DIGESTIVE ACTION OF SALIVA ON STARCH 353 



presence is to be taken into consideration afterward. Make the following 

 tests: 



Reaction. A slip of neutral litmus paper when introduced into freshly 

 collected saliva, or for convenience simply taken into the mouth during sali- 

 vary secretion, shows an alkaline reaction. 



Mucin. To 3 or 4 c.c. of saliva add 2 per cent acetic acid drop by drop 

 until distinct acidity is obtained. On stirring the saliva with a glass rod a 

 sticky mucin makes its appearance. 



Potassium Sulphocyanide. To 2 c.c. of saliva in a test tube add 2 or 3 

 drops of ferric-chloride solution, slightly acidulated with hydrochloric acid, 

 a reddish-brown coloration indicates the presence of potassium sulphocyanide. 

 One should run a blank test on distilled water for comparison. 



Chlorides. Add silver nitrate to 2 c.c. of saliva after first removing the 

 proteids. A white, cloudy precipitate, which disappears on adding ammonia 

 and reappears on adding nitric acid, indicates the presence of chlorides. 



Proteids. Remove the mucin from a sample of saliva, as above, and test 

 by the characteristic proteid reactions. A faint trace of proteid can usually 

 be demonstrated. 



5. Digestive Action of Saliva on Starch. Review the test for starch, 

 dextrin, and dextrose, as preparation for an identification of these prod- 

 ucts of salivary digestion. To 50 c.c. of i per cent starch paste in the 

 water bath at 40 C. add 5 c.c. of saliva, and mix thoroughly with a glass rod. 

 Immediately begin two series of tests: a, for the presence of starch; b, for 

 the presence of reducing sugar. The tests for starch can be made by adding 

 to 3 drops of starch, on a porcelain plate, an equal quantity of dilute iodine 

 in potassium iodide solution. Use a glass rod. Make the tests every 2 minutes 

 for 20 minutes. The tests for reducing sugar are best made by placing 2 c.c. 

 of Fehling's solution in each of a series of test tubes and adding, at intervals 

 of 5 minutes, i c.c. from a dropping-pipet and boiling. If the tests are set 

 away as fast as they are prepared, a reddish-yellow cuprous oxide will settle 

 out, and the series will give a rough comparison as to the quantity of reducing 

 sugar present. 



In the first series the deep blue of the starch reaction quickly changes 

 to a reddish-blue, a red, a reddish-brown, until finally no change in color 

 other than that produced by the mixture of the iodine occurs, showing that 

 the starch has passed the second stage of erythro-dextrin in its disappearance. 

 The indication of reducing sugar in the second series shows that this erythro- 

 dextrin has been transformed into reducing sugar, and also that the amount 

 of sugar is greatly increased during the progress of the test. 



6. The Influence of Temperature on Salivary Digestion. Prepare 

 three test tubes, a, b, c, containing 4 c.c. each of saliva. Boil a, place b in 

 a water bath at 40 C., and place c in ice water. After c has been cooled down 

 to the temperature of the ice bath add to each 2 c.c. of i per cent starch solu- 



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