The flexion is similar in the case of the
multipeds; only that the legs in between the extreme ends always
move in a manner intermediate between that of those in front and those
behind, and accordingly bend sideways rather than backwards or
forwards. But man bends his arms and his legs towards the same
point, and therefore in opposite ways: that is to say, he bends his
arms backwards, with just a slight inclination inwards, and his legs
frontwards. No animal bends both its fore-limbs and hind-limbs
backwards; but in the case of all animals the flexion of the shoulders
is in the opposite direction to that of the elbows or the joints of
the forelegs, and the flexure in the hips to that of the knees of
the hind-legs: so that since man differs from other animals in
flexion, those animals that possess such parts as these move them
contrariwise to man.
Birds have the flexions of their limbs like those of the
quadrupeds; for, although bipeds, they bend their legs backwards,
and instead of arms or front legs have wings which bend frontwards.
The seal is a kind of imperfect or crippled quadruped; for just
behind the shoulder-blade its front feet are placed, resembling hands,
like the front paws of the bear; for they are furnished with five
toes, and each of the toes has three flexions and a nail of
inconsiderable size.
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