Right superior view of internal auditory canal, inner and middle ear, and external auditory canal. For orientation, the upper right corner of the image is anterior; the upper left corner is medial. The roof of the internal auditory canal has been removed to reveal the facial nerve, nervus intermedius, geniculate ganglion, and greater superficial petrosal nerve. Posterior to the facial nerve within the canal is the superior vestibular nerve to the membranous labyrinth. These two nerves are separated at the fundus of the internal auditory canal by a bony vertical crest (Bill's bar). These nerves are separated from the cochlear and inferior vestibular nerves by a horizontal septum of bone called the transverse (falciform) crest. The thin roof of the tympanic cavity (tegmen tympani) has been drilled away to reveal the middle ear cavity (malleus and incus are exposed) and aditus leading posteriorly to the mastoid antrum. The lateral wall of the tympanic cavity is the tympanic membrane, which separates this cavity from the external auditory canal. The anterior wall of the tympanic cavity has been opened to reveal two canals, one above the other. The superior of the two contains the tensor tympani muscle and the lower one is the bony part of the Eustachian tube. The tendon of the tensor curves sharply laterally to attach to the upper part of the handle of the malleus. The two canals are separated by a thin bony septum called the septum canalis musculotubarii. Superior and medial to the tensor tympani is the internal carotid artery coursing within the carotid canal. Crossing the apex of the petrous portion of temporal bone is the trigeminal nerve, which expands as the trigeminal ganglion before giving rise to its three divisions. The motor root of the trigeminal nerve can be seen entering the foramen ovale where it fuses with its sensory root as the mandibular nerve. Posterolateral to the foramen ovale is the foramen spinosum and middle meningeal artery. (Image courtesy of AL Rhoton, Jr.)