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 ouranalysis 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 tbe conditions under which they are appointed to exist, we shall soon per ceive that both in texture and composition it offers very important varieties. In the Alyxine 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 vertebra; 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 vertebrae, 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 vertebra. In the generality of Fishes, indeed, the centml 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 lamitra 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 vertebra are so consolidated by an encrustment of bone as to resemble a single mass; and in both the Rays and Sharks there are many more laminw enclosing the spinal canal than there are bodies of vertebrw, bony plates being developed over the junctions of vertebral centres with each other as well as in the usual situation,—a circumstance which might at first sight seem to militate against the views adopted by modern physiologists concerning the elemental constitution of this part of the body, but from which, in reality, no legitimate inference is deducible, seeing the extremely confused and incomplete progress of ossification in all the cartilaginous Fishes.
Advancing to the osseous Fishes, such con fusion no longer exists, and every vertebra assumes a precise form corresponding with the particular uses assig-ned to it in the region which it occupies. Before, however, proceed ing further, it behoves us to resolve an isolated vertebra into the primary elements of which it may itself be made up, and then we shall understand how all the varieties of shape pre sented by these bones are easily obtainable by the simple exaggeration, diminution, or suppres sion of some of the elements composing it.
Geoffroy St. Hilaire was the first anatomist who pointed out the importance of thus analysing the different portions of the osseous system, and the views which were promulg,ated by that learned writer were generally adopted until Professor Owen, in the course of his researches concerning the composition of the skeletons of extinct British Reptiles, was led, as we think very justly, to modify considerably the views which had been previously entertained upon this subject; we cannot therefore do better than lay before the reader the conclusions deduced by Professor Owen from a very elaborate and exten sive survey of the various forms of the skeleton.