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.