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A TEXTBOOK OF PHYSIOLOGY 



ascend the gullet in one-third of the time occupied by solids in the 

 normal position of the body. 



The Nervous Mechanism of Swallowing. Swallowing is a reflex 

 act, the nervous centre controlling it being situated in the floor of the 

 fourth ventricle in the spinal bulb. The afferent impulses which 

 provoke the reflex arise in the neighbourhood of the pharynx ; in the 

 dog and cat, chiefly from the posterior wall of the pharynx, 

 opposite the opening from the mouth an area supplied by the glosso- 

 pharyngeal nerve. Impulses also arise from the upper part of the 

 soft palate, supplied by the ninth and the second part of the fifth 

 nerves, and from the base of the epiglottis, supplied by the superior 

 laryngeal division of the tenth nerve. 



.p/n 



Heart 



,/' Miephref-x 



FIG. 202. DIAGRAMS OF POSITION OF SHADOW IN (ESOPHAGUS AT INTERVALS 

 OF A SECOND AFTER SWALLOWING. (Hurst.) 



In monkeys, the swallowing reflex is most easily evoked in the region 

 of the tonsils; in man, from the back wall of the pharynx and round 

 about the base of the tongue. The ability to swallow depends upon 

 the presence of these special sensitive spots, as is shown by the fact 

 that if a sponge moistened with cocaine be swallowed, and then pulled 

 back by means of an attached thread, the power to swallow is lost for 

 a time. Whenever a bolus of food or saliva is made to stimulate 

 one of these sensitive spots, swallowing involuntarily occurs. 



The chief efferent or effector paths are fibres running in the hypo- 

 glossus to the hyoglossus, in the third branch of the fifth to the mylo- 

 hyoid, in the glosso-pharyngeal and the pharyngeal branches of the 

 vagus to the muscles of the palate and pharynx, and in the vagus 

 to the oesophagus itself. Stimulation of these fibres causes strong 

 contraction of the oesophagus; section of both vagus nerves produces 



