i6 4 TEXT-BOOK OF PHYSIOLOGY 



experimenter noticed, while observing the interior of the larynx, both from 

 above, through an opening in the hyothyreoid membrane, and from below, 

 through an opening in the trachea, that when an act of deglutition was excited 

 by touching the soft palate with a sound, there was simultaneously with the 

 contraction of the mylohyoid muscles, a firm closure of the glottis. This 

 was accomplished by an approximation of the true vocal bands, a close 

 approximation and a downward and forward movement of the arytenoid 

 cartilages, until they almost touched the anterior wall of the thyroid carti- 

 lage. This movement preceded the ascent of the larynx. When the larynx 

 was separated from all surrounding structures with the exception of the 

 laryngeal nerves, a touch of the palate excited the same phenomenon. Under 

 such circumstances the closure of the glottis must have been due to the con- 

 traction of its intrinsic muscles and in consequence of a reflex action through 

 the inferior laryngeal nerves. 



The Nerve Mechanism of Deglutition. Deglutition is almost exclu- 

 sively a reflex act throughout its entire extent, and requires for its inaugura- 

 tion merely a stimulus to some portion of the mucous membrane in the 

 anterior part of the deglutitory canal. The first stage is primarily voluntary, 

 but from inattention to the process may become secondarily reflex. The 

 origin and course of the afferent nerves, stimulation of which excite reflexly 

 the movements of the pharynx and esophagus, however, are practically un- 

 known. In the rabbit deglutition 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 degluti- 

 tory movements, it is obvious that the terminals of this nerve cannot be the 

 source of the natural afferent impulses. Stimulation of the glosso-pharyn- 

 geal 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 glosso-pharyngeal and vagus nerves derived in all 

 probability directly from the medulla oblongata. Inasmuch as the different 

 mechanisms of this reflex, act not only in a coordinate but sequential manner, 

 it would appear as if the deglutition center sent out, in response to the nerve 

 impulses coming from a single peripheral area, a series of nerve impulses 

 successively to successive portions of the canal, through the groups of nerve- 

 cells corresponding to the origins of the efferent nerves. . That this orderly and 

 progressive peristalsis usually observed is due to a sequence of changes in the 

 central nerve system is shown by the fact, that if the esophagus is divided or 

 a ring of it excised, the extremity in connection with the stomach will exhibit 

 a well-marked peristalsis after a short interval, when an act of deglutition is 

 excited in the customary manner. The efferent nerve-fibers, which stimulate 

 the esophageal muscles to action are contained in the trunk of the vagi nerves 

 for after their division the peristalsis is abolished. 



In addition to this primary reflex mechanism, the esophagus appears to 

 possess a secondary reflex mechanism consisting of a series of reflex arcs, 

 whose afferent and efferent paths are found in the trunk of the vagus and 

 both connected with successive portions of the esophagus. The first mechan- 



