DIGESTION. 175 



by the action of the levator palati and tensor palati muscles respec- 

 tively (Fig. 70). This septum is completed by the advance toward 

 the middle line of the posterior half arches caused by the contrac- 

 tion of the muscles which compose them the palato-pharyngei. 

 When these structures are impaired in their functional activity, 

 as in diphtheritic paralysis and ulcerations, there is not infre- 

 quently a regurgitation of food, especially liquids, into the nose. 

 The larynx is equally protected against the entrance of food during 

 deglutition under normal circumstances. That this accident occa- 

 sionally happens, giving rise to severe spasmodic coughing, and even 

 in extreme cases to suffocation, is abundantly shown by the records 

 of clinical medicine. Usually it does not occur, for in addition to 

 other methods there is a complete suspension of the act of inspiration 

 by which particles of food might be drawn into the larynx. At the 

 same time the larynx is drawn well up under the base of the tongue 

 and closed by the descent of the epiglottis. The glottis itself is also 

 closed by the constrictor muscles which surround it. 



Nerve Mechanism of Deglutition. Deglutition is almost ex- 

 clusively a reflex act throughout its entire extent, and requires for 

 its inauguration merely a stimulus to some portion of the mucous 

 membrane of the deglutitory canal. The first stage is primarily 

 voluntary, but from inattention to the process may become second- 

 arily reflex. The origin and course of the afferent nerves, stimu- 

 lation of which excite reflexly the movements of the pharynx and 

 esophagus, however, are practically unknown. In the rabbit deg- 

 lutition can be excited by stimulating the anterior central part of 

 the soft palate. In man it has not yet been possible to locate an 

 area stimulation of which will give rise to a reflex deglutitory act. 

 Though electric stimulation of the superior laryngeal -nerve will cause 

 reflex deglutitory movements, it is obvious that the terminals of this 

 nerve can not be the source of the natural afferent impulses. Stimu- 

 lation of the glossopharyngeal nerve causes an inhibition of the 

 movements. 



The center from which emanate nerve impulses which excite the 

 various muscles to action has been located experimentally in the 

 medulla oblongata just above the alae cinereae. The efferent nerves 

 comprise branches of the facial, hypoglossal, motor filaments of the 

 third division of the fifth nerve, motor filaments of the glossopharyn- 

 geal and vagus derived in all probability from the spinal accessory. 

 Inasmuch as the different mechanisms act not only in a coordinate 

 but sequential manner, it is possible that the deglutition center is not 

 a single circumscribed collection of cells, but a series of centers corre- 

 sponding to the origin of the efferent nerves, the activities of which 

 are coordinated by some single true deglutition center. 



