SACRAL AND COCCYGEAL VERTEBRA. 



155 



Sacral and Coccygeal Vertebrae. 



The Sacral and Coccygeal Vertebrae consist, at an early period of life, of nine 

 separate pieces, which are united in the adult so as to form two bones, five enter- 

 ing into the formation of the sacrum, four into that of the coccyx. Occasionally, 

 the coccyx consists of five bones. 1 



The Sacrum (sacer, sacred) is a large, triangular bone (Fig. 122), situated at 

 the lower part of the vertebral column, and at the upper and back part of the pelvic 



FIG. 122. Sacrum, anterior surface. 



cavity, where it is inserted like a wedge between the two innominate bones ; its 

 upper part or base articulating with the last lumbar vertebrae, its apex with the 

 coccyx. The sacrum is curved upon itself, and placed very obliquely, its upper 

 extremity projecting forward, and forming, with the last lumbar vertebra, a very 

 prominent angle, called the promontory or mcro-vertebral angle ; whilst its central 

 part is directed backward, so as to give increased capacity to the pelvic cavity. 

 It presents for examination an anterior and posterior surface, two lateral surfaces, 

 a base, an apex, and a central canal. 



The Anterior Surface is concave from above downward, and slightly so from 

 side to side. In the middle are seen four transverse ridges, indicating the original 

 division of the bone into five separate pieces. The portions of bone intervening 

 between the ridges correspond to the bodies of the vertebrae. The body of the 

 first segment is of large size, and in form resembles that of a lumbar vertebra ; the 

 succeeding ones diminish in size from above downward, are flattened from before 

 backward, and curved so as to accommodate themselves to the form of the sacrum, 

 being concave in front, convex behind. At each end of the ridges above mentioned 

 are seen the anterior sac-mi. f<./-<imina, analogous to the intervertebral foramina, 



1 Sir George Humphry describes this as the usual composition of the coccyx. On the Skeleton, 

 p. 456. 



