(Chapter 18) Footnote 16:

See De ossibus chapter 12 (§762-763): “At the end of the ‘broad bone’ [sacrum] is the coccyx, which also is of three parts. They are rather more cartilaginous than those of the ‘broad bone,’ particularly that at the end. Nerves pass through their junctions from back and front, the first pair where the coccyx adjoins the ‘broad bone,’ the second at the junction of the first and second, and the third at the junction of the second and third. What remains of the spinal cord passes through at the end of the third and it alone is unpaired.” (tr. Singer 1952 p. 773).