(Chapter 30) Footnote 19:

The 1555 edition postpones mention of the third muscle, and adds “to which, like the first, the sesamoid ossicle belongs. Two ossicles of this kind are readily visible behind the knee in deer, dogs, and hares, as they are dry animals, and in humans of advanced age besides. In addition, the muscle which we shall make the third of those responsible for motions of the foot comes out of the outer head of the femur.” On the dryness of certain quadrupeds relative to man, see, Ch. 18 n. 94.