Transmastoid view of left lateral labyrinth, posterior fossa, and brainstem. 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 long crus of the incus narrows to the lentiform process that articulates with the head of the stapes. The base of the stapes, in turn, attaches to the oval window inferior to the facial canal on the medial wall of the middle ear cavity. The tentorium cerebelli has been divided and the cerebellum reflected to expose further contents of the posterior fossa. The mesencephalon or midbrain can be seen and gives rise to the oculomotor (CNIII) and the trochlear (CNIV) nerves, the latter which originates from its dorsum. Leaving the lateral pons is the trigeminal (CNV) nerve and, from the lateral pontomedullary junction, the facial (CNVII) and vestibulocochlear (CNVIII) nerves. Similarly, portions of the anterior inferior cerebellar, superior cerebellar, and posterior cerebral arteries, all branches of the basilar artery, can be seen. The transverse sinus is contained within the posterior attachment of the tentorium cerebelli ( at right border of the image) to the occipital bone and continues as the sigmoid sinus. The jugular bulb, located at the jugular foramen, is the continuation of the sigmoid sinus. (Image courtesy of AL Rhoton, Jr.)