558 THE URINE [CH. XXXVII. 



in the urine. If creatine is an intermediate step, it must undergo 

 some further change before it leaves the muscle. Other ammo-acids, 

 such as glycine (ammo-acetic acid), leucine (amino-caproic acid), and 

 arginine are probably to be included in the same category ; there is, 

 however, no evidence that tyrosine acts in this way. The facts upon 

 which such a theory depends are (1) that the introduction of glycine 

 or leucine into the bowel, or into the circulation, leads to an increase 

 of urea in the urine ; and (2) that amino-acids appear in the urine of 

 patients suffering from acute yellow atrophy of the liver. Then, 

 again, it is perfectly true that, in the laboratory, urea can be obtained 

 from creatine, and also from uric acid, but such experiments do 

 not prove that creatine or uric acid are normally intermediate pro- 

 ducts of urea formation in the body. Still, if we admit, for the sake 

 of argument, that amino-acids are normally intermediate stages in 

 proteid metabolism, and glance at their formulae 



Glycine C 2 H 5 NO 2 , 



Leucine C 6 H 13 NO 2 , 



Creatine C 4 H 9 N 3 O 2 , 



we see that the carbon atoms are more numerous than the 

 nitrogen atoms. In urea, CON 2 H 4 , the reverse is the case. The 

 amino-acids must therefore be split into simpler compounds, which 

 unite with one another to form urea. Urea formation 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 CO 3 - 2H 2 O - 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 experi- 

 ment 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 

 amino-acids is a necessary preliminary to urea formation. The con- 

 version 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 



