The ligaments are five in number: two anterior, divided into superior and inferior; two posterior, divided in the same way. The fifth is an interosseous ligament. Now to these five must be added a sixth, the ilio lumbar ligament, which may be considered an appendage of the articula tion.
This ilio-lumbar ligament, extends from the transverse process of the last lumbar vertebra to the corresponding part of the iliac crest. It is thick and round within, becoming flattened and more slender without. It is strong and resisting, uniting the lumbar vertebras and the sacrum to the iliac bones, and completes above and behind the walls of the greater pelvis.
The antero-superior ligament is formed by the periosteum of the lateral parts of the base of the sacrum, and crosses the articular line while pass ing towards the internal iliac fossa.
The antero-inferior ligament is formed by the periosteum of first two anterior sacral foramina, and, like the superior ligament, becomes one with that of the iliac bones.
The postero-superior ligament is composed of fibres which pass from the posterior part of the iliac crest, and from the subjacent roughened surface, to the tubercles situated on the outside of the first two posterior sacral foramina, and to the space between them, blending with the inter osseous ligament.
The postero-inferior ligament is very thick, and may be divided into two layers: 1st, a superficial one, which is inserted above, on the posterior and superior iliac spine, and below, on the tubercle situated on the out side of the third posterior sacral foramen. This is the sacro-spinal liga ment of Bichat, and the posterior sacro-vertical of Cruveilliier. It blends at its internal edge with the aponeurosis which covers the spinal muscles, and, at its external edge, with the great sacro-sciatic ligament; 2d, a deep layer arising from the whole of the space between the tubercles situated outside of the second and third posterior sacral foramina. It is attached to the two posterior iliac spines and the notch between them.
The interosseous ligament, the strongest of all, occupies a deep cavity immediately behind the two articular facets. It is attached to the iliac tuberosity, and to two facets which are on the external apex of the first sacral foramen.
The Synovial Membrane.—The sacroiliac articulation has a synovial membrane which covers the internal face of the ligaments to the level of the articular line. Its size does not exceed that of the line of articulation.
Movements.—The sacro-iliac articulation is not absolutely immovable, having a little sliding motion in several directions.
This arrangement of ligaments is strengthened by the sacro-sciatic and obturator ligaments.
The Sacro-Sciatic Ligaments.—There are two on each side, the large
and the small.
The great sciatic ligament is attached by its base to the posterior ex tremity of the superior semi-circular line of the iliac bone, to the external edge of the superficial layer of the posterior and inferior sacro-iliac liga ment, to the lateral parts of the sacrum, and to the whole of the corre sponding edge of the coccyx. It becomes narrow in its middle, and larger at its apex, which is attached to the inferior part of the ischium, and to the external lip of the ascending branch.
By its posterior face it gives attachment to the gluteus maximus mus cle. By its anterior face it is united to the lesser sciatic ligament. Its superior edge joins the great sciatic notch, closing above the greater sci atic canal, and below the lesser sciatic canal. Its inferior edge forms the lateral and posterior parts of the strait.
The lesser sciatic ligament is attached by its base to the lateral and in ferior parts of the sacrum, and to the edges of the coccyx, by its apex, to the sciatic spine.
The two sciatic ligaments divide the sciatic notch into two canals, the greater and lesser sciatic foramina.
The obturator ligament, or obturator membrane, is a fibrous lamina which closes the sub-pubic canal, and is attached to the border of the ob turator canal in such a manner as to leave free, and to convert into a canal, the groove through which the obturator nerves and vessels pass.
Tim SACRO-ILIAC SYNCIIONDROSES.
The Sacro-coccggeal Symphysis.
It is formed by the articular facet of the apex of the sacrum, by that which is at the base of the coccyx, by an interosseous fibro-cartilage, and by some ligaments which are named the anterior posterior and lateral sacro-coccygeal ligaments. These are fibres which stretch over the whole of the coccyx from top to bottom.
The Sacro-vertebral Articulation.
It consists of one amphiarthrosis, and two arthrodize. The amphiar throsis is formed by the two articular surfaces of the inferior face of the lumbar vertebra, and of the base of the sacrum. The surfaces are united by a disc similar to the inter-vertebral discs.
The two arthrodiw are formed by two small vertical facets on the sides of the Superior opening of the sacritl canal, and by two similar facets situ ated on the inferior face of the lait lumbar vertebra, and which fit to gether.
They are united by ligamentous fibres, belonging to the common an terior and posterior vertebral ligament, to an inter-spinal ligament, and finally to a fascia which Bichat has called the sacro-vertebral fascia. It proceeds from the anterior and inferior part of the transverse process of the last lumbar vertebra, and is attached to the superior part of the sacrum.