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- Discussion:
    - upper end of sacrum, base of triangle, is directed forward & upward;
    - in middle is upper surface of body of the first sacral vertebra;
    - its anterior border forms the sacral promontory.
    - it is concave & fairly smooth on its pelvic surface, & convex & irregular on its dorsal surface.
    - consists of five fused vertebrae, is broadened proximally by fusion of  large costal elements and transverse processes into heavy pars
          lateralis on each side;
          - pelvic side has 4 ridges marking lines of fusion of vertebrae;
          - at ends of ridges are large pelvic sacral foramina through which ventral rami of sacral nerves emerge;
    - on dorsal side, there are 4 converging grooves for sacral nerves about an inch lateral to median sacral crest (rudimentary fused spinous 
         processes);
    - lateral to the sacral foramina are lateral sacral crests representing transverse processes;
    - behind the body is large vertebral foramen leading into sacral canal.
    - sacral canal is roofed by fused laminae of S1-S4;
    - superior articular processes of sacrum are supported by short heavy pedicles;
    - articular facets face medially and posteriorly.
    - sacrum is situated between the two hip bones; upper, wider part of pars lateralis articulates with the ilium on each side.