310 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



nerve-fibres, but the precise relationship between the changes underlying the 

 development of tone and those leading to the formation of an ordinary contrac- 

 tion has not been satisfactorily determined. 



The mode of contraction of the plain muscle in the walls of some of the 

 viscera, especially the intestine and ureter, is so characteristic as to be given 

 the special name of peristalsis. By peristalsis, or vermicular contraction as it 

 is sometimes called, is meant a contraction which, beginning at any point in 

 the wall of a tubular viscus, is propagated along the length of the tube in the 

 form of a wave, each part of the tube as the wave reaches it passing slowly 

 into contraction until the maximum is reached, and then gradually relaxing. 

 In viscera like the intestine, in which two muscular coats are present, the 

 longitudinal and the circular, the peristalsis may involve both layers, either 

 simultaneously or successively, but the striking feature observed when watching 

 the movement is the contraction of the circular coat. The contraction of this 

 coat causes a visible constriction of the tube, which may be followed by the 

 eye as it passes onward. 



MASTICATION. 



Mastication is an entirely voluntary act. The articulation of the mandi- 

 bles 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 combinations 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 pterygoids 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 mylo-hyoid and the genio-hyoid. The 

 two former receive motor-fibres 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 fibres 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 pterygoids, 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 



