236 THE URINE. 



Test for Nitrates. To demonstrate the presence of nitrates, 

 200 c.c. of urine are treated with 30 to 40 c.c. of chemically pure, 

 concentrated sulphuric acid or hydrochloric acid, and distilled upon 

 a sand-bath. The distillate is received into a dilute solution of 

 caustic alkali. Owing to the presence of reducing substances in the 

 urine, the nitric acid is thus transformed into nitrous acid, and 

 passes over as such. The presence of nitrites may then be demon- 

 strated as usual (see Saliva). 



THE ORGANIC CONSTITUENTS OF THE URINE. 



The organic constituents of the urine comprise the normal end- 

 products of the nitrogenous metabolism of the body, various products 

 of albuminous putrefaction which have found their way into the 

 general circulation from the intestinal canal, and certain pigments 

 which are more or less intimately related to the normal blood-pig- 

 ment. In addition, traces of various other substances may be 

 encountered, the origin of which is obscure. Under pathological 

 conditions we meet with certain normal constituents of the blood 

 which generally do not appear in the urine as such, or occur in 

 infinitesimally small amounts, and also with various abnormal 

 products of metabolism, all of which will be considered in detail. 



The Nitrogenous Constituents of the Urine. 

 UREA. 



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 research, however, 

 has shown that in man and the mammalian animals uric acid is 

 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 substances, 

 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 



