FEED CONSUMPTION A SOURCE OF HEAT. 



71 



CRITICAL TEMPERATURE. 



It is clear from the foregoing results that when the external tem- 

 perature falls below a certain limit the heat production of the animal 

 shows a marked increase. This point at which the physical regula- 

 tion gives way to or begins to be supplemented by the chemical regu- 

 lation has been called the " critical temperature " for the animal. 

 Above this temperature the radiating capacity of the body surface 

 is varied to meet the varying conditions; below it this method of 

 regulation is largely exhausted, and therefore the heat production 

 is varied to suit the needs. This latter so-called chemical regulation 

 is probably effected largely in the muscles, either by visible motion 

 or by 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 required for the various processes of physical regulation. 



Any conditions tending to facilitate the escape of heat from the 

 body would obviously act like a fall of temperature. Wind, for ex- 

 ample, by removing the layer of partially warmed air next to the 

 skin, tends to remove the 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 trying than a dry 

 heat. Cold, moist air, on the other hand, facilitates the escape of heat 

 from the body by increasing the conducting power of the clothing, 

 hair, or fur, so that a damp cold is more severe than a dry cold. The 

 direct rays of the sun may impart a considerable amount of heat to 

 the body, thus moderating the effect of low temperature and, on the 

 other hand, increasing that of high temperature. 



FEED CONSUMPTION A SOURCE OF HEAT. 



For the sake of simplicity, the foregoing paragraphs have dealt 

 especially with the case of the fasting animal, neglecting one im- 

 portant source of heat, viz, the consumption of feed. As was show T n 

 on pages 19-28, the latter results in increasing the katabolism of the 

 body, and whether this be considered the result of the work of digestion 

 or simply designated as specific dynamic effect, the fact is established 

 beyond question. This heat, however, once generated, while unavail- 

 able for the physiological processes of the body is just as useful as ex- 

 ternal heat for keeping it warm. In other words, the consumption of 

 feed will tend to have the same effect as a rise of external tempera- 



