40 Anatomy of Skeleton 



processes of the last sacral vertebra for connection with the Coccyx. As the last 

 laminar arch is wanting and sometimes the fourth also the articular crests are longer 

 than the spinal, and the interval between them, opening into the sacral canal, is 

 termed the hiatus sacralis. 



The lateral mass has a rough double hollow on its postero-lateral aspect, opposite 

 the first two segments, making a ligamentous area (lig.) for the strong posterior sacro- 

 iliac ligaments and in front of and below this, on its lateral aspect, a slightly concave 

 articular or auricular surface (art.) for the innominate bone : this corresponds in extent 

 with three sacral bodies. 



The upper aspect of the sacrum, or base (Fig. 32), shows the first sacral body 

 centrally, and the ala on each side of it, expanding outwards ; each ala is the upper 

 surface of the lateral mass, and has an inner and anterior smooth part slightly grooved 

 for the lumbo-sacral cord, and an outer and posterior rougher ligamentous part. 

 Behind the first body is the upper opening of the sacral canal, somewhat triangular in 

 shape, flanked by a large superior articular process on each side, and between this and 

 the body is an intervertebral notch. A mammillary tubercle is below the hinder edge 



of the articular surface on the back of the 

 bone : it is not always present. 



The front surface of the bone looks 

 more downwards than forwards in its 

 upper part, when the bone is in position, 

 as may be seen in the articulated skeleton, 

 and thus the front part of the upper border 



of the first body becomes the most promi- 



FIG. 33. To show the constitution of the lateral nent forward part of the bone and con- 

 piece of the sacrum. Tr. transverse ,-, , .-, ^ nrvn ] brnwin'ntnrM Whpn thp 

 element ; Cost, costal element ; Epiph. stl1 e sacrM P ro ' 



marginal epiphysis ; AT. neural arch ; lumbar Column is in position, the lumbo- 



fora F mina P - R "**** "* cral angle of about 120 degrees is formed 



here between the first sacral and last 

 lumbar segments, the angle being rounded by the intervening disc, which is much 

 thicker in front than behind. 



The female sacrum is broader than the male bone in proportion to its length. 

 The anterior curve of the bone from above down is a continuous one in the male, but 

 in the female there is usually a more sudden curve in the lower part, the upper being 

 nearly flat. This is not a constant feature. 



Examine the sacrum in conjunction with the rest of the column, and it becomes 

 evident that the lateral masses can be analysed into transverse and costal elements. 

 The transverse processes (Fig. 32, Tr.) of the third and fourth elements can be recognised 

 as prominent tubercles on the dorsal surface of the bone, just outside the foramina : 

 again, it is easy to see the transverse element of the first segment in the thick promi- 

 nence that forms the upper lateral angle of the bone, and the second transverse point 

 can be recognised without difficulty in the middle of the ligamentous sacro-iliac area. 

 Now look at the bone from the front, and there is no difficulty in seeing the costal 

 elements in the bars of bone that separate the anterior foramina : all this is simply 

 an expression of the fundamental structure of the column, and the anterior primary 

 divisions of the nerves come forward between the ribs while the posterior divisions 

 pass back between the transverse processes. 



Looking at the bone from above, it is easy to see that the lateral mass is built 

 up by the fusion of these costal elements and transverse processes, and the same 



