DIGESTION IN THE MOUTH. 289 



discharges the blue color of the latter. In neither of these substances is 

 there the property of converting starch into sugar, but the result is due 

 to oxidation of the iodide. There are two practical points to be drawn 

 from this demonstration : first, since the starch is not instantaneously 

 converted into sugar upon contact with the saliva, even though mas- 

 tication be prolonged, by no means all of the starch in the food can be 

 converted into sugar in the mouth; and, second, starch cannot be 

 considered as a conclusive test for iodine in the various secretions. 



It is often desired to test urine for iodine, as in cases of iodism, and all that 

 is deemed necessary is to add a solution of starch-mucilage to the suspected fluid, 

 and if the characteristic blue color does not appear it is concluded that no iodine 

 is present. This procedure is doubly fallacious, not only because these fluids 

 have the power of decolorizing solutions of the iodide of starch, but even when 

 iodine is present it is not in the form of free iodine but of hydriodic acid, the very 

 agent through which this decolorizationis effected. If, therefore, iodine is present 

 in such organic fluids, its presence can only be detected by the starch test by first 

 deoxidizing the hydriodic acid. This may be accomplished by soaking a piece of 

 filter-paper in starch-mucilage, drying, moistening with the suspected fluid, and 

 then allowing a drop of nitrous acid to fall upon it. If iodine is present in the 

 form of hydriodic acid, it will be deoxidized by the nitrous acid, and the free 

 iodine will form the characteristic blue color with the starch-paper. 



The old view as to the saccharification of starch was based upon the 

 assumption that the diastatic ferment first converted the starch into dex- 

 trin, and that then dextrin through hydration was converted into dex- 

 trose. This view lias been shown to be erroneous by Musculus, who 

 found that the subject is very much more complex. He stated that in 

 the conversion of starch into sugar all the starch was not first trans- 

 formed into dextrin and then into sugar, but that these two bodies were 

 simultaneously. formed, and he gives the following formula as represent- 

 ing this conversion : — 



3C 6 H 10 O 5 +2H 2 O = C 6 H 12 O +2C 6 H 10 O 5 

 Starch. Dextrose. Dextrin. 



Even this view has, however, been modified by subsequent observa- 

 tion. According to the view of Musculus, only 33 per cent, of sugar 

 could originate from the action of the diastatic ferments on starch, 

 but it has been found that dextrin also is converted partially into sugar, 

 :ind from 20 to 30 per cent, of sugar may be formed in this waj . 

 Estimates of the actual amount of sugar developed through the action 

 of the diastatic ferment on starch show that, instead of 33 per cent., over 

 50 per cent, of sugar will actually form ; so that, therefore, while the 

 starch may first be split up into dextrin and sugar, this dextrin also 

 undergoes partial conversion into a fermentable sugar. Consequently, 

 through the action of ptyalin, starch is first converted into dextrin and 

 sugar, and then the dextrin itself, through the action of the ferments, 

 undergoes subsequently a progressive hydration and results in the 



19 



