no. 3607 KINESIS OF AVIAN SKULL — ZUSI 19 



through the jaw articulation unfortunately has not been established 

 for any species to my knowledge. Manger Cats-Kuenen (1961, 

 pp. 18, 19) explained the coupling action of the jaw articulation in 

 detail and concluded that independent motion of the two jaws in the 

 hornbill Rhinoplax vigil was impossible because the rami had no 

 lateral flexibility. I believe, however, that the flexibility necessary 

 for independent depression of the mandible exists in Rhinoplax be- 

 cause even in a dried skull it is easy to depress the lower jaw by press- 

 ing on the retroarticular processes. When the kinetic mechanism 

 is immobilized in the dried skull, the rami nevertheless readily spread 

 apart during depression of the mandible. 



It is probable that the conformation of the jaw-quadrate articulation 

 played a role in protraction through the depressor mandibulae in the 

 evening grosbeak (lacking a postorbital ligament) and in the chicken 

 after removal of the postorbital ligament (fig. 4) although the mecha- 

 nism appears to be different in the two species. The condyles of the 

 jaw articulation of the domestic chicken are quite different from those 

 of Balaeniceps (fig. 9), but both species have lateral surfaces providing 

 broad support and medial surfaces providing guidance. In Gallus 

 domesticus the medial condyle of the quadrate slides along a well- 

 defined medial face (corresponding to the trough of Balaeniceps) of 

 the broad, flattened articular surface. The medial faces of the two 

 rami converge anteriorly. Depression of the lower jaw in the chicken, 

 therefore, requires spreading of the rami or anteromedial motion of 

 the quadrates just as in Balaeniceps. The possibility that resistance 

 to lateral spreading of the rami in the chicken is enough to raise the 

 upper jaw is suggested by the occurrence of protraction after removal 

 of the postorbital ligaments. 



The structure of the jaw articulation in the evening grosbeak differs 

 sharply from that of the domestic chicken in that the quadrate 

 possesses three knoblike condyles and in that the articular has a deep 

 depression between its two articular surfaces (fig. 9). There is no 

 well-defined trough of the articular that would serve to guide the 

 motion of the rounded medial condyle of the quadrate; rather, the 

 medial condyle rests on the top of a narrow medial articular surface 

 that slopes downward and terminates just in front of the condyle 

 when the jaws are closed. As the lower jaw is depressed, the medial 

 and posterior condyles of the quadrate slide forward on the inclined 

 surfaces of the articular to occupy the space or depression in front. 

 With contraction of the depressor mandibulae, protraction appears 

 to be caused by pressure of the articulation surfaces of the lower 

 jaw on the quadrate condyles. The quadrate slides forward easily 

 because of the slope of the articular surfaces. 



