Stepwise Dissection of the Roof of the Cavernous Sinus. A, Superior view showing the dura lining the superior surface of the anterior clinoid process, continuing medially above the optic canal to form the falciform ligament and below the optic nerve to form the upper dural ring, and blending further medially into the diaphragma sellae. The petroclinoid dural folds are a continuation of the tentorial edge, which divides at the petrous apex into the anterior and posterior petroclinoid dural folds. The anterior petroclinoid dural fold stretches from the petrous apex to the tip of the anterior clinoid process and the posterior petroclinoid dural fold extends from the apex to the posterior clinoid process. The interclinoid dural fold extends from the anterior to the posterior clinoid process. The internal carotid artery and the optic nerve are medial to the anterior clinoid process, and the carotid artery is inferolateral to the optic nerve. The oculomotor nerve penetrates the roof of the cavernous sinus in the oculomotor triangle located between the three folds. B, View showing the left optic nerve elevated to expose the ophthalmic artery arising from the medial part of the upper surface of the internal carotid artery and coursing anterolaterally along the floor of the optic canal. The right carotid artery was divided at the level of the roof of the cavernous sinus, which forms the upper dural ring. C, View of another specimen showing the anterior clinoid process, roof of the optic canal, and lesser wing of the sphenoid bone removed. Removing the anterior clinoid process exposes the clinoidal triangle, also called the clinoidal space. The optic strut, positioned at the anterior end of the clinoidal space, separates the optic canal from the superior orbital fissure. The clinoidal segment of the carotid artery rests against the posterior surface of the optic strut. The superior hypophyseal arteries arise from the medial surface of the carotid’s ophthalmic segment, which extends between the ophthalmic and posterior communicating artery origins. The oculomotor, trochlear, and abducens nerves and branches of the first trigeminal division (V1) pass through the superior orbital fissure. The lacrimal and frontal nerves are branches of the first trigeminal division. The maxillary nerve (V2) passes through the foramen rotundum at the lower edge of the cavernous sinus. D, Superolateral view of the left cavernous sinus in another specimen showing the oculomotor nerve, which pierces the oculomotor triangle between the anterior and posterior petroclinoid and interclinoid dural folds, traverses the short oculomotor cistern, and becomes incorporated into the lateral wall of the cavernous sinus just below the tip of the anterior clinoid process. The thin wall of the oculomotor cistern has been preserved. E, Superior view of another specimen showing the oculomotor triangle in the roof of the cavernous sinus opened, but the clinoidal triangle that sits below the anterior clinoid process has not been exposed. The oculomotor triangle through which the oculomotor nerve enters the roof of the cavernous sinus sits between the anterior, posterior, and interclinoid dural folds. F, View showing the anterior clinoid process removed to expose the clinoidal triangle or space. The inner layer of the lateral dural wall of the cavernous sinus, which covers the inferior surface of the anterior clinoid process, blends with the outer layer, which cover the upper surface of the anterior clinoid process, at the level of the tip of the anterior clinoid. The optic strut separates the superior orbital fissure from the optic canal. The clinoidal segment of the internal carotid artery sits against the posterior surface of the optic strut. (Images courtesy of AL Rhoton, Jr.)