28 MAINTENANCE RATIONS OF FARM ANIMALS. 



Using the data afforded by the experiment on fat, the availability 

 of the energy of the protein may be computed as follows : 



Availability of energy of protein — Rubner. 





Metabolizable 

 energy of feed 

 per kilogram 

 live weight. 



Loss by 

 body per 

 kilogram 

 live weight. 



Meat fed 



Calories. 



61.65 

 



61.65 

 4.95 

 56.70 

 Per cent. 



67.53 



Calories. 

 8.90 

 51.50 



42. 60 

 4.31 

 38.29 



Fasting 



Difference 



Difference due to fat 



Difference due to protein 



Percentage available 







The difference between the percentage available and 100 shows, of 

 course, the proportion of the metabolizable energy of the feed which 

 was expended in increasing the total metabolism as measured by the 

 heat production. This increase of the metabolism of the body is 

 called by Rubner the " specific dynamic effect " of the several nutri- 

 ents. Rubner's final average results are contained in the following 

 table. It should be clearly understood that they are not applicable 

 to the " digestible nutrients " of the feed of herbivora. 



Average availability — Rubner. 





Availa- 

 bility. 



Specific 

 dynamic 

 effect. 



Body protein 



Per cent. 

 68.1 



72! 



87.3 

 94.2 



Per cent. 

 31.9 

 30.9 

 28.0 

 12.7 

 5.8 



Meat protein 



Gelatin 



Fat 



Cane sugar 





CAUSES OF INCREASED METABOLISM. 



The foregoing paragraphs have dealt with the fact of the in- 

 creased metabolism and consequent heat production resulting from 

 the ingestion of feed without considering the cause of the increase. 

 Two explanations of it naturally suggest themselves. The first is 

 that the greater supply of the various nutrients directly stimulates 

 the metabolism of the body cells, while the second ascribes the in- 

 creased metabolism to the additional expenditure of energy required 

 for the digestion of the feed and its preparation for metabolism in 

 the actual vital processes. The latter explanation is the one which 

 has been generally accepted, although by no means without dissent, 1 



1 Compare Heilner, Zeitschrift fiir Biologie, vol. 48, p. 144 ; vol. 50, p. 488. 



