no. 3607 KINESIS OF AVIAN SKULL — ZUSI 13 



ducing the force of protraction. The attachment point of the post- 

 orbital ligament, however, is never far in front of the jaw articulation 

 as this would reduce the amount by which the lower jaw could be 

 depressed. 



During depression of the lower jaw by the depressor mandibulae 

 muscles, the resultant force of each depressor muscle shifts in a clock- 

 wise direction relative to the bones. It is possible that the geometry 

 of the entire ligarnentary mechanism is such that, in some birds, the 

 system reaches a neutral condition before the mandible is depressed 

 to the limits allowed by the jaw articulation alone. Full contraction 

 of the depressor mandibulae thus would stabilize both jaws in an 

 opened position without permitting excessive strain on the jaw 

 articulation or kinetic articulations that might occur in the absence 

 of the ligament. 



Zusi (1959) postulated that birds that force their jaws open against 

 environmental resistance (probers, gapers) may derive a protraction 

 effect through the depressor mandibulae in that the point of resistance 

 to depression of the lower jaw creates a new fulcrum, replacing that 

 of the postorbital ligament. (In figure 6, A and B would then extend 

 from the point of resistance near the tip of the jaw to the quadrate 

 and to the depressor mandibulae, respectively.) In the light of 

 the above analysis, it is clear that the mechanism would work effec- 

 tively only if F were shifted in a counterclockwise direction to offset 

 the loss of protraction force incurred by the relative increase in the 

 ratio of A to B, or if the ratio were reduced by elongation of the 

 retroarticular process of the mandible. I have not carried the 

 analysis for any particular species far enough to be able to say whether 

 or not the depressor mandibulae alone could produce protraction 

 of the upper jaw in the presence of environmental resistance on both 

 jaws. Bock (1964, p. 17) pointed out that the analysis is difficult 

 and must include both jaws. It seems likely, however, that species 

 with a strong forward component to the pull of the depressor mandib- 

 ulae would be capable of stronger gaping (forceful opening of both 

 jaws), not only by increasing the force of depression of the mandible, 

 but also by favoring conditions for protraction of the upper jaw as 

 well. The validity of the difference between the adaptations found 

 in the depressor mandibulae muscles of Sturnus vulgaris and gapers 

 of the Icteridae postulated by Zusi (1959) is thus strengthened by 

 the above analysis although the explanation originally proposed was 

 incorrect. 



In species lacking a functional postorbital ligament, any structure 

 that resists depression of the lower jaw will cause protraction of the 

 upper jaw through forces similar to those diagrammed in figure 6 but 

 only in proportion to the amount of resistance offered by the structure. 



