Stepwise Exposure of the Petrous Carotid. A, Superolateral view. The dura has been elevated from the floor of the left middle fossa to expose the trigeminal ganglion (3 divisions) and the cavernous carotid. B, Bone has been removed between the trigeminal divisions to expose the sphenoid sinus and mucosa. The anterior genu and anterior vertical segment, which underlie prominences in the lateral wall of the sphenoid sinus, are hidden on the medial side of the trigeminal nerve. The upper edge of the petrolingual ligament, which marks the boundary between the petrous and cavernous carotid, is positioned medial to the junction of the V1 and V2 (blue line) and the upper and middle thirds of Meckel’s cave. The site of Meckel’s cave, the arachnoid cistern around the trigeminal nerve, is outlined with a green line. Bone has been removed lateral to the trigeminal nerve to expose the horizontal segment of the petrous carotid and the tensor tympani muscle. The greater petrosal nerve courses above the anterior half of the horizontal segment of the petrous carotid. C, The trigeminal ganglion and adjacent part of the posterior trigeminal root and divisions have been removed to expose the distal part of the horizontal segment and the anterior genu and the anterior vertical segments. The lateral wall of the sphenoid sinus has been opened. The greater petrosal nerve passes medially along the anterior surface of the anterior genu toward the vidian canal. D, Bone has been removed from the floor of the sphenoid sinus to unroof the vidian canal. The vidian nerve is formed by the union of the greater petrosal and deep petrosal nerves, which join above the foramen lacerum and enter the vidian canal. The deep petrosal nerve arises from the periarterial carotid nerves. The anterior genu sits above the foramen lacerum. E, Superolateral views of another right middle fossa. The bone above the internal acoustic meatus has been removed and the dura opened to expose the facial and vestibulocochlear nerves. Bone has been removed below and anterior to the greater petrosal nerve to expose the horizontal segment of the petrous carotid and the tensor tympani muscle and eustachian tube. The petrous carotid is separated from the tensor tympani and eustachian tube by a thin shell of bone. The cochlea, which is positioned slightly posterior, superior, and lateral to the posterior genu, is exposed in the cochlear angle located between the labyrinthine segment of the facial nerve and the greater petrosal nerve. The tensor tympani, which passes along the anterior margin of the petrous carotid, is innervated by the trigeminal nerve and has a sharp bend around the cochleaform process, where it gives rise to a narrow tendon, which attaches to the malleus. The tensor tympani is separated from the roof of the eustachian tube, carotid canal, and floor of the middle fossa by a thin shell of bone. The tegmen has been opened and the mastoid air cells have been removed to expose the semicircular canals. F, Another specimen. Unroofing the carotid canal lateral to the trigeminal nerve exposes a sufficient length of the petrous segment to complete a bypass from the right petrous carotid to the supraclinoid carotid. The saphenous vein bypass has been anastomosed to the petrous carotid in an end-to-side fashion using 8-0 nylon interrupted sutures. The other end of the saphenous vein graft is anastomosed to the supraclinoid carotid in an end-to-side fashion between the ophthalmic and posterior communicating arteries using 8-0 suture. (Images courtesy of AL Rhoton, Jr.)