THE SALIVA. 123 



(erythrodextrin), and that still later no color-reaction whatever 

 occurs (achroodextrin). As soon as this point is reached a small 

 amount of the starch mixture is examined with Trommer's or 

 Fehling's test, when the presence of sugar can be established. The 

 sugar which thus results is maltose. The reaction which takes 

 place may be represented by the schema : 



(C 12 H 22 O n ) 



Starch/* 1 * 111086 l C \| H2Al) 



(C 12 H 20 10 ) n \Erythrodextrin. /MaltOSe 



[C 12 H 20 O, ]._ a C 12 H 22 O n \Achro6ctextrin 



Glucose may incidentally be also demonstrable among the end- 

 products of salivary digestion, but is not due to the action of 

 ptyalin, but of accompanying maltose. Its amount is rather insig- 

 nificant. 



We shall return to these reactions in the next chapter, when the 

 digestion of the food is considered in detail. 



To isolate the ptyalin from saliva, the following method, which 

 has been suggested by Gautier, may be employed : To a large 

 quantity of saliva 98 per cent, alcohol is added so long as a floccu- 

 lent precipitate is seen to form. This is collected on a small filter 

 and dissolved with a small amount of distilled water. The solu- 

 tion is treated with a few drops of a solution of bichloride of mercury, 

 in order to remove any albuminous material that may be present. 

 In the filtrate the excess of the bichloride is removed with hydrogen 

 sulphide, when the remaining liquid is evaporated to dryness at a 

 temperature not exceeding 40 C., and the residue is treated with 

 strong alcohol. The insoluble portion is then dissolved with a small 

 amount of distilled water, filtered, dialyzed in order to remove inor- 

 ganic salts, and finally precipitated with absolute alcohol, when the 

 ptyalin will separate out in light flakes. Obtained in this manner, 

 ptyalin is a white amorphous substance, which is soluble in water, 

 dilute alcohol, and glycerin. In neutral or slightly alkaline solu- 

 tion, but not in acid solution, it rapidly transforms boiled starch 

 into sugar at a temperature of from 36 to 40 C. Heated to a 

 temperature of 65 to 70 C., it is destroyed, and it is thus possible 

 to distinguish between ptyalin and vegetable diastase, for which the 

 optimum temperature lies at 50 C., and which is destroyed at 

 80 C. 



Of special interest is the fact that the transformation of starch 

 into sugar ceases as soon as the latter is present to the extent of 

 from 2 to 2.5 per cent. This phenomenon is common to all enzy- 

 matic processes, and is probably referable to the establishment of a 

 certain equilibrium of reaction. A complete transformation of the 

 starch could occur only if the resulting sugar were removed as 

 rapidly as it is formed. So long as it is present, the reversible 

 action of the enzyme becomes manifest. 



