THE SACRUM. 



95 



prominent rim, which in front curves downward to form 

 the promontory. The alse project laterally ; they are 

 trilateral, rough surfaces, continuous with the anterior 

 surface by a rounded border. Numerous foramina pene- 

 trate these surfaces and afford exit to a great number of 

 veins. 



The apex articulates with the coccyx. It is deficient 

 behind, exposing the termination of the sacral canal be- 

 tween two well-marked processes, which project down- 



FIG. 43. THE SACRUM, POSTERIOR SURFACE. 



ward, and known as the sacral cornua. The lateral bor- 

 ders of the sacrum serve principally for articulation with 

 the ossa innominata. The articular surfaces are ear-shaped, 

 and are designated the auricular surfaces ; they measure 

 about two and three-fourths inches long, by one inch in 

 width. Just behind the auricular surfaces, near the 

 upper part of the bone, is the digital fossa, for the attach- 

 ment of strong ligaments. Below the auricular surfaces 

 the bone is rough, for the attachment of the sacro-sciatic 



