(Chapter 9) Footnote 49:

See chapter 6, note 87. Having been denied access to a copy of Galen’s text in the original Greek, Vesalius was forced to use a Latin translation by Ferdinando Balamio. Vesalius' astringently ironic point here is that his “understanding of Galen” (viz., his own anatomical description) is based on actual observation (ex rei cognitione), not derived from a translation of Galen.