SALIVARY SECRETION. 43 



Of the stimuli which cause secretion by acting on sensory nerve 

 endings in the mouth, some influence the parotid, others the sub- 

 maxillary gland, and different stimuli produce different effects. 

 Even for pure mechanical stimulation of the buccal mucosa, a 

 marked degree of discrimination is shown; thus, smooth clean 

 pebbles may be rolled around in the mouth and yet cause no 

 saliva to be secreted, whereas dry sand will immediately cause 

 the parotid to discharge enormous quantities of thin watery 

 juice. Similarly dry bread crumbs invoke copious parotid secre- 

 tion, bread itself having little effect; water, ice, etc., are inert, 

 but if they contain a trace of acid an abundant secretion is in- 

 stantly poured out. It is plain in all these cases that the pur- 

 pose of the secretion is to assist in the removal or neutralization 

 of the substance which is present in the mouth. The thick 

 mucous secretion of the submaxillary and sublingual glands 

 seems to depend more on the chemical nature of the food than on 

 its mechanical state, boiled potatoes, hard boiled eggs, meat, etc., 

 causing the secretion of a thick slimy saliva, which by coating 

 the food assists swallowing. The relish for the food seems to be 

 of little account in influencing the secretion of saliva, for noxious 

 substances, or those that are acid, or very salty, call forth much 

 more secretion than do savory morsels. Although mere mechani- 

 cal stimulation is not in itself an adequate stimulus, yet move- 

 ment of the lower jaw is quite effective, as for example in chew- 

 ing, or when the mouth is kept open, as by a gag in a dental 

 operation. 



The stimulus does not, however, require to be applied to the 

 buccal mucosa itself; it may be psychic or associational, and 

 here again a remarkable discrimination is evident, although the 

 response is not so predictable as when the stimulus is local. 

 Thus, when flry bread or sand is shown to a dog to whom previ- 

 ously these substances have been given by mouth, salivation fol- 

 lows, but this is not the . case when moist bread or pebbles are 

 offered. Appetite plays an important part in this psychic reflex, 

 for when dry food is shown to a fasting animal, salivation is 

 marked, but may cause no secretion when it is offered to a well- 

 fed animal. It is possible in this case, however, that there may 



