858 PHYSIOLOGY 



In all these views it is assumed that before urea can make its appear- 

 ance in the urine there must be a complete destruction of the amino- 

 acid. If we compare the structure of any amino-acid with that of urea 

 we see that the proportion of carbon to nitrogen is very much greater 

 in the former than in the latter, and that even after splitting off from 

 two molecules of amino-acid the necessary elements to form urea, 

 CON 2 H 4 , almost the whole of the molecule will be left in an 

 unoxidised condition. A complete oxidation of the amino-acid would 

 result in the production of ammonia, carbon dioxide, and water, so 

 that if oxidation were the method adopted for the production of urea 

 the immediate precursor of this substance would be a combination 

 of ammonia and carbonic acid, either ammonium carbonate as 

 suggested by Schroder, or carbamate as thought by Dreschel. We 

 have distinct evidence that ammonia in one of these two forms is 

 an important precursor of urea. If ammonium carbonate or carba- 

 mate be administered to man or to an animal the whole of it is turned 

 out in the urine as urea. Although there is normally a small amount 

 of ammonia in the urine, it is not increased by injections of ammonium 

 carbonate. Schroder has shown that the liver, even after removal 

 from the body, has the power of transforming ammonium carbonate 

 into urea. Defibrinated blood mixed with ammonium carbonate was 

 passed through a surviving liver. After a little time it was found 

 that the ammonium carbonate had disappeared and that its place 

 was taken by urea. If the liver is necessary for this conversion to 

 take place and ammonia is a constant precursor of urea, we should 

 expect to find that the abolition of the hepatic functions would cause 

 the appearance in the urine of ammonium carbonate or carbamate 

 in the place of urea. The cutting out of the liver is not, however, 

 an easy matter in mammals. Ligature of the portal vein, which 

 would be a necessary step in the extirpation of the liver, causes the 

 blood to be dammed up behind the ligature in the portal area. The 

 intestinal wall gets full of effused blood, the blood pressure falls 

 steadily, and the animal dies within a few hours, being bled to death, 

 so to speak, into its portal vessels. A way of obviating this difficulty 

 was suggested by a Russian surgeon, Eck, and was successfully carried 

 out by Pawlow. Before ligature of the portal vein, this vessel was 

 joined to the vena cava and an artificial opening made connecting the 

 lumen of the two vessels, so that, after the ligature, the blood could 

 flow directly into the general circulation without passing through the 

 liver. Some animals operated on in this way showed no abnormal 

 symptoms whatsoever. There was a rapid formation of a collateral 

 circulation so that the blood could get round the ligature to the liver. 

 Under all circumstances a path to the liver was still open by the 

 hepatic artery, but to arrive here the blood from the alimentary 



