702 PHYSIOLOGY OF DIGESTION AND SECRETION. 



Effect of Reaction. The normal reaction of saliva is slightly 

 alkaline. Chittenden has shown, however, that ptyalin acts as well, 

 or even better, in a perfectly neutral medium. A strong alkaline 

 reaction retards or prevents its action. The most marked influence 

 is exerted by acids. Free hydrochloric acid to the extent of only 

 0.003 per cent. (Chittenden) is sufficient to practically stop the amy- 

 lolytic action of the enzyme, and a slight further increase in acidity 

 not only stops the action, but also destroys the enzyme. 



Condition of the Starch. It is a well-known fact that the conver- 

 sion of starch to sugar by enzymes takes place much more rapidly 

 with cooked starch for example, starch paste. In the latter ma- 

 terial sugar begins to appear in a few minutes, provided a good 

 enzyme solution is used. With starch in a raw condition, on the 

 contrary, it may be many minutes, or even several hours, before 

 sugar can be detected. The longer time required for raw starch is 

 partly explained by the fact that the starch grains are surrounded 

 by a layer of cellulose or cellulose-like material that resists the action 

 of ptyalin. When boiled, this layer breaks and the starch in the 

 interior becomes exposed. In addition, the starch itself is changed 

 during the boiling ; it takes up water, and in this hydrated condition 

 is acted upon* more rapidly by the ptyalin. The practical value of 

 cooking vegetable foods is evident from these statements. 



Functions of the Saliva. In addition to the digestive action of 

 the saliva on starchy foods it fulfills other important functions. By 

 moistening the food it enables us to reduce the material to a consis- 

 tency suitable for swallowing and for manipulation by the tongue and 

 other muscles. Moreover, the presence of mucin serves doubtless 

 as a kind of lubricator that insures a smooth passage along the 

 esophageal canal. Finally by dissolving dry and solid food it pro- 

 vides a necessary step in the process of stimulating the taste nerves, 

 and, as is described below, the activity of the taste sensations may 

 play an important part in the secretion of the gastric juice. 



