138 BIOLOGY. [Book ii. 



It is in the anatomical elements themselves that the urates 

 are formed, and thence they afterwards pass into the hlood. 

 They are created particularly in the fibrous and muscular tissues. 

 The formation of uric acid seems to respond to an incomplete, 

 imperfect oxydation of the tissues, at least in man, and yet we 

 find an enormous quantity of this acid in the urine of birds, 

 whose respiration is extremely active, — a paradoxical fact, of 

 which there are many in biology, proving that much remains to 

 be done by physiologists. 



We must put in the same category with uric acid a substance 

 yet more important, urea (C^Az^H^O^), proceeding, like the uric 

 acid, from an oxydation of the albuminoidal substances, but a 

 deeper and more complete oxydation. It is necessary to guard 

 against considering urea as an alimentary residuum ; this sub- 

 stance is incessantly and regularly formed, and is produced by 

 the dis-assimilation of the anatomical elements themselves. It is 

 produced even during abstinence, since Lassaigne found it in the 

 urine of a man executed after eighteen days of absolute absti- 

 nence ; and it exists also in the urine of the newly-born. As 

 it is constantly forming, it is also constantly met with in the 

 blood, which appropriates it afresh from the tissues. 



Urea and urates are not the only products of the molecular 

 dis assimilation of the anatomical elements. There is a whole 

 series of analogous compounds ; for, each kind of anatomical 

 element, having its own composition, assimilates and dis-assimi- 

 lates particular substances. The principal azotized quaternary 

 substances proceeding from proteic dis-assimilation are creatine, 

 creatinine, leucine, inosic acid, and inosates. All these sub- 

 stances are crystallizable. They are more specially produced by 

 such or such a tissue. Thus creatine seems to be almost 

 exclusively the product of the dis-assiinilation of the muscular 

 substance. Leucine is found rather in the tissue of the glands, 

 with or without an excretive canal, in the lymphatic gangUons, 

 and also in the grey substance of the brain.' 



' Ch. Eobin, loo. cif., pp. 686—688. 



