108 



DIGESTION 



gullet. The glottis opening is protected by a sort of 

 trapdoor called the epiglottis, which is closed during 

 swallowing but open at other times to permit the entrance 

 and exit of air in breathing. The whole cavity is 

 lined with mucous membrane, whose secretion of mucus 

 moistens and makes flexible its walls. (Excess of this 

 mucus produces catarrh.) When the food is swallowed 

 the epiglottis is closed and the food falls upon the top 

 of the gullet. Sometimes the epiglottis does not close 

 quickly enough, and some food enters the air passage. 



This produces choking, 

 =-S.M. 



that is to say, the for- 

 cible expiration of air 

 in the attempt to force 

 out the misplaced 

 food. If properly swal- 

 lowed, the food is 

 forced along the gullet 

 to the stomach. 



The esophagus. — 

 The esophag-us, or gul- 

 let, is the name given 

 to that part of the alimentary tract which begins at 

 the lower end of the pharynx cavity, and passing through 

 chest and diaphragm, enters the stomach. This portion 

 is in the form of a tube of about nine inches in length 

 and is formed by three layers of tissue. (See Fig. 33 

 and Ex. XLI.) The inside layer is a continuation of 

 the mucous membrane, and the secretion of mucus here 

 together with the saliva acts as a lubricator, making the 

 food slip down easily. The outer layer is formed of 

 muscles, while between this and the inside layer is a layer 



L.M. 



"^C.M. 



~-M.M. 



Fig. 33 — Diagrammatic section of the -wall 

 ' of the esophagus ; S. M., serous membrane ; 

 L. M., longitudinal muscles ; CI M.^ circu- 

 lar muscles ; M. M., mucous membrane. Be- 

 tween the circular muscles and the mucous 

 membrane is a thin layer of connective 

 tissue. 



