264 NUTRITION OF FARM ANIMALS 



of the room are entirely closed, nothing more can be effected 

 in this manner toward maintaining its temperature, and if the 

 weather continues to grow colder, the fire in the room must be 

 increased. Similarly, if the external cooling effect upon the 

 animal becomes so great as to exceed the limits of physical ad- 

 justment, more fuel material is katabolized, that is, more heat 

 is produced. This method of maintaining the body temperature 

 is commonly called chemical regulation. 



353. Mechanism of chemical regulation. The chemical 

 regulation is probably effected largely through muscular action, 

 by visible motion or by an increase in the muscular tonus, 

 either of which involves an increased heat production. This 

 has been clearly shown to be true of man and probably applies 

 also to other animals. Above the critical temperature, there 

 appears to be a slight increase in the heat production with 

 rising temperature, probably due to the additional energy ex- 

 pended in the various processes of physical regulation. 



354. Critical temperature. The temperature at which the 

 physical regulation gives way to or begins to be supplemented 

 by the chemical regulation has been called the critical tempera- 

 ture. 1 Above this temperature the radiating capacity of the 

 body ' surf ace is varied to meet the varying conditions; below 

 it, this method of regulation is largely exhausted and therefore 

 the heat production is varied to meet the need. The critical 

 temperature for man wearing ordinary clothing appears to be 

 about 15 C. ; for the dog it is about 20 C., for the guinea pig 

 3o-35, and for the hog, according to the results of Tangl 2 

 and of Von der Heide and Klein, 3 about 2o-23 C. 



355 . O ther thermal conditions . Any conditions tending to facili- 

 tate the escape of heat from the body will obviously act like a fall of 

 temperature. Wind, for example, by removing the layer of partially 

 warmed air next to the skin, tends to remove heat more rapidly from 

 the body, so that the cold is felt more severely on a windy day, while, 

 on the other hand, the effect of a high temperature is modified by 

 wind. A high percentage humidity of the air on a warm day hinders 

 the removal of heat by evaporation, so that a moist heat is more try- 

 ing than a dry heat. Cold moist air, on the other hand, facilitates 



1 The term refers, of course, to the temperature of the surroundings and not to 

 that of the animal itself. 



2 Biochem. Ztschr., 44 (1912), 252. 3 Ibid., 55 (1913), 195- 



