ACTION OF THE WOODPECKER. 353 



the head, the neck, and the spine as far as the lum- 

 bar vertebras (which have a little more motion in 

 this bird than in some others), perfectly free, so that 

 the point of the bill may command the largest pos- 

 sible surface which is compatible with the length of 

 the neck, or move with that force and velocity 

 which are necessary for hewing holes in the wood 

 with the greatest certainty and expedition. For this 

 purpose, the long sternum and coracoids, with the 

 keel and furcal bone on the exterior side of them, 

 form a flat arc with its chord, the former applied 

 to the tree, so that the fixed point upon which the 

 head and neck move in pecking may be brought 

 nearer to the surface, or moved farther from it, 

 according as may be necessary. If this part (which 

 may be called the base of the bird when in action) 

 had been straight, there would have been more sta- 

 bility in one position, but it would have been only 

 in one, and in that one only when the vertical line 

 of the bark happened to be straight, which is not 

 often the case in those gnarly and decaying trees 

 which afford the fattest pastures for woodpeckers. 

 This, however, would have made the bird work at a 

 disadvantage in excavating a hole to any considerable 

 depth; because, if the position of the centre of action 

 had been immoveably adjusted to any one distance, 

 the action of the bird would have been less effective 

 at every other. But the arched form of the keel ena- 

 bles the bird to keep the centre of action always 

 adjusted for the maximum effect, and that with so 

 slight a motion of the steady or pectoral part of its 

 body that it can hardly be perceived. 



A very little extension of the tarsal joints brings 

 the centre of action more to the tree, and a very 

 little bending of the same joints removes it farther 

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