SACRUM AND COCCYX. 29 



THE SACRUM is a triangular bone, situated at the lower extremity 

 of the vertebral column, and formed by the consolidation of five 

 false vertebra?. It is divisible into an anterior and posterior surface, 

 two lateral and a superior border, and an inferior extremity. 



The anterior surface is concave, and marked by four transverse 

 lines, which indicate its original constitution of five separate pieces. 

 At the extremities of these lines, on each side, are the four anterior 

 sacral foramina, which diminish in size from above downwards, and 

 transmit the anterior sacral nerves. The projection of the superior 

 piece is called the promontory of the sacrum. 



The posterior surface is convex. Upon the middle line is a rough 

 crest formed by the rudiments of four spinous processes, the fifth 

 remaining undeveloped, and exposing the lower termination of the 

 sacral canal. The rudiments of the fifth are situated one on each side 

 of the termination of the sacral canal ; they are named the sacral 

 cornua, and articulate with the cornua of the coccyx. Parallel with 

 the middle line, on each side, are the openings of the four posterior 

 sacral formina ; they are smaller than the anterior, and transmit the 

 posterior sacral nerves. Immediately external to each of the pos- 

 terior sacral foramina is a tubercle, representing a rudimentary 

 transverse process. The first transverse tubercle corresponds with 

 the angle of the superior border of the bone ; the second is small, 

 and enters into the formation of the sacro-iliac articulation ; the 

 third is large, and gives attachment to 

 the oblique sacro-iliae ligament ; the 

 fourth and fifth are smaller and serve for 

 the attachment of the sacro-ischiatic 

 ligaments. The lateral border presents 

 superiorly a broad and ear-shaped sur- 

 face to articulate with the ilium; and 

 inferiorly a sharp edge, to which the 

 greater and lesser sacro-ischiatic liga- 

 ments are attached. On the superior 

 border, in the middle line, is an oval 

 articular surface, which corresponds 

 with the under part of the body of the 

 last lumbar vertebra ; and on each side, a 

 broad triangular surface which supports 



the lumbo-sacral nerve and psoas magnus muscle. Immediately 

 behind the vertebral articular surface is the triangular entrance of 

 the sacral canal ; and on each side of this opening an articular 

 process, which looks backwards and inwards, like the superior ar- 

 ticular processes of the lumbar vertebrae. In front of each articular 



* The sacrum seen upon its anterior surface. 1, 1. The transverse lines marking 

 the original constitution of the bone of four pieces. 2, 2. The anterior sacral foramina. 3. 

 The promontory of the sacrum. 4. The ear shaped surface which articulates with the 

 ilium. 5. The sharp edge to which the sacro-ischiatic ligaments are attached. 6- The 

 vertebral articular surface. 7. The broad triangular surface which supports the psoas 

 muscle and lumbo-sacral nerve. 8. The articular process of the right side. 9. The 

 inferior extremity, or apex of the sacrum. 10. One of the sacral cornua. 11. The 

 notch which is converted into a foramen by the coccyx. 



