E, The opening of the choroidal fissure has been extended backward from the foramen of Monro to expose both internal cerebral veins and the medial posterior choroidal arteries coursing in the velum interpositum. The anterior septal vein crosses the septum pellucidum. The lower layer of tela choroidea, attached to the striae medullaris thalami deep to the internal cerebral veins, is intact. F, The lower layer of tela choroidea that forms the floor of the velum interpositum has been opened, exposing the massa intermedia and posterior commissure within the third ventricle. G, The internal cerebral veins have been separated to expose the anteroinferior part of the third ventricle. The upper end of the midbrain forms the posterior part of the floor of the third ventricle. The mamillary bodies are situated in the midportion of the floor. The floor anterior to the mamillary bodies and behind the infundibular recess in very thin and is the site commonly opened in a third ventriculostomy. The chiasmatic recess extends forward above the posterior edge of the optic chiasm and below the anterior commissure. H, Enlarged view of the inner sur-face of the anterior wall of the third ventricle. The columns of the fornix extend downward behind the anterior commissure toward the mamillary bodies. The lamina terminalis, chiasmatic recess, posterior edge of the chiasm, and the infundibular recess are located along the anterior and lower wall of the third ventricle. (Images courtesy of AL Rhoton, Jr.)