THERMAL PHENOMENA OF THE BODY 199 



B. THE REGULATION OF THE BODY TEMPERATURE 



14. How the balance of the heat account may be disturbed. 



Events both within the body and in its immediate surround- 

 ings tend to change the balance of the heat account ; that is, 

 to upset the equilibrium previously existing between heat 

 loss and heat production. The most important of these 

 events are (1) muscular activity and the digestion of food 

 within, and (2) changes of atmospheric or weather condi- 

 tions without. Let us consider how each of these acts. 



Muscular activity, by producing more heat within the body, 

 would tend to increase the heat balance ; and, unless measures 

 were taken at the same time to increase heat output, the tem- 

 perature of the body would rise. Muscular activity may double 

 or even treble the heat produced. The digestion of a meal 

 similarly liberates heat within the body and so tends to raise 

 its temperature, but the heat produced in this case is far less 

 in amount than that produced during muscular activity. 



Changes of atmospheric or weather conditions act by changing 

 the ease with which heat is lost ; and, remembering that heat 

 is lost in two ways, by transfer to colder surroundings and 

 by evaporation of perspiration, we must inquire how various 

 weather conditions influence each of these agents of heat out- 

 put. The three main weather conditions are the temperature, 

 movement, and moisture of the air, and the following tabular 

 form will aid in understanding the relation of each of these 

 conditions to the heat output of the body. 



I. TEMPERATURE OF AIR 



A. INFLUENCE ON HEAT B. INFLUENCE ON EVAPO- 



TRANSFER RATION 



Heat is transferred more rapidly The warmer the air, the more 



to colder surroundings than to water vapor it can take up. This 

 surroundings which are near the facilitates the evaporation of per- 

 temperature of the body. spiration on a warm day, when 



this is most needed to cool the body. 



