UREA AND ITS FORMATION. 345 



forming the carbamate of ammonium. This being dehydrated, 

 urea is formed, as shown in the following equation: 



That the conversion of ammonia compounds into urea does 

 occur in the liver is sustained by experimental physiology. 

 Ho well refers to the experiments of Schroder, in which this 

 is demonstrated as follows: "As further proof of the urea- 

 forming power of the liver, Schroder found that if ammonium 

 carbonate was added to the blood circulating through the liver 

 to that from the fasting as well as from the well-nourished 

 animal a very decided increase in the urea always followed. 

 It follows, from the last experiment, that the liver-cells are 

 able to convert carbonate of ammonium into urea. The re- 

 actions may be expressed by the equation: 



(NHJ 2 C0 3 2H 2 = CON 2 H 4 ." 



The foregoing facts, considered collectively, indicate that 

 the formation of urea in the liver is probably accomplished in 

 the following manner, taking DrechseFs series of reactions as 

 standard of the numerous ones of the same class that must 

 occur in this organ: 



Granting that the nitrogenous bodies are absorbed by the 

 venules of the intestinal villi and transmitted by the mesen- 

 teric veins to the portal vein, the ramifications of which would 

 then carry them to the hepatic lobules, the first reaction would 

 occur in the preldbular portal vessels: i.e., the nitrogenous bodies 

 would undergo hydrolysis, with the formation of amides, leucin, 

 aspartic acid, tyrosin, etc. The second reaction would follow 

 as soon as these bodies reached the pericellular capillaries, 

 owing to the presence therein of the oxidizing substance sup- 

 plied by the terminal branches of the hepatic artery; in other 

 words, further reduction of these bodies by oxidation to ammonia, 

 carbonic acid, and water would occur in the pericellular capillaries 

 of the lobule. The third reaction would seem, like the first, to 

 require comparatively inert surroundings: i.e., a fluid not 

 charged with oxidizing substance as is the blood of the peri- 

 cellular capillaries. Such a medium we have, in all likelihood, 



