282 DEGLUTITION. [BOOK n. 



sensations. The motor fibres of the fifth cranial nerve convey 

 motor impulses from the brain to the 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. 



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 stylo- 

 glossus 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 somewhat 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 mano- 

 meter 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 forms 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 

 being also raised so that they come very near to the false vocal 

 cords : the cushion at the base of the epiglottis covers the rima 

 glottidis, while the epiglottis itself is depressed over the larynx. 

 The thyroid cartilage is now, by the action of the laryngeal muscles, 

 suddenly raised up behind the hyoid bone, and thus assists the 

 epiglottis to cover the glottis. This movement of the thyroid can 

 easily be felt on the outside. Thus, both the entrance into the 

 posterior nares and that into the larynx being closed, the impulse 

 given to the bolus by the tongue can have no other effect than to 

 propel it beneath the sloping soft palate, over the incline formed 

 by the root of the tongue and the epiglottis. The palato-glossi or 

 constrictores isthmi faucium, which lie in the anterior pillars of the 

 fauces, by contracting, close the door behind the food which has 

 passed them. 



When the bolus of food is large, it is received by the middle 

 and lower constrictors of the pharynx which, contracting in 

 sequence from above downwards, thrust it into the oesophagus, 

 along which it is driven by a similar series of successive con- 



