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.
13
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.
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