Left posterolateral exposure of bony labyrinth, tympanic cavity, sigmoid sinus, and temporal lobe. The temporal lobe is reflected superiorly and petrosectomy has exposed the semicircular canals of the bony labyrinth. Inferior to the lateral semicircular canal is the prominence of the facial canal containing the facial nerve. Just anteroinferior to this is the stapes attached to the oval window. The malleus and incus remain articulated within the tympanic cavity. The superior petrosal sinus courses along the superior border of the petrous bone within the attachment of the tentorium cerebelli and drains posteriorly into the transverse sinus at its junction with the sigmoid sinus. The vein of Labbe, part of the superficial venous system of the brain, is seen crossing the lateral surface of the temporal lobe, which it drains, and also enters the transverse sinus. The trochlear nerve, the motor nerve to the superior oblique muscle, is the only cranial nerve to emerge from the dorsal surface of the brainstem. It passes forward typically above the superior cerebellar arteries. (Image courtesy of AL Rhoton, Jr.)