

MOUTH DIGESTION. 1-19 



The active principle which brings about this change is Ptyalin. 

 This is one of a series of ferments which exist in the body, and 

 to which most of the chemical changes in digestion are due. 

 Although each possesses certain peculiarities, yet as a group they 

 may be described as effecting by their presence various altera- 

 tions in the substances on which they act, while they themselves 

 do not undergo any perceptible change, and a small quantity 

 will do as much work as a larger quantity if allowed a propor- 

 tionately longer time. 



Ptyalin acts exclusively on starch, and hence is spoken of as 

 an amylolytic ferment, its action consisting in causing the starch 

 to unite chemically with one molecule of water, thus : 

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



Starch. Grape sugar. 



During this process, which takes, at the least, a few minutes to 

 complete, various stages can be detected : first, two substances 

 are formed which together are commonly spoken of as dextrin ; 

 one, erythro-dextrin, which gives a red color with iodine, and 

 easily passes into soluble sugar ; and the other, achroo-dextrin ; 

 gives no color with iodine, and is with difficulty converted into 

 sugar. As it gives no color with the ordinary test, its presence 

 is often overlooked. 



The sugar thus formed has been called Ptyalose, which, how- 

 ever, can be converted into ordinary grape sugar (glucose) by the 

 action of sulphuric acid. Some say the product is all maltose. 



The presence of starch, either in its soluble.or insoluble form, 

 is easily recognized by the blue color given by free iodine, which 

 color disappears on heating to about 100 C., but reappears on 

 cooling. ^ 



Very many tests have been recommended for the detection of 

 sugar. The most generally applicable one is Trommer's. The 

 liquid is made strongly alkaline with potash, and a few drops of a 

 dilute solution of cupric sulphate are added ; a clear, blue solution 

 results, which, on being raised to the boiling point, deposits a 

 yellow precipitate of cuprous oxide. Fehling's and Pavy's solu- 

 tions are modifications of the above test adapted for quantitative 

 analysis. 



