516 COMPOSITION OF UKINE. [BOOK n. 



superficial ; or on the other hand may take place in the very 

 depths of the tissues and be closely associated with the very 

 life of the tissues. We shall, however, have to return to these 

 matters later on, and may here briefly consider what substances 

 are, normally and abnormally, present in urine, and the chief 

 features of the fluid itself. 



321. Besides water, the constituents of urine are: 



Nitrogenous Crystalline Bodies. Neglecting the small pro- 

 portion of these bodies which, especially in the case of flesh 

 eaters, are introduced into the economy with the food, as 

 kreatin and the like, and so pass into the urine with no or with 

 comparatively little change, we may on the whole regard the 

 substances of this class as the products of the changes which 

 the proteid matters (and allied substances such as gelatin and 

 the like) present in food have undergone either while the food 

 was simply food, still in the alimentary canal for instance, or 

 after the food had been built up into the tissues of the body. 



Of these by far the most important, in the urine of man 

 and mammalia, is the body urea (N 2 H 4 CO). It is the chief 

 form in which, in these animals, nitrogen leaves the body. We 

 shall have to discuss the relations and formation of urea later 

 on, but meanwhile we will simply state that it has remarkable 

 double connections with two great groups. On the one 

 hand it is related to the ammonia group, and by hydration is 

 readily converted into ammonium carbonate (N 2 H 4 CO + 2H 2 O = 

 (NH 4 ) 2 CO 3 ). On the other hand it is related to the great 

 cyanogen group, ammonium cyanate and urea being isomeric, 

 and the former by simple heating being converted into the 

 latter (NH 4 .CNO = N 2 H 4 CO). 



Though a base, forming salts with acids, such as nitrates, 

 oxalates, &c. urea occurs in urine in a free and independent 

 condition. 



Closely allied to urea, occurring apparently as a bye product 

 of the same line of metabolism, is uric acid (C 5 H 4 N 4 O 3 ), which 

 is found always in 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 compound 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 (glycocoll). 



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 potas- 

 sium and sodium, and to a less extent with calcium and am- 



