406 THE ELIMINATION OF WASTE PRODUCTS. 



the urine of man, occurring in small but variable quantity. In the urine of 

 some animals, such as birds and reptiles, it occurs in abundance, and indeed 

 in these replaces urea as the chief nitrogenous excretion. Uric acid is a 

 more complex body than urea, one molecule of uric acid splitting up, under 

 the influence of certain reagents, into two molecules of urea and a com- 

 pound of oxalic acid. Its decomposition products, however, under different 

 reagents are very numerous and complex, though urea occurs among them 

 frequently and characteristically. Uric acid may be synthetically produced 

 out of urea and glycin (glycocol). 



It is a weak dibasic acid, and occurs in normal human urine, not as a 

 free acid but as an acid salt, being combined with potassium and sodium, 

 and to a less extent with calcium and ammonium. 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 "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 quanti- 

 ties of more or less nearly allied bodies, such as kreatinin, xanthin, hypo- 

 xanthin, 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 

 kreatinin, and kreatinin by dehydration into kreatin ; kreatin introduced 

 into the alimentary canal or into the blood appears in the urine as kreat- 

 inin ; 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, ty rosin, 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 243) ; 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 219). 



338. Inorganic salts. These for the most part exist in urine in natural 

 solution, the composition of the ash almost exactly 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 sub- 

 stances, 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-holding sub- 



