By far the rnost powerful of the ligaments of this articulation, and that which must be con sidered as the chief means of supporting the great downward pressure at this joint, are the posterior sacro-iliac ligaments. These are di vided into deep and supoficial layers of fibres.
The deep layer (fig. 89. page 144. e) passes from a well-marked prominence on the anterior surface of the iliac tuberosity, downwards and inwards, to the superior lateral part of the pos terior surface of the sacrum, principally to the two upper pieces, external to the foramina; the fibres spreading out in interlacing bundles to wards the broader surface of implantation on the sacrurn, becoming longer as they become more superficial, and leaving meshes for the interposition of masses of loose fat, and the passage of numerous small veins. The erector spinw muscles arise from the surface of this ligament, and cover it. To obtain a good view of these fibres, a transverse section along the brim of the true pelvis should be carried backward through the sacrum, as shown in the figures. This will show the manner in which the tuberosities of the ilium hang over the sacrum, suspended, as it were, between them by these ligaments. It will be rnore par ticularly explained when treating on the me chanics of the pelvis. The superficial posterior sacro-iliac ligament (fig. 81., next page, a) has been termed oblique, from the direction of its fibres ; or long, from the extent of them. It is attached above to the posterior superior spine of the ilium, and passes downwards and ob liquely a little inwards to be implanted into the fourth transverse tubercle of the sacrum ex ternal to the hole. To the sides of this liga ment, which is almost subcutaneous, are at tached the fascia lumborum and great gluteus muscle. This ligament is described by Cru veilhier to be attached to the third sacral vertebra. In all the ca.ses I have seen, it is attached to the fourth transverse tubercle, which is the most prominent tubercular pro jection in the dried bone. Bichat erroneously calls it "sacro-spinous." Attached to the sarne sacral tubercle, and passing horizontally outwards to be im planted into the posterior surface of the in ferior posterior spine of the ilium, a point exactly corresponding to the termination of the horizontal limb of the sacro-iliac articular surface, is another well-marked ligament (fig. 81. b), which, being. separated by a dis tinct cellular interval from the deep ligaments and distinguished by the more deeply seated position and horizontal direction of its fibres from the oblique ligament (a), and from the great sacro-sciatic ligament (c), I think merits the name of the inferior or short superficial posterior saero-iliac ligament. This ligament has been hitherto apparently confounded with the great sacro-sciatic, which is attached to its lower border by a thin fibrous extension.
The ligaments which may be considered as accessory to this articulation are three in number—the ilio-lumbar ligament above, and the greater and lesser sacro-sciatic ligaments below.
The ilio-lumbar ligament (fig. 80. c) is a
triangular fascicular ligament, thickest at the edges, and passing from the tip of the last lumbar transverse process, to which its apex is attached, horizontally outwards, and a little backwards to the posterior fifth of the inner lip of the crest of the ilium, along w hich its fibres spread as far forward as the inner projecting point of the posterior curve. To the outer side and behind this ligament is attached the quadratus lumborum muscle with the tendon of the transversalis abdo minis, and to its front the psoas magnus muscle. Meckel describes this ligament as sometimes reaching as high as the transverse process of the fourth lumbar vertebra. He also describes a second ligament lower than the preceding, but arising from the iliac crest a little behind it. They are called by him, respectively, the upper and lower anterior pelvic ligaments, the latter corresponding to the sacro-vertebral ligament before described.
The great sacro-sciatic ligament (ligamentum pelvis posticum magnum, fig. 81. c) is attached behind, to the posterior inferior spine of the ilium by a membranous expansion (e); to the superficial! posterior sacro-iliac ligaments with which its fibres are blended; to the posterior surface and borders of the two last pieces of the sacrum; and to the posterior sacro-coccy gean ligament and borders of the two or three upper coccygeal bones. From this broad at tachment its fibres pass downwards, forwards, and outwards to be implanted into the whole length of the raised inner border of the great tuberosity of the ischium. The fibres of this ligament are arranged in fasciculi, which cross each other in an X-like manner, so as to present, at the extremities, an expanded appearance, and in the centre a thick con tracted rounded outline. The fibres which are placed superiorly in one extremity of insertion cross at the contracted part to become inferior at the other extremity, while those which are internal cross in the opposite direction to become external. Its superior border, consequently, is directed outwards and forwards, and its inferior border inwards, and both present a curvilinear outline. At its insertion into the sciatic tuberosity, the fibres of the lower border present a falciforrn margin having the concavity directed upwards along the inner edge of the tuberosity, where it is united to the fascia covering the obturator in ternus muscle. Its superficial or external fibres are continued over the tuberosity in feriorly into the tendons of the biceps flexor cruris, and seini-tendinosus muscles. Near the posterior extremity, this ligament is almost invariably perforated by a small hole, through which passes the coccygeal branch of the ischiadic artery. To the whole length of its external or posterior surface is attached the great gluteus muscle, which causes it vvhen dissected to he very rough and flocculent. At the posterior half of its inner surface it is blended intimately with the lesser sacro-sciatic ligament, anterior to which it is smooth, and forms part of the boundary of the ischio rectal fosse.