Chap, iv.] METABOLIC PROCESSES OF THE BODY. 597 



mal any satisfactory physiological or clinical evidence that an 

 increase of uric acid is the result of deficient oxidation. The 

 absolute amount of uric acid discharged by man and its pro- 

 portion to the urea passed at the same time varies a good deal. 

 There is no positive evidence that the quantity excreted is 

 necessarily increased by nitrogenous diet, unless some disorder 

 supervenes ; indeed it is asserted that both absolutely and rela- 

 tively to the urea the quantity excreted is greater upon a mixed 

 diet than upon a highly proteid one. Alkalis in the food seem 

 undoubtedly to diminish it, and alcohol, at least in excess, to 

 increase it. 



So far from considering uric acid as a less oxidized antece- 

 dent of urea we ought perhaps rather to regard its appearance 

 as a result of a synthesis in which urea or some allied body 

 takes part. As we have said uric acid may be formed syn- 

 thetically by heating together urea and glycin ; and it has 

 more recently been similarly prepared from various allied 

 bodies. As to where or how such a synthesis is effected in the 

 living body, we know little or nothing for certain, and can only 

 make conjectures. The constant presence of uric acid in the 

 spleen however, and the frequently noted connection between 

 a rise and fall of uric acid in the urine and variations in the 

 volume and therefore presumably in the activity of the spleen, 

 suggest that the change may be brought about in this organ ; 

 but it must be remembered that in birds and reptiles the for- 

 mation of uric acid seems to be effected in the same organs as 

 that of urea and in an analogous manner ; and the arguments 

 which we have used concerning the formation of urea in the 

 liver of mammals may be applied to the formation of uric acid 

 in the livers of birds and reptiles. It is more probable there- 

 fore that in the mammal the turn to uric acid rather than urea 

 is given in the liver, the spleen however possibly playing its 

 part also in the matter. 



§ 389. Of the meaning of the appearance in the tissues of 

 such bodies as xanthin, hypoxanthin, guanin and the like, and 

 of the exact nature of the metabolism which gives rise to them 

 or which they themselves undergo, we know little or nothing. 

 The presence of these several bodies may be taken as illustrat- 

 ing the complex and varied nature of proteid metabolism to 

 which we referred above. Urea is the chief end-product of 

 proteid metabolism, but that end is probably reached in several 

 ways ; so that probably a very large number of nitrogenous 

 chemical substances make a momentary appearance in the body. 

 Some of these fail to become urea, and either without or after 

 further change make their appearance in the urine. But we do 

 not know whether their appearance is accidental, the result of 

 imperfect chemical machinery ; or whether they, though small 

 in quantity, serve some special ends in the economy. Perhaps 



