Chap, hi.] ELIMINATION OF WASTE PKODUCTS. 517 



monium. In quite normal urine these salts are soluble in the 

 urine, even after the fluid has cooled down to the ordinary 

 temperature of the 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 " de- 

 posit 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 (§ 59) as a constituent 

 of muscles. Kreatin by dehydration is readily converted into 

 kreatinin, and kreatinin by hydration 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, allan- 

 toin and ammonium oxalurate are occasionally found in urine, 

 but cannot be regarded as constituents of normal urine. 



In the urine of man hiphuric acid appears to be always pres- 

 ent 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 suc- 

 ceeding 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 mem- 

 ber of the aromatic group, tyrosin, is occasionally present in urine. 



A special interest belongs to certain compounds, which may 

 be regarded as, in small quantities, normal constituents of urine, 

 but which may occur in much larger quantities; these are cer- 

 tain phenol compounds such as phenyl-sulphuric acid, certain 

 indigo compounds, the so-called indican, and others. These 

 arise from bodies appearing in the alimentary canal as prod- 

 ucts of the decomposition of proteids, effected not by natural 

 juices, but by micro-organisms, §§ 210, 232. Their amount in the 

 urine may be taken as a measure of the extent to which proteids 

 are being changed by these agents in the alimentary canal. 



§ 322. Inorganic Salts. These for the most part exist in 

 urine in natural solution, the composition of the ash almost ex- 

 actly corresponding 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 



