THE SEAT OF FORMATION OF UREA 679 



possible seats of urea formation, using methods of estimation 

 which, were conclusive as to the presence of urea. He obtained 

 a positive result with one organ only, the liver. 



The Formation of Urea in the Liver. Schroder circulated through 

 the surviving liver of a dog blood to which ammonium carbonate 

 or formate had been added. The urea in the blood was estimated 

 after the blood had been circulated once through the liver, in 

 order to wash out any urea present, and again after circulating 

 for four hours. In most of his experiments he estimated the urea 

 by Bunsen's method, but in one experiment he separated it as 

 crystals of urea nitrate, and demonstrated finally that the liver 

 can form urea from ammonium salts. In this experiment he 

 found that the blood had gained 1 grm. of urea, representing an 

 increase of 220 per cent. Schroder's experiment has been ex- 

 tended to the herbivora, and found to give the same result in 

 the sheep. 



We have already seen that some of the intermediate products 

 of proteid katabolism are converted into ammonia, not only in 

 the liver but also in other tissues. The question arises, does 

 much N reach the liver in the form of ammonia, and, if so, 

 from what tissues does it come ? Analyses of the ammonia 

 content of the blood from different parts of the body by 

 Horodynski, Salaskin, and Zaleski have thrown light on the answer 

 to this question. According to their analyses, arterial blood 

 contains about 0'4 mgr. per cent, of ammonia. The blood from 

 a peripheral vein contains about twice as much as the correspond- 

 ing artery, and muscles contain even more than the venous blood. 

 From this it seems clear that muscles manufacture ammonia and 

 give it off freely to the blood. They also found that the portal 

 blood of a starving animal contained three times as much 

 ammonia as the blood in the hepatic vein or an artery. When 

 the animal was fed on meat the ammonia content of the portal 

 blood was four to five times greater than that of the blood in the 

 hepatic vein. These experiments show that the liver abstracts 

 ammonia from the blood, and that, besides the general tissues, 

 the portal area is an important source of ammonia which 

 might have two origins, the tissue of the alimentary canal and 

 proteid food. We know that the digestive ferments trypsin and 

 erepsin can outside the body form ammonia from proteid, but we 

 do not know to what extent they do so in the body. The rise 



