View source for Coccyx

{{Short description|Bone of the pelvis}}
{{About|the tailbone in apes and analogous structures|animals with tails|Caudal vertebrae|the moth genus|Cydia (genus)}}
{{Redirect|Coccyges|the clade of birds|Cuculiformes|and|Parrot}}
{{Infobox bone
| Name        = Coccyx
| Pronunciation   = {{IPAc-en|ˈ|k|ɒ|k|s|ɪ|k|s}} {{respell|KOK|siks}}
| Latin       = os coccygis
| Image       = Coccyx - anterior view02.png
| Width       = 
| Caption     = The coccyx
| Image2      =  File:Illu vertebral column.svg
| Width2      =
| Caption2    = The coccyx is the final bone in the [[vertebral column]] that surrounds the [[spinal cord]].
}}

The '''coccyx''' ({{Plural form}}: '''coccyges''' or '''coccyxes'''), commonly referred to as the '''tailbone''', is the final segment of the [[vertebral column]] in all [[ape]]s,<ref name="CNN-20240323">{{cite news |last=Weisberger |first=Mindy |title=Why don't humans have tails? Scientists find answers in an unlikely place |url=https://www.cnn.com/2024/03/23/world/humans-tails-genetic-mutation-junk-dna-scn/index.html |date=March 23, 2024 |work=[[CNN]] |url-status=live |archiveurl=https://archive.today/20240324031927/https://www.cnn.com/2024/03/23/world/humans-tails-genetic-mutation-junk-dna-scn/index.html |archivedate=March 24, 2024 |accessdate=March 24, 2024 }}</ref> and analogous structures in certain other mammals such as [[horse anatomy|horses]]. In tailless [[primates]] (e.g. [[human]]s and other [[great ape]]s) since ''[[Nacholapithecus]]'' (a [[Miocene]] [[hominoid]]),<ref name="Nakatsukasa-J-Anatomy">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.''.)</ref><ref><!-- due to an earlier error -->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)</ref> the coccyx is the remnant of a [[Human vestigiality#Coccyx|vestigial tail]]. In animals with bony tails, it is known as [[Rump (animal)|''tailhead'' or ''dock'']], in [[bird anatomy]] as ''tailfan''. It comprises three to five separate or fused coccygeal vertebrae below the [[sacrum]], attached to the sacrum by a [[fibrocartilaginous]] [[joint]], the [[sacrococcygeal symphysis]], which permits limited movement between the sacrum and the coccyx.

== Structure ==
[[File:Coccyx svg hariadhi depan belakang.svg|250px|thumb|A coccyx with four vertebrae below the sacrum]]
The coccyx is formed of three, four or five rudimentary [[vertebra]]e. It articulates superiorly with the [[sacrum]]. In each of the first three segments may be traced a rudimentary body and [[articular processes|articular]] and [[transverse process]]es; 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 [[pedicle of vertebral arch|pedicles]], [[Lamina of the vertebral arch|laminae]] and [[spinous process]]es. 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 [[Neuroanatomy#Orientation_in_neuroanatomy|rostrally]].<!-- From Gray's -->

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.<ref name="pmid28091426">{{cite journal| author=Tetiker H, Koşar MI, Çullu N, Canbek U, Otağ I, Taştemur Y| title=MRI-based detailed evaluation of the anatomy of the human coccyx among Turkish adults. | journal=Niger J Clin Pract | year= 2017 | volume= 20 | issue= 2 | pages= 136–142 | pmid=28091426 | doi=10.4103/1119-3077.198313 | doi-access=free | pmc= }} </ref>

=== 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 [[Levator ani|levatores ani]] and supports part of the [[rectum]].<!-- From Gray's --> The [[Posterior (anatomy)|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 cornu]]a''' they project [[Neuroanatomy#Orientation_in_neuroanatomy|caudal]]ly, and articulate with the cornua of the sacrum, and on either side complete the [[foramen]] for the transmission of the [[Lumbar_nerves#Posterior_divisions|posterior division]] of the [[Sacral spinal nerve 5|fifth sacral nerve]].<!-- From Gray's -->

=== 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 [[Ventral ramus of spinal nerve|anterior 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 the [[sacrotuberous ligament|sacrotuberous]] and [[sacrospinous ligament]]s, to the [[Coccygeus muscle|coccygeus]] and [[levator ani]] in front of the ligaments, and to the [[gluteus maximus]] behind them.<!-- From Gray's -->

=== Apex ===
The apex is rounded, and has attached to it the tendon of the [[external anal sphincter]]; it may be divided in two.<!-- From Gray's -->

=== 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]].{{what|date=April 2022}}<ref>{{cite journal | last1 = Cosmo | first1 = L | year = 2017 | title = Filling the Gap: on the New Micro-toponomic Phenomena and Partial Topologies | url = http://www.itspoa.com/itsadmin/Ll/LL.DE.asp?action=Paper_Information&id=1264 | journal = Health Research | volume = 1 | issue = 1| pages = 39–49 | doi = 10.31058/j.hr.2017.11004 | doi-access = free }}</ref> 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 (anatomy)|origin]] here.<ref>{{Cite book|title=Applied anatomy of the pelvis|last=Lierse|first=Werner|isbn=978-3-642-71368-2|page=40|date=2012-12-06|publisher=Springer }}</ref>

=== 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.<!-- -From Gray's- -->

=== 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.<ref>{{cite journal
 |author1=Maigne JY |author2=Molinie V |author3=Fautrel B | title = Anatomie des disques coccygiens
 | journal = Revue de Médecine Orthopedique
 | volume = 28 | year = 1992 | pages = 34–35
}}
</ref><ref>{{cite journal
 | author = Saluja PG
 | title = The incidence of ossification of the sacrococcygeal joint
 | journal = Journal of Anatomy
 | volume = 156 | year = 1988 | pages = 11–15
| pmid = 3138225
 | pmc = 1261909
 }}</ref>

===Attachments===
The anterior side of the coccyx has attachments to the [[levator ani muscle]], [[coccygeus muscle|coccygeus]], [[iliococcygeus muscle|iliococcygeus]], and [[pubococcygeus muscle|pubococcygeus]], [[anococcygeal raphe]].  Attached to the posterior side is the [[Gluteus maximus muscle|gluteus maximus]], which extends the [[thigh]] at the hip joint.<ref name="Foye-eMed-CocPain" /> The [[ligament]]s attached to the coccyx include the [[anterior sacrococcygeal ligament|anterior]] and [[posterior sacrococcygeal ligament]]s which are the continuations of the [[Anterior longitudinal ligament|anterior]] and [[posterior longitudinal ligament]]s that extend along the entire spine.<ref name="Foye-eMed-CocPain" />  The [[lateral sacrococcygeal ligament]]s complete the [[foramina]] for the [[Sacral spinal nerve 5|last sacral nerve]].<ref name="Morris-59">{{cite book
 | url = https://books.google.com/books?id=EwKCu-Fti0YC&pg=PA59
 | title = Low Back Syndromes: Integrated Clinical Management
 | first = Craig E. | last = Morris
 | publisher = McGraw-Hill | isbn = 978-0-07-137472-9 | year = 2005 | page = 59
}}</ref>  Some fibers of the [[sacrospinous ligament|sacrospinous]] and [[sacrotuberous ligament]]s (arising from the [[Ischial spine|spine of the ischium]] and the [[ischial tuberosity]] respectively) also attach to the coccyx.<ref name="Foye-eMed-CocPain" /> An extension of the [[pia mater]], the [[filum terminale]], extends from the apex of the [[conus medullaris|conus]], and inserts on the coccyx.

== Function ==
The coccyx is not entirely useless in humans,<ref name="Saladin-285">{{cite book
 | title = Anatomy & Physiology: The Unity of Form and Function
 | first = Kenneth S. | last = Saladin
 | publisher = McGraw-Hill | year = 2003 |edition=3rd | page = 268
}}</ref> 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 of [[coccygectomy]] procedures performed annually to treat [[coccydynia]]. Reviews of studies covering more than 700 coccygectomies found the operation was successful in relieving pain in 84% of cases.<ref name=aarby>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</ref><ref>Heum Dai Kwon et al.,  Coccygodynia and Coccygectomy. ''Korean Journal of Spine'', 9, 4 (2012), 326-333.</ref> 12% of the time, the only major complication faced was infection due to the proximity to the anus. One notable [[Complication (medicine)|complication]] of coccygectomy is an increased risk of [[perineal hernia]].

== 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, [[coccyx fracture|fractured tailbone]].<ref name="Maigne">{{cite journal
 | url = http://www.coccyx.org/medabs/spine.htm
 | title = Causes and Mechanisms of Common Coccydynia. Spine
 | year = 2000 | volume = 25 | issue = 23 | pages = 3072–3079
 | first1 = J-Y | last1 = Maigne |author2= Doursounian, L |author3= Chatellier, G
 | publisher = coccyx.org | doi = 10.1097/00007632-200012010-00015 | journal = Spine
| pmid = 11145819
 | s2cid = 25790826
 | url-access = subscription }}</ref><ref name="Foye-2006">{{cite journal
 |vauthors=Foye P, Buttaci C, Stitik T, Yonclas P | title = Successful injection for coccyx pain.
 | journal = Am J Phys Med Rehabil
 | volume = 85 | issue = 9
 | year = 2006 | pmid = 16924191
 | doi = 10.1097/01.phm.0000233174.86070.63
 | pages = 783–784
| doi-access = free
 }}</ref>  A number of [[tumor]]s are known to involve the coccyx;  of these, the most common is [[sacrococcygeal teratoma]].  Both coccydynia and coccygeal tumors may require surgical removal of the coccyx ([[coccygectomy]]).  One very rare complication of coccygectomy is a type of [[perineal hernia]] known as a coccygeal hernia.<ref name="pmid17889632">{{cite journal
|vauthors=Miranda EP, Anderson AL, Dosanjh AS, Lee CK | title = Successful management of recurrent coccygeal hernia with the de-epithelialised rectus abdominis musculocutaneous flap
| journal = J Plast Reconstr Aesthet Surg
| volume = 62| issue = 1| pages = 98–101
| date = September 2007
| pmid = 17889632
| doi = 10.1016/j.bjps.2007.08.002
}}</ref>

== Etymology ==
The term ''coccyx'' is derived from the [[ancient Greek]] word {{lang|grc|κόκκυξ}}<ref name="Klein1971">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.</ref><ref>{{OEtymD|coccyx}}</ref> {{Transliteration|grc|kokkyx}} "cuckoo";<ref name="Liddell & Scott">{{LSJ|ko/kkuc|κόκκυξ|ref}}.</ref> the latter is attested in the writings of the Greek [[physician]] [[Herophilus]] to denote the end of the vertebral column.<ref name="Hyrtl1880">Hyrtl, J. (1880). ''Onomatologia Anatomica. Geschichte und Kritik der anatomischen Sprache der Gegenwart.'' Wien: Wilhelm Braumüller. K.K. Hof- und Universitätsbuchhändler.</ref> This Greek name for the [[cuckoo]] was applied as the last three or four bones of the coccyx resemble the beak of this bird,<ref name="Klein1971"/><ref name="Hyrtl1880"/><ref name="Kraus1844">Kraus, L.A. (1844). ''Kritisch-etymologisches medicinisches Lexikon'' (Dritte Auflage). Göttingen: Verlag der Deuerlich- und Dieterichschen Buchhandlung.</ref><ref name="Panourias2011">{{cite journal | last1 = Panourias | first1 = I.G. | last2 = Stranjalis | first2 = G. | last3 = Stavrinou | first3 = L.C. | last4 = Sakas | first4 = D.E. | year = 2011 | title = The Hellenic and Hippocratic origins of the spinal terminology | journal = Journal of the History of the Neurosciences | volume = 20 | issue = 3| pages = 177–187 | doi = 10.1080/0964704X.2010.510180 | pmid = 21736439 | s2cid = 22256856 }}</ref> when viewed from the side.<ref name="Foye-eMed-CocPain">{{cite web
 | url = http://www.emedicine.com/pmr/topic242.htm
 | title = Coccyx Pain
 | first1 = Patrick M | last1 = Foye |author2 = Buttaci, Charles J
 | publisher = [[eMedicine]] | date = June 3, 2008
}}</ref><ref>{{Cite journal | doi = 10.1097/00007632-199502000-00024 | last = Sugar | first = Oscar | title = Historical Perspective Coccyx: The Bone Named for a Bird | journal =Spine | volume = 20 | issue = 3 | pages = 379–383 | date = February 1995 | issn = 0362-2436 | pmid = 7732478}}</ref>

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''<ref name="Hyrtl1880"/> (the cuckoo bone shows a likeness to the beak of the cuckoo bird). Vesalius used the Latin expression ''os cuculi'', with ''os'', bone<ref name="Lewis & Short">{{L&S|os2|os}}, {{L&S|cuculus}}, {{L&S|cauda|ref}}</ref> and ''cuculus'', the Latin name for the cuckoo.<ref name="Lewis & Short"/> 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''<ref name="Hyrtl1880"/> (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.<ref name="Hyrtl1880"/><ref name="Kraus1844"/>

Besides ''os cuculi'', ''os caudae'',<ref name="Hyrtl1880"/><ref name="Schreger">Schreger, C.H.Th.(1805). ''Synonymia anatomica. Synonymik der anatomischen Nomenclatur.'' Fürth: im Bureau für Literatur.</ref> with ''caudae'', of the tail<ref name="Lewis & Short"/> is attested. This Latin expression might be the source of the English, [[French language]], [[German language|German]] and [[Dutch language|Dutch]] terms ''tailbone'', {{Lang|fr|l'os de la queue}},<ref name="Schreger"/> {{Lang|de|Schwanzbein}}<ref name="Kraus1844"/><ref name="Schreger"/> and ''{{Lang|nl|staartbeen}}''.<ref name="Pinkhof2012">Everdingen, J.J.E. van, Eerenbeemt, A.M.M. van den (2012). ''Pinkhof Geneeskundig woordenboek'' (12de druk). Houten: Bohn Stafleu Van Loghum.</ref> In the current official anatomic Latin nomenclature, ''[[Terminologia Anatomica]]'',<ref name="FCAT">Federative Committee on Anatomical Terminology (FCAT) (1998). ''Terminologia Anatomica''. Stuttgart: Thieme</ref> ''coccyx'' and ''os coccygis'' is used.

== Additional images ==
<gallery>
 File:Gray404.png|The coccyx sits below the sacrum and behind the [[pelvic cavity]].
</gallery>

== See also ==
{{Anatomy-terms}}
* [[Bone#Terminology|Bone terminology]]
* [[Ganglion impar]]
* [[Human vestigiality]]
* [[Perineum]]
* [[Pygostyle]]

== References ==
{{Reflist}}

==Further reading==
* {{cite journal | title = A new Late Miocene great ape from Kenya and its implications for the origins of African great apes and humans |author1=Kunimatsu, Yutaka |author2=Nakatsukasa, Masato | doi = 10.1073/pnas.0706190104 | journal = [[Proceedings of the National Academy of Sciences|PNAS]] | pmc = 2148271 | pmid = 18024593 | volume = 104 | issue = 49 | pages = 19220–5 | date = December 2007|display-authors=etal|bibcode = 2007PNAS..10419220K|doi-access=free }}
* {{cite journal | title = Acquisition of bipedalism: the Miocene hominoid record and modern analogues for bipedal protohominids| first = Masato | last = Nakatsukasa| journal = Journal of Anatomy | volume = 204 | date = May 2004 | issue = 5 | pmid = 15198702 | pages = 385–402 | pmc = 1571308 | doi = 10.1111/j.0021-8782.2004.00290.x}}

== External links ==
{{Commons category|Coccyx}}
* [https://emedicine.medscape.com/article/309486-overview Coccydynia (coccyx pain, tailbone pain) at eMedicine] (Peer-reviewed medical chapter, available free online)

{{Bones of torso}}
{{Authority control}}

[[Category:Bones of the thorax]]
[[Category:Bones of the vertebral column]]
[[Category:Vestigial organs]]