physical work generates more than twice this amount. The heat generated within man's body is not wasted energy; it keeps his bodily tissues at the temperature (37° C.; 98.6° F.) at which nature has decreed that they shall do their best work. But more heat is produced within than is needed for this purpose, and if this excess were allowed to accumulate unchecked, man's tissues would very soon become unduly heated, the protoplasm of his living cells would become coagulated, and death would be the end of him. He possesses, however, a very efficient regulating mechanism by which his body is enabled to give off heat constantly and in quantity just sufficient to maintain an equilibrium, notwithstanding the varying amounts which he produces from minute to minute. This constant output of heat takes place partly through expired air, but chiefly by direct radiation into the air from the skin, by conduction from the skin to the clothing, and by the evaporation of perspiration poured upon the surface of the skin by the sweat glands. The skin is thus the medium by which the excess of the bodily heat is carried away. But the action of the skin is dependent upon the action of the nervous system in regulating both the amount of hot blood sent to it and the activity of the sweat glands. Whenever, therefore, the body works harder than before and produces more heat, not only does the breathing intensify, but through the nervous system the cutaneous blood vessels are dilated, more blood is sent to them, more perspiration is made in the glands, poured out and evaporated, and thus the excess of heat is passed out to the clothing and the air. By these provisions our bodily temperature is kept fairly constant, whether we do much or little work, whether we live indoors or outdoors, in summer or in winter, whether we labor beside molten metal at a temperature of 250° F. or are exposed to the polar air with its 75° F. below zero.
Nevertheless, it is obvious that there are external essentials to this physiological power of regulation and that these are the possibilities of the radiation and the conduction of the heat and the evaporation of the perspiration. The body is ever ready to do its share, but the surrounding air must be in such a physical condition as to supplement the body's activities. If the air be cool and moderately dry the best conditions exist for the body's well-being; if the air be hot and dry, or cool and moist, within certain limits the body can protect itself; but if the air be hot and at the same time contain much moisture a condition exists against which the body is imperfectly equipped. If the external temperature be as high as or higher than the bodily temperature, bodily heat can not be given out by radiation and conduction, and if at the same time the air be saturated with moisture, bodily heat can not be given out by the evaporation of perspiration; and thus with the two principal avenues of heat loss obstructed and with the fires still burning within, the temperature rises and the unfortunate individual passes
