(Chapter 18) Footnote 32:

Normally five; Vesalius illustrates the sacrum always with six bones, though later in this chapter he says the sacrum sometimes consists of only five. The sacrum varies from four to seven vertebrae. The decision to describe the sacrum with six bones instead of five, which is almost four times as frequent, is the more puzzling when we note that Vesalius had shown a five-piece sacrum in his Tabulae sex five years earlier (sixth table). Straus and Temkin 1943 pp. 625-27 comment: “Vesalius himself, in later life, tried to explain and justify his choice of the six-piece sacrum by stating that he happened to have a particularly good specimen of that type available; and that he selected this because it conformed more closely to Galen's description. It is difficult to see why Vesalius felt it so necessary to concur with Galen here, especially when he was in such violent and outspoken disagreement in other instances. ... A possible motive for his choice of the six-piece sacrum as normal is suggested in his teleological attitude toward the relation between sacral foramina and nerves. Obviously Vesalius regards the foramina as having the purpose of protecting the nerves as they pass from the spinal cord to the periphery. He is at pains to prove that this end also is attained with a five-piece sacrum, but only by the additional provision of special processes of the coccyx.” Vesalius explains his rationale below in the paragraph on the coccyx, pp. 303-304.