678 



PHYSIOLOGY 



These muscles, especially the palatoglossi, serve to close the isthmus faucium, 

 thus preventing any return of the food towards the mouth. 



As the food is passing through the upper part of the pharynx it traverses 

 a region common to the respiratory as well as the digestive passages. Its 

 passage through this region is therefore rapid, and is associated with a 

 closure of the two openings of the air passages into the pharynx. The nasal 

 cavity is shut off by a simultaneous contraction of the levator-palati and 

 palato-pharyngeus muscles and azygos uvulae, by which means the soft 

 palate is raised (Fig. 333) and the posterior pillars are proximated to the uvula. 



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FIG. 333. Diagram (after TIGEBSTEDT) to show the position of the soft palate. 

 I, during rest ; II, during the act of swallowing. 



The upper and back wall of the palate is thus formed into a tense sloping 

 roof which guides the bolus down the pharynx. 



More important is the shutting off of the lower air passages. The con- 

 traction of the mylohyoid muscles which initiate deglutition is followed 

 almost immediately (at an interval of 047 sec.) by an elevation of the larynx, 

 and this elevation is accompanied by closure of the glottis as well as of the 

 superior opening of the larynx. The laryngeal opening is bounded in front 

 by the epiglottis, behind by the tips of the arytenoid cartilages, and at the 

 sides by the aryteno-epiglottidean folds. When deglutition takes place the 

 arytenoid cartilages, which normally lie against the posterior wall of the 

 pharynx, are rotated and move inwards and forwards, so that the laryngeal 

 opening assumes the form of a tri-radiate fissure, the vertical limb being short, 

 while the transverse limb is rounded owing to the pulling inwards of the 

 margins of the epiglottis. At the same time both the true and false vocal 

 cords are approximated, while the movement of the dorsum of the tongue 

 backwards enables the closed laryngeal orifice to lie directly under the back 

 part of the tongue. The muscles which are actively involved in this closure 



