(Chapter 3) Footnote 26:

Vesalius cites Book 11 of De usu partium, where see 3.927.12ff: “Now when porosity and hollowness are both present together in the same bone, you may see at once that there is a head growing upon the end of it because it needs a lid and because this must be dense and solid, particularly when the bone ends at a movable joint; for bones that articulate need to be hard since they have to be continually in motion and to rub against one another.” —tr. May p. 543.