48 



THE SKELETON. 



division of the fifth sacral nerve. This lower, sharp border gives attachment to 

 the greater and lesser sacro-sciatic ligaments, and to some fibres of the (jluteus 

 maximus posteriorly, and to the Coccygeus in front. 



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 fibro-carti- 

 lao-inous 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 part of the foramen between the last 

 lumbar and first sacral vertebra. Lastly, on each side of the large oval articular 



of fifth 

 posterior sacral foramen, 



FIG. 17. Sacrum, posterior surface. 



plate is a broad and flat triangular surface of bone, which extends outward, sup- 

 ports 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 attach- 

 ment to a few of the fibres of the Iliacus muscle. The posterior part of the ala 

 represents the transverse process of the first sacral segment. 



The Apex, directed downward and slightly 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 

 and triangular in form above, small and flattened, from before backward, below. 

 In this situation its posterior wall is incomplete, from the non-development of the 

 laminae and spinous processes. It lodges the sacral nerves, and is perforated by 

 the anterior and posterior sacral foramina, through which these pass out. 



Structure. It consists of much loose, spongy tissue within, invested externally 

 by a thin layer of compact tissue. 



