(Chapter 14) Footnote 19:

Beginning “its nature resembles that of the brain,” this language is a translation of Galen’s De usu partium 4.51.1-4.52.1 (May pp. 574ff.), whose twelfth book Vesalius cites; Vesalius’ marginal quotation marks represent Galen’s reference to Hippocrates De articulis 46-48. The last two sentences of this section continue as a condensed paraphrase of what follows in Galen as far as 4.52.14 (May p. 575).