100 
MANUAL 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 effect 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. 
