4G6 



B I R D. 



which arc enlarged at their extremities, and, except 

 the middle one, padded with cartilaginous discs; 

 they thus protect the flanks of the bird far backward 

 with a strong cuirass, though flexible or elastic at the 

 sides. The continuation of the keel, however, renders 

 the central part stiff' and firm throughout its length, 

 so that it admits of flexure in the flank angles only. 



Upon examining the anterior part we find the 

 coracoid bones produced in the direction of the 

 sternum, and as long as that bone ; while the furcal 

 bone is nearly a continuation of the line of the keel, 

 and, though not w ide in proportion, it is longer than in 

 any example which we have yet adduced. The lateral 

 processes are also very much produced, and two of 

 the five ribs on each side are articulated upon them. 

 The blade bones are of very peculiar shape, being 

 curved downwards at the terminations in the shape of 

 blunt hooks, as if the coracoids, which lie nearly 

 parallel to the axis of the body, instead of approaching 

 it obliquely, as in most birds, were hooked to the 

 muscles near the spine. 



Woodpecker. 



And this is a highly typical sternum, and one 

 upon which, if space permitted, many observations 

 might be made. Though its peculiarities have much 

 less reference to the style and manner of flight than 

 to the action of the bird when upon the bark, with 

 closed wings, yet they show, and perhaps on that 

 account show more clearly, that the sternum of a 

 bird, that organisation which supports the body, having 

 the spinal column in a great measure bare, is the 

 grand characteristic portion of its structure the 

 essential part by which a bird is distinguished from 

 every other animal and the basis, as it were, upon 

 which all the rest of its organisation is built, and in 

 accordance with which the whole of the minor parts 

 are formed. 



The main office for which the .structure of a 

 woodpecker has to be adapted, is that of maintaining 



with the under part of the body a vertical position 

 on the bark of a tree, in such a manner as to have 

 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 mored 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 efl'cct, 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 

 away. Nor is this ready adaptation of the centre to 

 the greatest effect of the stroke the only result of 

 that action of the tarsal joints by which it is pro- 

 duced ; for there is the same nice adjustment of the 

 degree of hold taken on the bark to the varying 

 stability of the position. When the centre of action 

 is removed to the greatest distance, the centre of 

 gravity is thrown farthest out, in proportion to the 

 line of the axis, and therefore the weight tends more 

 to pull the bird from the bark ; but the very same 

 action of the tarsal joints which produces this, causes 

 the claws to take a firmer hold of the bark, and also 

 the stiff 1 feathers of the tail to bear more against it as 

 a strut. So also, when the centre of action is brought 

 nearer to the tree, and the hold by the foot not so 

 much required, the same extension of the tarsal joint 

 which brings the axis more parallel to the tree eases 

 the clutch of the foot in exactly the same proportion. 



Thus, by one of the most beautiful instances of 

 the harmony of parts with each other, the wood- 

 pecker is enabled to work with equal stability and 

 effect, while the axis of the body is at all possible 

 angles to the line of the tree, within the range that 

 is necessary for its habit ; and yet the different parts 

 of this very curious and apparently complicated 

 organisation are so flexible to other purposes that 

 the woodpecker can at once become a w ing bird or 

 a ground bird, when such a habit is necessr 



When on the tree, the woodpecker may be regarded 

 as consisting of three parts, all differently employed; 

 the first of these is the sternum and its apparatus, 

 which acts as the basal or pectoral part, arid has no 



