160 TEXT-BOOK OF PHYSIOLOGY. 



The immediate cause of the sudden rise of pressure was shown by 

 Meltzer to be the contraction of the mylohyoid muscles. When the nerves 

 going to these muscles were divided in a dog, deglutition was practically 

 abolished. These muscles are probably assisted in their action by the 

 contraction of the hyoglossus muscles as well as the tongue itself. 



It was also demonstrated in these experiments that the contraction of 

 the esophagus did not partake of the character of ordinary peristalsis. It 

 was found that the esophagus contracted in three distinct segments, corre- 

 sponding in all probability to the difference in the character of their muscular 

 fibers. The first segment, about six centimeters in length, was found to 

 begin to contract about 1.2 seconds after the beginning of the first curve and 

 acts for 2 seconds; the second segment, about twelve centimeters in length, 

 beginning to contract about 1.8 seconds or 3 seconds after the beginning of 

 the first section, and lasting for from 5 to 7 seconds; the third segment, 

 six centimeters in length, contracting from 6 to 7 seconds. The beginning 

 and the end of the contraction for each segment occurred simultaneously 

 throughout its entire extent. If, however, a series of deglutitory acts follow 

 each other in quick succession, there is an inhibition of the peristaltic con- 

 tractions until after the final swallow. 



An examination of the action of the esophagus during deglutition, made 

 by Cannon and Moser with x-rays and the fluoroscope, disclosed the fact 

 that the method of food transmission varied in different animals. In the 

 cat and dog the transmission was effected by peristalsis alone. The time 

 required for the food to reach the stomach varied in the cat from nine to 

 twelve seconds and in the dog from four to five seconds. The descent of 

 the bolus was more rapid in the upper than in the lower part of the esophagus. 

 In man, liquids descended rapidly, at the rate of several feet a second, in 

 consequence of the rapid and energetic contraction of the mylohyoid muscles. 

 A peristaltic contraction, passing over the entire esophagus, was necessary 

 to the passage of solid and semisolid food through it. 



Closure of the Posterior Nares and Larynx. Because of the rapid 

 rise of pressure in the deglutitory canal during the act of swallowing it is 

 essential that the openings into the nasal and laryngeal cavities be closed to 

 prevent the entrance of food into them, which would otherwise take place. 

 Under normal circumstances this is done so effectually that it is seldom that 

 any portion of the food, liquid or solid, ever enters the nasal chambers 

 or the cavity of the larynx. The mechanism by which these openings are 

 closed is as follows: 



At the moment the food passes into the pharynx the posterior nasal open- 

 ings are closed against the entrance of the food by a septum formed by the 

 pendulous veil of the palate and the posterior half arches. The palate is 

 drawn upward and backward by the levator palati muscles, until it meets 

 the posterior wall of the pharynx, which at this moment advances. At the 

 same time it is made tense, by the action of the tensor palati muscles. (Fig. 

 70). This septum is completed by the advance toward the middle line of the 

 posterior half arches caused by the contraction of the muscles, the palato- 

 pharyngei, which compose them. When these structures are impaired in 

 their functional activity, as in diphtheritic paralysis and ulcerations, there 

 is not infrequently a regurgitation of food, especially liquids, into the nose. 



