202 



THE HUMAN MECHANISM 



is abundantly secreted ; the cutaneous arterioles are also 

 widely dilated. Let us now suppose the day becomes cooler 

 and the temperature falls to 80 F. Heat production remains 

 unchanged; but more heat is now transferred to the cooler 

 surrounding objects, and less is lost by evaporation because 

 less perspiration is secreted. As the external temperature 

 falls further, still more heat is transferred to colder objects 



100 90 80 7fi 72 70. 68 64 60 50 40 30 20 10 -10* 



Heat loss 



checked 



solely by 



vasomotor 



Problem is to get 

 rid of the heat 



means 

 Heat productior 



Problem is to produce heat 



enough to compensate for 



the rapid loss 



* = Heat lost by transfer (conduction, convection, radiation) 



................ ss: Heat lost by evaporation of perspiration 



FIG. 87. Production and output of heat at different temperatures 



and correspondingly less is lost by evaporation of the per- 

 spiration until, somewhere about 68 to 70 F., exactly the 

 same amount of heat is lost by conduction, convection, and 

 radiation as is produced. At this point the secretion of the 

 perspiration ceases. 



Thus far the difficulty in maintaining a constant tempera- 

 ture has been that of getting rid of heat under atmospheric 

 conditions which are unfavorable for the ready conduction, 

 convection, and radiation of heat from the skin. Blood is 



