154 THE HUMAfl BODY. 



C 12 H ao 10 + 2H 2 = 2C 6 H 13 O fl 



Starch. Water. Grape Sugar. 



The Influence of Saliva in Promoting Digestion in the 

 Stomach. So far as chemical changes are concerned the 

 saliva is but of secondary importance in digestion: its main 

 use is to facilitate swallowing. It only changes starch into 

 grape sugar (at least rapidly) when no acid is present, and 

 food passes from the mouth to the stomach where it is 

 mixed with the acid gastric juice, before the saliva has time 

 to do much. Indirectly, however, the saliva promotes di- 

 gestion in the stomach. Weak alkalies stimulate the gastric 

 glands to pour forth more abundant secretion,* and the 

 saliva, being alkaline, acts in this way. This is one reason 

 why food should be well chewed before being swallowed; 

 its taste, and the movements of the jaws, excite a more 

 abundant salivary secretion, and this alkaline saliva, when 

 swallowed, helps to stir the stomach up to work. 



Swallowing or Deglutition. A mouthful of solid food 

 is broken up by the teeth and rolled about the mouth by 

 the tongue until it is thoroughly mixed with saliva and 

 made into a soft pasty mass. The muscles of the cheeks 

 keep this from getting between them and the gums.f The 

 mass is finally sent on from the mouth to the stomach by 



What is the chief use of saliva? Under what circumstances does 

 it change starch into sugar? In what portion of the digestive tract 

 is this action of the saliva stopped? Why? How does the saliva 

 promote digestion in the stomach? Why should food be thoroughly 

 chewed before swallowing? 



What is the technical term for swallowing? In how many stages 

 does swallowing occur? 



* Hence the efficacy of a little carbonate of soda or apollinaris water taken 

 before meals, in some forms of dyspepsia. 



t Persons with facial paralysis have from time to time to press out with the 

 finger food which has collected outside the gums, where it can neither be chewed 

 nor swallowed. 



