EXCRETION AND THE EXCRETORY ORGANS 463 



also of those exogenous excreta that are absorbed from the ali- 

 mentary tract into the blood. From the rectum are discharged 

 all exogenous excreta that fail of absorption, and likewise a num- 

 ber of endogenous excretory substances received into the intestine 

 from the liver, by way of the bile duct. The chapter on Respira- 

 tion contains the discussion of the excretory function of the lungs. 

 It is not necessary, therefore, to consider it here. 



The Liver as an Excretory Organ. To the functions previously 

 described of aiding the digestive and absorptive processes, and of 

 serving as a temporary storehouse for carbohydrates, the liver 

 adds a very important excretory function. This is in part direct, 

 the separation from the blood of waste materials contained in it, 

 and in part the working over of harmful excretory substances into 

 harmless ones which it does not excrete but returns to the blood 

 to be discharged through the urinary system and skin. This latter 

 function will be considered before the direct excretions of the liver 

 are discussed. It will be recalled that in the process of protein 

 absorption the "fuel-protein" is supposed to be split into a nitrog- 

 enous waste portion, and a non-nitrogenous oxidizable portion. 

 The nitrogenous part probably consists largely of ammonia com- 

 pounds, chief of which is ammonium carbonate (NH 4 ) 3 CO 3 . It 

 is well known that ammonia compounds are very poisonous to 

 animals into whose circulating blood they are introduced, and it 

 has been proven that an animal would be seriously affected if the 

 ammonia content of the general circulation should ever reach that 

 which is normal to the portal vein during protein absorption. It 

 is through the action of the liver that the Body is protected from 

 the harmful effects of this portal ammonia. During the passage 

 of the portal blood through the liver its ammonia is converted by 

 dehydration into urea, a compound harmless to the Body if not 

 present in the blood in too great concentration. The conversion of 

 ammonium carbonate by dehydration to urea is made clear if we 

 compare the chemical formulae of the two substances: 



NH 4 0-H 2 NH 2 



0< -NH 4 0-H 2 J< -NH 2 



(ammonium (urea) 

 carbonate) 



The urea formed thus from the ammonia compounds of the portal 

 blood belongs to the group of exogenous excreta, since it does not 



