This article is about the tailbone in apes and analogous structures. For animals with tails, seeCaudal vertebrae. For the moth genus, seeCydia (genus).
"Coccyges" redirects here. For the clade of birds, seeCuculiformes andParrot.
The coccyx is formed of three, four or five rudimentaryvertebrae. It articulates superiorly with thesacrum. In each of the first three segments may be traced a rudimentary body andarticular andtransverse 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 lackpedicles,laminae andspinous processes. The first segment is the largest; it resembles the lowestsacral vertebra, and often exists as a separate piece; the remaining ones diminish in sizerostrally.
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.[4]
Theanterior surface is slightly concave and marked with three transverse grooves which indicate the junctions of the different segments. It gives attachment to theanterior sacrococcygeal ligament and thelevatores ani and supports part of therectum. Theposterior 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 undevelopedarticular processes of the coccygeal vertebrae. Of these, the superior pair are the largest, and are called thecoccygeal cornua they projectcaudally, and articulate with the cornua of the sacrum, and on either side complete theforamen for the transmission of theposterior division of thefifth sacral nerve.
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 theanterior division of the fifth sacral nerve; the others diminish in size from caudally, and are often lacking. The borders of the coccyx are narrow, and give attachment on either side to thesacrotuberous andsacrospinous ligaments, to thecoccygeus andlevator ani in front of the ligaments, and to thegluteus maximus behind them.
Thecoccygeal fossa is a shallow depression on the posterior surface between thesacrum and theperineum, located in theintergluteal cleft that runs from just below the sacrum to theperineum.[clarification needed][5] It is not consistently present in all humans. The coccygeal fossa marks the deepest part of thepelvic floor, next to the coccyx. Thelevator ani has itsorigin here.[6]
Theextensor coccygis is a slendermuscle fascicle, which is not always present. It extends over the caudal portion of the posterior surface of thesacrum 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.
The coccyx is not entirely useless in humans,[11] because it has attachments to various muscles, tendons and ligaments. However, these muscles, tendons and ligaments are also attached at many other points, to stronger structures than the coccyx. It is doubtful that the coccyx attachments are important to the well-being of humans, given the large number ofcoccygectomy procedures performed annually to treatcoccydynia. Reviews of studies covering more than 700 coccygectomies found the operation was successful in relieving pain in 84% of cases.[12][13] 12% of the time, the only major complication faced was infection due to the proximity to the anus. One notablecomplication of coccygectomy is an increased risk ofperineal hernia.
Injuring the coccyx can give rise to a painful condition calledcoccydynia and one or more of the bones or the connections thereof may be broken,fractured tailbone.[14][15] A number oftumors are known to involve the coccyx; of these, the most common issacrococcygeal teratoma. Both coccydynia and coccygeal tumors may require surgical removal of the coccyx (coccygectomy). One very rare complication of coccygectomy is a type ofperineal hernia known as a coccygeal hernia.[16]
The termcoccyx is derived from theancient Greek wordκόκκυξ[17][18]kokkyx "cuckoo";[19] the latter is attested in the writings of the GreekphysicianHerophilus to denote the end of the vertebral column.[20] This Greek name for thecuckoo was applied as the last three or four bones of the coccyx resemble the beak of this bird,[17][20][21][22] when viewed from the side.[9][23]
This established etymological explanation can also be found in the writings of the 16th century anatomistAndreas Vesalius who wrote:os cuculi, a similitudine rostri cuculi avis[20] (the cuckoo bone shows a likeness to the beak of the cuckoo bird). Vesalius used the Latin expressionos cuculi, withos, bone[24] andcuculus, the Latin name for the cuckoo.[24] The 16th/17th century French anatomistJean 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[20] (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.[20][21]
Besidesos cuculi,os caudae,[20][25] withcaudae, of the tail[24] is attested. This Latin expression might be the source of the English,French language,German andDutch termstailbone,l'os de la queue,[25]Schwanzbein[21][25] andstaartbeen.[26] In the current official anatomic Latin nomenclature,Terminologia Anatomica,[27]coccyx andos coccygis is used.
^Nakatsukasa 2004,Acquisition of bipedalism (SeeFig. 5 entitledFirst coccygeal/caudal vertebra in short-tailed or tailless primates..)
^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, Nakatsukasaet al. Dec 2007)
^Saladin, Kenneth S. (2003).Anatomy & Physiology: The Unity of Form and Function (3rd ed.). McGraw-Hill. p. 268.
^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
^Heum Dai Kwon et al., Coccygodynia and Coccygectomy.Korean Journal of Spine, 9, 4 (2012), 326-333.
^Miranda EP, Anderson AL, Dosanjh AS, Lee CK (September 2007). "Successful management of recurrent coccygeal hernia with the de-epithelialised rectus abdominis musculocutaneous flap".J Plast Reconstr Aesthet Surg.62 (1):98–101.doi:10.1016/j.bjps.2007.08.002.PMID17889632.
^abKlein, 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.
^abcdefHyrtl, J. (1880).Onomatologia Anatomica. Geschichte und Kritik der anatomischen Sprache der Gegenwart. Wien: Wilhelm Braumüller. K.K. Hof- und Universitätsbuchhändler.
^abcKraus, L.A. (1844).Kritisch-etymologisches medicinisches Lexikon (Dritte Auflage). Göttingen: Verlag der Deuerlich- und Dieterichschen Buchhandlung.
^Panourias, I.G.; Stranjalis, G.; Stavrinou, L.C.; Sakas, D.E. (2011). "The Hellenic and Hippocratic origins of the spinal terminology".Journal of the History of the Neurosciences.20 (3):177–187.doi:10.1080/0964704X.2010.510180.PMID21736439.S2CID22256856.
