Basal Cisternal Membranes. A, Left frontotemporal craniotomy (inset). The oculomotor nerve is exposed on the left and the posterior communicating artery on the right of the carotid artery. The diencephalic membrane is exposed deep to the carotid and posterior communicating arteries. The diencephalic membrane may extend laterally to attach to the oculomotor nerves. B, The internal carotid artery has been elevated to open the carotid-oculomotor interval and expose the mesencephalic leaf of Liliequist’s membrane, which surrounds the upper part of the basilar artery just below the origin of the posterior cerebral arteries. C, View through another left frontotemporal craniotomy. The optic chiasm has been elevated to expose the medial carotid membrane through the opticocarotid triangle, the space between the optic nerve, the internal carotid artery, and the A1. The medial carotid membrane extends downward from the inferior surface of the optic chiasm, attaches to the outer arachnoidal membrane covering the lateral edge of the sella and posterior clinoid process, and incompletely separates the carotid and chiasmatic cisterns. D, The medial carotid membrane has been removed to expose the diencephalic membrane, which extends downward from the posterior edge of the mamillary bodies and attaches to the outer arachnoid membrane covering the dorsum sellae. (Images courtesy of AL Rhoton, Jr.)