(Chapter 18) Footnote 46:

The metaphor is Galenic. See De usu partium 4.55.12: “Nature made the lowest bone [os sacrum] the largest of all, like a base [krhpi/j, ‘foundation’] placed beneath all the vertebrae.” (tr. May 1968 p. 576); Cf. 4.65.3, 4.109.2 (May 1968 pp. 581, 602)