 526

frontal organ, 420

fundus of stomach, 292

furcula, 174

galea aponeurotica, 255

Galeopithecus, 37

gametes, 7, 49, 55

ganglion, or ganglia : — buccale,
456; ciliary, 447, 452, 464; gas-
serian, 448, 456; geniculare, 454,
456; jugulare, 453, 456; laterale,
453, 456; list of, in head, 456;
mandibulare, 456; of articulates,
428, 429, 463; of fly, 428, 429;
of myriapod, 428; otic, 451, 452,
455, 464; ophthalmicum profun-
dum, 456; ophthalmicum super-
ficiale, 456; petrosum, 454, 456;

INDEX

557

ganglion, or ganglia — (Continued)
semilunare, 448, 449; spinal, 436;
spheno-palatine, 452, 455, 456,
464; submaxillary, 452, 464;
sympathetic, 451, 463.

ganoids, 29, 44, 148, 149, 170, 171,
173, 176, 184, 230, 270, 311, 369,
416, 444

ganoid scales, 8l

Gartner's duct, 391

gastrocoele, 59, 258, 365

gastrula, 59, 61, 62, 257, 365

gall-bladder, 259

gecko, 487

geese, 296

genital cleft, 403

genital ridge, 403

genital tubercle, 403

germ cells, 7, 366, 368, 378, 379

germ glands, 49, 366, 367, 391, 394

germ layers, derivatives of, 74, 75

germ plasm, continuity of, 58

germinal epithelium, 378, 379

gibbon, 45

Gigantostraca, 524

gill arches, fifth pair, 312

gill-flap, 306

gill, innervation of, 453, 454; of
Amphioxus, 526, 527 ; of Balan-
oglossus, 533, 534; of tunicates,
530, 531-

gills, 302-307

gill-slits, 267, 268, 303, 305

gill system, 260

gland, or glands: — acinous, 97,
112; anal sacs, 113; beaker-cells,
no, 262; buccal, 285; bulbo-
urethral, 403; carotid, 288; cil-
iary, 505; cloacal, no; femoral,
of lizard, no, in; germ, 49;
glandulse ductus deferentis, 401 ;
glandulae vesicales, 401 ; harde-
rian, 504; integumental, 78, 107;
intermaxillary, 285; labial, 285;
lacrimal, 504; lingual, 285; mam-
mary, 33, 114-118; meibomian,
i*3» 5O5; mesenteric, 362; molar,
286; musk, 110; necrobiotic, no;
odoriferous, 114; of mouth cav-
ity, 285, 286; of mucosa, 262;
orbital, 286; parotid, 286; pre-
putial, 113, 403; prostate, 403;

gland, or glands— (Continued)
rectal, 114; retrolingual, 286;
salivary, 286; sebaceous, 113;
• sublingual, 285 ; submandibular,
285 ; submaxillary, 285 ; sweat,
112, 113; tarsal, 113, 505; thy-
mus, 286, 287, 290, 363 ; thyreoid,
'286, 289, 290, 363; types of, 108,
109; Tyson's, 113; tubular, 97,
109, 112, 262; urethral, 401;
uropygeal, ill; vitally secretory,
no.

glandular area, 114

glans penis, 404

Globicephalus, 183

glomeruli, 371, 374

glottis, 270, 310, 311, 312

Glyptodon, 37

gnathostoma, 267

gnathostomes, 29

Goethe, theories of the " Urbild ",
506, 507

gonads, 49, 63, 366

Gorilla, 45

gorilla rib, 138

Grandry's corpuscles, 474

grasshopper, 428

gray matter, 408, 428

growth, 2, 3

greater curvature of stomach, 291

gubernaculum, 395

Gymnophiona, 31, 84, 313, 373, 398,
442, 483

gynaecomastism, 118

gyri, of hemispheres, 417

H

haemal arches, 126, 508

haemal spine, 508, 510

haemal system, 318

Haemamoeba, 5

haemapophyses, 508, 510

hair, 469, 471, 472; arrangement of,
86, 87; development of, 96; di-
rection of, 101, 103, 104, 105;
distribution of, 99; of mammals,
95-105; structure of, 97, ioo;
varieties of, 97, 98.

hair currents, 101

hair groups, 87, 88

558

INDEX

hair of Man: — direction of, 102,
103, 104; racial differences, 100;
shape of cross section, 100.

harderian glands, 504

hard palate, 270, 271, 479

Harrimania, 535

Hatteria, 32

head cavities, 461

head, formation of, 124

head, relation to vertebral column,
134

heart 318, 320, 324, 352-3575 of
amphibians, 354, 355, 356; of
Amphioxus, 353, 528; of fishes,
353, 354; of mammals, 356, 357;
of reptiles, 356.

hedgehog, 491

Heptanchus, 305

heredity, material basis for, 53, 57,
58

hermaphrodites, 379

hermaphroditic, 49

hemispheres, of cerebrum, 411, 413

heterodont dentition, 275

Hexanchus, 305

hibernation of Dipnoi, 478

hind brain, 411

hip-girdle, 128

hippocampus, 417

Hippopotamus, 274, 292, 505

Holocephali, 444

Homo, 87, 88

homodont dentition, 275

homology, of body somites, 439;
of limbs, 177-180, 237-245; sex-
ual, 394, 404, 405

Homo neanderthalensis, 42

Homo primigenius, 42, 43, 45

Homo sapiens, 42, 45

honeycomb stomach, 292

hoofs, 106, 107

horn, 78

horns of mammals, 106

horny structures, 105, 106

horse, 182, 250, 391

horse-shoe crab, 522

human phylogenesis, 44, 45

hydatid of Morgagni, 392

Hydra, 60

Hydromys, 277
hyobranchial apparatus, 283
hyobranchial complex, 160, 283

hyoid apparatus, 160, 283

hyoid arch, 155, 157

hyoid bone, 314; body, 160;

cornua, separate elements of,

160, 161

hyomandibular, 155, 157
hyperdactylism, 182
hypermastism, 117
Hyperoodon, 277
hyperphalangy, 183
hyperthelism, 117
hypertrichosis, 99
hypobranchial groove, 289
hypospadias, 399
hypothenar pads, 91
hypocone, 279
hypoglossus, 427
hypomeres, 64, 65, 190, 245
hypomeric muscles, 192
hypophysis, 414, 423, 424, 477
Hyracoidea, 38
Hyrax, 38

Ichthyopsida, 473
Ichthyopterygium, 167, 184
Ichthyosaurus, 20, 183, 184, 185,

186

ilium, 128, 171
immortality of Protozoa, 5
incisors, 275
incus, 159, 150, 195
inferior turbinated bone (see max-

illo-turbinal)
infundibulum, 424
ingluvies, 290
inguinal ligament, 386, 387, 391,

395

inner ear, 469, 472, 473, 485-493
insect, insects, 259, 428, 460, 463,

506, 512
Insectivora, insectivores, 37, 40,

44, 45, H3, 175, 239, 250, 277,

291, 393, 394, 401, 495, 500
insertion, of a muscle, 195, 197
integument :— of amphibians, 83,

84; of Amphioxus, 76; of fishes,

80-83 5 of invertebrates, 76 ; of

mammals, 85; of reptiles, 84, 85;

pigmentation of, 118-121.
integumental glands, 78, 107
integumental muscles, 190, 192

INDEX

559

integumental respiration, 308
integumental sense-organs, 448
interclavicle, 141, 174
interdigital pads, 91
interdural space, 357
intermaxillary glands, 285
intermuscular lymph spaces, 358
interventricular foramina, 416, 420
intestinal diverticula, 266
intestinal respiration, 303
intestine, 266, 294
intestine, length of, 299, 300, 301
intestinum crassum, 294
intestinum tenue, 294
introitus vaginae, 404
intromittent organ, 398
intumescentiae of spinal cord, 432,

434

involuntary muscles, 189
iris, 501
ischium, 172
iter a tertio ad quartum ventricu-

lum, 412

Jacobson's cartilage, 483
Jacobson's nerve, 451
Jacobson's organ, 483
jaws, origin of, 153, 154
jumping mice, 298

karyokinesis, 55

keratin, 84

kidneys, 323, 345, 369, 385,

kiwi-kiwi, 165

Krause's corpuscles, 474

labial cartilages, 153, 155

labial glands, 285

labia minora, 404

labia majora, 404

labyrinth, 452, 485-493; bony, 492;
development of, 485-490; sen-
sory areas of, 488-490.

Lacerta, 287, 344

Lacertilia, 32

lacrimal apparatus, 504, 505

lacrimal apparatus, 504, 505

lacrimal bone, origin of, 83

lacrimal fluid, 504

lacrimal glands, 504

lacunae, 318

lacunar circulation, 317

Laemargus, 470

lagena, 488, 491, 492

language, development of, 256

lanugo, 98

large intestine, 294

larynx, 160, 270, 307, 310, 311, 312;

of amphibians, 312, 313; of

mammals, 314, 315; of Saurop-

sida, 313, 314
larynx dorsalis, 310, 311
larynx ventralis, 310, 311
lateral cartilages, of larynx, 160
lateral cerebral tissue, 417
lateral line, 192, 448
lateral line organs, 469
lateral ventricles, of brain, 411,

416

lemmings, 298
Lemuroidea, 41
Lemurs, 38, 45, 89, 90, 284, 298,

3H, 394

lesser curvature, of stomach, 291

lesser omentum, 295

lesser peritoneal cavity, 294

leucocytes, 318, 362

life cycle, 9, 10

ligamentum : — arteriosum, 330, 331 ;
Botalli, 330, 331 ; hepato-gastri-
cum, 295; hepato-umbilicale, 347;
inguinale, 386, 387, 391; nuchae,
134; suspensorium hepatis, 295;
teres hepatis, 347

limb muscles, 130

limb girdles, 128

limb plexuses, 438-442

limbs, 164-188; of primates, 238,
239 ; reduction of, 165, 166 ; re-
dundancy of, 166; serial homol-
ogy of, 177-180, 237-245

Limulus, 522, 523, 524

linea alba, 65

Lingula, 19

lion, 282

Lithobius, 429

Litopterna, 38

liver, 266, 294, 323, 348

lizards, no, in, 166, 287, 288, 342,
344, 398, 416, 420, 422, 483

560

INDEX

lobes, of lungs, 315, 316

lobi optici, 425, 426

Loncheres, 86

love antics of salamanders, 397

lumbar intumescence, 433 .

lumbar vertebrae, 130

lumbo-sacral plexus, 438

lungless salamanders, 308

lungs, 307, 310-316

lungs, origin of, 270

lung system, 260

lymph, 319

lymph glands, 318, 362

lymphatic nodes, 362

lymphatics, 65, 318, 358

lymphatic spaces, 318, 357, 358

lymphatic system, 318, 319, 357-
364; development of, 360; ori-
gin of, 363

lymphatic vessels, 318, 358

lymphocytes, 362

lymph hearts, 318, 358, 359

lymphoid tissue, 362

lyssa, 385

M

Macacus, 91, 92, 93
macrogametes, 7, 48, 49, 50, 58
maculae acusticae, 490
macula lutea, 500
malleus, 159, 45O, 495
Malpighian corpuscles, 373
mammae, 116, 117; inguinal, 116;

pectoral, 117; unusual position

of, 117.
Mammalia, mammals, 39, in, 113,

148, 172, 174, 175, 179, 198, 200,

201, 203, 206, 212, 215, 2l6, 226,

227, 229, 236, 247, 248, 249, 250,

263, 264, 268, 269, 271, 275, 280,

283, 284, 288, 290, 291, 294, 296,

301, 314, 315, 319, 322, 324, 331,

333, 337, 347, 349, 350, 351, 353,

354, 355, 356, 357, 361, 363, 373,

377, 385, 386, 388, 389, 390, 391,

395, 398, 399, 400, 401, 402, 409,

414, 415, 419, 425, 426, 431, 437,

439, 444, 450, 451, 452, 455, 465,

469, 471, 472, 475, 479, 481, 486,
488, 489, 491, 496, 504, 509
mammary glands, 33, 114-118
mammary pockets, 114, 115

mammary ridge, 116

mammuth, 42

mandibular articulation, 159

mandibular cartilages, 153, 156, 157

Manidae, 85

marrow, 363

marsupials, 33, 44, 203, 223, 276,

284, 292, 297, 357, 388, 389, 399,

400, 401, 402, 417
marsupium, 34, 116
maxillo-turbinal, 479, 480, 481
Meckel's cartilage, 156, 159
median fins, 163, 164
medulla, of brain, 427
Medusa, 60
Megalonyx, 37
Megatherium, 37
meibomian glands, 113, 505
Meissner's corpuscles, 473
membraneous labyrinth, 485-492
Merostomata, 522, 523
mesencephalon, 411, 425, 426
mesenchyme, 60, 64
mesenteries, 63, 261, 265, 367, 368
mesenteric glands, 362
mesodseum, 259
mesoderm, 60, 63, 66
mesodermic diverticula, 63-67
mesodermic somites, 68, 69
Mesodonta, 38, 40, 41, 45
mesomeres, 64, 65
meso-hypomeres, 64, 65, 68
mesogastrium, 293
mesonephridia, 373, 374
mesonephros, 370, 373-375 J 382,

385, 386, 390, 391, 394
mesonephros; remains in Amniota,

385
mesonephrotic duct, 374, 375, 376,

381, 383, 385, 387
mesonephrotic ligament, 386, 390,

391
mesonephrotic system, 373-375 ;

vestiges of in female, 391 ; ves-
tiges of in male, 392
mesonephrotic tubules, 373, 374
mesopterygium, 176, 185-187
mesorchium, 369-387
mesovarium, 369, 387, 391
metacoele, 60, 63, 64, 365, 367, 368,

38i
metacone, 278

INDEX

metaccelic sacs, 367, 368

metamerism in vertebrate head,
460

metanephridia, 376

metanephros, 370, 375-377, 3$3

metanephrotic system, 375, 377

metapleural folds, 527

metapterygium, 176, 185-187

Metazoa, 257, 317

metencephalon, 411, 426, 443

microgametes, 7, 48, 49, 50, 58

Midas, 86, 88

midbrain, 411

middle ear, 493-496

milk glands, 114-118

mitosis, 54, 55

molar glands, 286

molars, 275

mole, 277, 287, 284

mole, star-nosed, 90

monkeys, 90, 91, 92, 112, 223, 254,
277

Monodelphia, 36

Monodon, 277

monophyodont dentition, 280

monorrhine condition, 476

monotremes, 33, 44, 236, 255, 337,
357, 388, 393, 399, 400, 402, 491

motor nerves, 434

motor roots, of nerves, 435, 436

mouse, 316, 402

mouth, 466, 467

mouth cavity, 466

mouth, origin of, 258, 259

mucosa, 260, 262

Miiller's duct, 382, 387, 388, 391,
392, 394

Multituberculata, 35

Mus, 402

muscle, or muscles: — abductor cau-
dae dorsalis, 212 ; abductor cau-
dae ventralis, 212; abductor coccy-
gis, 214; abductores breves, 234;
abductors, of pollex and minimus,
236; achselbogen, 251; acromio-
deltoid, 226; adductores (fem-
oris), 229; adductor laryngis,
313; adductor mandibulae, 247;
adductor mandibulae, 495 ', an-
coneus, 222, 226; auricularis
anterior, 255; auricularis pos-
terior, 255; auricularis superior,

muscle — (Continued}

255; auriculo-labialis, 55; au-
riculo-occipitalis, 255 ; appen-
dicular, 190, 192, 193, 217-245;
axial, 190, 192, 200-217; axil-
lary arch, 251 ; biventer cervicis,
226, 227; biceps femoris, 229;
brachialis, 226, 227 ; brachiora-
dialis, 240; buccinator, 255; cau-
dal, 212-214; caninus, 255; cervi-
calis ascendens, 208; coccygeus,
214; coraco-brachialis ; 226; co-
raco-brachialis brevis, 222 ; coraco-
brachialis longus, 222; complexus,
210; crureus, 229; cremaster,
396 ; cucullaris, 223 ; curvatores
coccygis, 214; deltoid, 195, 226;
depressors of visceral arches,
245, 246, 247; diaphragm, 316;
diaphragma pelvis, 214; di-
gastricus, 196, 247, 248, 438;
dilatator laryngis, 313; dorsalis
antebrachii, 231, 235; dorsalis
scapulae, 222 ; dorso-laryngeus,
247, 248; extensor caudae later-
alis, 212; extensor caudae medi-
alis, 212; extensor carpi radialis,
237, 239; extensor carpi ulnaris,
237, 239; extensor communis dig-
itorum, 235; extensores breves,
231 ; extensor radialis, 231 ; ex-
tensor ulnaris, 231 ; facial mus-
cles, 254-256 ; f emero-fibularis,
228; flexores breves profundi,
234; flexores breves superficiales,
234; flexor caudae, 212; flexor
carpi ulnaris, 237, 239; flexor
carpi radialis, 237, 239; flexor
digitorum profundus, 236; flexor
digitorum sublimis, 236; flexor
pollicis longus, 236; flexor u'l-
naris, 234; flexor radialis, 234;
galea aponeurotica, 55; gastro-
chnemius, 239 ; genio-glossus, 248 ;
glutaeo-cruralis, 229; glutaei, 229;
gracilis, 229; hyo-glossus, 248;
hyo-laryngeus, 247; homology of,
194-199; humero-antebrachialis,
222, 227; ilio-coccygeus, 214;
ilio-costalis, 208; ilio-femoralis,
228, 229; ilio-fibularis, 228, 229;
ilio-extensorius, 228, 229; ilio-

562

INDEX

muscle — (Continued}

psoas, 229; in fishes, 200, 201;
infra-spinatus, 226; insertion,
195; integumental, 190, 192, 249-
256 ; intercostales, externi et in-
terni, 204 ; intermetacarpales,
234 ; intermandibularis anterior,
247; intermandibularis posterior,
247; interossei, 236; interossei
dorsales, 236; interossei pal-
mares, 236; interspinales, 210 ;
intertransversarii, 210, 211; inter-
transversarii caudae, 213; inter-
vertebral system, 210-212; invol-
untary, 189 ; ischio-cavernosus,
214, 215; laryngeus dorsalis, 247;
laryngeus ventralis, 247; laryngei,
247, 248, 313; latissimus dorsi,
199, 206, 221, 223, 226, 249, 250,
251; levator ani, 214; levator an-
guli oris, 255 ; levatores arcuum,
247 ; levatores costarum, 204 ;
levator menti, 255; levator of
visceral arches, 245, 246, 247 ; lev-
ator scapulae, 222, 223 ; lingualis,
249; longissimus, 208, 212, 215;
longus colli, 202; longus capitis,
204; masseter, 248; mimetic mus-
cles, 253-256, 451 ; multifidus, 210,
212; mylo-hyoideus, 248; nasal
muscles, 255; obliqui capitis,
198, 215; obliqui oculi, 216, 447;
obliquus capitis inferior, 212; ob-
liquus capitis superior, 212; ob-
liquus externus abdominis, 198,
203, 204 ; obliquus inferior oculi,
447; obliquus internus abdominis,
203, 204; obliquus superior oculi,
447; obturator externus, 229;
obturator internus, 229; occipito-
frontalis, 255; of eyeball, 216,
447 ; of hip girdle and thigh, 227,
230; of shoulder and upper arm,
217, 227; of the free limbs, 230,
237; opponens hallucis, 242; op-
ponentes of pollex and minimus,
236; orbicularis oris, 255; orbicu-
laris oculi (palpebrarum), 255;
origin, 195 ; palmaris, 236 ; pal-
maris profundus, 233; palmaris
longus, 236; palmaris superfici-
alis, 233 ; panniculus carnosus, 250,

muscle — ( Continued}

251; parietal, 190, 192; patagial
muscles, 249; pectoralis abdomi-
nalis, 251; pectoralis, 199, 222,

226, 249, 250, 251, 253; pectoralis
major, 226; pectoralis minor,
226 ; peronaeus brevis, 239 ; pero-
naeus longus, 239; peronaeus ter-
tius, 241; petro-hyoideus, 198;
pharyngeal constrictors, 249; piri-
formis, 230; platysma, 196, 253;
prevertebral, 202; principles of
muscle formation, 198; proco-
raco-humeralis, 222, 226; pro-
nator, 234, 237; pronator teres,
240; pronator quadratus, 240;
propatagialis, 249; pterygoideus
externus, 248; pterygoideus inter-
nus, 248; pubo-coccygeus, 214;
pubo-ischio-femoralis externus,

227, 229 ; pubo-ischio-femoralis
internus, 228, 229; pubo-ischio-
tibialis, 228, 229; pubo-tibialis,
228 ; pyramidalis, 203 ; quadratus
labii inferioris, 255; quadratus
lumborum, 203; quadriceps fem-
oris, 229; recti oculi, 216, 447;
rectus abdominis, 199, 203;
rectus capitis anterior, 204;
rectus capitis posterior major,
212; rectus capitis posterior
minor, 212; rectus femoris, 229;
rectus externus oculi, 447; rectus
inferior oculi, 447; rectus inter-
nus oculi, 447; rectus lateralis
capitis, 204; rectus superficialis,
222; rectus superior oculi, 447;
retractor bulbi, 447 ; rhomboidei,
206, 226 ; rhomboideus capitis,
226; rhomboideus dorsi, 226;
rhomboideus major, 206, 226;
rotatores, 210; sacro-coccygei an-
teriores, 214; sacro-coccygei,
posteriores, 214; sacro-lumbalis,
207 ; sacro-transverso-transversa-
lis system, 207, 208, 215; sarto-
rius, 229; scalenus anterior, 204;
scalenus medius, 204 ; scalenus
posterior, 204; semi-membra-
nosus, 229; semi-spinalis, 210;
semitendinosus, 229; serratus an-
terior (magnus), 223; serratus

IXDEX

563

muscle — (Continued)

magnus, 222, 223; serrati poste-
riores, 203, 206; soleus-gastro-
chnemius, 239; sphincter ani,
214; sphincter colli, 253; sphincter
cloacae, 250; sphincter marsupii,
250; spinalis capitis, 209; spinalis
cervicis, 208 ; spinalis dorsi, 208 ;
spino-deltoid, 226 ; spino-spinalis
system, 208, 215 ; spino-transver-
salis system, 207; splenius capi-
tis, 207; splenius cervicis, 207,
215; stapedius, 196, 248, 438, 495;
sternalis, 253; sterno-cleido mas-
toideus, 223; sterno-cleido mas-
toideus, 457; striated, 189, 190;
stylo-glossus, 248; stylo-hyoideus,
248; subclavius, 226; subcutaneus
faciei, 255; supinator, 231, 237;
supinator brevis, 240; supinator
longus, 240; supracoracoideus,
222, 226; supra-spinatns, 226;
temporalis, 190, 248; tenuissimus/
229; tensor tympani, 248, 495;
teres major, 223, 226; teres minor,
226; tibialis anterior, 239; tibialis
posterior, 239; trachelor-mastoid,
208; transversalis abdominis, 203,
204; transversalis, colli, 208;
transverso-spinalis system, 210,
215; transversus thoracis, 204;
trapezius, 206, 222, 223, 457; tri-
angularis sterni, 204; triceps, 222,
226; unstriated, 189; vastus
group, 229; visceral, 190, 192, 245-
249; voluntary, 189, 190; zygoma-
ticus, 255

muscles of Necturus, 191-193, 220-
222, 227-229, 230-235

muscles, of alimentary canal, 290,
291

muscle somites, 192

muscular homology, 194-199

musculosa, 260

musk glands, no

muskrat, 297

myelencephalon, 412, 426, 42;, 443

myocommata, 27, 192

myology, comparative, 195

myomeres, 27 (see also myotomes)

Myopotamus, 86

myotomes, 192, 461

myotomic buds, 217, 218, 219

myriapod, 428, 429, 460

myriapod, nervous system, 428

Myrmecophaga, 277

Myrmecophagidae, 33

Myxine, 437, 476, 488, 489, 516

myxinoids, 49

nails, 105, 106, 107

nares, anterior, 478

nares, posterior, 478

narwhal, 277

nasal capsules, 145

nasal cavities, 269

Nasodon, 281

naso-lacrimal duct, 504

naso-pahtine canal, 479

Neanderthal man, 42

neck, 132, 133

neck, formation of, 130

Necturus, 128, 138, 171, 172, 183,
186, 191, 220, 227, 228, 229, 230,
231, 232, 233, 234, 235, 237, 238,
240, 246, 247, 312, 313, 430, 442

nemerteans, 520, 521

nemertean theory of vertebrate an-
cestry, 520, 521, 522

nerve, or nerves : — abducens, 216,
443, 445, 446, 447, 461, 462 ; acces-
sorius, 443, 452, 455, 457; acusti-
cus, 443, 448, 452, 462; auditorius
(see acusticus) ; ansa hypoglossi,
458; brachial plexus, 438, 439-
441 ; buccalis, 449 ; chorda tym-
pani, 450, 451 ; communicans IX,
454, 456; facialis, 196, 246, 248,
256, 443, 448-452; 453, 454, 461,
502; glosso-pharyngeus, 246, 247,
443, 451, 452, 457, 461, 502; gus-
tatorius, 452 ; hyomandibularis,
449; hypoglossus, 443, 457-459,
462, 463; intestinalis, 454, 457;
Jacobson's, 451, 454, 456; later-
alis X, 453, 457; lingualis, 452;
454; lumbo-sacral plexus, 438;
mandibularis externus VII, 449;
mandibularis internus VII, 449;
mandibularis V, 449, 451 ; maxil-
laris, 449; motor, 434; motor
oculi, 216, 443, 445, 446, 447, 462;
occipital, <\/\/\ ; occipito-spinal,
444; olfactorius, 443, 444; oph-
thalmicus, 451; ophthalmicus

564

INDEX

profundus, 449, 462, 463; oph-
thalmicus superficialis V, 44§;
ophthalmicus superficialis VII,
448; opticus, 443, 444; palatinus,
449; palatinus major, 450,
456; patheticus (v. trochlearis)
peripheral, 434, 437; petrosus
profundus major, 456; petrosus
profundus minor, 456; petrosus
superficialis major, 450, 456; pe-
trosus superficialis minor, 456;
plexus, 196, 197; plexus brachi-
alis, 438, 439-441; plexus lumbo-
sacralis, 438; plexuses, of sym-
pathetic system, 464; pneumo-
gastric (v. vagus) ; posttrematici,
454, 462, praetrematici, 454, 462;
sensory, 434; spinal accessory
(v. accessorius) ; spino-occipi-
tal, 444; stapedialis, 451; sym-
pathetic plexuses, 464; sympa-
thetic system, 451, 463, 464; ter-
minalis, 445; trifacial (v. trig-
eminus) ; trigeminus, 247, 248,
443, 447, 448-452, 461, 495 ; troch-
learis, 216, 443, 445, 446, 447,
449, 462; tympanic, 451, 454,
456; vagus, 247, 443, 452-457,
461

nerves, motor, 434

nerves, peripheral, 434, 437

nerves, sensory, 434

nerve supply to muscles, 438

nervous system : — anlage of, 62 ;
origin of, 406, 407

neosternum, 138

neostoma, 154, 424

nephridia, 366, 368, 370, 371, 381,
382, 383

nephrostomes, 366, 373, 374, 382,
383

neural arches, 126, 508

neural processes, 126

neural spine, 508, 510

neural tube, 62, 406; development
of, 407, 408; early history of,
406, 407

neurapophyses, 508, 510

neurenteric canal, 258

neurilemma, 434

newt, 313

nictitating membrane, 504

nipples, 115-118; morphology of,

115; rudimentary, 117
noduli lymphatici aggregati, 362,

363

Nomarthra, 37

nose, cartilages of external, 151

nose, external, 151, 484

notochord (see under skeletal ele-
ments)

notochord : — of Amphioxus, 123,
527; of Balanoglossus, 535; of
tunicates, 531

nucleus, 3

O

obturator foramen, 172

occipital condyles, 133

odontoblasts, 273

odontoid process of axis, 133

oesophagus, 290

Oken, Lorenz, his theories, 506

olfactory buds, 485

olfactory lobes, 416, 445

olfactory nerve, 416

olfactory pit of Amphioxus, 445

olfactory surface, increase of, 479,

482

omasus, 292
omentum, 293
ontogenesis, 15
ontogenesis, laws of, 20-25
oogonium, 55, 56, 57
open type of circulation, 317
operculum, of ear, 493
operculum, of gills, 157, 306
Ophidia, 32
opossum, 33, 276
optic cup, 422, 423
optic lobes, 411, 425
optic stalks, 445
optic vesicle, 422, 423
optici thalami, 425
opisthocoelous vertebrae, 131
orang-utan, 45
orbital glands, 286
organs of Corti, 492
organs of Giraldes, 392
organ of hearing, 485-497
organ of Rosenmiiller, 391
origin, of a muscle, 195
Ornithorhynchus, 33, 90, 432
Orthagoriscus, 430, 431

INDEX

565

ossicula auditus, 159

ostium tubae, 382

ostrich, 165, 172, 433

otic capsules, 145

otic vesicle, 473, 486

otter, 505

ova, 366, 379, 382, 389

ovaries, 49, 367, 379, 383, 387, 389,

39i, 393, 394
oviduct, 382, 383, 385, 387, 388, 389,

39i, 394

ovum, 48, 49, 50, 51, 55, 58
Owen Sir Richard, his theories,

507-512
owl, 489
ox, 182, 491

Pacini's corpuscles, 474

pads, mammalian, 90, 91

Palaeostoma, 154, 424, 478

palatine cleft, 271

palingenetic characters, 16

pallium, 416

palm print of boy, 94

pancreas, 266,^294

Pantotheria, 35, 44

papilla acustica basilaris, 490, 491

papilla acustica legenae, 490

parachordal elements, 144, 145

parachordal region of head, 124,

144, 46o
paracone, 278
paradidymis, 392
Paramoecium, 4, 6
paraphysis, 420, 422
parapophyses, 508
pars olfactoria, of nose, 478
paraseptal cartilage, 483
parasternum, 141
pars respiratoria, of nose, 478
parathyreoid bodies, 289
paired fins, 163. 164
parietal foramen, 420
parietal eye, 422
parietal mesoderm, 63, 66
parietal muscles, 190, 192
parietal organ, 420
paroophoron, 391, 394
parotid glands, 286
parrots, 296
patagium, 249

patella, 178

paunch, 292

pearl organs, 472

pectoral fins, 164

pectoral girdle, 130

pedunculi cerebri, 426

pelvic girdle, 169-172; phylogenesis

of, 170-172

pelvis, of kidney, 376
penis, 398, 399, 400, 401
penis, position of, 399, 400
perennibranchiate amphibians, 307,

330

perilymph, 492
perilymphatic cavity, 492-493
perilymphatic space, 357
perinaeum, 389, 401
perineurium, 434
Perissodactyla, 39
peristalsis, 265

peritoneal cavity, 367-369, 380
peritoneal cavity, lesser, 294
peritoneum, 66, 261, 293, 294, 316,

367, 374

perivisceral fluid, 317
perspiration, 112
pes hippocampi, 417
petrel, 478

Petromyzon, 289, 420, 477, 488
Peyer's patches, 362
phallus, 398
pharyngeal pockets, 268
pharyngeal pouches, 265, 266
pharyngo-oesophageal respiration,

308, 309

pharynx, 265, 266
Phascoloarctus, 297
philthrum, 271
phylogenesis, 15
phylogenesis, laws of, 16-20
phylogenesis of man, 44, 45
phylogenetic tree, of mammals, 36,

39; of vertebrates, 28
pig, 182, 274, 361, 391, 437, 489
pigment, 78, 118-121
pigment speck, of Amphioxus, 445
pineal gland, 421
pineal organ, 420
pinna, of ear, 161, 496, 497
Pinnipedia, 38
Pisces, 29, 30
Pithecanthropus, 43

566

INDEX

pituitary body, 424

placenta, 34, 70, 71, 72, 73, 377, 388

placoderms, 29, 83, 524, 525

placoid scales, 80, 81, 267

placoid scales on teeth, 154

plasma, 318, 319

plastron of turtles, 105

platyhelminths, 520

Platyrrhini, 278

Pleisiosaurus, 185

pleura, 66, 316

pleurapophyses, 138, 508, 510

pleuro-peritoneal cavity, 64, 66

plexus, blood vessels in brain, 413,

419, 420

plexus brachialis, 438, 439-441
plexuses, chorioid, 413, 416, 419, 420
plexuses, of nerves, 438
plexuses, sympathetic, 464
plexus formation, nerves, 438
plexus lumbo-sacralis, 438
plica fimbriata, 284
plica semilunaris, 504
pneumatic cyst, 310, 311
poikilothermous animals, 356
poison fangs of serpents, 280
polar globules, 56, 57
Polypterus, 171, 186, 187, 188, 311
Polypterus, ribs of, 137
polyspermy, 53
pons Varolii, 426, 427
Pontoporia, 301

pori abdominalis, 369, 380, 381
porpoise, 478, 491, 505
postbranchial bodies, 288, 289
posterior limb, 169
posterior nares, 269, 478
post-minimus, 182, 242
praechordal elements, 144, 145
praechordal portion of head, 460
praechordal region of head, 124, 144
pre-hallux, 182
prehistoric man, 42
premolars, 275
pre-pollex, 182, 242
preputial glands, 113, 403
Prevertebrata, 26
primary body cavity, 365
Primates, 38, 40, 41, 90, 91, 92, 93,

165, 175, 216, 223, 238, 250, 277,

291, 394, 402, 417
Primates, limbs, 239

primitive dentition, 277

primordial brain, 411

primordial skull, 145

Proboscidea, 38, 39

Procavia, 38

processus vaginalis, 395

precocious vertebrae, 131

procoracoid, 174

proctodaeum, 259

pronephridia, 371

pronephros, 37<>373

pronephrotic duct, 370

pronephrotic tubules, 371, 374

Propithecus, 300

propterygium, 176, 185-187

prosencephalon, 411

prostate gland, 403

prostatic vesicle, 393, 394

Proteus, 231, 247, 300, 312, 313

Protochordata, 26

protocoele, 63, 64, 365, 367

protocone, 278

protoplasm, characteristics of, I

protostome, 59, 61

Prototheria, 35

Protozoa, 3, 257 ; immortality of, 5 ;

conjugation of, 6
pseudohypertrichosis, 99
Pterichthys, 524
Pterodactyl, 183, 184
pterygoid canal, 450
pubic bones, 172
pubo-ischiadic symphysis, 172
pulmonary system, 260, 270, 304,

308, 310-316
pulsating vessels, 317
pyloric coeca, 266
pyloric end of stomach, 291
pylorus, 265, 291

R

rabbit, 297, 334, 345, 346, 491

race history, 15

Rana, 313, 489, 526

Ranodon, 186

rat, 336, 338

ratio of surface to mass, 3, 261, 262

ray, 453

receptacula chyli, 362

recessus utriculi, 487

rectum, 298, 389

red blood corpuscles, 318, 364

INDEX

567

reduction of limbs, 165, 166

reduction of teeth, 277

Reissner's membrane, 492

renal corpuscle, 373

reproduction by fission, 4

reproductive system, 378-405

reptiles, 44, 172, 201, 203, 212, 220,
229, 248, 253, 255, 268, 272, 280,
296, 328, 330, 331, 352, 353, 355,
356, 358, 375, 377, 385, 386, 399,
409, 415, 4i6, *I9, 425, 426, 465,
472, 487, 488, 504

respiration, 301-316; in amphibians,
306-308, 312-313; in Amphioxus,
304; in Balanoglossus, 304; in
fishes, 304, 305, 306; integumen-
tal, 308; intestinal, 303; pharyn-
go-oesophageal, 308 ; pulmonary,
307, 310-316

reticulum, 292

retina, 409, 422, 497, 498

retrolingual glands, 286

rhinencephalon, 416, 445

Rhineura, 20

Rhinoceros, 394

rhombencephalon, 427

Rhynchocephalia, 32

ribs, 126, 135, 136, 137, 138; distri-
bution of, 137, 138; two types of,
135, 137; variation in, 129, 130

rodents, 37, 45, 175, 203, 223, 239,
274, 277, 297, 298, 316, 394, 401,
417, 480, 495, 500

roots, of nerves, 435

roots, of teeth, 273, 274

rods and cones, of retina, 422, 467,
498

rods of Corti, 492

rostral plates of skull, 146

round ligament of uterus, 387, 390

rumen, 292

ruminants, 292, 297, 484

sacculus, 487, 488
saccus endolymphaticus, 486
sacral vertebrae, 129, 130
sacrum, variation in, 128, 129
St. Hilaire, his theories, 506, 512
salamanders, 179, 185, 186, 192, 220,

223, 307, 3o8, 309, 354, 383, 388,

397, 429, 493

saliva, 285

salivary glands, 285

Sauropsida, no, 200, 264, 283, 284,
285, 294, 296, 313, 314, 319, 322,
323, 324, 347, 36o, 373, 375, 377,

391, 444, 457, 464, 479, 491, 495,
497, 499

scales : — ctenoid, 83 ; cycloid, 83 ;
epidermic, 84; of fishes, 80; of
ganoids, 81 ; of mammals, 85, 86,
90; of palmar and plantar sur-
faces, 95; of Stegocephali, 84;
placoid, 80; structure of, 80

Scaphyrhynchus, 171

schizocoele, 68

sclerotic coat, of eye, 423, 498, 501

scorpion, 524

scrotal raphe, 404

scrotal sac, 395, 396

scrotum, 394, 395

sculpin, 419

scutes, 79, 83, 146

seal, 282, 316

sebaceous glands, 113

segmentation of head, 458, 460, 461

selachians, 29, 44, 152, 153, 170, 172,
173, 184, 185, 217, 218, 219, 253,
272 288, 305, 324, 325, 326, 327,
328, 340 341, 369, 38o, 381, 382,

392, 416, 419, 420, 436, 460, 486,
488, 489, 490, 512, 515

selachians, circulation in, 324, 325

selachians, skull of, 145, 146, 153

sella turcica, 424

semicircular canals, 487

seminal fluid (see spermatic fluid)

seminal groove, 399

sensations, 467, 468

sense of contact, 468

sense-organs, 408, 465-468

sense-organs, accessory parts, 467

sense-organs, cells of, 466

sense organs of head, 143

sensory cells, 408, 466, 467

sensory nerves, 408, 434

sensory roots of nerves, 435, 436

septum atriorum, 354

septum linguae, 285

septum pellucidum, 418

serial homology of limbs, 178, 237-

245

serosa, 260

568

INDEX

serous cavities, 318

serpents, 285

sesamoid bones, 178

sex determination, 403

sexes, definition of, 49

sexual homologies, 393, 394, 404,

405

sexual kidney, 382, 383, 385
sharks, 388
sheep, 276
shoulder-girdle, 172-175; of Amni-

ota, 174, 175; of amphibians,

174; of ganoids, 173; of teleosts,

174; of selachians, 173
shrew-mouse, 92
sigmoid flexure, 298
Simplicidentata, 37
sinuses, in bones of face, 482
sinus maxillaris, 482
sinusoids, 318

sinus utriculi posterior, 487
sinus utriculi superior, 487
sinus, venosus, 324, 326, 340, 341,

353-357

Siren, 165, 246, 442, 471

Sirenia, 38, 39, 98, 113, H7, 3i6,
393

skates, 453

skeletal elements: — abdominal ribs,
141; accelous vertebrae, 131; ali-
sphenoids, 148; alveoli of jaws,
273; amphicoelous vertebrae, 127;
angulare, 156; appendicular skel-
eton, 122; appendicular skeleton,
162 ff. ; archisternum, 138 ; artic-
ulare, 159, 495; arytaenoids, 160,
313; atlas, 133; auditory ossicles,
159 ; auricula, 496, 497 ; axial skele-
ton, 122; axis, 133; basihyal, 160,
284; basioccipital, 150; basiptery-
gium, 169, 175 ; basisphenoid,
150; bone complexes of skull,
151; bony labyrinth, 492; bran-
chial arches, 152, 155 ; carpus, 177-
181 ; cartilage bones, 148 ; carti-
lago lateralis, 160; centers of os-
sification, 148; centrum of verte-
bra, 127; ceratohyal, 160, 284;
cervical rib, 138; cervical verte-

• brae, 130; chondrocranium, 145,
146, 153, 156; clavicle, 173-175 ;
cleithrum, 173; coccyx, 135; col-

skeletal elements — (Continue d^
umella auris, 494; conchse of
nose, 480-482 ; coracoid, 174 ', cos-
tal cartilages, 136; cricoid, 314;
dentary, 156; dentine, 79; der-
mal bones, 79, 82, 146, 147, 156;
dermal bones of skull, 82, 146,
147, 156; diapophyses, 138; dor-
sal vertebrae (see thoracic verte-
brae) ; ear, external, 496, 497;
ectoturbinalia, 480; endochondral
ossification, 148; endoturbinalia,
480 ; entoglossum, 283-285 ; epi-
glottis, 160, 313; epihyal, 160,
284; epiotics, 148; episternum,
141 ; ethmoid, 148 ; ethmo-turbi-
nal, 479, 480; exoccipitals, 148;
external ear, 496, 497; eyeball,
skeleton, elements of, 144, 145,
151; eye capsules, 145; falci-
forme, 182; fangs of serpents,
280; fin spines, 162; free-limb,
skeleton of, 176-178; f rentals,
82, 83, 147; furcula, 174;
ganoid scales, 81 ; gill arches,
312-314; haemal arches, 126;
hard palate, 270, 271, 479; hip-
girdle, 128; hyobranchial appa-
ratus, 283 ; hyobranchial complex,
160, 283; hyoid, 160, 161 ; hyoid
apparatus, 160, 283; hyoid arch,
155, 157; hyoid bone, 314; hyoid
complex, 314; hyomandibular,

155, 157; ilia, 171; ilium, 128;
incus, 159, 450, 495; inferior tur-
binated bone (see maxillo-turbi-
nal) ; interclavicle, 141, 174;
ischia, 172 ; Jacobson's cartilage,
483; jaws, origin of, 153, 154;
labial cartilages, 153, 155 ; laby-
rinth, bony, 492; lacrimal, 83,
147; lateral cartilages of larynx,
160; limb-girdles, 128; lumbar
vertebrae, 130; malleus, 159, 450,
495 ; mandibular cartilage, 153,

156, 157; maxillaries, 83, 156;
maxillo-turbinal, 479-481 ; Meek-
el's cartilage, 156, 159; meso-
pterygium, 176, 185-187; meta-
pterygium, 176, 185-187; nasal
capsules, 145; nasals, 82, 147;
naso-turbinal, 479, 480; neo-

INDEX

569

skeletal elements — (Continued}
sternum, 138; neural arches, 126;
neural processes, 126; nose, car-
tilages of external, 157; noto-
chord, 26, 27, 63, 122, 123; noto-
chord, of Amphioxus, 527; noto-
chord, of Balanoglossus, 535;
notochord, of tunicates, 531 ; oc-
cipital condyles, 133 ; odontoid
process of axis, 133; omoster-
num, 142; operculum, of ear,
493 ; operculum, of gills, 147, 157 ;
opercular bones, 147 ; opisthocoe-
lous vertebrae, 131 ; opisthotics,
148; optic capsules, 145; orbitals,
83, 147; orbitosphenoids, 148; os
entoglossum, 283-285; os falci-
forme, 182 ; ossicula auditus,
*59; ossification, centers of, 148;
otic capsules, 145; palate, 270;
271 ; palatines, 83, 147, 156 ; pa-
tella, 178; parabasal, 83, 147;
parachordal elements, 144, 145;
paraseptal cartilage, 483 ; para-
sphenoid, 148 ; parasternum, 141 ;
parietals, 82, 83, 147; pectoral
girdle, 130, 169-172; petrosal
bone, 493; petrosals, 148; pinna,
496, 497; placoid scales, 80, 81,
267; placoid scales, as teeth, 154;
plastron, of turtle, 105; pleura-
pophyses, 138; post-frontals, 82,
147 ; post-temporals, 174 ; prae-
choidal elements, 144, 145; prae-
f rentals, 82, 147; praemaxillary,
156; praesphenoid, 150; primor-
dial skull, 145; precocious verte-
brae, 131; procoracoid, 174; pro-
otics, 148; propterygium, 176,
185-187; pterygoids, 147, 156;
pubic bones, 172 ; quadrate,
156, 450, 495; quadrate- jugal,
496; ribs, 126, 129, 130, 135-
138; ribs, abdominal, 141; ribs,
distribution of, 137, 138; ros-
tral plates, 146; sacrum, 129;
sacral vertebrae, 129, 130;
scales, ganoid, 81 ; scales, placoid,
80, 81, 267; scapula, 174; scapulo-
coracoid, 173; sesamoid bones,
178; shoulder girdle, 173-175;
sinuses, in bones of face, 482;

skeletal elements — (Continued")
skull, amniote stage, 150, 151 ;
skull, amphibian stage, 150;
skull, bone complexes of, 151;
skull, development of, 142-145;
skull, ganoid stage, 146; skull, of
cyclostomes, 143; skull, of sela-
chians, 144, 145, 146, 153 ; skull,
primordial, 145; skull, selachian
stage, 144-146; spinous processes,
134; spiracular cartilage, 155;
squamosals, 82, 83, 147; stapes,
159, 337, 495; sternebrae, 140;
sternum, 138, 139, 142; stylo-hyal,
161, 284; styloid process, 161, 284;
supra-clavicles, 174 ; supra-clei-
thra, 174 ; supra-occipital, 82, 147 ;
suspensorium, of jaws, 155; tar-
sal cartilages, 504; tarsus, 177-
182; teeth, 267, 271-283; teeth,
evolution of shapes, 278-280;
teeth of birds, 84; teeth of se-
lachians, 81 ; teeth, origin of, 81,
I53> 154; teeth, replacement of,
280-283; thecae of jaws, 273;
thoracic vertebrae, 130, thyreo-
hyal, 284, 314; thyreoid, 314;
thyreoid cartilage, 160; tongue-
bars, of Amphioxus, 290; tooth,
Structure of, 80; trabeculae,
144; trachea, 270, 312; tracheal
pieces, 160, 313; tracheal
rings, 313, 314; trunk vertebrae,
130; turbinalia, 479-482; tympanic
bone, 496; tympanic bulla, 496;
tympano-hyal, 161, 284; typical
vertebrae, 507-511; uncinate pro-
cesses, 136; urostyle, 136; verte-
brae, 129, 130; vertebrae, develop-
ment of, 124-127; vertebrae, typi-
cal, 507-511; vertebral column,
130-133; vertebral column, devel-
opment of, 124-127; visceral
arches, 267; visceral skeleton,
122, 152-162; vomero-nasal car-
tilage, 483; vomers, 83, 147, 150;
Weber's apparatus, 486; wish-
bone, 141.

skeleton, relation to soft parts, 123

skin, 77

skin color in Man, 119

skin, pigmentation of, 118-121

INDEX

skull: — amniote stage, 150, 151;
amphibian stage, 150; develop-
ment of, 142-145; ganoid stage,
146; selachian stage, 144-146.

slime canals, 469

sloths, 113, 316, 393

small intestine, 294

smell, 468, 476-485

smell-buds, 471

snakes, 110, 166, 377, 398, 432, 433,
441, 483, 487, 505

soft palate, 266, 271

soma, 7, 11, 48, 58

somatic cells, 55

somites, 27

somites of head, 458-463

sparrow, 419

spermatic cord, 396

spermatic fluid, 49, 52, 380, 381, 383

spermatogonium, 55, 56, 57

spermatophores, 383

spermatozoon, 48, 49, 50, 51, 52, 55,
58, 366, 379, 382, 397, 401

Sphenodon, 32

spider, 259

spinal cord, 427, 428; caliber of,

432, 433 ; columns of, 434 ; length,
428-431 ; shape of cross-section,

433, 434

spinal ganglia, 436, 437
spinal nerves, 427-434
spinous processes, excessive devel-
opment of, 134
spiny ant-eater, 33
spiracular cartilage, 155
spiracular opening, 493, 494
spiraculum, 155, 269
spleen, 293, 363.
Squalus, 388, 446
squid, 499
squirrel, 316
squamosals, 82, 83, 147
stapedial artery, 159
stapes, 159, 337, 495
Stegocephali, 30, 31, 32, 44, 84, 230
Stegosaurus, 433
Stenson's canal, 479
sternebrae, 140
sternum, 138

sternum, morphology of, 139
sternum of monotremes, 142
stoma, 266
stomach, 265, 290, 291, 292

stomatodaeum, 259, 476
stomato-pharyngeal cavity, 266
stratum corneum, 77
stratum germinativum, 77
stratum lucidum, 77
stratum mucosum, 77
striated muscle, 189, 190
sturgeon, 29
stylo-hyal, 161, 284
stylo-hyoid ligament, 161
styloid process, 161, 284
sub-cutaneous lymph sacs, 358
subdural space, 357
sub-lingua, 284
sublingual glands, 285, 286
submandibular glands, 285, 286
submaxillary glands, 285, 286
submucosa, 260
subperitoneal space, 357
subvertebral space, 357, 363
sulci, of brain, 417
sulcus centralis, 417
superciliary structures, 505
supra-clavicles, 174
supra-cleithra, 174
suprapericardial bodies, ,288
Sus, 86, 489

suspensorium of jaw, 155
sweat-glands, 112, 113
swine, 484, 491
Sycandra, 50

sympathetic plexuses, 464
sympathetic system, 451, 463, 464,
symphysis pubic, 172
syntropists, 244
syrinx, 314.

tactile cells, 473

tactile corpuscles, 473

tactile sense, distribution of, 475

tactile spots, 473

taania ventriculi quarti, 427

taenise chorioides, 419, 420

tail, 130, 132, 134, 135

tail, nerves of, 431, 432

talon, 279

Talpa, 277

tapetum nigrum, 423

tapir, 394

tarsal cartilages, 504

tarsal glands, 113, 505

INDEX

571

Tarsipes, 292

tarsus: — nomenclature of, 177-180;
primitive condition of, 179; su-
pernumerary elements, 181, 182

taste, 475, 476

taste-beakers, 475

taste buds, 469, 471, 475

Taxeopoda, 38

teleosts, 30, 174, 287, 303, 305, 415,
416, 419, 430, 486, 489

teeth, 267, 271, 283. (V. also
tooth) : — evolution of shapes,
278-280; growth of, 273, 274;
kinds of, 275; of birds, 84; of
selachians, 81 ; origin of, 81, 153,
154, 271, 272; parts of, 273; re-
duction of, 277; replacement of,
280-283

tela chorioidea, 419

telencephalon, 411, 415-418, 443

temperature of animals, 356

tentorium, 417

testes, 49, 367, 379, 383, 394, 39$

testes, position of, 393, 395

thalami optici, 425

thecadont articulation, 273

thecse of jaws, 273

thenar pads, 91

theories of vertebrate ancestry : —
from annelids, 513-520; from
arachnoids, 522-525 ; from arche-
type, 506-512; from articulates,
512, 513; from insect, 506, 512;
from nemertean, 520-522; from
protochordata, 525-538

theromorphs, 32, 44

third ventricle of brain, 411

thoracic duct, 357, 360

thoracic vertebrae, 130

thymus gland, 286, 290, 363

thymus, origin of, 286-287

thyreoid cartilage, 160, 314

thyreoid gland, 286, 289, 290, 363

thyreo-hyal, 284, 314

Tillodontia, 37

toad, 285, 290, 397, 420, 442

tongue, 283-285

tongue bars, of Amphioxus, 290

tonsils, 363

tooth. (V. also teeth)

tooth generations, 281

tooth, primitive form of, 274

tooth, structure of, 80

Tornaria, 536, 537

tortoise-shell, 105

trabeculae, 144

trachea, 312

trachea, origin of, 270

tracheal pieces, 160, 313

tracheal rings, 313, 314

triconodont dentition, 279

trigonodont dentition, 279

trilobites, 19

triradii, 91

Triton, 19, 67, 313, 483

tritubercular theory, 278, 279

truncus arteriosus, 324

trunk vertebrae, 130

tuba auditiva, 269

tuberculum auriculi, 497

tubular glands, 97, 112

tunica dartos, 396

Tunicata, 26, 260, 303, 529, 530, 531,

532, 533, 535, 537, 538
tunica vaginalis communis, 396
tunica vaginalis propria, 396
turtles, 33, 179, 201, 253, 331, 360,

398, 399, 419, 431, 433, 483
Tylopoda, 39
tympanic bone, 496
tympanic bulla, 496
tympanic cavity, ossicles of, 159
tympanic membranes, 494
tympano-hyal, 161, 284
tympanum, 493-496
typical vertebra (Owen's), 507, 508,

Tyson's glands, 113
U

ultimo-branchial bodies, 289

umbilical arteries, 70, 71

umbilical cord, 377

umbilical veins, 70, 71

umbilicus, 347

umbilicus, relation to bladder, 378

uncinate processes, 136

ungulates, 394, 417, 480, 496

unstriated muscle, 189

ureter, 376, 383

urethra, 377, 393, 394, 401

urethral glands, 401

urinary bladder, 377, 378

urinary organs, 369-378

urinary papilla, 374

572

INDEX

urinary system, 369-378

urodeles, 31, 44, 174, 187, 201, 206,
227, 229, 238, 247, 287, 288, 346,
354, 355, 440, 442, 47i, 493, 5O3

urogenital sinus, 378, 387, 388, 393,

394

urogenital system, origin of, 64, 65
uropygeal gland, HI
urostyle, 135
Ursus, 87

uterine ligaments, 390
uterine mucous membrane, 72
uterus, 383, 387, 388, 389, 394;

bicornis, 390; bipartitus, 390;

duplex, 390; masculinus, 393;

simplex, 390

utriculo-saccular canal, 487
utriculus, 487.

vagina, 387, 389, 390, 394

vasa aberrantia, 392

vasa efferentia, 381, 383, 385

vascular system, 65, 317-319
Vas deferens, 377. (See ductus def-
erens)

vein or veins: — 318, 328; abdom-
inal, 342; allantoic, 322, 346, 347,
348; anterior cardinal, 321, 326,
349, 350; azygos, 344, 350, 351;
cardinal, 321, 326, 341 ; caudal,
327, 341, 343 ; Cuvierian duct, 322,
326, 346 ; duct of Cuvier, 322, 326,
346; ductus venosus Arantii, 348,
349; external jugular, 350; hemi-
azygos, 351; hepatic, 323, 327;
hepatic portal, 324, 327, 342,
344; iliac, 326, 346, 358; in-
ternal jugular, 350; jugular, 350,
358; lateral, 326, 346; omphalo-
mesenteric, 323, 347, 348; ova-
rian, 346; portal, 323, 327; post
cava, 342, 344, 345, 349; posterior
cardinal, 321, 326, 343, 344, 345,

349, 358; pulmocutaneous, 354;
renal, 345; renales advehentes,
324, 327; renales revehentes, 324,
327; renal portal, 324, 327, 341;
spermatic, 346; subclavian, 327,

350, 358; subintestinal, 340, 349;
subintestinal of Amphioxus, 528;

veins — ( Continued}

umbilical, 322, 346, 347, 348; vena
anonyma, 350, 352; vena cava
anterior, 352; vena cava pos-
terior, 342, 344, 345, 349; vitel-
line, 320, 340, 347, 353; yolk, 320,
340, 347

velum palati, 271

ventral fins, 164

ventral nerves, 436

ventricle, 324, 353-357

ventricles of brain, 62, 406, 411, 412

Vermes, 258

vertebrae, articulations of, 131

vertebrae, development of, 124-127

vertebral column, development of,
124-127

vertebral column of birds, 132

vetebral column, regional differen-
tiation of, 130

vertebral foramina, 138

vertebrate history, sketch of, 13-15

vertebrates, phylogenetic tree of, 28

vesicles of brain, 411

vesicula prostatica, 393, 394

vernix caseosa, 95

vibrissae, 472

vidian canal, 450

villi, chorionic, 70, 71

visceral mesoderm, 63, 66

visceral arches, 267

visceral muscles, 190, 192, 193

visceral skeleton, 122, 152-162

visceral skeleton, metamorphoses
of, 161

vitreous humor, 423, 498, 501

vomero-nasal cartilage, 483

vomero-nasal organ, 483

voluntary muscle, 65, 189, 190

W

Weber's apparatus, 486

whale, 165, 478, 491, 505

white blood corpuscles, 318, 362

white matter, 428

wish-bone, 141

Wolffian body, 373, 386

Wolffian duct, 64, 374, 375, 387, 392

woodchuck, 297

woodpecker, 283.

INDEX .573

X yolk stalk, 69, 70

Xenarthra, 37. Z

Y zona radiata, 52

zonary placenta, 72

yolk, 52, 53, 69, 73 zygapophyses, 508

yolk sac, 69, 70, 319 zygote, 7, 48, 58.

LAST DATE
CENTS

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2-57967

BiOLOGY

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