The vertebral column (spine or backbone) extends from the skull to the pelvis and forms a somewhat flexible but sturdy longitudinal support for the trunk. It is formed of 24 slightly movable vertebrae, the sacrum, and the coccyx. The vertebrae are separated from each other by intervertebral discs that serve as shock absorbers and allow bending of the spinal column. Four distinct curvatures can be seen on the lateral view of the vertebral column. From superior to inferior they are the cervical, thoracic, lumbar, and sacral curvatures. These curvatures provide flexibility and cushion, and allow the vertebral column to bear body weight more efficiently.

Structure of A Vertebra

Vertebrae are divided into three groups: cervical, thoracic, and lumbar vertebrae. Although each type has a distinctive anatomy, they have many features in common.

The anterior, drum-shaped mass is the body, which serves as the major load-bearing portion of a vertebra. A bony vertebral arch surrounds the large vertebral foramen through which the spinal cord and nerve roots pass. A spinous process
projects posteriorly and transverse processes project laterally from each vertebral arch.

A pair of superior articular processes
projects superiorly and a pair of inferior articular processes
projects inferiorly from the vertebral arch. The articular facet
(fa^set) of each superior articular process articulates with the articular fact of the inferior articular process of the adjacent vertebra superior to it. When joined by ligaments, the vertebrae form the vertebral canal
that protects the spinal cord.

Small intervertebral foramina
occur between adjacent vertebrae. They serve as lateral passageways for spinal nerves that exit the spinal cord.

Cervical Vertebrae

The first seven vertebrae are the cervical (ser^vi-kul) vertebrae (C1-C7) that support the neck. They are unique in having a transverse foramen
in each transverse process. It serves as a passageway for the vertebral arteries and veins, blood vessels involved in blood flow to and from the brain.

The first two cervical vertebrae are distinctly different from the rest. The first vertebra (C1), or atlas, whose superior articular facets articulate with the occipital condyles, supports the head. The second vertebra (C2), which is called the axis, has a prominent dens
that projects superiorly from the vertebral body, providing a pivot point for the atlas. When the head is turned, the atlas rotates on the axis.

Thoracic Vertebrae

The 12 thoracic vertebrae (T1-T12) are larger than the cervical vertebrae, and their spinous processes are longer and slope inferiorly. The ribs articulate with costal facets on the transverse processes and bodies of thoracic vertebrae.

Lumbar Vertebrae

The five lumbar vertebrae (L1-L5) have heavy, thick bodies to support the greater stress and weight that is placed on this region of the vertebral column. The spinous processes are blunt and provide a large surface area for the attachment of heavy back muscles.

Sacrum

The sacrum (sa-k’rum) is composed of five fused sacral vertebrae (S1-S5). It articulates with the fifth lumbar vertebra and forms the posterior wall of the pelvis. The spinous processes of the fused vertebrae form the median sacral crest on the posterior midline. On either side of the median sacral crest are the posterior sacral foramina, passageways for blood vessels and nerves. Anterior sacral foramina on the anterior surface serve a similar function. The sacral canal is a continuation of the vertebral canal that carries spinal nerve roots to the sacral foramina and the sacral hiatus, an inferior opening proximal to the coccyx.

Coccyx

The most inferior part of the vertebral column is the coccyx (kok’-six), or tailbone, which is formed of three to five fused coccygeal vertebrae.

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