The sensation of thirst is generally felt at the back of the throat, because, when there is a deficiency of water in the system, the throat and mouth especially become parched by evaporation of moisture from their surface. Thirst is increased by heat, and is a constant symptom of FEVER; it is also present in diseases which remove a considerable amount of fluid from the system, such as diarrhoea, DIABETES MELLITUS and DIABETES INSIPIDUS, and after great loss of blood by haemorrhage. A demand for water is also a feature of many conditions associated with prolonged exertion, severe exhaustion and DEHYDRATION.