In the middle fossa approach, the internal auditory canal is approached from above, through a temporal craniotomy located anterior to the ear and above the zygoma. The dura under the temporal lobe is elevated from the floor of the middle cranial fossa until the arcuate eminence and greater petrosal nerve are identified. The distance from the inner table of the skull to the facial hiatus, through which the greater petrosal nerve passes, ranges from 1.3 to 2.3 cm (average 1.7 cm). In separating the dura from the floor of the middle fossa, the surgeon should remember that bone may be absent over all or part of the geniculate ganglion. In a previous study of 100 temporal bones, it was found that all or part of the geniculate ganglion and genu of the facial nerve were exposed in the floor of the middle fossa in 15 bones (15 percent). In 15 other specimens, the geniculate ganglion was completely covered but there was no bone extending over the greater petrosal nerve, The greatest length of greater petrosal nerve covered by bone was 6.0 mm. It is also important to remember that the petrous segment of the internal carotid artery may be exposed without a bony covering in the area below where the greater petrosal nerve passes below the lateral margin of the trigeminal ganglion. The foramen spinosum and middle meningeal artery and the foramen ovale and third trigeminal division are situated at the anterior margin of the extradural exposure. Usually the extradural exposure can be completed without obliterating the middle meningeal artery at the foramen spinosum. The tensor tympani muscle and eustachian tube, although not exposed in this approach, are located beneath the floor of the middle fossa roughly parallel to, and in front of the horizontal portion of the petrous carotid artery.