222 THE URINE. 



The Nitrogenous Constituents of the Urine. 

 UKEA. 



While in birds and reptiles the greater portion of the urinary 

 nitrogen is excreted in the form of uric acid, urea constitutes the 

 most important end-product of the nitrogenous metabolism of the 

 remaining groups of vertebrate animals. In man, 86 per cent, of the 

 total nitrogen eliminated in the urine appears in this form. 



Origin. Formerly it was supposed that urea resulted from uric 

 acid through a process of oxidation, and that this was its only 

 source. We have seen that the formation of urea from uric acid is 

 possible, and we cannot deny that a certain proportion of the sub- 

 stance may be derived in this manner. Modern researches, how- 

 ever, have shown that in man and the mammalian animals uric acid 

 is largely derived from a destruction of the nucleins within the 

 body, and results from oxidation of the xanthin bases, which are 

 thus set free. In birds and reptiles, on the other hand, the greater 

 portion of the uric acid is formed synthetically from simpler sub- 

 stances, and is hence not directly comparable to the form which 

 is found in the higher animals. In these a synthetic formation is 

 also possible, but probably does not occur under normal conditions. 

 As we can therefore recognize one origin of uric acid only in the 

 mammal, and as this source of the nitrogen is insignificant when 

 compared with the large amount of urea actually found, we are 

 forced to the conclusion that the greater portion of the urea must 

 originate in a different way. 



It has been repeatedly shown that during the decomposition of 

 the albumins by means of acids and alkalies, as also during the 

 process of tryptic digestion and albuminous putrefaction, a large 

 amount of mono-amido-acids results. It has hence been supposed 

 that these bodies probably represent intermediary products in the 

 transformation of the albuminous nitrogen into urea, and it has 

 actually been demonstrated that in mammals and to these I shall 

 confine my remarks for the present the administration of such 

 acids in the food is followed by a corresponding increase in the 

 amount of urea. Under certain pathological conditions, moreover, 

 these acids appear in the urine as such, and it is then noted that the 

 elimination of urea is much diminished. In health, however, this 

 does not occur, and on examination of the different tissues and 

 organs of the body such acids are found only in traces. We must 

 hence assume that these acids, supposing them to occur as primary 

 products of albuminous decomposition within the body, are trans- 

 formed at once into other substances, which in turn give rise to urea. 

 As all these bodies on oxidation yield ammonium carbonate, this 

 substance would hence suggest itself as a probable antecedent of 

 urea. We find, as a matter of fact, that ammonium carbonate 

 when ingested by the mouth, or otherwise introduced into the body, 



