THE DIGESTIVE SYSTEM 77 



of digestion. It acts on starch, dextrin, and other carbohydrates, 

 converting them into simpler bodies. The first change is a conver- 

 sion of the huge starch molecule into dextrin and then into malt 

 sugar. Like other enzymes, ptyalin' is not destroyed or used up 

 in producing its reaction, but if heated to 60° C. it loses its virtue. 

 It is most active in a neutral medium at the body temperature. 

 In the presence of the free hydrochloric acid of the stomach ptyalin 

 is rapidly rendered inert. The chief part of the saUvary digestion 

 actually occurs in the stomach in spite of the hydrochloric acid 

 of the gastric juice, which requires some time to penetrate the 

 food. This is due to the low motility of the cardiac portion of the 

 stomach, the food does not mix with the hydrochloric acid very 

 promptly, and to the fact that this secretion is confined to the 

 fundus and pyloric regions. 



(B) Digestion in the Stomach. — The stomach is popularly sup- 

 posed to be the chief organ of digestion; as a matter of fact, in the 

 horse it acts principally as a temporary storehouse for food which 

 is undergoing preparation for the more complex digestive changes 

 designed to take place in the intestines. 



Gastric juice is the secretion of the glands of the stomach. 

 The difficulty of obtaining a pure sample from the horse has pre- 

 vented detailed study. However, the chief constituents are 

 known to be hydrochloric acid and the enzymes, pepsin and rennin. 

 Fresh gastric juice also contains much mucin, but this plays only a 

 passive part in digestion. 



Hydrochloric acid is found free in the gastric juice and gives the 

 secretion an acid reaction. It acts as an antiseptic by preventing 

 excessive fermentation and . putrefaction through destroying all 

 kinds of bacteria. The percentage is considerably lower in the 

 horse than in carnivora, so micro-organisms are not so readily 

 destroyed, and have a longer opportunity to act on the sugars 

 derived from salivary digestion of the carbohydrates, splitting them 

 into lactic acid. On a diet of starchy food the lactic acid may 

 exceed the hydrochloric acid. The rate of emptying the stomach is 

 controlled in a measure by the hydrochloric acid, but not to such an 

 extent as in the dog, for the pylorus is not usually closed in the 



^ To show the presence and action of ptyahn some starch paste should be 

 prepared. Five c.c. of the paste may be placed in each of as many test- 

 tubes and filtered saliva added in different quantities. By keeping them at 

 various temperatures and testing from time to time with iodin and Fehling's 

 solutions the progress of digestion in the mouth may be nicely demonstrated. . 



