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The sacrococcygeal joint is an amphiarthrodial joint, formed between the oval surface at the apex of the sacrum, and the base of the coccyx.
It is homologous with the joints between the bodies of the vertebræ, and is connected by similar ligaments:
A disk of fibrocartilage is interposed between the contiguous surfaces of the sacrum and coccyx; it differs from those between the bodies of the vertebræ in that it is thinner, and its central part is firmer in texture. It is somewhat thicker in front and behind than at the sides. Occasionally the coccyx is freely movable on the sacrum, most notably during pregnancy; in such cases a synovial membrane is present.
The Interarticular Ligaments are thin bands, which unite the cornua of the two bones.
The different segments of the coccyx are connected together by the extension downward of the anterior and posterior sacrococcygeal ligaments, thin annular disks of fibrocartilage being interposed between the segments. In the adult male, all the pieces become ossified together at a comparatively early period; but in the female, this does not commonly occur until a later period of life. At more advanced age the joint between the sacrum and coccyx is obliterated.