204 NUTRITION OF FARM ANIMALS 



291. Computation of protein. The conception of the sche- 

 matic body (280) upon which balance experiments are based 

 regards the total nitrogenous matter of the animal as consisting 

 substantially of protein. All the vast number of other substances 

 containing this element which have been identified as constituents 

 of the body are insignificant in amount as compared with the 

 great mass of protein which it contains. Accordingly, a gain 

 or loss of nitrogen is interpreted as signifying a gain or loss of 

 protein and the amount of the latter may be computed from 

 the former just as the protein of a feeding stuff is computed from 

 its protein nitrogen, it being only necessary to fix upon a suitable 

 factor or factors, i. e. } to know the average percentage of nitrogen 

 in body protein. 



From the results of analyses of entire bodies of animals cited in 

 Chapter II, the average nitrogen content of the fat- and ash-free dry 

 matter was computed (99) to be : 



In Lawes and Gilbert's experiments . . . . 16.11% 

 In Chaniewski's experiments 16.06% 



It is probable that in both cases the supposedly fat-free matter 

 still contained some fat, it having been subsequently shown that 

 extraction with ether does not remove the last trace's of it from ani- 

 mal tissues. 



Kohler's analyses (88) of the fat- and ash-free lean meat of vari- 

 ous species, after correction for the glycogen content of the horse flesh, 

 show an average nitrogen content of 16.64 per cent. Since the material 

 of Lawes and Gilbert's and of Chaniewski's experiments doubtless in- 

 cluded some residual fat and other non-nitrogenous substance, and 

 since the larger share of the protein of the body is contained in the 

 muscular tissues, it appears justifiable to regard Kohler's figures as 

 representing with substantial accuracy the average elementary com- 

 position of body protein as a whole, especially since they are the results 

 of direct analysis while the others are derived from slaughter experi- 

 ments in which the limits of error are somewhat wide. 



Assuming, on the basis of Kohler's results, that average body 

 protein contains 16.64 P er cent f nitrogen, the corresponding 

 protein factor is 6.0, and the gain or loss of nitrogen observed in 

 a nitrogen balance experiment multiplied by this factor gives the 

 gain or loss of protein. This is, of course, an approximation, 

 since protein is not the only nitrogenous substance contained 



