The Medial Carotid Membrane. A, Left frontotemporal craniotomy and transsylvian exposure (inset). The outer arachnoid membrane has been opened and the frontal lobe elevated to expose the outer arachnoid membrane, which has been opened above the optic nerve to expose the chiasmatic and carotid cisterns. The medial carotid membrane is exposed in the opticocarotid triangle, located between the carotid, A1, and optic nerve. B, Endoscopic view. The optic nerve has been elevated to expose the naked area along the initial segment of the supraclinoid carotid, where there is a paucity of arachnoid trabeculae. The medial carotid membrane is exposed above the naked area. C, Enlarged view of the multiple trabeculae forming the medial carotid membrane, which extends from the lower part of the optic nerve and chiasm and attaches to the outer arachnoid membrane covering the posterior clinoid process and lateral margin of the sella. D, Endoscopic view of the medial carotid membrane. (Images courtesy of AL Rhoton, Jr.)