COMMON CROSSBILL. 19 



brought into contact with the pterygoid on its own side, 

 the extreme points of the mandibles were separated la- 

 terally to the extent I have already mentioned, namely, 

 three-eighths of an inch. 



The temporal and pyramidal muscles on the right side 

 of the head, that being the side to which the lower jaw 

 inclined, were considerably larger than those on the left 

 side, as represented in figures 1, 2, and 4, letters a and #, 

 and indicated by their bulk the great lateral power this 

 bird is capable of exerting, to be hereafter noticed. The 

 unusually large size of the pterygoid muscles on each side 

 was very conspicuous, figure 2, letters 0, c ; the space for 

 them being obtained by the great distance to which the 

 articulated extremities of the lower jaw were removed, 

 and the food of the bird being small seeds, rendered a nar- 

 row pharynx sufficient for the purpose of swallowing. 



The muscles which depress the lower mandible are three 

 in number, only one of which, the great pyramidal, is 

 visible, figures 1 and 4, letter b. This large and strong 

 muscle covers two other small ones, the triangular and 

 square muscles, so called from their peculiar shape. These 

 three muscles, all of which have their origin on the occi- 

 pital portion of the cranium, are inserted by strong tendons 

 on the under and back part of each extremity of the 

 lower jaw, behind the centre of motion, and consequently 

 by their simultaneous contraction raise the point to which 

 they are attached, and depress the anterior part of the 

 mandible. The lower portions of the ossa quadrata are 

 pushed somewhat forward by this compression, assisted by 

 two small muscles not exhibited; but the situation of 

 which may be explained by a reference to figure 3. One 

 of these, a small flat muscle, arises from the septum of 



the orbits behind the small aperture observed in the sep- 



c 2 



