An Endoscopic and Transcranial Perspective of Basal Cisternal Membranes. E, Endoscopic view directed through the opticocarotid triangle. The chiasmatic cistern is positioned anterior to and the interpeduncular cistern is behind the diencephalic membrane. F, The endoscope has been advanced through the opticocarotid triangle shown in E. The diencephalic leaf of Liliequist’s membrane extends upward from the outer arachnoid membrane to attach to the floor of the third ventricle. The mesencephalic leaf of Liliequist’s membrane extends posteriorly and attaches to the pontomesencephalic junction and the basilar apex below the origin of the P1 segments. G, The endoscope has been advanced into the interpeduncular cistern. The trabeculated part of the mesencephalic membrane attaches to the basilar artery, its branches, and the oculomotor nerve. H, Inferior view. The outer arachnoid membrane has been removed, except at the site of attachment of the inner arachnoidal membranes, to expose the lamina terminalis, carotid, interpeduncular, oculomotor, crural, and prepontine cisterns. The mesencephalic leaf of Liliequist’s membrane extends posteriorly to surround the upper basilar artery. The interpeduncular cistern is located above the mesencephalic membrane and between the diencephalic and mesencephalic leaves of Liliequist’s membrane. (Images courtesy of AL Rhoton, Jr.)