110 



OSTEOLOGY 





fibrocartilage. Behind this is the large triangular orifice of the sacral canal, which 

 is completed by the laminae and spiiious process of the first sacral vertebra. The 

 superior articular processes project from it on either side; they are oval, concave;, 

 directed backward and medialward, like the superior articular processes of a lumbar 

 vertebra. They are attached to the body of the first sacral vertebra and to the 

 alee by short thick pedicles; on the upper surface of each pedicle is a vertebral 

 notch, which forms the lower part of the foramen between the last lumbar and first 

 sacral vertebrae. On either side of the body is a large triangular surface, which 

 supports the Psoas major and the lumbosacral trunk, and in the articulated 

 pelvis is continuous with the iliac fossa. This is called the ala; it is slightly concave 



Cornua 



""7-OR 



Anterior Surface 



Fia. 99. Median sagittal section of the sacrum. 



EXTERNUS 



Posterior surface 

 Fia. 100. Coccyx. 



from side to side, convex from before backward, and gives attachment to a few 

 of the fibers of the Iliacus. The posterior fourth of the ala represents the trans- 

 verse process, and its anterior three-fourths the costal process of the first sacral 

 segment. 



Apex (apex oss. sacri). The apex is directed downward, and presents an oval 

 facet for articulation with the coccyx. 



Vertebral Canal (canalis sacralis; sacral canal). The vertebral canal (Fig. 99) 

 runs throughout the greater part of the bone; above, it is triangular in form; 

 below, its posterior wall is incomplete, from the non-development of the laminae 

 and spinous processes. It lodges the sacral nerves, and its walls are perforated by 

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



