SECRETION. 445 



after a meal rich in proteids, which stimulate proteid metabolism, 

 there are two maxima in its excretion. The first takes place at the 

 third or fourth hour and the second at the sixth or seventh hour. 

 The urea comes from proteid metabolism, and from the food. Labor 

 greatly increases the exhalation of carbonic acid, but does not affect 

 to any great extent the excretion of urea. 



FORMATION OF UHEA. The chief source of urea is from the 

 metabolism of the muscles. The ingestion of a large amount of pro- 

 teid food stimulates metabolism. Muscles contain in their mass over 

 70 grams of creatin, while the amount of creatin excreted is only 

 about 1 gram. Urine contains about 30 grams of urea and muscles 

 only a trace. But all experiments to prove an actual relation 

 between creatin and urea have been failures. 



The other alloxuric bodies xanthin, hypoxanthin, and uric acid 

 are also to be regarded. They are members of a group of bodies 

 having as their base of formation the so-called purin-ring which con- 

 sists of two urea radicles linked together by a central chain of car- 

 bon atoms. They are probably split up in part into urea. 



I have already alluded to arginin as a source of urea. All the 

 proteids are probably split up into bodies which form ammonia. 

 Now, when we give by the mouth ammonia salts we find an increase 

 of urea. Further, when ammonia salts are perfused through the 

 liver we find that urea is generated. This leads us to believe that 

 the liver Is the chief manufactory of urea. 



In Eck's fistula, when an artificial communication has been 

 made between the portal vein and the inferior vena cava, the portal 

 vein may be tied and the animal lives. After the Eck fistula the 

 portal blood does not go to the liver, but goes to the vena cava. The 

 hepatic artery is still sufficient to nourish the liver after EckV fistula. 



The diversion of the portal blood to the vena cava markedly 

 diminishes the quantity of urea, whilst the ammonium salts in the 

 urine are increased. This experiment supports the view that urea 

 is formed in the liver from ammonium carbonate. In digestion I 

 have alluded to arginin being converted into urea in the liver by 

 the ferment, arginase. 



Von Nencki has shown that the portal vein contains three to 

 four times more ammonia salts than the hepatic vein or the hepatic 

 artery. The ammonia comes from the breaking up of the proteids 

 by trypsin into peptones, which, in turn, are broken up by erepsin 

 into the amido-acids and ammonia. 



The amido-acids are absorbed as such, and carried by the blood 



