NERVES CONCERNED IN DEGLUTITION. 307 



The closure of the glottis is effected in the following manner: (a.) The 

 whole larynx the lower jaw being fixed is raised upwards and 

 forwards, while at the same time the root of the tongue hangs over it. 

 The hyoid bone is raised forwards and upwards by the genio-hyoid, 

 anterior belly of the digastric and mylo-hyoid; the larynx is approxi- 

 mated close to the hyoid bone (Berengar, 1521) by the thyro-hyoid. 



(&.) When the larynx is raised so that it comes to lie below the over- 

 hanging root of the tongue, the epiglottis is pressed downwards over 

 the entrance to the glottis, and the bolus passes over it, In addition, 

 the epiglottis is pulled down by the special muscular fibres of the 

 reflector epiglottidis (Thiele) and aryepiglotticus. 



Injury to the Epiglottis. Intentional injury of the epiglottis in animals, or 

 its destruction in man may cause fluids to "go the wrong way," i.e., into the 

 glottis, whilst solid food can be swallowed without disturbance. In dogs, at any 

 rate, coloured fluids placed on the root of the tongue have been observed to pass 

 directly into the pharynx without coming into contact with, so as to tinge, the 

 upper surface of the epiglottis (Magendie, Schiff). 



(c.) Lastly, the closure of the glottis by the constrictors of the 

 larynx also prevents the entrance of substances into the larnyx 

 (Czermak). 



In order that the descending bolus may be prevented from carrying 

 the pharynx with it, the stylo-pharyngeus, salpingo-pharyngeus, and 

 baseo-pharyngeus contract upwards when the constrictors act. 



Nerves. Deglutition is voluntary only during the time the bolus is 

 in the mouth. When the food passes through the palatine arch into 

 the gullet the act becomes involuntary, and is, in fact, a well-regulated 

 reflex action. When there is no bolus to be swallowed, voluntary 

 movements of deglutition can be accomplished only within the mouth ; the 

 pharynx only takes up the movement provided a bolus (food or saliva) 

 mechanically excites the reflex act. The sensory nerves which, when 

 mechanically stimulated, excite the involuntary act of deglutition, are, 

 according to Schrceder van der Kolk, the palatine branches of the tri- 

 geminus (from the sphenopalatine ganglion) and the pharyngeal branches 

 of the vagus (Waller, Prevost). The centre for the nerves concerned 

 (for the striped muscles) lies in the superior olives of the medulla 

 oblongata. Swallowing can be carried out when a person is uncon- 

 scious, or after destruction of the cerebrum, cerebellum, and pons. 

 [Even in the deep coma of alcoholism, the tube of a stomach-pump 

 is readily carried into the stomach reflexly, provided the surgeon 

 passes it back into the pharynx to bring it within the action of the 

 constrictors of the pharynx.] The nerves of the pharynx are derived 

 from the pharyngeal plexus, which receives branches from the vagus, 

 glosso-pharyngeal, and sympathetic. 



