180 THE ARTICULATIONS OF THE PELVIS. 



pyriformis muscle, the gluteal, sciatic, and pudic vessels and nerves, and the nerves 

 to the obturator internus and quadratus femoris muscles. The part between the 

 greater and lesser sacro-sciatic ligaments, much smaller and bounded in front by 

 the smooth surface between the spine and tuberosity of the ischium, is the small 

 sacro-sciatic foramen, through which pass the obturator internus muscle, the pudic 

 vessels and nerve, and the nerve to the obturator internus. 



The PUBIC ARTICULATION, or symphysis pulis, is the connection of the pubic 

 bones in front, and is effected by an interpubic disc and ligaments. The interpubic 

 disc consists of a layer of cartilage on each side, closely adherent to the bony sur- 

 faces, and an intermediate stratum of fibrous tissue and fibro-cartilage. The inter- 

 mediate layer is thicker in front than behind, and generally contains a fissure 

 towards the upper and back part, which sometimes extends through the whole depth 

 of the articulation (fig. 207). The ligaments are named anterior, posterior, 

 superior, and inferior. The anterior pubic ligament is of considerable thickness ; it 

 consists of deep fibres passing transversely between the bones at the front of the 

 disc, and superficial, oblique, interlacing fibres derived mainly from the tendons of 

 the obliquus externus and rectus muscles of the abdomen, as well as of the super- 

 ficial adductors of the thigh. The superior and posterior ligaments consist of only 

 a few fibres on the upper and back parts of the articulation. The inferior or 

 sulpubic ligament, thick and triangular, is attached to the inferior rami of the pubic 

 bones, giving smoothness and roundness to the subpubic angle, and forming part 

 of the outlet of the pelvis. 



The fissure in the interpubic disc appears to be formed during life by the softening 1 and 

 absorption of the fibro-cartilage. It is not usually found before the seventh year, it increases 

 in extent with advancing age, and is more constant and of larger size in the female than in 

 the male. Its greater development in the female sex may be in part due to the pressure 

 exerted upon the joints of the pelvis during parturition, but it is not a regular accompaniment 

 or a direct consequence of pregnancy. 



The obturator membrane, or ligament, is a fibrous septum attached to the 

 border of the thyroid foramen, which it closes, except at the obturator groove, where 

 a small oval canal is left for the obturator vessels and nerve. The membrane is 

 fixed accurately to the bony margin at the outer side of the foramen, but to the 

 posterior surface at the inner side. The obturator muscles are attached to its 

 surfaces. 



Movements and mechanism. In ordinary circumstances there is very little movement 

 allowed between the bones of the pelvis. In the erect posture the sacrum is inclined so much 

 backwards that none of the advantage of the key-stone of an arch is obtained by the tapering 

 of its form from base to apex. It is only by the sinuosities of its auricular surfaces that it 

 directly presses on the hip-bones ; and as the width of the bone rather diminishes towards the 

 dorsal surface, the principal strain is borne by the posterior sacro-iliac ligaments, from which 

 the sacrum is in great measure suspended (see fig. 207). As the base of the sacrum, in the 

 upright posture, projects forwards much beyond the auricular surface (cf. fig. 14 and 18), this 

 bone will necessarily have a disposition to rotate about the place of support under the 

 influence of the pressure of the superposed column, the upper extremity tending to sink and 

 the lower extremity to rise. This tendency is restrained by the sacro-sciatic ligaments, which 

 tie the lower end of the sacrum to the ischium. The ilio-lumbar ligament acts similarly in 

 supporting the base of the column, and it also serves to prevent the fifth lumbar vertebra 

 from slipping forwards over the oblique base of the sacrum. The space which might be 

 gained by the small amount of movement allowed between the bones of the pelvis in the 

 ordinary state is increased during parturition in this way, that the fore part of the sacrum 

 being pressed backwards, the wider part of the wedge formed by this bone is forced 

 farther between the hip-bones so as to separate them to a greater degree, and thus to 

 increase the capacity of the pelvis. It is thought also by some that during pregnancy a 

 slight amount of separation may occur at the symphysis pubis from relaxation of the con- 

 necting parts. 



