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Post-Drilled View of Internal Auditory Canal and External, Middle, and Inner Ear Structures

Surgical Correlation


Post-drilled view of internal auditory canal and external, middle, and inner ear structures. Exposure of the proximal course of the facial nerve reveals its many intracranial and intraosseous segments. It leaves the pontomedullary junction as the cisternal segment which enters the internal auditory meatus to become its meatal segment. At the distal or fundic end of this canal it becomes the labyrinthine segment to the geniculate ganglion. At this genu, the greater superficial petrosal nerve arises, while the main portion curves posteriorly as the tympanic segment. It courses along the medial wall of the tympanic cavity inferior to the lateral semicircular canal. The nerve subsequently bends inferiorly past the pyramidal eminence to become the mastoid segment to the stylomastoid foramen where it exits the skull. Following entrance of the facial, nervus intermedius, and vestibulocochlear nerves into the internal auditory canal, branches of the latter divide in the fundic region where they are separated by vertical and transverse crests of bone. The transverse crest creates a superior compartment and an inferior compartment. The facial and superior vestibular nerves occupy the superior compartment (the facial nerve being anterior) and are separated by the vertical crest. There is no similar vertical crest in the inferior compartment, but the cochlear nerve lies anterior with respect to the inferior vestibular nerve. At the fundic end of the canal the cochlea is anterior while the vestibule and labyrinth lies posterior. The tegmen tympani has been removed to expose the interior of the tympanic cavity. The malleus, incus, and stapes are intact. The semicanal containing the tensor tympani muscle is located above and parallel to the Eustachian tube that open at the anterior wall of the cavity. The head of the malleus, incus, and its short limb project into the epitympanic recess. This opens posteriorly into the mastoid antrum via the aditus or entrance of the mastoid antrum. The lateral wall of  the tympanic cavity is bounded by the tympanic membrane that forms the medial boundary of the external auditory canal. Along the posterior surface of the petrous bone is the cerebellum and a loop of the anterior inferior cerebellar artery lies close to the entrance of the internal auditory canal. (Image courtesy of AL Rhoton, Jr.)