758 URIC ACID. [BOOK n. 



and its proportion 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 nitrogeneous diet, unless some 

 disorder supervenes ; indeed it is asserted that both absolutely and 

 relatively 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 antecedent 

 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 synthetically 

 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 con- 

 jectures. The constant presence of uric acid in the spleen how- 

 ever, 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 formation 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 therefore 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. 



491. 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 illustrating the 

 complex and varied nature of proteid metabolism to which we 

 referred above. Urea is the chief end-product of proteid meta- 

 bolism, 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 machin- 

 ery ; or whether they, though small in quantity, serve some special 

 ends in the economy. Perhaps sometimes or with some of them 

 it is the one case, at other times or with others it is the other 

 case. 



When proteid material undergoes outside the body, either by 

 the action of trypsin or as the result of decomposition or under 



