I always thought it was one of two things, and I never thought about which one I liked better.
One was that it was similar to a spinal column, as had been said. It would be able to bend in many directions, as well as a large degree of rotation.
Another thought was that they were a muscle similar to the tongue, except longer. This theoretically has more potential, but would be more biologically difficult.
And Broken, I'm pretty sure that the part of the human anatomy you were referring to can't move like that on its own. Correct me if I am wrong, or if you are a special case.