METABOLISM OF PROTEINS 585 



in the portal blood, as compared with the blood of the hepatic vein, 

 is greater than the difference between the arterial and the vena 

 cava blood. Now it has been proved that the liver does not store 

 amino-acids to an appreciable extent as such, and therefore it must 

 either have destroyed or condensed them into proteins. There is no 

 evidence that the liver forms a store of reserve protein from amino- 

 acids as it does of glycogen from carbohydrates, although this is 

 possible. But there is good reason to believe that a portion at least 

 of the amino-acids taken up by the liver is quickly broken down, 

 and- that urea is formed from them. For when amino-acids were 

 injected into a vein in such amount that the content of amino-acids 

 in all the tissues was considerably increased, they disappeared far 

 more rapidly from the liver than from muscle or kidney, and their 

 disappearance from the liver was accompanied by an increase in the 

 urea content of the blood (Van Slyke) . 



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

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

 and almost exclusively, in the liver. 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. 385) 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 acid 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. 



(5) 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. 385), 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 

 same extent, evidently because the substances from which urea is 

 formed still, for the most part, gain access to the liver through the 

 hepatic artery and by means of the back-flow which is known to 

 take place through the hepatic vein. Yet while in normal dogs the 

 proportion of ammonia to urea in the urine is only 1 :22 to i : 73, 



