THE MECHANISMS OF DIGESTION 481 



sphincter, ordinarily contracted, relaxes for a moment so that the bolus 

 passes into the stomach. In many animals, including man and the cat, 

 the peristaltic wave travels much more rapidly in the upper part of the 

 esophagus than lower down because of differences in the nature of the 

 muscular coat, this being of the striated variety above, and of the non- 

 striated below. The purpose of more rapid movement in the upper part 

 is no doubt that the bolus may be hurried past the regions where, by 

 distending the esophagus, it might interfere with the function of neigh- 

 boring structures, such as the heart. In other animals, as the dog, the 

 muscular fiber is striated all along the esophagus, and the bolus of food 

 correspondingly travels at a uniform, quick rate all the way. It takes 

 only about four seconds for the bolus to reach the stomach in the dog. 



The peristaltic wave of the upper part of the esophagus in the cat and 

 presumably in man, unlike that of the intestines (see page 501), is trans- 

 mitted by the esophageal branches of the vagus nerves. If these are 

 severed, but the muscular coats left intact, the esophagus becomes dilated 

 above the level of the section and contracted below, and no peristaltic 

 wave can pass along it; on the other hand, the muscular coat may be 

 severed (by crushing, etc.) but the peristaltic wave will continue to 

 travel, provided no damage has been done to the nerves. 



In the lower part of the esophagus, however, the wave of peristalsis, 

 like that of the intestines, travels independently of extrinsic nerves. 

 This has been observed in animals in which all of the extrinsic nerves 

 have been cut some time previously. This difference between the upper 

 and the lower portions is associated with the difference in the nature of 

 the muscular fibers above noted (Meltzer). 14 



The propagation of the wave by the nerves in the upper part of the 

 esophagus indicates that the second stage and the first part of the third 

 stage of deglutition must be rehearsed, as it were, in the medullary 

 centers from which arise the nerve fibers to the pharynx and the upper 

 levels of the esophagus. It is thought that the discharges from these 

 local centers are controlled by a higher swallowing center situated in the 

 medulla just above that of respiration, the afferent stimuli to which 

 proceed from the pharynx by the fifth, superior laryngeal, and vagus 

 nerves. The exact location of the sensory areas whose stimulation is 

 most effective in initiating the swallowing reflex varies considerably 

 in different animals. In man it is probably at the entrance to the 

 pharynx; in the dog it is on the posterior wall. A foreign body placed 

 directly in the upper portion of the esophagus of man has been observed 

 to remain stationary until the individual made a swallowing movement. 

 The afferent fibers in the glossopharyngeal nerve exercise a powerful 

 inhibitory influence on the deglutition center as well as on that of respira- 



