(Chapter 18) Footnote 12:

Vesalius refers marginally to books 12, 13, and 15 of De usu partium “and elsewhere.” Galen does in fact mention the sacrum (to\ i(ero\n o)stou=n) a number of times in books 9, 12, 13, and 14 (e.g. 4.109.9-10, 110.10-12, May 1968 pp. 602-603), and the coccyx is mentioned as an outgrowth (e)pi/fusij) of it in 4.110.12, though not by name. Galen mentions both the sacrum and the coccyx by name in De ossibus §762-763. On Galen’s inconsistency regarding the number of bones in the sacrum and the rebuke that Vesalius’ intemperate attack drew from Sylvius in 1555, see above, chapter 14 n. 6.