THE SACRO-ILIAC ARTICULATION 



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itself upon the sacral si3ines, where it mixes its fibres with the supra-spinous 

 ligameut of the lumbar vertebrfe. 



The inferior ilio-sacral ligavient is a triangular and very resisting mem- 

 branous band, formed of parallel fibres running obliquely from above 

 downwards, and from before backwards. It is attached by its antero- 

 inferior edge to the superior half of the ischiatic border and the intei'nal 

 angle of the ilium, mixing itself with the preceding ligament ; its superior 



^^ 



Fio. 44. — Profile View of the Ilio-Saceal and Sacro-Sciatic Ligaments. 



A. Os ilium. 



B. Os pubis. 



C. Os ischium. 



D. Femur. 



E. Troeiiauter major extemus. 



F. Large tubercle iit the heiid of femur. 



G. Head of the femur. 



H. Riin of the cotyloid cavity. 

 K. K. Sacral si.>iiR's. 



N. M. O. Tubercles on the antero-iuferior 

 spinous process of the ilium. 



1. 1. Superior ilio-sacral ligament. 



2. 3. -i. Sacro-sciatic ligament. 



border inserts itself upon the roughened ridge which bounds the sacrum 

 laterally ; its posterior border is united to the aponeuroses which cover the 

 coccygeal muscles. 



The sacro-sciatic ligament is a vast membranous expansion, stretched 

 upon the side of the pelvis, between the sacrum and the os innominatum ; 

 it serves rather as an inclosure for the pelvic cavity than as a means of 

 securing the firmness of the sacro-iliac articulation. Its form is irregu- 

 larly quadrilateral, presenting four borders — a superioi', attached to the 

 lateral roughened edge of the sacrum ; an inferior, inserted in the ridge 

 below the cotyloid cavity ; an anterior, unattached in a great part of 

 its course, and serving as a protection to the large vessels and nerves 

 which pass through the sciatic notch ; and lastly, a posterior margin, which 

 splits into two laminse, between which the semi-membranous muscle takes 

 its origin. 



A synovial viembrane covers the sacro-iliac ligament, but furnishes a small 

 quantity of synovia. 



Movements. — The two sacro-iliac articulations, through which all the 



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