596 UEIC ACID. [BOOK n. 



leucin, glycin, and other amides or amido-acids administered 

 by the alimentary canal no longer increase the uric acid of the 

 urine, as they do in the intact animal. In these animals, the 

 synthesis of ammonia compounds into uric acid, which is par- 

 allel to the synthesis into urea occurring in the mammal, seems 

 to take place in the liver, and we may infer is in some way or 

 other effected by the hepatic cells. 



As to the exact way in which ammonia either as such or in 

 form of an amide or amido-acid changes into urea we have no 

 certain knowledge. Ammonium carbonate, we know, is readily 

 formed out of urea by simple hydration, and we may imagine 

 that the living organism can carry out the reverse process and 

 dehydrate ammonium carbonate into urea. There is, however, 

 a certain amount of evidence that not ammonium carbonate but 

 ammonium carbamate is the immediate antecedent of urea ; and 

 indeed, out of the body, by electrolyzing a solution of ammo- 

 nium carbamate with alternating currents, a certain amount of 

 urea may be artificially produced. But this is a matter too 

 obscure to be discussed here. 



388. Uric Acid. This, like urea, is a normal constituent 

 of human urine, and, like urea, has been found in the blood, 

 in the liver and in the spleen ; it is a conspicuous constituent 

 of an extract of the latter organ. In various diseases the quan- 

 tity in the urine is increased ; and at times, as in gout, uric 

 acid accumulates in the blood, and a deposit of urates takes 

 place in the tissues. In some animals, such as birds and most 

 reptiles, uric acid takes the place of urea. Since by oxidation 

 a molecule of uric acid can be split up into two molecules of 

 urea, and a molecule of some carbon acid, uric acid is com- 

 monly spoken of as a less oxidized product of proteid metabo- 

 lism than urea. But there is no evidence whatever to shew 

 that the former is a necessary antecedent of the latter ; on the 

 contrary, all the facts known go to shew that the appearance 

 of uric acid is the result of a metabolism slightly diverging 

 from that leading to urea ; indeed it is probable that the 

 divergence occurs towards the end of the series of changes, for 

 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 rep- 

 tiles. 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 mam- 



