The sacrum is
formed by the fusion of five sacral vertebrae. It has a natural kyphosis. The
median sacral crest is seen to bear the spinous tubercles and this crest is the
fused spines of the sacral vertebrae. Below the spine of S4 is the sacral
hiatus which is due to failure of fusion of the S5 laminae. It is through this
hiatus that caudal epidural anaesthesia is performed. Note the four coccygeal
segments which, in this case, are existing as separate entities, but are often
fused together. The sacral spinal canal contains the cauda equina and the
spinal meninges. These are best seen, however, on a lateral lumbosacral
myelogram. The filum terminale from the pia mater emerges below the sacral
hiatus and passes downwards to insert into the coccyx. Babies of mothers
suffering from diabetes mellitus may occasionally have sacral agenesis.