Coccyx and lower back. Coloured X-ray of a side view of the coccyx and lower back of a healthy adult man. The front of the body is at right. At top, the bottom two lumbar vertebrae (orange, block-like) are seen. Below them at lower left is the sacrum, a bone formed from five fused vertebrae in the pelvis. At the base of the sacrum is the coccyx (bottom left), a small bone formed from four fused vertebrae that is an evolutionary remnant of a tail. The round ball- joint of the thigh-bone (femur) is at lower right.