The Serial Morphology of the Vertebrae
Although at first sight many of the vertebrae exhibit peculiarities, nevertheless a study of the mode by which they develop, and their variations, indicates the serial homology of the constituent parts of the vertebrae in each region of the column.
The centrum, or body of the vertebra, is that part which immediately surrounds the notochord. This part is present in all the vertebrae of man, but the body of the atlas is dissociated from its neural arch, and ankylosed to the body of the axis. The reasons for regarding the odontoid process as the body of the atlas are these: In the embryo the notochord passes through it on its way to the base of the cranium. Between the odontoid process and the body of the axis there is a swelling of the notochord in the early embryo as in other intervertebral regions. This swelling is later indicated by a small inter-vertebral disc hidden in the bone, but persistent even in old age. The odontoid process arises from primary canters, and in chelonians it remains as a separate ossicle throughout life; in Ornithorhynchus it remains distinct for a long time, and it has been found separate even in an adult man. Lastly, in man and manny mammals, an epiphyseal plate develops between it and the true body of the axis.
The anterior segment of the atlas is in no sense homologous with a centrum, and is most probably an enlarged hypapophysis or sub-vertebral wedge-bone, which, in lizards, exists on the ventral aspect of the column between individual centra. Similar ossicles occur in the lumbar region of the mole.
The neural arches and spinous processes are easily recognized throughout the various parts of the column in which complete vertebrae are present.
The articular processes are of no morphological value, and do not require consideration here.
The transverse processes offer more difficulty. They present themselves in the simplest form in the thoracic series. Here they articulate with the tubercles of the ribs. The transverse process and the neck of the rib enclose an arterial foramen, the costo-transverse. In the cervical region this rib, or costal element, and the transverse process are fused together, but the conjoint process thus formed is pierced by the costo-transverse foramen. The compound nature of the process is indicated by the fact that the anterior or costal processes in the lower cervical vertebrae arise from additional centers and occasionally retain their independence as cervical ribs. These processes in Sauropsida (birds and reptiles) are represented by free ribs. In the lumbar region the compound nature of the transverse process is further marked. The true transverse process is greatly suppressed, and its extremity is indicated by the accessory tubercle. Anterior to this in the adult vertebra a group of holes represent the costo-transverse foramen, and the portion in front of this is the costal element. Occasionally it will persist as an independent ossicle, the lumbar rib.
In the sacral series the costal elements are peculiarly modified in the first three vertebrae to form piers of bone for articulation with the ilium. The costo-transverse foramina are completely obscured. In rare instances the first sacral vertebra will articulate with the ilium on one side, but remain free on the other. Under such conditions the free process exactly resembles the elongated transverse process of a lumbar vertebra. The first three sacral vertebrae which develop a costal process (rib) for articulation with the ilium are true sacral vertebrae. Those ankylosed below these are pseudo-sacral. A glance at the spine will show the homology of the various parts of a vertebra from the cervical, thoracic, lumbar and sacral regions.
The mammillary processes are vestiges of the greatly elongated articular processes of such mammals as the dog, armadillo, &c.