E, Enlarged view. The anterior wall is made up of the lamina terminalis and the anterior commissure. The optic chiasm, mamillary bodies, and midbrain are in the floor. F, Enlarged view. The chiasmatic recess is located above the optic chiasm and behind the lamina terminalis. The infundibular recess is located below and behind the optic chiasm. The lamina terminalis blends into the rostrum of the corpus callosum. The anterior commissure is positioned between the rostrum of the corpus callosum and the columns of the fornix. The thalamus and hypothalamus form the lateral wall of the third ventricle. G, Enlarged view of the posterior part of the third ventricle. The posterior wall of the third ventricle is formed by the aqueduct, pineal, and habenular and posterior commissures. The pineal recess extends into the base of the pineal in the interval between the habenular and posterior commissures. H, Lateral view of the third ventricle with the hippocampus and fornix preserved. The body of the fornix forms the roof of the third ventricle. The velum interpositum, through which the internal cerebral veins course, is located between the body of the fornix and the striae medullaris thalami. The quadrigeminal cistern and pineal region are located anteromedial to the crus of the fornix, and the ambient cistern and posterior cerebral artery are located medial to the temporal horn and the fimbria. Opening the choroidal fissure adjacent to the body of the fornix exposes the third ventricle. The medial posterior choroidal arteries turn forward beside the pineal to reach the velum interpositum. (Images courtesy of AL Rhoton, Jr.)