DEGLUTITION 79 



ered in its upper part with columnar ciliated and its lower 

 part by pavement epithelium. 



Esophagus. The esophagus runs a course of about nine 

 inches from the end of the pharynx, at a point behind the 

 cricoid cartilage, to the stomach, which it enters a little to the 

 left of the median line. The coats of the esophagus are two, 

 an external muscular and an internal mucous. The external 

 coat has its fibers disposed in two layers, longitudinal 

 and circular. The circular layer is internal. In the upper 

 third of the esophagus the fibers of the muscular coat are all 

 striped, but at the beginning of the middle third they begin to 

 give place to plain fibers, and these latter progressively in- 

 crease, to constitute virtually the whole muscular coat at the 

 diaphragm. The internal mucous coat is lined by squamous 

 epithelium. This is thrown into longitudinal folds except 

 during the passage of substances through the esophagus. 

 The outside fibrous tissue attaches the whole esophagus to 

 the surrounding tissue. 



Mechanism of Deglutition. The first period of degluti- 

 tion is voluntary but automatic, like respiration. The mor- 

 sel of food is forced toward and through the fauces by the 

 tongue, which presses from before backward against the 

 hard palate, with the bolus above it. That the tongue is 

 mainly concerned in this act is shown by inability to swallow 

 when this organ is absent, unless the food is pushed far back 

 into the mouth by the finger or other means. 



The mechanism of the second period is much more com- 

 plex. The food must pass through the pharynx into the 

 esophagus, and must not be allowed to enter any of the other 

 openings communicating with the pharynx. The larynx es- 

 pecially is to be protected. Since the air enters through the 

 posterior nares above the isthmus and must enter through 

 the larynx in front of the esophagus, it follows that the cur- 

 rent of air would cross the current of food if swallowing 

 and respiration took place together. Consequently respira- 

 tion is suspended during deglutition. A.S soon as the food 



