The spine is a complex organ consisting of bones, ligaments, muscles and inter-vertebral discs. In addition, it surrounds part of the central nervous system and part of the peripheral nervous system. It has a musculo-skeletal function and also a protective function for the delicate nervous tissue it surrounds.
The bones of the spine are called vertebrae. There are generally 7 cervical vertebrae and these are conventionally labelled C1 to C7. There are 12 thoracic vertebrae and these are numbered and labelled T1 to T12 (occasionally they will be labelled D1 to D12 where D stands for dorsal). There are 5 lumbar vertebrae; L1 to L5. L5 articulates with the sacrum. The sacrum forms the lower part of the spine and is situated between the 2 sides of the pelvis. It is formed from 5 vertebral segments which generally have fused into a single bone during development but can still be labelled S1 to S5. The disc between L5 and the sacrum is labelled L5/S1. At the tip of the sacrum are a variable number of coccygeal segments: the coccyx. The central nervous system consists of the brain and the spinal cord. The spinal cord starts at the base of the brain and leaves through the base of the skull into the spinal canal. The spinal canal is a roughly circular canal that is formed by the vertebral bodies. The spinal cord extends from the base of the skull to approximately L1. At each spinal level, a pair of nerves is given off which pass beneath the pedicles and through the foramen. The foramen is a space on each side of the spine and at each level. It is bordered by the pedicles above and below of adjacent vertebrae, with the disc in front and the facet joint behind.
The pairs of nerves given off at each level of the spine are part of the peripheral nervous system.
These divide and reform outside the spine and go on to make other nerves including the main nerves that go up and down the limbs.