SEC. 7. THE MUSCULAR MECHANISMS OF DIGESTION. 



267. 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 must i<-;it inn, < 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. This in man consists chiefly of an up and down 

 movement of the lower jaw, combined, 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 mean.- 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 coudyles, 

 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 

 sensations. The motor fibres of the fifth cranial nerve convey 

 motor impulses from the brain to the above-mentioned muscles ; 

 but paralysis of the sensory fibres of the same nerve renders 

 mastication difficult by depriving the will of the aid of the usual 

 sensations. 



268. Deglutition. The food when sufficiently masticated is, 

 by the movements of the tongue, gathered up into a bolus on the 

 middle of the upper surface of that organ. The front of the 

 tongue being raised partly by its intrinsic muscles, and partly by 



