THE SACRUM. 83 
vertebra. Centrally and in front is placed the body, the upper surface of which 
articulates with the last lumbar vertebra through the medium of an intervertebral 
dise. The anterior margin is thin and projecting, overhanging the general con- 
cavity of the front of the bone, and forming what is called the promontory (promon- 
torium). Behind the body, the spinal canal, of tr iangular form with slightly 
appressed sides, is seen, whilst posteriorly is the short spinous process forming the 
highest tubercle of the median crest. Spreading out from the sides, and partly. from 
the back of the body on either 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 (ala sacralis), corresponds to the thick upper 
border of the lateral mass, and is formed, as will be explaimed hereafter, by elements 
Superior articular process 
Superior aperture of 
sacral canal 
Depression 
for posterior 
sacro-iliac 
ligaments 
Auricular 
surface 
process 
Spinous process 
_ Gluteus 
Transverse process = 
maximus 
Posterior sacral foramen 
— Erector spine 
Multifidus spine 
Inferior lateral angle— er 
Inferior aperture of sacral canal 
Sacral cornu Min. Groove for fifth sacral nerve 
Coccygeal articular surface 
Fic. 63.—THE Sacrum (posterior view). 
which correspond to the pedicles and transverse processes of the sacral vertebrae, to- 
gether with superadded structures—the sacral ribs. The external margin of the 
lateral mass, as seen from above, is sharp and outwardly convex, terminating behind 
in a prominent tubercle—the highest of the series of elevations seen on the poster ior 
surface of the bone, which have been already described as serially homologous with 
the true transverse processes of the lumbar vertebrae. Fused with the back of each 
lateral mass, and separated from it externally 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 concave from side to side, having a general direction 
backwards and a little inwards. 
The borders of the bone are thick above, where they articulate with the iha, 
thin and tapering below, where they furnish attachments for the powerful sacro- 
sciatic ligaments. The iliac articular surfaces are described as auricular in shape 
(facies auriculares), and overlie the lateral masses formed by the first three sacral 
vertebrae, though this arrangement is liable to considerable variation. Behind the 
f=) 
auricular surface, the bone is rough and pitted by three distinct depressions for the 
