90 THE STRUCTURE AND LIFE OF BIRDS chap. 



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. 1 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 

 role. 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. 



