8 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 123 



chicken after removal of the postorbital ligament; (3) protraction of 

 the upper jaw by the depressor mandibulae was reduced after removal 

 of the postorbital ligaments in the chicken; (4) depression of the lower 

 jaw occurred when the upper jaw was held closed in both species; 

 (5) protraction of the upper jaw occurred when the lower jaw was 

 held closed in both species; and (6) the postorbital ligament played a 

 role in holding the lower jaw partially closed at rest. In addition, 

 protraction in the chicken following removal of the postorbital liga- 

 ment was about half that of the normal condition. A weight of ten 

 grams suspended from midway along the exposed culmen of the 

 chicken reduced protraction in the normal bird and prevented pro- 

 traction in birds from which the postorbital ligaments had been re- 

 moved. Limiting or preventing motion of one jaw reduced but did 

 not prevent motion of the other jaw. The postorbital ligament alone 

 did not hold the lower jaw completely closed at rest (see fig. 4: A 1, 

 2, 3), but removal of the ligament shifted the jaw to a more depressed 

 resting position. That the adductor muscles also played some role 

 in support of the resting jaw is suggested by B 3, in which the jaw at 

 rest was most strongly depressed following removal of both the post- 

 orbital ligaments and adductor muscles. 



Discussion. — The fact that the postorbital ligament and the 

 depressor mandibulae can together produce protraction of the upper 

 jaw is demonstrated by the experiments on domestic chickens 

 described above. The ligament thus serves to coordinate motions 

 of both jaws during depression of the lower jaw. Coordination of 

 both jaws through the depressor mandibulae, however, does not 

 require the presence of the postorbital ligament, and the ligament 

 must be regarded as only one of several means (excluding other jaw 

 muscles) of producing coordination. Furthermore, coordination of 

 the jaws in the presence of a postorbital ligament is not obligatory; 

 either jaw may also be moved independently. 



Bock (1964) discussed the role of the postorbital ligament in 

 avian kinesis, basing his conclusions on the results of manipulation 

 of jaws in fresh birds and on inference from the anatomical structure 

 of the jaw mechanism. His major hypotheses were as follows: (1) 

 Two basically different kinetic mechanisms exist in birds — coupled 

 and uncoupled. The jaws are coupled in those species with a post- 

 orbital ligament (or a functionally equivalent ligament), and/or with 

 an interlocking arrangement of the jaw-quadrate articulation. Birds 

 lacking both of these features have uncoupled jaws. (2) When the 

 upper jaw of a coupled bird is held firmly in place, the lower jaw 

 cannot be depressed. (3) In uncoupled birds the depressor mandib- 

 ulae does not contribute to raising the upper jaw except perhaps when 

 the mandible is depressed against resistance. (4) In coupled kinesis 



