168 PHYSIOLOGY. 



Nerve Control of Salivary Glands. The glands are 

 doubly dependent on nerve control : 



1. Through the control of the arterial muscles by the 

 nerves the amount of blood sent to the glands is regulated. 



2. Nerves also go to the cells of the gland to control 

 their activity. When we taste, smell, see, or even when 

 we think of, some delicious food the mouth may " water," 

 as we say, i.e. the salivary glands are, by reflex action, 

 stimulated to activity ; on the other hand, some emotions, 

 such as fear, check the flow of saliva. 



Saliva and its Uses. The saliva is mostly water, and, 

 when we are not eating, serves to keep the mouth moist. 

 The water of the saliva soaks the food during mastication 

 and helps the process of grinding ; it enables us to taste 

 by dissolving any food that is soluble; it further enables 

 us to swallow what would otherwise be a dry powder. 

 The special .element of the saliva, ptyalin, has the power 

 of changing starch to sugar. 



Amount of Saliva. The amount of saliva secreted 

 daily is estimated at three pints. Of course the glands 

 should be allowed to rest between meals. The habit of 

 chewing gum, though supposed to aid digestion, undoubt- 

 edly does far more harm than good. During the resting 

 period the glands accumulate material for the active work 

 of secretion, for there is no sac in which to store the 

 saliva, and it must be made as fast as it is needed. 



Character of Salivary Ferment. "The character of 

 action of salivary ferment is further defined by experi- 

 ments showing: I, that it is destroyed by boiling; 2, that 

 its action is delayed or suspended at a low temperature, 

 most pronounced at about body temperature (37 C); 

 3, that it acts best in a neutral or in a faintly alkaline 



