A, The insert shows the temporo-occipital craniotomy and the mastoid exposure. The mastoidectomy has been completed and the dense cortical bone around the labyrinth has been exposed. The tympanic segment of the facial nerve and the lateral canal are situated deep to the spine of Henley. Trautman’s triangle, the patch of dura in front of the sigmoid sinus, faces the cerebellopontine angle. B, The presigmoid dura has been opened and the superior petrosal sinus and tentorium divided, taking care to preserve the vein of Labbe´ that joins the transverse sinus, and the trochlear nerve that enters the anterior edge of the tentorium. The abducens and facial nerves are exposed medial to the vestibulocochlear nerve. The posteroinferior cerebellar artery courses in the lower margin of the exposure with the glossopharyngeal and vagus nerves. The SCA passes below the oculomotor and trochlear nerves and above the trigeminal nerve. C, The semicircular canals have been opened. The superior canal is located under the middle fossa’s arcuate eminence and the posterior canal is located immediately lateral to the posterior wall of the internal acoustic meatus. D, Labyrinthine exposure in another specimen. The tympanic segment of the facial nerve courses below the lateral canal and turns downward as the mastoid segment where it gives origin to the chorda tympani, seen ascending along the inner surface of the tympanic membrane and neck of the malleus. The head of the malleus and incus are located in the epitympanic area above the level of the tympanic membrane. The mastoid antrum communicates through the aditus with the epitympanic area and tympanic cavity. (Images courtesy of AL Rhoton, Jr.)