That part of the fore-brain situated beneath, and linked with, the THALAMUS on each side and forming the floor of the third ventricle (see BRAIN). Also linked to the PITUITARY GLAND beneath it, the hypothalamus contains collections of nerve cells believed to form the controlling centres of (1) the sympathetic and (2) the parasympathetic nervous systems (see under NERVOUS SYSTEM). The hypothalamus is the nervous centre for primitive physical and emotional behaviour. It contains nerve centres for the regulation of certain vital processes – the metabolism of fat, carbohydrate and water; sleep; body temperature; and sexual function.