KINESIS OF AVIAN SKULL — ZUSI 



9 



it is impossible to depress the upper jaw beyond the closed position. 

 (5) The elevated upper jaw cannot be lowered without raising the 

 mandible. (6) Only those few groups of modern birds having un- 

 coupled kinesis could have given rise to an akinetic form. 



Although the experiments confirm the existence of coordinated jaw 

 motion through the postorbital ligament, they nevertheless contradict 



Gallus 

 domesticus 



Hesperiphona vesperfina 



Figure 5. — Tracings of jaws from superimposed photographs taken before (stippled) and 

 during contraction of the depressor mandibulae muscles. (Left: normal bird; middle: 

 normal bird; right: postorbital ligaments cut. Drawings do not present maximum opening 

 of either jaw.) 



Bock's assumptions 1, 2, and 3 directly and 4, 5, and 6 by implication. 

 The avian kinetic mechanism should not be regarded as either strictly 

 coupled or uncoupled ; rather, it is likely that many birds are coupled 

 weakly, but probably few are coupled so strongly that other muscles 

 cannot counteract the jaw linkage. Approaches toward complete 

 coupling or complete independence of jaw action would represent 



246-573—67 2 



