8O THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



has passed the fauces, the elevators of the hyoid raise that 

 bone, and with it the larynx. It is at the same time pulled 

 a little forward, and since the pharynx is attached to the 

 larynx posteriorly, the former necessarily follows the move- 

 ment of the latter, and is thus slipped under the base of the 

 tongue and the entering bolus. With elevation of the larynx 

 the superior constrictor of the pharynx contracts upon the 

 food, and passes it quickly to the grasp of the middle con- 

 strictor, which in turn hands it to the inferior constrictor and 

 thence to the esophagus. 



The posterior nares are protected by contraction of the 

 posterior pillars and the superior constrictor. The laryngeal 

 opening is protected by the epiglottis. When the tongue is 

 forced back and the larynx raised the natural effect would 

 be to fold the epiglottis down over the laryngeal opening. 

 At the same time contraction of the pharyngeal muscles 

 draws together the sides of the larynx and aids in closing 

 the glottis. Furthermore, the vocal cords fall together (as 

 they always lie except during inspiration and inspiration is 

 now suspended). 



The third period passes the food through the esophagus 

 into the stomach by contraction from above downward of 

 successive portions of its muscular wall. Contraction of the 

 longitudinal fibers draws the mucous membrane above the 

 bolus. Then the circular fibers, contracting in successive 

 segments from above downward, force the bolus before 

 them. These movements are continued until the food 

 reaches the stomach. The time consumed in swallowing a 

 given article is about six seconds. 



This is the mechanism which carries all materials through 

 the alimentary canal from the esophagus to the anus. It is 

 called peristalsis, or vermicular (worm-like) action. 



Nervous Control. While nearly all the muscular tissue 

 concerned in deglutition is of the striated variety, the whole 

 process, except the first, which is automatic, must be consid- 

 ered as reflex. The mechanism of deglutition is one of the 



