Not P but Moby-Dick (82)

[Mike Jing]

>From Chapter 103:

To me this vast ivory-ribbed chest, with the long, unrelieved spine,
extending far away from it in a straight line, not a little resembled the
hull of a great ship new-laid upon the stocks, when only some twenty of her
naked bow-ribs are inserted, and the keel is otherwise, for the time, but a
long, disconnected timber.

What does "unrelieved" mean here?