E, The labyrinthectomy has been completed to expose the internal acoustic meatus. F, The dura lining the meatus has been opened and the facial nerve has been transposed posteriorly. The facial segments are the cisternal segment located in the cistern medial to the meatal porus, the meatal segment that extends laterally from the porus to the meatal fundus, the labyrinthine segment that is located between the fundus and the geniculate ganglion, the tympanic segment that arises at the ganglion and the sharp turn, the genu, and passes between the lateral semicircular canal and the oval window, and the mastoid segment that descends to exit the stylomastoid foramen. The labyrinthine segment courses between the semicircular canals and vestibule on its posterolateral side and the cochlea on its anteromedial margin. The superior and inferior vestibular nerves have lost their end organs with the drilling of the semicircular canals and vestibule. The cochlear nerve passes laterally to enter the cochlea, which is still preserved in the bone anteromedial to the fundus of the meatus. G, The cochlear nerve has been divided and reflected and bone removed to expose the cochlea. H, The transcochlear exposure, completed by removing the cochlea and surrounding petrous apex, provides access to the front of the brainstem and vertebrobasilar junction, but at the cost of loss of hearing due to the labyrinthectomy and almost certain temporary or permanent facial weakness associated with the posterior transposition of the facial nerve. (Images courtesy of AL Rhoton, Jr.)