8.5.6: Sacrum and Coccyx
- Page ID
- 53929
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Above: The sacrum is shown in red. It is located in the posterior of the pelvic girdle and is the second-to-most inferior bone of the vertebral column (coccyx is the most inferior bone of the vertebral column).
The sacrum is part of both the vertebral column and the pelvic girdle. The sacrum articulates with the body of the L5 vertebra above it and articulates to the inferior articular processes and facets of L5 with the superior articular processes and facets of the sacrum. The sacrum also articulates with two coxal bones (hip bones) lateral to it at the auricular surfaces. In young people, the sacrum is actually five sacral vertebrae, but in late adolescence/early adulthood these sacral vertebrae fuse together to form one bone, the sacrum. This fusion is not complete until somewhere between the 18th and 30th year of life.
Above: The sacrum and coccyx from (A) anterior, (B) anterolateral, and (C) posterior views.
The coccyx is a vestigial tailbone (vestigial = remnant of a structure that used to have a function). Coccyx is the evolutionary remnant of ancestral species to humans that did have tails. It no longer serves as a functional tail, but some muscles, tendons, and ligaments do attach to it, making it useful. It forms from the fusion of usually three vertebrae, but a small proportion of the population have four or even five vertebrae in their coccyx.
Attributions (All Vertebral Column Sections)
- "Anatomy 204L: Laboratory Manual (Second Edition)" by Ethan Snow, University of North Dakota is licensed under CC BY-NC 4.0
- "Anatomy and Physiology" by J. Gordon Betts et al., OpenStax is licensed under CC BY 4.0
- "BIOL 250 Human Anatomy Lab Manual SU 19" by Yancy Aquino, Skyline College is licensed under CC BY-NC-SA 4.0
- "BodyParts3D/Anatomography" by The Database Center for Life Science is licensed under CC BY-SA 2.1
- "Cervical vertebra english.png" by user:debivort is licensed under CC BY-SA 3.0
- "Sacrum - animation02.gif" by BodyParts3D is made by DBCLS is licensed under CC BY-SA 2.1
Updated 2025.