CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 373 



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 isth- 

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

 is raised by the levator palati, and so brought to touch the poste- 

 rior 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 in- 

 troducing 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 

 tilled 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 

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

 cords ; and 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 contrac- 

 tions, that is to say, by peristaltic action. This comparatively 

 slow descent of the food from the pharynx into the stomach, may 

 be readily seen if animals with long necks such as horses and 

 dogs be watched while swallowing. When however the morsel 

 is not large or when the substance swallowed is liquid, the move- 



