On The Gait Of Animals   

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. If, however, the flexion of the hind legs were forwards

the lifting of these feet would be similar to that of the forefeet

(for the hind legs, too, would in this case have only a little room

for their lifting inasmuch as both the thigh and the knee-joint

would fall under the position of the belly); but the flexion being

backwards, as in fact it is, nothing comes in the way of their

progression with this mode of moving the feet. Moreover, it is

necessary or at least better for their legs to bend thus when they are

suckling their young, with a view to such ministrations. If the

flexion were inwards it would be difficult to keep their young under

them and to shelter them.

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Now there are four modes of flexion if we take the combinations in

pairs. Fore and hind may bend either both backwards, as the figures

marked A, or in the opposite way both forwards, as in B, or in

converse ways and not in the same direction, as in C where the fore

bend forwards and the hind bend backwards, or as in D, the opposite

way to C, where the convexities are turned towards one another and the

concavities outwards. Now no biped or quadruped bends his limbs like

the figures A or B, but the quadrupeds like C, and like D only the

elephant among quadrupeds and man if you consider his arms as well

as his legs. For he bends his arms concavely and his legs convexly.

In man, too, the flexions of the limbs are always alternately

opposite, for example the elbow bends back, but the wrist of the

hand forwards, and again the shoulder forwards. In like fashion,

too, in the case of the legs, the hip backwards, the knee forwards,

the ankle in the opposite way backwards. And plainly the lower limbs