ACTION OF SALIVA. 205 



walls and surrounds the gland ; it is now called lymph ; from 

 the lymph the gland directly gets its material. 



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. 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 dis- 

 solving 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. 



Besides the salivary glands, there are great numbers of 

 simple glands in the Mucous Membrane lining the mouth. 

 These secrete a glairy substance called Mucus. 



The amount of saliva secreted daily is estimated at three 

 pints. Of course the glands should be allowed to rest be- 

 tween meals. The habit of chewing gum, though supposed 

 to aid digestion, undoubtedly 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. 



" The character of action of salivary ferment is further 

 defined by experiments showing : 1, 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 alka- 



