(Chapter 15) Footnote 87:

In Celsus De medicina 8.1.12, the description of the neck vertebrae begins with a text which is corrupt and has been variously emended. The following, to which Vesalius refers, is however undamaged: “To secure the rotation of the head, the top [of the second vertebra] ends in a narrower round process, so that the first vertebra encircling the top of this allows the head to turn sideways as well.” Loeb tr. by W.G. Spencer.