The process of maintaining constancy or equilibrium in the physiological activities of the organism. The normal functioning of all parts of the body is dependent on this balance; it is essential for both physical survival and psychological well-being.Two general types of activity are involved in keeping the internal environment within favorable limits: homeostatic mechanisms (Cannon, 1932), and regulatory behavior (Richter, 1942-43). The homeostatic mechanisms are largely automatic processes which maintain constancy of body temperature, blood sugar level, calcium-phosphate balance, salt and water balance, blood pressure, heart rate, and hydrogen-ion (PH) concentration. Practically all the organs and systems of the body participate in these homeostatic processes, and frequently a number of them work together—particularly the lungs, the digestive tract, the kidneys, the liver, the circulatory system, and the nervous system which directly or indirectly regulates all the other mechanisms.Here are some examples of these activities. If body temperature becomes too high or too low, a “homeostat” located in the hypothalamus initiates certain body activities: we either perspire, and the resultant evaporation cools the body, or we generate heat through stepped-up metabolism and shivering. If our blood-sugar level falls, some endocrine secretions release glucose from our liver, while others regulate the rate at which the intestine absorbs and utilizes carbohydrates.These “automatic” physiological mechanisms are sometimes insufficient. The body cannot maintain homeostasis for any length of time if it is not getting enough heat, food, water, or air from outside itself. In that case the animal or human being is motivated to meet its needs through overt, regulatory behavior. To maintain warmth, animals roll into a ball or move to a warm area; and to ensure adequate nourishment, they hoard food, increase their intake, or select a diet of high caloric value. Human beings behave in similar ways. We counteract the cold by wearing warm clothing, insulating our houses, or moving south; and we guard against insufficient oxygen at high altitudes by designing pressurized airplane cabins and special masks for astronauts to wear. In spite of the fact that there are two general approaches to adjustment, homeostatic mechanisms and regulatory behavior, they have a common denominator: they are equally dependent on the co-ordinated functioning of the nervous system. Without this factor it would be impossible to bring together all the activities that are necessary to keep the internal environment within normal limits.