Having studied foodstuffs and foods through chemical experimentation, the student is now in a position to study the digestion of foods under the influence of the digestive fluids secreted into mouth, stomach, and intestines.

1. Materials

Bread, fibrin, olive oil, starch paste.

2. Preparation

Chew a piece of rubber or paraffin. The flow of saliva is stimulated; catch the secretion in a beaker; dilute and filter. Label: Salivary Secretion.

3. Experiments And Observations

(1) Subject saliva to the Fehling's test. It will be found that the secretion will not reduce the CuS04 of Fehling's solution.

(2) Subject the starch paste to the same test. The result is negative.

(3) Mix equal volumes of starch paste and salivary extract in a beaker. Keep the mixture at approximately blood temperature for a time. After ten or fifteen minutes subject the mixture to a test with Fehling's solution. If the conditions are normal a copious precipitate of CuO indicates that a change has been wrought in the mixture. The starch has been changed to a reducing sugar by the ptyalin of the saliva.

(4) Put a few crumbs of bread in a test-tube. Starch is an important constituent of bread. Add saliva; keep warm. After ten minutes, test for sugar.

(5) In a similar way proceed to test the action of saliva upon bits of fibrin or shreds of lean meat, a bit of fat, or a drop of oil, noting results.

(6) Boil a few cubic centimeters of saliva; add starch paste; keep warm for ten minutes; test for reducing sugar. What is the verdict ?

(7) To Determine The Course Of Salivary Digestion

Mix salivary secretion with an equal amount of starch paste. Test a portion at once with iodin and another portion at once with Fehling's solution. Test a portion of the mixture every minute with iodin and another portion every minute with Fehling's solution through a period of at least five minutes.

What is the first change noted in the digestion of the starch? How many steps may be made out with the means used and under the conditions existing in the experiment? In what order do the changes occur?