MAINTENANCE THE ENERGY REQUIREMENTS 277 



production resulting from the consumption of feed by herbivora 

 cannot be doubted. Besides proteins, carbohydrates and fats, 

 however, the feed of herbivora contains a great variety of other 

 substances and the results upon steers obtained by Armsby and 

 Fries l seem to indicate that among these there may be com- 

 pounds acting specifically as stimuli to the cell metabolism or 

 to the minor muscular movements of the animal. Among the 

 feeding stuffs examined, this appeared to be notably true of 

 alfalfa hay and maize meal. 



368. " Work of digestion." The expenditure of energy 

 by the body which results from the ingestion of feed has been 

 somewhat loosely, and perhaps not altogether fortunately, 

 designated as " work of digestion." While there may be ob- 

 jections to the term and while it must not be interpreted too 

 literally, it may nevertheless serve a useful purpose as a col- 

 lective expression for the energy cost to the organism of all the 

 various processes involved in the digestion and assimilation of 

 the feed. Its total amount is equal, of course, to the extra 

 heat produced above that generated by the fasting animal. 

 Rubner's specific, dynamic action, or Kellner's thermal energy, 

 is equivalent to the " work of digestion " in this broad mean- 

 ing. The considerations presented in the previous paragraphs 

 serve to indicate some of the factors of the " work of digestion " 

 and render it evident that it is by no means all work in the 

 mechanical sense. In herbivora this factor is an important 

 one, while with man and carnivora it apparently plays a small 

 part. A similar difference is strikingly shown in the case of the 

 digestive fermentations, which are very extensive in ruminants 

 but play a subordinate role in other animals. 



369. Significance of expenditure of energy in feed con- 

 sumption. Whatever the part played by various factors in 

 the increase of metabolism due to feed ingestion, the existence 

 of that increase and the consequent augmented heat production 

 is a fully established fact which has an important bearing upon 

 the value of the feed as a source of energy. 



Recurring once more to the comparison of the animal body 

 with an internal combustion motor (274), if a gasoline engine 

 has to obtain its supply of fuel by hoisting it from a lower 

 level, it is evident that the energy spent in this way diminishes 



1 Jour. Agr. Research, 3 (1915), 479. 



