The upper and under sides of the body, that is of what in quadrupeds
is called the trunk, present in birds one unbroken surface, and they
have no arms or forelegs attached to it, but in their stead wings,
which are a distinctive peculiarity of these animals; and, as these
wings are substitutes for arms, their terminal segments lie on the
back in the place of a shoulder-blade.
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.
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