Groove for vertebral artery - Sulcus arteriae vertebralis
Immediately behind each superior articular process on the posterior arch, is the groove for vertebral artery (sulcus arteriæ vertebralis), sometimes converted into a foramen or canal by a delicate bony spiculum which arches backward from the posterior end of the superior articular process. This groove represents the superior vertebral notch, and serves for the transmission of the vertebral artery, which, after ascending through the foramen in the transverse process, winds around the lateral mass in a direction backward and medialward; it also transmits the suboccipital (first spinal) nerve.
This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from http://www.bartleby.com/107/).
General Anatomy > Bones; Skeletal system > Axial skeleton > Vertebral column > Cervical vertebrae [C I-C VII] > Atlas [C I] > Posterior arch of atlas > Groove for vertebral artery