The Mesencephalic Membrane. E, The A1 has been elevated to expose the intracarotid membrane through the opticocarotid triangle. The intracarotid membrane is positioned above and lateral to the medial carotid membrane and extends from the optic tract to the uncus in the upper part of the carotid cistern. F, Endoscopic view of the intracarotid membrane. G, The medial carotid membrane has been opened, and the mesencephalic leaf of Liliequist’s membrane has been exposed. The diencephalic membrane is hidden under the optic chiasm. H, The A1 was divided to obtain this view of the mesencephalic membrane. (Images courtesy of AL Rhoton, Jr.)