(Chapter 18) Footnote 20:

Vesalius cites the fifteenth book in the margin. See De usu partium 4.260.10-12: “The muscles moving the femur toward the outside are one part of the muscle [gluteus maximus] mentioned first of all and the muscle [piriformis] which arises from the broad bone [os sacrum] and which I have said rotates the femur slightly.” (tr. May 1968 p. 679).