Left anterolateral view of brainstem and exposure of the Sylvian fissure. The Sylvian (lateral) fissure is opened to show the first two segments of the middle cerebral artery, the larger of the two terminal branches of the internal carotid. The occipital lobe is sectioned sagittally and is separated from the cerebellum by the tentorium cerebelli. The midline of this dural fold contains the straight sinus at its intersection with the falx cerebri. The posterior edge of the tentorium contains the transverse sinus and at its midpoint, the torcula (of Herophili) or confluens of sinuses. The cerebellum is supplied by three pairs of arteries; posterior inferior cerebellar artery from the vertebral artery, the anterior inferior cerebellar artery from the proximal portion of the basilar artery, formed by union of the right and left vertebral arteries, and the superior cerebellar artery from the distal portion of the basilar. The latter vessel courses posteriorly in a subtentorial direction. The basilar artery ascends on the pons midline beginning at the pontomedullary junction. Between the anterior inferior cerebellar and superior cerebellar arteries arise smaller transverse running pontine arteries. Distally, the basilar artery terminates with the posterior cerebral vessels. These course toward the occipital lobes in a supratentorial direction. Near its origin, the posterior cerebral artery receives the posterior communicating artery from the internal carotid as part of the circle of Willis arterial anastomoses. The intradural portion of the internal carotid artery gives off the ophthalmic artery which enters the optic canal with the optic nerve. The anterior cerebral artery, the smaller terminal branch of the ICA, course medially around the optic nerve to enter the longitudinal interhemispheric fissure. At the interpeduncular fossa the oculomotor nerves emerge from the midbrain and pass between the superior cerebellar and posterior cerebral arteries. The trochlear nerves originate on the dorsal surface of the midbrain and wrap around the cerebral peduncles coursing forward toward the cavernous sinus. The larger sensory root and smaller motor root of the trigeminal nerve emerge from the anterolateral aspect of the pons. The abducens nerves emerge at the pontomedullary junction near the midline, while the facial and vestibulocochlear nerves emerge at the cerebellopontine angle near the choroid plexus of the fourth ventricle. The lower cranial nerves (glossopharyngeal, vagus, and accessory) are outlined as a group and arise from the lateral medulla. (Image courtesy of PA Rubino)