Left presigmoid and retrosigmoid exposure after partial petrosectomy. Drilling of the petrous and mastoid portions of the temporal bone reveals the middle ear including the malleus, incus and stapes, and the facial nerve’s geniculate ganglion. The chorda tympani is visible running within the petrous bone superior to the tympanic membrane. The superior petrosal vein is visible alongside cranial nerve V entering the superior petrosal vein within the dura overlying the petrous ridge. Cranial nerves IX, X and XI course through the jugular foramen along with the jugular bulb and the inferior petrosal sinus. Cranial nerve VI (abducens nerve) runs rostrally in the pontine cistern before entering Dorello’s canal just inferior to Meckel’s cave and the tentorium. Cranial nerve III can be identified by its course between the posterior cerebral artery and the superior cerebellar artery, and cranial nerve IV exits the dorsal midbrain to course laterally around the midbrain ultimately entering the cavernous sinus. The greater superficial petrosal nerve and a petrosal branch artery (arising from the middle meningeal artery after it enters the cranium vai the foramen spinosum) course through the hiatus of the facial canal within the temporal bone. (Image courtesy of AL Rhoton, Jr.)