Regulation of Heat of the Body.--The bodily temperature of a person engaged in manual labor will be found to be but little higher than the temperature of the same person at rest. We know from our previous experiments that heat is released. Muscles, nearly one half the weight of the body, release about five sixths of their energy as heat. At all times they are giving up some heat. How is it that the bodily temperature does not differ greatly at such times? The temperature of the body is largely regulated by means of the activity of the sweat glands. The blood carries much of the heat, liberated in the various parts of the body by the oxidation of food, to the surface of the body, where it is lost in the evaporation of sweat. In hot weather the blood vessels of the skin are dilated; in cold weather they are made smaller by the action of the nervous system. The blood thus loses water in the skin, the water evaporates, and we are cooled off. _The object of increased perspiration, then, is to remove heat from the body._ With a large amount of blood present in the skin, perspiration is increased; with a small amount, it is diminished. Hence, we have in the skin an automatic regulator of bodily temperature. Sweat Glands under Nervous Control.--The sweat glands, like the other glands in the body, are under the control of the sympathetic nervous system. Frequently the nerves dilate the blood vessels of the skin, thus helping the sweat glands to secrete, by giving them more blood. "Thus regulation is carried out by the nervous system determining, on the one hand, the _loss_ by governing the supply of blood to the skin and the action of the sweat glands; and on the other, the _production_ by diminishing or increasing the oxidation of the tissues."--FOSTER AND SHORE, _Physiology_. Colds and Fevers.--The regulation of blood passing through the blood vessels is under control of the nervous system. If this mechanism is interfered with in any way, the sweat glands may not do their work, perspiration may be stopped, and the heat from oxidation held within the body. The body temperature goes up, and a fever results. [Illustration: _A_, blood vessels in skin normal; _B_, when congested.] If the blood vessels in the skin are suddenly cooled when full of blood, they contract and send the blood elsewhere. As a result a congestion or cold may follow. Colds are, in reality, a congestion of membranes lining certain parts of the body, as the nose, throat, windpipe, or lungs. When suffering from a cold, it is therefore important not to chill the skin, as a full blood supply should be kept in it and so kept from the seat of the congestion. For this reason hot baths (which call the blood to the skin), the avoiding of drafts (which chill the skin), and warm clothing are useful factors in the care of colds.