CH. XXXVII.] FORMATION OF UREA. 555 



is thus, in part, synthetic. There have been various* theories 

 advanced as to what these simpler compounds are. Some have 

 considered that cyanate, others that carbamate, and others still 

 that carbonate of ammonium is formed. Schroder's work, which 

 has been confirmed by subsequent investigators, proves that 

 ammonium carbonate is one of the urea precursors, if not the 

 principal one. The equation which represents the reaction is 

 as follows: 



(NH 4 ) 2 C0 3 - 2 H 2 = CON 2 H 4 



[Ammonium [Water] [Urea], 



carbonate] 



Schroder's principal experiment was this : a mixture of blood and 

 ammonium carbonate was injected into the liver by the portal 

 vein ; the blood leaving the liver by the hepatic vein was found 

 to contain urea in great abundance. This does not occur when 

 the same experiment is performed with any other organ of the body, 

 so that Schroder's experiments also prove the great importance 

 of the liver in urea formation. 



There is, however, no necessity to suppose that the formation 

 of amido-acids is a necessary preliminary to urea formation. The 

 conversion of the leucine and arginine formed in the intestine 

 into ammonium salts and then into urea does certainly occur, but 

 this only accounts for quite an insignificant fraction of the urea 

 in the urine. If this also occurs in tissue metabolism, we ought 

 to find considerable quantities of leucine, glycocine, creatine, 

 arginine, and such substances in the blood, leaving the various 

 tissues and entering the liver ; but we do not. We do, however, 

 constantly find ammonia which, after passing into the blood or 

 lymph, has united with carbonic acid to form either carbonate 

 or carbamate of ammonium. It is quite probable that the nitro- 

 genous waste that leaves the muscles and other tissues is split off 

 from them as ammonia, and not in the shape of large molecules 

 of amido-acid, which are subsequently converted into ammonia. 

 The experiments outside the body which most closely imitate 

 those occurring within the body are those of Drechsel, in which 

 he passed strong alternating currents through solutions of 

 proteid-like materials. Such alternating currents are certainly 

 absent in the body, but their effect, which is a rapidly changing 

 series of small oxidations and reductions, are analogous to meta- 

 bolic processes ; under such circumstances the carbon atoms are 

 burnt off as carbon dioxide, and the nitrogen is split off in the 

 form of ammonia ; by the union of these two substances ammonium 

 carbonate is formed. 



The following structural formula; exhibit the relationship 



