<P> Each vertebra is an irregular bone . The size of the vertebrae varies according to placement in the vertebral column, spinal loading, posture and pathology . Along the length of the spine the vertebrae change to accommodate different needs related to stress and mobility . </P> <P> Every vertebra has a body, which consists of a large anterior middle portion called the centrum (plural centra) and a posterior vertebral arch, also called a neural arch . The body is composed of cancellous bone, which is the spongy type of osseous tissue, whose micro-anatomy has been specifically studied within the pedicle bones . This cancellous bone is in turn, covered by a thin coating of cortical bone (or compact bone), the hard and dense type of osseous tissue . The vertebral arch and processes have thicker coverings of cortical bone . The upper and lower surfaces of the body of the vertebra are flattened and rough in order to give attachment to the intervertebral discs . These surfaces are the vertebral endplates which are in direct contact with the intervertebral discs and form the joint . The endplates are formed from a thickened layer of the cancellous bone of the vertebral body, the top layer being more dense . The endplates function to contain the adjacent discs, to evenly spread the applied loads, and to provide anchorage for the collagen fibers of the disc . They also act as a semi-permeable interface for the exchange of water and solutes . </P> <P> The vertebral arch is formed by pedicles and laminae . Two pedicles extend from the sides of the vertebral body to join the body to the arch . The pedicles are short thick processes that extend, one from each side, posteriorly, from the junctions of the posteriolateral surfaces of the centrum, on its upper surface . From each pedicle a broad plate, a lamina, projects backwards and medialwards to join and complete the vertebral arch and form the posterior border of the vertebral foramen, which completes the triangle of the vertebral foramen . The upper surfaces of the laminae are rough to give attachment to the ligamenta flava . These ligaments connect the laminae of adjacent vertebra along the length of the spine from the level of the second cervical vertebra . Above and below the pedicles are shallow depressions called vertebral notches (superior and inferior). When the vertebrae articulate the notches align with those on adjacent vertebrae and these form the openings of the intervertebral foramina . The foramina allow the entry and exit of the spinal nerves from each vertebra, together with associated blood vessels . The articulating vertebrae provide a strong pillar of support for the body . </P> <P> There are seven processes projecting from the vertebra; a spinous process, two transverse processes, and four articular processes . A major part of a vertebra is a backward extending spinous process (sometimes called the neural spine) which projects centrally . This process points dorsally and caudally from the junction of the laminae . The spinous process serves to attach muscles and ligaments . </P>

The pedicles laminae and spinous process of a vertebra form the