PHYSIOLOGY OP THE DIGESTIVE GLANDS 431 



THE NORMAL SECRETION OF SALIVA 



The duct fistula can in this case be made either for the submaxillary 

 gland, representing a mucous gland, or for the parotid, representing a 

 serous gland. Under ordinary conditions there is very little secretion 

 from either duct. When secretion occurs, it is, of course, caused by 

 influences acting on a nerve center or centers in the medulla oblongata, 

 the exact location of which for the different glands has* been worked out 

 in recent years by Miller. 9 The impulses acting on these centers may be 

 transmitted along afferent nerves coming from the mucous membrane of 

 the mouth, nares, etc., or by impulses which we may call psychic, trans- 

 mitted from the higher nerve centers. The reflex secretions caused by 

 impulses traveling by the afferent nerve from the mouth, etc., have been 

 called unconditioned, and those from the higher nerve centers, condi- 

 tioned. With regard to the former, there is considerable discrimination 

 in the type of stimulus that will be effective. Thus, if the dog for most 

 of the experiments have been performed on this animal is given meat, 

 a secretion of thick, mucous saliva will be observed to occur (submaxil- 

 lary gland). On the other hand, if the meat is dried and pulverized, 

 the secretion which it calls forth will be very copious and watery (par- 

 otid gland). There is, then, an obvious association between the nature 

 of the secretion and the function it will be called upon to perform when 

 it becomes mixed with the food. The mucous secretion called forth by 

 meat will serve to lubricate the bolus of food and thus facilitate its 

 swallowing, whereas the thin watery secretion produced by the dry 

 powder will have the effect of washing the powder from the mouth. 



It is evident that the mechanical condition of the food partly deter- 

 mines its exciting quality. Mechanical stimulation of the mucosa alone is, 

 however, not an adequate stimulus, for if pebbles are placed in the mouth, 

 little secretion occurs, but if sand is placed in the mouth, secretion immedi- 

 ately becomes copious. The nerve endings also respond to chemical stimuli. 

 Thus, weak acid causes a copious secretion, while alkali has no effect; 

 disagreeable, nauseous substances also excite secretion. The above dif- 

 ferences in the response of the glands according to the mechanical condi- 

 tion of the food has been observed only in the case of the parotid gland, 

 increase in the submaxillary secretion being obtained only when actual 

 foodstuffs are placed in the mouth. 



The investigations that have been made on the conditions of psychic 

 secretion of saliva are still more interesting and important. Their im- 

 portance depends not so much on the information they give us concern- 

 ing the secretion of saliva as such, as on the methods they furnish us for 

 investigating the various conditions that affect the psychic processes 



