Chapter 8 - Section 8.3 - Study Guide - Assess Your Learning Outcomes - Page 271: 9

The sacrum in adults (18+) is a single bone that was formed by the fusion of 5 sacral vertebrae that were separate bones in infants and children. This trianguloid bone is positioned at the inferior end of the vertebral column, wedged between the two coxal bone, where it forms the posterior wall of the pelvis. There are several notable features of the sacrum; some of these are the foramina, crests, canal and hiatus, auricular surface, sacroiliac joint, promontory, and alae, On the anterior surface of the sacrum there are four pairs of anterior sacral foramina. These openings permit the passage of nerves and arteries to the pelvic organs. On the posterior surface of the sacrum there are also four pairs of openings --posterior sacral foramina-- that permit the passage of spinal nerves to supply the gluteal region and lower limbs. The median sacral crest and the lateral sacral crest are rough osseous ridges formed by fusion of spinous processes, and fusion of transverse processes, respectively. The less prominent lateral crests are located on each side of the median crest. A canal-- the sacral canal --runs through the sacrum and ends inferiorly in an opening called the sacral hiatus. The sacral canal houses spinal nerve roots. The sacroiliac joint of the of the pelvis is a very strong synovial plane joint formed at the articulation of the ear-shaped surface on each side of the sacrum, and a similar articular surface on the coxal bones. This joint supports much of the weight of the body. The sacral promontory is an anterior projection of the body of S1; this projection supports the body of S5. S1 also has a pair of superior articular processes that articulate with L5. These are situated medially to the alae which are a pair of large winglike bony extensions that articulate with the ilium at the sacroiliac joints.

The sacral promontory is an anterior projection of the body of S1; this projection supports the body of S5. S1 also has a pair of superior articular processes that articulate with L5. These are situated medially to the alae which are a pair of large winglike bony extensions that articulate with the ilium at the sacroiliac joints.