CONDITIONS INFLUENCING HEAT DISSIPATION 189 



urine and feces (disregarding the small amount which goes to 

 warm ingested articles). 



Heat is radiated from the body just as from a hot stove. 

 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 in- 

 creasing heat dissipation. 582 calories of heat are consumed 

 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, res- 

 piration, secretion and other functions. When there is a tend- 

 ency for the body temperature to rise, the circulation 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 animals 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 conductor of heat. It is less 

 when the body is covered with clothing which is a poor con- 

 ductor of heat than when the covering conducts heat readily. 



