22 THE STRUCTURE OF THE FOWL 



The quadrate bone is irregularly quad- 

 rangular and intervenes between a concavity 

 on the temporal bone, just in front of the 

 tympanic cavity, and the mandible. In ad- 

 dition it is movably articulated with the 

 quadrato-jugal and pterygoid. 



The presence of the quadrate bones permits 

 of the mechanical elevation of the skeleton of 

 the upper part of the beak whenever the 

 mandible is depressed. This is rendered 

 possible by the absence of rigidity in the 

 neighbourhood where the frontal, nasal, and 

 incisive bones meet, and adds materially to 

 the freedom with which the fowl can widely 

 open the mouth. The elevation of the upper 

 beak is brought about in the following manner. 

 Depression of the mandible causes a forward 

 motion to the lower end of the quadrate, and 

 this is transmitted to the pterygoid, the inner 

 end of which is movably articulated to the 

 sphenoid. The palatine, being jointed to the 

 pterygoid, is necessarily pushed forwards at 

 the same time ; thus the movement is com- 

 municated to the upper beak with which the 

 anterior end of the palatine is connected. 



Beneath the skull is the hyoid bone (Fig. 5), 

 consisting of three medial segments and a pair 



