Chapter 8 - Section 8.3 - Study Guide - Assess Your Learning Outcomes - Page 271: 6

The first vertebra, C1, supports the head (skull) . It is atypical in shape in that it has no body. It has the form of a bony ring which is composed of two bony arches-- the anterior arch and the posterior arch. The anterior arch bears a median tubercle anteriorly, and posteriorly and has a facet that articulates with the dens of the axis vertebra (C 2) when one moves the head from side, as in the nonverbal expression of "No." The posterior arch of the atlas --which bears bears a median posterior tubercle-- forms the greater part of the atlas; in addition the atlas bears a pair of small transverse processes. Probably the most important structural feature of thee atlas are two bony protuberances with anterior concave articulating surfaces. The importance of these facets is that they articulate with the occipital condyles of the when one nods the head back and forth (up and down) in the nonverbal expression of the affirmative answer, "Yes." The second vertebra, C2, is usually called the Axis, but it has other names-- the epitropheus and the vertebra dentata. Also like the Atlas, this vertebra has a has an atypical shape. Its most distinctive feature is the dens or odontoid process dens) which is a bony projection , fused is the body of the axis and which rises anteriorly through the vertebral foramen of the atlas. Other features of the axis include the transverse process (with a transverse foramen); superior and inferior articular facets, laminae, pedicles (of the posterior arch), and a spinous process. The dens represents the body of C1 which lost its way and became attached (fused) to the body of C 2. The ring-shaped atlas rotates around the dens when one rotates the head laterally, as in the making the non-verbal gesture that means "No," in English (Anglicized) cultures. The dens is held in position by the transverse ligament of the atlas.

The odontoid process of the axis starts as an independent ossification center in the first year of the life of the infant. Later (during the period 3-6 years), this peg of bone becomes fused to the body of the axis.