(Chapter 23) Footnote 32:

De usu partium 3.180.10ff: “It is clear that four-footed animals cannot sit, because in all of them the hind legs bend forward. The forelimbs are attached to the shoulder blades, just as in man, and the hind limbs to the hip bones; in both cases, however, flexion is in the opposite direction to the flexion of the corresponding member in man, that is, the front legs bend back and the hind legs forward, ...” (tr. May 1968, pp. 159f. and note 11: like Aristotle, Galen confuses the articulation of the femur and tibia with that of the leg bones and the tarsus in quadrupeds).