652 METABOLISM 



animal with an Eck fistula, a more pronounced decrease in urea and 

 increase in ammonia occur during the short period of time that the 

 animal survives the operation. 



The results observed after the removal or diminution of liver function 

 fail to occur when other viscera are removed from the animal, which 

 would at least tend to indicate that the liver is very important in the 

 manufacture of urea out of ammonia. This does not, however, warrant 

 the conclusion that the liver is the only place in the animal body in which 

 such a process occurs. 



In corroboration of these observations on mammals, it may be of in- 

 terest to note that when the liver is removed from birds, which is a com- 

 paratively simple operation on account of a natural anastomosis between 

 the portal and renal veins, there is a marked decrease in the excretion 

 of uric acid and a corresponding increase in the excretion of ammonia 

 during the twelve hours or so that the birds survive. In birds and 

 reptiles urea is excreted as uric acid, being produced by a synthetic 

 process in the liver (see page 677). The changes in this experiment are 

 of considerable magnitude; thus, before the operation the amount of 

 ammonia nitrogen relative to total nitrogen has been found to vary be- 

 tween 10 and 18 per cent; after the operation it may be increased to 

 between 45 and 60 per cent. The uric-acid nitrogen normally varies be- 

 tween 60 and 70 per cent of the total nitrogen ; after the operation it may 

 fall to between 3 and 6 per cent. 



In animals with an Eck fistula and with the hepatic artery ligated, 

 an increase in the urea output occurs when amino acids are injected under 

 the skin. This result corroborates the conclusion that the liver can not 

 alone be responsible for the conversion of ammonia into urea. 



(2) PERFUSION OF ORGANS. This method consists in removing the or- 

 gan into a warm chamber or bath and perfusing it, through cannulac 

 inserted in its main artery and vein, with a solution of defibrinated blood 

 or of defibrinated blood mixed with saline solution. The perfusion 

 liquid is kept at body temperature and is saturated with oxygen. By 

 means of a pump it is made to circulate in a pulsatile flow, and the total 

 amount of urea or other metabolite in the circulating fluid is determined 

 before and after the fluid has been circulated several times through the 

 organ. When the liver is perfused, urea gradually accumulates in the 

 fluid, particularly after the addition of one of its known precursors 

 for example, ammonium carbonate. When other organs or viscera are 

 perfused, no urea is formed. The evidence shows that the liver is an 

 important seat of urea formation, but not necessarily that other organs 

 are unable to form it in the intact animal, for there are many sources 

 of inaccuracy in perfusion experiments. Even though w r e exercise the 



