VERTEBRAL COLUMN. The axial skeleton in cludes the vertebral column, ribs, sternum, and head-skeleton. The vertebral column, or `back bone,' first appears in cyclostomes, where it oc curs as fibrous tissue, surrounding the notochord, which thus comes to lie as a rod in the axis of the primitive vertebrate column and is known as the `skeletogenous layer.' From this point on it be comes a more and more important organ, while the chorda takes less and less part in the com position of the body of the adult. In the lowest vertebrates the skelctogenous layer is replaced at intervals by cartilage, which forms arches around the neural canal. In ganoids and higher forms these consist of five cartilaginous pieces for each smite, the fifth or unpaired piece form ing the dorsal spinous process. Ventral car tilaginous pieces also occur ventral to the chorda. The bodies (centra ) of the vertebrae definitely appear, and the chorda, becomes constricted in travertebrally, giving the vertebre an hour-glass form. The rings of cartilage formed by intra vertebral constrictions are biconcave or 'amphi cmlons' in all fishes with bony vertebra' and in most trrodela; also in a few fossil and living rep tiles (q.v.), and in a few fossil birds. So long as the separate of the vertebral column are amphiccelous their connection with one an other must depend upon something else than the bony vertebrae themselves. In the lower fishes this union is effected by the chorda and chordal sheath. In the lower Urodela it is effected by the intervertebral, non-ossitied cartilage. In the higher Urodela„ the Anura and almost all the reptiles, however, the vertebrae are linked to gether by means of a ball-and-socket joint. The concavity may be on the posterior and the con vexity on the anterior end (opisthocadous) or conversely (proccelons). In crocodiles, birds, and mammals the opposed faces of the vertebrm are approximately plane surfaces. In the develop
ment of the of man the phylogenetie stages are recapitulated. The typical vertebra of man consists of a centrum from which an arch arises dorsally to protect the spinal cord. These arches together constitute the neural canal. Each half arch is composed of the rounded pedicle and the broad flat lamina. There are three kinds of processes: (1) the dorsal or neuropophysis; (2) the transverse process, serving for the attach ment of the muscles which keep the vertebra, to gether; (3) the forward and backward articulat ing processes (zygopophyses). The relation of the eentra to the somites of the body is an in teresting one. They do not arise one in the mid dle of each somite, but at the plane of separation of adjacent somites, thus insuring flexibility in the column.
The number of in mammals is highly variable in different species. With one or two exceptions all mammals have seven cervical (non rib-bearing) vertebrae. All the artiodactyls pos sess nineteen thoracico-hunbar The smallest number (fourteen) occurs in arma dillos: the largest (thirty) in hyracoids. Since the number of vertebra; corresponds to that of the somites of the body, it seems necessary to conclude that the latter are highly vari able in number. If we seek for an inter pretation of the differences in the vertebral column we may find it in the different tasks the parts perform, and the differentiation of vertebra, is a. late acquisition, gradually ac quired with advancing age. The sacral bones begin their fusion only at sixteen years, and this is not completed until the age of thirty. The sacrum is composed of four or five caudal verte fused together.