OSSEOUS SYSTEM. (Comp. Anat.) 
expect, in the construction of this skeleton, va- 
rieties correspondingly great both in the mate- 
rials employed and their mechanical arrange- 
ment, inasmuch as the machinery employed 
for effecting progression under circumstances 
so dissimilar must be changed in every race, 
and adapted to the peculiarities of habit con- 
ferred upon any given creature. 
The substance of which the internal ske- 
leton of a vertebrate animal is composed differs 
moreover very remarkably from that employed 
to build up the organs of support in any of the 
other divisions of the animal kingdom. In all 
the great group of Radiata (Cuv.), wherever 
a hard material is employed, it is built up by 
the slow external accretion of earthy particles 
deposited in successive layers from the living 
substance of the body, arranged not unfre- 
quently with admirable precision; but, when 
once formed, such a skeleton is entirely devoid 
of vascularity, and almost placed beyond the 
reach of vital influences. Throughout all the 
Articulata the skeleton is an external crust 
exuded from the surface of the skin, which is 
so entirely destitute of all capability of growth 
or expansion, that it must be cast off frequently 
during the life of the animal, to be renewed 
again and again as the bulk of its body is 
enlarged. In all the Mollusca, too, with the 
exception of the Cephalopods, in which a true 
bony structure begins for the first time to be 
_ developed, all the hard parts of the body are 
_ Cuticular and composed of shell. In the Ver- 
tebrata alone is found a real osseous skeleton 
nourished by bloodvessels, consisting essen- 
tially of a living tissue that is capable of con- 
_ Stant growth and renovation, having its texture 
hardened in proportion to the necessities of the 
case by an interstitial deposit of various earths, 
especially of phosphate of lime, which is con- 
tinually removed and renovated as age ad- 
vances, and, in short, is subject, during the 
whole existence of the creature, to vital in- 
fluences, its hardness and composition being 
Subject to great variations. In making use of 
the terms bone and osseous tissue, we must 
therefore be understood by no means to employ 
these words as indicating portions of the animal 
fabric endowed with any particular degree of 
density or firmness, that being entirely an ad- 
ventitious circumstance depending upon the 
‘greater or less abundance of the earthy matters 
deposited in the living tissues, and even in the 
Same animal, in this respect, offering at dif- 
ferent periods of its life the most opposite con- 
ditions. 
_ In the lowest and most feeble Fishes, which, 
im consequence of their sluggish movements 
through an element that buoys them up on all 
sides, no firmness is required in any part of their 
construction, and few of the locomotive levers 
met with in more highly-gifted forms are pre- 
Sent, the whole osseous system consists per- 
Maunently of the softest cartilage undivided as 
yet into distinct pieces; and it is only as we 
ascend from this point through successive 
groups of Cartilaginous Fishes as they are 
called, the Sharks, Rays, Sturgeons, &c., that, 
Owing to an increased deposit of the hardening 
earths within the cartilaginous web, firm- 
821 
ness and solidity are slowly given. Even in 
the most perfect Fishes the bones remain soft 
in comparison with theircondition in terres- 
trial Vertebrata, whilst it is only in Carnivorous 
Mammalia, and more especially in Birds, that 
the maximum of hardness is conferred upon 
the osseous system, a density and a strength 
commensurate with the powerful muscular 
exertions required by the conditions under 
which those races live. Equally remarkable 
are the differences observable in the texture 
of the osseous skeleton at different ages in 
the same creature The Tadpole of the Ba- 
trachian Reptile, for example, at the time 
when it commences its earliest struggles in 
the element wherein it passes the first por- 
tion of its existence, is, as relates to the condi- 
tion of this part of its economy, inferior even to 
the My.xine and the Lamprey amongst Fishes, 
consisting of the most delicate cellulosity 
or of the softest gristle. As growth proceeds, 
osseous particles accumulate, and the condition 
approximates that of the more perfect Fishes. 
Lastly, as the anterior and posterior extremities 
sprout, the bones acquire progressively the den- 
sity essential to the construction of a terrestrial 
animal, and the whole internal framework 
becomes consolidated to an extent proportioned 
to the vigorous movements of the perfect Frog. 
In the higher Mammalia the succession of the 
phases of developement is still further pro- 
longed. At its first appearance, the osseous 
system is represented by a mere web of cellular 
tissue, which slowly attains to a cartilaginous 
texture; this cartilage, during foetal growth, is 
converted into bone by the deposition of earth 
in its substance; but it is not till long after 
birth, when adult age has need to exert all the 
energies of life, that the bones are fully formed, 
hardened, and lightened to the utmost required 
extent by consolidating their substance to the 
ma.cimum, and excavating the caverns and can- 
celli that characterize the most perfectly ma- 
tured conditions of the osseous framework. 
But passing from these general views, for a 
more complete consideration of which the reader 
is referred to another article, (Osszous Tis- 
SUE,) we proceed to examine more closely the 
composition and developement of the skeleton, 
and here we find difficulties to be encountered of 
nocommonkind. Did the skeleton invariably 
consist of the same number of bones, only modi- 
fied in their shape or position according to'the 
necessities of the different races of Vertebrata, 
the task of the comparative anatomist would be 
easy when he came to investigate their analo- 
gies and relations with each other ; but this is 
far from being the case: the skeleton of the 
adult animal does not present the same 
number of pieces as that of the same creature 
in a less advanced condition, numerous parts, 
originally distinct, having become fused and 
consolidated into one; and, on the other hand, 
the juvenile being differs from the embryo from 
circumstances precisely the reverse, seeing that 
the full complement of bones or centres of 
ossification has not as yet been developed. 
Now, as in ascending the scale of living beings 
belonging to the Vertebrate division of the ani- 
mal creation, we find that nature can arrest the 
