CHAP, iv.] METABOLIC PKOCESSES OF THE BODY. 595 



in a normal state of things converted into urea by the liver. 

 And in this connection it may be remarked that not only is leu- 

 cin found in nearly all the tissues after death, 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 con- 

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

 ure to suppose that when such bodies as leucin, glycin, &c., 

 introduced into the alimentary canal appear in the urine as 

 urea the transformation has taken place in the liver. The body 

 tyrosin which so often accompanies leucin, belonging as it does 

 to the aromatic series, stands on a different footing from leucin 

 and the like. 



387. 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 con- 

 ceive of no other way in which leucin can be converted into 

 urea than by the complete reduction of the former to the am- 

 monia condition (the caproic acid residue being either elabo- 

 rated 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 am- 

 monia 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 corre- 

 sponding 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 converted 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 a certain amount of experimental evidence. 

 Birds may be kept alive after total extirpation of the liver for 

 a longer time than can mammals; and when in geese the liver 

 is removed the uric acid (representing in these animals the urea 

 of the mammal) is largely decreased, while the ammonia of the 

 urine is largely increased. After the removal of the liver also, 



