The discs are commonly referred to as spongy or jelly-like material. However, they should really be described as having two concentric layers; a soft inner layer and a tough outer layer. Some people describe them as a doughnut with a jelly like material on the inside and a tougher ‘bread’ layer on the outside.
However, it may be easier to think of them like oranges. The outer layer of a disc is a tough material called the annulus fibrosus which is made up of 25 or more layers of very tough collagen sheets, similar to the peel of an orange and the material of your skin.
The outer layer attaches to the vertebra bone above and below, and keeps all the anatomy in place, including the inner pulp and liquid. The inner layer of a disc is made up of a soft jelly-like material called the nucleus pulposus (yes, its like the pulp of an orange but softer).
Normally, the outer tough sheet layer (ie. Annulus fibrosus), keeps the nucleus pulposus inside, just like how the outer peel of an orange protects the inner fruit. As long as the annulus fibrosus is intact, there are very few problems with the disc. However, when you have a ‘tear’ in this outer disc layer (ie. Annular fissure), this is when the inner nucleus pulposus ‘herniates’ out and becomes a problem.
It can start off as a ‘contained’ fissure just within the inner layers of the annulus fibrosus. But overtime, can spread so that the tear breaks through all layers of the annulus and allows the inner nucleus pulposus to ‘herniate’ outwards onto the surrounding nerves.