104 



DIGESTION 



Impulse 

 From Sense Organs ^ 



Impulse Controlling Size Of Blood 

 Tube And Amount Of Blood Supply 



Fig. 31 — Diagram to illustrate mouth control. 



cated in the diagram. (See Fig. 31.) The results of this 

 arrangement are as follows: The nerves may be stimu- 

 lated in several ways: by the contact of the food in the 

 mouth with the nerve ends, by the odor of the food which 

 stimulates the nerve ends in the nose, or by the stimula- 

 tions of the nerves 



impulse con trolling secretin ,_„„ ^^ ^^^^ ^^^.^ ^^^^ ^^ 



by thought of food. 

 When the nerves 

 are stimulated in 

 any of these three 

 ways the impulse 

 they carry is com- 

 municated to two 

 points. One impulse goes to the blood vessels and causes 

 them to dilate, and thus supply more blood and food. 

 The other impluse is given to the cells, and stimulates them 

 to greater secretive activity. The result of these two 

 directing impulses is an increased flow of saliva. 



In a similar way, this threefold nerve connection also 

 accounts for the dryness of the mouth when certain con- 

 ditions are present. For example, fright, a bad smell, or 

 unpleasant taste, may start impulses along the nerve 

 which, instead of increasing the flow of blood and secre- 

 tion, may constrict the blood vessel and diminish the 

 secretive activity of the cells. 



The saliva. — The secretion produced by the salivary 

 glands is called the saliva. The liquid found in the mouth 

 and constituting the spit is not pure saliva, but a mix- 

 ture of this liquid ^\■ith a secretion of the mouth lining 

 called the mucus, together with a number of other more or 

 less solid elements. This mixture is a colorless, cloudy, 



