(Chapter 4) Footnote 43:

Vesalius cites Chapter 9, Book 2 of Aristotle’s De partibus animalium; but Aristotle does not speak of particulars here, saying only that there is a type of joint where two hollow bones “hold a huckle-bone [a)stra/galoj] between them (as it might be a bolt [go/mfoj]), to admit of bending and extension, since these movements would be quite impossible or at any rate unsatisfactory without such an arrangement.”—654b.21ff., Loeb translation by A. L. Peck. One meaning of a)stra/galoj may be specifically the ball of the ankle-joint [astragalus, os tali], e.g. in Galen, De usu partium 3.195.5, May p. 167; hence Vesalius’ interpolation.