SALIVARY GLANDS 7 1 



a partial vacuum in front, and liquids are drawn into the 

 mouth. The mechanism of drinking is the same. 



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 re- 

 mark 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 be- 

 tween the teeth. The muscles which depress the lower jaw 

 are the diagastric, 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 con- 

 traction, from side to side. The tongue is active during mas- 

 tication 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 



