DEGLUTITION. 305 



(1.) Shortening and broadening by the longitudinal muscle, aided by 

 the hyo-glossus. 



(2.) Elongation and narrowing, by the transversus linguse. 



(3.) The dorsum rendered concave by the transversus and the 

 simultaneous action of the median vertical fibres. 



(4.) Arching of the dorsum : (a) transversely by contraction of the 

 lowest transverse bundles ; (b) longitudinally by the action of the lowest 

 longitudinal muscles. 



(5.) Protrusion, by the genio-glossus, while at the same time the 

 tongue usually becomes narrower and longer (2^. 



(6.) Eetraction, by the hyo-glossus and stylo-glossus, and (1) usually 

 occurring at the same time. 



(7.) Depression of the tongue into the floor of the mouth, by the hyo- 

 glossus. The floor of the mouth may be made deeper by simultaneously 

 depressing the hyoid bone. 



(8.) Elevation of the tongue towards the gums : (a) At the tip by 

 the anterior part of the longitudinal fibres ; (6) in the middle by eleva- 

 ting the entire hyoid bone by the mylo-hyoid (N. trigeminus) ; (c) at the 

 root by the stylo-glossus and palato-glossus, as well as indirectly by the 

 stylo-hyoid (N. facialis). 



(9.) Lateral movements, whereby the tip of the tongue passes to the 

 right or the left; these are caused by the contraction of the longitudinal 

 fibres of one side. 



Motor Nerves. The proper motor nerve of the tongue is the hypoglossal. 

 When this nerve is divided or paralysed on one side, the tip of the tongue lying 

 in the floor of the mouth is directed towards the sound side, because the tonus of 

 the non-paralysed longitudinal fibres shortens the sound side slightly. If the 

 tongue be protruded, however, the tip passes towards the paralysed side. This 

 arises from the direction of the genio-glossus (from the middle downwards and 

 outwards), and the tongue follows the direction of its action. The tongues of 

 animals which have been killed exhibit fibrillar contractions of the muscles, some- 

 times lasting for a whole day (Cardanus, 1550). 



156. Deglutition. 



The onward movements of the contents of the digestive canal are 

 effected by a special kind of action whereby the tube or canal 

 contracts upon its contents, and as this contraction proceeds along the 

 tube, the contents are thereby carried along. This is the "peristaltic 

 movement" or peristalsis. 



In the first and most complicated part of the act of deglutition, we 

 distinguish in order the following individual movements : 



(1.) The aperture of the mouth is closed by the orbicularis oris (N. 

 facialis). 



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