764 PHYSIOLOGY 



the pharyngeal branches of the superior laryngeal nerve. We can 

 excite a single act or a whole series of acts of deglutition by electrical 

 stimulation of the central end of the last-named nerve. The efferent 

 fibres which determine the contraction of muscles engaged in the 

 act of deglutition travel by the hypoglossal nerve to the muscles 

 of the tongue, by the fifth to the mylohyoid, by the glossopharyngeal, 

 the vagus, and the spinal accessory nerves to the muscles of the fauces 

 and pharynx. The closure of the larynx is effected by impulses which 

 travel through the superior and inferior laryngeal branches of the 

 vagus. The centre for the act is situated in the medulla oblongata, 



FIG. 333. Tracings of respiratory movements to show effect of stimulating the 



central end of the glossopharyngeal nerve. (MARCKWALD.) 



The point of stimulation is marked with a cross. Note that the stoppage 



may occur at any phase of the respiratory movement. 



and can be considered as consisting of a chain of centres stimulation of 

 one of which involves the firing off of all the others in orderly sequence. 

 Thus, as Meltzer has shown, the propulsion of the contraction down 

 the oesophagus is determined by the intracentral nervous connections, 

 and does not require the integrity of the muscular tube itself. If 

 the cesophageal nerves be divided, the act of deglutition is abolished, 

 the upper part of the oesophagus becoming permanently relaxed, 

 while the lower part, including the cardiac sphincter, enters into 

 a state of tonic contraction. On the other hand, the oesophagus 

 may be ligatured or cut across without interfering with the propulsion 

 of the wave of contraction, started in the pharynx, from one end of 

 the tube to the other. Stimulation"applied to w the mucous surface 

 of the cesophageal tube is without effect. 



There is an important interdependence between the functions 

 of respiration and deglutition. If an inspiratory or expiratory 

 movement were going on during the act of deglutition, food might 



