Endoscopic and Transcranial Perspective of Posterior Fossa Cisternal Membranes. I, Inferior view of the mesencephalic membrane in another specimen. The sheet-like and trabeculated parts of the mesencephalic membrane are well developed and surround the upper part of the basilar artery. J, Endoscopic view of the mesencephalic membrane in another specimen. The trabecula part of the mesencephalic membrane is poorly developed, so that the interpeduncular and prepontine cisterns have almost no barrier between them. (Images courtesy of AL Rhoton, Jr.)