CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 755 



actually take place in the living body ; it may take 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 may be 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. 



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 may go so far perhaps as to suspect that it is largely 

 or wholly confined to the liver, but we have no convincing demon- 

 stration. 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 ; in the liver of birds the urea is represented by 

 urates. 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 



482 



