SACRUM. 57 



nor transverse processes of the true vertebrse ; beyond these is a shallow 

 groove in which the four posterior sacral foramina open, and farther out- 

 wards, a range of five tubercles corresponding with the anterior or costal 

 transverse processes of the lumbar vertebrse. The lowest pair of the pos- 

 terior transverse tubercles bound on each side the termination of the sacral 

 canal, and send each a process downwards to articulate with the coccyx. 

 The two descending processes are the sacral cornua. The posterior sacral 

 foramina are smaller than the anterior, and transmit the posterior sacral 

 nerves. Of the anterior transverse tubercles the first corresponds with the 

 angle of the superior border of the bone ; the second is small, and enter? 

 into the formation of the sacro-iliac articulation ; the third is large, and 

 gives attachment to the oblique sacro-iliac ligament ; the fourth and fifth 

 are smaller, and serve for the attachment of the p i 2 i. 



sacro-ischiatic ligaments. The lateral border 

 of the sacrum presents superiorly a broad and 

 ear-shaped (auricular) surface to articulate with 

 the ilium ; and inferiorly a sharp edge, to which 

 the greater and lesser sacro-ischiatic ligaments 

 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 sur- 

 face 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 articular processes of the lumbar vertebras. In front of each 

 articular process is an intervertebral notch. The inferior extremity of the 

 bone presents a small oval surface which articulates with the coccyx ; and 

 on each side a notch, which, with a corresponding notch in the upper 

 border of the coccyx, forms the foramen for the transmission of the fifth 

 sacral nerve. 



The sacrum presents some variety in respect of curvature, and of the 

 number of pieces which enter into its structure. The curve is often very 

 slight, and is situated only near the lower part of the bone ; while in other 

 subjects it is considerable, and occurs at the middle of the sacrum. The 

 sexual differences in the sacrum relate to its greater breadth, and the 

 greater angle which it forms with the rest of the vertebral column in the 

 female, rather than to any peculiarity in shape. It is sometimes composed 

 of six pieces, more rarely of four, and, occasionally, the first and second 

 pieces remain permanently separate. 



Development. By twenty-one points of ossification ; five for each of the 

 three first pieces, viz. one for the body, one for each lateral portion, and 

 one for each lamina ; and three for each of the two last, namely, one for 



* 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 tho sacral cornua 

 1 1. The notch which is converted into a foramen by the coccyx. 



