INCOME OF MATTER 93 



heat of the mouth. Mastication is a physical 

 process carried on by the movements of the 

 jaws bearing the teeth. Saliva is a fluid 

 poured forth from three pairs of salivary 

 glands and by numerous small glands in 

 the membrane lining the various parts of the 

 mouth. Chemically, the saliva acts only on 

 carbo-hydrates in the form of starch: it has 

 no chemical action on proteins or fats. The 

 saliva, in addition to being a watery solvent, 

 contains an enzyme called ptyalin, which con- 

 verts starch into dextrose or grape sugar, thus 

 rendering the carbo-hydrate soluble. The 

 saliva also lubricates the mouthful with 

 mucus, and thus facilitates swallowing. 



48. The food is then swallowed (Degluti- 

 tion) and by a muscular mechanism (con- 

 trolled by nerves) it is prevented from escaping 

 by the mouth, or into the nose by the posterior 

 apertures of the nostrils, or into the chink 

 between the true vocal cords which is the 

 entrance into the trachea, or windpipe, and 

 the respiratory passages. It is propelled 

 into the gullet (oesophagus), and carried into 

 the stomach by a series of contractile move- 



