- OSSEOUS SYSTEM. (Comp. Anat.) 
1. The spinal column, the centre of the whole 
fabrie enclosing in a canal formed by arches 
surmounting its dorsal aspect the medulla spi- 
nalis or the axis of the spinal portion of the 
nervous system. 
_ 2. The cranium, essentially composed of 
vertebre ; but here, in consequence of the 
enlarged size of that of the cerebro-spinal 
axis of the nervous apparatus placed within 
them, exaggerated in size and modified in form. 
3. Of a frame of bones appended to the 
anterior part of thecranium for the lodgement of 
the organs of those senses that are immediately 
in connection with the encephalon, forming 
_ what, taken collectively, is called the face. 
4. Of a hyo-branchial apparatus forming the 
framework of the throat, and supporting the 
organs connected with aquatic respiration. 
These last of course are only present in animals 
breathing by gills, and can only be expected to 
exist in a state of complete developement in 
the class of Fishes. 
5. Of the thoracic apparatus, composed of 
two sets of ribs—a dorsal and a sternal series— 
and of the sternum, which itself, when fully 
developed, is made up of numerous bones. 
_ 6. Ofa pair of anterior extremities, divisible 
into shoulder, arm, forearm, carpus, meta- 
carpus, and digits. 
7. Of a pair of posterior extremities, con- 
_ structed after the same model as the last, and 
_ presenting corresponding parts, to which the 
names pelvis, thigh, leg, tarsus, metatarsus, 
; _ and toes are respectively appropriated. 
The most complete skeleton with which we 
_ are acquainted among existing Vertebrata is that 
_ of the Crocodiles, the study of which cannot be 
too strongly recommended *to the comparative 
osteologist, as in these creatures all its parts 
_fTemain permanently in a medium condition, 
so that the arbitrary divisions of the skeleton 
adopted by the human anatomist are at once 
recognisable, although we find others which in 
Man have noexistence. Thespineis divisible into 
acervical region (fig. 432, a, b) interposed be- 
tween the cranium and the thorax, although ribs 
© are appended even to the cervical vertebre. 
e dorsal region (b, c) supporting the thoracic 
ribs, the lumbar (c,d), the sacrum (e), and the 
caudal (f') are distinguishable for the same rea- 
_ sons as in the human subject, notwithstanding 
__ that the caudal portion resembles anything rather 
_ than the human os coccygis; for here, so far 
_ from its being formed merely of the rudiments 
_ of the bodies of almost obliterated vertebre, 
__ the processes form very powerful levers, and of 
there are some developed inferiorly (g) 
_ of which no vestiges exist in the human skeleton. 
_ The bones of the cranium and face are far more 
_ Mumerous than in the skull of our own species, 
_ as we shall explain more minutely hereafter: 
_ see fig. 441, where they are delineated on an 
enlarged scale. The thorax consists of dorsal 
ribs (/) and of sternal ribs (m), which are equally 
important elements of the skeleton and of the 
sternum, here situated much as in the human 
subject. Behind the sternum, moreover, and 
extending from it quite to the pubic bones, 
_ there is in the Crocodile a set of ventral ribs () 
| towhich in Man there is nothing analogous, 
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except, perhaps, the tendinous intersections still 
lingering in the recti muscles of the abdomen. 
The shoulder (p, g) consists, like the pelvis 
(A, z), of three distinct and important bones, 
while all the pieces entering into the formation 
of the extremities very nearly resemble what is 
met with in the human subject. 
Having premised thus much, we may now, 
without further preface, plunge more deeply 
into our subject, and, taking in detail all the 
elements that are recognised by modern anato- 
mists as belonging to the osseous system, exa- 
mine them separately in the various aspects 
under which they present themselves in the 
different classes composing the Vertebrate por- 
tion of animated nature. 
Spinal column.—Commencing our analysis of 
the skeleton by an examination of the spine as 
being the most essential portion of the osseous 
system, the primary or central part to which all 
others that are met with in the different classes 
of Vertebrata may or may not be superadded in 
accordance with the conditions under which 
they are aspomies to exist, we shall soon per- 
ceive that both in texture and composition it 
offers very important varieties. In the Myxine 
and Lampreys it is a simple stem of extremely 
soft cartilage, almost gelatinous in its con- 
sistence, which traverses the axis of the body, 
presenting, when superficially examined, no 
appearance of division into separate vertebre ; 
and it is not uninteresting to observe how, 
advancing from this simplest form of spine 
through various tribes of Fishes, its separation 
into distinct pieces is gradually effected. But 
even in the Lamprey, on strict examination, 
there are perceptible in the arches that embrace 
the spinal canal and on the surface of the soft 
cord that represents the bodies of the vertebra, 
slight indications of an incipent division into ver- 
tebral pieces, which are represented by slender 
rings of ossific matter that encircle at intervals 
the soft cartilage upon which they sensibly 
encroach. In a more advanced form of the 
spine, these ossified rings are considerably 
increased in their relative proportions, and en- 
croach further and further upon the cartilaginous 
stem until they penetrate even to its centre, and 
are then no longer dubiously the representatives 
of the bodies of so many vertebre. In the 
generality of Fishes, indeed, the central part 
remains unossified, so that a cartilaginous axis 
traverses the vertebral column from end to end. 
At last even this is obliterated, and the vertebral 
centres are completely formed. 
But even before the bodies of the vertebra 
are thus perfected, the lamine destined to 
enclose and protect the spinal cord are fully 
formed by the deposition of osseous matter, as 
may be readily seen in the Sharks and Rays 
and many other cartilaginous Fishes, in which, 
although the complete consolidation of the 
body has not yet been achieved, the spinous 
and other processes destined to form the fulcra 
upon which muscular action is to be exerted 
are so ossified as to afford the needful solidity 
and strength. In these races of Fishes, indeed, 
the condition of the spinal column is not a 
little remarkable, inasmuch as in the Skates the 
anterior vertebre are so consolidated by an 
