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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
  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 therefore 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.


FIG. 322– Coronal section of anterior sacral segment. (See enlarged image)



FIG. 323– Coronal section of middle sacra segment. (See enlarged image)

  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 downward. The middle segment (Fig. 323) is a narrow band across the centers of the articulations. Its dorsal width is distinctly greater than its ventral, so that the segment is more definitely wedge-shaped, the truncated apex being again directed downward. 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.


FIG. 324– Coronal section of posterior sacral segment. (See enlarged image)

  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

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