Sacrum

In vertebrate anatomy the sacrum (plural: sacrums or sacra) is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx (tailbone). It consists of usually five initially unfused vertebrae which begin to fuse between ages 16–18 and are usually completely fused into a single bone by age 34.

It is curved upon itself and placed obliquely (that is, tilted forward). It is kyphotic — that is, concave facing forwards. The base projects forward as the sacral promontory internally, and articulates with the last lumbar vertebra to form the prominent sacrovertebral angle. The central part is curved outward towards the posterior, allowing greater room for the pelvic cavity. The two lateral projections of the sacrum are called ala (wings), and articulate with the ilium at the L-shaped sacroiliac joints.

Etymology
The name is derived from the Latin (os) sacrum, (sacer, sacra, sacrum, "sacred"), a translation of the Greek hieron (osteon), meaning sacred or strong bone. Since the sacrum is the seat of the organs of procreation, animal sacra were offered in sacrifices. In Slavic languages and in German this bone is called the 'cross bone', in Dutch 'holy bone' (Heiligbeen).

Parts

 * The pelvic surface of the sacrum is concave from above downward, and slightly so from side to side.
 * The dorsal surface of the sacrum is convex and narrower than the pelvic.
 * The lateral surface of the sacrum is broad above, but narrowed into a thin edge below.
 * The base of the sacrum, which is broad and expanded, is directed upward and forward.
 * The apex (apex oss. sacri) is directed downward, and presents an oval facet for articulation with the coccyx.
 * The vertebral canal (canalis sacralis; sacral canal) runs throughout the greater part of the bone; above, it is triangular in form; below, its posterior wall is incomplete, from the non-development of the laminae and spinous processes. It lodges the sacral nerves, and its walls are perforated by the anterior and posterior sacral foramina through which these nerves pass out.

Articulations
The sacrum articulates with four bones:
 * the last lumbar vertebra above
 * the coccyx (tailbone) below
 * the illium portion of the hip bone on either side

Rotation of the sacrum forward a few degrees vis-à-vis the ilia is sometimes called "nutation" (L. "nodding"), and the reverse motion "counter-nutation."

It is called the sacrum when referred to all of the parts combined, but sacral vertebrae when referred individually.

Sexual dimorphism
The sacrum is noticeably sexually dimorphic (differently-shaped in males and females).

In the female the sacrum is shorter and wider than in the male; the lower half forms a greater angle with the upper; the upper half is nearly straight, the lower half presenting the greatest amount of curvature. The bone is also directed more obliquely backward; this increases the size of the pelvic cavity and renders the sacrovertebral angle more prominent.

In the male the curvature is more evenly distributed over the whole length of the bone, and is altogether larger than in the female.

Variations
In some cases the sacrum will consist of six pieces or be reduced in number to four. The bodies of the first and second vertebrae may fail to unite.

Sometimes the uppermost transverse tubercles are not joined to the rest of the ala on one or both sides, or the sacral canal may be open throughout a considerable part of its length, in consequence of the imperfect development of the laminae and spinous processes.

The sacrum also varies considerably with respect to its degree of curvature.