KINESIS OF AVIAN SKULL — ZUSI 



11 



quadrate 



postorbital 

 ligament 



m depressor 

 mandibulae 



Figure 6. — Diagram of forces produced by the depressor mandibulae in a simplified sup- 

 porting system of the lower jaw. (F=resultant force of depressor mandibulae; V and 

 H = rectangular components of F; H'=H transferred to quadrate articulation; V'=V 



transferred to quadrate articulation and increased in relation V'= ; F'=resultant 



A 



of components V and H'; P and C=rectangu!ar components of F'; A=work arm from 

 fulcrum at attachment of postorbital ligament to quadrate; B = force arm from post- 

 orbital ligament to insertion of depressor mandibulae.) 



a 



b 



Figure 7. — Effect of the angle of the depressor mandibulae upon motion of the upper jaw 

 through the postorbital ligaments: a, protraction force P increased by counterclockwise 

 shift of F (compare with fig. 6); b, retraction component R produced by clockwise shift of 

 F (compare with fig. 6). (Symbols defined in fig. 6.) 



