Stepwise Dissection of the Right Middle Fossa. A, Superolateral lateral view with the dura intact. The 3 divisions of the trigeminal nerve are seen through the intact dura. The cavernous carotid exits and the oculomotor nerve enters the roof of the cavernous sinus. The arcuate eminence is in the lateral margin of the exposure. B, The dura has been elevated from the floor of the middle fossa. Removal of the lateral wall of Meckel’s cave exposes the posterior root of the trigeminal nerve. The petrous carotid is exposed below a dehiscence in the floor of the middle fossa lateral to the third trigeminal division as occurs in approximately 16% of middle fossae. The greater petrosal nerve passes medially along the upper surface of the petrous carotid. C, The internal acoustic meatus has been unroofed to expose the facial and vestibulocochlear nerves. The greater petrosal nerve arises from the geniculate ganglion and passes medially above the petrous carotid. The cochlea has been exposed by drilling the bone in the cochlear angle located between the labyrinthine segment of the facial nerve and the greater petrosal nerve. The posterior genu of the petrous carotid is positioned a few millimeters below, anterior, and medial to the cochlea. D, Bone has been removed above and anterior to the petrous carotid to expose the tensor tympani muscle and the eustachian tube, which slope downward as they proceed medially along the anterior surface of the petrous carotid. The petrous apex and additional bone have been removed from the floor of the middle fossa to expose the external acoustic meatus, inferior petrosal sinus, and a tortuous basilar artery. The greater petrosal nerve passes medially above the petrous carotid. (Images courtesy of AL Rhoton, Jr.)