DIGESTION 307 



receptor spots of less importance also exist, from which 

 swallowing may be elicited when food gets lodged upon 

 them. 



(2) The excito-reflex nerves are the fifth, via the spheno- 

 palatine ganglion, and the vagus. The glossopharyngeal also 

 may contain fibres wliich act ; but section of this nerve 

 causes a sustainetl tonic contraction of the gullet, and stimu- 

 lation of its central end inhibits swallowing. It probably 

 acts chiefly to prevent a second act of swallowing occurring 

 while one is already in progress. 



(3) The centre is situated in the medulla oblongata, and 

 swallowing is readily induced in a decerebrated cat by pressing 

 a bolus of cotton wool through the pillars of the fauces upon 

 the receptor points in the pharynx. 



(4) The effector mechanism leads to the following changes : 

 — (a) The hyoid and larynx are pulled upwards by the muscles 

 which act upon these structures, 



(6) The upper orifice of the larynx is closed by the action 

 of the lateral crico-arvtenoidei, the arvtenoidei, and the thyreo- 

 arytenoidei, which pull forward the arytenoids against the 

 posterior surface of the epiglottis. The whole larynx is 

 pulled forwards as well as upwards, and thus the upper 

 part of the oesophagus is opened, and the food slides over the 

 back of the epiglottis and down the posterior aspect of the 

 larynx into the gidlet. 



(c) The passage of food into the naso-pharynx is pre- 

 vented by the contraction of the glossopharyngeus, palato- 

 pharyngeas, and the levator and tensor palati muscles, 

 which approximate the soft palate and the back wall of the 

 pharynx. 



((/) The constrictors of the pharynx contract from above 

 downwards and force the bolus on into the true oesophagus, 

 which may be said to begin at the level of the cricoid 

 cartilage. 



(5) The outgoing nerves involved are the hypoglossal, the 

 third branch of the fifth, the glossopharyngeal to the stylo- 

 pharj-ngeus and middle constrictor, and the vagus which 

 supplies both the pharynx and the oesophagus. 



Section of the vagi nerves paralyses the upper part of the 



