(Chapter 14) Footnote 17:

By modern count, the spine is composed of seven cervical vertebrae, twelve thoracic, and five lumbar, with the sacrum (five fused vertebrae) and the coccyx (four fused vertebrae) bringing the total to 26 bones or 33 segments. Vesalius is counting the segments instead of individual bones. Since he counts six sacral segments instead of five, he comes up with 34. If he had counted the bones, he would have come up with 26.