ORGANS AND PROCESSES OF DIGESTION 171 



To demonstrate the Teeth. Procure from the dentist a collection of 

 different kinds of teeth, both sound and decayed. 



(a) Examine external surfaces of different kinds of teeth, notinf 



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general shape, cutting or grinding surfaces, etc. Make a drawing of an 

 incisor and also of a molar. 



() After soaking some of the teeth for a couple of days in warm 

 water saw one of them in two lengthwise, and another in two crosswise, 

 and smooth the cut surfaces with fine emery or sand paper. Examine 

 both kinds of sections, noting arrangement and extent of dentine, 

 enamel, and pulp. Make drawings. 



(c) Examine a decayed tooth. Which substance of the tooth 

 appears to decay most readily ? Why is it necessary to cut away a part 

 of the tooth before filling ? 



(<t) Test the effect of acids upon the teeth by leaving a tooth over 

 night in a mixture of one part hydrochloric acid to four parts water, and 

 by leaving a second tooth for a couple of days in strong vinegar. 

 Examine the teeth exposed to the action of acids, noting results. 



To show the Importance of Mastication. Fill two tumblers each 

 half full of water. Into one put a lump of rock salt. Into the other 

 place an equal amount of salt that has been finely pulverized. Which 

 dissolves first and why ? 



To illustrate Acid and Alkaline Reactions. To a tumbler half 

 full of water add a teaspoonful of hydrochloric or other acid, as 

 vinegar. To a second tumbler half full of water add an equal amount 

 of cooking soda. Taste each liquid, noting the sour taste of the acid, 

 and the alkaline taste of the soda. Hold a piece of red litmus paper 

 in the soda solution, noting that it is turned blue. Then hold a piece 

 of blue litmus paper in the acid solution, noting that it is turned 

 red. Add acid to the soda solution, and soda to the acid solution, 

 until the conditions are reversed, testing with the red and blue litmus 

 papers. 



Hold, for a minute or longer, a narrow strip of red litmus paper in 

 the mouth, noting any change in the color of the paper. Repeat, using 

 blue litmus paper. What effect, if any, has the saliva upon the color of 

 the papers ? Has the mouth an acid or an alkaline reaction ? 



To show the Action of Saliva on Starch. i (Optional). Prepare 

 starch paste by mixing half a teaspoonful of starch in half a pint of 

 water and heating the mixture to boiling. Place some of this in a test 

 tube and thin it by adding more water. Then add a small drop of 



