Transmastoid view of left lateral labyrinth, petrosal surface of the cerebellum, and cerebellopontine angle. The petrous bone has been drilled to expose the semicircular canals, the facial nerve, and the ossicles. The malleus, incus, and stapes are intact as an ossicular chain. 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 along the petrous ridge to the incisura and the petrosal surface of the cerebellum reflected to expose the midbrain, pons, and cerebellopontine angle. The distal portion of the basilar artery gives rise to the superior cerebellar and posterior cerebral arteries. The oculomotor (CNIII) and trochlear (CNIV) nerves leave the ventral and dorsal surface, respectively, of the midbrain and course forward between these two vessels. The trigeminal (CNV) nerve can be seen to emerge from the lateral pons. The proximal portion of the basilar artery gives rise to the anterior inferior cerebellar artery and pontine branches. The petrosal surface of the cerebellum is drained by the anterior hemispheric vein, one of the superficial veins that drain the posterior fossa. The tentorium separates the occipital lobes of the cerebrum from the cerebellum and contains the transverse sinus within the posterior attachment of it to the occipital bone. It continues laterally as the sigmoid sinus, which is visible in this image. (Image courtesy of M Nunez)