VI MASTICATION AND SWALLOWING 235 



almost resemble an epithelium. They are, however, in 

 reality connective-tissue cells, and the layer is merely a 

 slightly modified condition of the stratum of undifferen- 

 tiated connective tissue, which lies at the surface of 

 every dermic structure, and from them long filamentous 

 processes can be traced into the dentinal tubules. 



The muscles of the parts which have been described 

 have such a disijosition that the lower jaw can be de- 

 pressed, so as to open the mouth and separate the teeth ; 

 or raised, in such a manner as to bring the teeth together ; 

 or more obliquely from side to side, so as to cause the 

 face of the gi'inding teeth and the edges of the cutting 

 teeth to slide over one another. And the muscles which 

 perform the elevating and sliding movements are of great 

 strength, and confer a corresponding force upon the 

 grinding and cutting actions of the teeth. 



When solid food is taken into the mouth, it is cut and 

 ground by the teeth, the fragments which ooze out upon 

 the outer side of their crowns being pushed beneath them 

 again by the muscular contractions of the cheeks and 

 lips ; while those which escape on the inner side are 

 thrust back by the tongue, until the whole is thoroughly 

 rubbed down. 



While mastication is proceeding, the salivary glands 

 pour out their secretion in great abundance, and the 

 saliva mixed with the food, which thus becomes inter- 

 penetrated not only with the salivary fluid, but with the 

 air which is entangled in the bubbles of the saliva. 



When the food is sutticiently ground it is collected, 

 enveloped in saliva, into a mass or bolus, which rests 

 upon the back of the tongue, and is carried backwards to 

 the aperture which leads into the pharynx. Thr. ugh this 

 it is thrust, the soft palate being lifted and its pillars being 

 brought together, while the backward movement of the 

 tongue at once propels the mass and causes the epiglottis 

 to incline backwards and downwards over the glottis, 



