(Chapter 18) Footnote 26:

See De ossibus §762, where Galen counts three pairs of nerves issuing from the spinal cord through the foramina of the sacrum, and §763: “Nerves pass through the junctions [of the coccyx] from back and front, the first pair where the coccyx adjoins the ‘broad bone,’ the second at the junction of the first and the 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 unimpaired.” (tr. Singer 1952 p. 773).