Organ component of neuraxis, each instance of which is the ventral part of some diencephalon, forming the floor and part of the lateral wall of some third ventricle. Anatomically, it includes some preoptic area, optic tract, optic chiasm, mammillary bodies, tuber cinereum, infundibulum, and neurohypophysis, but for physiological purposes the neurohypophysis is considered a distinct structure.