This is the way then the limbs bend, and for the reasons given.

But the hind limbs move criss-cross with the fore limbs; after the off fore they move the near hind, then the near fore, and then the off hind.The reason is that (a) if they moved the forelegs together and first, the animal would be wrenched, and the progression would be a stumbling forwards with the hind parts as it were dragged after.

Again, that would not be walking but jumping, and it is hard to make a continuous change of place, jumping all the time.Here is evidence of what I say; even as it is, all horses that move in this way soon begin to refuse, for example the horses in a religious procession.For these reasons the fore limbs and the hind limbs move in this separate way.Again, (b) if they moved both the right legs first the weight would be outside the supporting limbs and they would fall.If then it is necessary to move in one or other of these ways or criss-cross fashion, and neither of these two is satisfactory, they must move criss-cross; for moving in the way we have said they cannot possibly experience either of these untoward results.And this is why horses and such-like animals stand still with their legs put forward criss-cross, not with the right or the left put forward together at once.In the same fashion animals with more than four legs make their movements; if you take two consecutive pairs of legs the hind move criss-cross with the forelegs; you can see this if you watch them moving slowly.Even crabs move in this way, and they are polypods.They, too, always move criss-cross in whichever direction they are making progress.For in direction this animal has a movement all its own; it is the only animal that moves not forwards, but obliquely.Yet since forwards is a distinction relative to the line of vision, Nature has made its eyes able to conform to its limbs, for its eyes can move themselves obliquely, and therefore after a fashion crabs are no exception but in this sense move forwards.