332 



HUMAN PHYSIOLOGY 



. TIE7H 



reached the act becomes involuntary and reflex, and the 

 food is forced through the pillars of the fauces into the 

 pharynx. It is prevented from passing up into the posterior 

 nares by the contraction of the palato-pharyngeus muscle, 

 and of the levator palati. The larynx as a whole is pulled 

 upwards by the stylo-hyoid and stylo-thyroid and the thyro- 



hyoid, and its entrance 

 into the larynx is pre- 

 vented by the closure of 

 upper part of aperture. 

 The arytenoid carti- 

 lages are pulled forward 

 by the thyro-arytenoid 

 muscles, and approxi- 

 mated by the aryte- 

 noidei, while a cushion 

 on the posterior surface 

 of the epiglottis be- 

 comes applied to their 

 tips, forming a tri- 

 radiate fissure or chink 

 through which food 

 cannot pass. The lateral 

 crico - arytenoids also 

 approximate the vocal 

 cords, and close the 

 glottis (Fig. 147). 



The constrictors of 

 the pharynx contract 

 from above downwards, 

 and force the food into the grasp of the oesophagus, and this 

 by a slow peristaltic contraction sends the food onwards to 

 the stomach. Under normal conditions this peristalsis is not 

 essential. It is abolished by section of the vagi. In swallow- 

 ing liquids, the peristalsis of the oesophagus is not brought 

 into play, but the fluid is forced by the tongue down the 

 relaxed oesophagus into the stomach. The passage of the 

 food down the gullet may be heard by applying a stetho- 

 scope to the right side of the spinal column, and any delay 

 caused by a stricture may thus be determined. 



FIG. 147. Vertical Mesial Section of Mouth and 

 Pharynx to show how, in swallowing, the food 

 slips along the back of the Epiglottis. 



