(Chapter 16) Footnote 62:

See De usu partium 4.80.1-6: “For of course when the spine becomes convex, the vertebrae lying below must shift downward and those lying above move up. Moreover, when we lift our heads and straighten up, the vertebrae must reverse their motion, those lying above moving down and those places below moving up.” (tr. May 1968. p. 589).