JAW MUSCLES 23 



powerful influence upon the bones to which they are attached 

 are seen not only in passing from genus to genus, but in tracing 

 the life-history of the individual from youth to old age. 



Masseter lateralis. This muscle consists of two distinct 

 portions ; one, anterior and superficial, partly concealing the other, 

 posterior and deep . 



The anterior portion arises by a strong tendon from the lower 

 border of the maxilla just in front of and below the mouth of the 

 infraorbital canal. Becoming fleshy and wider, its fibres pass 

 backwards obliquely to their insertion which occupies the thick- 

 ened lower border of the angular process of the mandible. Along 

 the inner edge of that border the fibres meet the ends of those 

 of the ^pterygoid internus muscle. Owing to the oblique, almost 

 horizontal, course of its fibres this portion of the masseter is that 

 principally concerned in drawing the jaw forwards when the 

 animal is gnawing. 



The posterior portion rises by fleshy fibres from the whole 

 outer surface of the outer wall of the infraorbital canal and from 

 the lower border of the zygomatic arch throughout its entire 

 length. The muscle is inserted into the whole length of the 

 masseteric crest of the mandible from a point below m^ to the 

 end of the angular process behind. The anterior fibres become 

 tendinous towards their insertion, and they have a more nearly 

 vertical direction. The fibres become more and more oblique 

 posteriorly, the hindermost winding round the bases of the angular 

 and condylar processes to be inserted upon the inner surfaces of 

 the upper part of the angular process and the hinder base of the 

 condylar process. This portion of the masseter is very powerful, 

 drawing the jaw forwards and upwards. 



Masseter medialis. This is a comparatively feeble muscle 

 arising by fleshy fibres from the inner surface of the zygomatic 

 arch, the inner surface of the outer wall of the infraorbital canal, 

 and by a small slip, which, passing through the infraorbital canal, 

 arises from the side of the maxilla in the prezygomatic fossa. The 

 anterior fibres, from the infraorbital canal and floor of the orbit, 

 pass downwards and backwards and are inserted by tendon into 

 the fore-part of the masseteric crest under cover of the posterior 

 portion of the masseter lateralis. The fibres from the zygomatic 

 arch pass vertically downwards to their insertion by tendon into 

 a groove which rises from the crista masseterica below and ascends 

 parallel with the front border of the coronoid process to a point 

 a little below the base of the sigmoid notch ; thence the line of 

 insertion passes backwards to the alveolar protuberance of the 

 lower incisor and finally ascends the condylar process to a point 

 a little below the head. The small posterior portion of the muscle 

 arising from the squamosal root of the zygoma and inserted into 

 the fine around the sigmoid notch, is usually separable from the 

 chief or anterior portion, a branch of the facial nerve passing out 

 between them. The masseter medialis assists the temporalis in 



