SACRUM. 



51 



Fig. 15. Vertical Section of the Sacrum. 



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 downwards, 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 dowmvards, are flattened from before 

 backwards, and curved so as to accom- 

 modate 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, four in number on each side, 

 somewhat rounded in form, diminishing 

 in size from above downwards, and di- 

 rected outwards and forwards; they 

 transmit the anterior branches of the 

 sacral nerves. External to these fora- 

 mina is the lateral mass, consisting, at 

 an early period of life, of separate seg- 

 ments, which correspond to the anterior 

 transverse processes ; these become blend- 

 ed, in the adult, with the bodies, with 

 each other, and with the posterior trans- 

 verse processes. Each lateral mass is 

 traversed by four broad shallow grooves, 

 which lodge the anterior sacral nerves as 

 they pass outwards, 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 bone (fig. 15), the bodies 

 are seen to be united at their circumfe- 

 rence by bone, a wide interval being left 

 centrally, which, in the recent state, is 

 filled by intervertebral substance. In 

 some bones, this union is more complete 

 between the lower segments than between 

 the upper ones. 



The Posterior Surface (fig. 16) is convex, and much narrower than the anterior. 

 In the middle line, are three or four tubercles, which represent the rudimentary 

 spinous processes of the sacral vertebras. Of these tubercles, the first is usually 

 prominent, and perfectly distinct from the rest ; the second and third are either 

 separate, or united into a tubercular ridge, which diminishes in size from above 

 downwards; the fourth usually, and the fifth always, remaining undeveloped. Ex- 

 ternal to the spinous processes on each side are the laminse, broad and well marked 

 in the three first pieces ; sometimes the fourth, and generally the fifth, being un- 

 developed ; in this situation the lower end of the sacral canal is exposed. External 

 to the laminae is a linear series of indistinct tubercles representing the articular 

 processes; the upper pair are large, well developed, and correspond in shape and 

 direction to the superior articulating processes of a lumbar vertebra; the second 

 and third are small ; the fourth and fifth (usually blended together) are situated 

 on each side of the sacral canal: they are called the sacral cornua, and articulate 

 with the cornua of the coccyx. External to the articular processes are the four 



