Some of the dorsal vertebræ are peculiar in the arrangement of their facets and demi-facets, while among the cervical vertebræ are several whose peculiarities should be more carefully noted. Thus, the first cervical vertebra or atlas supports the head and has practically no body, the place of the body being taken by a narrow anterior arch of bone and an opening, continuous with the spinal foramen, into which the odontoid process of the axis fits, being held in place by ligaments. At either side on top is a facet for articulation with the occipital bone. There is almost no spine. The second vertebra or axis has surmounting the body the odontoid process, with a facet in front for articulation with the atlas and one behind for the transverse ligament to move over. The seventh cervical vertebra or vertebra prominens has a very long spinous process—hence name—to which is attached the ligamentum nuchæ. It can be felt very distinctly on the living.
Running from the skull down through the spinal column into the sacral vertebræ and formed by the joining of the spinal foramina of the individual vertebræ is an opening called the spinal canal, which holds the cord. The cord, however, stops practically at the first lumbar vertebra, where it splits up into the cauda equina, only the filum terminate extending farther down.
Occasionally the laminæ do not form completely and the membranes of the cord may bulge out and form a tumor, or the cord itself may come out also. This generally occurs in the lumbar region, where it is known as spina bifida. If in case of fracture of a vertebra there is paralysis of the parts below due simply to the pressure of a fragment of bone upon the cord, it may be completely cured by removal of the fragment. If, however, the cord suffers injury, the paralysis will remain. Humpback or Pott's disease is caused by the tubercle bacillus, which eats away the bodies of the vertebræ so that the column caves in and the spinous processes are thrown out in a hump or kyphos.
