FATE OF THE FOOD ABSORBED 383 



Elimination of Heat. Heat is got rid of by three channels. 

 A. Skin. Since the body is generally warmer than the 

 surrounding air, heat is constantly lost by conduction, con- 

 vection, and radiation, and the extent of this loss depends 

 mainly upon the difference between the temperature of the 

 body and that of the air. Kadiation plays the most im- 

 portant part when a person is sitting quiet in still air ; 

 conduction and convection when the exchange of air over 

 the surface is rapid. The temperature of the skin is increased 

 when, from dilatation of the cutaneous vessels, more blood is 

 brought to the surface and conversely it is lowered by con- 

 striction of these -vessels. The influence of variations in the 

 temperature of the air is generally minimised in man by the 

 covering of clothes, and in animals by the covering of fur or 

 feathers, which retains a stationary layer of air at about 25 

 to 30 C. over the skin. It has been calculated that in man 

 over 70 per cent, of all the heat is lost by conduction and 

 radiation. 



The loss of heat by radiation may be determined by finding 

 to what extent a thin metal grill fixed at a definite distance 

 from the surface of the body is heated. This can be 

 done by determining the change produced in its electric 

 conductivity. 



By the evaporation of sweat, heat is rendered latent, and is 

 taken from the body, which is thus cooled just as the hand 

 may be cooled by allowing ether' to evaporate upon it. If 

 the amount of sweat vaporised is known, it is possible to 

 calculate the amount of heat removed from the body in this 

 way. The loss is comparatively small in man only about 14 

 per cent, of the whole. The extent depends upon the rapidity 

 with which evaporation goes on, and this is governed by the 

 amount of sweat secreted, and by the dryness and temperature 

 of the atmosphere. Thus a warm dry climate is better borne 

 than one which is warm and moist, since in the former the 

 loss of heat by evaporation is so much greater. Of the various 

 factors increasing sweat secretion, heat is probably the most 

 important. 



In the lower animals the loss of heat by evaporation is much 

 increased if the skin is wet, and the temperature of the horse 

 may fall distinctly if the coat remains moist. 



