790 ORIGIN OF UREA. [BOOK n. 



though it still contains an excess of carbon may be regarded as an 

 antecedent of urea ; for when introduced into the alimentary canal 

 in not too great a quantity it, though absorbed, does not reappear 

 as such in the urine, but leads to a proportionate increase of urea ; 

 it appears to be transformed into urea. And the same is the case 

 with analogous bodies such as glycin, asparagin and the like. We 

 seem therefore justified in supposing that the sudden increase of 

 the urea in the urine which follows proteid food is at least largely 

 due to the splitting up somewhere in the body (begun even within 

 the alimentary canal) of the proteid into a non-nitrogenous and a 

 nitrogenous moiety, the latter being some antecedent or other of 

 urea (it may be leucin, it may be some other body), speedily con- 

 verted into urea itself. 



488. We have seen reason to think that the proteids of a 

 meal are absorbed not by the lacteals but by the portal blood 

 vessels, 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 may be 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 have however a convergence of evidence that the 

 last stage of the process, namely the conversion into urea of some 

 or other 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. 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 however 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 previously 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, 



