A, The vertical line shows the site of the scalp incision and the stippled area outlines the bone flap bordering the middle fossa floor. B, The dura has been elevated to expose the middle meningeal artery, the greater petrosal nerve, and the arcuate eminence. C, Bone has been removed to expose the junction of the greater petrosal nerve and the geniculate ganglion. A portion of the upper wall of the internal meatus has been removed. The upper surface of the arcuate eminence has been drilled to expose the superior semicircular canal. In the middle fossa approach, for an acoustic neuroma, the cochlea and semicircular canal are not opened, as seen in this dissection illustrating some of the important structures that are to be avoided in opening the meatus. D, Enlarged view. The cochlea, located below the middle fossa floor in the angle between the facial and greater petrosal nerves, has been opened in the area anteromedial to the meatal fundus. The roof of the meatus has been opened to expose the superior vestibular nerve, which innervates the ampullae of the superior and lateral canals and the meatal segment of the facial nerve. E, The vestibule and semicircular canals are located posterolateral and the cochlea is located anteromedial to the meatal fundus. The tensor tympani is layered along the anterior edge and the greater petrosal nerve above the petrous carotid. F, Enlarged view. The vertical crest (Bill’s bar) separates the facial and superior vestibular nerves at the meatal fundus. The superior and inferior vestibular nerves are located posteriorly and the facial and cochlear nerves anteriorly in the meatus, with the cochlear nerve passing below the facial nerve to enter the modiolus. The labyrinthine segment of the facial nerve courses superolateral to the cochlea. (Images courtesy of AL Rhoton, Jr.)