(Chapter 18) Footnote 19:

Vesalius is probably referring to a passage in De anat. adm. where Galen discusses the muscles of the hip: “All which passes toward the hip-joint and is continuous with it is fleshy, and fuses with the head that issues from the coccyx. ... Lay bare the top of the femur and find the twofold termination of the muscle, one [part] uniting with the back parts of the femur set roughly in a line with the fibres from the ischium to the coccyx ...” (310.12-311.4, tr. Singer 1956 p. 45).