Beginning at the base of the brain and curving gently down to what remains of the human tail, the spine is a stack of 33 building blocks called vertebrae.

It is strong enough to withstand forces of hundreds of pounds, but flexible enough to bend nearly double.

The interlocking bones of the spinal column, which protect the spinal cord, are held in place by a web of muscles, tendons and ligaments that keeps the column from collapsing.

Of the 33 vertebrae, seven are in the neck (cervical), 12 behind the chest (thoracic) and five in the lower back (lumbar). The other nine include five fused together in the back of the pelvis (sacrum) and four in the vestigial tail (coccyx). Joints called facets connect the vertebrae with the knobby outer bones of the spine (which you can feel by touching the back or neck) and help line each vertebra up with the next.

The most vulnerable portion of the back -- the part that takes most of the wear and tear when you lift a bag of groceries or bend over to tie your shoes or slump in a chair -- is the lower back, the five lumbar vertebrae between the rib cage and the pelvis.

The vertebrae of the back don't normally rub against one another, because they're separated by shock-absorbing disks -- rubbery cushions filled with soft, jelly-like tissue. The disks allow the back to compress (as when you bend over) or stretch.

It is because of the compressibility of back disks that you're slightly taller when you wake up in the morning than you are at night, and slightly taller at age 20 than at age 60.

Disks take a beating, especially in people who eat too much, let their abdominal muscles weaken and spend most of their waking hours sitting down.

"Mechanically, the disks are not designed to withstand the trauma of everyday living," says Dr. Michael Dennis, chief of neurosurgery at the Washington Hospital Center.

The two most common back injuries are herniated ("slipped") disk, in which the disk ruptures and presses against a nerve, and facet joint syndrome, in which the facet joints slightly dislocate. In both, the pressure against the spinal nerves can cause severe pain.

Back disks tend to lose their softness and compressibility with age, leading to intermittent stiffness beginning in middle age. But usually the symptoms are mild, unless aggravated by injury.

"Animals, for the most part, don't get back problems," says Dr. Peter Kenmore, chief of orthopedics at Georgetown University Medical Center. "Dachshunds do, because their backs are too long, but most animals don't.

"Humans do, because of their upright posture."