278 THE ACT OF SWALLOWING. 



6. Responding to the successive contractions of the superior, middle, 

 and inferior constrictors of the pharynx and the esophageal muscles, 

 the bolus is forced downward. During this time the entrance to the 

 larynx must be kept closed, to prevent food from passing into the 

 trachea. 



7. According to Kronecker and Falk, semisolid foods and fluids in the 

 mouth are forced through the pharynx and the esophagus by vigorous 

 contraction of the muscles closing the mouth, particularly the mylo- 

 hyoid muscles. If the act of swallowing is repeated several times in 

 rapid succession, as in drinking, only the last is followed by movements 

 of contraction in the pharynx and the esophagus, for every act of 

 swallowing in the mouth exerts an inhibitory effect upon the lower 

 portions of the esophagus, through stimulation of the glossopharyngeal 

 nerve. That solid and semisolid articles of food are, however, pushed 

 slowly through the esophagus, by peristalsis alone, has been demonstrated 

 by the Rontgen rays on admixture of bismuth subnitrate with the bolus. 



According to Meltzer and Kronecker, the duration of the act of deglutition in 

 the mouth is 0.3 second. Then the constrictors of the pharynx contract; 0.9 

 second later the superior, 1.8 seconds later the middle, and 3 seconds later the 

 inferior constrictor of the pharynx. The constriction of the cardiac orifice, after 

 the food has passed into the stomach, is th^Jinal movement of the series. 



On auscultation of the stomach two^^fcads are heard during deglutition: (i) 

 the squirting sound, which is due to the^^^Pthat the material swallowed is forced 

 irjgp the stomach, and (2) the squeezing !^md, due to peristalsis occurring at the 

 end of deglutition forcing the contents of the esophagus through the cardia. The 

 latter is a rale and, as such, is dependent on the presence of air in the mass swal- 

 lowed. 



Closure of the larynx is brought about as follows: (a) The lower jaw 

 being fixed, the larynx is drawn upward and forward beneath the root of 

 the tongue, which is arched over it. This is effected by a movement of 

 the hyoid bone forward and upward through the action of the geniohyoid, 

 the anterior belly of the digastric, and the mylohyoid muscles together 

 with an approximation of the larynx to the hyoid bone, through the thyro- 

 hyoid muscle, (b) While the tongue, besides, is drawn somewhat backward 

 by the styloglossus muscles, it presses the epiglottis over the entrance to 

 the larynx, so that food can now slide over it. (c) The epiglottis, 

 further, is pulled down over the entrance to the larynx by the action 

 of the reflector epiglottidis and the aryepiglottic muscle. 



Intentional injuries of the epiglottis in animals, or destruction of the epiglottis 

 in human beings, cause choking readily from the entrance of liquids into the 

 larynx, while solid foods can be swallowed with scarcely any trouble. In dogs, 

 however, colored liquids pass directly from the back of the root of the tongue 

 downward into the pharynx, without necessarily staining the upper surface of 

 the epiglottis, hidden under the overhanging root of the tongue. 



(d) Finally, closure of the glottis by the constrictors of the larynx 

 prevents the entrance of swallowed substances into the larynx. This 

 closure is brought about through reflex influences. 



In order that the pharynx itself shall not be drawn down with the de- 

 scending bolus it is drawn upward by the stylopharyngeal, salpingo- 

 pharyngeal and basopharyngeal muscles during the activity of the 

 pharyngeal constrictors. 



Nervous Supply. The nerves of the pharynx are comprised in the pharyngeal 

 plexus, formed by branches from the pneumogastric, the glossopharyngeal and 

 the sympathetic. The act of deglutition' is voluntary- only in so far as it takes 



