" A vertebra," says Professor Owen,'" may be traced through its various degrees of com plication, either during the progressive stages of its developetnent, or by taking permanently formed vertebr& of different grades of com plexity in different animals; or in many instances by comparing the vertebrx in different parts of the spine in the same animal." The terminal vertebrx of the tail in most species exhibit the simplest condition of these bones. The most complicated vertebrx are those of the lower part of the neck of certain birds, as the Pelican, or at the beginning of the tail of a Python or other large Serpent.
The parts or processes of such a vertebra may be divided into autogenous, or those which are independently developed in separate carti lages, and exogenous, or those which shoot out as continuations from these independent con stituents. The autogenous or true elements are- 1. The centrum or body of the vertebra (4. 433, d,) which in Mammalia, as Cuvier lias observed, is complicated by two epiphyses.
2. Two superior laminm (b, b) developed to protect the great nervous cord which rests on the upper surface of the centrum, and which Professor Owen therefore proposes to call Neurapophyses.
3. Two inferior laminw (e, e) developed generally to protect the great bloodvessels on the under surface of the centrum, and which may be called Hemapophyses.
4. The superior spinous process (a) which is connected and generally anchylosed with the distal extremities of the neurapophyses, and forms, in conjunction with those processes, the superior arch of the vertebra.
5th. An inferior spinous process which is connected and commonly anchylosed with the distal extremities of the Hirmapophyses, form ing in conjunction with these a chevron or V-shaped bone.
To the category of autogenous vertebral pieces likewise belong the ribs (cc), which are generally anchylosed to the other vertebral elements in the cervical, sacral, and caudal vertebrte of the warm-blooded Vertebrate classes.
The propriety of regarding the ribs as verte bral elements is well illustrated in the Plesio saurus, in the cervical, sacral, and caudal vertebr& of which they have been genemlly described as transverse processes, although they are separate bones.
True transverse processes are always exoge nous, or mere projections from the centrum or the neurapophyses, and are of secondary impor tance. They are of two kinds superior and inferior ; both are present in the cervical ver tebrae in most classes of the vertebrated animals ; the inferior transverse processes alone are deve loped in Fishes.
The oblique or articulating processes are also exogenous, and may be developed either from the neurapophyses or the base of the superior spines of the vertebrm.
As in other complicated bones resulting from au association of several osseous pieces, certain elements of a vertebra may be modified in position and proportions so as to perform the ordinary functions of others which may be atmphied or absent : thus in Fishes the inferior transverse processes are gradually bent down wards until in the dorsal region their extremities meet and perform the functions of the hama pophyses.
From the vertebral elements named above every possible variety is presented by these bones throughout all the races of animals possessing them. The body alone (fig. 433, d) may be de veloped without the addition of any of the other parts, as in the terminal bones of a -Mammal's tail or of the human us cueggis, or the neura pophyses may exist without an ossified body, as in sorne cartilaginous Fishes. The vertebra of the human skeleton present body, neurapophy ses, neural spine, and transverse processes, as do the rib-bearing vertebra of the Fish. The caudal vertebra of the Fish, in order to give the great vertical expansion required in this region of their skeleton, have the centre, the neum pophyses, and neural spine as well as the hamapophyses and liminal spine to the ex-. elusion of the transverse, while in the earlier caudal vertebra of the tail of the Crocodile (fig. 433, g ) all the elements enumerated exist in a medium state of developement.
From these data, therefore, the osteologist is enabled to explain the composition of any vertebra that may be offered to his inspection ; nevertheless there are numerous apparent ex ceptions which are well calculated to puzzle the student, met with, especially in the vertebra of Serpents or of the neck of some Birds, where the processes are so complicated by the bifurcation of their extremities, or by foramina passing through their roots, or the great size of the articulating processes, or lastly, by the exuberant deposition of osseous matter in particular parts of the bone, that the greatest possible distortion may easily be produced without at all violating the prescribed laws in accordance with which the osseous system is organized. Not unfrequently indeed stunted ribs, or even derivations from the exoskeleton, may become consolidated with the proper vertebral elements in such a manner as not to be readily distinguishable from them, producing additional complications which are sometimes very embarrassing.