156 



THE SKELETON. 



four in number on each side, somewhat rounded in form, diminishing in size 



from above downward, and directed out- 

 ward and forward ; 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 ; 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 anterior sacral 

 nerves as they pass outward, 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. 123), the 

 bodies are seen to be united at their cir- 

 cumference 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. 124) 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 un- 

 developed. External to the spinous processes on each side are the lamince, broad 

 and well marked in the first three pieces ; sometimes the fourth, and generally 

 the fifth, being undeveloped : in this situation the lower end of the sacral canal 

 is exposed, 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 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 eornua, and articulate 

 with the cornua of the coccyx. 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 rudiment- 

 ary transverse processes of the sacral vertebrae. The first pair of transverse 

 tubercles are large, very distinct, and correspond with each superior lateral angle 

 of the bone ; the second, small in size, enter into the formation of the sacro-iliac 

 articulation ; 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 Erector 

 spinae. 



FIG. 123. Vertical section of the sacrum. 



