(Chapter 3) Footnote 30:

Here the 1555 edition adds the following: “Therefore, if we rightly describe the system of the epiphyses, we will not only prove that they are by no means covers of the hollow parts, but we shall state with good reason that the Maker of things used epiphyses when he saw fit to fashion some bone softer in one place than in the others — whether he required an area of bone that was thick for a suitable and convenient joint to be thereafter not so solid and compact, or for the origin and insertion of softer bodies, or for some other such purpose. Galen, on the other hand, believed in the eleventh book of De usu partium that the material of the epiphyses is more solid and harder than the rest of the bone of which they are a part, but he did not consider the epiphyses in newborns, which are mostly cartilaginous, just as in the old the bones are elsewhere (as I have already said) softer and less dense.” Vesalius is referring to De usu partium 3.927.12 ff.: “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 moveable joint, etc.” (tr. May 1968, p. 543).