I4 6 THE VITAL PROCESSES 



by the passageway for the air. To keep the food from 

 passing out of its natural channel, the openings into the 

 air passages have to be carefully guarded. This is accom- 

 plished through the soft palate and epiglottis, which are 

 operated somewhat as valves. The muscular coat of the 

 pharynx is made up of a series of overlapping muscles 

 which, by their contractions, draw the sides together and 

 diminish the cavity. The mucous membrane lining the 

 pharynx is smooth, like that of the mouth, being covered 

 with a layer of flat epithelial cells. 



The Esophagus, or gullet, is a tube eight or nine inches 

 long, connecting the pharynx with the stomach. It lies 

 for the most part in the thoracic cavity and consists chiefly 

 of a thick mucous lining surrounded by a heavy coat of 

 muscle. The muscular coat is composed of two layers 

 an inner layer whose fibers encircle the tube and an outer 

 layer whose fibers run lengthwise. 



Steps in Deglutition. The process of deglutition varies with the 

 kind of food. With bulky food it consists of three steps, or stages, 

 as follows': i. By the contraction of the muscles of the cheeks, the 

 food ball, or bolus, is pressed into the center of the mouth and upon 

 the upper surface of the tongue. Then the tongue, by an upward and 

 backward movement, pushes the food under the soft palate and into the 

 pharynx. 



2. As the food passes from the mouth, the pharynx is drawn up to 

 receive it. At the same time the soft palate is pushed upward and 

 backward, closing the opening into the upper, pharynx, while the 

 epiglottis is made to close the opening into the larynx. By this means 

 all communication between the food canal and the air passages is 

 temporarily closed. The upper muscles of the pharynx now contract 

 upon the food, forcing it downward and into the esophagus. 



3. In the esophagus the food is forced along by the successive con- 

 tractions of muscles, starting at the upper end of the tube, until the 

 stomach is reached. 



Swallowing is doubtless aided to some extent by the force of gravity. 



