300 Biological Chemistry. 



by the action of a substance contained in the mucous 

 membrane of the intestine known as enterokinase, the 

 proteoclastic enzyme trypsin is set free. This is capable 

 of degrading into simpler products the proteoses and pep- 

 tones produced in gastric digestion. Finally, the small 

 intestine and other organs contain an enzyme known as 

 erepsin which can hydrolyze the peptones, producing 

 therefrom a mixture of amino-acids. The fats also undergo 

 hydrolysis in the small intestines. The undigested food, 

 and the partially digested food which is not absorbed, pass 

 farther down the intestine, where they are acted upon by 

 the bacterial flora which exist normally in the gut. Here 

 they undergo further changes, being acted upon by the 

 bacterial enzymes. All the reactions described above can 

 be studied in vitro. It is easy to study the digestive 

 action of saliva on starch ; pepsin can be prepared from 

 extracts of the mucous membrane of the stomach ; pan- 

 creatic juice can be obtained by the insertion of a cannula 

 into the pancreatic duct of an animal, and commences to 

 flow after injection into the veins of an extract made with 

 dilute hydrochloric acid of the mucous membrane of the 

 small intestine (which extract must be neutralized before 

 injection). The pancreatic juice, if obtained without con- 

 taminating the small intestine, possesses no tryptic activity ; 

 this is only exhibited after the juice has been treated 

 with extract of the small intestine which contains the 

 enterokinase, which is also supposed to be an enzyme. 

 The mechanism of the conversion of trypsinogen into 

 trypsin is not completely understood. The proteoclastic 

 action of the various ferments can be investigated by the 

 chemical methods already described (p. 259 and seq.). The 

 mucous membrane of the small intestine of young animals, 

 the chief diet of which is milk, contains an enzyme lactase, 

 which converts the milk sugar into a mixture of glucose 



