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The legs are two in number, as in man; not however, as in man,
bent outwards, but bent inwards like the legs of a quadruped. The
wings are bent like the forelegs of a quadruped, having their
convexity turned outwards. That the feet should be two in number is
a matter of necessity. For a bird is essentially a sanguineous animal,
and at the same time essentially a winged animal; and no sanguineous
animal has more than four points for motion In birds, then, as in
those other sanguineous animals that live and move upon the ground,
the limbs attached to the trunk are four in number. But, while in
all the rest these four limbs consist of a pair of arms and a pair
of legs, or of four legs as in quadrupeds, in birds the arms or
forelegs are replaced by a pair of wings, and this is their
distinctive character. For it is of the essence of a bird that it
shall be able to fly; and it is by the extension of wings that this is
made possible. Of all arrangements, then, the only possible, and so
the necessary, one is that birds shall have two feet; for this with
the wings will give them four points for motion. The breast in all
birds is sharp-edged, and fleshy. The sharp edge is to minister to
flight, for broad surfaces move with considerable difficulty, owing to
the large quantity of air which they have to displace; while the
fleshy character acts as a protection, for the breast, owing to its
form, would be weak, were it not amply covered
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