G, The vertical and transverse crest are exposed at the meatal fundus. The common crus and adjacent part of the superior and posterior canals have been exposed. The endolymphatic duct and sac are situated inferolateral to the internal acoustic meatus. H, Another dissection showing the relationships of the junction of the posterior and superior canals and common crus to the meatus. The endolymphatic duct extends downward and backward from the vestibule and opens into the endolymphatic sac, which sits under the dura in the area below and lateral to the meatus. The jugular bulb can be seen through the bone medial to the endolymphatic sac. I, Fundus of the left internal acoustic meatus. The transverse crest separates the superior vestibular area and facial canal above from the inferior vestibular and cochlear areas below. The vertical crest separates the superior vestibular area from the entrance into the facial canal. The multiple cochlear nerve filaments penetrating the tiny openings in the lamina cribrosa at the meatal fundus can easily be torn with traction on the nerve from lateral to medial, therefore, we try to direct the strokes of dissection from medial to lateral when there is an opportunity to preserve hearing. J, Closure after removing the posterior wall of the internal acoustic meatus. Bone wax on a microdissector is carefully placed into open air cells in the posterior meatal lip and then a pledget of crushed subcutaneous abdominal fat is laid over the drilled meatal area. This has prevented cerebrospinal fluid leaks after removal of the posterior wall of the internal acoustic meatus in more than 200 consecutive operations for acoustic neuroma by the author. (Images courtesy of AL Rhoton, Jr.)