662 PHYSIOLOGY 



saliva a definite red colour is obtained. So far as we know it is formed in the body when- 

 ever 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 

 the 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 arid 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 temperature 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 hydrolysed into 

 an achroodextrin (giving no colour with iodine) and maltose, and the achroo- 

 dextrin 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 represented in the following Table : 



Starch 

 soluble starch 



erythro-) dextrines Maltose 



(achroo- ) dextrines maltose 



The process by which the huge starch molecule is converted into dextrins 

 and maltose is a very complicated one, and a number of intermediate com- 

 pounds of dextrins and maltose can exist between those whose presence is 

 revealed by their varying reaction to iodine. 



Ptyalin is most active in a neutral medium, so that the addition of 

 minute traces of acid to the saliva increases its diastatic power. In the 

 presence of free mineral acid ptyalin is rapidly destroyed, -003 per cent. 



