The atlas (C1) is the first cervical vertebra, and its major distinguishing feature is the lack of a vertebral body and a spinous process. Also, its shape is different from the other cervical vertebrae, and it appears ring-shaped when viewed from above.

Instead of a vertebral body, the atlas presents with an arch called the anterior arch. Therefore, the first cervical vertebra is composed of two vertebral arches named the anterior arch and posterior arch. Both are interconnected by two lateral masses.

Each lateral mass is formed by articular processes, and it contains the superior and inferior articular facets. The superior facet articulates with the occipital bone of the skull at the atlanto-occipital joint. The inferior articular facet of the atlas and the superior articular facet of the axis (C2) form the atlanto-axial joint. The structural features of the first two cervical vertebrae allow performing a great degree of neck and head movements. The atlanto-occipital joint is responsible for head shaking (neck flexion) when saying “yes.”

The inner or posterior aspect of the anterior arch contains an impression for the dens (known as the fovea for dens) of the second cervical vertebra. The fovea for dens contains an articular facet for dens. The superior surface of the posterior arch posterior to the lateral masses has a paired groove for vertebral artery. The central aspects of both arches present with anterior and posterior protrusions known as the anterior and posterior tubercles.

The two transverse processes of the atlas are large and project further laterally than those of other cervical vertebrae. They extend from the lateral masses and act as levers for muscle action.

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Atlas (C1)