360 ACTION OF SALIVA. [BOOK n. 



action of saliva, especially at a somewhat high temperature such as 

 35 or 40 C., it is found that the subsequent addition of iodine 

 gives no blue colour at all, or very much less colour, shewing that 

 the starch has disappeared or diminished ; on the other hand the 

 mixture readily gives a precipitate of cuprous oxide when boiled 

 with Fehling's fluid, shewing that maltose or dextrose is present. 

 That is to say the saliva has converted the starch into maltose 

 or dextrose. The presence of the previously absent sugar may also 

 be shewn by fermentation and by the other tests for sugar. 

 Moreover, if an adequately large quantity of starch be subjected 

 to the charge, the sugar formed may be isolated, and its characters 

 determined. When this is done it is found that while some 

 dextrose is formed the greater part of the sugar which appears is 

 in the form of maltose. As is well known starch may by the 

 action of dilute acid be converted into dextrin, and by further 

 action into sugar; but the sugar thus formed is always wholly 

 dextrose, and not maltose at all. The action of saliva in this 

 respect differs from the action of dilute acid. 



While the conversion of the starch by the saliva is going on the 

 addition of iodine frequently gives rise to a red or violet colour 

 instead of a pure blue, but when the conversion is complete no 

 coloration at all is observed. The appearance of this red colour 

 indicates the presence of dextrin (erythrodextrin) ; the violet 

 colour is due to the red being mixed with the blue of still un- 

 changed starch. 



The appearance of dextrin shews that the action of the saliva 

 on the starch is somewhat complex ; and this is still further 

 proved by the fact that even when the saliva has completed its 

 work the whole of the starch does not reappear as maltose or 

 dextrose. A considerable quantity of the other dextrin (achroo- 

 dextrin) always appears and remains unchanged to the end ; 

 and there are probably several other bodies also formed out of 

 the starch, the relative proportions varying according to circum- 

 stances. The change therefore, though perhaps we may speak 

 of it in a general way as one of hydration, cannot be exhibited 

 under a simple formula, and we may rest content for the present 

 with the statement that starch when subjected to the action of 

 saliva is converted chiefly into the sugar known as maltose with 

 a comparatively small quantity of dextrose and to some extent 

 into achroodextrin (erythrodextrin appearing temporarily only in 

 the process), other bodies on which we need not dwell being 

 formed at the same time. 



Raw unboiled starch undergoes a similar change but at a much 

 slower rate. This is due to the fact that in the curiously formed 

 starch grain the true starch, or gramdose, is invested with coats 

 of cellulose. This latter material, which requires previous treat- 

 ment with sulphuric acid before it will give the blue reaction 

 on the addition of iodine, is apparently not acted upon by saliva. 



