A, Superior view. The dura covering the upper surface of the right anterior clinoid process, optic canal, and adjacent part of the planum has been removed. The roof of the cavernous sinus is formed anteriorly by the dura lining the lower margin of the anterior clinoid and posteriorly by the dura covering the oculomotor triangle located between the anterior and posterior petroclinoidal and intraclinoidal dural folds. The oculomotor nerve enters the roof of the cavernous sinus through the oculomotor triangle. B, The anterior clinoid and roof of the optic canal has been removed. The optic nerve has been elevated to expose the ophthalmic artery entering the optic foramen. Removing the anterior clinoid exposes the floor of the clinoidal triangle located between the optic and oculomotor nerves. The dura separating the lower surface of the clinoid from the oculomotor nerve and extending medially around the carotid artery, referred to as the carotidoculomotor membrane, forms the floor of the clinoidal triangle and the anterior part of the roof of the cavernous sinus. The dura extending medially off the upper surface of the clinoid forms the upper dural ring, and the carotidoculomotor membrane extending medially from the lower surface of the clinoid forms the lower dural ring. C, The dura in the floor of the clinoidal triangle and roof of the oculomotor triangle, which together form the roof of the cavernous sinus, have been removed to expose the upper part of the cavernous sinus. The dura has been elevated from the lateral wall of the cavernous sinus and middle fossa floor to expose the nerves coursing in the lateral wall of the cavernous sinus. D, The sinus has been cleared to expose the clinoid segment of the internal carotid artery in the clinoidal triangle and the posterior bend of the intercavernous carotid below the oculomotor triangle. The anterior part of the roof is formed by the dura that separates the anterior clinoid and oculomotor nerve and that extends medially to form the lower dural ring. The posterior part of the roof is formed by the dura forming the oculomotor triangle. (Images courtesy of AL Rhoton, Jr.)