METABOLISM, NUTRITION AND DIETETICS 501 



half as much urea as that of the renal artery. So that the whole 

 of the urea in the urine may be simply separated by the kidney 

 from the ready-made urea of the blood. 



Another line of evidence leads to the same conclusion : that 

 the kidney is, at all events, not an important seat of urea- 

 formation. When both renal arteries are tied, or both kidneys 

 extirpated, in a dog, urea accumulates in the blood and tissues ; 

 and, upon the whole, as much urea is formed during the first 

 twenty-four hours of the short period of life which remains to 

 the animal as would under normal circumstances have been 

 excreted in the urine. 



Where, then, is urea chiefly formed? If the main source of 

 urea were the decomposition of tissue -protein, we should naturally 

 look first to the muscles, which contain three-fourths of the 

 proteins of the body ; but we should look there in vain for any 

 great store of urea only a trace is normally present. The 

 liver contains a relatively large amount, and there is very strong 

 evidence that it is the manufactory in which the greater part of 

 the nitrogenous relics of broken-down proteins reach the final 

 stage of urea. This evidence may be summed up as follows : 



(1) An excised ' surviving ' liver forms urea from ammonium 

 carbonate mixed with the blood passed through its vessels, 

 while no urea is formed when blood containing ammonium car- 

 bonate is sent through the kidney or through muscles. Other 

 salts of ammonium, such as the lactate, the formate, and the 

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

 difficult, in the light of this experiment, to resist the conclusion 

 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 while 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 amino-acids of 

 the fatty series, like glycocoll or glycin, and aspartic acid, and 

 it has been shown by perfusion experiments that this change 



