backbone
Meaning of backbone: A stack of small bones with shock absorbers between them, rigid enough to carry weight and flexible enough to bend in every direction.
Definition of backbone
The backbone is the column of small bones, called vertebrae, running down the middle of the back from the base of the skull to the tailbone. Cushioning discs sit between them and ligaments strap them together. Each bone carries a ring at the rear, and stacked rings form a bony canal that shelters the spinal cord and the nerve roots leaving it.
What is it made of?
Vertebrae, stacked one on another, with a tough pad of cartilage between each pair working as both shock absorber and pivot. Ligaments bind the stack and layers of muscle hold it upright. Seen from the side the column is not straight but curves in alternating directions, so load is absorbed like a spring instead of being passed as a jolt straight up to the skull. Muscles and tendons move the column but are not part of it, a distinction blurred in loose definitions.
How is it divided?
| Region | Bones | What it does |
|---|---|---|
| Neck | Seven | Carries the skull and turns the head |
| Chest | Twelve | Anchors the ribs and moves little |
| Lower back | Five, the largest | Bears most of the body weight |
| Sacrum | Five, fused solid | Wedges into the pelvis |
| Tailbone | Three to five, fused | Anchors pelvic floor muscles |
Where does the spinal cord end?
Higher than most people assume. The spinal cord tapers and finishes near the top of the lower back, while the bony canal carries on down to the sacrum. Below that point the canal holds a loose bundle of nerve roots trailing downward through fluid. That is precisely why a needle can enter the canal low down, as in a spinal tap, to sample fluid or place anesthetic: the roots drift aside and no cord lies in the path. Injury in that lower zone damages roots rather than cord, producing a distinct pattern of weakness, numbness and bladder trouble.
Why does trouble concentrate low down?
The lower back is the junction between a mobile column and a fixed pelvis, and it carries the weight of everything above. Its discs are the thickest and most heavily loaded, so they are the ones that bulge or tear and press on a root as it leaves the canal. Wear in the small joints at the rear narrows the canal with age, and the leg pain on walking that follows is among the commonest reasons for imaging the spine.
Also known as
Used in a sentence
Imaging showed degenerative change through the lower backbone without cord compression.
Good to know
The spinal cord ends near the top of the lower back, so the lower canal contains a bundle of nerve roots rather than cord, which is what makes a needle entered below that level a route to the surrounding fluid rather than a threat to the cord.