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.
Sacrum κλόνις (Latinized "clonis"); this term is cognate to Latin "clunis" "buttock", Sanskrit "" "haunch" and Lithuanian "šlaunis" "hip, thigh". The Latin word is found in the alternative Latin name of the sacrum, "ossa clunium", as it were "bones of the buttocks". Due to the fact that the os sacrum is broad and thick at its upper end, the sacrum is alternatively called "os latum", "broad bone". In dogs the sacrum is formed by three fused vertebrae. The sacrum in the horse is made up of five fused vertebrae. In birds the sacral vertebrae are fused with the lumbar and some