Located in the ventralmost aspect of the diencephalon, the hypothalamus is bounded by the lamina terminalis anteriorly, the hypothalamic sulcus superiorly, and the tegmentum of the midbrain posteriorly (eFig. 9.87). Laterally the hypothalamus is bordered by the substantia innominata rostrally and the posterior limb of the internal capsule caudally. It forms the floor in addition to the lower portion of the lateral walls in the third ventricle. When viewing the external surface of the ventral brain, the area containing the hypothalamus is circumscribed by the optic chiasm, optic tract, crus cerebri, and caudal edge of the mammillary bodies (eFig. 9.88).