The spine is made up of many bones called vertebrae. These are roughly circular and between each vertebra is a disc. The discs are made of strong rubber-like tissue which allows the spine to be fairly flexible. A disc has a stronger fibrous outer part and a softer jelly-like middle part. The middle part is called the nucleus pulposus.
This is a companion discussion topic for the original article at https://patient.info/bones-joints-muscles/back-and-spine-pain