Coccyx
Os coccygis
Also known as: tailbone, coccygeal vertebrae
The small triangular "tailbone" at the end of the vertebral column, formed by 3–5 (usually 4) rudimentary fused coccygeal vertebrae.
Overview
The first coccygeal segment has a rudimentary body, small transverse processes and coccygeal cornua (vestigial articular processes) that articulate with the sacral cornua; the lower segments are simple nodules of bone. The coccyx articulates with the sacral apex at the sacrococcygeal joint, a symphysis that allows some movement — it is pushed back during defecation and childbirth.
Although vestigial, it is functionally important: it gives attachment to gluteus maximus (posteriorly), coccygeus and the anococcygeal body/ligament receiving fibres of levator ani (pelvic floor), the sacrotuberous and sacrospinous ligaments' margins, and the external anal sphincter. In sitting it forms part of a tripod with the two ischial tuberosities, bearing more load when leaning back.
- Location
Inferior end of the vertebral column, behind the anal canal, in the natal cleft.
Anatomy
- Articulations
Sacrum (sacrococcygeal symphysis); intercoccygeal joints between segments (often fused).
- Ossification
One primary centre per segment, appearing from ≈1–4 years (first segment) to ≈10–20 years (last); segments fuse progressively in adulthood.
Function
- Attachment for the pelvic floor (levator ani, coccygeus) and gluteus maximus
- Weight-bearing when sitting and leaning back
- Remnant of the embryonic tail
Clinical significance
Coccydynia
Pain over the coccyx on sitting, often after a fall onto the buttocks or childbirth; due to fracture, subluxation or hypermobility of the sacrococcygeal joint.
Sacrococcygeal teratoma
The commonest neonatal tumour, arising from primitive streak remnants near the coccyx; complete excision includes the coccyx to prevent recurrence.
Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.