Healthy spine. Coloured magnetic resonance imaging (MRI) scan of a healthy human spine seen from the side (sagittal side). The front of the spine is on the left. The spine is a flexible column of 33 bones called the vertebrae (light green blocks), which are separated by intervertebral discs (dark green) made of cartilage. It extends from the base of the skull to the bottom of the back, supporting the head and trunk as well as enclosing and protecting the spinal cord.