CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 757 



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 parallel 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 electrolysing a solution of ammonium 

 carbamate with alternating currents, a certain amount of urea may 

 be artificially produced. But this is a matter too obscure to be 

 discussed here. 



490. 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 some animals, such as birds and 

 most reptiles, it takes the place of urea. In various diseases the 

 quantity 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. 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 commonly spoken of as a less oxidised product 

 of proteid metabolism than urea. But there is no evidence / 

 Avhatever 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 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 

 fin id, 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 



