The Sacro-coccygeal Articulation
Class. - False Synchondrosis.
The last piece of the sacrum and first piece of the coccyx enter into this union [symphysis sacrococcygea] and are bound together by the following ligaments: - Anterior sacro-coccygeal. Deep posterior sacro-coccygeal.
Superficial posterior sacro-coccygeal. Lateral sacro-coccygeal. Intervertebral substance.
The intervertebral fibro-cartilage is a small oval disc, three-quarters of an inch (about 2 cm.) wide, and a little less from before backward, closely connected with the surrounding ligaments. It resembles the other discs in structure, but is softer and more jelly-like, though the laminae of the fibrous portion are well marked.
The anterior sacro-coccygeal ligament is a prolongation of the glistening fibrous structure on the front of the sacrima. It is really the lower extremity of the anterior longitudinal ligament, which is thicker over this joint than over the central part of either of the bones.
The posterior sacro-coccygeal ligament is divided into two layers of which one (the deep) is a direct continuation of the posterior longitudinal ligament of the column, consisting of a narrow band of closely packed fibers, which become blended at the lower border of the first segment of the coccyx with the filum terminate and deep posterior ligament.
The superficial layer of the posterior sacro-coccygeal ligament (or supra-cornual ligament), (fig. 274) is the prolongation of the supraspinous which becomes inseparably blended with the aponeurosis of the sacro-spinalis (erector spinae) opposite the laminae of the third sacral vertebra, and is thus prolonged downward upon the back of the coccyx, passing over and roofing in the lower end of the spinal canal where the laminae are deficient.
The median fibers (the supraspinous ligament) extend over the back of the coccyx to its tip, blending with the deep fibers of the posterior sacro-coccygeal ligament and filum terminale; the deeper fibers run across from the stunted laminae on one side to the next below on the opposite side, and from the sacral cornua on one side to the coccygeal on the opposite, some passing between the two cornua of the same side, and bridging the aperture through which the fifth sacral nerve passes. Its posterior surface gives origin to the gluteus maximus muscle.
The lateral sacro-coccygeal or intertransverse ligament (fig. 274) is merely a quantity of fibrous tissue which passes from the transverse process of the coccyx to the lateral edge of the sacrum below its angle. It is connected with the sacrosciatic ligaments at their attachments, and the fifth sacral nerve escapes behind it. It is perforated by twigs from the lateral sacral artery and the coccygeal nerve.
The arterial supply of the sacro-coccygeal joint is from the lateral sacral and middle sacral arteries.
The nerves come from the fourth and fifth sacral and coccygeal nerves.
The movements permitted at this joint are of a simple forward and backward, or hinge-like character. In the act of defecation, the bone is pushed back by the fecal mass, and, in parturition, by the fetus; but this backward movement is controlled by the upward and forward pull of the levator ani and Coccygeus. The external sphincter also tends to pull the coccyx forward.