Inferior view of the midbrain (cut), the inferior cerebrum, and the posterior Circle of Willis. The basilar artery is shown dividing into both posterior cerebral arteries; the visualized posterior communicating arteries demarcate the P1 and P2 junction. The oculomotor nerves can be seen inferior to the posterior cerebral arteries. The posterior cerebral arteries continue in the crural and ambient cisterns with the Basal Vein of Rosenthal. The medial and lateral posterior choroidal arteries arise from the P2 segment in the ambient cistern. The lateral posterior choroidal arteries can be seen traversing the choroidal fissure, and the medial posterior choroidal artery courses anterosuperiorly to supply the choroid plexus of the third ventricle. In the midline of the quadrigeminal cistern, basal veins merge with the internal cerebral veins to form the vein of Galen. (Image courtesy of AL Rhoton, Jr.)