DIGESTION IS CONTINUED IN THE STOMACH 93 



increase the flow of saliva by thorough chewing because chewing 

 stimulates the salivary glands and causes them to secrete more 

 abundantly. We can increase the flow of saliva by serving 

 foods in such a way that their appearance and odor are pleas- 

 ant. Often the mere sight and odor of food stimulate the flow 

 of saliva so quickly and effectively that the mouth waters. We 

 can increase the salivary secretion by agreeable conversation 

 and good cheer during meals, because these excite the glands 

 to activity, while anger, worry, and distress paralyze the glands 

 and lessen their secretions. Thorough mastication and well- 

 cooked and well-served foods aid salivary secretion and assist 

 digestion. 



Ptyalin does not produce an immediate change of starch into 

 sugar ; it acts slowly and requires from ten to twenty minutes 

 to transform cooked starch, and from I to ij hours to trans- 

 form raw or uncooked starch. Since food remains but a short 

 time in the mouth, the digestion of starch is not completed in 

 the mouth, but is only begun there. From the mouth, food 

 passes by the gullet to the stomach where the ptyalin acts until 

 it is stopped by the gastric juice. Saliva is an alkaline liquid, 

 and its ferment ptyalin cannot act in an acid medium ; gastric 

 juice is an acid, and as soon as it is well mixed with the food, 

 it interferes with the ptyalin and stops the transformation of 

 starch into sugar. Food should be thoroughly chewed because 

 the ptyalin will then be abundant and will act on the starchy 

 substances before it is stopped by the gastric juice. 



Digestion is continued in the stomach. Gastric juice, like 

 saliva, is largely water, and like saliva it plays an important 

 part in digestion. It contains two ferments, rennin and pepsin. 

 Proteins cannot pass through cell walls, but rennin and pepsin 

 change them into soluble peptones, substances which can 

 readily diffuse through cell walls and reach the blood. Gas- 

 tric glands are stimulated by an appetizing appearance, a 



