(Chapter 9) Footnote 25:

Though the language is imprecise, this can only refer to the cire perdue or investment casting process by which bronze statues are cast in the space left by wax melted from between a plaster core and an outer covering or investment. Vesalius compares the thin bones of the sinus areas to the resultant hollow bronze castings, but he describes wax being poured into the mould instead of melted out of it. Cf. bottom of Fabrica p. 98 and our note 15, chapter 21