nucleus pulposus, nucleus gelatinosus or vertebral pulp

Soft fibrocartilage central part of the intervertebral disk surrounded by the anulus fibrosis. It is thought to be a derivative of the notochord. The nucleus pulposus is prone to herniation when the surrounding anulus fibrosis is compromised.

References

Stedman's Medical Dictionary 27th ed, Williams & Wilkins, Baltimore, 1999