CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 793 



urea given by the mouth to birds appears in the urine as uric acid, 

 and, conversely, uric acid given to mammals appears in the urine 

 as urea. We have no evidence to prove that the cause of the 

 divergence lies in an insufficient supply of oxygen to the organism 

 at large ; on the contrary, uric acid occurs in the rapidly breathing 

 birds as well as in the more torpid reptiles. Nor can the fact 

 that in the frog again urea replaces uric acid be explained by 

 reference to that animal having so large a cutaneous in addition to 

 its pulmonary respiration. The final causes of the divergence are 

 to be sought rather in the fact that urea is the form adapted to a 

 fluid, and uric acid to a more solid excrement. Nor is there in 

 man or the mammal 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 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 nitrogenous 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 un- 

 doubtedly 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 



F. II. 51 



