The spine (also called the vertebral column or spinal column) is composed of a series of bones called vertebrae stacked one upon another. There are four regions of the spine:
- cervical (neck)
- thoracic (chest/trunk)
- lumbar (low back)
- sacral (pelvic)
The cervical spine is made up of seven cervical vertebrae. The main function of the cervical spine is to support the weight of the head which is approximately 10-12 pounds. The cervical spine has the greatest range of motion, in part because of two specialized vertebra that move with the skull. Cervical vertebrae are the smallest of the vertebrae. The first cervical vertebra is called the atlas and is significantly different from the other vertebrae. It is ring-like in shape with two large protrusions on the sides to support the weight of the head. The second cervical vertebra is called the axis.The axis is also unique in that it has a bony peg-like protrusion, called the dens or odontoid on its upper surface that fits within the ring of the atlas. The curve of the neck is described as a lordosis or lordotic curve, and looks like a “C” in reverse.
The main function of the thoracic spine is to protect the organs of the chest, especially the heart and lungs. There are 12 thoracic vertebrae with one rib attached on each side, to create a thoracic cage, which protects the internal organs of the chest. The thoracic spine has a normal kyphosis, or “C” curve. The thoracic spine is less mobile than the cervical and lumbar spine because of the thoracic cage.
The lumbar spine has five lumbar vertebrae, which are the largest vertebrae. These vertebrae are also aligned in a reverse “C” like the cervical spine, creating a normal lumbar lordosis. The five lumbar vertebral bodies are the weight-bearing portion of the spine and are the largest in diameter compared to the thoracic and cervical vertebral bodies. They sit atop the sacrum, which is formed by five vertebrae fused together into a solid unit. There are usually no identifiable disc spaces between the sacral segments. At the end of the spinal column is the coccyx or tailbone. Most people have 33 vertebrae in total, although there may be 32 or 34. Variations are usually found in the lumbar or sacral regions.