Introduction:
The large, triangular-shaped sacrum bone (os sacrum) is located inferior to the fifth lumbar vertebra (L5). It is composed of five vertebrae (S1-S5), which fuse during the late teens to the early twenties.
Each sacral vertebra contains a large central opening (or foramen). Together, the five openings form a sacral canal that extends the length of the bone. Inside the canal are nerve roots that originate from the end of the spinal cord (at L1). The nerve roots give rise to the sacral nerves, which enter and exit the sacral canal through anterior and posterior sacral foramina.
Below the sacrum is the coccyx (os coccygis). Also known as the tailbone, the coccyx consists of 3-5 small bones that are often fused.
The sacrum and coccyx are located between the left and right os coxae (=hips bones) and form the posterior portion of the bony pelvis (= os coxae, sacrum, and coccyx bones).
The concave anterior surface and convex posterior surface of the sacrum and coccyx also serve as attachment points for serveral muscles and ligaments.