DIGESTION 151 



ment has demonstrated that it has a two-fold action; viz., physical and 

 chemical. 



Physically, saliva softens and moistens the food, unites its particles into 

 a consistent mass by means of its contained mucin, and thus facilitates 

 swallowing. 



Chemically it converts starch into sugar. This action is more marked 

 with boiled than with raw starch, a fact which depends on the physical 

 structure of the starch grain. In the natural condition each starch grain con- 

 sists of a cellulose envelope or stroma in the meshes of which is contained the 

 true starch material, the granulose. When boiled for some minutes, the 

 starch grain absorbs water, the granulose swells and ruptures the cellulose 

 envelope, after which it passes into an imperfect opalescent solution more or 

 less viscid, depending on the relative amounts of water and starch. This is 

 the change largely brought about by the process of cooking. If a portion of 

 this hydrated starch be kept in the mouth for a few minutes it will be con- 

 verted into sugar, a fact made evident by the sense of taste. 



The chemic action of saliva in converting starch into sugar, as well as the 

 intermediate stages, can be experimentally shown in the following manner: 

 To 5 volumes of a thin starch solution in a test-tube add two volumes of 

 filtered saliva. Place the mixture in a water-bath at a temperature of 35 C. 

 In a few minutes the starch passes into a soluble condition and the fluid 

 becomes clear and transparent. On testing the solution from time to time 

 with iodin the characteristic blue reaction will be found to disappear, the 

 color gradually passing from blue to violet, to red, to yellow. If now the 

 solution be boiled with a solution of cupric hydroxid (Fehling's solution) a 

 copious red or yellow precipitate of cuprous oxid is formed, which indicates 

 the presence of sugar. The polariscope shows that this sugar is dextro- 

 rotatory. During the conversion of the starch intermediate substances 

 are formed to which the term dextrin is applied. After the starch has been 

 rendered soluble it undergoes a cleavage into maltose and a dextrin, which, 

 as it gives rise to a red color with iodin, is termed erythrodextrin. At a 

 later stage this erythrodextrin also undergoes a cleavage into maltose 

 and a second variety of dextrin, which, as it does not give rise to any 

 color with iodin, is termed achroodextrin. It is claimed by some investi- 

 gators that this form can also in time be transformed into sugar. It is 

 possible that a small quantity of dextrose is also formed. 



The successive stages of the conversion of starch into sugar may be 

 represented by the following schema: 



r T? *T,^J ^- / Achroodextrin. 

 Starch - Soluble Starch - { Maltose " I Maltose - 



This change consists in the assumption by the starch of a molecule of water, 

 and for this reason the process is termed hydrolysis. The nature of the 

 chemic change is shown in the following formula: 



alo5 a1?2a n a10a 

 Starch + Water- Maltose* Dextrin. 



The amylolytic 1 , amyloclastic, or starch-changing action of saliva depends 



1 The term amylolytic has been objected to on the ground that it does not correctly express 

 the fact, but is analogous with electrolytic and means a transformation by means of starch. 

 Armstrong has suggested the use of the term amyloclastic as well as proteoclastic and lipoclastic 

 for the terms now generally employed. 



