Left anterior inferior endoscopic view of the upper pons and interpeduncular fossa. In this magnified view, the distal end of the basilar artery gives rise to the superior cerebellar artery before terminating into right and left posterior cerebral arteries. The oculomotor nerve is seen arising from the midbrain and coursing between the superior cerebellar and posterior cerebral arteries within the interpeduncular cistern to the cavernous sinus. The upper border of the pons represents the posterior border of the rhomboid-shaped interpeduncular fossa. Here, a very thin arachnoid membrane, Liliequist's membrane, forms a barrier within the basilar cisternal complex. It is described as having three components or sheets: sellar, diencephalic, mesencephalic that separate the chiasmatic cistern, interpeduncular cistern, and prepontine cistern. A perforator artery is shown arising from the P1 segment of the posterior cerebral and supplies the midbrain and thalamus. (Image courtesy of AL Rhoton, Jr.)