196 ANATOMY FOE NUKSES. [CHAP. XVII. 



Bodily heat. In order that the bodily functions may be prop- 

 erly performed, it is necessary for the body to maintain a certain 

 temperature. Just as plants are killed by the frost, or withered 

 by the heat of the sun, so our tissues die if the bodily tempera- 

 ture falls below, or rises above, a certain limit. Our bodies, 

 however,, differ from plants in that they generate and regu- 

 late their own temperature, and possess the power of adapting 

 themselves to extremes of external heat and cold, without 

 necessarily suffering any vital injury. But, although the ex- 

 ternal temperature of the atmosphere may vary considerably 

 without hurting us, the bodily temperature must be kept at 

 an average standard of 98.6 F. (37 C.) if we are to remain in 

 a state of health. Slight variations are compatible with health, 

 the temperature being normally a trifle higher after eating or in 

 the evening of the day, but any variation over a degree above or 

 below 98.6 F. is indicative of danger. 



Production of heat. Heat in the body is produced by the 

 chemical changes that are constantly going on in the tissues. 

 Wherever metabolic changes are taking place, there heat is set free. 

 These changes take place more rapidly in some tissues than in 

 others, and in the same tissues at different times. The muscles 

 always manifest a far higher rate of activity than the connec- 

 tive tissues, and consequently the former evolve a larger pro- 

 portion of the bodily heat than the latter. We might liken the 

 different tissues of the body to so many fireplaces stored with 

 fuel, the fuel in some of the fireplaces being more easily ignited 

 and burning more rapidly than in others. The muscles and the 

 secreting glands, especially the liver, are supposed to be the 

 main sources of heat, as they are the seats of a very active 

 metabolism. 



Loss of heat. The heat thus continually produced is as con- 

 tinually leaving the body by the skin and the lungs, and by the 

 urine and feces. It has been calculated that in every 100 parts 

 about : 



88 per cent is lost by conduction and radiation from the surface of 

 the skin and the evaporation of the perspiration. 



9 per cent is lost by warming the expired air and the evaporation 

 of the water of respiration. 



3 per cent is lost by warming the urine and feces. 



