ON UREA AND ON NITROGENOUS METABOLISM. 461 



place to a very slight extent and it may under certain circumstances take 

 place to a considerable extent. But in any case it illustrates the way in 

 which a somewhat similar disruption of proteid material, a disruption which 

 may be broadly described as a splitting up of the proteid into a nitrogenous 

 and a non-nitrogenous moiety, may take place somewhere in the body and so 

 lead to the sudden formation of some antecedent of urea. The antecedent 

 may be leucin or may be some other body or bodies. 



In support of this view maybe urged the fact that such bodies as leucin, 

 glycin, asparagin, and many others when introduced into the alimentary 

 canal are transformed into urea. When these bodies are administered in 

 not too great quantities they do not reappear in the urine, but the urea is 

 proportionately increased. 



402. We have seen reason to think that the proteids of a meal are 

 absorbed not by the lacteals but by the portal bloodvessels, and such bodies 

 as leucin probably take the same course. This being so, all these bodies pass 

 through the liver and are subjected to such influences as maybe exerted by 

 the hepatic cells. Now, we have no positive evidence that the liver does or 

 can exert such an action on proteid material itself as to separate a relatively 

 simple nitrogen compound from the remaining constituents, leaving these to 

 form a body rich in carbon ; we have no positive proof that the increase of 

 proteid metabolism just spoken of as leading to an increase of urea takes 

 place in the liver rather than in the tissues at large ; we may go so far per- 

 haps as to suspect that it is largely or wholly confined to the liver, but we 

 have no convincing demonstration. We have, however, a convergence of 

 evidence that the last stage of the process, namely, the conversion into 

 urea of some product of proteid metabolism, which though allied to is not 

 exactly urea, does occur in the liver. In the first place, a large quantity 

 of urea seems to be present in the liver of mammals ; in this respect the liver 

 presents a strong contrast to the muscles ; in the liver of birds the urea is 

 represented by u rates. Moreover, when a stream of fresh blood is passed 

 several times through the liver of an animal recently killed, the percentage 

 of urea in the blood so used is found to be decidedly increased. This, how- 

 ever, does not prove that urea is formed in the liver, since the increased 

 quantity of urea in the blood which had been circulated might have been 

 simply urea which had been washed out from the liver, where it had pre- 

 viously been staying. Still as far as it goes it is suggestive. In the second 

 place, in certain cases of a form of disease of the liver known as acute yellow 

 atrophy in which the hepatic cells are so changed that their functional 

 activity is largely diminished, the urea of the urine not only undergoes a 

 very marked decrease but appears to be replaced to a very large extent by 

 leucin. This fact suggests that leucin (and not for instance kreatin) is the 

 chief immediate product of the nitrogenous metabolism of the body, and 

 that the leucin thus produced is in a normal state of things converted into 

 urea by the liver. And in this connection it may be remarked that not 

 only is leucin 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 

 gelatiniferoue substances. Without going, however, so far as to conclude 

 that leucin is the chief antecedent of urea, we may take the above observa- 

 tion as indicating that the normal liver has, in some way or other, the power 

 of converting leucin into urea. If this be so we may also venture to suppose 

 that when such bodies as leucin, glycin, etc., 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. 



