Vertebral column: The 33 vertebrae that fit together to form a flexible, yet extraordinarily tough, column that serves to support the back through a full range of motion. The vertebral column also protects the spinal cord, which runs from the brain through the hollow space in the middle of the vertebral column. There are 7 cervical (C1'C7), 12 thoracic (T1'T12), 5 lumbar (L1'L5), 5 sacral (S1'S5), and 3 to 5 coccygeal vertebrae in the vertebral column, each separated by intervertebral disks. The first cervical vertebra, known as the atlas, supports the head. It pivots on the odontoid process of the second cervical vertebra, the axis. The cervical vertebrae end at their juncture with the thoracic vertebrae. The seventh cervical vertebra (the prominent vertebra, so named because of its long spiny projection) adjoins the first thoracic vertebra. The thoracic vertebrae provide an attachment site for the true ribs and make up part of the back of the chest (thorax). This part of the spine is very flexible, to permit bending and twisting. The thoracic vertebrae join the lumbar vertebrae, which are particularly sturdy and large because they support the entire structure. The lumbar vertebrae are nonetheless quite flexible. At the top of the pelvis, the lumbar vertebrae join the sacral vertebrae. By adulthood, the five sacral vertebrae have usually fused to form a triangular bone called the sacrum. At the tip of the sacrum, the final part of the vertebral column projects slightly outward. This is the coccyx, better known as the tailbone. It is made up of 3 to 5 coccygeal vertebrae: small, rudimentary vertebrae that fuse together. Also known as the spinal column.