The |sacrum| is the portion of the |vertebral column| between the |lumbar vertebrae| and the structures of the |coccyx|. It is composed of five |vertebrae| which are fused together to form a single bone structure. The |sacrum| features a median crest (running down the back, or ~posterior~ of the |sacrum|) which is made of the fused |spinous processes| of its component |vertebrae|. Beneath this crest is the sacral canal, a tunnel which runs lengthwise from the top of the |sacrum| to a hiatus (opening) near the bottom. Four pairs of holes (sacral foramina) pierce the |sacrum|, flanking the medial (center) line, where the intermediate sacral crests are formed by the fused articular ~processes~ of the component |vertebrae|. To the outside of the intermediate sacral crests are the lateral crests, formed by the fused |transverse processes| of the component |vertebrae|. In the |sacrum|, therefore, unlike the upper |vertebrae| in the |spine|, the intertransverse ~ligaments~ have been replaced by fusion of these ~processes~ together. The crests are not represented on the front (pelvic) surface of the |sacrum|, though the sacral foramina are evident.
