SYNDESMOLOGY 



The movements of the sacrum are regulated by its form. Viewed as a whole, it presents the 

 shape of a wedge with its base upward and forward. The first component of the force is there- 



FIQ. 322. Coronal section of anterior sacral segment. 



fore acting against the resistance of the wedge, and its tendency to separate the iliac bones is 

 resisted by the sacroiliac and iliolumbar ligaments and by the ligaments of the pubic symphysis. 



Fid. 323. Coronal section of middle sacra segment. 



If a series of coronal sections of the sacroiliac joints be made, it will be found possible to divide 

 the articular portion of the sacrum into three segments: anterior, middle, and posterior. In 

 the anterior segment (Fig. 322), which involves the first sacral vertebra, the articular surfaces 



show slight sinuosities and are almost parallel 

 to one another; the distance between their 

 dorsal margins is, however, slightly greater 

 than that between their ventral margins. 

 This segment therefore presents a slight 

 wedge shape with the truncated apex down- 

 ward. The middle segment (Fig. 323) is a 

 narrow band across the centers of the articu- 

 lations. Its dorsal width is distinctly 

 greater than its ventral, so that the segment 

 is more definitely wedge-shaped, the trun- 

 cated apex being again directed downward. 

 FIG. 324. Coronal section of posterior sacral segment. Each articular surface presents in the center 



a marked concavity from above downward, 



and into this a corresponding convexity of the iliac articular surface fits, forming an interlocking 

 mechanism. In the posterior segment (Fig. 324) the ventral width is greater than the dorsal, 

 so that the wedge form is the reverse of those of the other segments i. e., the truncated apex 

 is directed upward. The articular surfaces are only slightly concave. 



Dislocation downward and forward of the sacrum by the second component of the force applied 

 to it is prevented therefore by the middle segment, which interposes the resistance of its wedge 



