THE DIGESTIVE APPARATUS AND NUTRITION 229 



nerves, the nervous center in the brain sends impulses to 

 the special center in the medulla oblongata from which 

 efferent secretory im- 

 pulses are reflected, 

 and they pass along 

 the fibers of a branch 

 of the seventh cranial 

 nerve, which probably 

 contains fibers from 

 the ninth nerve, with 

 which it communi- 

 cates. The impulse 

 finally reaches the 

 cells of the submaxil- 

 lary and sublingual 

 glands, and a flow of 

 saliva results. Even 

 the thought of food, 

 by stimulating a center in the brain, may produce nervous 

 impulses having the same effect. Nerve branches from 

 the sympathetic system also carry stimulus to these 

 glands. 



For the parotid glands the chief secretory nerve fibers 

 arise in the glossopharyngeal nerves (ninth cranial). 



324. Action of Saliva. A part of the digestion of food 

 takes place in the mouth. Saliva is mixed with the food 

 by mastication, and serves to moisten the mass and lubri- 

 cate it for swallowing. It also causes a chemical action, 

 due to the presence of its active principle, ptyalin, which 

 affects the starch in food, converting it into malt sugar. 

 If one chews slowly a few grains of wheat, he will notice 

 that the paste becomes sweet. This is because some of the 

 starch in the wheat is changed by the saliva into sugar. 



Fig. 115. 



The salivary glands of the 

 right side. 



