In general, a vertebral bone (vertebra) consists of a body, a vertebral arch, and special bony processes arising from the arch. The vertebral body is a thick columnar-shaped bone comprising the front of each vertebra. The vertebral arch is a partial circle of bone that is connected to the back of the body; it is made up of the pedicles and laminae (parts of the vertebra important to certain surgical procedures). The vertebral body and the vertebral arch complete a circle to form the spinal canal, through which the spinal cord and spinal nerves pass. Specialized bony processes arising from the arch include a spinous process (the hard projections one can feel in the middle of the back), two transverse processes, and four facet joints (articular processes). [Figure 1 ]
Figure 1: Parts of Vertebrae, lumbar example
Throughout the spinal column, the size and shape of vertebral parts differ to accommodate the function of that particular spinal region. In the neck, the cervical vertebrae have short spinous processes and the transverse processes have a special canal (transverse foramen) for blood vessels passing up to the brain. Of the first two cervical vertebrae, the atlas (C1), which supports the head, has no vertebral body. It is the upper articulation of the atlas with the skull that allows the head to nod, as when saying the word "yes". The axis (C2) has a unique vertical-shaped projection called the dens, or odontoid process, that acts as a pivot for rotation of the atlas and skull, as when a person turns their head to each side to indicate the word "no" . [Figure 2 ] The position of the facet joints in the thoracic vertebrae allows for significant rotation of the spine in this region, as might be used during a golf swing, but the long spinous processes of these vertebrae (in addition to the rib cage) act to limit the degree of motion in the midback. Lumbar vertebrae have facet joint positions that are oriented in the sagittal plane, and act to block almost all rotation in the lower back. This helps to protect the lumbar discs, which can be damaged by rotation. The orientation of the lumbar facets, however, allows for a large amount of forward (flexion) and backward (extension) bending. The spinous processes of the lumbar spine are short and stubby, and the bodies are progressively larger to support the increased weight from above.
Figure 2: Specialized cervical vertabrae: atlas and axis