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

right-angled 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 right-angled

triangle. That which goes forward therefore must bend, and while