Superior Transtentorial Perspective of Brainstem and Cerebellum.  The internal anatomy of the midbrain is well demonstrated; from dorsal-to-ventral can be seen the tectum, midbrain tegmentum, substantia nigra and crus cerebri.  The anterior lobe of the cerebellum has been retracted to demonstrate the superior cerebellar artery and its branches.  Note the proximity of the trochlear nerve and the superior cerebellar artery within the ambient cistern. The posterior cerebral artery and basal vein of Rosenthal would also pass within this space (not present). This perspective can orient the surgeon during a subtemporal approach to the internal auditory canal, given that you can see the course of the facial and vestibulocochlear nerves as they traverse the cerebellopontine cistern. (Image courtesy of AL Rhoton, Jr.)