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Other Terms: Pelvis, Tail bone, Tailbone, Coccyx [coccygeal vertebrae I-IV], Coccyx [vertebrae coccygeae I-IV], Os coccygis
This is the terminal end of the vertebral column. It is a triangular bone that forms from the fusion of three to five vertebral segments, most commonly from four fused vertebrae. The first segment is characteristic of the vertebral plan, with succeeding segments becoming highly reduced. The first segment has small costotransverse processes. It also has superior projecting cornua that represent modified pedicles and superior articular processes. The superior surface of the first segment's body forms an oval articular surface with the inferior surface of the fifth sacral segment. The bone provides attachment for the gluteus maximus, levator ani, external anal sphincter, coccygeus, and various coccygeal ligaments.
The word coccyx comes from the Greek word meaning cuckoo for its resemblance to the beak of the cuckoo bird.
The bone articulates with one other bone: the sacrum. The superior articular surface of the first segment's body forms a cartilaginous symphyseal joint with the inferior surface of the fifth sacral segment. The coccygeal cornua form a synovial joint with the sacral cornua.
The bone usually ossifies from four centers, one for each segment with the first segment often forming separate ossification centers for the cornua. The center for the first segment appears just before or soon after birth. Remaining centers are highly variable in their appearance ranging from around puberty to the twentieth year. They unite as one bone as late as the thirtieth year. In later life, the coccyx may fuse with the sacrum.
Coccyx [vertebrae coccygeae I-IV]