ADVANGE13 PHYSIOLOGY 



FIG. 42. SHOWING THE STRUCTURE 

 OP A SALIVARY GLAND 



ed as less compact than in reality, B, 



The salivary glands really receive stimuli from the brain. 

 The taste of food starts a nerve impulse that goes to the brain; 

 there certain nerve centers are excited and from them another 

 impulse passes to the salivary glands, causing them to se- 



crete saliva. This passage of 

 the impulse from the brain 

 to the glands is an uncon- 

 scious one, and we certainly 

 do not secrete saliva by any 

 volition of our own. Such an 

 action is called a reflex action. 

 Saliva is more than 99% 

 water. This moistens the 

 food and makes it easy to 

 swallow. Indeed, this is one 



f the m st important func- 

 tionS of Saliva. One Cannot 



swallow a dry cracker until 

 he has thoroughly wet it with 

 saliva or water. When people are much frightened their sali- 

 vary glands sometimes refuse to secrete and at such times 

 they find it difficult or impossible to swallow. 



While this is a very important function, saliva has also 

 a digestive action on the food. This is due to an enzyme called 

 ptyalin, present only in very minute quantity, but having 

 a powerful effect on starch, which it converts into sugar. 

 This change is absolutely necessary, since starch unchanged 

 cannot be absorbed from the digestive tract and hence cannot 

 be used in the body. The conversion of starch into sugar, as 

 we have already noticed, is brought about by the addition of 

 water to starch. (See page 63.) But this combination will not 

 occur except under the influence of some outside agent such 

 as ptyalin. Sugar is a substance that is easily absorbed 

 through the intestine while starch cannot be absorbed at ail. 

 Hence this change from starch into sugar is true digestion. 



