Transverse view of medulla oblongata in posterior fossa. The left vertebral artery can be seen curving around the lateral mass of the atlas and crossing the superior surface of its posterior arch to enter the posterior fossa through foramen magnum. The C1 spinal nerve passes out of the vertebral canal between the artery and the arch. The posterior inferior cerebellar arteries arise from each vertebral artery. Anteriorly, the vertebral arteries join to form the basilar artery that ascends on the anterior pons. It gives rise to anterior inferior cerebellar arteries, pontine arteries, and superior cerebellar arteries before terminating with the posterior cerebral arteries. The oculomotor nerves emerge from the midbrain at the interpeduncular fossa and course in its cistern medial to the tentorial edge toward the roof of the cavernous sinus. The abducens nerves can be seen ascending on the clivus before they penetrate dura to course toward the cavernous sinus. The trigeminal nerves penetrate dura near the petrous apex and expand in Meckel's cave as the Gasserian ganglion, a sensory ganglion. From it arises its three divisions: ophthalmic nerve to the orbit via the superior orbital fissure, the maxillary nerve to the pterygopalatine fossa via the foramen rotundum, and the mandibular nerve to the infratemporal fossa via the foramen ovale. The facial and vestibulocochlear nerves leave the brainstem at the cerebellopontine angle and enter the internal auditory canal. The facial nerve courses laterally within the canal between the cochlea and semicircular canals. At its first genu (location of the geniculate ganglion) it gives rise to the greater superficial petrosal nerve carrying preganglionic parasympathetic fibers to the pterygopalatine ganglion. Portions of the glossopharyngeal, vagus, and spinal accessory nerves can be seen leaving the lateral medulla and coursing toward the jugular foramen. Also, slightly more anterior are rootlets of the hypoglossal nerve coursing toward the hypoglossal canals. (Image courtesy of PA Rubino)