EXCRETION AND THE EXCRETORY ORGANS 517 



those exogenous excreta that are absorbed from the alimentary 

 tract into the blood. From the rectum are discharged all exog- 

 enous excreta that fail of absorption, and likewise a number of 

 endogenous excretory substances received into the intestine from 

 the liver, by way of the bile duct. The chapter on Respiration 

 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 by the process of deaminiza- 

 tion the " fuel-protein " is split into a nitrogenous waste portion, 

 and a non-nitrogenous oxidizable portion. The nitrogenous part 

 takes the form largely of ammonia compounds, chief of which is 

 ammonium carbonate (NH 4 ) 2 CO 3 . These ammonia compounds 

 are discharged into the blood. In connection with the putrefactive 

 processes that go on in the large intestine there is a considerable 

 production of ammonia which is also absorbed into the blood. 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 all 

 the ammonia produced in the Body were allowed to remain in 

 the circulation in that form. It is through the action of the liver 

 that the Body is protected from the harmful effects of ammonia. 

 During the passage of the 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: 



C0< 



NH 4 0-H 2 NH 2 



NH 4 O-H 2 O~ NH 2 



(ammonium (urea) 



carbonate) 





