204 DIGESTION. 



cannot be sufficiently displaced from side to side, but, with the condyle on one side fixed 

 or moved slightly backward, the other may be brought forward against the articular 

 eminence, producing a movement of rotation. The pterygoid muscles are largely de- 

 veloped in the herbivora, in which the lateral movements of mastication are so important. 

 The above explanation of the lateral movements of the jaw presupposes the possi- 

 bility of movements in an antero-posterior direction. Movements in a forward direction, 

 so as to make the lower teeth project beyond the upper, are effected by the pterygoids, 

 the oblique fibres of the masseter, and the anterior fibres of the temporal. By the 

 combined action of the posterior fibres of the temporal, the digastric, mylo-hyoid, and 

 genio-hyoid, the jaw is brought back to its position. By the same action it may also be 

 drawn back slightly from its normal position while at rest. 



Action of the Tongue, Lips, and Cheeks, in Mastication. Experiments on living 

 animals and phenomena observed in cases of lesions of the nervous system in the human 

 subject have fully demonstrated the importance of the tongue and cheeks in mastication. 

 The following observations of Panizza on the effects of section of both hypoglossal 

 nerves in dogs show the importance of the tongue, both in mastication and deglutition : 

 "After the section of the hypoglossal the movements of the tongue cease immediately, 

 but the general sensibility of that organ and the taste was not less marked. Indeed, if 

 milk, or bread moistened in the liquid, were presented to the dog, he made ineffectual 

 efforts to lap and to masticate, moving the head and the lower jaw ; the tongue, when 

 displaced, remaining in the same position, and even when a bolus of meat or bread was 

 put on its anterior surface, it was found for a long time after in the same place, which 

 proves that section of the hypoglossals destroys not only the movements necessary to 

 mastication, but also those of deglutition." We have lately had occasion to verify most 

 of these observations in a dog in which both sublingual nerves were divided. The experi- 

 ment, however, was made chiefly with reference to the action of the tongue in deglutition. 



Section of the facial nerves is now a common physiological experiment. Opera- 

 tions of this kind and cases of facial palsy, which are not uncommon in the human 

 subject, show that when the cheek is paralyzed the food accumulates between it and the 

 teeth, producing great inconvenience. In animals, like the herbivora, which use the lips 

 and tongue extensively in the prehension of food, division of the facial and hypoglossal 

 nerves interferes materially with this function. 



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

 fibres, is capable of a great variety of important movements. By the action of what are 

 called the extrinsic muscles 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 aro 

 attached to the chin and the posterior part of the tongue, the whole organ is carried for- 

 ward and may be protruded 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 centre so as to render it concave from side to side ; or the transverse 

 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 farther shortened by the contraction of the stylo-glossus and the interior 

 fibres of the hyo-glossus. The general action of the hyo-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 pharnyx, and they are thus useful in the first processes of deglutition. By the com- 

 bined 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 



