Anterior inferior view of atlantoaxial junction and pontomedullary surface. The anterior aspect of the body of the axis (C2 vertebra) with its upward projecting dens into the vertebral foramen of the atlas is in view. The anterior arch of the atlas (C1 vertebra) has been drilled away to demonstrate this relationship. The dens articulates with the posterior surface of this arch and is firmly held in place by the transverse ligament of the atlas. The foramen magnum is continuous with the vertebral canal. At this junction the vertebral arteries pass posterior to the rootlets of the hypoglossal nerve and anterior to the spinal root of the spinal accessory nerve. Each artery gives rise to a posterior inferior cerebellar artery. Near the pontomedullary junction the vertebral arteries unite to form the basilar artery that ascends on the ventral surface of the pons. Near its origin it gives off the anterior inferior cerebellar arteries. (Image courtesy of PA Rubino)