Superior view of the lateral and third ventricles through the velum interpositum after partial resection of the fornices. At the level of the foramen of Monro, the paired septal veins merge with the thalamostriate veins to form both internal cerebral veins, which course posteriorly along the roof of the third ventricle. The internal cerebral veins are retracted laterally here to demonstrate the medial posterior choroidal arteries. (Image courtesy of AL Rhoton, Jr.)