The Vertebral Column and Vertebrae
Vertebral Column Overview
The Vertebral column (spinal column) supports the head and encloses the spinal cord.
The spinal column is comprised of 26 individual bones, these bones are referred to as vertebrae. The spinal column is divided into 5 different areas containing groups of vertebrae and are grouped as follows:
7 cervical vertebrae in the neck.
12 thoracic vertebrae in the upper back corresponding to each pair of ribs.
5 lumbar vertebrae in the lower back.
5 sacral vertebrae which are fused together to form 1 bone called the sacrum.
4 coccygeal vertebrae that are fused together to form the coccyx or tail-bone.
The vertebrae are referred to by their name and number, counting down from the top of the spinal column as follows:
The cervical vertebrae are C1 - C7
The thoracic vertebrae are T1 - T12
The lumbar vertebrae are L1 - L5
The sacrum and coccyx do not have numbers and each is thought of as one bone. Spinal nerves exit the sacrum and coccyx at levels (Foramen) within the main structure of each vertebra.
General Characteristics of a Vertebra
A typical vertebra consists of two essential parts, an anterior segment - the body, and a posterior part - the vertebral or neural arch; these enclose a foramen, the vertebral foramen. The vertebral arch consists of a pair of pedicles and a pair of laminae, and supports seven processes, four articular, two transverse, and one spinous.
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), while between every pair of vertebrae are two apertures, the inter-vertebral foramina, one on either side, for the transmission of the spinal nerves and vessels.
Body (corpus vertebrae): The body is the largest part of a vertebra, and is more or less cylindrical in shape. Its upper and lower surfaces are flattened and rough, and give attachment to the inter-vertebral fibrocartilages, and each presents a rim around its circumference. In front, the body is convex from side to side and concave from above downward. Behind, it is flat from above downward and slightly concave from side to side. Its anterior surface presents a few small apertures, for the passage of nutrient vessels; on the posterior surface is a single large, irregular aperture, or occasionally more than one, for the exit of the basi-vertebral veins from the body of the vertebra.
Pedicles (radices arci vertebrae): The pedicles are two short, thick processes, which project backward, one on either side, from the upper part of the body, at the junction of its posterior and lateral surfaces. The concavities above and below the pedicles are named the vertebral notches; and when the vertebrae are articulated, the notches of each contiguous pair of bones form the inter-vertebral foramina, already referred to.
Laminae: The laminae are two broad plates directed backward and medial ward from the pedicles. They fuse in the middle line posteriorly, and so complete the posterior boundary of the vertebral foramen. Their upper borders and the lower parts of their anterior surfaces are rough for the attachment of the ligamenta flava.
Processes - Spinous Process (processus spinosus): The spinous process is directed backward and downward from the junction of the laminae, and serves for the attachment of muscles and ligaments.
Articular Processes: The articular processes, two superior and two inferior, spring from the junctions of the pedicles and laminae. The superior project upward, and their articular surfaces are directed more or less backward; the inferior project downward, and their surfaces look more or less forward. The articular surfaces are coated with hyaline cartilage.
Transverse Processes (processus transversi): The transverse processes, two in number, project one at either side from the point where the lamina joins the pedicle, between the superior and inferior articular processes. They serve for the attachment of muscles and ligaments.
Structure of a Vertebra (Fig. 2): The body is composed of cancellous tissue, covered by a thin coating of compact bone; the latter is perforated by numerous orifices, some of large size for the passage of vessels; the interior of the bone is traversed by one or two large canals, for the reception of veins, which converge toward a single large, irregular aperture, or several small apertures, at the posterior part of the body. The thin bony lamellae of the cancellous tissue are more pronounced in lines perpendicular to the upper and lower surfaces and are developed in response to greater pressure in this direction (Fig. 2). The arch and processes projecting from it have thick coverings of compact tissue.