The annulus fibrosus is the strong wrapping that makes up the outside portion of the intervertebral disc. Its job is to contain and protect the soft material located in the center of the disc. This soft center is called the nucleus pulposus. The nucleus pulposus (and the entire intervertebral disc) provides shock absorption for the spine.
The annulus, as it is called for short, consists of several concentric rings of fibrous cartilage. The fibers run on a diagonal angle. Fibers of each separate layer run at a right angle to the fibers in the ring next to it. This design increases the strength of the annulus.