C1 vertebra. C1, also known as the atlas, is unique among the cervical vertebrae in that it lacks a vertebral body.  It has a ring-like structure consisting of an anterior arch, a posterior arch, and two lateral masses.  The posterior arch lacks a spinous process and instead has a small posterior tubercle.  The anterior arch articulates with the dens of C2 (axis).  Each lateral mass has a smooth superior articular surface that articulates with the occipital condyle of the skull.  A transverse process extends laterally from each lateral mass and features a transverse foramen that transmits the vertebral artery. (Image courtesy of PA Rubino)