Coccyx

Structure

The coccyx is formed of three, four or five rudimentary vertebrae. It articulates superiorly with the sacrum. In each of the first three segments may be traced a rudimentary body and articular and transverse processes; the last piece (sometimes the third) is a mere nodule of bone. The transverse processes are most prominent and noticeable on the first coccygeal segment. All the segments lack pedicles, laminae and spinous processes. The first segment is the largest; it resembles the lowest sacral vertebra, and often exists as a separate piece; the remaining ones diminish in size caudally.

Most anatomy books incorrectly state that the coccyx is normally fused in adults. It has been shown that the coccyx may, in some people, consist of up to five separate bony segments, the most common configuration being two or three segments.

Surfaces

The anterior surface is slightly concave and marked with three transverse grooves which indicate the junctions of the different segments. It gives attachment to the anterior sacrococcygeal ligament and the levatores ani and supports part of the rectum. The posterior surface is convex, marked by transverse grooves similar to those on the anterior surface, and presents on either side a linear row of tubercles – the undeveloped articular processes of the coccygeal vertebrae. Of these, the superior pair are the largest, and are called the coccygeal cornua they project caudally, and articulate with the cornua of the sacrum, and on either side complete the foramen for the transmission of the posterior division of the fifth sacral nerve.

Borders

The lateral borders are thin and exhibit a series of small bony protrusions, which represent the transverse processes of the coccygeal vertebrae. Of these, the first is the largest; it is flattened anteriorly, and often extends to join the lower part of the thin lateral edge of the sacrum, thus completing the foramen for the transmission of the anterior division of the fifth sacral nerve; the others diminish in size caudally, and are often lacking. The borders of the coccyx are narrow, and give attachment on either side to the sacrotuberous and sacrospinous ligaments, to the coccygeus and levator ani in front of the ligaments, and to the gluteus maximus behind them.

Apex

The apex is rounded, and has attached to it the tendon of the external anal sphincter; it may be divided in two.

Coccygeal fossa

The coccygeal fossa is a shallow depression on the posterior surface between the sacrum and the perineum, located in the intergluteal cleft that runs from just below the sacrum to the perineum. It is not consistently present in all humans. The coccygeal fossa marks the deepest part of the pelvic floor, next to the coccyx. The levator ani has its origin here.

Extensor coccygis

The extensor coccygis is a slender muscle fascicle, which is not always present. It extends over the caudal portion of the posterior surface of the sacrum and coccyx. It arises by tendinous fibers from the last segment of the sacrum, or first piece of the coccyx, and passes downward to be inserted into the lower part of the coccyx. It is an evolutionary relic of the extensor muscle of the caudal vertebrae of other animals, enabling limited coccygeal motion.

Sacrococcygeal and intercoccygeal joints

The joints are variable and may be: (1) synovial joints; (2) thin discs of fibrocartilage; (3) intermediate between these two; (4) ossified.{{cite journal

Attachments

The anterior side of the coccyx has attachments to the levator ani muscle, coccygeus, iliococcygeus, and pubococcygeus, anococcygeal raphe. Attached to the posterior side is the gluteus maximus, which extends the thigh at the hip joint. The ligaments attached to the coccyx include the anterior and posterior sacrococcygeal ligaments which are the continuations of the anterior and posterior longitudinal ligaments that extend along the entire spine. The lateral sacrococcygeal ligaments complete the foramina for the last sacral nerve.{{cite book

Function

The coccyx is not entirely useless in humans,{{cite book

Clinical significance

Injuring the coccyx can give rise to a painful condition called coccydynia and one or more of the bones or the connections thereof may be broken, fractured tailbone.{{cite journal | url-access = subscription }}{{cite journal | doi-access = free

Etymology

The term coccyx is derived from the ancient Greek word κόκκυξ grc "cuckoo"; the latter is attested in the writings of the Greek physician Herophilus to denote the end of the vertebral column. This Greek name for the cuckoo was applied as the last three or four bones of the coccyx resemble the beak of this bird, when viewed from the side.{{cite web

This established etymological explanation can also be found in the writings of the 16th century anatomist Andreas Vesalius who wrote: os cuculi, a similitudine rostri cuculi avis (the cuckoo bone shows a likeness to the beak of the cuckoo bird). Vesalius used the Latin expression os cuculi, with os, bone and cuculus, the Latin name for the cuckoo. The 16th/17th century French anatomist Jean Riolan the Younger gives a rather hilarious etymological explanation, as he writes: quia crepitus, qui per sedimentum exeunt, ad is os allisi, cuculi vocis similitudinem effingunt (because the sound of the farts that leave the anus and dash against this bone, shows a likeness to the call of the cuckoo). Riolan's explanation is not considered credible.

Besides os cuculi, os caudae, with caudae, of the tail is attested. This Latin expression might be the source of the English, French language, German and Dutch terms tailbone, l'os de la queue, Schwanzbein and staartbeen. In the current official anatomic Latin nomenclature, Terminologia Anatomica, coccyx and os coccygis is used.

Additional images

File:Gray404.png|The coccyx sits below the sacrum and behind the pelvic cavity.

References

References

1. Weisberger, Mindy. (March 23, 2024). "Why don't humans have tails? Scientists find answers in an unlikely place". [[CNN]].
2. Nakatsukasa 2004, ''Acquisition of bipedalism'' (See [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571308/ Fig. 5] entitled ''First coccygeal/caudal vertebra in short-tailed or tailless primates.''.)
3. Note: Nacholapithecus and Nakaliphitecus nakayamai are two different species of Miocene hominoids (specimens from Nakali and Nachola respectively). See for example "Comparisons with Other Hominoids" in (Kunimatsu, Nakatsukasa ''et al.'' Dec 2007)
4. Tetiker H, Koşar MI, Çullu N, Canbek U, Otağ I, Taştemur Y. (2017). "MRI-based detailed evaluation of the anatomy of the human coccyx among Turkish adults.". Niger J Clin Pract.
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7. Coccygektomi kan være en behandlingsmulighed ved kronisk coccygodyni (Coccygectomy may be a treatment option for chronic coccydynia) Ugeskr Læger 2011 Feb 14; 173(7): 495-500. In Danish. Aarby, Nanett Skjellerup (1), Trollegaard, Anton Mitchell (2) and Hellberg, Steen (2) https://www.coccyx.org/medabs/aarby.htm
8. Heum Dai Kwon et al., Coccygodynia and Coccygectomy. ''Korean Journal of Spine'', 9, 4 (2012), 326-333.
9. Klein, E. (1971). ''A comprehensive etymological dictionary of the English language. Dealing with the origin of words and their sense development thus illustration the history of civilization and culture.'' Amsterdam: Elsevier Science B.V.
10. {{OEtymD. coccyx
11. {{LSJ. ko/kkuc. κόκκυξ. ref.
12. Hyrtl, J. (1880). ''Onomatologia Anatomica. Geschichte und Kritik der anatomischen Sprache der Gegenwart.'' Wien: Wilhelm Braumüller. K.K. Hof- und Universitätsbuchhändler.
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14. (2011). "The Hellenic and Hippocratic origins of the spinal terminology". Journal of the History of the Neurosciences.
15. Sugar, Oscar. (February 1995). "Historical Perspective Coccyx: The Bone Named for a Bird". Spine.
16. {{L&S. os2. os, {{L&S. cuculus, {{L&S. cauda. ref
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