OSSEOUS SYSTEAL (CotrrArtAnv AxxTomv.)--One of the most striking an distinctive characters peculiar to the highes grades of animal existences, the VERTEBRATA is that they have their bodies supported b and as it were moulded upon, an internal frame-work, which is generally made up of numerous pieces, very various in their forms and uses, which are called the bones ; and the assemblage of them, whatever their modifica tion, constitutes the skeleton.
Seeing the great diversity of forms and habits in the innumerable races of animals constituting this great group of living beings, some being specially appointed to occupy the waters of . . . . . .
our globe, others to inhabit the marsh and me swamp, whilst others again tread the firm sur face of the ground, or raise themselves into the regions a the thin air; and that under all the diversified shapes of Fishes, Reptiles,Birds, and Mammifers, we are prepared, a priori, to 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 org,ans 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. lhroughout 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 ag,ain and again as the bulk of its body is enlarg,ed. In all the Molluscs, 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 arid 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 svith theirs%mdition 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, al 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 illyrine 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 fcetal 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 forrned, hardened, and lightened to the utmost required extent by consolidating their substance to the maximum, and excavating the caverns and can celli that characterize the most perfectly ma tured conditions of the osseous framework.