260 



DIGESTION 



ally becomes fatigued, so that the delivery of secretory products exceeds the 

 new formation of specific constituents. 



Under normal circumstances the secretion of saliva is caused by a reflex act 

 induced chiefly from the mouth, and Pawlow has shown that the quantity as 

 well as the quality of saliva in the dog is adapted with extraordinary nicety 

 to the properties of the substances introduced into the mouth. 



Mechanical stimulation of the buccal mucous membrane does not always 

 produce a flow of saliva. If, for example, a handful of pebbles be thrown into 



B 



FIG. 100. Parotid gland of the rabbit as seen in a fresh, unstained preparation, after Langley. 

 A, resting state. B, after injection of a slight quantity of pilocarpine. C, after stimulation 

 of the cervical sympathetic. D, the same, only a stronger effect. 



a dog's mouth, the dog moves them to and fro in his efforts to get rid of them, 

 but no saliva, or at most only a drop or two, is poured out. If on the other hand 

 sand be used instead of pebbles, a copious flow of saliva is set up, because the 

 sand cannot be removed from the mouth without a stream of fluid. Nor is there 

 any discharge of saliva from application of water or snow, but with acid, salty, 

 bitter or caustic substances which require to be diluted or washed out of the 

 mouth, a discharge at once occurs. 



In all these cases the saliva is thin, watery and contains only a trace of 

 mucin. But with all kinds of edible substances a viscid, mucous saliva is 

 secreted such as is necessary to facilitate swallowing the bolus. Besides, the 

 quantity of saliva depends upon the dryness of the food: the drier it is, the 

 more saliva. 



It is unnecessary actually to place the stimulant into the mouth in order 

 to produce a flow of saliva. Sight or smell of it is sufficient, or indeed, as 

 our own experiences prove, imagination even of savory substances will produce 

 the effect. With regard to the quality and quantity of saliva, the same differ- 

 ences are observed as when the stimulus is applied to the mouth cavity : from 

 which we may conclude that a psychical influence of no small value is involved, 

 although this cannot be exercised by direct effort of the will. 



