(Chapter 6) Footnote 125:

Lat. galea. Vesalius is Latinizing krani/on, the Greek term derived from kra/noj “helmet.” This metaphoric nomenclature for the upper part of the head is at least as old as Aristotle (who defined it as the hairy part of the head). Galen compared the skull to a helmet in Book 8 of De usu partium (3.661.1-2, May 1968 p. 411) and again in the beginning of Book 9 (3.688.17, May 1968 p. 426), which Vesalius had in mind when writing the 1555 additions to this chapter.