SALIVA AND SALIVARY DIGESTION. 145 



dilute solution of ferric chloride. This gives a more or less marked red 

 color from the formation of ferric thiocyanate. A very strong reaction 

 must not be expected. Make a comparative test by adding a like amount 

 of ferric chloride to dilute solutions of potassium thiocyanate. 



The addition of solution of mercuric chloride discharges the color. This 

 test is of value in distinguishing between a thiocyanate and a meconate, 

 which sometimes has value in medico-legal work. 



Test the reaction of saliva with neutral litmus paper. It will be found 

 slightly alkaline. Now add two or three drops of dilute acetic acid and 

 note that a stringy precipitate of mucin separates. Filter off this pre- 

 cipitate and test the filtrate for proteins by boiling with Millon's reagent 

 or by the xanthoproteic reaction. 



Make a thin starch paste, about a gram to 200 cc. of water, and observe 

 that it does not respond to the Fehling sugar test already described. Mix 

 10 cc. of this paste with 5 cc. of the filtered saliva and warm to a tem- 

 perature not above 40 cc. for about 15 minutes. At the end of this time 

 apply the sugar test again. A yellow or red precipitate will appear now, 

 showing that the starch has been converted, in part at least, into sugar. 



The saliva alone fails to reduce the copper solution, as should be shown 

 by trial. 



Pour about 5 cc. of the clear saliva into a test-tube and boil a few 

 minutes; add the starch paste and allow to stand as in the above experi- 

 ment. On testing with the copper solution no sugar will be found, show- 

 ing that heat destroys the activity of the ferment. 



The digesting power of the saliva is destroyed also by the addition of 

 a small amount of strong acid or alkali solution, which the student should 

 prove by experiment. 



Saliva is practically without action on raw starch, as may be shown in 

 this way. Stir a small amount of uncooked potato starch into 5 cc. of 

 saliva, and allow to stand 15 minutes at 35-4O, and filter. Now apply 

 the Fehling test, and note the absence of precipitated copper suboxide. 



THE CONVERSION OF STARCH. 



The action of ptyalin on starch is a complicated one and in 

 all details cannot be satisfactorily described. In many re- 

 spects the digestive behavior of the enzymes of the saliva and 

 of malt is similar to that of weak acid. The complex insolu- 

 ble starch molecule is in some manner broken up and partly 

 soluble bodies result. This change is at first unaccompanied 

 by hydration, but later the normal enzymic reaction of water 

 addition follows and the dextrin bodies first produced become 

 sugars. Malt sugar is formed first, and in the case of acids 

 this gives rise finally to glucose by further conversion. But 

 ii 



