286 AN AMERICAN TEXT- HOOK OF PHYSIOLOGY. 



of' treatment, ptyalin is the first of this group of ferments to be encountered. 

 It is interesting also to remember in this connection that starch can be con- 

 verted into sugar by a process of hydrolytic cleavage by boiling with dilute 

 mineral aeids. Although the general action of dilute acids and of amylolytic 

 enzymes is similar, the two processes are not identical, since in the first process 

 dextrose is the sugar funned, while in the second it is maltose. Moreover, 

 variations in temperature affect the two reactions differently. 



Conditions Influencing - the Action of Ptyalin. — Temperature. — As in 

 the ease of the other enzymes, ptyalin is very susceptible to changes of temper- 

 ature. 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 of the ptyalin 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 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 prac- 

 tically stop the amylolytic action of enzyme, and a slight increase in acidity not 

 only stops the action, but also destroys the enzyme. The latter fact is of 

 practical importance because it indicates that the action of ptyalin on starch 

 must be suspended after the food reaches the stomach. 



Condition of the Starch. — It is a well-known fact that the conversion of starch 

 to sugar by enzymes takes place much more rapidly with cooked starch — for 

 example, starch paste. In the latter condition sugar begins to appear in a 

 lew minutes (one to four), 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 well-known 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 without further comment. 



Physiological Value of Saliva. — Although human saliva contains ptyalin, 

 and this enzyme is known to possess very energetic amylolytic properties, yet 

 it is probable that it has an insignificant action in normal digestion. The time 

 that food remains in the mouth is altogether too short to suppose that the starch 

 is profoundly affected by the ptyalin. Indeed, the saliva of dogs and cats is 

 said to contain no ptyalin, while horse's saliva is free from ptyalin, although it 

 contains a zymogen that may give rise to ptyalin. it would seem that what- 



