234 



NUTRITION 



There are two modes of regulating the amount and degree of heat in a 

 homothermous animal changing the production and altering the outgo 

 of the heat. Since the actual temperature at any time throughout the 

 body is the balance of these two phases, regulation consists in alteration 

 of either or of both in the way circumstances at the time require. In 

 practice both of these opposed processes are always in action at the same 

 time. When the temperature tends to become too high, for example, not 

 only is thermogenesis (heat-making) checked in one of various ways, but 

 thermolysis (heat-loss) is increased. When body-heat trends unduly 

 downward, the two processes work in just the opposite ways. In this 



FIG. 128 



AREA, 

 CONTENTS, 



AREA, 54 

 CONTENTS, 27 



This diagram shows that animal bodies of the same volume and mass (heat-production) 

 may greatly differ in surface-area (heat-loss). 



way the balance is kept so perfectly that in health the temperature of 

 the human body varies less than 2 from the adult mean of about 37. 

 One sees the immediate working of thermotaxis in the involuntary 

 shivering which often ensues on exposure to cold, and Lowy has shown 

 that this marked increase of rhythmic muscular contraction may even 

 double the body's heat-producing metabolism. Another immediate proof 

 of the presence of such a function in the body is seen in cold bathing, 

 which, in a normally reacting organism raises the temperature. Again, 

 one's appetite is normally somewhat less in a hot day of summer than 

 on a cold winter day, metabolism and heat-production being thereby 

 lessened. The arrangements for controlling body-heat consists, in 



