PASSAGE OF FOOD FKOM MOUTH TO STOMACH 727 



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, 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 contrac- 

 tion, started in the pharynx, from one end of the tube to the other. 

 Stimulation applied to the mucous surface of the cesophageal tube is 

 without effect. 



There is an important interdependence between the functions of respira- 

 tion and deglutition. If an inspiratory or expiratory movement were 

 going on during the act of deglutition, food might be drawn into the lungs 

 or driven into the nasal cavities. Such an accident is prevented by the 

 fact that every act of swallowing inhibits a respiratory movement. This 

 inhibition is effected reflexly through the glossopharyngeal nerve. Stimula- 

 tion of the central end of this nerve at once causes cessation of respiration 

 in whatever phase it may happen to be (Fig. 347). This cessation lasts for 

 five or six seconds, i. e. a sufficient length of time for a whole series of acts 

 of deglutition. Respiration then recommences, and the inhibition cannot be 

 prolonged by continuing the stimulation of the glossopharyngeal nerve. This 

 inhibition of the activity of the respiratory centre can be shown on oneself. 

 If the breath be held until the feeling of dyspnoea, i. e. the need to breathe, 

 becomes insistent, relief is at once experienced by swallowing, and the feeling 

 of relief will last for three or four seconds. 



