This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The First And Second Cervical Vertebrę are termed the atlas and axis, and are specially modified to facilitate movements of the head, which rests on them. The atlas, instead of presenting a body in front and an arch behind, has the hollow of its arch prolonged forwards, between the articular portions which carry the skull; and in the recent state, the anterior part of this hollow is converted into a separate ring by a transverse ligament. Through this anterior ring projects a process which surmounts the body of the axis, namely, the odontoid process, and round this the atlas, carrying with it the skull, revolves as on a pivot. The motion, however, is limited by two lateral bands, the check ligaments, which pass out from the top of the odontoid process to be attached to the sides of the foramen magnum, the opening in the skull by which the cranial cavity is made continuous with that of the spinal column. By the study of development and the anatomy of different animals, it is well ascertained that the odontoid process is really the body of the atlas, which has become fastened to the top of the body of the axis, and remained separated by a joint from the other parts of the bone to which it in one sense belongs.
Fig. 16. Atlas and Axis. A, Upper surface of atlas; above is the ring for the spinal cord, and, separated from it by the transverse ligament, is the ring for the odontoid process below; to the sides of this are the surfaces which articulate with the skull. B, Front view of the axis, with the arch seen in perspective behind the odontoid process.