THE SACRO-ILIAC ARTICULATION 407 



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

 ligament of the lumbar vertebra?. 



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

 branous band, formed of parallel fibres running obliquely from above 

 (lownwai'ds, and from before backwards. It is attached by its antero- 

 inferior edge to the superior half of the ischiatic border and the internal 

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



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



A. Os ilium. H. Rim of the cotyloid cavity. 



B. Os pubis. K. K. Sacral spines. 



C. Os ischium. N. M. O. Tubercles on the antero-inferior 



D. Femur. spinous process of the ilium. 



E. Trochanter major extemus. 1. 1. Superior ilio-sacral lignnient. 



F. Large tubercle at the head of femur. 2. 3. -4. Sacro-sciatic ligament. 



G. Head of the femur. 



border inserts itself upon the roughened ridge which bounds tlie 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 superior, attached to the 

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

 beluw 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 memhrane covers the sacro-iliac ligament, but furnishes a small 

 quantity of synovia. 



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



