In anatomy, the axis is the second cervical vertebra (C2) of the spine, immediately inferior to the atlas, upon which the head rests. The spinal cord passes through the axis.
Axis
Repair of a dens fracture
Axis on X-ray taken through an open mouth, teeth visible
Unfused arch of atlas at CT
In tetrapods, cervical vertebrae are the vertebrae of the neck, immediately below the skull. Truncal vertebrae lie caudal of cervical vertebrae. In sauropsid species, the cervical vertebrae bear cervical ribs. In lizards and saurischian dinosaurs, the cervical ribs are large; in birds, they are small and completely fused to the vertebrae. The vertebral transverse processes of mammals are homologous to the cervical ribs of other amniotes. Most mammals have seven cervical vertebrae, with the only three known exceptions being the manatee with six, the two-toed sloth with five or six, and the three-toed sloth with nine.
Scrollable computed tomography images of normal cervical vertebrae
X-ray of cervical vertebrae
X-ray of cervical spine in flexion and extension
Image: X ray of vertebral lines