Superior perspective of the third ventricle through the velum interpositum. The velum interpositum is the double layer of tela choroidea which contains a potential space above the internal cerebral veins in the third ventricle. Opening the fornices and this membrane allows visualization from above into the third ventricle. Here, the medial posterior choroidal arteries and internal cerebral veins are retracted laterally to expose the midline pineal gland, the posterior commissure, the floor of the third ventricle and the massa intermedia. Prominent features of the lateral ventricles include the superior choroidal vein adjacent to the choroid plexus, the thalamostriate veins running between the thalamus and the caudate, and the septal veins anteriorly along the septum pellucidum. (Image courtesy of AL Rhoton, Jr.)