METABOLISM OF PROTEINS 575 



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) f Uric acid which in birds is the chief end-product of protein 

 metabolism, as urea is in mammals ds formed in the goose largely, 

 and almost exclusively, in the livey This has been most clearly 

 shown by the experiments of Minkowski, who took advantage of 

 the communication between the portal and renal-portal veins 

 (p. 379) to extirpate the liver in geese. When the portal is ligated 

 the blood from the alimentary canal can still pass by the round- 

 about road of the kidney to the inferior cava, and the animals 

 survive for six to twenty hours. While in the normal goose 50 to 

 60 per cent, of the total nitrogen is eliminated as uric acid in the 

 urine, and only 9 to 18 per cent, as ammonia, in the operated goose 

 uric acid represents only 3 to 6 per cent, of the total nitrogen, and 

 ammonia 50 to 60 per cent. A quantity of lactic acjd equivalent 

 to the ammonia appears in the urine of the operated animal, none 

 at all in the urine of the normal bird. The small amount of urea in 

 the normal urine of the goose is not affected by extirpation of the 

 liver. And while urea, when injected .into the blood, is in the 

 normal goose excreted as uric acid, it is in the animal that has lost 

 its liver eliminated in the urine unchanged. 



(4) After removal of the liver in frogs, or in dogs which have 

 survived the previous connection of the portal vein with the inferior 

 vena cava by an Eck's fistula (p. 379), the quantity of urea excreted 

 is markedly diminished, and the ammonium salts in the urine are 

 increased. When the Eck's fistula is established and the portal 

 vein tied, without any further interference with the hepatic circula- 

 tion, the amount of urea in the urine is not lessened to nearly the 



