

THE SACRAL AND COCCYGEAL VERTEBRAE 



107 



versed by four broad, shallow grooves, which lodge the anterior divisions of the 

 sacral nerves, and are separated by prominent ridges of bone which give origin 

 to the Piriformis muscle. 



If a sagittal section be made through the center of the sacrum (Fig. 99), the 

 bodies are seen to be united at their circumferences by bone, wide intervals being 

 left centrally, which, in the fresh state, are filled by the intervertebral fibro- 

 cartilages. In some bones this union is more complete between the lower than 

 the upper segments. 



Dorsal Surface (fades dorsalis). The dorsal surface (Fig. 96) is convex and 

 narrower than the pelvic. In the middle line it displays a crest, the middle sacral 

 crest, surmounted by three or four tubercles, the rudimentary spinous processes 



Promontory 





Fio. 95. Sacrum, pelvic surface. 



of the upper three or four sacral vertebrae. On either side of the middle sacral 

 crest is a shallow groove, the sacral groove, which gives origin to the Multifidus, 

 the floor of the groove being formed by the united laminae of the corresponding 

 vertebrae. The laminae of the fifth sacral vertebra, and sometimes those of the 

 fourth, fail to meet behind, and thus a hiatus or deficiency occurs in the posterior 

 wall of the sacral canal. On the lateral aspect of the sacral groove is a linear 

 series of tubercles produced by the fusion of the articular processes which together 

 form the indistinct sacral articular crests. The articular processes of the first 

 sacral vertebra are large and oval in shape; their facets are concave from side to 

 side, look backward and medialward, and articulate with the facets on the inferior 

 processes of the fifth lumbar vertebra. The tubercles which represent the inferior 

 articular processes of the fifth sacral vertebra are prolonged downward as rounded 



