CONDITIONS INFLUENCING HEAT DISSIPATION 189 



Radiation is affected by the conductivity of the surrounding 

 medium. For instance, in media of water and air of the 

 same temperature the radiation is greater in water, because 

 it is a better conductor of heat. 



Evaporation from the skin is of very great importance in 

 increasing heat dissipation. 582 calories of heat are con- 

 sumed when one gram of water is vaporized ; and when this 

 evaporation takes place on the skin the heat is abstracted 

 largely from the body. This is said to represent nearly 15 

 per cent, of the total heat dissipation. Hence the value of 

 perspiring in hot weather. Evaporation also takes place 

 from the moist surfaces of the lungs and, moreover, when 

 as is usually the case, the inspired air is cooler than the lung 

 structure, a certain amount of heat is consumed in warming 

 it. 



But it is not to be inferred that loss of heat takes place 

 from the body just as from an inanimate object. On the 

 other hand, it is intimately connected with and influenced by 

 circulation, respiration, secretion and other functions. When 

 there is a tendency for the body temperature to rise, the cir- 

 culation becomes more active and sends more blood to the 

 periphery to be cooled; respiration is augmented, causing a 

 greater abstraction in the lungs ; the secretion of sweat, for 

 instance, is increased. 



There may be distinct centers governing the loss of heat. 



Conditions Influencing Heat Dissipation. These have 

 been suggested in a previous section. Heat dissipation is 

 greater in proportion to weight in small than in large ani- 

 mals because the radiating surface is relatively larger. It is 

 less in the female than in the male because she has, as a rule, 

 a larger proportion of subcutaneous fat, which is a poor con- 

 ductor of heat. It is less when the body is covered with 

 clothing which is a poor conductor of heat than when the 

 covering conducts heat readily. It is increased when the 

 internal temperature is raised and when the external temper- 

 ature is lowered. Any general increase in vascular or 



