METABOLISM 159 



addition to gaseous products, to the formation of various or- 

 ganic acids. In particular, the constituents of the cell walls 

 of plants appear to owe their apparent digestibility chiefly to 

 this action of the organized ferments of the alimentary canal. 

 While, therefore, the organic acids are chemically distinct from 

 the carbohydrates, and while some of these acids are contained 

 as such in the feed, the amounts produced from carbohydrates 

 are so considerable that this would appear an appropriate 

 point at which to consider their metabolism. 



Unfortunately, however, little is known of the metabolism of 

 the simpler organic acids, beyond the fact that such of them 

 as have been subjected to experiment are katabolized to carbon 

 dioxid and water, not more than traces of them at most ap- 

 pearing in the excreta. A portion of the carbon dioxid pro- 

 duced unites with alkalies and appears in the urine as carbonates. 



223. Analogy with carbohydrates. It is interesting to re- 

 call in this connection that the carbohydrates themselves 

 undergo cleavage, producing lactic or even acetic and formic 

 acids, before their final oxidation (219). If it be true that these 

 latter comparatively simple substances are those whose oxida- 

 tion yields most of the energy supplied by the carbohydrates, 

 there would seem to be no reason why the same acids resorbed 

 directly from the digestive tract should not follow the same 

 general course of metabolism and have substantially the same 

 nutritive value. If this view be correct, there is after all a con- 

 siderable similarity between the metabolism of the carbohy- 

 drates and that of their fermentation products. 



The non-nitrogenous matter of the urine 



224. Products of incomplete katabolism. It has been im- 

 plied in the foregoing pages that the digested carbohydrates of 

 the feed, whatever the intermediate stages through which they 

 may pass, are ultimately oxidized to carbon dioxid and water. 

 Of the ordinary hexose carbohydrates this is doubtless true, but 

 with some of the large variety of substances ordinarily grouped 

 together in the conventional scheme of feeding stuffs analysis 

 as " carbohydrates and related bodies," or as " crude fiber " 

 and " nitrogen-free extract," the case appears to be otherwise. 



It has been shown that the urine, in addition to the nitrogenous 



