Midsagittal Perspective of Third and Lateral Ventricle. The vein of Galen can be seen along the posterior aspect of the dissection (image left). The medial posterior choroidal artery is visible along the posterior aspect of the third ventricle. The internal cerebral vein is visible within the superior margin of the third ventricle. The hypothalamus is visible inferior to the massa intermedia and forms the floor and anterior part of the lateral wall of the third ventricle. (Image courtesy of M Nunez)