The sacred bone at the base of your spine starts out as five
The sacrum, the triangular bone wedged between the hip bones, begins as five separate vertebrae that fuse only between the ages of 18 and 30. Its name comes from the Latin os sacrum, sacred bone, a translation of a Greek term used by Galen, and English speakers once simply called it the holy bone.
It sits at the upper back of the pelvic cavity and meets four bones: the last lumbar vertebra above, the coccyx below, and the ilium on each side through ear-shaped surfaces that form the L-shaped sacroiliac joints. Its side projections are called alae, or wings. The bone curves on itself, with the broad upper base tipping forward as the sacral promontory and the middle bowing backward to give the pelvis more room. The promontory meets the spine at about 30 degrees from horizontal, a landmark used when placing a sling implant.
Traces of its origins stay visible. Four transverse ridges across the front mark where the five vertebrae once met, and four pairs of openings, the sacral foramina, let the sacral nerves out and arteries in. Along the back, three or four bumps on the median crest are leftover spinous processes. The canal inside continues the spinal canal, and where the lowest arches fail to close, a gap called the sacral hiatus opens. Some people have six pieces or only four, and the first two may never unite.
Although fused, it is not rigid. The sacrum can tilt slightly in the sagittal plane: rocking forward and up relative to the ilium is called nutation, from the Latin for nodding, and the reverse is counter-nutation. When a person bends backward, the top of the bone shifts forward. Its embryonic building blocks, the sacral somites, appear around day 29 of development, a day before the last pairs that will form the coccyx.
Other animals fuse different numbers: five in the horse, three in the dog, four pelvic vertebrae in the rat. Birds merge sacral vertebrae with neighbours into a single synsacrum, and in frogs a mobile joint between the long ilium and the sacrum works like an extra limb for jumping. In humans the bone is a common site for spina bifida and for chordomas, sarcomas arising from notochord remnants, while fractures are uncommon but often damage nerves.
Source: Sacrum
