THE HI'. AD AND XKCK. 



95 



third of the zygoma. This portion is inserted into the upper half of the outer 

 surface of the ramus. (Fig. 5'S.) Its action is to assist the posterior segment 

 of the temporal muscle in drawing the mandible backward. 



The external pterygoid muscle (Fig. 57) originates inhere un,t is IUKC 

 inserted ' 



The function of this muscle is second to none, hence its bony and cartil- 

 aginous attachments merit special notice. Here, as in the preceding case, there 

 are in reality two muscles. They are, however, described as upper and lower 

 heads of one muscle. 



Tlie it/>/>t-r head arises from the pterygoid ridge on the outer and under surface 

 of the greater ala of the sphenoid bone. Its specific insertion is, according to my 

 experience, confined to the interarticular fibro-cartilage of the temporo-mandibular 

 articulation. The function would seem to be to regulate the movements of this 



External pterygoid 



Internal pterygoid 



flbro-cartilaee 



KIG. 57. THK I'TKRYcoii) Mi'sci.KS. 





cartilage to the sliding of the condyle forward on to the eminentia articularis 

 when the mouth is opened. The only apparent use of the interarticular cartilage 

 is, when the mouth is opened, to furnish a cup concavity for the condyle while 

 this latter is gliding over the convex eminentia articularis. (Fig- 58.) 



A cracking noise on opening the mouth is frequently both a subjective and 

 an objective symptom. This noise is beyond doubt produced by temporary 

 paralysis of this part of the external pterygoid, the condyle overriding the margin 

 of cartilage -that normally should precede the same. The rational treatment 

 would seem to be fixation of the cartilage in the position it occupies when the 

 mouth is open. 



Conclusion : The function of the upper head of the external pterygoid is to 

 make tense and carry forward the interarticular fibro-cartilage. The function 

 of the lower head is to most powerfully draw the jaw forward. 



The Aw; 1 /- head of the external pterygoid muscle arises from the outer surface 



