The sacrum originated by fusing the 5 sacral vertebra and the intervertebral disks in-between (discus intervertebrales). Children only have primary cartilaginous joints, and with adults you can only see transverse lines on the front side of the sacrum.
The sacrum had a surface concave towards the front. The side towards the pelvis is designated as the facies pelvina and the side towards the rear is designated as the facies dorsalis. The surface bordering on the lumbar vertebral column is the basis ossis sacri. A tip projecting downwards (apex ossis sacri) forms the connection to the coccyx (os coccygis).
The rear surface of the sacrum has an even convex curve. There are five bone-combs (longitudinal ridges) on it that come from the spine, articular and transverse processes. The crista sacralis mediana is in the middle and it develops from the spines of the vertebrae fused with one another (processus spinosi).
There are two irregular ridges from the articular processes (processus articulares) developing from cristae sacrales intermediae. The original transverse processes (processus transversi) are fused with the two cristae sacrales laterals, which project somewhat more. Two articular processes (Processus articulares) come from the sacral basis.
The vertebral canal (canalis vertebralis) is continued on the inside of the sacrum as the sacral canal (canalis sacralis). The transverse lines of the fused bodies of the vertebrae (lineae transversae) may be found in the surface towards the pelvis just as well as the openings (foramina sacralia pelvina) for the passageway for the spinal cord nerves (nervi spinales).
The sacrum of the man is somewhat longer and narrower than that of the women and is also a little more curved.
The coccyx developed from three to five not completely formed vertebrae. They get smaller according to distal (removed from the middle of the body). Only the first vertebra is still similar with the typical vertebral form. Here there may be two lateral processes, which used to form the transverse processes (processus transversi).
Two bony articular processes, which are also called coccygeal horns (cornua coccygea), come from the first coccygeal vertebra opposite the bony protrusion on the rear side of the sacrum (os sacrum).
Please click on the image below for a 3D view of the sacrum
Please click on the image below for a 3D view of the coccyx
