Spine anatomy. Historical anatomical artwork of the spine (backbone) seen from the front (left), side (middle) and rear (right). For the side view, the rear of the spine is at left. The spine is a flexible column of small compact bones called vertebrae, separated by discs of fibrocartilage. Each vertebra has bony processes protruding on either side for muscle attachments. The bottom two vertebrae are fused structures called the sacrum and coccyx. The spine extends from the base of the skull to the pelvis, enclosing and protecting the spinal cord. In an adult there are 26 vertebrae. Artwork from The Bones and Ligaments of the Human Body (Ed. Jones Quain, London, 1842).