762 PHYSIOLOGY OF DIGESTION AND SECRETION. 



or. more untouched by the acid secretion. There is every reason to 

 believe, therefore, that salivary digestion may be carried on in the 

 stomach to an important extent. 



Conditions Influencing the Action of Ptyalin. — Temperature. 

 — As in the case of the other enzymes, ptyalin is very susceptible to 

 changes of temperature. At 0° C. its activity is said to be suspended 

 entirely. The intensity of its action increases with increase of 

 temperature from this point, and reaches its maximum at about 

 40° C. If the temperature is raised much beyond this point, the 

 action decreases, and at from 65° to 70° C. the enzyme is destroyed. 

 In these latter points ptyalin differs from diastase, the enzyme of 

 malt. Diastase shows a maximum action at 50° C. and is destroyed 

 at 80° C. 



Effect of Reaction. — The normal reaction of saliva is neutral or 

 slightly acid (pH 6.6 to 7.1). Chittenden has shown that ptyalin 

 acts well in a perfectly neutral medium. A strong alkaline reac- 

 tion 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 

 amylolytic 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-Uke 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 Idnd 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 ma}/ 

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



