DIGESTION. 171 



bone; inferiorly it becomes continuous with the esophagus. The 

 anterior wall of the pharynx is imperfect and presents openings which 

 communicate with the nasal chambers, the mouth, and the larynx. 

 The lateral wall on either side presents the opening of the Eustachian 

 tube which leads directly into the cavity of the middle ear. The 

 interior of the pharynx is lined by mucous membrane. The pharynx 

 is partially separated from the mouth by the velum pendulum palati, 

 a muscular structure attached above to the hard palate; its lower 

 edge or border is directed downward and backward and presents in 

 the middle line a conical process, the uvula. On either side the palate 

 presents two curved arches, the anterior and posterior, formed re- 

 spectively by the palato-glossei and palato-pharyngei muscles. The 

 laryngeal orifice or glottis is placed just beneath the base of the 

 tongue. It is triangular in shape, wide in front, narrow behind, and 

 directed downward and backward. It is bounded above by a thin 

 plate of cartilage, the epiglottis, placed just behind the tongue and so 

 arranged that it can easily be depressed and elevated. 



The esophagus, the continuation of the deglutitory canal, extends 

 downward from the lower border of the cricoid cartilage for a dis- 

 tance of from 22 to 25 centimeters, to a point opposite the ninth 

 thoracic vertebra, where it expands into the stomach. Its walls are 

 composed of an internal or mucous and an external or muscular coat, 

 united by areolar tissue. ^The muscular coat consists of an external 

 layer of longitudinal fibers arranged in three bands and of an internal ||/ ' 

 layer composeJ^oT fibers arranged circularly in the upper pa"rT"a*rld ^ 

 obliquely in the lower part of the esopKagus. In the upper third 

 tKenbers areT striated ; in trT6* middle third they are a mixture of both 

 striated and non- striated; in the lower third they are entirely non- 

 striated. 



The deglutitive act may be for convenience divided into three 

 stages, viz. : 



1. The passage of the food from the mouth into the pharynx. 



2. The passage of the food through the pharynx into the esophagus. 



3. The passage of the food through the esophagus into the stomach. 



In the first jtage the bolus of food is placed on the superior surface 

 of the tongue? The mouth is then closed and respiration is momen- 

 tarily suspended. The tip of the tongue is placed against the pos- 

 terior surfaces of the teeth. The tongue, because of its intrinsic 

 musculature, then arches from before backward against the roof of 

 the mouth and pushes the bolus of food through the isthmus of the 

 fauces into the pharynx. This completes the first stage. It is a 

 voluntary effort and accomplished partly by the tongue, though, 

 as shown by Meltzer, mainly by the mylojiyj:>idjnuscles. 



The second and third stages, or theTpassage of the food through 

 the pharynx and esophagus into the stomach, have been attributed 



