CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 791 



especially in the glandular tissues, but also appears with striking 

 readiness in almost all decompositions of proteids, and is moreover 

 a product of decomposition of gelatiniferous substances. Without 

 going however so far as to conclude that leucin is the chief 

 antecedent of urea, we may take the above observation as indicating 

 that the normal liver has, in some way or other, the power of 

 converting leucin into urea. If this be so then we may- also 

 conclude, and indeed have, as we shall see, direct evidence, that 

 when such bodies as leucin, glycin, &c., introduced into the alimen- 

 tary canal appear in the urine as urea, the transformation has 

 taken place in the liver. The body tyrosin which so often 

 accompanies leucin stands on a different footing from leucin and 

 the like, since it is of a different nature, belonging as it does to the 

 aromatic series. 



489. The transformation however of leucin into urea raises 

 a new point of view. Leucin, as we know, is amido-caproic acid ; 

 and, with our present chemical knowledge, we can conceive of no 

 other way in which leucin can be converted into urea than by the 

 complete reduction of the former to the ammonia condition (the 

 caproic acid residue being either elaborated into a fat or oxidized 

 into carbonic acid) and by a reconstruction of the latter out of the 

 ammonia so formed. We have a somewhat parallel case in glycin, 

 which is amido-acetic acid ; here too a reconstruction of urea out 

 of an ammonia phase must take place. Moreover when ammonium 

 chloride is given to a dog a very large portion reappears as urea, 

 i.e. there is an increase in the urea of the urine corresponding to a 

 large portion of the nitrogen contained in the ammonium chloride. 

 And in the case of other animals also, indeed of man himself, there 

 is evidence that somewhere in the body ammonia may be con- 

 verted into urea. Hence in all these cases where ammonia or 

 ammonia compounds are changed into urea the last step at all 

 events is one of synthesis ; and this suggests the possibility that 

 in the ordinary proteid metabolism also, the downward katabolic 

 series of changes may finish off with a synthetic effort, the last 

 stage of the former being the appearance of an ammonia compound 

 which is subsequently reconstructed into urea. 



This synthesis, like the transformation of leucin and other 

 bodies, probably takes place in the liver ; and in support of this 

 view we have experimental evidence. When, after death, an 

 artificial circulation is kept up through the liver of an animal 

 (dog), the addition of ammonia salts to the blood so circulated gives 

 rise to a very considerable increase of the urea in that blood ; but 

 no such increase is observed when the blood is circulated through 

 muscles or through the kidney instead of through the liver, or when 

 the blood of a starving animal without the addition of ammonia 

 salts is circulated through the liver of an animal which had been 

 starved. Further, in a dog when the liver is by ligatures separated 

 from the general blood stream, ammonia salts injected into the 



