The coccyx (: coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the

vertebral column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...

in all apes, and analogous structures in certain other mammals such as

horses The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 milli ...

. In tailless

primates Primates is an order of mammals, which is further divided into the strepsirrhines, which include lemurs, galagos, and lorisids; and the haplorhines, which include tarsiers and simians ( monkeys and apes). Primates arose 74–63 ...

(e.g.

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s and other

great ape The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); '' Gorilla'' (the ...

s) since '' Nacholapithecus'' (a

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

hominoid),Nakatsukasa 2004, ''Acquisition of bipedalism'' (Se
Fig. 5
entitled ''First coccygeal/caudal vertebra in short-tailed or tailless primates.''.) the coccyx is the remnant of a vestigial tail. In animals with bony tails, it is known as ''tailhead'' or ''dock'', in

bird anatomy Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capa ...

as ''tailfan''. It comprises three to five separate or fused coccygeal vertebrae below the

sacrum The sacrum (: sacra or sacrums), in human anatomy, is a triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part of the pelvic cavity, ...

, attached to the sacrum by a

fibrocartilaginous Fibrocartilage consists of a mixture of white fibrous tissue and Cartilage, cartilaginous tissue in various proportions. It owes its inflexibility and toughness to the former of these constituents, and its Elasticity (physics), elasticity to the ...

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

, the sacrococcygeal symphysis, which permits limited movement between the sacrum and the coccyx.

Structure

The coccyx is formed of three, four or five rudimentary

vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

e. It articulates superiorly with the

. In each of the first three segments may be traced a rudimentary body and articular and

transverse process Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

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 pedicles, laminae and

spinous process Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

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 rostrally. 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 Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

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 rectum (: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult ...

. 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 In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, artery, ...

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 from caudally, and are often lacking. The borders of the coccyx are narrow, and give attachment on either side to the

sacrotuberous The sacrotuberous ligament (great or posterior sacrosciatic ligament) is situated at the lower and back part of the pelvis. It is flat, and triangular in form; narrower in the middle than at the ends. Structure It runs from the sacrum (the lower ...

and

sacrospinous ligament The sacrospinous ligament (small or anterior sacrosciatic ligament) is a thin, triangular ligament in the human pelvis. The base of the ligament is attached to the outer edge of the sacrum and coccyx, and the tip of the ligament attaches to the i ...

s, to the coccygeus and

levator ani The levator ani is a broad, thin muscle group, situated on either side of the pelvis. It is formed from three muscle components: the pubococcygeus, the iliococcygeus, and the puborectalis. It is attached to the inner surface of each side of the ...

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

and the

perineum The perineum (: perineums or perinea) in placentalia, placental mammals is the space between the anus and the genitals. The human perineum is between the anus and scrotum in the male or between the anus and vulva in the female. The perineum is ...

, located in the intergluteal cleft that runs from just below the sacrum to the

. It is not consistently present in all humans. The coccygeal fossa marks the deepest part of the

pelvic floor The pelvic floor or pelvic diaphragm is an anatomical location in the human body which has an important role in urinary and anal continence, sexual function, and support of the pelvic organs. The pelvic floor includes muscles, both skeletal and ...

, next to the coccyx. The

has its origin here.

Extensor coccygis

The extensor coccygis is a slender

muscle fascicle A muscle fascicle is a bundle of skeletal muscle fibers surrounded by perimysium, a type of connective tissue. Structure Muscle cells are grouped into muscle fascicles by enveloping perimysium connective tissue. Fascicles are bundled toge ...

, which is not always present. It extends over the caudal portion of the posterior surface of the

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 Fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions. It owes its inflexibility and toughness to the former of these constituents, and its elasticity to the latter. It is the only type of ...

; (3) intermediate between these two; (4) ossified.

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

ligament A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...

s attached to the coccyx include the

and posterior sacrococcygeal ligaments which are the continuations of the

and posterior longitudinal ligaments that extend along the entire spine. The lateral sacrococcygeal ligaments complete the

foramina In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, arter ...

for the last sacral nerve. Some fibers of the sacrospinous and

sacrotuberous ligament The sacrotuberous ligament (great or posterior sacrosciatic ligament) is situated at the lower and back part of the pelvis. It is flat, and triangular in form; narrower in the middle than at the ends. Structure It runs from the sacrum (the lowe ...

s (arising from the spine of the ischium and the ischial tuberosity respectively) also attach to the coccyx. An extension of the

pia mater Pia mater ( or ),Entry "pia mater"
in
filum terminale, extends from the apex of the

conus ''Conus'' is a genus of venomous and predatory cone snails.Bouchet, P.; Gofas, S. (2015). Conus Linnaeus, 1758. In: MolluscaBase (2015). Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&i ...

, and inserts on the coccyx.

Function

The coccyx is not entirely useless in humans, 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.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 12% of the time, the only major complication faced was infection due to the proximity to the anus. One notable 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, fractured tailbone. A number of

tumor A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

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.

Etymology

The term ''coccyx'' is derived from the

ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

word 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. "cuckoo";. the latter is attested in the writings of the Greek

physician A physician, medical practitioner (British English), medical doctor, or simply doctor is a health professional who practices medicine, which is concerned with promoting, maintaining or restoring health through the Medical education, study, Med ...

Herophilus Herophilos (; ; 335–280 BC), sometimes Latinised Herophilus, was a Greek physician regarded as one of the earliest anatomists. Born in Chalcedon, he spent the majority of his life in Alexandria. He was the first scientist to systematically p ...

to denote the end of the vertebral column.Hyrtl, J. (1880). ''Onomatologia Anatomica. Geschichte und Kritik der anatomischen Sprache der Gegenwart.'' Wien: Wilhelm Braumüller. K.K. Hof- und Universitätsbuchhändler. This Greek name for the

cuckoo Cuckoos are birds in the Cuculidae ( ) family, the sole taxon in the order Cuculiformes ( ). The cuckoo family includes the common or European cuckoo, roadrunners, koels, malkohas, couas, coucals, and anis. The coucals and anis are somet ...

was applied as the last three or four bones of the coccyx resemble the beak of this bird,Kraus, L.A. (1844). ''Kritisch-etymologisches medicinisches Lexikon'' (Dritte Auflage). Göttingen: Verlag der Deuerlich- und Dieterichschen Buchhandlung. when viewed from the side. This established etymological explanation can also be found in the writings of the 16th century anatomist

Andreas Vesalius Andries van Wezel (31 December 1514 – 15 October 1564), latinized as Andreas Vesalius (), was an anatomist and physician who wrote '' De Humani Corporis Fabrica Libri Septem'' (''On the fabric of the human body'' ''in seven books''), which is ...

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'',Schreger, C.H.Th.(1805). ''Synonymia anatomica. Synonymik der anatomischen Nomenclatur.'' Fürth: im Bureau für Literatur. with ''caudae'', of the tail is attested. This Latin expression might be the source of the English,

French language French ( or ) is a Romance languages, Romance language of the Indo-European languages, Indo-European family. Like all other Romance languages, it descended from the Vulgar Latin of the Roman Empire. French evolved from Northern Old Gallo-R ...

, German and Dutch terms ''tailbone'', , and '.Everdingen, J.J.E. van, Eerenbeemt, A.M.M. van den (2012). ''Pinkhof Geneeskundig woordenboek'' (12de druk). Houten: Bohn Stafleu Van Loghum. In the current official anatomic Latin nomenclature, '' Terminologia Anatomica'',Federative Committee on Anatomical Terminology (FCAT) (1998). ''Terminologia Anatomica''. Stuttgart: Thieme ''coccyx'' and ''os coccygis'' is used.

Additional images

File:Gray404.png, The coccyx sits below the sacrum and behind the

pelvic cavity The pelvic cavity is a body cavity that is bounded by the bones of the pelvis. Its oblique roof is the pelvic inlet (the superior opening of the pelvis). Its lower boundary is the pelvic floor. The pelvic cavity primarily contains the reproduc ...

References

External links

Coccydynia (coccyx pain, tailbone pain) at eMedicine
(Peer-reviewed medical chapter, available free online) {{Authority control Bones of the thorax Bones of the vertebral column Vestigial organs