E, Inferior view with the floor of the third ventricle removed to expose the roof. The pituitary stalk has been reflected forward to expose the ventricular side of the infundibular recess and lamina terminalis. The lamina terminalis slopes upward from the upper edge of the chiasm to the area in front of the anterior commissure where it blends into the rostrum of the corpus callosum. The columns of the fornix cross above and anterior to the foramen of Monro and descend toward the mamillary bodies. The massa intermedia crosses the midportion of the third ventricle. The velum interpositum, in which the internal cerebral veins and medial posterior choroidal arteries course, is positioned between the thalami in the roof of the third ventricle. The posterior commissure is exposed below the pineal gland. The vein of Galen, into which the basal veins empty, is located just behind the third ventricle. F, Enlarged view. The infundibular recess is located below the optic chiasm in the base of the pituitary stalk, and the chiasmatic recess is located above the optic chiasm. The lamina terminalis forms the anterior wall of the chiasmatic recess. The anterior commissure crosses the anterior wall in front of the columns of the fornix. The foramina of Monro open upward into both lateral ventricles. The lower wall of the velum interpositum is formed by the layer of tela choroidea, in which the choroid plexus in the roof of the third ventricle arises, and which is attached laterally to the striae medullaris thalami. The internal cerebral veins can be seen through the layer of tela forming the lower wall of the velum interpositum. G, Another specimen with the floor of the third ventricle removed. The posterior cerebral arteries, from which the lateral and medial posterior choroidal arteries arise, passes around the midbrain. The lamina terminalis is exposed above the optic chiasm and slopes upward toward the anterior commissure. The columns of the fornix pass along the anterior and superior margins of the foramen of Monro and behind the anterior commissure. The lower layer of tela choroidea in the velum interpositum has been removed to expose the vascular layer in the roof of the third ventricle formed by the internal cerebral veins and medial posterior choroidal arteries. Another layer of tela, which spans the interval above the internal cerebral veins and below the body of the fornix, separates the vascular layer from the body of the fornix. H, Enlarged view. The upper layer of tela choroidea that spans the interval below the body of the fornix has been removed. The body of the fornix, exposed by removing the upper layer of tela, blends anteriorly into the columns of the fornix that pass along the anterior and superior margin of the foramen of Monro. The lamina terminalis has been opened in the interval between the optic chiasm and anterior commissure to expose the perforating branches of the anterior cerebral artery. (Images courtesy of AL Rhoton, Jr.)