SACRUM. 



127 



Fig. 82. Vertical Section of the Sacrum. 



where it is inserted like a wedge between the two ossa innominata; its upper 

 part, or base, articulating with the last lumbar vertebra, its apex with the 

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

 upper extremity projecting forwards, and forming, with the last lumbar verte- 

 bra, a very prominent angle, called the promontory or sacro-vertebral angle, whilst 

 its central part is directed backwards, 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, arid 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 vertebra. The 

 body of the first segment is of large size, and in form resembles that of a lum- 

 bar vertebra ; the succeeding ones diminish in size from above downwards, are 

 flattened from before backwards, 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, four in 

 number on each side, somewhat rounded 

 in form, diminishing in size from above 

 downwards, and directed outwards and 

 forwards ; they transmit the anterior 

 branches of the sacral nerves. External 

 to these foramina is the lateral mass, 

 consisting, at an early period of life, of 

 separate segments, which correspond to 

 the anterior transverse processes. 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 ante- 

 rior sacral nerves as they pass outwards, 

 the grooves being separated by promi- 

 nent ridges of bone, which give attach- 

 ment to the slips of the Pyriformis 

 muscle. 



If a vertical section is made through 

 the centre of the bone (Fig. 82), the 

 bodies are seen to be united at their 

 cirumference by bone, a wide interval 

 being left centrally, which, in the recent 

 state, is filled by intervertebral sub- 

 stance. In some bones, this union is 

 more complete between the lower segments, than between the upper ones. 

 _ The Posterior Surface (Fig. 83) is convex and much narrower than the ante- 

 rior. In the middle line, are three or four tubercles, which represent the rudi- 

 mentary 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, remain- 

 ing undeveloped. External 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 undeveloped ; in this situation the lower end of 



