Intervertebral discs consist of an outer fibrous ring, the anulus (or annulus) fibrosus disci intervertebralis, which surrounds an inner gel-like center, the nucleus pulposus.

[medical citation needed] The intervertebral disc functions to separate the vertebrae from each other and provides the surface for the shock-absorbing gel of the nucleus pulposus.

These events can occur during peak physical performance, during traumas, or as a result of chronic deterioration (typically accompanied with poor posture), and has been associated with a Propionibacterium acnes infection.

[8] Both the deformed anulus and the gel-like material of the nucleus pulposus can be forced laterally or posteriorly, distorting local muscle function and putting pressure on the nearby nerve.

Another kind of herniation, of the nucleus pulposus, can happen as a result of the formation of Schmorl's nodes on the intervertebral disc.

Beyond age 40, more than 60% of people show evidence of disc degeneration at one or more levels on magnetic resonance imaging (MRI).

This general shrinking of disc size is partially responsible for the common decrease in height as humans age.

[5] Since the fissures are formed in the anulus fibrosus due to osteo-arthritic bones or degeneration in general, the inner nucleus pulposus can seep out and put pressure on any number of vertebral nerves.

[13](see also: Intervertebral disc arthroplasty) Other degeneration of the vertebral column includes diffuse idiopathic skeletal hyperostosis (DISH)[14] which is the calcification or ossification of the ligaments surrounding the vertebrae.

[5] In addition to scoliosis, which is the lateral 'S' curvature of the spine, the fused vertebrae can also experience other abnormalities such as kyphosis[12] (hunchback) which shows in old age, or lordosis[12] (swayback), which is often present in pregnancy and obesity.

However, modern English also spells the word more phonetically annulus, as with the term annular eclipse, where the moon blocks the sun except for a bright ring around it.