Of all the bones in your body, few are as important as the stack of 33 that forms your spine.
Your spine (or vertebral column, as it’s known to biologists and Trivial Pursuit players) is the centerpiece of your skeletal system—your body’s equivalent of the center pole in a circus tent. It anchors your body, allowing you to stand upright, support your head and arms, and keep your balance. It’s a central attachment point for countless muscles, and it encases your critically important spinal cord, which ferries commands from your brain to the far regions of your body (and sends sensory information in the reverse direction, from your body parts back to your brain).
Your spine is also one of the easiest parts of your body to injure—poor posture, repetitive actions, or heavy lifting can easily drive you to the medicine cabinet or leave you lying helplessly under an office table. In fact, back pain affects 80 percent of adults at some point, and nearly half of them suffer at least one bout of back pain every year. In the U.S., lower-back pain is the fifth most common reason for doctor visits.
Part of the reason your spine is so vulnerable is its complexity. Of its 33 bones, 24 are bony disks called vertebrae. Every two vertebrae form a separate joint, each with its own ligaments holding it together and its own tendons binding it to the appropriate muscles. This design lets you freely bend and twist your spine. Your body limits your range of motion not with the ligaments themselves, but with the bony protrusions of each vertebra, which lock together if you try to bend them past a certain point. This important safeguard prevents a strained back from causing life-threatening paralysis.
Between each pair of vertebrae is a spongy disk of cartilage. These disks become temporarily more compressed throughout the day, and permanently more compressed as you age. They’re the reason you go to bed a little bit shorter than when you woke up, and why you’re likely to lose an inch of height by the time you reach old age. They’re also the source of more serious, long-term back ailments. For example, the infamous “slipped disk” occurs when you inadvertently crush a piece of cartilage out of shape, making it bulge into a nearby nerve.
Source of Information : Oreilly - Your Body Missing Manual