(Chapter 7) Footnote 6:

Cf. Galen, De usu partium 3.851.13 (tr. May p. 508): “Indeed, this arch (for that is what anatomists call it) is not just ordinary bone but has no marrow and is dense and hard as rock; for Nature has contrived to establish in front of these muscles a rampart as insensitive as possible.” Vesalius rewrote the section on the use of the zygomatic bone as follows in the 1555 edition: “Each process or area which we call the cheekbone is without marrow, solid, and fit by its hardness for repelling injuries so as to make it a suitable bulwark for the temporal muscle, convex on the outside but concave on the inside.”