MUSCLES OF THE JAW. 181 



MUSCLES OF THE JAW. 



The masseter, temporal, and two pterygoid muscles form a group of 

 muscles of mastication, which may be properly considered together. 



The masseter is a thick quadrate muscle, whose fibres are arranged so as to 

 form two portions differing in size and direction. The superficial part arises 

 from the anterior two-thirds of the lower border of the zygomatic arch, chiefly 

 by thick tendons projecting down between the muscular fasciculi, to which 

 they afford an extensive surface of origin : its fibres proceed downwards and 

 a little backwards to be inserted into the lower half of the ramus of the jaw, 

 extending as far as the angle. The fibres of the deep part of the muscle, 

 much shorter than those of the superficial part, and directed downwards 

 and forwards, arise from the posterior third of the lower border and from 

 all the deep surface of the arch, and, becoming united with the superficial 

 part, are inserted into the upper half of the ramus of the jaw, including the 

 coronoid process : only the upper and back part of this portion of the muscle 

 is left uncovered by the superficial portion. 



The external surface of the masseter muscle is covered for the most part only by 

 the skin and fascia ; it is, however, overlapped behind by the parotid gland, and 

 crossed by its duct ; the branches of the facial nerve and the transverse facial artery 

 also rest upon it. Its inner surface is towards the buccinator, from which it is separated 

 by some soft adipose tissue ; it is in intimate contact with the ramus of the jaw, and 

 covers a nerve and vessels which enter it over the sigmoid notch of that bone. 



The temporal muscle (crotaphite Winslow) is fan-shaped, occupies the 

 temporal fossa, and arises from the whole surface of that fossa, with the 

 exception of the anterior or malar wall ; it likewise takes origin from the 

 deep surface of the temporal fascia, which passes down over it to the zygoma, 

 and some of its posterior fibres arising from this fascia are blended with the 

 deep fibres of the masseter muscle. The direction of the anterior fibres is 

 nearly vertical, that of the middle fibres oblique, and that of the posterior 

 fibres at first horizontal. The fasciculi from this extensive origin converge 

 as they descend, and all terminate in a tendon, which, emerging from the 

 interior of the muscle, is implanted into all the inner surface as well as the 

 anterior border of the coronoid process of the lower jaw-bone, as far down 

 as the union of the body and ramus of the jaw, where they are blended with 

 the origin of the buccinator muscle. 



The upper part of the muscle is in contact with the temporal fascia; the lower and 

 anterior part is imbedded in fat continuous with that which lies between the masseter 

 and buccinator muscles; the insertion of the tendon is mainly concealed by the lower 

 jaw. Between the muscle and the temporal fossa are the deep temporal arteries and 

 the temporal nerves, which penetrate its substance. In contact with the deep surface 

 of the muscle near its insertion the buccal nerve descends, and at the posterior border 

 of the insertion the masseteric nerve and artery emerge. 



The internal pterygoid muscle is related to the inner surface of the ramus 

 of the jaw, somewhat in the same manner as the masseter is to the outer. 

 It arises from the pterygoid fossa ; its fibres, tendinous and fleshy, being 

 attached mostly to the inner surface of the external pterygoid plate, and 

 that portion of the tuberosity of the palate-bone which is situated between 

 the pterygoid plates. Thence it is inclined downwards, with a direction 



