Below the fourth spinous tubercle is an inverted U-shaped defect in the posterior wall of the sacral canal (vertebral foramen), where the laminae of the fifth sacral vertebra have not fused.  This is the inferior opening of the sacral canal and is referred to as the ‘sacral hiatus’.  The S5 and coccygeal spinal nerves leave the sacral canal through the sacral hiatus with the filum terminale.