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The vertebral column (spinal column) is the main support structure for the body, providing attachments for the skull, the thorax, and the pelvis. Although the major role of the spine is that of support, its construction is such that it permits the trunk of the body to have appreciable flexibility. In addition, the vertebral column protects the spinal cord while providing openings between adjacent vertebrae for the passage of spinal nerves.

The vertebral column consists of 33 bones and is divided into:

• 7 cervical vertebrae which form the upper part of the spine between the skull and the chest
• 12 thoracic vertebrae which are found between the upper chest and lower back
• 5 lumbar vertebrae which are the largest and strongest of the vertebrae and are the source of attachment of many strong back muscles
• 5 vertebrae fused together form the sacrum, which is of triangular shape, connects the spine to the pelvis, and also provides support for the spine
• 4 fused vertebrae form the coccyx (tail bone) which has little known function

The vertebrae are connected much like the links of a chain. Although there are differences among the vertebrae of the various regions of the spinal column, there is a basic design common to all of them.

A typical vertebra contains several parts including:

• Centrum - the thick anterior body of the vertebra
• Vertebral (neural) arch - arises from the posterior surface of the centrum
• Vertebral foramen - the enclosed canal through which the spinal cord passes. It is formed by each neural arch as it combines with the posterior surface of the centrum
• Pedicles - small lateral projections off the right and left sides of each vertebra that function as attachment sites for muscles and ligaments of the spine and extend to the sides of each neural arch
• Spinous process - a projection from the posterior, midline region of the neural arch and also a site of attachment for back muscles and ligaments
• Superior articulating processes and inferior articulating processes - each is a pair and projects upward (superior) or downward (inferior) from each side of the neural arch. Each articular process has a smooth surface that interlocks and provides a "joint" between the vertebra above or below it which facilitates movement of the spine and also increases the rigidity of the vertebral column.
• Pars interarticularis (pars) - a thin isthmus of bone which is part of the lumbar spine and connects the upper and lower facet joints to the vertebrae

Between each pair of vertebrae is a cartilaginous intervertebral disk that acts as a shock absorber for any movement of the spine.

Normally, posterior facets, ligaments, and intervertebral disks provide resistance or forward displacement of the vertebrae. When there is a defect, fracture, or elongation of the pars, spondylolysis develops. If the defect in the pars is bilateral, instability results and the vertebra is able move past the one below and slip forward. This is called spondylolisthesis. If the posterior elements also slide stenosis (narrowing of the spinal canal) and compression on nearby nerves occurs.