202 TUK ARricriATJoys 



lust inch of tlie iliac crest to the side of the sacrum external to the second and tliird 

 ])osterior sacral foramina. To the outer edge of this ligament is attached the fascia 

 covering the eret-tor sjMna' muscle. 



The inferior sacro-iliac ligament (tig. 207; is covered l>ehind hy the upper 

 end of the great sacro-sciatic ligament; it consists of strong. <libres extending from 

 the lateral border of the sacrum below the articular facet, to the posterior iliac 

 spines; some of the fibres are attached to the deep surface of the ilium and join the 

 interosseous ligament. 



The interosseous ligament is the strongest of all, and consists of fil)res of 

 different lengths passing in various directions between the two bones. Immediately 

 above the interspinous notch of the ilium the fibres of this ligament are very strong, 

 and form an open network, in the interstices of which is a quantity of fat in Avhich 

 the articular vessels ramify. 



The ear-shaped cartilaginous plate, which unites the bones firmly, is accu- 

 rately applied to the auricular surfaces of the sacrum and ilium. It is about one- 

 twelfth of an inch (2 mm. ) thick in the centre, but becomes thinner towards the 

 edges. Though closely adherent to the bones, it tears away from one entirely, or 

 from both iiartialh', on the application of violence, sometimes breaking irregularly 

 so that the greater portion remains connected with one bone, leaving the other bone 

 rough and bare. It is really one mass, and is only occasionally formed of two plates 

 with a synovial cavity between them. Because of the occasional presence of a more 

 or less extensive synovial cavity within the fibro-cartilage, and also of a synovial 

 lining to the ligaments passing in front and behind the articulation, the term 

 ' Diarthro-amphiarthrosis ' has been given to this joint, and also to the symphysis 

 pubis. Testut mentions certain folds of synovial membrane filling up gaps which 

 here and there occur at margin of the fibro-cartilage, but I have not myself seen 

 such. 



The great or posterior sacro-sciatic ligament (figs. 206, 207 and 208) is 

 attached al>ove to the ])osterior extremity of the crest of the ilium and the external 

 aspect of the posterior iliac spines. From this attachment some of its fibres pass 

 downwards and backwards to be attached to the outer ])orders and posterior sur- 

 faces of the lower three sacral vertebrte and up}ier two segments of the coccyx; 

 while others, after passing for a certain distance backwards, curve forwards and 

 downwards to the ischium forming the anterior free margin of the ligament where 

 it limits posteriorly the sacro-sciatic foramina. These fibres are joined by others 

 which arise from the posterior surfaces of the lower three sacral vertebrae and upper 

 pieces of the coccyx. At the ischium it is fixed to the inner border of the great 

 tuberosity, and sends a thin sharp process upwards along the ramus of the ischium 

 which is called the falciform process (fig. 208), and is a prolongation of the pos- 

 terior edge of the ligament. A great many fibres pass on directly into the tendon 

 of the biceps muscle, so that traction on this muscle braces up the whole liga- 

 ment, and the coccyx is thus made to move on the sacrum. The ligament may 

 not unfairly be described as a tendinous expansion of the muscle, whereby its 

 action is extended and a more advantageous leverage given. It is broad and flat at 

 its attached ends, but narrower and thicker in the centre, looking like two triangular 

 expansions joined by a flat band, the larger triangle being at the ilium, and the 

 smaller at the ischium. The fibres of the ligament are twisted upon its axis at the 

 narrow part, so that some of the superior fibres pass to the lower border. 



The posterior surface gives origin to the cjhiteus mnximu.s muscle, and on it 

 ramify the loops from the jiosterior branches of the sacral nerves; its anterior sur- 

 face is closely connected at its origin with the small sacro-sciatic ligament, and some 

 fibres of Uw pi/riformi.i nuiscle arise from it; below, the obturator mfernus passes out 

 of the i)elvis under its cover, and the internal pndic re,s.-ieh and verve pass in. At the 

 ilium, its })Ost.erior t-dge is continuous witli the v.ertebral aponeurosis; while to the 

 anterior edge is attached the thick fascia covering the gluteus medius. The obtura- 

 tor fiiscia is attached to its falciform edge. It is pierced by the coccygeal branches of 

 the sciatic, and a branch from i\\c fonrfh literal nerre. 



The small sacro-sciatic ligament (figs. 206, 207 and 208) is triangular and 

 thin, springing l»y a broad l)ase from the lateral border of the sacrum and coccyx, 

 from the front of the sacrum both above and below the level of the fourth sacral 



