Heat Regulation 169 



in kind and quantity would be impracticable in the 

 tropics, and this too, even if there is no apparent in- 

 crease of internal or external work. This would seem 

 to indicate the direct oxidation of food for heating 

 purposes. In any case, animal heat is sustained either 

 directly or indirectly by the burning of the nutrients. 



NOTE. Recent investigations indicate that under given condi- 

 tions there is an air temperature, called the critical temperature, 

 at which metabolism (oxidation) reaches a minimum. If the air 

 temperature falls below this point, thus causing a greater radiation 

 of heat from the body surface, increased oxidation occurs. If the 

 temperature rises above this point there is no diminution of oxida- 

 tion but rather a slight increase, hence the conclusion that there 

 is a minimum oxidation necessary to the maintenance of the vital 

 functions which must go on however much the demands for the 

 radiation of heat may be lessened by a rise of the air temperature. 

 It is evident then that at the higher air temperatures there is an 

 excess of oxidation above that which is required for warming the 

 animal, so that some heat must be thrown off as a waste product. 

 Whichever way the air temperature moves from the critical point 

 there is heat regulation, this being chemical for the lower temper- 

 atures and physical for the higher. 



The critical temperatures for our various farm animals have 

 not been determined, so that we are not yet able to draw therefrom 

 conclusions as to the influence of given temperatures upon pro- 

 duction. 



