MUSCLES OF MASTICATION. 14-9 



the contraction of those of the posterior muscles of the neck, 

 which have for their chief function the elevation of the head. 



Action of the Muscles which elevate the Lower Jaw, and 

 move it laterally and antero-joosteriorly. The temporal, 

 masseter, and internal pterygoid muscles are chiefly con- 

 cerned in the simple act of closing the jaws. As this is al- 

 most the only movement of mastication in many of the car- 

 nivora, in this class of animals these muscles are most largely 

 developed. Their anatomy alone gives a sufficiently cleai 

 idea of their mode of action ; and their immense power, even 

 in the human subject, is explained by the number of their 

 fibres, by the attachments of many of these fibres to the 

 strong aponeuroses by which they are covered, and the fact 

 that the distance from their origin to their insertion is very 

 short. 



The attachments of the internal and external pterygoids 

 are such that by their alternate action, on either side, the 

 jaw may be moved laterally, as their points of origin are 

 situated in front and within the temporo-maxillary articu- 

 lation. As was first shown by Ferrein, 1 the articulation of 

 the lower jaw is of such a nature that, in its lateral move- 

 ments, the condyles themselves cannot be sufficiently dis- 

 placed 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 move- 

 ment 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 possibility 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, 



1 FERREIN, Memoire sur les mouvemenls de la MAdioire inferieure. Memoires 

 de fAcad. des Sciences, Paris, 1744, p. 434. 



