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" (from the Latin term nutatio which means "nodding"), and the reverse (posterior) motion "counter-nutation." In upright vertebrates, the sacrum is capable of slight independent movement along the sagittal plane. When you bend backward the top (base) of the sacrum moves forward relative to the ilium; when you bend forward the top moves back.
The sacrum is called so when referred to all of the parts combined. Its parts are called sacral vertebrae when referred individually.
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