470 



BIRD. 



united. The shoulder bones are rather large, broader 

 in the middle than toward the extremities, and a little 

 bent. 



The whole structure of this sternal apparatus shows 

 that its strength lies much more in the vertical plane 

 than in the horizontal, and consequently that it is 

 better adapted for supporting the body of the bird, 

 while walking or running, than for yiving steadiness 

 to the wing in flight. The coracok' and the clavicle 

 stand upon the sternum like two sides of a triangle, 

 of which the anterior parts of that bone and its keel 

 form the third side or base, and thus the shoulder- 

 joint is kept at the same vertical distance from the 

 sternum. The enlargement of the blade-bone, too, 

 gives that joint a firm suspension in the muscles of 

 the back, and this, with the other structure of the 

 bones which have been mentioned, causes the sternum, 

 and the viscera which it supports, to hang, in a great 

 measure, upon the upper part of the back, from which 

 the support is continued through the bones, and fur- 

 nishing a larger supply of mnscles for the support of 

 the legs than in air birds. But this very arrangement, 

 which makes these bones steady, or comparatively stiff, 

 in the vertical direction, and connects them with the 

 legs as the grand organs of motion, renders them 

 more flexible, or less able to resist a strain in the 

 cross direction. This is a general principle in 

 mechanics, that when any arrangement of pieces, 

 framed by carpentry, as one would say, have the 

 greatest possible strength in any one plane, they have 

 always the least possible in the plane at right angles 

 to that one. A truss which supports the most exten- 

 sive roof, as, for instance, one of those beautifully 

 scientific ones which support the roof of Westminster 

 Hall, would break in pieces by its own weight, if an 

 attempt were made to support it horizontally by the 

 ends of the rafters ; and even if it were laid on its 

 side, with the ends of these and the vertex on two 

 walls of equal height, all the framing, which makes it 

 so stiff and strong, when it stands upright, would be 

 just so much of a load upon the rafters, by which they 

 would be weakened to its full weight, from the strength 

 which they would have if laid single at their full 

 length upon two walls, to the general lines at which 

 they were at right angles. 



This structure which throws the strength of the 

 sternal apparatus into the plane in which the legs are 

 moved in walking, is the grand characteristic of a 

 ground bird, and the very opposite of that of a bird 

 of powerful wing, which has the plane of greatest 

 strength in its sternal apparatus nearly at right angles 

 to the plane of walking ; and therefore the power of 

 walking in the latter bird is the smallest possible in 

 proportion to the articulation and muscles of its legs, 

 just as the flying power of the gallinaceous, or ground 

 birds, is the smallest possible in proportion to the 

 articulation and muscles of their wings. 



Birds of this order are numerous ; and, though there 

 is perhaps fully as much similarity of habit among 

 them as among the different species of any other order 

 equally numerous, yet there are slight differences. 

 The common partridge, as one which is among the 

 fleetest runners, runs almost the instant it is out of the 

 shell, always rests or squats as well as nestles on 

 the ground, and seldom takes the wing, may be con- 

 sidered as one of the most typical ; and it has the 

 farther advantage of being almost the only one which, 

 in a state of nature, may be seen and studied alive 

 in every habitable district of the British islands. The 



following figures represent its sternal apparatus, half 

 the lineal dimensions of nature. 



Common Partridge. 



Sterna of the Gallinules. Birds of this tribe may 

 be considered as in their habits forming a connecting 

 link between the gallinidae and those swimming or 

 diving birds which have the feet lobed or webbed, 

 and they may be taken as an average type of the 

 Long-toed birds of Cuvier's arrangement. 



The following figures represent the sternal appa- 

 ratus of the common gallinule, half the lineal dimen- 

 sions of nature. 



Gallinule. 



This sternum, as may be seen from the figures, is 

 very long and narrow, broader toward the ends than 

 at the middle, divided posteriorly into three processes, 

 by means of two deep and wide notches, which are 

 filled by cartilaginous membrane. The middle pro- 

 cess is rather longer than the solid portion of the 

 sternum, and terminates in a blunt point. The lateral 

 ones are more than one and a half times as long, and 

 are a little broadened at the points, and the edge of 

 the membrane between them is concave. These 

 lateral processes arc flexible, and may be regarded as 

 oblique sternal ribs for supporting the planks of 

 the bird, rather than as portions of the sternum in 

 supporting the organs of flight. The middle pro- 

 cess is rendered stiffer by the keel, which, although 

 not high as compared with that of many other 

 birds, is high in proportion to the width of the 



