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MANUAL OF PHYSIOLOGY. 



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 backwards and forwards. By these move- 

 ments the bruised food is soon pushed from between the teeth, 

 and passes towards either the tongue or cheek. The morsel is 

 soon replaced between the teeth 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 



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 tubercles of hard dentine. 

 (Beale.) 



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 pressed 

 against the hard palate, and by the successive elevations of the 

 different parts of the dorsum of the tongue the bolus is gradually 

 pushed backwards towards the isthmus of the fauces. The root , 



