480 MASTICATION. [BOOK n. 



268. 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 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 

 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. 



269. 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 

 the styloglossus the bolus is thrust back between the tongue and 

 the palate through the anterior pillars of the fauces or isthmus 

 faucium. Immediately before it arrives there, the soft palate is 

 raised by the levator palati, and so brought to touch the posterior 

 wall of the pharynx, which, by the contraction of the upper 

 margin of the superior constrictor of the pharynx, bulges some- 

 what forward. The elevation of the soft palate causes a distinct 

 rise of pressure in the nasal chambers; this can be shewn by 

 introducing a water manometer into one nostril, and closing the 

 other just previous to swallowing. By the contraction of the 

 palato-pharyngeal muscles which lie in the posterior pillars of the 

 fauces, the curved edges of those pillars are made straight, and 

 thus tend to meet in the middle line, the small gap between them 

 being filled up by the uvula. Through these manoeuvres, the 

 entrance into the posterior nares is blocked, while the soft palate 

 is formed into a sloping roof, guiding the bolus down the pharynx. 

 By the contraction of the stylo-pharyngeus and palato-pharyngeus, 

 the funnel-shaped bag of the pharynx is brought up to meet the 

 descending morsel, very much as a glove may be drawn up over 

 the finger. 



Meanwhile in the larynx, as shewn by the laryngoscope, the 

 arytenoid cartilages and vocal cords are approximated, the latter 



