462 Mr. Yarrell on the Beak of the Crossbill. 



This strong muscle covers two other small ones, the triangular and 

 square muscles, so called from their particular shape. These three mus- 

 cles, all of which have their origin on the occipital 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 con- 

 sequently by their simultaneous contraction raise the point to w^hich they 

 are attached, and depress the anterior part of the mandible. The lower 

 portions of the ossa quadrata are pushed somewhat forwards by this 

 compression, assisted by two small muscles not exhibited, but the situa- 

 tion 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 septum, and passes downwards to be 

 inserted upon the projecting styloid process of the os quadratum ; the 

 second is a small pyramidal shaped muscle arising also from the septum, 

 anterior to the other muscle, and passing downwards and backwards is 

 inserted upon the omoideum, both by their contraction pulling the os 

 quadratum forwards and thus elevating the other mandible. The de- 

 pressors of the lower jaw, and the elevators of the upper, therefore act 

 together to separate the mandibles. 



To close the mandibles the temporal and pterygoid muscles elevate the 

 lower jaw, assisted by the slender slips marked d. d. figure 2, which 

 extending forwards to the superior maxillary bones, act in concert by 

 bringing them down. 



When the lateral motion is required, the great pyramidal muscle on 

 the right side pulls the extremity of the lower jaw to which it is attached 

 backwards, the pterygoid muscles of the left side at the same time power- 

 fully assisting by carrying that side of the lower jaw inwards. 



Having thus described the muscles of the mandibles in birds generally, 

 and their peculiar mode of action in the Crossbill, I shall quote Mr. 

 Townson's description of the manner in which they are made subser- 

 vient to the use of the bird in feeding. " The great pine forests, such 

 " as the Hartz in Germany, are the natural places of residence of the 

 " Crossbeaks, and the seed of the cones of these trees their food, and 

 " it is to pull out the seeds from between the squarase, or scales of the 

 " cones, that this structure is given them. Their mode of operation is 

 " thus: they first fix themselves across the cone, then bring the points of 

 " the maxillae from their crossed or lateral position, to be immediately 

 " over each other. In this reduced compass, they insinuate their beaks. 



