Interventricular foramen of Monro and roof of the third ventricle. In this axial dissection, rostral is towards the top of the figure and medial is to the left.  The fornix has been retracted medially to expose the velum interpositum, a vascular layer located in the space between the superior and inferior layers of the tela choroidea (just above the roof of the third ventricle). The thalamostriate vein, which lies in a groove between the caudate and the thalamus, joins the choroidal vein to form the internal cerebral vein. The internal cerebral vein passes posteriorly along the superior surface of the third ventricle.  Further posteriorly (bottom of the image), the internal cerebral vein runs between the layers of the tela choroidea of the third ventricle.  The choroid plexus in the floor of the lateral ventricle courses into the foramen of Monro to become continuous with choroid plexus suspended from the roof of the third ventricle (hidden from view).  Medial posterior choroidal arteries are visible within the roof of the third ventricle adjacent to the internal cerebral veins. (Image courtesy of AL Rhoton, Jr.)