A main function of the spine is to protect the nerve tissue of the spinal cord. The spinal cord is a rope-like bundle of countless millions of tiny nerve fibers that run from the base of the brain to the lumber spine. The cord is protected from harm by a bony tunnel that runs behind the vertebral bones.

As the spinal cord travels down from the brain, spinal nerves at each vertebral level exit the spine through a small bony tunnel on either side of the vertebral bones (arrow). Overall, there are thirty-one pairs of spinal nerves (eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal). Once the spinal nerves exit the spine, they group together to form the main nerves, which then travel to the arms and legs.