(Chapter 6) Footnote 137:

That is to say, the cavities of the sinus function as acoustical chambers in phonation while the thickness of this porous bone provides a structural rigidity for neighboring bones without adding unduly to the weight of the head. Cf. Ch. 9 appendix A and note 59. It appears that between 1543 and 1555 Vesalius became more interested in the role of bone cavities in phonation.