MUSCLES OF MASTICATION. 279 



projecting domi between the muscular ftisciculi, to which tliey afford an 

 extensive surface of oritciu : 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 deep part of the muscle, of a tri- 

 angular form, consists of fibres which are much shorter tlian those of 

 the superficial part, and are directed downwards and forwards. They 

 arise from the posterior third of the lower border and from all the deep 

 surface of the zj'goraatic arch, and, becoming united with the superficial 

 part, are inserted into the upper half of the ramus of the jaw, includ- 

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

 of the muscle is left uncovered by the superficial portion. 



Itdations. — The external surface of the masseter muscle is covered for the most 

 part only by the skin ami fascia ; it is, however, overlapped behind by the parotid 

 gland, and crossed by its duct ; the branches of the facial nerve and the trans- 

 verse facial arteiy also rest upon it. Its inner sui-face is towards the buccinator, 

 from which it is separated by soft adipose tissue ; it is in close contact with the 

 ramus of the jaw, and covers a nerve and vessels wliich enter it over the sigmoid 

 notch of that bone. 



The temporal fascia is a dense white shining aponeurotic structure, 

 which covers the temporal muscle above the zygoma, and gives attach- 

 ment to some of its fibres of origin. It is attached superiorly to the 

 posterior border of the malar bone and to the temporal ridge on the 

 frontal, parietal, and temporal bones ; while inferiorly, where it is sepa- 

 rated by a quantity of fat into two layers, it is attached to the zygoma, 

 the superficial layer to the outer surface, and the deep layer to the inner 

 surface of that process. This dense fascia is separated from the integu- 

 tnents by the layer of thin membrane descending from the epicranial 

 aponeurosis, and by the auricular muscles; and from the temporal 

 muscle below, by a layer of fat. 



The temporal muscle is fan-shaped, and arises from the whole 

 surface of the temporal fossa, with the exception of the anterior or 

 malar wall, and from the deep surface of the temporal fascia, as it 

 passes down over it to the zygoma ; some of its posterior fibres arising 

 from this fascia blend with the deep fibres of the masseter muscle. 

 The direction of the anterior fibres is nearly vertical, that of the middle 

 fibres oblicpie, and that of the posterior fibres at first horizontal. The 

 fibres converge as they descend, and all terminate in a tendon, which, 

 emerging from the interior of the muscle, is implanted into the inner 

 surface and 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. 



Eelntinns. — Tire upijer part of the muscle is closely covered by the temporal 

 fascia ; the lower and anterior part is imbedded in fat continuous with that 

 which lies between the masseter and Ijuccinator muscles ; the insertion of the 

 tendon is mainly concealed by the lower jaw. Between the muscle and the bone 

 of the temporal fossa are the deep temporal arteries and neiwes, 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 insei-tion the 

 masseteric nerve and artery emerge. 



The internal pterygoid muscie arises from the pterygoid fossa ; its 

 fibres, tendinous and fieshy, being attached mostly to the inner surfiice 

 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 backwards and outwards, and 



