324 TEE POPULAR SCIENCE MONTHLY 



physical work generates more than twice this amount. The heat gen- 

 erated 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 cutane- 

 ous blood vessels are dilated, more blood is sent to them, more perspira- 

 tion 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 pro- 

 visions 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 surround- 

 ing 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 tem- 

 perature 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 



