SACRAL AND COCCYGEAL VERTEBRAE. 



47 



some bones this union is more complete between the lower segments than between 

 the upper ones. 



The Posterior Surface (Fig. 17) 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 sac- 

 ral vertebrae. 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 downward ; the fourth usually, 

 and the fifth always, remaining undevel- 

 oped. External to the spinous processes 

 on each side are the laminae, broad and 

 well marked in the first three pieces ; 

 sometimes the fourth, and generally the 

 fifth, are only partially developed and 

 fail to meet in the middle line. These 

 partially developed lamina? are prolonged 

 downward as rounded processes, the 

 ^ncral cornua, and are connected to the 

 cornua of the coccyx. Between them the 

 bony wall of the lower end of the sacral 

 canal is imperfect, and is liable to be 

 opened in the sloughing of bed-sores. 

 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 artic- 

 ulating 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 

 and assist in forming the sacral cornua. 

 External to the articular processes are the four posterior sacral foramina ; they 

 are smaller in size and less regular in form than the anterior, and transmit the 

 posterior branches of the sacral nerves. On the outer side of the posterior sacral 

 foramina is a series of tubercles, the rudimentary transverse processes of the sacral 

 vertebra;. The first pair of transverse tubercles are large, very distinct, and 

 correspond with each superior angle of the bone; they together with the second 

 pair, which are of small size, give attachment to the horizontal part of the sacro- 

 iliac ligament ; the third give attachment to the oblique fasciculi of the posterior 

 sacro-iliac ligaments ; and the fourth and fifth to the great sacro-sciatic ligaments. 

 The interspace between the spinous and transverse processes on the back of the 

 sacrum presents a wide, shallow concavity, called the sacral groove ; it is 

 continuous above with the vertebral groove, and lodges the origin of the Multifidus 

 spinrc. 



The Lateral Surface, broad above, becomes narrowed into a thin edge below. 

 Its upper half presents in front a broad, ear-shaped surface for articulation with 

 the ilium. This is called the auricular surface, and in the fresh state is coated 

 with n'bro-cartilage. It is bounded posteriorly by deep and uneven impressions, 

 for the attachment of the posterior sacro-iliac ligaments. The lower half is thin 

 and sharp, and terminates in a projection called the inferior lateral angle ; below 

 this angle is a notch, which is converted into a foramen by articulation with the 

 transverse process of the upper piece of the coccyx, and transmits the anterior 



FIG. 16. Vertical section of the sacrum. 



