The leaser or internal sacro-sciatic ligament (ligamentuni pelvis posticum parvum, fig. 81. d) lies internal to the last, in common with which it is attached posteriorly to the side of the two last pieces of the sacrum and of the two upper pieces of the coccyx. At its an terior extremity it is contracted into a pointed insertion into the spine of the ischium. The direction of this ligament is horizontally for wards and outwards,and its shape is triangular, so that its anterior contracted portion diverges from the great sacro-sciatic ligament, leaving a triangular opening between them through which pass the obturator muscle out of, and the pudic vessels and nerves into the pelvis This ligament, thus passing from the sacrum across to the ischium, converts the sacro sciatic notch into a triangular or oval foramen through which pass the pyramidalis muscle, the gl u teal, ischiadic and pudic vessels, and the superior. gluteal and great and lesser sciatic and pudic nerves out of the pelvis. Withits anterior or internal surface are blended the fibres of the ischio-coccygeus muscle, which exclude it from the ischio-rectal fossa, and render it rough when dissected.
Scemmering describes the lower part of the powerful lumbar fascia as a ligament connect ing the ilia to each other posteriorly and to the lower spines of the sacrum. This fascia does, doubtless, act powerfully in clasping the ilia upon the sacrum between them. He calls it the lateral sacra-iliac ligament, or the posterior lateral iliac ligament.
The important part which these three ac cessory lig,aments play in the mechanism of the pelvis will be hereafter shown.
The movements of the sacro-iliac joint are very lirnited indeed, its principal characteristic being compactness and strength, with just sufficient sliding motion downwards and back wards to break the shock of concussion pass ing from the lower extremities to the trunk. This is said by some to be increased in preg nancy and by parturition.
The pubic symphysis (fig. SO. 2) is an azygos joint uniting the innominate bones by their pubic portions in front. The osseous surfaces composing it are oval, with the long diameter directed downwards and backwards, and ge nerally an inch and a half long, by three quarters broad. The planes of these sur faces not being directly opposed to each other, leave a larger interval of separation in front than behind. This interval is filled by a fibro cartilaginous disc, which is correspondingly thicker in front, where the fibrous components are so numerous and strong as to constitute almost an interosseous ligament, and pass from one bone to the other in an oblique and concentric direction. Towards the central
and posterior part this disc is generally mainly cartilaginous in structure, and is often, in females, separated in the middle by a chink forming two smooth, plane, oval contiguous articular smfaces, of various dirnensions, some times irregularly laminar, at others with a de licate investing membrane. In parturient wo men these surfaces often extend over nearly the whole of the articulation, and are well marked in a figure given by Dr. Hunter, in the second volutne of Medical Observaliims and Inquiries. In males, this separation is seldom present. The whole of the disc may, however, by maceration, generally be separated into two plates (fig. 82. a, a), or a denser and more cartilaginous structure than the rest, each strongly adherent to the bone by mammil lifbrm fibrous processes (b), which pass into rorresponding depressions in the osseous sur faces (c), and are connected to each other on opposite sides, by continuation of their fibres, arranged in oblique and concentric layers, which interlace obliquely with each other. (d) Dr. W. Hunter remarks, with Sandifort and Albinus,that the two cartilaginous plates (a,a), covering the opposed surfaces of the ossa pubis, are usually connected by a structure rather liga mentous than cartilaginous ; and in a memoir on the pubic symphysis, gives an engraving of this arrangement. In several instances I have seen the fibrous processes which connect the plates with the bone very well marked, leav ing on the bone, after maceration, deep conical pits. The above figure was taken from a ma cerated preparation of this joint. According to the observations of Tenon, these processes are directed into the bone downward and backward, as well as outward, and tend to prevent displacement of the cartilage in that direction. The inter-laminar fibro-cartilagin ous tissue is very elastic and yielding, swelling out on the cut surface when lateral pressure is made on the bone, somewhat in the manner of the intervertebral discs. It often e‘inces a tendency to split in a lamellar direction after maceration. Around the circumference the concentric fibres become much more numerous and strona, and are continued into the peri pheral ligaments. These are an anterior, pos terior, a superior, and an inferior ligament.
The anterior pubic ligament (fig. SO. d) is a thick layer, passing between the anterior sur faces of the bones, strengthened by and blended with the oblique fibres of the aponeurosis of the external oblique muscle continued to the opposite pubic bone in front of the joint.