DIGESTIVE CHANGES IN THE SMALL INTESTINE 371 



of secretion, the succus entericus, can be obtained by this means. Intestinal 

 juice is a yellowish alkaline fluid with a specific gravity of i .on and contains 

 about 2 . 5 per cent, of solid matters. 



Intestinal juice has only slight digestive action. It contains a weak pro- 

 teolytic enzyme and a weak amylolytic enzyme. Maltase is also present. 

 But the chief and most profound importance is given to the intestinal juice 

 by the discovery of the activating enzyme, enter okinase. This specific 

 activating enzyme for the trypsinogen of the pancreatic juice places the in- 

 testinal secretion in the rank of necessary secretion for efficient digestion. 

 Enterokinase can be prepared by extracting the superficial scrapings of 

 the intestinal mucous coat. The duodenal region is richest in enterokinase, 

 but the secretion of the lower intestinal lengths also contains the enzyme. 



Extracts of the mucosa of the intestine have been found to contain an- 

 other substance which has the specific action of splitting peptones into 

 simpler polypeptids and amino-acids. This substance has been called 

 erepsin. 



There are, therefore, three important new substances in the succus en- 

 tericus (or in the extract of the glands), secretin (page 360), erepsin, and 

 enterokinase, in addition to the proteolytic and diastatic enzymes. 



Summary of the Digestive Changes in the Small Intestine. The 

 thin chyme, which, during the whole period of gastric digestion, is being con- 

 stantly squeezed or strained through the pyloric orifice into the duodenum, 

 consists of albuminous matter that is breaking down, dissolving and half 

 dissolved; of fatty matter that is mechanically separated and melted, but 

 not dissolved at all; of starch in various stages of the process of con- 

 version into sugar, and as it becomes sugar dissolving in the fluids 

 with which it is mixed; and with these are mingled gastric juice and 

 fluid that has been swallowed, together with such portions of the food as 

 are not digestible. 



The chyme in the duodenum is subjected to the influence of the bile and 

 pancreatic juice and also to that of the succus entericus. All these secretions 

 have a more or less alkaline reaction, and at once neutralize the acid of the 

 gastric chyme. 



The special digestive changes in the small intestine are: (i) The fats are 

 changed by the bile and pancreatic juice in two ways: (a) They are 

 chemically decomposed by the alkaline secretions, and a soap and glycerin 

 are the result. (6) They are emulsified; i.e., their particles are minutely 

 subdivided and diffused, so that the mixture assumes the condition of a 

 milky fluid or emulsion. (2) The albuminous substances which have been 

 partly dissolved in the stomach are subjected chiefly to the action of the 

 pancreatic juice. The pepsin is rendered inert by the bile. The pancreatic 

 trypsin proceeds with the further conversion of the proteoses into peptones, 

 and, with the erepsin, of the peptones into leucin, tyrosin, and the other 



