440 A MANUAL OF PHYSIOLOGY 



carbonate mixed with the blood passed through its vessels, 

 Xvhile no urea is formed when blood containing ammonium 

 carbonate i-s sent through the kidney or through muscles. 

 Other salts of ammonium, such as the lactate and the car- 

 bamate, undergo a like transformation in the liver. It is 

 difficult, in the light of this experiment, to resist the con- 

 clusion 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 

 Xvhile 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 amido-acids of the fatty 

 series, like glycocoll or glycin, and asparaginic acid, and it 

 has been shown by perfusion experiments that this change 

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

 proteid 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 Min- 

 kowski, who took advantage of the communication between 

 the portal and renal-portal veins (p. 328) to extirpate the 

 liver in geese. When the portal is ligatured the blood from 

 the alimentary canal can still pass by the roundabout road 

 of the kidney to the inferior cava, and the animals survive 



