Digestion — The Mouth 107 



sublingual. Other glands of this character are scat- 

 tered in the cheeks and at the base of the tongue. 

 The anatomj^ and arrangement of these organs are not 

 essential to our subject. We are chiefly interested in 

 the liquid which they secrete. 



The saliva is a transparent and somewhat slimy 

 liquid, and contains generally not less than 99 parts 

 in 100 of water, and one part or less of solid matter. 

 It is alkaline in reaction, because of the presence of 

 compounds of the alkalies. The specific chemical effect 

 exerted by this liquid on the food constituents may 

 be illustrated by subjecting starch to its action. When 

 this is done, the starch gradually disappears as such 

 and is replaced by maltose, the same sugar that we 

 find in barley malt. The chemist has learned that 

 the agent which is active in causing this change is 

 a ferment, to which the name ptyalin has been given, 

 and which is always present in the saliva of man and 

 of some animals. It is classed among the diastatic 

 ferments, because it has an office similar to that of 

 diastase in the germination of seeds; viz., the trans- 

 formation of the starch into a sugar. This change 

 begins in the mouth and probably continues in the 

 stomach until the food becomes so acid that the fer- 

 ment ceases to act, for ptyalin is inactive except in 

 an alkaline medium. There is no reason for supposing 

 that any considerable proportion of the starch of a 

 ration is transformed by the saliva, but this solvent 

 action which continues later in the digestive processes 

 certainly begins in the mouth in the manner described. 



The saliva also moistens the food, which is a most 



