SECTION VII. 

 PHYSIOLOGY OF DIGESTION AND SECRETION. 



CHAPTER XXXIX. 

 MOVEMENTS OF THE ALIMENTARY CANAL. 



Mastication. Mastication is an entirely voluntary act. The 

 articulation of the mandibles with the skull permits a variety of 

 movements; .the jaw may be raised and lowered, may be projected 

 and retracted, or may be moved from side to side, or various com- 

 binations of these different directions of movement may be effected. 

 The muscles concerned in these movements and their innervation 

 are described as follows: The masseter, temporal, and internal 

 ptery goids raise the jaw; these muscles are innervated through the 

 inferior maxillary division of the trigeminal. The jaw is depressed 

 mainly by the action of the digastric muscle, assisted in some cases 

 by the mylohyoid and the geniohyoid. The two former receive 

 motor fibers from the inferior maxillary division of the fifth cranial, 

 the last from a branch of the hypoglossal. The lateral movements 

 of the jaws are produced by the external pterygoids, when acting 

 separately. Simultaneous contraction of these muscles on both 

 sides causes projection of the lower jaw. In this latter case forcible 

 retraction of the jaw is produced by the contraction of a part of the 

 temporal muscle. The external pterygoids also receive their motor 

 fibers from the fifth cranial nerve, through its inferior maxillary 

 division. The grinding movements commonly used in masticating 

 the food between the molar teeth are produced by a combination of 

 the action of the external pteryogids, the elevators, and perhaps 

 the depressors. At the same time the movements of the tongue 

 and of the muscles of the cheeks and lips serve to keep the food 

 properly placed for the action of the teeth, and to gather it into 

 position for the act of swallowing. 



Deglutition. The act of swallowing is a complicated reflex 

 movement which may be initiated voluntarily, but is, for the most 

 part, completed quite independently of the will. The classical 

 description of the act given by Magendie divides it into three stages, 



i719 



