400 PHYSIOLOGY CHAP. 



the hepatic cells (even if slight and fugitive), such as usually 

 arises from gas tro- enteric disorders. 



Of the chromogens contained in the urine, the most important 

 from its physiological significance is undoubtedly indican, which 

 is readily oxidised, and gives rise to indigo-blue and its isomer 

 indigo-red. Jaffe was the first who recognised, by special experi- 

 ments with diet, that indole is the mother-substance of the 

 indican of the urine. Baumann proved that indoxyl is formed 

 by oxidation of indole, and 'that it is, as we have seen, normally 

 present in urine in combination with sulphuric acid and potassium 

 in the form of potassium indoxyl-sulpliate (indican). From this 

 compound the indoxyl resists oxidation : it is liberated by hydrolysis 

 with acid, and on oxidation to indigo the urine may be stained 

 blue or red. 



The practical importance of the indican reaction of the urine 

 rests on the fact that indole, its mother-substance, is the most 

 characteristic product of the protein putrefaction that takes place 

 in the intestine. From the relative indican content of the urine 

 we can therefore make an approximate estimation of the putre- 

 factive processes in the intestine. Quantitative estimation of the 

 whole of the ethereal sulphates, however, gives a more exact 

 criterion of the degree and intensity of intestinal putrefaction, 

 because indole is not the only aromatic body present in the urine. 



VII. Small quantities of non-nitrogenous organic acids are 

 very frequently present in normal urine, among which we must 

 confine ourselves to noting oxalic acid, lactic acid, and various 

 fatty acids. 



Although not constant (Neubauer), the presence of oxalic 

 acid in the form of calcium oxalate in the urine is held to be 

 normal. It crystallises readily a few hours after the urine has 

 been passed, probably in consequence of an acid fermentation. 

 Normally it is held in solution by the acid sodium phosphate. 



According to Flirbringer the oxalic acid content of human 

 urine fluctuates from a hardly perceptible trace to 20 mgrms. per 

 diem ; according to Schultzen, on the contrary, it may amount to 

 10 cgrms. per diem. In the urine of certain animals (horses, pigs) 

 it is found in much higher quantities. 



Part of the oxalic acid of the urine is undoubtedly alimentary 

 in origin. Many fruits and vegetables, as well as herbage, contain 

 oxalic acid, which resists oxidation in the body, and is largely 

 excreted again in the urine (sorrel, spinach, asparagus, grapes, 

 apples, rhubarb, gentian, etc.). Still it is a well-established fact 

 that oxalic acid can be detected in the dog's urine independent of 

 vegetable* food during an exclusive diet of meat and fat (Mills). 

 The katabolic processes of the tissues can thus give rise to a 

 formation of oxalic acid. The specific conditions of this process 

 are not yet known. According to some interesting observations 



