The sacral canal  is formed by the vertebral foramina of the sacral vertebrae.  The sacral canal is enclosed by the sacral vertebral bodies anteriorly, the transverse processes laterally and the laminae, spinous processes and ossified ligamenta flava posteriorly.  Each lateral wall contains intervertebral foramina through which the sacral canal is continuous with the ventral and dorsal sacral foramina.  Inferiorly the sacral canal opens through the sacral hiatus.  This is an inverted U-shaped defect in the posterior wall of the spinal canal, just below the fourth spinous tubercle of the median sacral crest.  

The sacral canal is triangular in section and contains the internal vertebral venous plexuses, sacral spinal nerves, fat and the lower dural sac, which contains the cauda equina, filum terminale, spinal meninges and cerebrospinal fluid (CSF).  At the lower border of the second sacral vertebra the subarachnoid and subdural spaces cease, with the lower sacral spinal roots and filum terminale piercing the arachnoid and dura mater.