20 



PROCEEDINGS OF THE NATIONAL MUSEUM 

 Discussion 



I have been concerned primarily with the functional properties of 

 only one paired muscle as it acts within the kinetic systems of various 

 species of birds (see fig. 11). The depressor mandibulae muscles serve 

 principally to open the upper and lower jaws simultaneously. In 

 most species the most powerful agents for protracting the upper jaw 

 are undoubtedly the protractor quadrati et pterygoidei muscles, which 



D 



A 



C 



/ 



Jauus coordinated by 



1. jaw articulation 



2. postorbital ligament 



Jaais complete!/ 

 uncoordinated 



^*C^ 



/ 



Jatus complete// 

 coordinated 



B 



Jaius independent by 

 1. spreading of rami 

 2- shift of fulcrum 



Specialised protraction by 

 1. postorbifal ligament 

 Z. larger depressor mandibulae 



3. longer retroarticular process 



4. depressor angled forward 



Figure 11. — Diagram of kinetic possibilities through the depressor mandibulae in the avian 

 jaw: Species A has weakly coordinated jaws that may be moved independently. Species 

 B has developed stronger protraction of the upper jaw through the mechanisms of coordi- 

 nation but can also overcome coordination. Species C has no capacity for independent 

 jaw action, whereas species D has no coordination through the jaw-quadrate complex. 

 (C and D are hypothetical.) 



almost certainly come into play during maximum or forceful gaping. 

 The depressor mandibulae, however, may be the sole or principal 

 muscles opening both jaws when extensive or powerful opening of the 

 jaws is not required, as in taking small food items from a substrate. 

 The mechanisms by which the depressor mandibulae can protract 

 the upper jaw involve the angle of the muscle, the angle of sliding of 

 the jaw-quadrate surfaces, the rigidity of the rami of the lower jaw 

 and of the kinetic mechanism, the presence of a postorbital ligament 

 or a comparable ligament, the length of the retroarticular process of 



