I 



MOUTH DIGESTION. 141 



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. 



When yeast is added to grape-sugar and placed in an inverted 

 test-tube, the sugar is converted into alcohol and carbon-dioxide. 

 The latter rises to the top of the test-tube, and can be used as an 

 indication of the quantity present. Experiments may be carried 

 out with saliva obtained from any of the glands directly, but the 

 mixture of the secretion of all is found to be more efficacious than 

 that of any single one. The ordinary mouth-fluid filtered serves 

 well for ordinary experiment. 



An effective glycerin solution of ptyalin may be obtained by 

 steeping chopped salivary glands in alcohol, and then extracting 

 for some days with glycerin and water. 



The following facts must be borne in mind concerning the 

 amylolytic action of ptyalin : 



1. The extremely small amount of the ferment required to 



make the fluid effective. 



2. There is no appreciable diminution in the amount of fer- 



ment, so that it cannot be said to be used up in the 

 process. 



3. The action takes place most readily in alkaline solutions, 



such as the saliva, slowly in neutral solution, and not at 

 all in acids of the strength of .2 per cent, of hydrochloric 

 acid. 



4. Temperature has a marked effect on the process. Cold 



(0 C.) quite checks the action ; heat (75 C.) destroys 



