100 MANUAI. OF NATURE STUDY. 



Again, let the pupil try to swallow when the 

 mouth and throat are perfectly dry. What result? 

 Saliva, then, is necessary to swallowing. 



Let the pupils chew grains of wheat for a few 

 minutes. Note the appearance of a sweetish, sug- 

 ary taste due to the action of the saliva on the 

 starch of the wheat. Note further that starch is 

 insoluble in saliva, and, therefore, has no taste, 

 while sugar is both soluble and sweet. 



The saliva, then, is used to change starch into 

 sugar, and hence in solution to make them more 

 pleasant to the taste and to facilitate digestion. 



Let the pupils now enumerate the four uses of 

 saliva, and state what eflfect the use of tobacco 

 would have upon each use. 



The food has quite a series of processes to go 

 through before it can be of any service to us. 

 After leaving the mouth, it passes over a little 

 draw-bridge that always falls across the opening to 

 the wind-pipe whenever it sees any solid or liquid 

 coming. You know the windpipe is so constructed 

 that nothing but air, or other substance in the 

 form of gas, can pass along its track without caus- 

 ing violent strangling and coughing. Poisonous 

 gases, at least some kinds, will also cause strang- 

 ling, and even death, if introduced into the system 

 in any way, but especially through the windpipe. 



