Coccyx

At a glance

English

Coccyx

Latin (TA2)

Os coccygis

System

Bone

Region

Abdomen

Side

Single, on the midline

About the coccyx

The coccyx, or tailbone, is the last part of the vertebral column, a small triangular bone hanging from the tip of the sacrum. It is the human remnant of a tail. It is built from three to five rudimentary coccygeal vertebrae, most often four. These may stay as separate pieces joined by fibrocartilage or small joints, or may fuse partly or wholly; two or three separate segments is a common pattern. Its name comes from the Greek word for a cuckoo, because its curved outline viewed from the side was thought to resemble the bird's beak.

The segments get smaller from above down. The first is the largest and most vertebra-like, with a small body, short transverse processes and a pair of upward-pointing coccygeal horns that represent articular processes; it often remains separate. The lower pieces lose their processes, and the last is just a nodule of bone. No segment has pedicles, laminae or a spine, so there is no vertebral canal. The base joins the apex of the sacrum at the sacrococcygeal joint, a disc of fibrocartilage that lets the coccyx move a little forwards and back.

The front surface is slightly hollow and supports the end of the rectum; the levator ani, coccygeus and anterior sacrococcygeal ligament all attach to it, the two muscles forming part of the pelvic floor. The back surface gives origin to gluteus maximus and receives the posterior sacrococcygeal ligaments and the end of the filum terminale. The side borders take the sacrotuberous and sacrospinous ligaments, and the tip anchors the anococcygeal body and the external anal sphincter. The coccygeal horns meet the sacral horns and close the gap through which the fifth sacral nerve passes.

The coccyx moves back slightly when the pelvic floor relaxes, as in sitting down hard. Pain at the tailbone, coccydynia, follows falls onto the buttocks or strain, and may come from a fracture or a loose joint. A tumour of fetal origin, sacrococcygeal teratoma, arises here in newborns. When pain does not settle, the bone may be removed.

Function and supply

Function

Gives a fixed point for the pelvic floor muscles and the gluteus maximus and helps carry weight when sitting leans back.

Nerve supply

Not innervated as a muscle is; the coccygeal plexus, from the fourth and fifth sacral and coccygeal nerves, supplies the skin over it and the sacrococcygeal joint.

Blood supply

The median sacral artery runs down its front, with branches of the lateral sacral and inferior gluteal arteries.