CHAPTER LVI 

 THE BIOCHEMICAL PROCESSES OF DIGESTION (Cont'd) 



DIGESTION IN THE INTESTINES 



The further changes which the half-digested foodstuffs in the chyme 

 undergo in the intestinal canal depend on the enzymes present in the 

 secretion of the various glands and on the presence of bacteria. The 

 most important of the digestive juices are the pancreatic juice and bile. 

 The latter, however, does not contain any enzyme, its influence on diges- 

 tion being entirely adjuvant. 



Pancreatic Digestion 



When we were considering the mechanism of secretion of the pan- 

 creatic juice, we saw that the juice produced by the action of secretin on 

 the gland cells does not contain any active proteolytic enzyme, although 

 it contains one capable of acting on polysaccharides and another, on fat. 



THE ACTION OF TRTPSIN 



When this juice is mixed with the secretion of the duodenum or of 

 the upper part of the small intestine, it immediately develops powerful 

 proteolytic power. The same result may also be obtained by mixing it 

 with an extract of the mucous membrane of the duodenum made with 

 dilute bicarbonate solution. A very small amount of the extract is 

 capable of increasing the digestive activity of a very considerable quan- 

 tity of pancreatic juice, showing that the action depends on the presence 

 of an enzyme which has been called enterokinase. This influence of the 

 intestinal secretion is readily destroyed by heating. 



Large quantities of alkali are contained in the pancreatic juice and 

 bile, so that in the upper reaches of the intestine the acidity of the 

 chyme is practically neutralized. A little lower down, however, an acid 

 reaction may again develop (see page 505). On account of these facts it 

 has been concluded that the activity of trypsin is most rapid in the pres- 

 ence of a slight excess of hydroxyl ions; i. e., in a weakly alkaline solu- 

 tion. It is interesting to note that, as a result of the great secretion of 

 alkali by the pancreas, extracts of this organ after death show a very 

 high degree of acidity in comparison with extracts from other organs 



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