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 relieving 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 
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