Superior View of the Middle Cranial Base and Petrous Carotid. A, The horizontal segment of the petrous carotid is exposed lateral to the trigeminal nerve. The internal acoustic meatus, eustachian tube, and cochlea have been unroofed. The anterior part of the floor of the middle fossa is formed by the greater sphenoid wing and the posterior part of the floor is formed by the upper surface of the temporal bone. The dural roof and lateral wall of the cavernous sinus have been removed. B, Enlarged view. The anterior genu and anterior vertical segments of the petrous carotid are located below the trigeminal ganglion. The upper edge of the petrolingual ligament is located at the level of a line extending posteriorly along the upper edge of the second trigeminal division. The petrolingual ligament can be seen deep to the fascicles of the posterior trigeminal root. C, The floor of the middle fossa has been removed to show the relationships below the floor. The infratemporal fossa is located below the greater sphenoid wing and contains the pterygoid muscles, venous plexus, and branches of the mandibular nerve and maxillary artery. The roof of the temporal bone, which forms the posterior part of the floor of the middle fossa, has been opened to expose the mastoid antrum, eustachian tube, semicircular canals, cochlea, nerves in the internal acoustic meatus, and mandibular condyle. D, The trigeminal nerve has been reflected forward to expose the petrolingual ligament. The abducens nerve passes below the petrosphenoid ligament and through Dorello’s canal. The junction of the petrous and cavernous carotid is located at the upper edge of the petrolingual ligament. The greater petrosal nerve is joined by the deep petrosal branch of the carotid neural plexus to form the vidian nerve, which passes forward in the vidian canal, which has been unroofed. The lesser petrosal nerve crosses the floor of the middle fossa anterior to the greater petrosal nerve. The tensor tympani muscle and eustachian, with the former above the latter, are layered along and separated from the anterior surface of the petrous carotid by a thin layer of bone. E, Posterosuperior view of another specimen. The petrous apex medial to the internal acoustic meatus has been partially removed to expose the petrous carotid. The posterior genu, located at the junction of the posterior vertical and horizontal segments, is situated below, medial, and anterior to the cochlea, which is enclosed in the bone anteromedial to the fundus of the internal acoustic meatus. The jugular bulb extends upward, toward the posterior meatal wall, vestibule, and semicircular canals. The inferior petrosal sinus courses along the petroclival fissure. F, Posterosuperior view after drilling of the bone above and posterior to the petrous carotid. The cochlea, which is located above, posterior, and lateral to the posterior genu of the petrous carotid, is exposed in the angle between the greater petrosal nerve and the pregeniculate segment of the facial nerve. The chorda tympani is also in the exposure. (Images courtesy of AL Rhoton, Jr.)