METABOLISM AND FOOD 57 



body. From protein this portion was 71-3%, from 

 fat 87-3%, and from cane sugar 94%, the remain- 

 ders, 187%, 127%, and 6% respectively, appear 

 under all circumstances in the form of heat, and 

 cannot be used for work by the animal or its organs. 

 Thus there are two forms of energy to be considered, 

 the one which takes the form of work and is called 

 " dynamic " energy, and the other which takes the 

 form of heat and is known as " thermic " energy. 



The portion of dynamic energy which comes to 

 herbivorous animals from their food is much less 

 than the amounts given above for carnivorous 

 animals receiving pure nutrients. A considerable 

 proportion of the food of ruminants and horses is 

 coarse and hard, and some part of it must always 

 be used up to furnish energy for the work of mastica- 

 tion and digestion. In addition, some of the food 

 materials undergo so much bacterial decomposition 

 in the stomach and intestines that the products 

 which thereby arise hardly yield anything but heat. 

 Experiments relative to these conditions have 

 shown that when timothy-hay is fed at least 40% 

 of the digestible nutrients reappear in the form of 

 heat only, and in coarsely ground maize the percent- 

 age is still considerable — 22%. 



With pigs it is different, for they get much less 

 coarse food, and with them bacterial activity is 

 weaker. In these animals a much greater pro- 

 portion of the energy of the digested nutrients re- 



