The vertebral column (backbone or spine) usually consists of 24 articulating vertebrae and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs, and it houses and protects the spinal cord in its spinal canal.

The illustration at right shows the five regions of the spine – cervical, thoracic, lumbar, sacral, and coccygeal (click on the image to expand it). There are normally 33 vertebrae in humans, including the five that are fused to form the sacrum (the others are separated by intervertebral discs) and the four coccygeal bones that form the tailbone. The upper three regions comprise the remaining 24: seven cervical vertebrae, 12 thoracic vertebrae, and five lumbar vertebrae. Some people have an extra vertebra in one region or may be missing a vertebra in another. The number of cervical vertebrae, however, very rarely increases or decreases.

Structure of Vertebrae

A typical vertebra consists of two essential parts:

  • Anterior (front) segment, which is the vertebral body
  • Posterior part – the vertebral (neural) arch – which encloses the vertebral foramen

The vertebral arch is formed by a pair of pedicles and a pair of laminae, and supports seven processes, four articular, two transverse, and one spinous, the latter also being known as the neural spine.

When the vertebrae are articulated with each other, the bodies form a strong pillar for the support of the head and trunk, and the vertebral foramina constitute a canal for the protection of the medulla spinalis (spinal cord). In between every pair of vertebrae are two apertures, the intervertebral foramina, one on either side, for the transmission of the spinal nerves and vessels. Two transverse processes and one spinous process are posterior to the vertebral body. The spinous process comes out the back, one transverse process comes out the left, and one on the right. The spinous processes of the cervical and lumbar regions can be felt through the skin.

Superior and inferior articular facets on each vertebra act to restrict the range of movement possible. These facets are joined by a thin portion of the neural arch called the pars interarticularis.

Curves

Viewed laterally, the vertebral column presents several curves, which correspond to the different regions of the column, and are called cervical, thoracic, lumbar, and pelvic.

  • The cervical curve, convex forward, begins at the apex of the odontoid process, and ends at the middle of the second thoracic vertebra; it is the least marked of all the curves.
  • The thoracic curve, concave forward, begins at the middle of the second and ends at the middle of the 12th thoracic vertebra. Its most prominent point behind corresponds to the spinous process of the seventh thoracic vertebra. This curve is known as a tt curve.
  • The lumbar curve is more marked in the female than in the male; it begins at the middle of the last thoracic vertebra, and ends at the sacrovertebral angle. It is convex anteriorly, the convexity of the lower three vertebrae being much greater than that of the upper two. This curve is described as a lordotic curve.
  • The pelvic curve begins at the sacrovertebral articulation and ends at the point of the coccyx. Its concavity is directed downward and forward.

The thoracic and pelvic curves are termed primary curves because they alone are present during fetal life. The cervical and lumbar curves are compensatory or secondary, and are developed after birth, the former when the child is able to hold up its head (at 3 or 4 months) and to sit upright (at 9 months), the latter at 12 to 18 months, when the child begins to walk.

The vertebral canal follows the different curves of the column; it is large and triangular in those parts of the column which enjoy the greatest freedom of movement, such as the cervical and lumbar regions, and small and rounded in the thoracic region, where motion is more limited.

Regions

Cervical

There are seven cervical bones (but eight cervical spinal nerves) that in general, are small and delicate. They are numbered top to bottom, from C1 to C7. Their spinous processes are short and often bifurcated, with the exception of C2 and C7, which have palpable spinous processes. Cervical vertebrae possess triangular-shaped transverse foramina that allow vertebral arteries to pass through on their way to the foramen magnum to end in the circle of Willis.

Atlas (C1) and axis (C2) are the vertebrae that allow the neck and head so much movement. For the most part, the atlanto-occipital joint allows the skull to move up and down, while the atlanto-axial joint allows the upper neck to twist left and right. The axis also sits on the first intervertebral disk of the spinal column.

The term cervicothoracic is often used to refer to the cervical and thoracic vertebrae together, and sometimes also their surrounding areas.

Thoracic

The 12 thoracic bones and their transverse processes have surfaces that articulate with the ribs. Some rotation can occur between the thoracic vertebrae, but their connection with the rib cage prevents much flexion or other excursion. The bodies of the thoracic bones are roughly heart-shaped and are about as wide anterio-posterioly as they are in the transverse dimension. Vertebral foramina are roughly circular in shape.

The term thoracolumbar is sometimes used to refer to the thoracic and lumbar vertebrae together, and sometimes also their surrounding areas.

Lumbar

The five lumbar vertebrae are very robust in construction, as they must support more weight than other vertebrae. They allow significant flexion and extension, moderate lateral flexion (side-bending), and a small degree of rotation. The discs between these vertebrae create a lumbar lordosis (curvature that is concave posteriorly) in the human spine.

The term lumbosacral is often used to refer to the lumbar and sacral vertebrae together, and sometimes also their surrounding areas.

Sacral

There are five sacral vertebrae (S1-S5) and they are fused in maturity, with no intervertebral discs.

Coccygeal

There are usually four and rarely three or five coccygeal vertebrae (Co1-Co5), with no intervertebral discs.

Surfaces

Anterior surface

When viewed from in front, the width of the bodies of the vertebrae is seen to increase from the second cervical to the first thoracic. There is then a slight diminution in the next three vertebrae, and below this, there is again a gradual and progressive increase in width as low as the sacrovertebral angle. From this point, there is a rapid diminution, to the apex of the coccyx.

Posterior surface

The posterior surface of the vertebral column presents in the median line the spinous processes:

  • In the cervical region (with the exception of the second and seventh vertebrae), these are short and horizontal with bifid extremities.
  • In the upper part of the thoracic region they are directed obliquely downward; in the middle they are almost vertical; and in the lower part they are nearly horizontal.
  • In the lumbar region they are nearly horizontal.

The spinous processes are separated by considerable intervals in the lumbar region, by narrower intervals in the neck, and are closely approximated in the middle of the thoracic region. Occasionally one of these processes deviates a little from the median line — a fact to be remembered in practice, as irregularities of this sort are attendant also on fractures or displacements of the vertebral column.

On either side of the spinous processes is the vertebral groove formed by the laminae in the cervical and lumbar regions, where it is shallow, and by the laminae and transverse processes in the thoracic region, where it is deep and broad; these grooves lodge the deep muscles of the back.

Lateral to the vertebral grooves are the articular processes, and still more laterally, the transverse processes. They stand backward in the thoracic region, on a plane considerably behind that of the same processes in the cervical and lumbar regions:

  • In the cervical region, the transverse processes are placed in front of the articular processes, lateral to the pedicles and between the intervertebral foramina.
  • In the thoracic region, they are posterior to the pedicles, intervertebral foramina, and articular processes.
  • In the lumbar region, they are in front of the articular processes, but behind the intervertebral foramina.
Lateral surfaces

The lateral surfaces are separated from the posterior surface by the articular processes in the cervical and lumbar regions, and by the transverse processes in the thoracic region. They present, in back, the sides of the bodies of the vertebrae, marked in the thoracic region by the facets for articulation with the heads of the ribs. More posteriorly are the intervertebral foramina, formed by the juxtaposition of the vertebral notches, oval in shape, smallest in the cervical and upper part of the thoracic regions, and gradually increasing in size to the last lumbar. They transmit the special spinal nerves and are situated between the transverse processes in the cervical region, and in front of them in the thoracic and lumbar regions.

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