SALIVARY GLANDS . 'JI 



Digestion in the Mouth. 



Mastication. The object of mastication is to grind up the 

 food so that it may be swallowed more easily and the various 

 digestive fluids, particularly the saliva and gastric juice, may 

 have more ready access to its parts. The proper mastication 

 of the food is an important factor in its complete digestion later 

 on. 



Mechanically, mastication is effected by the action of the 

 lower jaw, aided by the tongue, lips and cheeks. This remark 

 presumes of course that the teeth are intact. Lateral and 

 antero-posterior movements of the lower jaw combine with its 

 simple elevation to compress and grind the food between the 

 teeth. The muscles which depress the lower jaw are the digas- 

 tric, mylohyoid, geniohyoid and platysma. Those which elevate 

 it are the temporal, masseter, internal and external pterygoids. 

 The attachments of the external pterygoids are such that by 

 their simultaneous action the mandible can be thrown forward 

 and, by their alternate contraction, from side to side. The 

 tongue is active during mastication in carrying the mass of 

 food to this or that part of the buccal cavity so that it may be 

 ground up completely. It also gives accurate information as to 

 the size (of the mass) and stage of mastication. The cheeks, as 

 is shown in facial palsy, are quite important in keeping the food 

 from between them and the teeth. The lips prevent the escape 

 of liquids from the mouth, in addition to assisting in prehension. 



The Salivary Glands and their Secretion. 



The first of the digestive juices with which the food comes in 

 contact is the saliva which is the mixed secretion of the large 

 salivary glands and the various smaller mucous and serous 

 glands which open into the mouth cavity. The chief salivary 

 glands are three in number on each side of the mouth the 

 parotid, submaxillary and sublingual. Besides these, there 



