SEC. 3. THE MUSCULAR MECHANISMS OF DIGESTION. 



From its entrance into the mouth until such remnant of it as is 

 undigested leaves the body, the food is continually subjected to 

 movements having for their object the trituration of the food as in 

 mastication, or its more complete mixture with the digestive juices, 

 or its forward progress through the alimentary canal. These various 

 movements may briefly be considered in detail. 



Mastication. Of this it need only be said that in man it con- 

 sists chiefly of an up and down movement of the lower jaw, com- 

 bined, in the grinding action of the molar teeth, with a certain 

 amount of lateral and fore-and-aft movement. The lower jaw is 

 raised by means of the temporal, masseter, and internal pterygoid 

 muscles. The slighter effort of depression brings into action chiefly 

 the digastric muscle, though the mylohyoid and geniohyoid probably 

 share in the matter. Contraction of the external pterygoids pulls 

 forward the condyles, and thrusts the lower teeth in front of the 

 upper. Contraction of the pterygoids on one side will also throw 

 the teeth on to the opposite side. The lower horizontally placed 

 fibres of the temporal serve to retract the jaw. 



During mastication the food is moved to and fro, and rolled 

 about by the movements of the tongue. These are effected by the 

 muscles of that organ governed by the hypoglossal nerve. 



The act of mastication is a voluntary one, guided, as are so 

 many voluntary acts, not only by muscular sense but also by contact 



