INTESTINAL DIGESTION. 75 



lar quantitative differences exist in the secretion of pancreatic 

 juice and this is probably to be explained by the varying quanti- 

 ties of acid chyme coming in contact with the duodenal mucosa. 



Chemical Changes Produced by Intestinal Digestion. In the 

 lower portion of the duodenum and in the jejunum, the digestive 

 < tiii/mes of the pancreatic juice act on the food in full intensity. 

 The trypsin rapidly hydrolyzes the proteins to peptone, which if 

 it is not immediately absorbed may become further broken down 

 to amino bodies and aromatic compounds. The lipase hydrolyses 

 fat to glycerine and fatty acid, which are absorbed, the former 

 as such, the latter, after combining with alkali to form soap, or, 

 if no alkali be available, with bile salts to form compounds which 

 like the soap are soluble in water. Amylopsin converts into mal- 

 tose any starch or dextrines which the ptyalin of saliva has failed 

 to act on. The maltose thus formed, and the other disaccharides, 

 cane sugar and lactose, although soluble in water, do not become 

 absorbed into the blood as such but become further hydrolyzed 

 by the action of so-called inverting enzymes, of which there is one 

 for each disaccharide (see p. 25). These inverting enzymes are 

 more plentiful in extracts of the mucosa than in the intestinal 

 juice itself, from which we conclude that it is only after they 

 have been absorbed into the cells of the intestines that the disac- 

 charides are inverted. The process, in other words, is an in- 

 tracellular one. 



One other enzyme exists in the intestinal juice, namely, erepsin. 

 It acts on partially hydrolyzed proteins and on caseinogen, so 

 as to hydrolyze them completely into the amino compounds. 

 Erepsin is a widely distributed enzyme in the animal body, be- 

 ing present in practically every tissue, although it is absent from 

 blood plasma. It is present in much greater concentration in ex- 

 tracts of the intestinal mucosa than in succus entericus, so that, 

 like the inverting enzymes, it possibly displays its action while 

 the protein is being absorbed as proteoses and peptones. It serves 

 as the last barrier against the entry into the blood of protein in 

 any other form than as a mixture of amino bodies. Less com- 

 pletely digested protein is poisonous when added to 'the blood 

 (p. 152). 



