650 COMPOSITION OF URINE. [BOOK 11. 



air; but not infrequently the urates, soluble in the urine at the 

 temperature at which it leaves the body, are precipitated when 

 the fluid cools, forming the well known " deposit of urates." On 

 further standing the salts are apt to be decomposed and thus to 

 give rise to crystals of uric acid. 



Besides urea and uric acid the urine contains small but 

 variable quantities of more or less nearly allied bodies such as 

 kreatinin, xanthin, hypoxanthin, and guanin. Concerning these 

 we will at present only say that kreatinin is a hydrated form of 

 the body kreatin which we spoke of ( 62) as a constituent of 

 muscles. Kreatin by hydration is readily converted into krea- 

 tinin, and kreatinin by dehydration into kreatin ; kreatin 

 introduced into the alimentary canal or into the blood appears 

 in the urine as kreatinin ; and in flesh eaters some at least of the 

 kreatinin of the urine is derived directly from the kreatin present 

 in the meat eaten as food ; but we shall discuss the subject of 

 kreatin later on. 



Besides the above, such bodies as leucin, taurin, cystin, 

 allantoin and ammonium oxalurate are occasionally found in 

 urine, but cannot be regarded as constituents of normal urine. 



In the urine of man hippuric acid appears to be always 

 present in small quantities, and in the urine of herbivora occurs 

 in large quantities. In these latter it is derived more or less 

 directly, by changes of which we shall have to speak in a 

 succeeding chapter, from constituents of the food containing 

 bodies belonging to the aromatic group (benzoic acid series) ; but 

 the small quantity present in man and other carnivora appears to 

 come from the metabolism of proteid matter which, as we have 

 already seen, contains an aromatic constituent. Another member 

 of the aromatic group, tyrosin, is occasionally present in urine ; and 

 as more regular constituents of normal urine may be mentioned 

 certain phenol compounds, such as phenylsulphuric acid, the 

 phenol constituents of which are derived from the action of 

 micro-organisms in the alimentary canal, see 282 ; these 

 substances though they no longer contain nitrogen take origin 

 from bodies of the aromatic series. Similar changes are also the 

 source of indigo compounds (indican) in the urine, derived from 

 indol, see 249. 



402. Inorganic Salts. These for the most part exist in urine 

 in natural solution, the composition of the ash almost exactly cor- 

 responding with the results of the direct analysis of the fluid ; in 

 this respect urine contrasts forcibly with blood, the ash of which is 

 largely composed of inorganic substances, which previous to the 

 incineration existed in peculiar combination with proteid and other 

 complex bodies. In the ash of urine there is rather more sulphur 

 than corresponds to the sulphuric acid directly determined ; this 

 indicates the existence in urine of some sulphur-holding complex 

 body. And there are traces of iron, pointing to some similar iron- 



