480 SALIVA [CH. XXX. 



The action of saliva is twofold, physical and chemical. 



The physical use of saliva consists in moistening the mucous 

 membrane of the mouth, assisting the solution of soluble substances 

 in the food, and in virtue of its mucin, lubricating the bolus of food 

 to facilitate swallowing. 



The chemical action of saliva is due to its active principle, ptyalin. 

 This substance belongs to the class of unorganised ferments, and to 

 that special class of unorganised ferments which are called ainylolytic 

 (starch splitting) or diastatic (resembling diastase, the similar ferment 

 in germinating barley and other grains). 



The starch is first split into dextrin and maltose ; the dextrin is 

 subsequently converted into maltose also : this occurs more quickly 

 with erythro-dextrin, which gives a red colour with iodine, than with 

 the other variety of dextrin called achroo-dextrin, which gives no 

 colour with iodine. Brown and Morris give the following equa- 

 tion : 



10(C H 10 O 5 )n + 4nH 2 O 



[Starch.] [Water.] 



4nC 1 ,H 22 p ]1 + (C,H 10 S ). + (C 6 H 10 5 ),, 



[Maltose.] [Achroo-dextrin.] [Erythro-clextriii.] 



Ptyalin acts in a similar way, but more slowly on glycogen : it has 

 no action on cellulose; hence it is inoperative on uncooked starch 

 grains, for in them the cellulose layers are intact. 



Ptyalin acts best at about the temperature of the body (35-40 C.). 

 It acts best in a neutral medium ; a small amount of alkali makes 

 but little difference ; a very small amount of acid stops its activity. 

 The conversion of starch into sugar by swallowed saliva in the 

 stomach continues for a certain time. It then ceases owing to the 

 hydrochloric acid secreted by the glands of the stomach. The acid 

 which is first poured out neutralises the saliva, and combines with 

 the proteids of the food, but when free acid appears ptyalin is de- 

 stroyed, and so it cannot resume work when the acid is neutralised 

 in the duodenum. Another amylolytic ferment contained in pan- 

 creatic juice (to be considered later) continues the digestion of starch 

 in the intestine. 



Cannon has recently shown that salivary digestion continues in 

 the stomach for longer than one supposed. The food lying in the 

 fundus of the stomach undergoes amylolysis for at least two hour?, 

 because the absence of peristalsis in this region until quite late 

 stages in digestion prevents admixture with gastric juice, especially 

 in the interior of the swallowed masses. 



