- 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
- 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.