(Chapter 12) Footnote 97:

Vesalius is perhaps referring to students of anatomy who overheat bones of the vertex to dry them out prior to dissection and inadvertently cause portions of bone tissue to cave in; others will mistake the effects of their own cutting and drilling for natural declivities in the bone. Richardson 1998, p. 129 interprets these words as a warning against cautery and unwarranted drilling to relieve fractures.