CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 683 



404. Pigments. Urine is always coloured, the tint varying 

 from a light to a dark yellow with an admixture of brown. In 

 the course of twenty-four hours, a not inconsiderable quantity of 

 pigment must leave the body by the urine ; but the nature of the 

 normal pigment or pigments of urine is at present obscure and 

 the subject of much controversy. The matter is further com- 

 plicated by the presence in urine of ' chromogens,' that is to say, 

 bodies which are not coloured themselves but which readily give 

 rise to pigments upon oxidation ; and it is probable that some of 

 these ' chromogens ' of the urine are reduction products of the 

 respective pigments, the reduction taking place in the urine after 

 or during secretion, or occurring even before secretion. A sub- 

 stance called urobilin, a derivative of bilirubin, appears to be 

 normally present in urine either as such or in the phase of 

 its chromogen ; since it possesses no great colouring power it 

 cannot contribute largely to the colour of urine. In some cases 

 of disease, a colouring substance closely allied to the above but 

 giving a different spectrum has been found in the urine ; it 

 has been called pathological urobilin, and it also may occur 

 as a chromogen. Hsemato-porphyrin (iron-free haBmatin) or an 

 allied urohsemato-porphyrin is also a frequent if not a constant 

 constituent of urine. The conspicuously red urine of certain 

 diseases, such as acute rheumatism, contains a colouring matter 

 which has been called uroerythrin. But it may be doubted 

 whether the real colouring matters (or matter) of normal urine 

 have as yet been isolated and their nature determined. 



405. Ferments and other bodies. Even normal urine has 

 frequently been found to contain a small quantity, hardly amount- 

 ing to more than a trace, of proteid material, apparently an 

 albumin ; but the normal presence of even this small quantity 

 has been disputed. Urine, however, certainly contains ferment 

 bodies. 



When urine is treated with many times its volume of alcohol, 

 a granular or flocculent precipitate is thrown down, consisting 

 chiefly of phosphates, together with some other substance or 

 probably several other substances, in very small quantities. An 

 aqueous solution of the precipitate, which may be freed from 

 the phosphates, is both amylolytic and proteolytic. Ferments 

 may also and more readily be extracted from urine by allowing 

 shreds of fibrin to soak in the urine for a few hours, and then 

 removing and washing them. The ferments become entangled in 

 the fibrin in such a way as not to be easily removed by washing. 

 The washed shreds will convert starch into sugar ; and when 

 treated with dilute hydrochloric acid digest themselves so rapidly 

 as to shew the presence of pepsin. By this method it has been 

 ascertained that an amylolytic ferment and pepsin are present 

 in quantities which vary in the twenty-four hours according to 

 the meals. Rennin has also been found, and at times at least, 



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