CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 681 



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 dehydrated form 

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

 of muscles. Kreatin by dehydration is readily converted into 

 kreatinin, and kreatinin by hydration into kreatin ; kreatin intro- 

 duced 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 can- 

 not 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 aro- 

 matic group, tyrosin, is occasionally present in urine. Of special 

 interest are certain bodies which in some cases are in small 

 quantities almost constant constituents of urine and which from 

 time to time are found in larger quantities ; these are such bodies 

 as certain phenol compounds, phenyl-sulphuric acid for instance, 

 certain indigo compounds, the so-called indican, and others. They 

 take their origin from the decomposition of proteids carried on in 

 the alimentary canal not by the digestive juices but by micro- 

 organisms ( 249, 282); the products so formed are absorbed, 

 undergo . further changes and appear in the urine as the above 

 bodies. The amount of these bodies appearing in the urine may 

 be taken as a measure of the extent to which proteids are being 

 changed by micro-organisms within the alimentary canal. 



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 



p. ii. 44 



