SACRAL AND COCCYGEAL VERTEBRAE. 



157 



The Lateral Surface, broad above, becomes narrowed into a thin edge below. 

 Its upper half presents in front a broad, ear-shaped surface for articulation with 



Upper half of fifth 

 posterior sacral foramen. 



FIG. 124. Sacrum, posterior surface. 



the ilium. This is called the auricular surface, and in the fresh state is coated 

 with fibro-cartilage. It is bounded posteriorly by deep and uneven impressions, 

 for the attachment of the posterior sacro-iliac ligaments. The lower half is thin 

 and sharp, and ends in a prominence, the inferior lateral angle ; the border gives 

 attachment to the greater and lesser sacro-sciatic ligaments, and to some fibres 

 of the Glutens maximus : below the angle is a deep notch, which is converted 

 into a foramen by the transverse process of the upper piece of the coccyx, and 

 transmits the anterior division of the fifth sacral nerve. 



The Base of the sacrum, which is broad and expanded, is directed upward and 

 forward. In the middle is seen a large oval articular surface, which is connected 

 with the under surface of the body of the last lumbar vertebra by a fibre-carti- 

 laginous disk. It is bounded behind by the large, triangular orifice of the sacral 

 canal. The orifice is formed behind by the laminae and spinous process of the 

 first sacral vertebra: the superior articular processes project from it on each side; 

 they are oval, concave, directed backward and inward, like the superior articular 

 processes of a lumbar vertebra : and in front of each articular process is an inter- 

 vertebral notch, which forms the lower half of the last intervertebral foramen. 

 Lastly, on each side of the large oval articular surface is a broad and flat triangular 

 surface of bone, which extends outward, supports the Psoas magnus muscle and 

 lumbo-sacral cord, and is continuous on each side with the iliac fossa. This is 

 called the ala of the sacrum, and gives attachment to a few of the fibres of the 

 Iliacus muscle. 



The Apex, directed downward and forward, presents a small, oval, concave 

 surface for articulation with the coccyx. 



The Spinal Canal runs throughout the greater part of the bone ; it is large 



