STERNA OF THE GALLINID^. 367 



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 muscles 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, w^ould 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 



