(Chapter 15) Footnote 129:

The ligaments become ossified. In this paragraph Vesalius describes fusion of vertebrae, with concomitant immobility of the vertebral column, in the aged. He states that “this fusion is effected by certain processes,” which in modern medical science we understand is caused by a rupture of the anulus fibrosus resulting from either degenerative changes or trauma, allowing the nucleus pulposus to protrude into the vertebral canal, causing a herniated intervertebral disk. Today we also know that other causes of immobility or abnormalities in the aged vertebral column are the result of degenerative changes in the joints of the spine and back muscles, several forms of arthritis, and fracture. The end result of these degenerative changes is bony fusion.