8o THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



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

 entering bolus. With elevation of the larynx the superior con- 

 strictor of the pharynx contracts upon the food, and passes it 

 quickly to the grasp of the middle constrictor, which in turn 

 hands it to the inferior constrictor and thence to the esophagus. 



The posterior nares are protected by contraction of the pos- 

 terior 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 to- 

 gether 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 con- 

 sumed 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 con- 

 cerned in deglutition is of the striated variety, the whole proc- 

 ess, except the first, which is automatic, must be considered as 

 reflex. The mechanism of deglutition is one of the best ex- 

 amples of finely coordinated muscular action to be found. The 

 afferent fibers concerned are from the 5th, pth, and loth, and the 

 superior laryngeal branch of the last. The efferent fibers are 

 from the 5th, yth, pth, loth, and i2th. The center for the re- 

 flex is supposed to be far forward in the medulla. 



