46 



THE SKELETON. 



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

 prominent angle, called the promontory or sacro-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 sacral foramina, analogous to the intervertebral foramina, 



FIG. 15. Sacrum, anterior surface. 



four in number on each side, somewhat rounded in form, diminishing in size 

 from above downward, and directed outward and forward ; they transmit the 

 anterior branches of the sacral nerves and the lateral sacral arteries. External 

 to these foramina is the lateral mass, consisting at an early period of life of 

 separate segments; these become blended, in the adult, with the bodies, with each 

 other, and with the posterior transverse processes. Each lateral mass is traversed 

 by four broad, shallow grooves, which lodge the anterior sacral nerves as they 

 pass outward, the grooves being separated by prominent ridges of bone, which give 

 attachment to the slips of the Pyriformis muscle. 



If a vertical section is made through the centre of the sacrum (Fig. 16), the 

 bodies are seen to be united at their circumference by bone, a wide interval being 

 left centrally, which, in the recent state, is filled by intervertebral substance. In 



