In man the knee and the elbow bend contrary ways; the same is the case, too, with the bear and the ape, and it is for this reason that they are not so swift of foot as other animals. Those quadrupeds which are oviparous, such as the crocodile and the lizard, bend the knee of the fore-leg back- wards, and that of the hind-leg forwards; their thighs are placed on them obliquely, in a similar manner to a man's thumb; which is the case also with the multipede insects, the hind-legs only excepted of such as leap. Birds, like quadru- peds, have the joints of the wings bending forwards, but those of the legs backwards.