sacrum

The sacrum is a triangle shaped bone that consist of five segments (S1 to S5). The five segments are actually fused together during a baby's development in the womb.
There are two series of four openings on each side of the sacrum, through which nerves and blood vessels run.
The sacrum makes up the back wall of the pelvis and connects the pelvis to the spine. The sacrum is wider and shorter in women, making the opening in the pelvis wider to easier accomodate birth.