(Chapter 6) Footnote 128:

The metaphor is Galenic: De usu partium 3.688.10-11 (May 1968 p. 426), on which this discussion relies. Vesalius would perhaps have thought here of a roof of thatch, whose permeable fabric of plant stalks keeps off rain while allowing smoky vapors to escape. A similar double function is provided by cedar shakes, which swell when wet to repel rain but are porous when dry to permit a degree of ventilation.