ENERGY IN FEEDINGSTUFFS AND ITS USES 99 



glycogen and fat to furnish energy for the deficiency. After 

 the body fat is used up, the animal uses the protein tissues 

 of the body as a source of energy for the running of the vital 

 organs, and then soon dies of starvation. 



Work of Digestion and Absorption. — The consumption 

 of feed by an animal is always accompanied and followed 

 by a large increase in the amount of energy expended. This 

 increase was formerly thought to be due entirely to the energy 

 expended in the work of mastication, swallowing, rumination, 

 fermentation, peristalsis, secretion of the digestive juices, 

 and the distribution of the absorbed nutrients. Hence, it 

 was spoken of as the energy expended for the work of diges- 

 tion. Later experiments with dogs, cats, and men, however, 

 appear to have shown that the energy expended in the 

 mechanical work of digestion is but a small part of the in- 

 creased expenditure of energy due to the consumption of 

 food. In case of the carnivora and man, the greater part 

 of the increased expenditure of energy seems to be due to an 

 effect which the absorbed nutrients, particularly the amino 

 acids, have upon the metabolism of the cells, stimulating 

 them to higher oxidation and consequent liberation of energy 

 in the form of heat. Thus the term, ^' work of digestion," as 

 applied to the increased expenditure of energy after food 

 consumption, is not strictly true, but in lieu of a better term 

 we shall continue to use it. In herbivora, and especially in 

 ruminants, the expenditure of energy due to actual work of 

 digestion is much greater than in carnivora, due to the large 

 amount of energy expended in the work of mastication and 

 rumination, and to the loss of energy as heat of fermentation. 

 However, whether the increased expenditure of energy after 

 food consumption is due to actual work of digestion or to 



