DEGLUTITION 79 



coid 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 cir- 

 cular. 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 increase, to consti- 

 tute virtually the whole muscular coat at the diaphragm. The 

 internal mucous coat is lined by squamous epithelium, and, 

 except during the passage of substances through the esophagus, 

 is thrown into longitudinal folds. The outside fibrous tissue 

 attaches the whole esophagus to the surrounding tissue. 



Mechanism of Deglutition. The first period of deglutition 

 is voluntary but automatic, like respiration. The morsel 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 be pushed far back into the mouth by the finger 

 or other means. 



The mechanism of the second period is much more complex. 

 The food must pass through the pharynx into the esophagus, 

 and must not be allowed to enter any of the other openings com- 

 municating with the pharynx. The larynx especially is to be 

 protected. Since the air enters through the posterior nares 

 above the isthmus and must enter the larynx in front of the esoph- 

 agus, it follows that the current of air would cross the current 

 of food if swallowing and respiration took place together. Con- 

 sequently respiration is suspended during deglutition. As soon 

 as the food 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 movement of the 



