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 overlying the fundus of the internal auditory canal has been removed to reveal the facial nerve, 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 here by a bony vertical crest (Bill's bar). These nerves, in turn, are separated from the cochlear and inferior vestibular nerves by a horizontal septum of bone called the transverse (falciform) crest. The cochlear nerve lies inferior to the facial nerve. 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. In view as well is the motor nerve to the tensor (a branch of the mandibular nerve, CNV3). Its tendon curves sharply laterally to attach to the upper part of the handle of the malleus. Note that the chorda tympani crosses superior to the tendon. Superior and medial to the tensor tympani is the internal carotid artery coursing within the carotid canal. The middle meningeal artery can be seen emerging through the foramen spinosum. (Image courtesy of AL Rhoton, Jr.)