Stepwise Dissection of the Roof of the Cavernous Sinus. G, View of the roof of the cavernous sinus showing an anterior portion and a posterior portion. The anterior portion is formed by the dura lining the lower surface of the anterior clinoid process. The posterior portion is formed by the oculomotor triangle. The falciform ligament is a medial extension of the dura lining the upper surface of the anterior clinoid process. H, View showing the left anterior clinoid process and the lateral wall of the cavernous sinus removed. Removing the anterior clinoid process exposes the clinoidal space or triangle. The structures in the clinoidal space, from anterior to posterior, are the optic strut, the clinoid segment of the carotid, and the thin roof of the anterior part of the cavernous sinus. The clinoidal segment of the carotid rests against the posterior surface of the optic strut. The thin carotidoculomotor membrane formed by the dura that lines the lower surface of the anterior clinoid separates the lower surface of the clinoid from the oculomotor nerve. This membrane, after removing the clinoid, separates the venous contents of the cavernous sinus from the subarachnoid space and extends medially to form the lower or proximal ring and the carotid collar around the clinoidal segment. I, Lateral view after opening the optic sheath and elevating the optic nerve showing the ophthalmic artery arising from the medial part of the upper surface of the internal carotid artery inferomedial to the optic nerve and passing anterolateral to reach the inferolateral aspect of the optic nerve at the posterior end of the optic canal. The carotidoculomotor membrane extends above the oculomotor nerve and around the carotid artery to form the lower ring and turns upward around the clinoidal segment to form the carotid collar. The venous contents of the cavernous sinus can be observed through this thin semitransparent oculomotor membrane. The circular sinus extending inside the diaphragma sellae and around the superior aspect of the pituitary gland is formed by the anterior and posterior intercavernous sinuses and the upper part of the paired cavernous sinuses. The oculomotor nerve traverses a short cistern as it enters the roof of the cavernous sinus, and becomes incorporated into the fibrous lateral wall of the sinus below the anterior clinoid process. J, View showing the carotidoculomotor membrane opened with a microdissector introduced between the clinoidal segment of the carotid and the lower dural ring and carotid collar, which are not as tightly adhered to the artery as is the upper dural ring. The oculomotor triangle on the medial side of the anterior petroclinoid fold has been opened, and the posterior clinoid process has been exposed. The oculomotor nerve courses lateral to the posterior clinoid process and medial to the trochlear nerve. The trochlear nerve penetrates the roof of the cavernous sinus near the junction of the anterior and posterior petroclinoid dural folds at the posterior apex of the oculomotor triangle. (Images courtesy of AL Rhoton, Jr.)