UREA AND ITS FORMATION. 



343 



tained in it was distinctly increased by passing it through 

 the liver,, thus indicating that the blood of an animal after 

 digestion contains something that the liver can convert to 

 urea." 



Considered from our standpoint this experiment has an- 

 other meaning. During digestion, especially after copious feed- 

 ing, as stated above, the entire organism is, to a certain degree, 

 involved in the digestive process, as shown by the general sense 

 of heat often experienced under these conditions. As liver, 

 intestines, pancreas, and spleen, even after gastric digestion 

 has passed, are all operating together, the suprarenal activity 

 is doubtless enhanced. In other words, at such times the blood 

 contains either in its corpuscles or in its serum a more or less 

 marked increase of oxidizing substance. Conversely, the fast- 

 ing dog's blood especially if the fasting has been prolonged 

 is really abnormal blood, in which the oxidizing substance 

 is unusually low, since suprarenal activity is depressed with 

 that of the rest of the tissues. We have also seen that, under 

 these conditions, the tissues nevertheless continue to absorb 

 their normal supply of oxygen, the blood being thus actively 

 depleted while insufficiently oxygenated. It seems clear, there- 

 fore, that the blood of the well-fed dog contained more oxidiz- 

 ing substance than that of the fasting one. 



That the injected blood taken from the well-fed animal 

 should have been the source of the urea-forming substance is 

 unlikely. Since the liver alone receives alimentary waste- 

 products, it is only with blood from the portal vein that such 

 substances could have been obtained. This is not specified. 

 The urea-forming agent must, therefore, have been in the 

 excised liver's portal channels, and the only available agency 

 capable of inducing the reactions involved appears to be the 

 oxidizing substance. Experimental evidence may be adduced 

 to show that such is the case. We consider the blood of the 

 hepatic artery, as previously stated, as the source of supply of 

 the oxidizing substance, since it is directly derived from the 

 cceliac axis. Stewart states that, "although the portal vein 

 carries a much greater supply of blood than the hepatic artery, 

 ligation of the latter causes a greater diminution in the ratio 

 of the amount of urea to the total nitrogen in the urine than 



