Chap. XVIII] BODY HEAT 353 



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 connective tissues, and consequently the 

 former evolve a larger proportion of the bodily heat than the latter. 

 We might liken the different tissues of the body to so many fire- 

 places 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 sup- 

 posed 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 sur- 

 face of the skin and the evaporation of the perspiration. 



9 per cent is lost by warming the expired air and the evapora- 

 tion of the water of respiration. 



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



The temperature and humidity of the atmosphere may cause 

 considerable difference in the per cents given above. A low tem- 

 perature will increase the loss of heat by radiation and decrease 

 that by evaporation. A high temperature will decrease the loss 

 of heat by radiation and increase that by evaporation owing to 

 the greater production of sweat. From the above figures it is 

 evident that the skin is the important factor in getting rid of body 

 heat. This is due: (1) to the large surface offered for radiation, 

 conduction, and evaporation ; and (2) to the large amount of blood 

 which it contains. 



Distribution of heat. — The blood, as we know, permeates all 

 the tissues in a system of tubes or blood-vessels. Wherever 

 oxidation takes place and heat is generated, the temperature of 

 the blood circulating in these tissues is raised. Wherever, on 

 the other hand, the blood-vessels are exposed to evaporation, as 

 in the moist membranes in the lungs, or the more or less moist 

 skin, the temperature of the blood is lowered. The gain and 

 loss of heat balance one another with great nicety, and the blood, 

 circulating rapidly, now through warmer, and again through cooler, 

 tubes, is kept at a uniform temperature of about 100° F. (37.8 C). 

 2a 



