CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 453 



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 

 1'auces, 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 

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

 oords ; 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 con- 

 tractions which we shall speak of immediately as 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 movement of the back part of the tongue 

 may be sufficient not merely to introduce the food into the grasp of 

 the constrictors of the pharynx, but even to propel it rapidly, to 



