34 



A MANUAL OF ANATOMY 



the attachment of the posterior sacro-iliac Ugament, and it is known 

 as the ligamentous surface. The lower part of the lateral surface 

 corresponds with at least the lower two sacral vertebrae, and may 

 include more or less of the third. It gives attachment to fibres 

 of the great and small sacro-sciatic ligaments and a portion of the 

 coccygeus muscle, whilst the adjacent portion of the posterior 

 aspect gives origin to fibres of the gluteus maximus, Inferiorly 

 the lower part is thinned away to a mere margin, and presents a 

 process, called the inferior lateral angle. Below this is the trans- 

 verse process of the first coccygeal vertebra when that is in position, 

 a notch being thus formed on the side of the fifth sacral vertebra. 

 The inferior lateral angle inclines towards the coccygeal transverse 

 process, with which it is usually connected by a hgament, though 

 in some cases the two processes become ankylosed. There is thus 

 constructed a fifth anterior sacral foramen at either side for the 



Body 



Articular Process, i 



with Mammillary 



Process 



Sacral Canal 



Spine 



Fig. 25. — The Base of the Sacrum. 



passage of the anterior primary division of the fifth sacral 

 nerve. 



The base presents a central and two lateral divisions. The 

 central division corresponds in its characters with the superior 

 surface of a lumbar vertebra. Each superior vertebral notch 

 lodges a fifth lumbar spinal nerve, and the superior articular pro- 

 cesses stand wide apart. The lateral divisions of the base are 

 called the alee. Each ala is triangular with the apex directed 

 backwards. The alar surface is depressed, concave from side .to 

 side, and convex from behind forwards. It gives attachment to 

 fibres of the iliacus, the lateral lumbo-sacral and anterior sacro- 

 iliac ligaments, and it supports the lumbo-sacral nervous cord and 

 the internal iliac vessels. The ala is formed by the fusion of the 

 pedicle, transverse process, and costal element of the first sacral 

 vertebra. 



The apex is transversely oval, and articulates with the first 



