72 



Bones of the Spiue. 



Basis ossis sacri 



Processus articularis superior 



Pars 

 lateralis 



Lineae _' 

 transversae 



Foramina 

 -* sacralia 

 anteriora 



- Apex ossis sacri 



90. Sacrum, OS Sacrum, from in front and below. 



The five vei'tebrae sacrales are distinctly separated from one another in youth only: 

 in the adult they are fused to a single bone, the OS sacrum (see also Figs. 91 93), which 

 is situated below the lumbar vertebrae and is the largest and broadest bone of the verte- 

 bral column. 



The os sacrum is wedge-shaped, broad above (basis), pointed below (apex) ; its sagittal 

 diameter also diminishes essentially from above downward. Its anterior inferior surface, fades 

 pelvina, looking into the pelvis, is concave, its posterior upper surface, fades dorsalis, is convex. 



The anterior inferior surface, fades pelrina, is in general smooth and concave. In 

 the middle, four lineae transversae are to be seen, lines corresponding to the places where 

 the five individual bodies of the sacral vertebrae have grown together. Lateral from them 

 on each side lying in a vertical series over one another are the four foramina sacralia an- 

 teriora, which behind and medianward lead into the canalis sacralis (see Fig. 92 a ), but, 

 lateralward, run out into grooves on the anterior surface of the partes laterales. The foramina 

 sacralia anteriora diminish in size from above downward ; below the last one, near the apex 

 ossis sacri, is a notch which is closed by the processus transversus ossis coccygis and by the 

 ligamentum sacrococcygeum laterale to form a fifth foramen sacrale. The apex ossis sacri 

 presents a small oval, flat surface which is formed by the body of the fifth sacral vertebra 

 only and is united at the symphysis sacrococcygea with the upper surface of the coccyx. 



