DEGLUTITION 



281 



of the oesophagus) are excited in a perfectly definite order by impulses 

 coming from the central nervous system. 



The nerves for the upper part of the oesophagus are found in the recurrent 

 laryngeals, for the lower part in the pulmonary and cesophageal plexuses. 



Since the pharynx is in open communication not only with the oesophagus 

 but with the nose and the larynx, the pressure developed in swallowing might 

 easily force the food into the wrong opening. But because of the different 

 protective mechanisms brought into play this does not happen. The return 

 of the bolus into the mouth cavity' is prevented by contraction of the palato- 

 glossal muscles, which approximates the tongue to the palate and narrows the 

 isthmus of the fauces. This is assisted also by the styloglossal muscles, which 

 raise the tongue and press it forward against the soft palate. The nasal 

 cavity is shut off from the mouth and pharynx by elevation of the soft palate. 



i m 



FIG. 112. Sagittal, optical sections of the floor of the mouth and of the neck, reconstructed from 

 X ray photographs, after P. H. Eykman. A, position of quiet inspiration; B, position in 

 swallowing; the passageway for food into the cesophagus is open. The observer is supposed 

 to be looking into the left half of the larynx. In A the epiglottis is shown in the erect posi- 

 tion. In B it is depressed, by the backward motion of the tongue, against the posterior wall 

 of the pharynx (only the edge of the epiglottis is seen). In B the cricoid cartilage is raised to 

 the upper border of the fourth cervical vertebra. 



When the palate muscles are paralyzed, water sometimes passes out the wrong 

 way through the nose. 



The larynx is raised in swallowing both for the purpose of releasing the 

 pressure on the oesophagus so that it may be freely open, and for the purpose 

 of preventing the passage of food into the larynx itself. This movement may 

 be described as follows: 



The geniohyoid, mylohyoid and anterior belly of the digastric muscle draw 

 the hyoid bone and the larynx, the lower jaw being fixed, forward and upward. 

 The hyothyroid muscles serve to keep the larynx close up to the hyoid bone. At 

 the same time the base of the tongue moves downward and backward, a move- 

 ment already described as the effect of the hyoglossal and genioglossal muscles. 

 By this means the cushion of fat which is found immediately over the epiglottis 

 is pressed together from above downward, so that with the epiglottis it is pushed 

 in as far as the bottom of the supraepiglottidean space, and the aryepiglottidean 



