For, if it be forwards, the
stretching out of the leg will be while the body is going forwards,
but, if the other way, while it is going backwards. And again, if
the flexion were backwards, the placing of the foot would be made by
two movements and those contrary to one another, one, that is,
backwards and one forwards; for in the bending together of the limb
the lower end of the thigh would go backwards, and the shin would move
the foot forwards away from the flexion; whereas, with the flexion
forwards, the progression described will be performed not with
contrary motions, but with one forward motion.
Now man, being a biped and making his change of position in the
natural way with his two legs, bends them forward for the reasons
set forth, but his arms bend backwards reasonably enough. If they bent
the opposite way they would be useless for the work of the hands,
and for taking food. But quadrupeds which are also viviparous
necessarily bend their front legs forwards. For these lead off first
when they move, and are also in the forepart of their body. The reason
that they bend forward is the same as in the case of man, for in
this respect they are like mankind. And so quadrupeds as well as men
bend these legs forward in the manner described. Moreover, if the
flexion is like this, they are enabled to lift their feet high; if
they bent them in the opposite way they would only lift them a
little way from the ground, because the whole thigh and the joint from
which the shin-bone springs would lie under the belly as the beast
moved forward.
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