440 URIC ACID. [BOOK n. 



they serve as a warning not to neglect the possible synthetic 

 operations of the animal body, we may sum up our imperfect 

 knowledge concerning the history of urea as follows. We have 

 evidence, not exactly complete but fairly satisfactory, that a part 

 at least of the urea is simply withdrawn from the blood by the 

 renal epithelium. The activity of the protoplasm of the secreting 

 cells must therefore, as far as this part of the urea is concerned, 

 be confined to absorbing the urea from the renal blood, and 

 to passing it on into the cavities of the renal tubules. The 

 mechanism by which this is effected we cannot at present 

 fathom, but it seems more comparable to a selection of food than 

 to anything else ; the cells appear to treat urea much in the same 

 way as they treat sodium sulphindigotate. The antecedents of the 

 urea in the blood are, we may at present suppose, partly the 

 kreatin formed in muscle and elsewhere, partly the leucin and 

 other like bodies formed in the alimentary canal as well as in 

 various tissues. The transformation of these bodies into urea may 

 take place in the liver and possibly in the spleen, but we have no 

 exact proof of this, nor can we say exactly in what way the 

 transformation is effected. There is no proof of any body existing 

 in the blood capable of effecting this transformation ; and we may 

 probably rest assured that in this, as in other metabolic events, 

 the activity exercised in the change comes from some tissue, and 

 cannot be manifested by simple blood plasma. 



Lastly, it is possible that the kidney may, besides the simpler 

 duty of withdrawing ready formed urea from the blood, be exercised 

 in transforming various nitrogenous crystalline bodies to serve as 

 part of the supply of urea which passes from it. 



Uric Acid. This, like urea, is a normal constituent of urine, 

 and, like urea, has been found in the blood, and in the liver 

 and spleen ; we have already, p. 434, referred to its relations with 

 this 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 is deposited in the tissues. By oxidation a 

 molecule of uric acid can be split up into two molecules of urea, 

 and a molecule of mesoxalic acid. It may therefore be spoken of 

 as a less oxidised product of proteid metabolism 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. And 

 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 urea again replaces uric acid be explained by reference to that 

 animal having so large a cutaneous in addition to its pulmonary 



