DEGLUTITION. 



115 



upper. In this action they are opposed by the digastric and 

 hyoid muscles. One external pterygoid on either side acting 

 alone, advances that side of the lower jaw only, and thereby 

 causes the lower teeth to incline toward the opposite side in a 

 lateral direction. The two muscles acting alternately cause a 

 horizontal motion from side to side. Thus, while the lower teeth 

 are pressed firmly against the upper ones they are at the same 

 time made to glide over them, either from side to side or back- 

 ward and forward. By these movements the bruised food is 

 soon pushed from between the teeth, and passes toward either the 

 tongue or cheek. The morsel is soon replaced between the teeth 



FIG. 49. 



Section through a portion of Dentine next the pulp cavity of a growing tooth. (a) An 

 isolated odontoblast ; (6) Growing part; (c) Odontoblasts; (d) Filaments of protoplasm 

 projecting from the tubercules of hard dentine. (Beale.) 



by the action of the tongue on the one hand and the buccinator 

 muscle in the cheek on the other. 



While the process of mastication is going on, the food becomes 

 thoroughly moistened with the fluid secreted within the mouth. 



Deglutition. The next step is swallowing. When the food is 

 sufficiently triturated and moistened it is collected together by 

 means of the tongue, and placed upon the upper surface of that 

 organ, which becomes concave and presses or rolls the soft pulp 

 against the hard palate so as to shape it into an oblong mass or 

 bolus (Fig. 51). The apex of the tongue is now raised and 



