Sacrum

: sacra or sacrums[1]), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1–S5) between ages 18 and 30.The two projections at the sides of the sacrum are called the alae (wings), and articulate with the ilium at the L-shaped sacroiliac joints.[3] The Stegosaurus dinosaur had a greatly enlarged neural canal in the sacrum, characterized as a "posterior brain case".[clarification needed] The sacral promontory marks part of the border of the pelvic inlet, and comprises the iliopectineal line and the linea terminalis.Its middle part is crossed by four transverse ridges, which correspond to the original planes of separation between the five sacral vertebrae.At each end of the transverse ridges, are the four anterior sacral foramina, diminishing in size in line with the smaller vertebral bodies.Each part at the sides of the foramina is traversed by four broad, shallow grooves, which lodge the anterior divisions of the sacral nerves.If a sagittal section be made through the center of the sacrum, the bodies are seen to be united at their circumferences by bone, wide intervals being left centrally, which, in the fresh state, are filled by the intervertebral discs.Behind it is a rough surface, the sacral tuberosity, on which are three deep and uneven impressions, for the attachment of the posterior sacroiliac ligament.Medial to this angle is a notch, which is converted into a foramen by the transverse process 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 somites that give rise to the vertebral column begin to develop from head to tail along the length of the notochord.[12] The sacrum is one of the main sites for the development of the sarcomas known as chordomas that are derived from the remnants of the embryonic notochord.In the frog, the ilium is elongated and forms a mobile joint with the sacrum that acts as an additional limb to give more power to its leaps.