Endoscopic postclival view of anterior brainstem. The caudal portion of the diencephalon (indicated by the mammillary bodies), midbrain, pons, and medulla are in view in this image. Horizontal strips of dura mater indicate levels of the clivus. The internal carotid arteries, laterally, are exposed following removal of petrous bone. The vertebral arteries unite to form the basilar artery near the pontomedullary junction after having given off the small anterior spinal arteries. The basilar artery gives rise proximally to the anterior inferior cerebellar arteries and then provides several pontine branches that radiate onto the pons. Distally, the basilar artery gives off the superior cerebellar arteries before terminating with its posterior cerebral arteries. Between these two arteries pass the oculomotor nerve that emerges from the midbrain. The abducens nerve is shown leaving the pons near the pontomedullary junction. It takes a subarachnoid course along the clivus before penetrating the dura to pass through Dorello's canal near the petrous apex. The facial and vestibulocochlear nerves leave the brainstem laterally at the pontomedullary junction and course toward the internal auditory meatus. Rootlets of the glossopharyngeal, vagus, and spinal accessory nerves leave the lateral medulla and converge on the jugular foramen. The caudal most cranial nerve, the hypoglossal nerve, emerges as several rootlets from the anterolateral medulla and course toward the hypoglossal canal.  (Image courtesy of AL Rhoton, Jr.)