A, At this stage of the course, part of the frontal and temporal lobes has been removed and the sylvian fissure has been exposed. B, The sphenoid ridge has been removed and dura has been elevated from the superior and lateral wall of the orbit to expose the superior orbital fissure. At the lateral margin of the superior orbital fissure, the dura becomes thick and tough at the point it blends into the periorbita, making it necessary to make a sharp shallow cut in this dura at the lateral margin of the fissure to continue peeling the outer layer of dura away from the wall of the cavernous sinus. C, The outer layer of dura has been pealed back to expose the anterior clinoid process and the oculomotor and trochlear nerves and trigeminal divisions. The nerves course in the thin inner layer of the lateral wall of the cavernous sinus. D, The anterior clinoid has been removed to expose the clinoid segment of the internal carotid artery. The thin inner layer of dura within the wall of the sinus has been removed and the ophthalmic nerve has been depressed to expose the abducens nerve coming through Dorello’s canal, located below the petrosphenoid (Gruber’s) ligament. The lower margin of the posterior wall of the sinus is located at the petrous apex just below where the abducens nerve enters Dorello’s canal (red arrow). The anterior edge of the lower margin is located at the lower margin of the superior orbital fissure (yellow arrow). E, The bone in the anteromedial and anterolateral middle fossa triangles has been drilled to open into the lateral wing of the sphenoid sinus. The motor root of the trigeminal nerve courses along the medial side of the trigeminal ganglion and enters the medial part of the foramen ovale with the mandibular nerve. F, The bone behind the greater petrosal nerve and lateral to the trigeminal nerve in Kawase’s triangle has been drilled to expose the nerves and anteroinferior cerebellar artery in the internal acoustic meatus. (Images courtesy of AL Rhoton, Jr.)