DIGESTION OF STARCH 103 



Cheeks. The cheeks, which are so necessary for keep- 

 ing the food between the teeth, consist chiefly of power- 

 ful muscles. These draw the lower jaw upward, so as 

 to bring the teeth firmly together for biting and chewing. 



Salivary glands. The glands of the mouth secrete 

 a large amount of watery fluid, the saliva, which 

 moistens the food as it is chewed. There are three pairs 

 of salivary glands, one pair of which, the sublingual, 

 lies under the sides of the tongue. Another pair, the 

 submaxillary, lies just beneath the angles of the lower 

 jaw, at each side. The third pair, the parotid, lies in 

 the cheeks just in front of the ears. In mumps, the 

 parotids become swollen. 



Saliva. The saliva is a thin alkaline solution of vari- 

 ous substances, the most important of which is a fer- 

 ment 1 called ptyalin. Ptyalin has the power of chang- 

 ing insoluble starch into a soluble kind of sugar, called 

 maltose, without undergoing any change itself. Since 

 ptyalin, like all other ferments, requires time to do its 

 work, starchy food should not be swallowed without a 

 good deal of chewing. Of all foods, starch alone is thus 

 acted upon by the saliva. The others are merely mois- 

 tened, that chewing and swallowing may be made easy. 



Pharynx. When a mouthful of food has been prop- 

 erly chewed, it is reduced to a thin watery mixture. The 

 tongue and the cheeks then squeeze it into the cavity 

 at the back of the mouth, the pharynx, from which it 



1 A ferment or enzyme is a substance produced by living cells, 

 which causes chemical changes in other substances without itself 

 undergoing change. There are many ferments, each one of which 

 can act only upon a single kind of substance, as ptyalin upon 

 starch, pepsin upon proteids, amylopsin of the pancreatic juice 

 upon starch, and lipase of the pancreatic juice upon fat. Many 

 ferments are also found in the various tissues of the body and 

 assist the cells in the oxidation of food and the development of 

 energy therefrom. 



