Transmastoid view of left lateral labyrinth and posterior fossa. Removal of temporal (mastoid and petrous portions) and occipital bone reveals the bony labyrinth and the posterior, superior, and lateral semicircular canals. The tympanic segment of the facial (CNVII) nerve courses within the facial canal inferior to the lateral semicircular canal before bending and descending as the mastoid segment. From this latter segment arises the chorda tympani nerve, which enters the middle ear cavity through the posterior canaliculus and courses between the malleus (on its medial surface) and the incus. The tentorium cerebelli has been divided and the cerebellum reflected to expose further contents of the posterior fossa. Leaving the brainstem are portions of the trigeminal (CNV), facial (CNVII), vestibulocochlear (CNVIII), glossopharyngeal (CNIX), and vagus (CNX) nerves. Similarly, portions of the posterior inferior cerebellar artery, a branch of the vertebral artery, and anterior inferior cerebellar and superior cerebellar arteries, branches of the basilar artery, can be seen. The transverse sinus is contained within the posterior attachment of the tentorium cerebelli to the occipital bone and continues as the sigmoid sinus. It receives tributaries of the inferior cerebral veins. The jugular bulb, located at the jugular foramen, is the continuation of the sigmoid sinus. (Image courtesy of AL Rhoton, Jr.)