The discs have an inner core which is composed of a spongy gel (the nucleus pulposus) surrounded by a capsule of fibrous tissue (the annulus) which holds it in place. The gel is normally full of water, but when it is damaged it loses its water and becomes dry and less spongy. If the annulus is damaged disc material can ooze though the cracks and produce a disc herniation. Disc herniations can produce pain in two ways. The water lost from the damaged disc contains a witch's brew of inflammatory chemicals which irritate the membranes covering the spinal nerves, causing them to send pain impulses. The herniaton can also cause pain by pressing on the spinal nerves. When the spinal nerves are irritated or pressed upon you may feel pain going down the leg (sciatica). With time the herniation loses its water and shrinks, relieving the irritation and pressure (think of a juicy grape shriveling into a dry raisin).