THE SACRO-ILIAC LIGAMENTS. 429 



lumborum is attached ; and its posterior surface is covered by the 

 quadratus lumborum. 



This ligament supports the upright moveahle portion of use. 

 the spinal column, and resists the tendency of the last lumbar 

 vertebra to slip forwards over the inclined base of the sacrum. 



SACRO-ILIAC ARTICULATION. The irregular surfaces by which union of 

 the sacrum and the hip-bone articulate are covered with cartilage, 

 and are maintained in contact by anterior and posterior sacro-iliac 

 ligaments. Inferiorly the bones are further connected, without 

 being in contact, by the strong sacro-sciatic ligaments. 



Dissection. To see the posterior ligaments, the mass of muscle TO dissect 

 at the back of the sacrum is to be removed on the side on which 

 the hip bone remains. The thin anterior bands will be visible on 

 the removal of some areolar tissue. The small sacro-sciatic ligament 

 will be brought into view by removing the fleshy fibres of the 

 coccygeus ; and the large ligament has been dissected with the 

 lower limb. 



The anterior saci'o-iliac ligament consists of a few thin scattered Anterior 

 fibres between the bones, near their articular surfaces. 



The posterior sacro-iliac ligament is very strong, and consists of Posterior 

 bundles of fibres which pass obliquely from the rough part of the 

 inner side of the ilium above the auricular surface to the depressions 

 on the back of the first and second pieces of the sacrum. A distinct 

 band, longer and more superficial than the rest, runs from the a special 

 posterior superior iliac spine to the third and fourth pieces of the long band * 

 sacrum ; it is named the long posterior ligament. 



Articular cartilage. This maybe seen after the sacro-sciatic and A layer of 

 ilio-lumbar ligaments have been examined, by opening the articula- Sch brae? 1 

 tion and separating the bones. It covers the articular surfaces of 

 both sacrum and ilium, but is much thicker on the sacrum. Its 

 surface is generally uneven ; and the intermediate cleft is some- 

 times partly interrupted by transverse fibres uniting the two layers. 



Mechanism. There is scarcely any appreciable movement in this Use of joint 

 articulation, owing to the tightness with which the two bones are 1S 

 bound together by ligaments, and the irregular form of the articular 

 surfaces, which are consequently unable to glide over one another. 

 In the erect posture the sacrum is suspended between the two hip- to render 

 bones by the thick posterior sacro-iliac ligaments, and the upper 

 arch of the pelvis is thereby rendered less rigid than would be the 

 case if it were formed of continuous bone. The sacro-iliac articula- 

 tion thus serves to give elasticity to the pelvis, and to diminish the 

 effect of shocks passing to the spine. 



PUBIC ARTICULATION (symphysis pubis ; fig. 159, a). The two Symphysis 

 pubic bones are united by an interpubic disc, by ligamentous fibres P ubls - 

 in front and above, and by a strong subpubic ligament. 



The anterior pubic ligament is composed of interlacing fibres Anterior 

 which are mixed with fibres of the tendon of the external oblique llgam 

 muscle. 



There is not any definite posterior band ; but the periosteum is Few fibres 

 thickened by a few scattered fibres. 



