THE MUSCLES OF MASTICATION. 293 



end to the orbital margin in common with the lateral expansions, and serves to support the 

 eyeball in its socket (Sappey, " Traite d'Anatom. descrip.," t. ii ; v. Gerlach, "Beitrage zur 

 norm. Anat. des menschl. Auges.," Leipzig, 1880 ; C. B. Lockwood, Journ. Anat., xx, 1885 ; 

 Merkel, " Handb. d. topog. Anat.," Bd. i, 300.) 



Certain collections of involuntary muscular fibres that are contained in the eyelids and 

 wall of the orbit will be noticed in connection with the anatomy of the eye in Vol. III. 



THE MUSCLES OF MASTICATION. 



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

 mastication, which may be properly considered together. 



The masseteric fascia is a continuation upwards of the deep fascia of the neck 

 over the masseter muscle. It is firmly bound down to the outer surface of the 

 muscle, and is attached above to the zygoma. Farther back the fascia invests 

 closely the parotid gland (parotid fascia), on the posterior and deep surfaces of 

 which a process is also sent upwards ; a strong band of this process, the stylo- 

 maxillary ligament, extending from the angle of the jaw to the styloid process, 

 separates the parotid and submaxillary glands. 



The masseter (fig. 267, 13) is a thick quadrate muscle, the fibres of which 

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

 four-sided in form, arises from the lower border of the zygomatic arch for the 

 anterior two-thirds, chiefly by thick tendinous bundles projecting down between the 

 muscular fasciculi, to which they afford an extensive surface of origin : its fibres 

 proceed downwards and 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 shorter than those of the superficial part, 

 and are directed nearly vertically downwards. They arise from the posterior 

 third of the lower border and from all the deep surface of the zygomatic 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. 



Relations. The external surface of the massster muscle is covered for the most part only 

 by the skin and fascia, together with, in the lower half, the platysma myoides ; it is, however, 

 overlapped behind by the parotid gland, and crossed by its duct ; some branches of the facial 

 nerve and the transverse facial artery also rest upon it. The fore part of its inner surface 

 overhangs the buccinator ; the greater part is in close contact with the ramus of the jaw, and 

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



Buccal fat-pad. Occupying the interval between the fore part of the masseter and the 

 buccinator is a definitely circumscribed mass of fat, which extends outwards around the 

 anterior border of the masseter, and behind is prolonged into the zygomatic fossa between the 

 lower part of the temporal and the pterygoid muscles. It is especially well developed in the 

 infant, and has thence received the inappropriate name of the sucking-pad. 1 In front of this 

 the buccinator is covered by the looser subcutaneous fat. in which the parotid duct and the 

 vessels and nerves of the cheek are embedded. 



The temporal fascia is a dense white shining aponeurosis, which covers the 

 temporal muscle above the zygoma, and gives attachment to some of its fibres of 

 origin. It is attached above to the temporal crest of the frontal bone and to the 

 lower of the two lines on the parietal bone ; while below, it is divided into two 

 layers which are separated by a small quantity of fat, and are attached respectively 

 to the inner and outer surfaces of the zygomatic arch close to its upper border. 

 This dense fascia is separated from the integuments by the layer of thin 

 membrane descending from the epicranial aponeurosis, and by the superior and 



1 Ranke, "Em Saugpolster in der menschlichen Backe," Virchow's Archiv, xcvii, 1884. 



u 2 



