A, Inferior view of the posterior cerebral artery. The medial posterior choroidal artery arises from the P1 and encircles the brainstem on the medial side of the P2 and P3, giving off small branches to the brainstem along its course. The P3 is formed by the branches in the quadrigeminal cistern. B, Enlarged view. The medial posterior choroidal artery encircles the brainstem in the crural, ambient, and quadrigeminal cisterns and turns forward beside the pineal in the quadrigeminal cistern to reach the roof of the third ventricle. C, Inferior view of the posterior cerebral arteries in another specimen, with the floor of the third ventricle removed. The medial posterior arteries encircle the mid-brain and turn forward in the quadrigeminal cistern to reach the roof of the third ventricle. Some of the medial part of the right parahippocampal gyrus has been removed to expose the branches arising from the P2. D, Enlarged view. The lower layer of tela in the roof of the third ventricle has been opened to expose the medial posterior choroidal arteries coursing in the velum interpositum with the branches of the internal cerebral vein. The choroid plexus in the body of the lateral ventricle is continuous at the posterior margin of the foramen of Monro with the choroid plexus in the roof of the third ventricle, which has been removed. (Images courtesy of AL Rhoton, Jr.)