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 of the first piece of the coccyx, and this transmits the anterior division of the fifth sacral nerve. The thin lower half of the lateral surface gives attachment to the sacrotuberous and sacrospinous ligaments, to some fibers of the gluteus maximus at the back and to the coccygeus in the front. The sacrum articulates with four bones: Rotation of the sacrum superiorly and anteriorly whilst the coccyx moves posteriorly relative to the ilium is sometimes called "nutation" (from the Latin term nutatio which means "nodding") and the reverse, postero-inferior motion of the sacrum relative to the ilium whilst the coccyx