(Chapter 18) Footnote 74:

In the margin Vesalius writes “mostly in books 2 and 6” [of On Anatomical Procedures] and refers to book 3 of Galen’s Commentary on Hippocrates’ Book ‘On the joints’. See note 13 above for the passages in De anat. adm. Galen’s commentary on Hippocrates’ On the Joints refers several times to the sacrum or “wide bone” (e.g., 543.5-544.17), once to the coccyx (648.16)