SALIVA 123 



maxillary gland on to the outer surface of the cheek in 

 such a manner that the secretion could be collected. He 

 observed that, provided the dog desired food, sensations 

 arising from the presentation to it of food evoked a secre- 

 tion, even though the food did not come into contact with 

 the mouth. Pavlov further showed that stimuh which 

 normally were not connected with salivary activity could, 

 by prolonged association, become effective. If, for instance, 

 the exhibition of food was repeatedly accompanied by the 

 ringing of a bell, after a time ringing the bell alone caused 

 secretion. We can therefore readily understand how in 

 human beings, in whom association of ideas is so much 

 greater than it is in dogs, the range of stimuli may be 

 very wide. Not merely the sight and smell of food, but 

 the sounds and other sensations which we associate with 

 the immediate prospect of gratification will effectively 

 prepare the mouth for the reception of food. 



The first cause of sahvary secretion, then, is the com- 

 bination of two stimuh : one, arising from within, the 

 need for food ; the other, arising from without, the sensation 

 associated with the prospect of gratification. 



But the food having entered the mouth, a fresh path for 

 sensation becomes possible in the nerves of taste. These, 

 too, as has been shown by Pavlov, cause reflex secretion 

 of sahva. 



In the sahvary glands there exist two kinds of cells, 

 differing in their histological appearances and in the 

 secretion which they produce. There are the mucous 

 cells, which secrete a viscid fluid containing mucin, and 

 the serous cells, which secrete an albuminous fluid con- 

 taining the enzyme ptyahn. The mixed secretion is 

 alkaline in reaction. 



The character of the saliva varies with the nature of the 

 sensory stimulus from the mouth. Dry sand, for instance, 

 provokes a profuse thin, meat a scanty thick secretion. 

 It is said, too, that the amount of ptyahn increases with 

 the amount of carbohydrate eaten. 



