CAUSES OF INCREASED METABOLISM. 29 



and the expenditure of energy for these purposes has been somewhat 

 loosely and perhaps not altogether fortunately designated as the 

 " work of digestion." A consideration of some of the processes con- 

 nected with the consumption of feed which lead to the liberation 

 of energy may serve to clarify the conception. 



MECHANICAL WORK. 



Digestion requires more or less mechanical work in the prehension and 

 mastication of the feed and in moving it through the digestive organs. In this 

 connection, too, it should be remembered that the feed in this sense includes 

 the water as well, three or four parts of water being usually consumed by 

 herbivora for each part of dry matter in the feed. As noted on p. — , Zuntz 

 and Hagemann have compared the metabolism of the horse while eating with 

 that of the same animal while at rest and computed from the difference the 

 amount of energy expended in mastication. The following recapitulation of 

 some of their results shows the number of calories of energy expended in the 

 mastication of 1 kilogram of the material named : 



Calories. 



Hay 167.5 



Green alfalfa 30.4 



Oats 47. 



Maize 13. 8 



Kellner 1 has investigated the effect of the grinding of straw upon its value 

 in a productive ration. He finds that, the practical elimination in this way of 

 the work of mastication reduces the expenditure of energy by approximately 

 0.66 calorie for each gram of crude fiber present in the straw. 



That the movement of the masticated feed through the digestive tract must 

 also require an expenditure of energy is obvious, but no data are available 

 as to its amount. 



SECRETION. 



The secretion of the digestive fluids likewise requires some expenditure of 

 energy. This has been shown by direct experiment to be true of the salivary 

 glands and the pancreas and is also true, doubtless, of the other digestive 

 glands. Apparently, however, the amounts of energy thus expended are com- 

 paratively small. 



FERMENTATION. 



The extensive fermentations occurring in the digestive tract of herbivora 

 result in a considerable evolution of heat. The most important of these is the 

 methane fermentation. Assuming on the basis of Tappeiner's results 2 that 100 

 grams of carbohydrates yield 4.7 grams of methane and 33.5 grams of carbon 

 dioxid, and assuming further that two-thirds of the carbon of the organic 

 acids produced is contained in acetic acid and the remainder in butyric, it may 

 be computed that the heat evolved amounts to 12.5 per cent of the total energy 

 of the digested carbohydrates or 0.523 calorie per gram. It should be noted 

 that this estimate does not refer to the potential energy carried off in the 

 methane, but to the heat evolved in the fermentation. The latter is part of 

 the metabolizable energy of the carbohydrates, since it is liberated in the 



1 Die Ernahrung der Landwirtschaftliehe Nutztiere, 5th ed., p. 163. 

 2Zeitschrift fur Biologie, vol. 20, p. 52. 



