A, The upper part of the hemispheres has been removed to expose the frontal horn and body of the lateral ventricle. The choroid plexus is attached along the choroidal fissure located between the body of the fornix and the thalamus. The superior choroidal veins course along the choroid plexus. The thalamostriate veins pass through the posterior margins of the foramen of Monro. The columns of the fornix pass anterior and superior to the foramen of Monro. The body of the fornix forms the upper part of the roof of the third ventricle. B, The right lateral edge of the fornix has been removed to expose the upper layer of tela choroidea that spans the interval below the body of the fornix and forms the upper wall of the velum interpositum in the roof of the third ventricle. The velum is positioned between an upper layer of tela attached to the lower surface of the body of the fornix and a lower layer of tela attached below the internal cerebral veins to the striae medullaris thalami. The internal cerebral veins and medial posterior choroidal arteries course in the velum interpositum. C, The body of the fornix has been folded backward. The upper layer of tela that rests against the lower surface of the body of the fornix has been preserved. The tela is a thin, arachnoid-like membrane, through which the internal cerebral veins and the medial posterior choroidal arteries can be seen. The anterior septal veins pass above the foramen of Monro. D, The upper layer of tela has been removed to expose the internal cerebral veins and medial posterior choroidal arteries. The internal cerebral veins have been retracted laterally. The anterior septal veins course along the septum and join the internal cerebral veins near the foramen of Monro. (Images courtesy of AL Rhoton, Jr.)