Right lateral view of brainstem at level of tentorial notch. The midbrain has been transected to reveal the cerebral peduncles and substantia nigra anteriorly, and the cerebral aqueduct and tectum (consisting at this level of the inferior colliculus) posteriorly. The basilar artery ascends on the anterior pons and terminates in the interpeduncular cistern by bifurcating into the two posterior cerebral arteries. The oculomotor nerve emerges from the lateral wall of the interpeduncular fossa and passes forward beneath the posterior cerebral artery to enter dura at the roof of the cavernous sinus. The trochlear nerve emerges from the dorsal surface of the midbrain immediately behind the tectum. It wraps around the cerebral peduncle to enter dura along the lateral wall of the cavernous sinus. The tentorium cerebelli attaches to the clinoid processes of the sphenoid bone anteriorly and to the superior border of the petrous temporal bone. Its medial edge represents the lateral boundary of the tentorial notch. Medial to the anterior clinoid process is the internal carotid artery, which lies just lateral to the optic nerve entering the optic canal. A portion of the tentorium has been cut away to expose the trigeminal nerve emerging from the anterolateral surface of the pons. The abducens nerve arises from the anterior pons at the pontomedullary junction and ascends along the clivus, which has been opened to demonstrate the basilar venous plexus. At the cerebellopontine angle are the facial and vestibulocochlear nerves with a loop of the anterior inferior cerebellar artery passing between them. These structures border the flocculus of the cerebellum located just inferiorly. (Image courtesy of PA Rubino)