It springs from the edges of the notched in ferior opening of the sacral canal by a thick band of fibres, which includes also, as an in vesting capsular ligament, lined by a synovial membrane, the articular extremities of the sacral horns, and, gradually narrowing down wards, is attached to, and extends over, the whole posterior surface of the coccyx and its articular processes; covering in the inferior aperture of the sacral canal, and connecting the several pieces of the coccyx when they remain separate. In the latter case also are found intra-coccygeal articulations, small fibro cartilaginous discs intervening between the several bones. This, according to Levret, is most constant between the first and second pieces of the coccyx, where it is sometimes met with in advanced age.
The motions of this joint, and those of the coccygeal bones, are simply antero-posterior flexure, and are sometimes, especially in females and young subjects, very extensive, forming, Cruveilhier says, a complete anterior projecting angle. This anatomist also mentions having seen many times anterior sacro-coccygean mus cles ; other anatomists also mention posterior sacro-coccygean muscles blended with the fibres of the ligament.
The sacro-iliac joints (fig. SO. 1.), one on each side, are composed of an anterior or inferior portion, in which the opposing bones are covered with cartilages of an auricular or angular shape, and a posterior or superior portion where they are united by powerful inter-osseous ligaments, which fill up the re tiring angle left by the cartilages. These are inclosed by anterior, posterior, and superior sacro-iliac investing liganzents.
The cartilages lining these articulations differ from those in the pubic symphysis in being al most totally wanting in the fibrous elements which are in the latter joint intermingled with them. Under the microscope a section of the sacro-iliac cartilages presents the ordinary ap pearance of cartilage incrusting the surfaces of arthrodial joints. They have been said by many writers to be completely incorporated together so as to form but one mass; but such is not the conclusion I have come to, except in a few cases, after many examinations care fully made in subjects recently deceased. The cartilages are very strongly adherent to, and follow exactly the shape of, the auri cular articulating surfaces of the sacrum and ilium before described, their rounded pro jecting angles being the most depending part of the articulation. That on the sacrum is almost double the thickness of the iliac cartilage, which is somewhat less than one eighth of an inch thick. In the male, in a few instances, the two seem to project into each other by irregular prominences, and to be con nected without the intervention of a regular synovial membrane. ln these cases it has been
remarked, that on the application of force, the cartilage separates from the ilium, leaving its auricular surface denuded. Much more frequently in the male, however, and always in the female and child, I have found, extend ing between them throughout, a completely srnooth surface, apparently lined by a delicate menzbrane, and containing much thick synovia. The opposing cartilaginous surfaces are, in these cases, wavy or f-shapecl, when seen in a cross section, the sacral part being convex, and the iliac part concave in front, the re verse arrangement having place behind, the greatest depression being in the iliac surface, exactly at the angular junction of the two limbs of the auricular surfaces, at the most de pending point of the articulation, and through which passes directly the line of pressure here after to be noticed (see fig. 89. page 144. e, f). At this point also, the breadth of the cartilaginous surface is the greatest, being generally about an inch. Towards the extre mity of each limb the width gradually becomes less. The inferior or horizontal portion is longer than the other, generally being about two inches long, the superior or vertical poi,. tion being an inch and a half long.
Immediately posterior to, or rather above, this cartilage-covered surface, and filling up the digital depressions found there on each bone, are firmly implanted the inter-osseous ligaments, , composing the remainder of the articulation. These consist of very strong and coarsely interlacing fibres passing almost directly from bone to bone, inclosing large meshes which are filled with a soft loose synovial looking fat, and containing many veins. Behind, these are continued into the deep posterior sacro-sciatic ligaments.
The superior sacro-iliac ligament (fig. 80. a) is a strong layer of fibres passing from the lateral masses of the sacral base to the pos terior edge of the internal iliac fossa. It is continued in front to the anterior sacro-iliac ligament (b), similar in character to the last, but thinner and more feeble, passing from the first three bones of the sacrum to the superior border of the iliac notch. The former of these assist to prevent downward and backward displacement, and the latter upward and backward displacement ; the position of the former being more anterior than superior, and the position of the latter more inferior than anterior in the proper position of the pelvis.