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As in the intervertebral articulations, so in the union of the first portion of the sacrum with the last lumbar vertebra, there are two sets of joints -viz. (a) Class. -False Synchondrosis. (b) Class. -Diarthrosis. Subdivision. -Arihrodia (a) a synchondrosis, between the bodies and intervertebral disc; and (b) a pair of arthrodial joints, between the articular processes. The union is effected by the following ligaments, which are common to the vertebral column: -(i) anterior, and (ii) posterior longitudinal; (iii) lateral or short vertebral; (iv) capsular; (v) ligamenta flava; (vi) supraspinous and (vii) interspinous ligaments. Two special accessory ligaments on either side, viz., the sacro-lumbar and the ilio-lumbar, connect the pelvis with the fourth and fifth lumbar vertebrae.

The sacro-lumbar ligament is strong, and triangular in shape. Its apex is above and medial, being attached to the whole of the lower border and front surface of the transverse process of the fifth lumbar vertebra, as well as to the pedicle and body. It is intimately blended with the ilio-lumbar ligament. Below, it has a wide, fan-shaped attachment, extending from the edge of the ilio-lumbar ligament forward to the brim of the true pelvis; blending with the periosteum on the base of the sacrum and in the iliac fossa, and with the superior sacro-iliac ligament.

By its sharp medial border, it limits laterally the foramen for the last lumbar nerve. It is pierced by two large foramina, which transmit arteries to the sacro-iliac synchondrosis. This ligament is in series with the inter-transverse ligaments of the spinal column. It is sometimes described as a part of the ilio-lumbar ligament.

The ilio-lumbar ligament is a strong, dense, triangular ligament connecting the fourth and fifth lumbar vertebrae with the iliac crest.

It springs from the front surface of the transverse process of the fifth lumbar vertebra as far as the body, by a strong fasciculus from the posterior surface of the process near the tip, and also from the front surface and lower edge of the transverse process and pedicle of the fourth lumbar vertebra, as far medialward as the body. Between these two lumbar vertebrae it is inseparable from the intertransverse ligament.

At its origin from the transverse process of the fifth lumbar vertebra it is closely inter-woven with the sacro-lumbar ligament, and some of its fibers spread downward on to the body of the fifth vertebra, while others ascend to the disc above. At the pelvis, it is attached to the inner lip of the crest of the ilium for about two inches (5 cm.). The highest fibers at the column form the upper edge of the ligament at the pelvis, those which come from the posterior portion of the transverse process of the fifth lumbar vertebra forming the lower, while the fibers from the front of the same process pass nearly horizontally lateralward. Near the column, the surfaces look directly backward and forward, but at the ilium the ligament gets somewhat twisted, so that the posterior surface looks a little upward, and the anterior looks a Mttle downward. The anterior surface forms part of the posterior boundary of the major (false) pelvis, and over-lies the upper part of the posterior sacro-iliae ligament; the posterior surface forms part of the floor of the spinal groove, and gives origin to the mullifidus muscle. Of the borders, the upper is oblique, has the anterior lamella of the lumbar fascia attached to it, and gives origin to the quadralus lumborum; the lower is horizontal, and is adjacent to the upper edge of the sacro-lumbar ligament; while the medial is crescentic, and forms the lateral boundary of a foramen through which the fourth lumbar nerve passes.

The arterial supply is very free, and comes from the last lumbar, ilio-lumbar, and lateral sacral.

The nerve-supply is from the sympathetic, as well as from twigs from the fourth and fifth lumbar nerves.

Movements of the sacro-vertebral articulation

The angle formed by the sacrum with the spinal column is called the sacro-vertebral angle. The pelvic inclination does not depend entirely upon this angle, but in great part upon the obliquity of the coxal (innominate) bones to the sacrum, so that in males in whom the average pelvic obliquity is a little greater, the average sacro-vertebral angle is considerably less than in females.

The sacro-vertebral angle in the male shows that there is a greater and more sudden change in direction at the sacro-vertebral union than in the female. A part of this change in direction is due to the greater thickness in the anterior part of the intervertebral fibro-cartilage between the last lumbar vertebra and the sacrum. Owing to the greater thickness of the intervertebral disc here than elsewhere, the movements permitted at this joint are very free, being freer than those between any two lumbar vertebrae. As the diameter of the two contiguous bones is less in the sagittal than in the frontal plane, the forward and backward motions are much freer than those from side to side. The backward and forward motions take place every time the sitting is exchanged for the standing position, and the standing for the sitting posture; in rising, the back is extended on the sacrum at the sacro-lumbar union; in sitting down it is flexed.

The articular processes provide for the gliding movement incidental to the extension, flexion, and lateral movements; they also allow some horizontal movement, necessary for the rotation of the vertebral column on the pelvis, or pelvis on the column. The inferior articular processes of the fifth differ considerably from the inferior processes in the rest of the lumbar vertebrae, and in direction they resemble somewhat those of the cervical vertebrae; while the superior articular processes of the sacrum differ in a similar degree from the superior processes of the lumbar vertebrae. This difference allows for the freer rotation which occurs at this joint.

The sacro-vertebral angle averages 117° in the male, and 130° in the female; while the pelvic inclination averages 155° in the male, and 150° in the female.

As already stated, the movements at the sacro-vertebral joint are the same as those in other parts of the spinal column, but more extensive, and the muscles which produce the movements are those mentioned in the preceding groups which cross the plane of the articulation.

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