584 METABOLISM, NUTRITION AND DIETETICS 



There is no reason to suppose that the hepatic cells, so far as the 

 repair of their own protoplasm or the supply of energy for their own 

 special work is concerned, require to metabolize particularly large 

 quantities of amino-acids as compared, for instance, with the 

 muscles. Glycin, however, they must have for the manufacture of 

 glycocholic.andcystin for the manufacture of thetaurin of taurocholic 

 acid. In addition, the liver is known to possess the power of utilizing 

 amino-acids for the formation of dextrose and eventually of glyco- 

 gen, and a portion of the surplus amino-acids of the food may be 

 withdrawn from the blood of the portal vein for this purpose, just 

 as the surplus of dextrose is withdrawn. 



The liver contains a relatively large amount of urea, and there is 

 strong evidence that it is the manufactory in which a great part of 

 the nitrogenous relics of broken-down proteins, including amino- 

 acids, reach the final stage of urea. This evidence may be summed 

 up as follows: 



(1) An excised ' surviving ' liver forms urea from ammonium 

 carbonate mixed with the blood passed through its vessels, while 

 no urea is formed when blood containing ammonium carbonate is 

 sent through the kidney or through muscles. Other salts of am- 

 monium, such as the lactate, the formate, and the carbonate, under- 

 go a like transformation in the liver. It is difficult, in the light of 

 this experiment, to resist the conclusion that the increase in the 

 excretion of urea in man, when salts of ammonia are taken by the 

 mouth, is due to a similar action of the hepatic cells. 



(2) If blood from a dog killed during digestion is perfused through 

 an excised liver, some urea is formed, which cannot be simply 

 washed out of the liver-cells, because when the blood of a fasting 

 animal is treated in the same way there is no apparent formation 

 of urea (v. Schroeder). This suggests that during digestion certain 

 substances which the liver is capable of changing into urea enter the 

 blood in such amount that a surplus remains for, a time unaltered. 

 These substances may come directly from the intestine ; or they 

 may be products of general metabolism, which is increased while 

 digestion is going on ; or they may arise both in the intestine and in 

 the tissues. Leucin which, as we have seen, is constantly, or, at 

 least, very frequently, present in the intestine during digestion 

 can certainly be changed into urea in the body. So can other 

 amino-acids of the fatty series, like glycocoll or glycin, and aspartic 

 acid, and it has been shown by perfusion experiments that this 

 change can take place in the liver. Further, the blood of the portal 

 vein during digestion contains several times as much ammonia as 

 the arterial blood, and the excess disappears in the liver. 



(3) During digestion the blood loses a greater proportion of its 

 amino-nitrogen (amino-acids) in passing through the liver than in 

 passing through other organs, as shown by the fact that the excess 



