OF THE PELVIS 



203 



foramen, and from the coccyx nearly as far as its tip. By its apex it is attat-hed to 

 the front surface and the borders of the ischial spine as far outwards as its base. Its 

 fibres decussate so that the lower ones at the coccyx become the liighest at the 

 ischial spine; muscular til)res are often seen intermingled with the ligamentous. 



It is situated in front of the great sacro-sciatic ligament, with which it is 

 closely connected at the sacrum, and separates the greater from the lesser sacro- 

 sciatic foramen. Its front surface gives attachment to the coccygeus muscle, which 

 overlies it. Behind, it is connected with, and hidden by, the great sacro-sciatic 

 ligament, so that only the outer inch or less (2 cm.) and a small part of its attach- 

 ment to the coccyx, can be seen; the internal pudic nerve also passes over the 

 posterior surface. 



The arterial supply comes from the gluteal, ilio-lumbar, and lateral sacral. 



The nerve-supply is from the superior gluteal, sacral plexus, and external 

 twigs of the ]»()sterior divisions of the first and second sacral nerves. 



The movements. — It is quite clear, from the nature of the osseous surfaces. 



Fig. 20^. — Sacro-sciatic Ligaments. (Posterior view. 



Anterior or small aaero- 

 Bciatic ligament 



Falciform process of the 

 great ligament 



Tendon of biceps muscle, continuous with 

 the great saero-seiatie ligament 



from the wedge-shape of the sacrum, and the manner in which it is locked in 

 between the hip-bones, as well as from the amphiarthrodial character of the articu- 

 lation, that there can be no movement at the sacro-iliac joint. "\Mnle the joint 

 serves the useful purpose of breaking shocks, the cartilage is too thin and too 

 finnly fixed to the bones to allow even of appreciable yielding, such as occurs upon 

 the intervertebral discs. 



The double wedge-shape of the sacrum, Mith its broader surface at the base and 

 in front, prevents dislocation from forces acting from above downwards, and from 

 before backwards. 



The sinuous character of the oi)posed surfaces of the sacrum and ilium, the for- 

 ward and inward direction of the til)rcsof the posterior sacro-sciatic ligament which 

 pass from ilium to sacrum, as well as the ilio-lumbar and sacro-lumbar ligaments, 

 prevent forward displacement of the base of the sacrum; while the sacro-sciatic 

 ligaments prevent the tilting backwards of its ai)ex. Thus rotation forwards is 

 entirely prevented. 



The shape of the sacrum prevents its downward displacement, while the weight 



