no. 3607 KINESIS OF AVIAN SKULL — ZUSI 3 



most commonly stated action of M. depressor mandibulae is to open 

 the lower jaw, but emphasis herein will be placed on the interaction 

 between the postorbital ligament and the depressor mandibulae in 

 protraction (raising) of the upper jaw. Several authors have postu- 

 lated such a protraction effect through the postorbital ligament 

 (Kripp, 1933, pp. 556-559; Starck, 1940, pp. 618-620; Barnikol, 1952, 

 pp. 382-384; Zusi, 1962, p. 47; Bock, 1964, pp. 19-22), explaining its 

 action, with some variations, as follows: the ligament is a virtually 

 unstretchable band running from the cranium to the mandible and 

 attaching anterior to the quadrate articulation, where the ligament 

 provides a fulcrum around which the lower jaw pivots (see fig. 2). 

 The depressor mandibulae rotates that portion of the lower jaw lying 



M depressor 

 mandibulae 



attachment of 

 postorbital ligament 



Figure 2. — Diagram of protraction of the upper jaw through the depressor mandibulae 

 and postorbital ligament. The lower jaw pivots about the attachment of the ligament. 

 As the lower jaw opens, the quadrate is pushed forward and the upper jaw is raised. 



behind the attachment upward and forward, rocking the quadrates 

 forward and thereby protracting the upper jaw. The important point 

 is that the lower jaw pivots about the ligamentary attachment rather 

 than about the quadrate articulation as is commonly stated. Bock 

 (1964) has explored the significance of the postorbital ligament and 

 has presented a number of hypotheses about avian kinesis that will 

 be discussed below. 



The group of structures that support the lower jaw form a closely 

 interacting functional complex that is part of the larger complex of the 

 entire kinetic mechanism. The support system includes the quadrate, 

 the rami of the lower jaw, the jaw-quadrate articulation, and the 

 muscles and ligaments associated with these structures. Most of the 

 jaw muscles have the potential for moving the quadrate either through 



