126 



HUMAN ANATOMY. 



Articular process 



Ligamentous 

 surface 



Spinous 

 process 



Rudimentary 

 transverse 

 processes 



and inward. Thus the sacrum is broader before than behind. The apex is nothing 

 but the under side of the body of the very small fifth sacral vertebra. 



The anterior surface is a triangular concavity formed by the bodies and lat- 

 eral masses of the. five sacral vertebrae. It has a double row of four openings, the 

 anterior sacral foramina, one on each side of the ridges, representing the ossified 

 disks connecting the bodies of the fused sacral vertebrae. The sacral nerves, like 

 the other spinal nerves, divide into an anterior and a posterior division on leaving 

 the spinal canal ; in the case of the sacral nerves, however, this takes place inside 

 the bone, the anterior divisions escaping by these foramina. The bodies and the 

 foramina grow smaller from above downward, and the latter are nearer together. A 

 transverse depression across the body of the third vertebra usually marks a rather 



sudden change in the 

 Fig. 154. curvature of the anterior 



Transverse process surfacc. The irregular 



outline of the lateral bor- 

 ders may be divided into 

 two parts : the upper, 

 rather concave, ends be- 

 low in a litde point on a 

 level with the third verte- 

 bral body, and represents 

 the extent of the articu- 

 lar surface. Below this 

 the border slants down- 

 ward and inward until 

 opposite the lower part 

 of the fifth sacral seg- 

 ment, when it suddenly 

 turns inward, forming a 

 notch over the anterior 

 division of the fifth sacral 

 nerve, which emerges be- 

 tween it and the coccyx. 

 The posterior sur- 

 face is composed of the 

 fused lainiuiz and their 

 modifications. The up- 

 per borders of the first 

 laminae slant downward, 

 and below their junction 

 is a well-marked spine} 

 Below this the laminae of 

 the sacral vertebrae arc 

 fused and the spines 

 small. The laminae of 

 the fifth sacral never join, 

 and those of the fourth 

 frequently do not, thus leaving the lower end of the canal uncovered. The laminae 

 that do not meet end in tubercles each representing one-half of a spinous process. 

 The lowest two project downward at the sides of the open canal, and are called the 

 sacral cornua. Four posterior sacral foramina for the exit of the posterior divisions 

 of the nerves appear on each side of the laminae. Outside of these are some irregu- 

 lar tubercles representing the transverse processes,^ and internal to the first three 

 foramina are tubercles in line with the articular processes} 



The lateral surface begins just outside of the transverse tubercles. It is 

 broad above, but below the third vertebra is merely a line. The upper part is 

 divided into two portions : the front one is the auricular surface, from a slight 

 resemblance to an ear, which joins, by fibro-cartilage, the corresponding surface on 

 the ilium. It is broader above than below, convex in front, indented behind, with 



' Crista media. - Cristac latcrales. ^ Cristae articulares. 



Auricular 

 (articular) 

 surface 



Fourth posterior sacral foramen 



Notch 



Sacral cornu 

 The sacrum, lateral view. 



