DIGESTIVE ACTION OF SALIVA ON STARCH 379 



4. The Chemical Composition of Saliva. Collect several cubic cen- 

 timeters of saliva as follows: Wash the mouth thoroughly with water, then 

 induce secretion of saliva by chewing a bit of paraffin or a piece of thoroughly 

 washed rubber. The inhalation of ether vapor will often facilitate the reflex 

 secretion. One should avoid strong acids to induce secretion unless their 

 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 sulpho- 

 cyanide. 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 proteins. A white, cloudy precipitate, which disappears on adding 

 ammonia and reappears on adding nitric acid, indicates the presence of 

 chlorides. 



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

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

 be demonstrated. 



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

 dextrin, and dextrose, as preparation for an identification of these products 

 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. Immedi- 

 ately 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. portions of the digest from a dropping pipet and boil- 

 ing. 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- 



