[hypo- + thalamus]. /hyÂ·poÂ·thalÂ·aÂ·mus/ (-thalÂ´ah-mus) the part of the diencephalon forming the floor and part of the lateral wall of the third ventricle, including the optic chiasm, mammillary bodies, tuber cinereum, and infundibulum; the pituitary gland is also in this region but is physiologically distinct.he part of the brain that lies below the thalamus, forming the major portion of the ventral region of the diencephalon, and that regulates bodily temperature, certain metabolic processes, and other autonomic activities.