480 



DIGESTION 



ture projects beyond and serves as a ledge to guide the bolus safely past 

 this critical part of its course. (4) As a further safeguard against any 

 entry of food into the air passages, the laryngeal opening is narrowed by 

 approximation of the true and the false vocal cords. 



So far the force which propels the bolus is mainly the contraction of 

 the mylohyoid, assisted by the movements of the root of the tongue. 

 When it has reached the lower end of the pharynx, however, the bolus 

 readily falls into the esophagus, which has become dilated on account 

 of a reflex inhibition of the constrictor muscles of its upper end. This so- 

 called second stage of swallowing is, therefore, a complex coordinated 

 movement initiated by afferent stimuli and involving reciprocal action 



Fig. 153. The changes which take ' place in the position of the root of the tongue, the soft 

 palate, the epiglottis and the larynx during the second stage of swallowing. The thick dotted line 

 indicates the position during swallowing. 



of various groups of muscles: inhibition of the respiratory muscles and 

 of those that constrict the esophagus, and stimulation of those that 

 elevate the palate, the root of the tongue, and the larynx. It is purely 

 an involuntary process. 



The third stage of deglutition consists in the passage of the swallowed 

 food along the esophagus. The mechanism by which this is done de- 

 pends very much on the physical consistence of the food. A solid bolus 

 that more or less fills the esophagus excites a typical peristaltic wave, 

 which is characterized. by a dilatation of the esophagus immediately in 

 front of and a constriction over and behind the bolus. This wave travels 

 down the esophagus in man at such a rate that it reaches the cardiac 

 sphincter in about five or six seconds. On arriving here the cardiac 



