Atlas is that piece of the human vertebral column which articulates with the skull; in other words, it is the first cervical vertebra. It may be distinguished from the other six by its being without a body and spinous process, by its being a mere irregular bony ring, divided into two unequal parts by a constriction; this division in the recent subject is completed by a ligament, the segment in front being occupied by the tooth-like process of the second cervical vertebra or axis, and that behind, by the spinal marrow and its coverings. On either side the ring is very thick; each lateral mass is smooth and cupped above to receive the condyles of the occipital bone. The corresponding parts below are flat, and rest on the second cervical vertebra.
The atlas, with the occipital bone, forms the joint on which the head moves in bowing; and turns on the pivot of the second cervical vertebra, when we look from side to side.