174 



TEXT-BOOK OF PHYSIOLOGY. 



B 



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



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



f 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 contrac- 

 tion of the esophagus did not partake of the character of ordinary 

 peristalsis. It was found that the esophagus contracted in three 

 distinct segments, corresponding 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 lasting 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 seg- 

 ment, six centimeters in length, 

 contracting from 6 to 7 sec- 

 onds. The beginning and the 

 end of the contraction for each 

 segment occurred simultane- 

 ously throughout its entire ex- 

 tent. If, however, a series of 

 deglutitory acts follow each 

 other in quick succession, 

 there is an inhibition of the 

 peristaltic contractions until 

 after the final swallow. 



Closure of the Posterior 

 Nares and Larynx. Not- 

 withstanding the rise of pressure in the pharynx during the act of 

 swallowing, it is seldom under normal circumstances that any por- 

 tion of the bolus ever finds its way either into the larynx or nasal 

 chambers, for the reason that the openings of these cavities are fully 

 closed by appropriate means. 



At the moment the food passes into the pharynx the posterior 

 nasal openings 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 until it meets 

 the posterior wall of the pharynx, and at the same time is made tense, 



Trachea/ 



FIG. 70. DIAGRAM SHOWING THE MANNER 

 OF CLOSURE OF THE POSTERIOR NARES 

 AND LARYNX DURING DEGLUTITION. 



