THE VERTEBRAL COLUMN
The spine is composed of the vertebral bones and intervertebral disks (Figure 45–1). There are 7 cer-vical (C), 12 thoracic (T), and 5 lumbar (L) vertebrae (Figure 45–2). The sacrum is a fusion of 5 sacral (S)
vertebrae, and there are small rudimentary coccy-geal vertebrae. The spine as a whole provides struc-tural support for the body and protection for the spinal cord and nerves and allows a degree of mobil-ity in several spatial planes. At each vertebral level, paired spinal nerves exit the central nervous system (Figure 45–2).
Vertebrae differ in shape and size at the various levels. The first cervical vertebra, the atlas, lacks a body and has unique articulations with the base of the skull and the second vertebra. The second verte-bra, called the axis, consequently has atypical artic-ulating surfaces. All 12 thoracic vertebrae articulate with their corresponding rib. Lumbar vertebrae have a large anterior cylindrical vertebral body. A hollow ring is defined anteriorly by the vertebral body, lat-erally by the pedicles and transverse processes, and posteriorly by the lamina and spinous processes (Figure 45–1B and C). The laminae extend between the transverse processes and the spinous processes, and the pedicle extends between the vertebral body and the transverse processes. When stacked verti-cally, the hollow rings become the spinal canal in which the spinal cord and its coverings sit. The indi-vidual vertebral bodies are connected by the inter-vertebral disks. There are four small synovial joints at each vertebra, two articulating with the vertebra above it and two with the vertebra below. These are the facet joints, which are adjacent to the transverse processes (Figure 45–1C). The pedicles are notched superiorly and inferiorly, these notches forming the intervertebral foramina from which the spinal nerves exit. Sacral vertebrae normally fuse into one large bone, the sacrum, but each one retains discrete anterior and posterior intervertebral foramina. The laminae of S5 and all or part of S4 normally do not fuse, leaving a caudal opening to the spinal canal, the sacral hiatus ( Figure 45–3).
The spinal column normally forms a double C, being convex anteriorly in the cervical and lum-bar regions (Figure 45–2). Ligamentous elements provide structural support, and, together with supporting muscles, help to maintain the unique shape. Ventrally, the vertebral bodies and inter-vertebral disks are connected and supported by the anterior and posterior longitudinal ligaments (Figure 45–1A). Dorsally, the ligamentum flavum,
interspinous ligament, and supraspinous ligament provide additional stability. Using the midline approach, a needle passes through these three dor-sal ligaments and through an oval space between the bony lamina and spinous processes of adjacent ver-tebra (Figure 45–4).