98 



OSTEOLOGY. 



laterales), and which are serially homologous with the true transverse processes 

 of the lumbar vertebrae. The posterior surface of the bone furnishes an extensive 

 surface for the origin of the sacro-spinalis, whilst the edge of the bone lateral to 

 the third and fourth foramen gives attachment to the glutaeus maximus. 



The base of the bone displays features more in accordance with a typical 

 vertebra. Centrally, and in front, is placed the body, the superior surface of which 

 articulates with the last lumbar vertebra through the medium of an intervertebral 

 fibro-cartilage. The anterior margin is thin and projecting, overhanging the 

 general concavity of the pelvic surface of the bone, and forming what is called the 

 promontory. Posterior to the body, the sacral canal, of triangular form but slightly 

 compressed dorso-ventrally, is seen, whilst still more posteriorly is the short spinous 



Superior aperture of 

 sacral canal 



Superior articular process 



Auricular 

 surface 



Articular 

 process 



Spinous process 



Transverse process 

 Posterior sacral foramen 



Inferior lateral angle 



Inferior aperture of sacral canal 

 Groove for fifth sacral nerve 



Coccygeal articular surface 

 FIG. 113. THE SACRUM (posterior view). 



process, forming the highest tubercle of the median crest. Spreading out from 

 the sides, and partly from the back of the body on each side, is a fan-shaped mass 

 of bone, the upper surface of which is slightly concave from side to side, and convex 

 from above and behind downwards and forwards. This, the ala sacralis, corresponds 

 to the thick upper border of the lateral part, and is formed, as will be explained 

 hereafter, by elements which correspond to the roots of the vertebral arches (O.T. 

 pedicles) and the transverse processes of the sacral vertebrae, together with superadded 

 structures the sacral ribs. The lateral margin of the lateral part, as seen from above, 

 is sharp and laterally convex, terminating posteriorly in a prominent tubercle 

 the highest of the series of elevations seen on the posterior surface of the bone, which 

 have been already described as serially homologous with the true transverse processes 

 of the lumbar vertebrae. Fused with the dorsal surface of each lateral part, and 

 separated from it laterally by a narrow but deep notch, is the superior articular process. 

 This supports a vertical articular surface, which is of circular or oval form, and con- 

 cave from side to side, having a general direction backwards and a little medially. 

 The borders of the sacrum are thick above, where they articulate with the ilia, 



