Mesencephalic leaf of the basal intercisternal membrane

The mesencephalic leaf (mesencephalic segment; mesencephalic membrane) of the Liliequist membrane extends from the diaphragma sellae or dorsal clinoid process to the pontomesencephalic junction separating the interpeduncular cistern from the prepontine cistern. The oculomotor nerves separates the mesencephalic leaf into a medial part (attached to the distal basilar artery or the pontomesencephalic junction) and lateral part (attached to the tentorium and the mesial temporal lobe).

As soon as the basal intercisternal membrane (Liliequist's membrane) leaves the Posterior Clinoidal Process (with its posteriorly oriented trajectory), it spreads into two leafs (membranes):

• Diencephalic leaf of the Liliequist membrane (Diencephalic membrane): continuing a slightly posterior-superior direction (with intimate adhesion to the III nerve cuff at both sides) this membrane finally arrives at brainstem levels. Here, it attaches to Mamillary Bodies. This membrane is thick, regular, and uniform along its surface, forming the roof of the Interpeduncular Cistern. When this membrane is surgically perforated (i.e. while making a Third Ventriculocisternostomy, after the Tuber Cinereum was traversed) the Third Ventricle and basal cisterns are then communicated. After this perforation, the Basilar Artery should be easily seen among arachnoid trabeculae and beneath the mesencephalic membrane

• Mesencephalic leaf of the Liliequist membrane (Mesencephalic membrane): From the same anterior insertion as the diencephalic membrane, this one instead, runs in an oblique manner (with a posteroinferior direction). Once at the brainstem level, it attaches to the Pontomesencephalic Sulcus, just above the Basilar Artery tip and bifurcation. PComA end should be encountered here, dividing P1 from P2 segment of the posterior cerebral artery (PCA). Laterally, the Medial Carotid Sheet, descending up to the oculomotor nerve cuff, is also seen and joins this Mesencephalic Membrane of Liliequist. They continue as one membrane over the Pontomescencephalic Sulcus (forming the Lateral Pontomesencephalic Membrane). The Liliequist mesencephalic membrane is very thin and mostly incomplete, but it has several attachments to the oculomotor nerve and PComA sheath.