Inferior view of the base of the cerebrum. The midbrain has been cross-sectioned at the level of the superior colliculi. Prominent arterial anatomy at this level include the bilateral posterior communicating arteries, basilar apex, and P1 segments of the posterior cerebral arteries. The oculomotor nerves course inferolaterally to the posterior communicating arteries, and just medial to the uncus of the parahippocampal gyrus. Anterior choroidal arteries are the next major branch of the internal carotid arteries after the posterior communicating arteries, and course posterolaterally to enter the temporal horn of the lateral ventricle in the choroidal fissure along with the lateral posterior choroidal arteries (branches of the P2 segment of the PCA). The medial posterior choroidal arteries are branches of P2 that course around the midbrain within the crural and ambient cisterns before ascending toward the third ventricle. (Image courtesy of AL Rhoton, Jr.)