The spine is a complex structure, the core of which is a stack of small bones (vertebrae). Cushioning disks separate each vertebral body, and the bones are connected to each other at small joints (facet joints) between bony protrusions at the back of each vertebra. The spinal cord runs through the center of the spine. Nerves exit this spinal canal through openings at most levels of the spine. Layers of muscles, ligaments and tendons surround the spine.