Sacrum

The sacrum is a large triangular bone at the base of the lower spine. Its broad upper part joins the lowest lumbar vertebrae and its narrow lower part joins the coccyx or "tail bone". The sides are connected to the iliums (the largest bones forming the pelvis). The sacrum is a strong bone and rarely fractures. The five vertebrae that make up the sacrum are separated in early life, but gradually become fused together between the eighteenth and thirtieth years. The spinous processes of these fused bones are represented by a ridge of tubercles. The sacrum is wedged between the coxal bones of the pelvis and is united to them by fibrocartilage at the sacroiliac joints. The weight of the body is transmitted to the legs through the pelvic girdle at these joints.