The spine is a column of small bones, or vertebrae,
that supports the entire upper body. The column is grouped into three
sections: the cervical vertebrae are the five spinal bones that support
the neck; the thoracic vertebrae are the twelve spinal bones that connect
to the rib cage; and the lumbar vertebrae are the five lowest and largest
bones of the spinal column. Most of the body's weight and stress falls on
the lumbar vertebrae. Below the lumbar region is the sacrum, a
shield-shaped bony structure that connects with the pelvis. At the end of the sacrum are two to four tiny partially
fused vertebrae known as the coccus or "tail bone".
Natural Curves
The spinal column form three natural curves: the
Cervical curve, the Thoratcic curve and the Lumbar Curve.
Vertebrae in the spinal column are separated from each
other by small cushions of cartilage known as intervertebral discs. Inside
each disc is a jelly-like substance (called the nucleus pulposus), which
is surrounded by a fibrous structure. The disc is 80% water, which makes
it very elastic. It has no blood supply of its own, however, but relies on
nearby blood vessels to keep it nourished.
Muscles and Joints
Each vertebra in the spine has a number of bony
projections, known as processes. The spinal and transverse processes
attach to the muscles in the back and act like little levers, allowing the
spine to twist or bend. The articular processes form the joints between
the vertebrae themselves, meeting together and interlocking at the facet
joints.
Nerves
Each vertebra and its processes surround and protect an arch-shaped
central opening. These arches, aligned to run down the spine, form the
spinal canal, which encloses the spinal cord, the central trunk of nerves
that connects the brain with the rest of the body. Each nerve root passes
from the spinal column to other parts of the body through small openings
bounded on one side by the disc and the other by the facets. When the
spinal cord reaches the lumbar region, it splits into four bundled strands
of nerve roots (called the cauda equina).
