56 PHYSIOLOGY FOR DENTAL STUDENTS. 



pillars of the fauces. This marks the beginning of the second 

 stage, the first event of which is that the bolus, by stimulating 

 sensory nerve endings, acts on nerve centers situated in the me- 

 dulla oblongata so as to cause a coordinated series of movements 

 of the muscles of the pharynx and larynx and an inhibition for 

 a moment of the respiratory center (p. 219). The movements 

 alter the shape of the pharynx and of the various openings into 

 it in such a manner as to compel the bolus of food to pass into 

 the oesophagus: (see Fig. 4) thus, (1) the soft palate becomes 

 elevated and the posterior wall of the pharynx bulges forward 

 so as to shut off the posterior nares, (2) the posterior pillars of 

 the fauces approximate so as to shut off the mouth cavity, and 

 (3) in about a tenth of a second after the mylohyoid has con- 

 tracted, the larynx is pulled upwards and forwards under the. 

 root of the tongue, which by being drawn backwards becomes 

 banked up over the laryngeal opening. This pulling up of the 

 larynx brings the opening into it near to the lower half of the 

 dorsal side of the epiglottis, but the upper half of this structure 

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

 past this critical part of its course. (4) To further safeguard 

 any entry of food into the air passages, the laryngeal opening is 

 narrowed by approximation of the true and false vocal cords. 



The force which propels the bolus, so far, is mainly the con- 

 traction 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 oesophagus, 

 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 ini- 

 tiated by afferent stimuli and involving reciprocal action of 

 various groups of muscles : inhibition of the respiratory muscles 

 and of those that constrict the oesophagus, and stimulation of 

 those that elevate the palate, the root of the tongue y and the 

 larynx. It is purely an involuntary process. 



The third stage of deglutition consists in the passage of the 

 swallowed food along the oesophagus. The way in which this is 

 done depends very much on the physical consistence of the food. 



