Lower back. Coloured X-ray of a normal lower back (lumbosacral region) seen from the side; the front of the body is at left. The upper blocks are the bones of the lower spine (vertebrae). They are separated by intervertebral discs of soft cartilage, which are not shown clearly on the X- ray. At lower right is the upper part of the sacrum, a triangular bone composed of five fused vertebrae, which forms the rear of the pelvis.