DIGESTION AND RESORPTION 1 19 



digested and resorbed and this fact constitutes a serious dif- 

 ficulty in the study of the ash metabolism. 



165. Digestibility of ether extract. Among the excretory 

 products contained in the feces are included ether-soluble 

 substances, especially those derived from unresorbed bile con- 

 stituents. While their total amount is small, the feed of farm 

 animals is also usually poor in ether extract and consequently 

 the error in the computation of the percentage digestibility 

 may be relatively large. Indeed, not a few instances are on 

 record in which the ether extract of the feces has exceeded 

 that of the feed. Little definite knowledge is available, however, 

 as to the actual extent of the error thus introduced, but it is of 

 relatively less importance in view of the small role which fat 

 plays in the ordinary rations of farm animals. 



166. Digestibility of nitrogenous substances. Most of the 

 excretory products in the feces (154) are nitrogenous substances 

 and it is particularly with reference to their influence upon the 

 determination of the digestibility of the nitrogenous constituents 

 of feeding stuffs that investigation has been active. That they 

 may seriously affect it is evident from the results obtained in 

 numerous experiments upon feeding stuffs poor in protein, such 

 as straw, in which a negative digestibility of the crude protein 

 has been observed, i.e., in which the feces have contained 

 more nitrogen than the feed. Moreover, experiments upon 

 rations containing no nitrogen at all have shown that under 

 these conditions nitrogen continues to be excreted in the feces. 



Various methods for distinguishing between the nitrogen of 

 feed residues and the nitrogen of excretory products have been 

 proposed at different times, but the one which has proved most 

 satisfactory and which is generally accepted at present is based 

 upon the solubility of the nitrogenous excretory products in 

 the solution of pepsin and hydrochloric acid employed in Stut- 

 zer's method for the laboratory determination of the digesti- 

 bility of protein described in a previous paragraph (162). 



By treatment of a sample of the fresh feces with such a solu- 

 tion under proper conditions the excretory nitrogenous products 

 are dissolved, and it has been shown that very close agreement 

 can be obtained between the artificial and natural digestion of 

 protein if the comparison in the latter case be made upon the 

 pepsin-insoluble nitrogen of the feces. In other words, the 



