AN AMERICAN TENT-BOOK OE PHYSIOLOGY. 287 



ever change takes place must be confined to the initial stages. After the mixed 

 saliva and food are swallowed it is usually supposed that the acid reaction of 

 the gastric juice soon stops completely all further amylolvtic action, although 

 this point is often disputed. 1 The complete digestion of the carbohydrates 

 takes place after the food (chyme) has reached the small intestine, under the 

 influence of the amylopsin of the pancreatic secretion. For these reasons it is 

 usually believed that the main value of the saliva, to the human being and to 

 the carnivora at least, is that it facilitates the swallowing of food, it is ini pos- 

 sible to swallow perfectly dry food. The saliva, by moistening the food, not 

 only enables the swallowing act to take place, but its viscous consistency must 

 aid also in the easy passage of the food along the oesophagus. In addition 

 the solution of parts of the food in the saliva gives occasion for the stimula- 

 tion of the taste nerves, and, as we shall see in studying the mechanism of 

 gastric secretion, the conscious sensations thus produced are very important 

 for gastric digestion. 



O. Gastric Digestion. 



After the food reaches the stomach it is exposed to the action of the secre- 

 tion of the gastric mucous membrane, known usually as the gastric juice. The 

 physiological mechanisms involved in the production and regulation of this 

 secretion, and the important part played in gastric digestion by the movements 

 of the stomach, will be found described in other sections (Secretion, Move- 

 ments of Alimentary Canal). It is sufficient here to say that the secretion 

 of gastric juice begins with the entrance of food into the stomach. By means 

 of the muscles of the stomach the contained food is kept in motion for several 

 hours and is thoroughly mixed with the gastric secretion, which during this 

 time is exerting its digestive action upon certain of the food-stuffs. From time 

 to time portions of the liquefied contents, known as chi/nic, are forced into the 

 duodenum, and their digestion is completed in the small intestine. Gastric 

 digestion and intestinal digestion go more or less hand in hand, and usually 

 it is impossible to tell in any given case just how much of the food will 

 undergo digestion in the stomach and how much will be left to the action of 

 the intestinal secretions. It is possible, however, to collect the gastric secre- 

 tion or to make an artificial juice and to test its action upon food— lull- In- 

 digestions in vitro. Much of our fundamental knowledge of the digestive 

 action of the gastric juice has been obtained in this way, although this has 

 been supplemented, of course, by numerous experiments upon lower animals 

 and human being's. 



Methods of Obtaining' Normal Gastric Juice. — The older methods used 

 for obtaining normal gastric juice were very unsatisfactory. For instance, an 

 animal was made to swallow a clean sponge to which a string was attached so 

 that the sponge could afterward be removed and its contents be squeezed out ; 

 or there was given the animal to eat some indigestible material, to start the 

 6ecretion of juice by mechanical stimulation, the animal being killed at the 

 'Austin: Boston Medical and Surgical Journal, 1899. 



