INTESTINAL DIGESTION 209 



is believed to work upon the liver as well as the pancreas. 

 Bile scarcely deserves to rank as a digestive juice. It 

 has little virtue by itself. But it is certain that it 

 creates a favorable environment for the action of the 

 pancreatic enzymes. When it is prevented from enter- 

 ing the intestine, as in ordinary jaundice, the capacity 

 to digest and particularly to absorb food is reduced. 

 This is especially true of fats. 



While the presence of bile favors digestion, the secre- 

 tion contains some constituents which are regarded as 

 wholly useless wastes. This is the character of the 

 pigments which give bile its pronounced color. They are 

 chemically related to the red coloring matter of the 

 blood and are undoubtedly derived from it. Two chief 

 pigments are recognized, one which is red and one which 

 is green. The green pigment is predominant in the 

 bile of most animals; human bile varies from green to 

 orange. On the whole it may be said that bile occupies 

 a place intermediate between that of a true digestive 

 juice like the gastric, which is secreted on occasion, and 

 that of the urine which is a vehicle for waste and formed 

 continuously. 



The Colon. The value to man of this part of the 

 tract is dubious. We know that the average rate of 

 progress for its contents is slower than that in the small 

 intestine. During a period which is often more than 

 twelve hours matter received from above is retained 

 in this region. No fresh digestive juice of any conse- 

 quence is added to it and the changes taking place are 

 mainly induced by bacteria. Some of the products may 

 be available for nutrition if absorbed but others are 

 certainly injurious. A limit is set to the bacterial proc- 

 esses by the drying of the contents which is a marked 

 feature of the interval passed in the large intestine. 

 Whatever questions may be raised with reference to 

 its other uses, it serves as a retriever of water. 



In animals which eat coarse, woody food the large 

 intestine is capacious and probably plays a considerable 



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