To get to the 'spine', you must first gently lift off the two plastic strips that run on the long edges of the glass.
Use a knife to prise them up, they are glued down, and have plastic clips on the outside that allow them to slide towards the rear (USB end) of the scanner once the glue is up.
Alternate method: The ends near the hinge are not glued and have a tab that slides under the back edge. You can bend each strip up with a knife near the hinge, and slip the tab from underneath the back edge. The strip then lies on top of the back edge.