90 THE STRUCTURE AND LIFE OF BIRDS cuap. 
when the dead weight hanging on the ribs produces the 
result described. And the movement of the shoulder 
blades during flight will help to produce the same 
effect. In the down stroke the rotating or twisting of 
the wing, by which its front margin is lowered and its 
under surface made to look backward, causes a very 
slight lowering of the anterior end of the shoulder 
blade, and this necessitates a considerable upward 
swing of the hinder end, a point that can be best 
made out by moving the wing of a dead bird as it 
moves in flight. This raising of the shoulder blade 
acts upon the ribs since they are connected with it by 
muscle. They straighten out, chiefly at the joint 
which each of them has in its lower half, and so tend 
to raise the backbone. And not only this, but, being 
pliable, they bend outwards, and so broaden the roof of 
the cage which they form. Thus in birds, as in the 
lobster and crayfish, progression itself aids greatly 
the process of breathing. The external intercostals, 
which we saw moved the breastbone forward when 
the bird was standing or walking, now play a different 
réle. During flight the bird’s breast is practically 
immovable. The entire weight of the body is hang- 
ing on the wings and the wings are pressing inwards 
upon the coracoid bones and clavicles- which are 
firmly fixed to the breastbone. The dead weight and 
the pressure inwards do not allow the breast to move. 
Let us see then what will happen when the external 
intercostals contract. It must be remembered that the 
action of a muscle is to shorten the distance between 
its two ends, and that of the two bones which it 
1 See Fig. 2 on p. 8. 
