A, The floor of the third ventricle is located medial to the uncus and anterior perforated substance and above the midbrain. From anterior to posterior, the floor includes the lower margin of the optic chiasm, the pituitary stalk surrounded by the tuber cinereum, mamillary bodies, and the midbrain. The interpeduncular fossa is located below the posterior part of the floor. The anterior part of the optic tract extends along the lateral margin of the floor, but further posteriorly, the tracts deviate laterally away from the floor to pass around the upper margin of the cerebral peduncle. B, Enlarged view. The tuber cinereum is situated around the pituitary stalk. The infundibular recess extends into the base of the stalk. A third ventriculostomy is commonly performed by opening through the thin area (yellow arrow) in the floor just in front of the mamillary bodies. The oculomotor nerves arise behind the mamillary bodies below the posterior part of the floor of the third ventricle. C, Other specimen showing the thin area in front of the mamillary bodies (yellow arrow) through which a third ventriculostomy is completed. The anterior perforated substance and optic tracts are positioned lateral to the anterior part of the floor of the third ventricle. The mamillary bodies and upper midbrain are positioned below the posterior part of the floor. D, View of another third ventricle from below with the vascular structure preserved. The internal carotid, posterior communicating, anterior choroidal, and posterior cerebral arteries all give rise to branches that reach the walls of the lateral and third ventricles. The thalamoperforating branches of the posterior cerebral artery supply some of the posterior part of the floor of the third ventricle. (Images courtesy of AL Rhoton, Jr.)