DEGLUTITION. 



311 



pharynx from the posterior nasal fossa, the uvula serving still further 

 to close the chink between their two free borders. An inclined plane 

 is thus formed, down -which the bolus is pressed by the backward move- 

 ment of the tongue. At this stage the elevation of the soft palate 

 may readily be demonstrated by placing a light straw along the floor 

 of the nose, so that its posterior end rests on the soft palate (Landois). 

 If now a motion of swallowing is made, the end which projects from the 

 nose will descend, showing an elevation of the end which rests on the 

 soft palate. At the same time there is a distinct rise of pressure within 

 the nasal chambers. This may be shown by introducing a water ma- 

 nometer into one nostril and closing the other just before swallowing. 

 As the food passes behind the anterior palatine arch it is subjected 

 to the action of the pharyngeal constrictor muscles, which propel it 

 downward. The longitudinal fibres of the pharyngeal constrictors con- 

 tract and cause an elevation (or more strictly shortening) of the walls 





Fis. 132.— Distribution of Fibres of the Palato-Phaevngeai, Muscles. (Luschka.) 



A anterior. B posterior view. 



of the pharynx, together with the elevation of the larynx, this elevation 

 being produced by a contraction of the stylo-pharyngeal and palato- 

 pharyngeal muscles, the lower jaw coming in contact with the upper 

 jaw through the action of the muscles of mastication. The food then 

 passes within the grasp of the upper constrictor of the pharynx, which, 

 contracting, serves to squeeze the bolus of food downward, passage 

 into the nasal chamber being prevented by the mechanism above alluded 

 to, and the bolus being propelled downward by successive contraction 

 of the upper, lower, and middle constrictors of the pharynx until it 

 passes into the oesophagus. The elevation of the larynx occurs when 

 the bolus enters the pharynx, and is clue to the action of the genio-hyoid 

 and mylo-hyoid muscles. It is very perceptible in man, less so in animals 

 in which the larynx is very near or very far from the base of the skull, 



