14 



PROCEEDINGS OF THE NATIONAL MUSEUM 



The large adductor muscles of the jaw, and in particular M. pseudo- 

 temporalis superficialis, which in many species lies in a position 

 comparable to that of the postorbital ligament but medial to it, 

 may serve this function in a passive manner. The pseudotemporalis 

 superficialis, often highly tendinous, might functionally replace the 

 ligament by active contraction as well. There is clearly a great 

 need for experimental approaches to the problems of avian kinesis 

 to test the important specific questions that now can be asked. 



Although the effect of muscle angle on protraction of the upper jaw 

 in the absence of any restraining forces anterior to the jaw articulation 



a 



b 



Figure 8. — Diagram of forces on the lower jaw and quadrate from the depressor mandibulae 

 in the absence of the postorbital ligament: a, with unrestricted rotation of the lower jaw, 

 H serves to retract the quadrate and V serves primarily to rotate the lower jaw (H reaches 

 zero when F is perpendicular to the lower jaw); b, with reduction or loss of rotation at the 

 jaw articulation, H retracts the jaw-quadrate unit and V has no rotational effect (H 

 reaches zero when F passes through the otic articulation of the quadrate). (F= resultant 

 force of depressor mandibulae; H and V= rectangular components of F; curved arrows= 

 motion of quadrates under conditions illustrated.) 



probably seldom has a bearing on jaw motion in birds, I shall discuss 

 it briefly because it has been mentioned in the literature. If the 

 struts and hinges of the model shown in figure 8a were weightless 

 and frictionless, the effect of muscle angle would probably be that 

 described by Bock (1964, p. 17) — that is, protraction or retraction 

 would depend only on the muscle's pull forward or backward from 

 a 90-degree angle (the neutral angle) to the mandible. Were there 

 friction or any other resistance to rotation at the jaw-quadrate hinge, 

 protraction would occur if the line of pull of the muscle passed ahead 

 of the cranial articulation of the quadrate and retraction would occur 

 if it passed behind the articulation (fig. 86). The neutral angle would 



