Sacrum lumbar vertebra, and its lower part with the coccyx (tailbone) via the sacral and coccygeal cornua. The sacrum has three different surfaces which are shaped to accommodate surrounding pelvic structures. Overall it is concave (curved upon itself). The base of the sacrum, the broadest and uppermost part, is tilted forward as the sacral promontory internally. The central part is curved outward toward the posterior, allowing greater room for the pelvic cavity. In all other quadrupedal vertebrates, the pelvic vertebrae undergo a similar developmental process to form a sacrum in the adult, even while the bony tail (caudal) vertebrae remain unfused.
The Singing Bone learns of the younger brother's adventure. They then set out to deliver the body to the king, but on passing a bridge, the older kills the younger and buries his body beneath it. He takes the boar himself to the king and marries the king's daughter as prize. One day a shepherd sees a bone under the bridge and uses it to make a mouthpiece for a horn, which sings of the brother's fate: "Ah! Dear shepherd, you are blowing your horn With one of my bones, which night and morn Lie still unburied, beneath the wave Where I was