Bathing is a very efficient aid to cleanliness; when baths are for this purpose, warm or hot water, with an abundance of soap, should be used; the choice of soap is an individual matter determined largely by the sensitiveness of the skin. Chemically there is no very great difference between one kind of soap and another. Some of them contain materials which are more irritating than those found in others and are to be avoided by persons whose skins are easily irritated. The common belief that a bath should not be taken immediately after eating rests on the feeling that the flushing of the skin that results from the contact with the warm water may divert blood unduly from the alimentary tract and so interfere with the process of digestion. This may very well be true if a very hot bath is taken; where the bath is only a degree or two above body temperature little trouble is to be feared from this cause. A cold bath shortly after eating sometimes gives rise to cramps either in the stomach or in the muscles. Not much is known about these cramps or about the causes which bring them about. In connection with its use as the protective surface of the body the skin has scattered over it numerous glands known as the _sebaceous glands_, which secrete an oily substance that is spread more or less completely over the skin. This oil probably helps to keep the skin soft and also to some degree to make the skin waterproof. One of the reasons why soap has to be used in bathing is to dissolve off this thin film of oil which is ordinarily over the surface of the body. Another use of the skin is as a great sense organ; as we have already seen, the structures for touch, for temperature, and for pain are present in the deeper layers of the skin. These consist of tiny cell masses into which the tips of the sensory nerves pass. In the case of the nerves of pain there are believed to be no special end organs, but the nerves end nakedly among the cells of the skin.