The third use of the skin, and that with which we are particularly concerned here, is as the fine regulator of body temperature. We have already seen in part how the skin works in this connection, through being warmer when there is more blood flowing through it and cooler when there is less. Its other heat-regulating mechanism is made up of the _sweat glands_. These, as the name implies, are tiny glands in the skin; they consist of tubes opening on the surface and at their inner end coiled up into a sort of knot. In this knot are a great many fine blood vessels, so that each sweat gland has an abundant blood supply. So far as we can tell, the sweat glands act as filters, corresponding in that respect to the filtering tips of the kidney tubes. One interesting thing about them is that the filtering action is controlled by nerves. These nerves belong to the system which controls the smooth muscles and glands; when the nerves become active the sweat glands are also active, and there is rapid filtration of water and inorganic salts from the blood into the sweat glands and out to the outside of the body. Small amounts of organic materials come out also in the sweat, and these are responsible for its characteristic odor. They evidently differ in different people, since all dogs and some persons are able to recognize individuals by their odor, and body odor is largely the odor of sweat. The sweat glands are carrying on their filtering activity most if not all of the time; we are not usually aware of it because the sweat comes out in exceedingly fine drops which evaporate as fast as they are formed; it is only when the sweat is poured out faster than it evaporates that we become aware of its presence. If the air is very full of moisture, sweat will stand on our bodies even though it is not being formed more abundantly than usual. This is because of its inability to evaporate into air already laden with moisture. Usually, however, when sweat appears on the surface of the body it is because the sweat glands have become more active. A couple of paragraphs ago we saw that the sweat glands are under the control of nerves; these arise like the other nerves that control smooth muscles and glands from a center in the brain stem; this particular one is called the sweat center. It may become active through various influences. The one most commonly arousing it is a rise in the temperature of the blood. Whenever we begin to exercise, heat is produced very rapidly, warming up the tissues in which the activity is going on and, since the blood is flowing through them rapidly, warming the blood up as well. This warm blood circulates all through the body, tending to raise all parts of it to the temperature of the active regions, or, as it passes through the skin, to cool by loss of heat from the surface. When this warm blood enters the brain stem it arouses the sweat center and an increased secretion of sweat results.