The posterior arch of C1 (atlas) forms most of a ring that represents the pedicles and laminae of a typical cervical vertebra.  Immediately behind the lateral mass, the posterior arch is grooved on its superior surface by the vertebral artery and vein and first cervical spinal nerve.  This groove may be converted into a foramen (arcuate foramen) by cartilaginous or bony tissue (posterior ponticle) that bridges the posterior aspect of the lateral mass and the posterior arch.  The posterior tubercle represents a rudimentary spinous process and gives attachment to the ligamentum nuchae.  The rectus capitis posterior minor muscles attach lateral to the posterior tubercle.  The superior and inferior borders give attachment to the posterior atlanto-occipital membrane and ligament flava, respectively.