126 HUMAN PHYSIOLOGY 



The opening into the mouth is closed by the posterior part of 

 the tongue being pressed against the hard palate and against the 

 neighboring anterior pillars of the fauces. The nasal cavity is 

 separated from the pharynx by the elevating of the soft palate (by 

 means of the levator palati mollis), by the arching of the posterior 

 walls of the pharynx (by means of the superior constrictors of the 

 pharynx), and by the meeting of the two posterior pillars of the 

 fauces at the median line. The opening into the larynx is closed 

 by the elevation of the larynx by means of the mylohyoid and the 

 geniohyoid and the digastric, to such an extent that it can be 

 covered by the root of the tongue and the epiglottis. 



The first act of deglutition here described may occur with 

 so much force that by means of it the food reaches the 

 stomach. This is especially true for liquids and soft foods. 

 Only solid and dry foods remain in the pharynx or upper 

 part of the esophagus till the second step in deglutition 

 carries them downward. 



The second part of deglutition consists of a peristaltic 

 movement, that is, a constriction of the esophagus beginning 

 at the top and travelling downward. The pharynx first- 

 constricts by means of its constrictors, then the esophagus 

 by means of the constriction of the circular muscles. Thus, 

 the parts of the pharynx and esophagus, successively con- 

 stricted, push the bolus toward the stomach. 



The propagation of the contraction from one part of the. 

 esophagus to another does not take place by the direct conduction 

 in the muscles, but is dependent upon the central nervous system. 

 After the esophagus has been cut across, the wave of contraction 

 is set up in the lower part when it has ceased in the upper 

 segment. 



The innervation of the muscles of the buccal and pharyngeai 

 cavities is brought about by : 



1. The third branch of the trigeminus: masseter, temporal,, 

 internal and external pterygoid, tensor palati mollis, mylohyoid, 

 anterior belly of the digastric. 



2. The facial: muscles of the face, buccinator, posterior belly 

 of the digastric, levator palati mollis, azygos uvulas. 



3. The glossopharyngeal and vagus: stylopharyngeal, con- 

 strictors of pharynx and muscles of esophagus. 



4. Hypoglossus: the tongue-muscles collectively and the genio- 

 hyoid and thyrohyoid. 



The nerve centres which govern the processes of chewing, suck- 



