Only by this arrangement would flight be possible, for on the 

 upstroke of the wing through the air the quills act like the 

 shutters of the sails of a windmill, allowing the wind to pass 

 between them and so reUeving pressure on the uplifting 

 wing-stroke. On the down-stroke, the opposite effect is 

 produced. The full force of the stroke is conserved, because, 

 owing to the overlap, the several feathers are now pressed 

 closely together to form an impervious sheet. 



How are they fixed to the skeleton ? To see this all the 

 smaller feathers and the muscles, or "flesh" of the wing 

 must be removed. It will then be found that the flight- 

 feathers are divisible into two series. One, widely spaced, 

 runs along the upper surface of the forearm : the other, 

 closely packed, along what answers to the back of the hand. 

 In effect this is but a single rod of bone, but it is composed of 

 three elements, answering to three of the digits of the human 

 hand — ^the thumb and the first and second fingers. But 

 they are scarcely recognizable as such, for the thumb is 

 reduced to a mere stump, while the two fingers have become 

 welded together. The third finger, indeed, has become 

 reduced to the palm-bone, and a short stump answering to 

 the first finger-joint. To this frame- work, which can be folded 

 up into the shape of a Z when the bird is at rest, the quills are 

 fixed by their base by means of slender, but very strong elastic 

 tendons. In birds which have a long upper arm-bone, like 



