Slightly convex anteriorly, the short flattened anterior arch of C1 (atlas) has an anterior tubercle in the midline, giving attachment to the anterior longitudinal ligament, either side of which is the attachment of the superior oblique part of longus colli.  Posteriorly, a concave circular facet articulates with the dens (odontoid process) of C2 (axis).  The superior and inferior borders give attachment to the anterior atlanto-occipital membrane and anterior longitudinal ligament respectively.