WOODPECKERS. 
7 
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 ill pecking may be brought nearer to the surface, 
or moved farther from it, according as may be neces- 
sary. If this part (which may be called the base of 
the bird when in action) had been straight, there 
would have been more stability in one position, but it 
would have been only in one, and in that one only 
where the vertical line of the bark happened to be 
straight, which is not often Ihe case in* those gnarly 
and decaying trees which aflbrd the fattest pastures 
for AV oodpeckers. 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 immovably 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 enables 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. 
