254 PRACTICAL PHYSIOLOGY 



for the reasons given above, probably also endogenous creatin are 

 transformed by the tissues into urea. If we compare their formulae 

 with that of urea, however, we find that they contain too many 

 carbon atoms 



Leucin, C 6 H 13 N0 2 Creatin, C 4 H 9 N 3 2 



Arginine, C G H 14 N 4 2 Urea, CH 4 N 2 



in relation to their nitrogen, so that before such a transformation can 

 take place they must break off some of their carbon as other bodies 

 fat or carbohydrate the nitrogenous moiety being then liberated 

 probably as a salt of ammonia. 



Other probable precursors of urea are the alloxuric bodies (see p. 206). 



Regarding the Site of Formation of Urea, there can be no doubt that 

 it is the liver. The products of tissue metabolism are conveyed here, 

 and changed into urea which is then excreted by the kidneys. The 

 proofs of this are the following : 



(1) If the liver be excised and perfused with defibrinated blood, urea 

 is formed if ammonium salts, or amido acids be added to the blood. To 

 do this experiment an animal is bled, and its blood defibrinated. The 

 liver is then excised, and cannulae inserted into the portal and hepatic 

 veins. The former of these is connected with a pressure bottle contain- 

 ing the defibrinated blood to which some precursor of urea is added, 

 and the latter with a dish to collect the perfused blood. The urea is 

 estimated in a sample of blood from the pressure bottle i.e. before per- 

 fusion and another one from the collecting dish i.e. after perfusion. 

 It is found that the latter contains much more urea than the former. 



If this experiment be repeated with any other organ, or with the 

 muscles, no urea formation results. 



(2) If the liver be removed from the circulation the urinary excretion 

 of urea is diminished, this being accompanied by a corresponding 

 increase in the ammonia. In mammals, the liver extirpation is soon 

 followed by the death of the animal, but it can be removed to a large 

 extent from the circulation by attaching the portal vein to the inferior 

 vena cava, and thus diverting the blood stream so that no blood gets to 

 the liver by this path Eck's fistula. After such an operation it has 

 usually been found that the diminution of urea is very slight, the liver 

 being still supplied by blood from the hepatic arteries, and by blood 

 eddying back through the hepatic veins. In certain cases, however, 

 the diminution has been quite marked. 



In birds and reptiles, uric acid takes the place of urea as the end pro- 

 duct of proteid metabolism. In these animals, extirpation of the liver 

 is practicable on account of a natural anastomosis between the portal 

 vein and the vena advehens going to the kidneys. If, now, the portal 



