Vertebral column

The total number of vertebrae during early development is 33. Then, several vertebrae in the sacral and coccygeal regions fuse. As a result, the adult vertebral column typically contains 26 vertebrae.

■7 cervical vertebrae in the neck region. ■ 12 thoracic vertebrae posterior to the thoracic cavity. ■ 5 lumbar vertebrae support the lower back. ■ 1 sacrum consists of five fused sacral vertebrae. ■1 coccyx usually consists of four fused coccygeal vertebrae.

When viewed from the side, the vertebral column shows four slight bends called normal curves.

Relative to the front of the body, the cervical and lumbar curves are convex (bulging out), and the thoracic and sacral curves are concave (cupping in).

The curves of the vertebral column increase its strength, help maintain balance in the upright position, absorb shocks during walking and running, and help protect the vertebrae from breaks .

The cervical, thoracic, and lumbar vertebrae are movable, but the sacrum and coccyx are immovable.

Between adjacent vertebrae from the second cervical vertebra to the sacrum are intervertebral discs.

Each disc has an outer ring of fibrocartilage and a soft, pulpy, highly elastic interior.

The discs form strong joints, permit various movements of the vertebral column, and absorb vertical shock.

In the fetus, there is a single concave curve.

At about the third month after birth, when an infant begins to hold its head erect, the cervical curve develops.

Later, when the child sits up, stands, and walks, the lumbar curve develops.

The body, the thick, disc-shaped front portion, is the weight-bearing part of a vertebra.

-The vertebral arch extends backwards from the body of the vertebra. -It is formed by two short, thick processes, the pedicles, which project backward from the body to unite with the laminae. -The laminae are the flat parts of the arch and end in a single sharp, slender projection called a spinous process. -The hole between the vertebral arch and body contains the spinal cord and is known as the vertebral foramen. -Together, the vertebral foramina of all vertebrae form the vertebral cavity. -When the vertebrae are stacked on top of one another, there is an opening between adjoining vertebrae on both sides of the column. -Each opening, called an intervertebral foramen, permits the passage of a single spinal nerve.

-Seven processes arise from the vertebral arch. -At the point where a lamina and pedicle join, a transverse process extends laterally on each side. -A single spinous process (spine) projects from the junction of the laminae. -These three processes serve as points of attachment for muscles. -The remaining four processes form joints with other vertebrae above or below. -The two superior articular processes of a vertebra articulate with the vertebra immediately above them. -The two inferior articular processes of a vertebra articulate with the vertebra immediately below them. -The smooth articulating surfaces of the articular processes are called facets, which are covered with hyaline cartilage.

Vertebrae in each region are numbered in sequence from top to bottom.

The seven cervical vertebrae are termed C1 through C7.

The spinous processes of the second through sixth cervical vertebrae are often bifid, or split into two parts.

All cervical vertebrae have three foramina: one vertebral foramen and two transverse foramina.

Each cervical transverse process contains a transverse foramen through which blood vessels and nerves pass.

The first two cervical vertebrae differ considerably from the others.

The first cervical vertebra (C1), the atlas, supports the head and is named for the mythological Atlas who supported the world on his shoulders.

The atlas lacks a body and a spinous process.

The upper surface contains superior articular facets that articulate with the occipital bone of the skull. This articulation permits you to nod your head to indicate “yes.”

The inferior surface contains inferior articular facets that articulate with the second cervical vertebra.

-The second cervical vertebra (C2), the axis, does have a body and a spinous process. -A tooth-shaped process called the dens projects up through the vertebral foramen of the atlas. -The dens is a pivot on which the atlas and head move, as in side-to-side movement of the head to signify “no.”

-The third through sixth cervical vertebrae (C3 through C6), represented by the vertebra, correspond to the structural pattern of the typical cervical vertebra. -The seventh cervical vertebra (C7), called the vertebra prominens, is somewhat different. -It is marked by a single, large spinous process that can be seen and felt at the base of the neck.

-Thoracic vertebrae (T1 through T12) are considerably larger and stronger than cervical vertebrae. -Distinguishing features of the thoracic vertebrae are their facets for articulating with the ribs. -Movements of the thoracic region are limited by the attachment of the ribs to the sternum.

-The lumbar vertebrae (L1 through L5) are the largest and strongest of the unfused bones in the vertebral column. -Their various projections are short and thick, and the spinous processes are well adapted for the attachment of the large back muscles.

-The sacrum is a triangular bone formed by the fusion of five sacral vertebrae, indicated in as S1 through S5. -The fusion of the sacral vertebrae begins between ages 16 and 18 years and is usually completed by age 30. -The sacrum serves as a strong foundation for the pelvic girdle. -It is positioned at the back of the pelvic cavity where its lateral surfaces fuse to the two hip bones.

-The anterior and posterior sides of the sacrum contain four pairs of sacral foramina. -Nerves and blood vessels pass through the foramina. -The sacral canal is a continuation of the vertebral cavity. -The lower entrance of the canal is called the sacral hiatus. -The anterior top border of the sacrum has a projection, called the sacral promontory, which is used as a landmark for measuring the pelvis prior to childbirth.

-The coccyx, like the sacrum, is triangular in shape and is formed by the fusion of the four coccygeal vertebrae. -These are indicated in as Co1 through Co4. -The top of the coccyx articulates with the sacrum.