CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 049 



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 1 briefly consider what substances are, normally and abnor- 

 mally, present in urine, and the chief features of the fluid itself. 



401. Besides water, the constituents of urine are : 



nitrogenous Crystalline Bodies. Neglecting the small propor- 

 tion of these bodies which, especially in the case of flesh eaters, 

 are introduced into the economy with the food, as kreatiri 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 mean- 

 while we will simply state that it has remarkable double con- 

 nections 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 = (NHJ 2 C0 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 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 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 



