Lower spine. Coloured profile magnetic resonance imaging (MRI) scan of a healthy lower spine. The front of the spine is at left. The spine is composed of cylindrical bones, called vertebrae (central segments, blue), that support the body and protect the spinal cord. The spinal cord is made up of nerve fibres (solid blue line tapering off towards the bottom) and is surrounded by cerebrospinal fluid (white). The bony projections (blue) to the right of the spinal cord are attachment sites for the muscles and ligaments (dark blue) that hold the spine together. At top are the lower two thoracic vertebrae, followed by five lumbar vertebrae. At bottom is the sacrum, which forms the back of the pelvis and leads to the coccyx (tailbone, not seen).