(Chapter 23) Footnote 31:

At the beginning of De incessu animalium, Aristotle says that “a man ... bends his legs and his arms in opposite directions, the arms concavely and the knees convexly. And viviparous quadrupeds bend their limbs in the opposite way to a man’s and in opposite ways to one another; for they bend their front legs convexly and their back legs concavely.” (704a20-704b5, cf. 711a8-712a22. Loeb translation by E. S. Forster).