
On The Gait Of Animals
relative to one point. Moreover, without flexion there could not be
walking or swimming or flying. For since limbed creatures stand and
take their weight alternately on one or other of the opposite legs, if
one be thrust forward the other of necessity must be bent. For the
opposite limbs are naturally of equal length, and the one which is
under the weight must be a kind of perpendicular at right angles to
the ground.
When then one leg is advanced it becomes the hypotenuse of a
rightangled triangle. Its square then is equal to the square on the
other side together with the square on the base. As the legs then
are equal, the one at rest must bend either at the knee or, if there
were any kneeless animal which walked, at some other articulation. The
following experiment exhibits the fact. If a man were to walk parallel
to a wall in sunshine, the line described (by the shadow of his
head> would be not straight but zigzag, becoming lower as he bends,
and higher when he stands and lifts himself up.
It is, indeed, possible to move oneself even if the leg be not bent,
in the way in which children crawl. This was the old though
erroneous account of the movement of elephants. But these kinds of
movements involve a flexion in the shoulders or in the hips. Nothing
at any rate could walk upright continuously and securely without
flexions at the knee, but would have to move like men in the wrestling
schools who crawl forward through the sand on their knees. For the
upper part of the upright creature is long so that its leg has to be
correspondingly long; in consequence there must be flexion. For
since a stationary position is perpendicular, if that which moves
cannot bend it will either fall forward as the right angle becomes
acute or will not be able to progress. For if one leg is at right
angles to the ground and the other is advanced, the latter will be
at once equal and greater. For it will be equal to the stationary
leg and also equivalent to the hypotenuse of a rightangled
triangle. That which goes forward therefore must bend, and while
