Transoccipital view of medulla and associated neurovasculature. Bone of the lower clivus and adjacent parts of the occipital bone were removed to expose the medulla and its associated neurovasculature. The anterior margin of the foramen magnum has been preserved. Through this foramen the vertebral arteries ascend and subsequently unite at the pontomedullary sulcus to form the basilar artery. Prior to this union, they give rise to the posterior inferior cerebellar arteries, the largest branches of the vertebral arteries. The vertebral arteries give rise to anterior spinal arteries that will descend onto the ventral surface of the spinal cord. Along the rostral lateral medulla emerge in sequence the glossopharyngeal (CNIX), vagus (CNX), and spinal accessory (CNXI) nerves. These nerves converge on the jugular foramen. The petroclival fissure has been drilled to expose the inferior petrosal sinus that passes through the pars nervosa of the jugular foramen to enter the internal jugular vein. The hypoglossal (CNXII) nerve exits the sulcus between the pyramid and olive and passes toward and through the hypoglossal canal. (Image courtesy of AL Rhoton, Jr.)