Basal intercisternal membrane

The basal intercisternal membrane (Liliequist membran) is a thin structure (≤ 1 mm) with a thickness that is ever inferior to that of the tuber cinereum, located under the floor of the third ventricle, anteriorly extending from the dorsum sellae to the mammillary bodies. 

It may divided into two leaves:

• The diencephalic leaf of the Liliequist membrane extends from the diaphragma sellae or dorsal clinoid process to the at the anterior edge of mammillary bodies, separating the interpeduncular cistern from the suprasellar cistern.

• 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).

Contents:

• Basilar tip and bifurcation: This arterial complex is exactly situated where the mesencephalic membrane starts its insertion. Just a few millimeters downward, dissecting arachnoid space, the Prepontine Cistern is reached

• PComA: This artery can be seen coming from the Carotid Cistern (serving as a guide for dissection). It arrives and ends at the P1-P2 level of the PCA

• PCA and superior cerebellar artery (SCA): Mesencephalic membrane insertion involves both arteries, side to side of the midbrain anterior surface, marked laterally by the apparent origin of the oculomotor nerve

• Oculomotor nerve: This nerve can be seen up to its apparent origin at the brainstem surface. It passes between PCA and SCA

• Venous complex: It includes drainage veins from posterior communicating, pontomesencephalic, and thalamoperforating veins.

Relationships:

The roof of this cistern is composed of the Mamillary Bodies and the diencephalic membrane of Lieliquist. This roof is directly situated behind the Chiasmatic Cistern. Only a thin layer of grey matter (the Tuber Cinereum, part of the floor of the third ventricle, point of perforation for the previously named Third Ventriculo-cisternostomy) is situated between this cistern and the third ventricle above. On both sides, the Cistern Ambiens occupy the middle part of the Tentorial Incisura. The Posterior Perforated Substance (perforated by small branches of the Basilar and P1 arteries) is at the back of the Interpeduncular cistern. Downward, beyond the incomplete mesencephalic membrane of Liliequist, the cisternal space is continued by the Prepontine Cistern.