THE REGULATION OF SALIVARY SECRETION. 183 



means by which such impulses are made to traverse these 

 nerves. It has been a long-recognized fact that the flow of 

 saliva, which, under ordinary circumstances is not more than 

 sufficient to keep the mouth comfortably moist, is enormously 

 increased as soon as sweet, bitter, or dry substances are taken 

 into the mouth, a piece of rubber is chewed, or the mind 

 dwells for a few seconds upon the enjoyment of some food. 

 That so many and such diverse stimuli are capable of bring- 

 ing about an increased flow of saliva has naturally led to the 

 conclusion that any stimulus is effective in this direction. 

 This is, however, not the case, as can be shown very well on 

 dogs having permanent salivary fistulse, operated on as de- 

 scribed in the first section of this chapter. 



A mechanical stimulus is either unable to bring about a 

 flow of saliva, or at the best the secretion of only a few drops. 



If some pebbles are thrown to a dog from some distance, 

 so that the mechanical stimulus is fairly strong, he catches 

 them, may move them about in his mouth, even swallow a 

 few of them, and yet no saliva flows. Or if ice-water is poured 

 into the mouth, or some snow is thrown in, no saliva flows. 

 But let some sand be thrown into the mouth and the saliva 

 flows in quantities. The same is true of all substances which 

 the dog rejects acids, alkalies, salts, or bitters. There 

 seems, therefore, to be a purposeful element in the secretion 

 of the saliva, for it does not flow when substances which do 

 not need it, such as water, are taken into the mouth, but it 

 does as soon as substances which are to be rejected, or which 

 need to be neutralized, diluted, or washed out of the mouth f 

 are taken. The degree of dryness of the food determines 

 the amount of saliva poured out upon it the drier the food 

 the larger the amount of saliva that is secreted. This pur- 

 poseful element seems still more striking when it is found, 

 is experiment shows, that a thin and watery saliva is always 

 poured out upon substances which are to be rejected, while 

 jipon edible substances a saliva rich in mucin, one, therefore, 

 lubricates the bolus to be swallowed and facilitates 



