NERVOUS MECHANISM OF DEGLUTITION 315 



deglutition, forces the food into the stomach, the sphincter having previously 

 relaxed. The interval of time between the commencement of the act of degluti- 

 tion and the arrival of the more fluid food at the cardiac orifice of the stomach 

 may not be more than one-tenth second, though it remains at the cardiac 

 orifice without entering the stomach until the first parts of the act of swal- 

 lowing is reinforced by the subsequent contraction of the constrictors of the 

 pharynx and the passage of a peristaltic wave down the esophagus. In 

 some cases, however, the liquid food is not stopped at the cardiac orifice, 

 but is sent through the relaxed sphincter by the original force of the 

 mylo-hyoid contraction. 



In man the esophagus was said to contract in three separate segments, 

 the first segment lying in the neck and being about six centimeters long, the 

 second being the next ten centimeters of the tube, and the third the remaining 

 portion to the stomach. But the later Roentgen-ray observations show no 

 break in the continuous passage of the food, though the movement of the food 

 is slower in the lower segment of the esophagus. 



The act of swallowing consists, then, of the contraction in sequence of 

 the mylo-hyoids, the constrictors of the pharynx, and the esophagus. The 

 computed time of contraction is as follows: 



Seconds. 



Contraction of mylo-hyoids and constrictors of the pharynx 0.3 



Contraction of the first part of the esophagus 0.9 



Contraction of the second part of the esophagus 1.8. 



Contraction of the third part of the esophagus * 3.0 



6.0 



If a second attempt at swallowing be made before the first has been com- 

 pleted (that is, before six seconds have elapsed), the remaining portion of the 

 first act is inhibited, and the contraction wave reaches the stomach six sec- 

 onds after the commencement of the second act. 



During the act of deglutition the posterior nares are closed through the 

 action of the levator palati and tensor palati muscles, which raise the velum; 

 the palato-pharyngei, drawing the posterior pillars of the fauces together; 

 and the azygos uvulae, which raises the uvula thus forming a complete curtain. 

 Otherwise the food would pass into the nose, as happens in the case of cleft 

 palate. At the same time the larynx is closed by the adductor muscles of the 

 vocal cords and the descent of the epiglottis, the larynx being drawn upward 

 as a whole through the action of the mylo-hyoid, genio-hyoid, thyro-hyoid, 

 and digastric muscles. The presence of the epiglottis is not necessary for the 

 completion of the act of deglitition. 



Nervous Mechanism of Deglutition. The sensory nerves engaged 

 in the reflex act of deglutition are branches of the fifth cerebral supplying the 

 soft palate; glosso-pharyngeal, supplying the tongue and pharynx; the superior 

 laryngeal branch of the vagus, supplying the epiglottis and the glottis. The 



