26 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



expected to broaden our understanding of structural diversity in the 

 avian skull. 



Summary 



Experiments on the action of M. depressor mandibulae in Galhis 

 domesticus and Hesperiphona vespertina demonstrate that the muscle 

 causes protraction of the upper jaw as well as depression of the 

 mandible. The coordination of both jaws is enhanced by the presence 

 of a postorbital ligament and/or by modifications of the jaw-quadrate 

 articulation. The mechanism of protraction through the postorbital 

 ligament and depressor mandibulae is explained through force analysis 

 and it is shown that the force and amount of protraction vary with 

 muscle angle and length of the retroarticular process of the mandible. 

 Independence of jaw motion may be achieved by a backward shift of 

 the mandible or by spreading of the mandibular rami. The role of the 

 postorbital ligament in the evolution of coordination of the jaws 

 through the jaw articulation is presented. Two general properties 

 of avian skull structure are shown to diversify the manipulative 

 capabilities of the bill in a kinetic skull. 



