THE SACRUM 



41 



Together they form on each side of the sacrum an irregular ridge [crista sacralis lateralis]. 

 The space between the spinous and transverse processes presents a shallow concavity known as 

 the sacral groove, continuous above with the vertebral groove of the movable part of the 

 column, and, like it, lodging the mullifidus muscle. Bridging across the groove and attached 

 to the sacral spines medially, and to the lower and back part of the sacrum laterally, is the flat 

 tendon of origin of the sacro-spi7ialis {erector spince) . The gluteus maximus takes origin from the 

 back of the lower two pieces of the sacrum. 



The base or upper surface of the sacrum bears considerable resemblance to the 

 upper surface of the fifth lumbar vertebra. It presents in the middle the body, of 

 a reniform shape, posterior to which is the upper end of the sacral canal bounded 

 by two laminae. On each side of the canal are two articular processes bearing well- 

 marked mammillary tubercles. The conjoined transverse and costal processes 

 form on each side a broad surface, the wing or ala of the sacrum, continuous with 

 the iliac fossa of the hip bone, and giving attachment to a few fibres of the iliacus. 



Fig. 45. — Left Lateral View of Sacrum and Coccyx. 



The lateral margins.— It has already been noted that the lateral portion of 

 the sacrum is the part lateral to the foramina. It is broad and thick above, 

 where it forms the ala, but narrowed below. The lateral aspect of the upper 

 part presents in front a broad irregular surface, covered in the recent state with 

 fibro-cartilage, which articulates with the ilium and is known as the auricular 

 surface. It is bounded posteriorly by some rough depressions for the attachment 

 of the posterior sacro-iliac ligaments. Below the auricular surface, the lateral 

 margin is rough for the sacro-tuberous (greater) and sacro-spmous (lesser sacro- 

 sciatic) ligaments, and terminates in the projection known as the inferior lateral 

 angle. Immediately below the angle is a notch, converted into a foramen by the 

 transverse process of the first coccygeal vertebra, and a ligament connecting this 

 with the inferior lateral angle of the sacrum. Through this foramen passes the 

 anterior branch of the fifth sacral nerve. u • r • 



The apex is directed downward and forward and is formed by the inferior 

 aspect of the body of the fifth sacral vertebra. It is transversely oval and 



