OSSEOUS SYSTEM. (Coiwp. ANAT.) 



821 



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 crtature. 



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, 

 in 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- 

 manently 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- 



ness and solidity are slowly given. Even in 

 the most perfect Fishes the bones remain soft 

 in comparison with their condition 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 Myxine 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 developernent 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- 

 cel li 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, (OSSEOUS TIS- 

 SUE,) we proceed to examine more closely the 

 composition and developement of the skeleton, 

 and here we find difficulties to be encountered of 

 no common kind. 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 c:an arrest the 



