190 BILL OF THE CROSSBILL. 



The upper mandible is united to the frontal bone 

 of the skull by plates of bone which are a little flexi- 

 ble, so that that mandible has a little lateral motion. 

 This agrees with that very general law in the animal 

 economy, in virtue of which every part which is 

 subject to violent motion or pressure is always so 

 formed as to give a little, by which means a sudden 

 strain upon the moving part is not so forcibly propa- 

 gated to the rest of the body ; but beyond this limited 

 motion, the base of the upper mandible is very 

 strongly supported. The lower jaw, the sides of 

 which are deep and strong, and the coronoid process 

 near the middle of their length.'is so much elevated as 

 to give .them all the stiffness of triangles, and is arti- 

 culated by concave sockets upon spherical processes 

 on the os quadratum, and thus they admit of lateral 

 and oblique as well as vertical motion in that mandi- 

 ble. The pterygoid processes of the upper jaw are 

 very long, and the pterygoid muscles correspondingly 

 large ; and when the jaws are fully closed, that is, 

 when the coronoid process on each side is brought 

 into contact with the pterygoid on the same, the man- 

 dibles are crossed to the full extent. The muscles 

 of that side of the head to which the lower mandible 

 curves, are larger than those of the other side, so that 

 their action has a tendency to draw the mandible 

 toward that side ; but as the closing muscles, which 

 have their insertion before the centre of motion, pull 

 the mandible to that side, the opening ones, which 

 are inserted behind that centre, pull it to the other ; 

 and thus the parts come in contact when the bill is 

 opened. The action of the bill thus resembles that 

 of a wedge, by the mandibles sliding upon each other, 

 and thrusting the points asunder at once with great 

 force and great firmness. 



