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The space between two vertebrae (vertebras) is filled by a disc. This disc is made up of several parts. The central portion is the nucleus. When we are young this is a viscous material not that much different in consistency from silly putty. As we get older, the nucleus tends to dry out. If a disc is injured, then this drying out takes place quicker than it would with just aging.
The outer portion of the disc is called the annulus. It is made up of fibrous connective tissue. Outside the annulus there is a thin layer of stronger connective tissue known as ligament. Depending upon which direction you look, the ligamentous tissue can be very thick or very thin or somewhere in between.
Gravity tends to squeeze vertebrae together. Because of this, almost everyone on planet Earth will have bulging discs as they age. This process is generally painless. Should you squeeze a double stuff Oreo (TM) cookie, you will see how the white filling bulges out in all directions. If we kept our spine straight, our discs would bulge in all directions, too, but we don't. When we sit, or bend forward, we put much more pressure on the front of the disc, pushing the nucleus backward.
Fortunately, one of our strongest ligaments is directly behind the disc and helps to prevent large bulges in that direction, since this is where the spinal cord is located. Unfortunately, this strong ligament doesn't protect the entire rear portion of the dics. The edges closest to the sides of the disc are relatively weak. It's in these areas that the disc will occasionally rupture, allowing a portion of the nucleus to escape. This is known as a herniated disk. A herniated disc can be painless, too. Sometimes, however, the nucleus will put pressure on nerves causing pain.