A, This is the dissection that the participants complete in our microsurgery courses to demonstrate the intradural approach to the cavernous sinus. At this stage of the course, the cerebral hemisphere has been removed and the suprasellar area and the lateral wall of the cavernous sinus have been exposed. The optic nerve has been elevated to expose the ophthalmic artery entering the optic canal. B, The dura over the upper surface of the anterior clinoid, optic canal, and planum has been removed in preparation for anterior clinoidectomy and removal of the roof of the optic canal. The falciform ligament extends across the optic nerve just proximal to the nerve’s entrance to the optic canal. C, The anterior clinoid and optic strut have been removed to expose the clinoid segment of the carotid artery enclosed in the dural carotid collar. The carotid artery, within this cavernous sinus, is quite tortuous and bulges upward medial to the oculomotor nerve to distort the roof of the sinus. Removal of the optic strut exposed the sphenoid sinus mucosa extending into the base of the strut. The carotid collar is the cuff of dura that encloses the clinoid segment between the upper and lower dural rings. The lower dural ring is loosely adherent to the artery, but the upper dural ring adheres tightly to the artery. D, A dural incision extending around the margin of the lateral wall of the cavernous sinus and the trigeminal nerve has been completed, and the outer layer of dura in the lateral sinus wall has been removed to expose the thin inner layer in which the nerves course. The pericavernous venous plexus extends around all three trigeminal divisions. The greater petrosal nerve is exposed lateral to the trigeminal ganglion. E, The outer layer of the dural roof of sinus has been removed, while the thin layer investing the nerves has been preserved. The oculomotor nerve enters the dura through the oculomotor triangle located between the anterior and posterior clinoid processes and the petrous apex, and sits in a narrow cistern in the sinus roof that extends a variable length along the course of the nerve. The dura, which lines the lower surface of the anterior clinoid and separates the clinoid and the oculomotor nerve, referred to as the carotidoculomotor membrane, extends medially around the lower edge of the clinoid segment to form the lower dural ring. F, The thin inner layer of dura remaining over the lateral wall of the oculomotor triangle has been removed. The dura covering the lower margin of the anterior clinoid process and in the oculomotor triangle forms the roof of the cavernous sinus. The level at which the oculomotor nerve enters the oculo-motor cistern is marked with a green arrow and the level at which the cistern ends and the nerve become tightly invested by dura is shown with a yellow arrow. (Images courtesy of AL Rhoton, Jr.)