A triangular bone that consists of the five fused sacral vertebrae (S1- S5).

The sacrum articulates with the fifth lumbar vertebra and the coccyx.

A triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted between the two hip bones.

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.

The pelvic surface of the sacrum is concave, 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 is directed upward and forward, and the apex is directed downward and articulates with the coccyx.

The vertebral 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.

The sacrum contains the sacral nerves, and its walls are perforated by the anterior and posterior sacral foramina through which these nerves pass out.

The sacral hiatus and the posterior superior iliac spines form an equilateral triangle pointing inferiorly.

The sacral hiatus is identified by palpating the coccyx, then sliding the finger cephalad until a depression in the skin is felt.

The sacrum articulates with four bones: the last lumbar vertebra above, the coccyx below, the illium portion of the hip bone on either side.

The spinal cord reaches L3-4 in the neonate and the dural sac can be found at S3-4.

In adult the spinal cord levels of L1 and dural sac S1 are usually reached by 1 year of age.

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