from the sounds, sights, and smells that are 

 regularly associated with eating. 



The Pharynx and Esophagus. Most of 

 man's digestive organs lie in the abdominal 

 cavity, that is, below the diaphragm; and the 

 pharynx and esophagus carry the food 

 quickly through the neck and thorax to the 

 stomach (Fig. 16-11). 



The pharynx is a very complex portion of 

 the digestive tract, which conducts not only 

 food from the mouth to the esophagus, but 

 also air from the nasal passages to the 

 trachea (Fig. 16-11). The pharynx originates 

 immediately behind the palate, where the 

 nasal passages join the digestive tract, and 

 terminates in the upper part of the neck, 

 where the trachea forks off from the food 

 passage (Fig. 16-11). Moreover, the pharynx 

 communicates with the middle ear chamber 

 via the paired Eustachian tubes. The upper 

 part of the trachea, where the wall is re- 

 inforced by a conspicuous housing of carti- 

 lage, is called the larynx, or Adam's apple" 

 — in which lie the vocal cords. The opening 

 that conducts air from the pharynx to the 

 larynx is a narrow channel, the glottis (Fig. 

 16-11), above which lies a valvelike flap, the 

 epiglottis. 



The esophagus, or gullet, is a thick-walled 

 muscular tube that leads straight downward 

 through the neck and thorax, from the 

 pharynx to the stomach. The muscular layers 

 of the esophagus are well developed — being 

 composed of skeletal muscle,, in the upper 

 third of the tube, and of visceral muscle, in 

 the lower two-thirds of the length. 



Swallowing is a complex series of uncon- 

 ditioned reflexes that sweep the bolus of 

 food rapidly from the mouth to the stomach. 

 The tongue initiates swallowing by molding 

 the food mass into a bolus and projecting the 

 bolus from the mouth into the pharynx. 

 This part of the reflex is under voluntary 

 control, but the succeeding events, once 

 started, cannot be stopped at will. 



When a bolus comes in contact with the 

 wall of the pharynx, it initiates a series of 

 reflex movements that (1) closes off the air 



The Digestive System - 303 



passages and (2) propels the bolus into the 

 esophagus. The soft palate is elevated, clos- 

 ing off the nasal passages, and the larynx is 

 raised, bringing the glottis under cover of 

 the epiglottis and posterior part of the 

 tongue. Then, while the tongue is pressed 

 firmly against the roof of the mouth, the 

 whole pharyngeal wall constricts, forcing the 

 bolus into the esophagus. 



In the esophagus the bolus is seized by a 

 wave of muscular movement, which sweeps 

 downward toward the stomach. This wave- 

 like type of movement is called a peristaltic 

 wave, and is a characteristic movement in 

 most parts of the digestive tract. Each typical 

 peristaltic wave represents a slowly moving 

 wave of constriction, involving only a short 

 length of the wall of the digestive tract; but 

 a similar wave of dilatation runs along the 

 tract, immediately in front of the wave of 

 constriction (Fig. 16-18). The constriction 

 represents a contraction of the circular layer 

 of the muscularis, which squeezes the bolus 

 before it; and the dilatation, which chiefly 

 involves the longitudinal muscle, paces along 

 ahead of the bolus, opening the tube at each 

 moment to receive the moving food. In the 

 esophagus the waves are unusually rapid, so 

 that the food takes only about 6 seconds in 

 passing through the esophagus to the stom- 

 ach. Liquids are swallowed even more 

 quickly, because the esophagus tends to re- 

 main dilated while liquids are being swal- 

 lowed, allowing them to flow mainly under 

 gravity. 



The Stomach. The stomach is a thick- 

 walled muscular sac, which plays a very im- 

 portant role in digestion. The muscularis of 

 the stomach contains a layer of diagonal 

 fibers, in addition to the circular and longi- 

 tudinal layers, and the gastric mucosa is 

 pitted with a great number of simple glandu- 

 lar outpocketings, the gastric glands. While 

 the food remains in the stomach it is churned 

 and mixed with the gastric juice, and the 

 stomach does not pass its contents on to the 

 small intestine until the meal has been 

 thoroughly liquefied as a result of many 



