Stepwise Dissection of the Middle Fossa Floor. A and B, Superior view of the left temporal bone in a cadaveric specimen, stepwise dissection. A, The superior vestibular nerve has been retracted to show the cranial nerves approaching to the IAM. The petrous apex has been partly removed and the horizontal segment of the internal carotid artery has been exposed. The facial nerve arises at the pontomedullary junction anteromedial and below the vestibulocochlear nerve. The nervus intermedius initially attaches to the vestibulocochlear nerve and joins the facial nerve near the IAM. The trigeminal nerve passes the Meckel's cave toward its ganglion. The abducens nerve traverses through the Dorello's canal. B, Part of the labyrinth has been further removed and 5 segments of the facial nerve have been exposed to demonstrate the surgical limitations of bone removal at the lateral part of the IAM. The positions of the facial and the vestibulocochlear nerves are constant in the lateral portion of the IAM, and the Bill's bar divides the superior compartment of the meatus into an anterior smaller facial and a posterior larger superior vestibular compartments. The labyrinthine segment, the narrowest and shortest segment, courses anterolaterally and makes an abrupt angulation forward toward the geniculate ganglion and is related to the apical turn of the cochlea (blue arrow) medially and the superior SCC posterolaterally. The tympanic segment extends from the geniculate ganglion to the level of the stapes, where the nerve turns downward below the lateral SCC and the mastoid segment of the facial nerve begins inferior to the short process of the incus. The cochlea (blue arrow) lies in the angle between the labyrinthine segment and the GSPN. The vestibule (green arrow), a small cavity at the confluence of the SCCs, is located posterolateral to the fundus of the IAM. C, A drawing of the middle fossa floor based on a cadaveric dissection to demonstrate the middle fossa floor in relation to the middle fossa approach (left side). The bone over the petrous carotid and the IAM has been removed, and the SCCs have been unroofed to expose the facial and vestibulocochlear nerves, geniculate ganglion, cochlea, and vestibule. The surface of the floor of the middle fossa, as exposed through the middle fossa approach to the IAM, can be divided into 2 regions anteroposteriorly separated by a vertical plane passing through the anterior border of the cochlea. The anterior part contains the eustachian tube, tensor tympani muscle and horizontal segment of the petrous carotid artery, all of which lie side by side parallel to the GSPN in the space between the V3 anteriorly and the cochlea posteriorly. The posterior part of the middle fossa floor, the bony labyrinth, contains the cochlea, the vestibule, and the SCCs, which are all closely related to the meatal, labyrinthine, and tympanic segments of the facial nerve and the geniculate ganglion. The posterior border of the cochlea (blue arrow) is the fundus of the IAM, and, posterolaterally, it is related to the geniculate ganglion. The vestibule (green arrow), a small cavity at the confluence of the SCCs, communicates below the fundus with the cochlea. The superior, lateral, and posterior SCCs are situated posterosuperior to the vestibule. The superior SCC, the one that is most closely related to the middle fossa approach, projects toward the middle fossa floor. (Images courtesy of AL Rhoton, Jr.)