Atlas and Axis

The atlas (named after the mythical giant who carried the earth on his shoulders) supports the globe of the skull. It lacks a body and forms a ring consisting of shorter anterior and longer posterior arches, with two lateral masses. The enclosed vertebral foramen is relatively large.

The anterior arch is slightly curved, with an anterior midline tubercle and a posterior midline facet for articulation with the dens of the axis. The lateral masses bear superior and inferior articular facets and transverse processes. The superior articular facets are concave and ovoid (often waisted, or reniform) and are directed upward and inward as shallow cups, or foveae, for the reception of the occipital condyles. Nodding movements of the head mainly occur at these atlanto-occipital joints. The inferior articular facets are almost circular, gently concave, and face downward and slightly medially and backward; they articulate with the superior articular facets on the axis. The transverse processes are each pierced by a foramen for the vertebral artery, and project so far laterally that they can be easily palpated by pressing inward between the mandibular angles and the mastoid processes. They provide attachments and levers for some of the muscles involved in head rotation. On the anteromedial aspect of each lateral mass is a small tubercle for the attachment of the transverse ligament of the atlas.

The posterior arch is more curved than the anterior and has a small posterior tubercle, which is a rudimentary spinous process. Just behind each superior articular facet is a shallow groove for the vertebral artery and first cervical spinal nerve, the nerve lying between the artery and the bone.

The axis, or second cervical vertebra, has a toothlike process, or dens, projecting upward from its body. The dens is really the divorced body of the atlas that has united with the axis to form a pivot around which the atlas and the superjacent skull can rotate. Its anterior surface has an oval anterior facet for articulation with the facet on the back of the anterior arch of the atlas, and a smaller posterior facet lower down on its posterior surface, which is separated from the transverse ligament of the atlas by a small bursa. The apex of the dens is attached to the lower end of the apical ligament, and the alar ligaments are attached to its sides.

The body of the axis has a lower liplike projection that overlaps the anterosuperior border of the third cervical vertebra. Its anterior surface shows a median ridge separating slight depressions for slips of the longus colli muscles. The posteroinferior border of the body is less prominent, and attached to it are the tectorial membrane and the posterior longitudinal spinal ligament. The pedicles and laminae are stout, and the latter end in a strong, bifid spinous process. The vertebral foramen of the axis is somewhat smaller than that of the atlas. On each side of the body are superior and inferior articular and transverse processes. The articular processes are offset, because the superior pair is anterior in position to the inferior pair. They articulate with the adjoining processes of the atlas and third cervical vertebra. The transverse processes are smaller and shorter than those of the atlas, and their foramina are inclined superolaterally to allow the contained vertebral arteries and nerves to pass easily into the more widely spaced transverse foramina of the atlas.

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