Left anterolateral view of brainstem at tentorial notch. The left edge of the tentorial notch extends from the clinoid processes anteriorly and intersects with the falx cerebri posteriorly. The optic nerves emerge from the optic canals and their medial portions join at the optic chiasm. Between the optic nerves and anterior clinoid processes the internal carotid arteries emerge and shortly divide into their terminal anterior cerebral and middle cerebral arteries. The anterior cerebral artery sweeps medially toward the interhemispheric fissure. In view are its A1, precommunicating, and A2, postcommunicating, segments. The oculomotor nerve arises from the lateral wall of the interpeduncular fossa of the midbrain where it passes between the superior cerebellar and posterior cerebral branches of the basilar artery. It passes forward to penetrate the dura overlying the roof of the cavernous sinus. The trochlear nerve exits the dorsum of the midbrain, wraps around the cerebral peduncles and enters dura along the lateral wall of the cavernous sinus. The posterior cerebral artery is in view. From its P1 segment arises the quadrigeminal artery which courses around the midbrain to supply the colliculi on its dorsal surface. The P2 segment also passes around the midbrain to the quadrigeminal cistern. Here, its P3 segment gives off anterior and posterior temporal branches to this lobe before continuing posteriorly and medially as the P4 segment, which enters the calcarine fissure of the occipital lobe to supply the visual cortex. The basal vein of Rosenthal courses posteriorly and medially near the posterior cerebral artery and drains blood from the midbrain and medial parts of the temporal lobe. It empties into the great vein of Galen. This latter vein is formed by the union of the internal cerebral veins draining deep internal areas and empties into the straight sinus. The superior cerebellar vein drains the superior aspect of the cerebellum and also empties into the straight sinus. (Image courtesy of AL Rhoton, Jr.)