The spine is
the path along which the steps in a blended object are aligned.
By default, the spine forms a straight line.
To adjust the shape of the spine, drag
the anchor points and path segments on the spine with the Direct
Selection tool.
To replace the spine with a different path, draw an object
to use as the new spine. Select the spine object and the blended
object, and choose Object > Blend > Replace
Spine.
To reverse the order of a blend on its spine, select
the blended object and choose Object > Blend >
Reverse Spine.