Posterosuperior view of the axis (C2) vertebra. The inferior surface of its body is united to the body of the third cervical vertebra by the first intervertebral disc. From its superior surface the odontoid process extends upward behind the anterior arch where it is held in place by ligaments. The superior articular facet of the axis slopes downward and laterally and articulates with the inferior articular facet of the atlas to form the atlanto-axial joint. The atlanto-axial articulation consists of three synovial joints: a single median atlanto-axial joint between the odontoid process and anterior arch and paired atlanto-axial joints between the lateral masses of each. The principal action at this joint is axial rotation. Extending posteromedially from each lateral mass are laminae that fuse in the midline at the bifid spinous process. Each transverse process contains a transverse foramen for the vertebral artery. (Image courtesy of AL Rhoton, Jr.)