MOVEMENTS OF THE TONGUE. 22/ 



numerous dentifrices, such as powdered chalk or charcoal. Astringents such as catechu and 

 areca-nut are sometimes used. Mineral acids attack the teeth, and ought, when taken, to be 

 sucked through a tube. ] 



155. MOVEMENTS OF THE TONGUE. The tongue, being a muscular 

 organ, and extremely mobile, plays an important part in the process of mastica- 

 tion : (1) It keeps the food from passing from between the molar teeth. (2) It 

 forms into a bolus the finely-divided food after it is mixed with saliva. (3) When 

 the tongue is raised, the bolus lying on its dorsum is pushed backwards into the 

 pharynx and oesophagus. 



The course of the fibres is threefold longitudinally, from base to tip ; 

 transversely, the fibres for the most part proceeding outwards from the vertically- 

 placed septum linguae ; vertically, from below upwards. Some of the muscles are 

 confined to the tongue (intrinsic), while others (extrinsic) are attached beyond it 

 to the hyoid bone, lower jaw, the styloid process, and the palate. 



Microscopically, the fibres are transversely striated, with a delicate sarcolemma, and very 

 often they are branched where they are inserted intot he mucous membrane. The muscular 

 bundles cross each other in various directions, and in the interspaces fat-cells and glands occur. 



Changes in form and position : 



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

 glossus. 



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



(3) The dorsum is rendered concave by the transversus and the simultaneous action 

 of the median vertical fibres. 



(4) Arching of the dorsum : (a) Transversely, by the lowest transverse bundles ; 

 (b) longitudinally, by the lowest longitudinal muscles. 



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

 becomes narrower and longer (2). 



(6) Retraction, by the hyo-glossus and stylo-glossus, and (1) usually occurring at 

 the same time. 



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

 mouth may be made deeper by depressing the hyoid bone. 



(8) Elevation of the tongue towards the palate : (a) At the tip by the anterior 

 part of the longitudinal fibres ; (b) in the middle by elevating 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 (i\T. facialis). 



(9) Lateral movements, the tip of the tongue passing to the right or left ; these 

 are caused by the longitudinal fibres of one side. 



Motor Nerves. The 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 out- 

 wards), and the tongue follows the direction of its action. The tongues of animals which have 

 been killed exhibit fibrillar contractions of the muscles, sometimes lasting for a whole day. 

 [Stirling has frequently found nerve-ganglia in the nerves of the tongue]. 



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), c 



(2) The jaws are pressed against each other by the muscles of mastication 

 (N. trigeminus), while at the same time the lower jaw affords a fixed point for the 

 action of the muscles attached to it and the hyoid bone. . 



