The peculiar structure of the locomotive apparatus is one of the
features that are most distinctive of the vertebrate stem. The chief
part of this apparatus is formed, as in all the higher animals, by the
active organs of movement, the muscles; in consequence of their
contractility they have the power to draw up and shorten themselves.
This effects the movement of the various parts of the body, and thus
the whole body is conveyed from place to place. But the arrangement of
these muscles and their relation to the solid skeleton are different
in the Vertebrates from the Invertebrates.
(FIGURE 2.325. The human skeleton. From the right.
FIGURE 2.326. The human skeleton. Front.)
In most of the lower animals, especially the Platodes and Vermalia, we
find that the muscles form a simple, thin layer of flesh immediately
underneath the skin. This muscular layer is very closely connected
with the skin itself; it is the same in the Mollusc stem. Even in the
large division of the Articulates, the classes of crabs, spiders,
myriapods, and insects, we find a similar feature, with the difference
that in this case the skin forms a solid armour--a rigid cutaneous
skeleton made of chitine (and often also of carbonate of lime). This
external chitine coat undergoes a very elaborate articulation both on
the trunk and the limbs of the Articulates, and in consequence the
muscular system also, the contractile fibres of which are attached
inside the chitine tubes, is highly articulated. The Vertebrates form
a direct contrast to this. In these alone a solid internal skeleton is
developed, of cartilage or bone, to which the muscles are attached.
This bony skeleton is a complex lever apparatus, or PASSIVE apparatus
of movement. Its rigid parts, the arms of the levers, or the bones,
are brought together by the actively mobile muscles, as if by
drawing-ropes. This admirable locomotorium, especially its solid
central axis, the vertebral column, is a special feature of the
Vertebrates, and has given the name to the group.
(FIGURE 2.327. The human vertebral column (standing upright, from the
right side). (From H. Meyer.))
In order to get a clear idea of the chief features of the development
of the human skeleton, we must first examine its composition in the
adult frame (Figure 2.325, the human skeleton seen from the right;
Figure 2.326, front view of the whole skeleton). As in other mammals,
we distinguish first between the axial or dorsal skeleton and the
skeleton of the limbs. The axial skeleton consists of the vertebral
column (the skeleton of the trunk) and the skull (skeleton of the
head); the latter is a peculiarly modified part of the former. As
appendages of the vertebral column we have the ribs, and of the skull
we have the hyoid bone, the lower jaw, and the other products of the
gill-arches.
The skeleton of the limbs or extremities is composed of two groups of
parts--the skeleton of the extremities proper and the zone-skeleton,
which connects these with the vertebral column. The zone-skeleton of
the arms (or fore legs) is the shoulder-zone; the zone-skeleton of the
legs (or hind legs) is the pelvic zone.
(FIGURE 2.328. A piece of the axial rod (chorda dorsalis), from a
sheep embryo. a cuticular sheath, b cells. (From Kolliker.))
The vertebral column (Figure 2.327) in man is composed of thirty-three
to thirty-five ring-shaped bones in a continuous series (above each
other, in man's upright position). These vertebrae are separated from
each other by elastic ligaments, and at the same time connected by
joints, so that the whole column forms a firm and solid, but flexible
and elastic, axial skeleton, moving freely in all directions. The
vertebrae differ in shape and connection at the various parts of the
trunk, and we distinguish the following groups in the series,
beginning at the top: Seven cervical vertebrae, twelve dorsal
vertebrae, five lumbar vertebrae, five sacral vertebrae, and four to
six caudal vertebrae. The uppermost, or those next to the skull, are
the cervical vertebrae (Figure 2.327); they have a hole in each of the
lateral processes. There are seven of these vertebrae in man and
almost all the other mammals, even if the neck is as long as that of
the camel or giraffe, or as short as that of the mole or hedgehog.
This constant number, which has few exceptions (due to adaptation), is
a strong proof of the common descent of the mammals; it can only be
explained by faithful heredity from a common stem-form, a primitive
mammal with seven cervical vertebrae. If each species had been created
separately, it would have been better to have given the long-necked
mammals more, and the short-necked animals less, cervical vertebrae.
Next to these come the dorsal (or pectoral) vertebrae, which number
twelve to thirteen (usually twelve) in man and most of the other
mammals. Each dorsal vertebra (Figure 1.165) has at the side,
connected by joints, a couple of ribs, long bony arches that lie in
and protect the wall of the chest. The twelve pairs of ribs, together
with the connecting intercostal muscles and the sternum, which joins
the ends of the right and left ribs in front, form the chest (thorax).
In this strong and elastic frame are the lungs, and between them the
heart. Next to the dorsal vertebrae comes a short but stronger section
of the column, formed of five large vertebrae. These are the lumbar
vertebrae (Figure 1.166); they have no ribs and no holes in the
transverse processes. To these succeeds the sacral bone, which is
fitted between the two halves of the pelvic zone. The sacrum is formed
of five vertebrae, completely blended together. Finally, we have at
the end a small rudimentary caudal column, the coccyx. This consists
of a varying number (usually four, more rarely three, or five or six)
of small degenerated vertebrae, and is a useless rudimentary organ
with no actual physiological significance. Morphologically, however,
it is of great interest as an irrefragable proof of the descent of man
and the anthropoids from long-tailed apes. On no other theory can we
explain the existence of this rudimentary tail. In the earlier stages
of development the tail of the human embryo protrudes considerably. It
afterwards atrophies; but the relic of the atrophied caudal vertebrae
and of the rudimentary muscles that once moved it remains permanently.
Sometimes, in fact, the external tail is preserved. The older
anatomists say that the tail is usually one vertebra longer in the
human female than in the male (or four against five); Steinbach says
it is the reverse.
(FIGURE 2.329. Three dorsal vertebrae, from a human embryo, eight
weeks old, in lateral longitudinal section. v cartilaginous vertebral
body, li inter-vertebral disks, ch chorda. (From Kolliker.)
(FIGURE 2.330. A dorsal vertebra of the same embryo, in lateral
transverse section. cv cartilaginous vertebral body, ch chorda, pr
transverse process, a vertebral arch (upper arch), c upper end of the
rib (lower arch). (From Kolliker.))
In the human vertebral column there are usually thirty-three
vertebrae. It is interesting to find, however, that the number often
changes, one or two vertebrae dropping out or an additional one
appearing. Often, also, a mobile rib is formed at the last cervical or
the first lumbar vertebra, so that there are then thirteen dorsal
vertebrae, besides six cervical and four lumbar. In this way the
contiguous vertebrae of the various sections of the column may take
each other's places.
In order to understand the embryology of the human vertebral column we
must first carefully consider the shape and connection of the
vertebrae. Each vertebra has, in general, the shape of a seal-ring
(Figures 1.164 to 1.166). The thicker portion, which is turned towards
the ventral side, is called the body of the vertebra, and forms a
short osseous disk; the thinner part forms a semi-circular arch, the
vertebral arch, and is turned towards the back. The arches of the
successive vertebrae are connected by thin intercrural ligaments in
such a way that the cavity they collectively enclose represents a long
canal. In this vertebral canal we find the trunk part of the central
nervous system, the spinal cord. Its head part, the brain, is enclosed
by the skull, and the skull itself is merely the uppermost part of the
vertebral column, distinctively modified. The base or ventral side of
the vesicular cranial capsule corresponds originally to a number of
developed vertebral bodies; its vault or dorsal side to their combined
upper vertebral arches.
(FIGURE 2.331. Intervertebral disk of a new-born infant, transverse
section. a rest of the chorda. (From Kolliker.))
While the solid, massive bodies of the vertebrae represent the real
central axis of the skeleton, the dorsal arches serve to protect the
central marrow they enclose. But similar arches develop on the ventral
side for the protection of the viscera in the breast and belly. These
lower or ventral vertebral arches, proceeding from the ventral side of
the vertebral bodies, form, in many of the lower Vertebrates, a canal
in which the large blood-vessels are enclosed on the lower surface of
the vertebral column (aorta and caudal vein). In the higher
Vertebrates the majority of these vertebral arches are lost or become
rudimentary. But at the thoracic section of the column they develop
into independent strong osseous arches, the ribs (costae). In reality
the ribs are merely large and independent lower vertebral arches,
which have lost their original connection with the vertebral bodies.
If we turn from this anatomic survey of the composition of the column
to the question of its development, I may refer the reader to earlier
pages with regard to the first and most important points (Chapter
1.14). It will be remembered that in the human embryo and that of the
other vertebrates we find at first, instead of the segmented column,
only a simple unarticulated cartilaginous rod. This solid but flexible
and elastic rod is the axial rod (or the chorda dorsalis). In the
lowest Vertebrate, the Amphioxus, it retains this simple form
throughout life, and permanently represents the whole internal
skeleton (Figure 2.210 i). In the Tunicates, also, the nearest
Invertebrate relatives of the Vertebrates, we meet the same
chorda--transitorily in the passing larva tail of the Ascidia,
permanently in the Copelata (Figure 2.225 c). Undoubtedly both the
Tunicates and Acrania have inherited the chorda from a common
unsegmented stem-form; and these ancient, long-extinct ancestors of
all the chordonia are our hypothetical Prochordonia.
Long before there is any trace of the skull, limbs, etc., in the
embryo of man or any of the higher Vertebrates--at the early stage in
which the whole body is merely a sole-shaped embryonic shield--there
appears in the middle line of the shield, directly under the medullary
furrow, the simple chorda. (Cf. Figures 1.131 to 1.135 ch). It follows
the long axis of the body in the shape of a cylindrical axial rod of
elastic but firm composition, equally pointed at both ends. In every
case the chorda originates from the dorsal wall of the primitive gut;
the cells that compose it (Figure 2.328 b) belong to the entoderm
(Figures 2.216 to 2.221). At an early stage the chorda develops a
transparent structureless sheath, which is secreted from its cells
(Figure 2.328 a). This chordalemma is often called the "inner
chorda-sheath," and must not be confused with the real external
sheath, the mesoblastic perichorda.
(FIGURE 2.332. Human skull.
FIGURE 2.333. Skull of a new-born child. (From Kollmann.) Above, in
the three bones of the roof of the skull, we see the lines that
radiate from the central points of ossification; in front, the frontal
bone; behind, the occipital bone; between the two the large parietal
bone, p. s the scurf bone, w mastoid fontanelle, f petrous bone, t
tympanic bone, l lateral part, b bulla, j cheek-bone, a large wing of
cuneiform bone, k fontanelle of cuneiform bone.)
But this unsegmented primary axial skeleton is soon replaced by the
segmented secondary axial skeleton, which we know as the vertebral
column. The provertebral plates (Figure 1.124 s) differentiate from
the innermost, median part of the visceral layer of the coelom-pouches
at each side of the chorda. As they grow round the chorda and enclose
it they form the skeleton plate or skeletogenetic layer--that is to
say, the skeleton-forming stratum of cells, which provides the mobile
foundation of the permanent vertebral column and skull (scleroblast).
In the head-half of the embryo the skeletal plate remains a
continuous, simple, undivided layer of tissue, and presently enlarges
into a thin-walled capsule enclosing the brain, the primordial skull.
In the trunk-half the provertebral plate divides into a number of
homogeneous, cubical, successive pieces; these are the several
primitive vertebrae. They are not numerous at first, but soon increase
as the embryo grows longer (Figures 1.153 to 1.155).
(FIGURE 2.334. Head-skeleton of a primitive fish, n nasal pit, eth
cribriform bone region, orb orbit of eye, la wall of auscultory
labyrinth, occ occipital region of primitive skull, cv vertebral
column, a fore, bc hind-lip cartilage, o primitive upper jaw
(palato-quadratum), u primitive lower jaw, II hyaloid bone, III to
VIII first to sixth branchial arches. (From Gegenbaur.)
FIGURE 2.335. Roofs of the skulls of nine Primates (Cattarrhines),
seen from above and reduced to a common size. 1 European, 2 Brazilian,
3 Pithecanthropus, 4 Gorilla, 5 Chimpanzee, 6 Orang, 7 Gibbon, 8
Tailed ape, 9 Baboon.)
In all the Craniotes the soft, indifferent cells of the mesoderm,
which originally compose the skeletal plate, are afterwards converted
for the most part into cartilaginous cells, and these secrete a firm
and elastic intercellular substance between them, and form
cartilaginous tissue. Like most of the other parts of the skeleton,
the membranous rudiments of the vertebrae soon pass into a
cartilaginous state, and in the higher Vertebrates this is afterwards
replaced by the hard osseous tissue with its characteristic stellate
cells (Figure 1.6). The primary axial skeleton remains a simple chorda
throughout life in the Acrania, the Cyclostomes, and the lowest
fishes. In most of the other Vertebrates the chorda is more or less
replaced by the cartilaginous tissue of the secondary perichorda that
grows round it. In the lower Craniotes (especially the fishes) a more
or less considerable part of the chorda is preserved in the bodies of
the vertebrae. In the mammals it disappears for the most part. By the
end of the second month in the human embryo the chorda is merely a
slender thread, running through the axis of the thick, cartilaginous
vertebral column (Figures 1.182 ch and 2.329 ch). In the cartilaginous
vertebral bodies themselves, which afterwards ossify, the slender
remnant of the chorda presently disappears (Figure 2.330 ch). But in
the elastic inter-vertebral disks, which develop from the skeletal
plate between each pair of vertebral bodies (Figure 2.329 li), a relic
of the chorda remains permanently. In the new-born child there is a
large pear-shaped cavity in each intervertebral disk, filled with a
gelatinous mass of cells (Figure 2.331 a). Though less sharply
defined, this gelatinous nucleus of the elastic cartilaginous disks
persists throughout life in the mammals, but in the birds and most
reptiles the last trace of the chorda disappears. In the subsequent
ossification of the cartilaginous vertebra the first deposit of bony
matter ("first osseous nucleus") takes place in the vertebral body
immediately round the remainder of the chorda, and soon displaces it
altogether. Then there is a special osseous nucleus formed in each
half of the vertebral arch. The ossification does not reach the point
at which the three nuclei are joined until after birth. In the first
year the two osseous halves of the arches unite; but it is much
later--in the second to the eighth year--that they connect with the
osseous vertebral bodies.
(FIGURE 2.336. Skeleton of the breast-fin of Ceratodus (biserial
feathered skeleton). A, B, cartilaginous series of the fin-stem. rr
cartilaginous fin-radii. (From Gunther.)
FIGURE 2.337. Skeleton of the breast-fin of an early Selachius
(Acanthias). The radii of the median fin-border (B) have disappeared
for the most part; a few only (R) are left. R, R, radii of the lateral
fin-border, mt metapterygium, ms mesopterygium, p propterygium. (From
Gegenbaur.)
FIGURE 2.338. Skeleton of the breast-fin of a young Selachius. The
radii of the median fin-border have wholly disappeared. The shaded
part on the right is the section that persists in the five-fingered
hand of the higher Vertebrates. (b the three basal pieces of the fin:
mt metapterygium, rudiment of the humerus, ms mesopterygium, p
propterygium.) (From Gegenbaur.))
The bony skull (cranium), the head-part of the secondary axial
skeleton, develops in just the same way as the vertebral column. The
skull forms a bony envelope for the brain, just as the vertebral canal
does for the spinal cord; and as the brain is only a peculiarly
differentiated part of the head, while the spinal cord represents the
longer trunk-section of the originally homogeneous medullary tube, we
shall expect to find that the osseous coat of the one is a special
modification of the osseous envelope of the other. When we examine the
adult human skull in itself (Figure 2.332), it is difficult to
conceive how it can be merely the modified fore part of the vertebral
column. It is an elaborate and extensive bony structure, composed of
no less than twenty bones of different shapes and sizes. Seven of them
form the spacious shell that surrounds the brain, in which we
distinguish the solid ventral base below and the curved dorsal vault
above. The other thirteen bones form the facial skull, which is
especially the bony envelope of the higher sense-organs, and at the
same time encloses the entrance of the alimentary canal. The lower jaw
is articulated at the base of the skull (usually regarded as the XXI
cranial bone). Behind the lower jaw we find the hyoid bone at the root
of the tongue, also formed from the gill-arches, and a part of the
lower arches that have developed as "head-ribs" from the ventral side
of the base of the cranium.
Although the fully-developed skull of the higher Vertebrates, with its
peculiar shape, its enormous size, and its complex composition, seems
to have nothing in common with the ordinary vertebrae, nevertheless
even the older comparative anatomists came to recognise at the end of
the eighteenth century that it is really nothing else originally than
a series of modified vertebrae. When Goethe in 1790 "picked up the
skull of a slain victim from the sand of the Jewish cemetery at
Venice, he noticed at once that the bones of the face also could be
traced to vertebrae (like the three hind-most cranial vertebrae)." And
when Oken (without knowing anything of Goethe's discovery) found at
Ilenstein, "a fine bleached skull of a hind, the thought flashed
across him like lightning: 'It is a vertebral column.'"
(FIGURE 2.339. Skeleton of the fore leg of an amphibian. h upper-arm
(humerus), ru lower arm (r radius, u ulna), rcicu apostrophe,
wrist-bones of first series (r radiale, i intermedium, c centrale, u
apostrophe ulnare). 1, 2, 3, 4, 5 wrist-bones of the second series.
(From Gegenbaur.)
FIGURE 2.340. Skeleton of gorilla's hand. (From Huxley.)
FIGURE 2.341. Skeleton of human hand, back. (From Meyer.))
This famous vertebral theory of the skull has interested the most
distinguished zoologists for more than a century: the chief
representatives of comparative anatomy have devoted their highest
powers to the solution of the problem, and the interest has spread far
beyond their circle. But it was not until 1872 that it was happily
solved, after seven years' labour, by the comparative anatomist who
surpassed all other experts of this science in the second half of the
nineteenth century by the richness of his empirical knowledge and the
acuteness and depth of his philosophic speculations. Carl Gegenbaur
has shown, in his classic Studies of the Comparative Anatomy of the
Vertebrates (third section), that we find the most solid foundation
for the vertebral theory of the skull in the head-skeleton of the
Selachii. Earlier anatomists had wrongly started from the mammal
skull, and had compared the several bones that compose it with the
several parts of the vertebra (Figure 2.333) they thought they could
prove in this way that the fully-formed mammal skull was made of from
three to six vertebrae.
The older theory was refuted by simple and obvious facts, which were
first pointed out by Huxley. Nevertheless, the fundamental idea of
it--the belief that the skull is formed from the head-part of the
perichordal axial skeleton, just as the brain is from the simple
medullary tube, by differentiation and modification--remained. The
work now was to discover the proper way of supplying this philosophic
theory with an empirical foundation, and it was reserved for Gegenbaur
to achieve this. He first opened out the phylogenetic path which here,
as in all morphological questions, leads most confidently to the goal.
He showed that the primitive fishes (Figures 2.249 to 2.251), the
ancestors of all the Gnathostomes, still preserve permanently in the
form of their skull the structure out of which the transformed skull
of the higher Vertebrates, including man, has been evolved. He further
showed that the branchial arches of the Selachii prove that their
skull originally consisted of a large number of (at least nine or ten)
provertebrae, and that the cerebral nerves that proceed from the base
of the brain entirely confirm this. These cerebral nerves are (with
the exception of the first and second pair, the olfactory and optic
nerves) merely modifications of spinal nerves, and are essentially
similar to them in their peripheral expansion. The comparative anatomy
of these cerebral nerves, their origin and their expansion, furnishes
one of the strongest arguments for the new vertebral theory of the
skull.
(FIGURE 2.342. Skeleton of the hand or fore foot of six mammals. I
man, II dog, III pig, IV ox, V tapir, VI horse. r radius, u ulna, a
scaphoideum, b lunare, a triquetrum, d trapezium, e trapezoid, f
capitatum, g hamatum, p pisiforme. 1 thumb, 2 index finger, 3 middle
finger, 4 ring finger, 5 little finger. (From Gegenbaur.))
We have not space here to go into the details of Gegenbaur's theory of
the skull. I must be content to refer the reader to the great work I
have mentioned, in which it is thoroughly established from the
empirico-philosophical point of view. He has also given a
comprehensive and up-to-date treatment of the subject in his
Comparative Anatomy of the Vertebrates (1898). Gegenbaur indicates as
original "cranial ribs," or "lower arches of the cranial vertebrae,"
at each side of the head of the Selachii (Figure 2.334), the following
pairs of arches: I and II, two lip-cartilages, the anterior (a) of
which is composed of an upper piece only, the posterior (bc) from an
upper and lower piece; III, the maxillary arches, also consisting of
two pieces on each side--the primitive upper jaw (os palato-quadratum,
o) and the primitive lower jaw (u); IV, the hyaloid bone (II);
finally, V to X, six branchial arches in the narrower sense (III to
VIII). From the anatomic features of these nine to ten cranial ribs or
"lower vertebral arches" and the cranial nerves that spread over them,
it is clear that the apparently simple cartilaginous primitive skull
of the Selachii was originally formed from so many (at least nine)
somites or provertebrae. The blending of these primitive segments into
a single capsule is, however, so ancient that, in virtue of the law of
curtailed heredity, the original division seems to have disappeared;
in the embryonic development it is very difficult to detect it in
isolated traces, and in some respects quite impossible. It is claimed
that several (three to six) traces of provertebrae have been
discovered in the anterior (pre-chordal) part of the Selachii-skull;
this would bring up the number of cranial somites to twelve or
sixteen, or even more.
(FIGURES 2.343 TO 2.345. Arm and hand of three anthropoids.
FIGURE 2.343. Chimpanzee (Anthropithecus niger).
FIGURE 2.344. Veddah of Ceylon (Homo veddalis).
FIGURE 2.345. European (Homo mediterraneus). (From Paul and Fritz
Sarasin.))
In the primitive skull of man (Figure 2.323) and the higher
Vertebrates, which has been evolved from that of the Selachii, five
consecutive sections are discoverable at a certain early period of
development, and one might be induced to trace these to five primitive
vertebrae; but these sections are due entirely to adaptation to the
five primitive cerebral vesicles, and correspond, like these, to a
large number of metamera. That we have in the primitive skull of the
mammals a greatly modified and transformed organ, and not at all a
primitive formation, is clear from the circumstance that its original
soft membranous form only assumes the cartilaginous character for the
most part at the base and the sides, and remains membranous at the
roof. At this part the bones of the subsequent osseous skull develop
as external coverings over the membranous structure, without an
intermediate cartilaginous stage, as there is at the base of the
skull. Thus a large part of the cranial bones develop originally as
covering bones from the corium, and only secondarily come into close
touch with the primitive skull (Figure 2.333). We have previously seen
how this very rudimentary beginning of the skull in man is formed
ontogenetically from the "head-plates," and thus the fore end of the
chorda is enclosed in the base of the skull. (Cf. Figs 1.145 and
Chapters 1.13 and 1.14.)
The phylogeny of the skull has made great progress during the last
three decades through the joint attainments of comparative anatomy,
ontogeny, and paleontology. By the judicious and comprehensive
application of the phylogenetic method (in the sense of Gegenbaur) we
have found the key to the great and important problems that arise from
the thorough comparative study of the skull. Another school of
research, the school of what is called "exact craniology" (in the
sense of Virchow), has, meantime, made fruitless efforts to obtain
this result. We may gratefully acknowledge all that this descriptive
school has done in the way of accurately describing the various forms
and measurements of the human skull, as compared with those of other
mammals. But the vast empirical material that it has accumulated in
its extensive literature is mere dead and sterile erudition until it
is vivified and illumined by phylogenetic speculation.
Virchow confined himself to the most careful analysis of large numbers
of human skulls and those of anthropoid mammals. He saw only the
differences between them, and sought to express these in figures.
Without adducing a single solid reason, or offering any alternative
explanation, he rejected evolution as an unproved hypothesis. He
played a most unfortunate part in the controversy as to the
significance of the fossil human skulls of Spy and Neanderthal, and
the comparison of them with the skull of the Pithecanthropus (Figure
2.283). All the interesting features of these skulls that clearly
indicated the transition from the anthropoid to the man were declared
by Virchow to be chance pathological variations. He said that the roof
of the skull of Pithecanthropus (Figure 2.335, 3) must have belonged
to an ape, because so pronounced an orbital stricture (the horizontal
constriction between the outer edge of the eye-orbit and the temples)
is not found in any human being. Immediately afterwards Nehring showed
in the skull of a Brazilian Indian (Figure 2.335, 2), found in the
Sambaquis of Santos, that this stricture can be even deeper in man
than in many of the apes. It is very instructive in this connection to
compare the roofs of the skulls (seen from above) of different
primates. I have, therefore, arranged nine such skulls in Figure
2.335, and reduced them to a common size.
(FIGURE 2.346. Transverse section of a fish's tail (from the tunny).
(From Johannes Muller.) a upper (dorsal) lateral muscles, a
apostrophe, b apostrophe lower (ventral) lateral muscles, d vertebral
bodies, b sections of incomplete conical mantle, B attachment lines of
the inter-muscular ligaments (from the side).)
We turn now to the branchial arches, which were regarded even by the
earlier natural philosophers as "head-ribs." (Cf. Figures 1.167 to
1.170). Of the four original gill-arches of the mammals the first lies
between the primitive mouth and the first gill-cleft. From the base of
this arch is formed the upper-jaw process, which joins with the inner
and outer nasal processes on each side, in the manner we have
previously explained, and forms the chief parts of the skeleton of the
upper jaw (palate bone, pterygoid bone, etc.) (Cf. Chapter 2.25.) The
remainder of the first branchial arch, which is now called, by way of
contrast, the "upper-jaw process," forms from its base two of the
ear-ossicles (hammer and anvil), and as to the rest is converted into
a long strip of cartilage that is known, after its discoverer, as
"Meckel's cartilage," or the promandibula. At the outer surface of the
latter is formed from the cellular matter of the corium, as covering
or accessory bone, the permanent bony lower jaw. From the first part
or base of the second branchial arch we get, in the mammals, the third
ossicle of the ear, the stirrup; and from the succeeding parts we get
(in this order) the muscle of the stirrup, the styloid process of the
temporal bone, the styloid-hyoid ligament, and the little horn of the
hyoid bone. The third branchial arch is only cartilaginous at the
foremost part, and here the body of the hyoid bone and its larger horn
are formed at each side by the junction of its two halves. The fourth
branchial arch is only found transitorily in the mammal embryo as a
rudimentary organ, and does not develop special parts; and there is no
trace in the embryo of the higher Vertebrates of the posterior
branchial arches (fifth and sixth pair), which are permanent in the
Selachii. They have been lost long ago. Moreover, the four gill-clefts
of the human embryo are only interesting as rudimentary organs, and
they soon close up and disappear. The first alone (between the first
and second branchial arches) has any permanent significance; from it
are developed the tympanic cavity and the Eustachian tube. (Cf.
Figures 1.169 and 2.320.)
It was Carl Gegenbaur again who solved the difficult problem of
tracing the skeleton of the limbs of the Vertebrates to a common type.
Few parts of the vertebrate body have undergone such infinitely varied
modifications in regard to size, shape, and adaptation of structure as
the limbs or extremities; yet we are in a position to reduce them all
to the same hereditary standard. We may generally distinguish three
groups among the Vertebrates in relation to the formation of their
limbs. The lowest and earliest Vertebrates, the Acrania and
Cyclostomes, had, like their invertebrate ancestors, no pairs of
limbs, as we see in the Amphioxus and the Cyclostomes to-day (Figures
2.210 and 2.247). The second group is formed of the two classes of the
true fishes and the Dipneusts; here there are always two pairs of
limbs at first, in the shape of many-toed fins--one pair of
breast-fins or fore legs, and one pair of belly-fins or hind legs
(Figures 2.248 to 2.259). The third group comprises the four higher
classes of Vertebrates--the amphibia, reptiles, birds, and mammals; in
these quadrupeds there are at first the same two pairs of limbs, but
in the shape of five-toed feet. Frequently we find less than five
toes, and sometimes the feet are wholly atrophied (as in the
serpents). But the original stem-form of the group had five toes or
fingers before and behind (Figures 2.263 to 2.265).
The true primitive form of the pairs of limbs, such as they were found
in the primitive fishes of the Silurian period, is preserved for us in
the Australian dipneust, the remarkable Ceratodus (Figure 2.257). Both
the breast-fin and the belly-fin are flat oval paddles, in which we
find a biserial cartilaginous skeleton (Figure 2.336). This consists,
firstly, of a much segmented fin-rod or "stem" (A, B), which runs
through the fin from base to tip; and secondly of a double row of thin
articulated fin-radii (r, r), which are attached to both sides of the
fin-rod, like the feathers of a feathered leaf. This primitive fin,
which Gegenbaur first recognised, is attached to the vertebral column
by a simple zone in the shape of a cartilaginous arch. It has probably
originated from the branchial arches.* (* While Gegenbaur derives the
fins from two pairs of posterior separated branchial arches, Balfour
holds that they have been developed from segments of a pair of
originally continuous lateral fins or folds of the skin.)
We find the same biserial primitive fin more or less preserved in the
fossilised remains of the earliest Selachii (Figure 2.248), Ganoids
(Figure 2.253), and Dipneusts (Figure 2.256). It is also found in
modified form in some of the actual sharks and pikes. But in the
majority of the Selachii it has already degenerated to the extent that
the radii on one side of the fin-rod have been partly or entirely
lost, and are retained only on the other (Figure 2.337). We thus get
the uniserial fin, which has been transmitted from the Selachii to the
rest of the fishes (Figure 2.338).
(FIGURE 2.347. Human skeleton. (Cf. Figure 2.326.)
FIGURE 2.348. Skeleton of the giant gorilla. (Cf. Figure 1.209.))
Gegenbaur has shown how the five-toed leg of the Amphibia, that has
been inherited by the three classes of Amniotes, was evolved from the
uniserial fish-fin.* (* The limb of the four higher classes of
Vertebrates is now explained in the sense that the original fin-rod
passes along its outer (ulnar or fibular) side, and ends in the fifth
toe. It was formerly believed to go along the inner (radial or tibial)
side, and end in the first toe, as Figure 2.339 shows.) In the
dipneust ancestors of the Amphibia the radii gradually atrophy, and
are lost, for the most part, on the other side of the fin-rod as well
(the lighter cartilages in Figure 2.338). Only the four lowest radii
(shaded in the illustration) are preserved; and these are the four
inner toes of the foot (first to fourth). The little or fifth toe is
developed from the lower end of the fin-rod. From the middle and upper
part of the fin-rod was developed the long stem of the limb--the
important radius and ulna (Figure 2.339 r and u) and humerus (h) of
the higher Vertebrates.
In this way the five-toed foot of the Amphibia, which we first meet in
the Carboniferous Stegocephala (Figure 2.260), and which was inherited
from them by the reptiles on one side and the mammals on the other,
was formed by gradual degeneration and differentiation from the
many-toed fish-fin (Figure 2.341). The reduction of the radii to four
was accompanied by a further differentiation of the fin-rod, its
transverse segmentation into upper and lower halves, and the formation
of the zone of the limb, which is composed originally of three limbs
before and behind in the higher Vertebrates. The simple arch of the
original shoulder-zone divides on each side into an upper (dorsal)
piece, the shoulder-blade (scapula), and a lower (ventral) piece; the
anterior part of the latter forms the primitive clavicle
(procoracoideum), and the posterior part the coracoideum. In the same
way the simple arch of the pelvic zone breaks up into an upper
(dorsal) piece, the iliac-bone (os ilium), and a lower (ventral)
piece; the anterior part of the latter forms the pubic bone (os
pubis), and the posterior the ischial bone (os ischii).
There is also a complete agreement between the fore and hind limb in
the stem or shaft. The first section of the stem is supported by a
single strong bone--the humerus in the fore, the femur in the hind
limb. The second section contains two bones: in front the radius (r)
and ulna (u), behind the tibia and fibula. (Cf. the skeletons in
Figures 2.260, 2.265, 2.270, 2.278 to 2.282, and 2.348.) The
succeeding numerous small bones of the wrist (carpus) and ankle
(tarsus) are also similarly arranged in the fore and hind extremities,
and so are the five bones of the middle-hand (metacarpus) and
middle-foot (metatarsus). Finally, it is the same with the toes
themselves, which have a similar characteristic composition from a
series of bony pieces before and behind. We find a complete parallel
in all the parts of the fore leg and the hind leg.
When we thus learn from comparative anatomy that the skeleton of the
human limbs is composed of just the same bones, put together in the
same way, as the skeleton in the four higher classes of Vertebrates,
we may at once infer a common descent of them from a single stem-form.
This stem-form was the earliest amphibian that had five toes on each
foot. It is particularly the outer parts of the limbs that have been
modified by adaptation to different conditions. We need only recall
the immense variations they offer within the mammal class. We have the
slender legs of the deer and the strong springing legs of the
kangaroo, the climbing feet of the sloth and the digging feet of the
mole, the fins of the whale and the wings of the bat. It will readily
be granted that these organs of locomotion differ as much in regard to
size, shape, and special function as can be conceived. Nevertheless,
the bony skeleton is substantially the same in every case. In the
different limbs we always find the same characteristic bones in
essentially the same rigidly hereditary connection; this is as
splendid a proof of the theory of evolution as comparative anatomy can
discover in any organ of the body. It is true that the skeleton of the
limbs of the various mammals undergoes many distortions and
degenerations besides the special adaptations (Figure 2.342). Thus we
find the first finger or the thumb atrophied in the fore-foot (or
hand) of the dog (II). It has entirely disappeared in the pig (III)
and tapir (V). In the ruminants (such as the ox, IV) the second and
fifth toes are also atrophied, and only the third and fourth are well
developed (VI, 3). Nevertheless, all these different fore-feet, as
well as the hand of the ape (Figure 2.340) and of man (Figure 2.341),
were originally developed from a common pentadactyle stem-form. This
is proved by the rudiments of the degenerated toes, and by the
similarity of the arrangement of the wrist-bones in all the pentanomes
(Figure 2.342 a to p).
If we candidly compare the bony skeleton of the human arm and hand
with that of the nearest anthropoid apes, we find an almost perfect
identity. This is especially true of the chimpanzee. In regard to the
proportions of the various parts, the lowest living races of men (the
Veddahs of Ceylon, Figure 2.344) are midway between the chimpanzee
(Figure 2.343) and the European (Figure 2.345). More considerable are
the differences in structure and the proportions of the various parts
between the different genera of anthropoid apes (Figures 2.278 to
2.282); and still greater is the morphological distance between these
and the lowest apes (the Cynopitheca). Here, again, impartial and
thorough anatomic comparison confirms the accuracy of Huxley's
pithecometra principle (Chapter 1.15).
The complete unity of structure which is thus revealed by the
comparative anatomy of the limbs is fully confirmed by their
embryology. However different the extremities of the four-footed
Craniotes may be in their adult state, they all develop from the same
rudimentary structure. In every case the first trace of the limb in
the embryo is a very simple protuberance that grows out of the side of
the hyposoma. These simple structures develop directly into fins in
the fishes and Dipneusts by differentiation of their cells. In the
higher classes of Vertebrates each of the four takes the shape in its
further growth of a leaf with a stalk, the inner half becoming
narrower and thicker and the outer half broader and thinner. The inner
half (the stalk of the leaf) then divides into two sections--the upper
and lower parts of the limb. Afterwards four shallow indentations are
formed at the free edge of the leaf, and gradually deepen; these are
the intervals between the five toes (Figure 1.174). The toes soon make
their appearance. But at first all five toes, both of fore and hind
feet, are connected by a thin membrane like a swimming-web; they
remind us of the original shaping of the foot as a paddling fin. The
further development of the limbs from this rudimentary structure takes
place in the same way in all the Vertebrates according to the laws of
heredity.
The embryonic development of the muscles, or ACTIVE organs of
locomotion, is not less interesting than that of the skeleton, or
PASSIVE organs. But the comparative anatomy and ontogeny of the
muscular system are much more difficult and inaccessible, and
consequently have hitherto been less studied. We can therefore only
draw some general phylogenetic conclusions therefrom.
It is incontestable that the musculature of the Vertebrates has been
evolved from that of lower Invertebrates; and among these we have to
consider especially the unarticulated Vermalia. They have a simple
cutaneous muscular layer, developing from the mesoderm. This was
afterwards replaced by a pair of internal lateral muscles, that
developed from the middle wall of the coelom-pouches; we still find
the first rudiments of the muscles arising from the muscle-plate of
these in the embryos of all the Vertebrates (cf. Figures 1.124, 1.158
to 1.160, 2.222 to 2.224 mp). In the unarticulated stem-forms of the
Chordonia, which we have called the Prochordonia, the two
coelom-pouches, and therefore also the muscle-plates of their walls,
were not yet segmented. A great advance was made in the articulation
of them, as we have followed it step by step in the Amphioxus (Figures
1.124 and 1.158). This segmentation of the muscles was the momentous
historical process with which vertebration, and the development of the
vertebrate stem, began. The articulation of the skeleton came after
this segmentation of the muscular system, and the two entered into
very close correlation.
The episomites or dorsal coelom-pouches of the Acrania, Cyclostomes,
and Selachii (Figure 1.161 h) first develop from their inner or median
wall (from the cell-layer that lies directly on the skeletal plate
[sk] and the medullary tube [nr]) a strong muscle-plate (mp). By
dorsal growth (w) it also reaches the external wall of the
coelom-pouches, and proceeds from the dorsal to the ventral wall. From
these segmental muscle-plates, which are chiefly concerned in the
segmentation of the Vertebrates, proceed the lateral muscles of the
stem, as we find in the simplest form in the Amphioxus (Figure 2.210).
By the formation of a horizontal frontal septum they divide on each
side into an upper and lower series of myotomes, dorsal and ventral
lateral muscles. This is seen with typical regularity in the
transverse section of the tail of a fish (Figure 2.346). From these
earlier lateral muscles of the trunk develop the greater part of the
subsequent muscles of the trunk, and also the much later "muscular
buds" of the limbs.* (* The ontogeny of the muscles is mostly
cenogenetic. The greater part of the muscles of the head (or the
visceral muscles) belong originally to the hyposoma of the vertebrate
organism, and develop from the wall of the hyposomites or ventral
coelom-pouches. This also applies originally to the primary muscles of
the limbs, as these too belong phylogenetically to the hyposoma. (Cf.