DIGESTION IN THE MOUTH 741 



present to the extent of '01 per cent, so that on the addition of a drop of ferric 

 chloride to saliva a definite red colour is obtained. So far as we know it is formed 

 in the body whenever cyanides or organic nitriles in small quantities make their 

 appearance in the circulating fluid, either as the result of administration or 

 perhaps as by-products in the normal processes of metabolism. The conversion of 

 the poisonous cyanides into tin almost innocuous sulphocyanates seems to be a 

 means by which the organism protects itself against the poisonous effects of the 

 former. The channels of excretion of the sulphocyanate are by the salivary 

 glands, the kidneys, and possibly by the gastric juice. 



THE USES OF SALIVA 



The main function of saliva is to moisten the food and so facilitate 

 its mastication and deglutition. The presence of the mucin is 

 of special value for the latter process since it renders the mass of 

 food slippery. In animals, such as dogs, where the saliva is devoid of 

 any digestive ferment, this must represent its sole function. In man 

 and some of the herbivora the saliva exerts a well-marked digestive 

 effect on one of the food-stuffs, namely, starch. If a warm solution 

 of starch be taken into the mouth, kept there for one minute, and 

 then expelled into a test-tube, the starch will be found to have entirely 

 disappeared, its place being taken by a reducing sugar. The stages 

 of the action of saliva on boiled starch can be followed more easily 

 if its action is retarded by keeping the mixture cool, or at a tempera- 

 ture not above 25 C. The first change is a conversion of the 

 opalescent gelatinising starch solution into a clear solution which 

 no longer sets on cooling, but still gives a blue colour with iodine. 

 The fluid contains what is known as soluble starch. The soluble 

 starch then undergoes hydrolytic dissociation into a dextrin, which 

 gives a red colour with the iodine and is therefore known as erythro- 

 dextrin, together with maltose. The erythrodextrin is then hydro- 

 lysed into an achroodextrin (giving no colour with iodine) and maltose, 

 and the achroodextrin is still further broken up into dextrin and 

 maltose. The conversion of starch into maltose is never complete, 

 though if the maltose be removed by dialysis as it is formed, it was 

 found by Lee to be possible to convert as much as 95 per cent, of the 

 starch into reducing sugar. The stages in the conversion are repre- 

 sented in the following Table : 



Starch 



I 



soluble starch 



(erythro-) dextrins maltose 



(achroo-) dextrins maltose 



