>o. 3607 KINESIS OF AVIAN SKULL — ZUSI 15 



pass through the cranial articulation. In the most common circum- 

 stances there is some form of resistance to depression of the mandible 

 anterior to its articulation, and the line of pull of the depressor then 

 has relevance for protraction only if it angles forward from the neutral 

 angle, which is determined by the geometry of the whole system as 

 already explained. In any case, an evolutionary shift in the direction 

 of pull of the depressor mandibulae in a counterclockwise direction 

 represents a functional change along the continuum "retraction — 

 neutral — -protraction" with respect to the muscle's influence on the 

 upper jaw. 



Coordination and Independence of Jaw Action 



Postorbital ligament. — Coordination of the jaws through the 

 postorbital ligament (or its functional equivalent, as in the lacrymo- 

 mandibular ligament in certain ducks) requires that the ligament 

 attach on the mandible anterior to the jaw articulation. Should the 

 ligamentary attachment approach the jaw articulation (or should A 

 in figure 6 be reduced) the amount of protraction obtained through the 

 ligament would be correspondingly reduced. Coordination would 

 reach zero when the ligamentary attachment lay opposite the jaw 

 articulation. By manipulation of the jaws of fresh specimens of the 

 sooty tern (Sterna fuscata) , I found that a pull on the depressor muscles 

 caused very little forward or backward displacement, relative to the 

 braincase, of the postorbital ligament and lower jaw during depression 

 of the jaw; instead, the quadrate was displaced forward and the upper 

 jaw raised. On the other hand, when motion of the upper jaw was 

 restricted and the quadrates thereby held stationary, depression of 

 the lower jaw was accompanied by a backward shift of the lower jaw 

 and postorbital ligament relative to the braincase and quadrates. 

 (Strict limits to this backward shift were set by the articular jugo- 

 mandibular ligament.) Whether the jaw shifts backward or the 

 quadrate forward, the shift is enough to bring the attachment point 

 of the postorbital ligament closer to the rotation center of the jaw 

 articulation and to reduce the coordinating action of the ligament. 

 (Manipulation of the jaws was accomplished through pulling on the 

 depressor mandibulae muscles or by applying pressure on the jaw at 

 their points of insertion. Manipulation of the jaws by their tips may 

 give different results that do not correspond to actions of the depressor 

 mandibulae muscles.) 



The ability of the lower jaw to shift backward with respect to the 

 quadrate may be deduced from the jaw motions of an evening grosbeak 

 that is hulling sunflower seeds. The bud moves its lower jaw to the 

 right or left of the upper jaw to facilitate manipulation of the seeds. 



