Magnified view into the third ventricle through the velum interpositum. The fornix is retracted medially in this image and the velum interpositum opened to reveal the major anatomic structures of the lateral and third ventricles. The septal vein, superior choroidal vein and the thalamostriate vein drain into the internal cerebral veins of the third ventricle, which are paired with the medial posterior choroidal arteries. The commissural fibers of the posterior commissure are visible, as is the massa intermedia. (Image courtesy of AL Rhoton, Jr.)