378 A TEXTBOOK OF PHYSIOLOGY 



The Action of the Ptyalin. The enzyme ptyalin, when present in 

 the saliva of an animal, acts upon boiled starch, and converts it through 

 the stages of erythrodextrin and achroodextrin to maltose. The stages 

 can well be demonstrated by taking a series of test-tubes con- 

 taining iodine solution, and a test-tube containing some Fehling's 

 solution. Another tube containing some starch solution, preferably 

 at about 37 C., is taken, and some saliva placed in it. A drop of this 

 mixture, added at once, gives with the iodine a blue colour; after 

 a short time, the addition of a drop to another tube gives a red colour 

 (erythrodextrin) ; later, no colour is obtained when a drop of the 

 solution is added to the iodine. A little later, some of the solution 

 borled with the Fehling solution will give the reduction due to the 

 presence of sugar (maltose). With very active saliva, sugar may 

 appear in half to one minute. 



The rate of action of ptyalin depends upon the kind of starch; 

 also the reaction in which it is acting. It acts best in neutral or very 

 weak acid 0-003 per cent. HC1. It is inhibited by stronger acid, such, 

 for example, as 0-3 per cent. HC1. Organic acids do not stop the action 

 of ptyalin until they reach a strength about ten times that of inorganic 

 acids. 



Common salt greatly favours the action of ptyalin, making it 

 about ten times as active. Other chlorides have a similar but less 

 marked action. Alkaline carbonates hinder the action. 



The action of ptyalin continues in the stomach, digestion of poly- 

 saccharides proceeding until inhibited by acid. This may be as long 

 as forty-five minutes. The saliva has an immunizing effect, protect- 

 ing the teeth from decay. Mouth-breathing and the habit of eating 

 too often and too much soft and sticky food, whereby chronic slight 

 derangement of the alimentary canal is induced, together contribute to 

 the decay of teeth. The decay is the result of bacterial action, 

 favoured by acid fermentation of food allowed to stick between the 

 teeth. 



