MUSCLES OF MASTICATION. 151 



In animals, like the herbivora, that use the lips and tongue 

 extensively in the prehension of food, division of the facial 

 and hypoglossal nerves interferes materially with this func- 

 tion. 



The tongue is a muscular organ which, by virtue of the 

 complex arrangement of its fibres, is capable of a great va- 

 riety of important movements. Reference has already 

 been made to the importance of- these movements in suc- 

 tion. By the action of what are called the extrinsic mus- 

 cles of the tongue, the organ is moved in various directions, 

 while the intrinsic muscles are capable at the same time 

 of producing many changes in its form. For example, by 

 the action of those fibres of the genio-hyo-glossal muscles 

 which are attached to the chin and the posterior part of the 

 tongue, the whole organ is carried forward and may be pro- 

 truded* to a considerable extent. At the same time the 

 whole length of the muscles may act upon the middle line of 

 the tongue, to which they are attached, and depress the cen- 

 tre so as to render it concave from side to side ; or the trans- 

 verse fibres of the tongue may act so as to make it longer 

 and narrower. The tongue is drawn into the mouth by the 

 action of the anterior fibres of the genio-hyo-glossus on either 

 side, and may be still further shortened by the contraction 

 of the stylo-glossus, and the interior fibres of the hyo-glossus 

 its intrinsic and superior longitudinal fibres. The general 

 action of the stylo-glossus, on either side, is to draw down 

 the sides of the tongue and make it convex from side to side. 

 The stylo-glossus and the palato-glossus draw the back of 

 the tongue upward and backward toward the pharynx, and 

 are thus useful in the first processes of deglutition. By the 

 combined and varied actions of these and other muscles, the 

 tongue is made to perform the numerous movements which 

 take place in connection with phonation, suction, mastica- 

 tion, deglutition, etc. 



The varied and complicated movements of the tongue 

 during mastication are not easily described. After solid 



