Posterolateral view of the right side of the atlas (C1) and axis (C2) vertebra. Several features of these vertebrae necessitate their having unique names. The atlas is essentially a ring of bone and lacks a body. It consists of an anterior and a posterior arch that join the lateral masses, from which extend the transverse processes containing a transverse foramen for passage of the vertebral artery and veins. After passing through the C1 transverse process, the vertebral artery courses around the lateral mass before penetrating the posterior atlantooccipital membrane and ascending through the foramen magnum into the posterior cranial fossa. Unique features of the axis include the odontoid process or dens that extends superiorly from its body behind the anterior arch of the atlas. The odontoid process is held in place here by ligaments, principally the transverse ligament of the atlas. The spinous process of the axis is commonly bifid. (Image courtesy of AL Rhoton, Jr.)