An Organ of Adaptation.—Forming, as it does, the boundary between the body and its physical environment, the skin is perhaps the most important agent through which the body is adapted to its immediate surroundings. Evidence of this is found in the great variety of influences which are able to affect the body through their action upon the nerves in the skin, and in the changes which the epidermis undergoes on exposure. The latter function is especially marked in the lower animals, the coverings of epidermal tissue (hair, scales, feathers, etc.) adapting each species to the physical conditions under which it lives. In man the most striking example of adaptation through the skin is seen in the variations in the quantity of blood circulating through it, corresponding to the changes in the temperature outside of the body. These variations are of great importance, having to do with the
Maintenance of the Normal Temperature.—It is necessary to the continuance of life that the temperature of the body be kept at a nearly uniform degree, called the normal temperature, which is about 98.6° F. The maintenance of the normal temperature depends mainly upon four conditions: the chemical changes at the cells, the circulation of the blood, the nervous system, and the skin. The chemical changes produce the heat, the blood in its circulation distributes the heat over the body, and the nervous system controls the heat-producing and distributing processes (page 320). The skin is the chief means by which the body[pg 270] gets rid of an excess of heat and, by so doing, avoids overheating. [91]
How the Skin cools the Body.—The skin is a means of ridding the body of an excess of heat in at least two ways:
1. By the conduction and radiation of heat from its surface as from a stove. This goes on all the time, but varies with the amount of heat brought to the surface by the blood.
2. By the evaporation of the perspiration. It is a well-established and easily demonstrated principle that liquids in evaporating use up heat.(See Practical Work.) It is also a matter of everyday experience that the perspiration has a cooling effect upon the body and that its flow increases with the amount of heat to be gotten rid of. The quantity of perspiration secreted, and of heat disposed of through its evaporation, also varies with the amount of blood circulating through the skin.
Temperature Regulation by the Skin.—Variations in the quantity of blood circulating through the skin enable this organ to throw off just the right amount of heat for keeping the body at the normal temperature. If it is necessary for the body to rid itself of an excess of heat, the quantity of blood circulating in the skin is increased. This brings the blood near the surface, where more heat can be radiated and where it may cause an increase in the perspiration. On the other hand, if the body is in danger of losing too much heat, the circulation diminishes in the skin and increases in the internal organs. This stops the rapid loss of heat from the surface. The skin in this work[pg 271] is of course made to cooperate with other parts of the body. That it is not the only organ concerned in regulating the escape of heat is seen in the results that follow sensations either of chilliness or of heat at the surface.
Effects of Heat and Cold Sensations.—Sensations, or feelings, of heat and cold are made possible through the nerves which connect the brain with the temperature corpuscles, found in the skin (page 343). As the warm blood recedes from the skin, a sensation of cold is felt, but when the blood returns, there is again the feeling of warmth. The sensation of cold prompts one to seek a warmer place, or to put on more clothing; while the sensation of heat, if it be oppressive, leads to activities of an opposite kind. Prompted in this way by the sensations from the skin, one voluntarily supplies the external conditions, such as clothing and heat, that affect the body temperature.
Alcohol and the Regulation of Temperature.—Alcohol, through its effect upon the nervous system, interferes seriously with the regulation of the body temperature. By dilating the capillaries, it increases the circulation in the skin and leads to an undue loss of heat. At the same time the excess of blood in the skin causes a feeling of warmth which has led to the erroneous belief that alcohol is a heat producer. If taken on a cold day, it deceives one about his true condition and leads to a wasting of heat when it should be carefully economized. Not only is alcohol of no value in maintaining the body temperature, but if taken during severe exposure to cold, it becomes a menace to life itself. Arctic, explorers and others exposed to severe cold have found that they withstand cold far better when no alcohol at all is used.[92]