PASSAGE OF FOOD FROM MOUTH TO STOMACH 761 



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 of the lower air passages 

 are the external thyro-arytenoid, arytenoid, ary-epiglottidean, and 

 the lateral crico-arytenoid muscles. Since the approximation of the 

 posterior to the anterior boundary of the laryngeal opening is only 

 rendered possible by the elevation of the whole larynx under the 



c 



FIG. 331. Diagram (after TIGERSTEDT) to show the position of the soft palate. 

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



hyoid bone, the act of deglutition cannot be carried out unless the 

 larynx is free to move. 



The two openings from the back of the pharynx into the air 

 passages being thus closed, the bolus is shot rapidly past them into 

 the region of the middle and inferior constrictors of the pharynx. 

 If the bolus be liquid or semi-fluid, the movement of the back part 

 of the tongue may be sufficient to propel the substance past the 

 constrictors through the lax oesophagus to its lower end. It is on 

 this account that, when corrosive fluids are swallowed by accident, 

 we very often find the damage to the oesophagus limited to the three 

 points where it is narrowed and where, therefore, there is a slight 

 hindrance to the onward flow of fluid. If the bolus be large and 

 solid or semi-solid, it is seized in the grasp of the middle constrictors 

 on passing through the upper part of the pharynx, and is thrust 

 by successive contractions of this muscle and of the inferior 



