The coccyx (
: coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the
vertebral column
The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...
in all
ape
Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...
s, and analogous structures in certain other mammals such as
horses. In tailless
primates (e.g.
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
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
''Nacholapithecus kerioi'' was an ape that lived 14-15 million years ago during the Middle Miocene. Fossils have been found in the Nachola formation in northern Kenya. The only member of the genus ''Nacholapithecus'', it is thought to be a key g ...
'' (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
Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...
),
[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
The tail is the section at the rear end of certain kinds of animals’ bodies; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals ...
. 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 cap ...
as ''tailfan''. It comprises three to five separate or fused coccygeal vertebrae below the
sacrum, attached to the sacrum by a
fibrocartilaginous
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 ...
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
The sacrococcygeal symphysis (sacrococcygeal articulation, articulation of the sacrum and coccyx) is an amphiarthrodial joint, formed between the oval surface at the apex of the sacrum, and the base of the coccyx.
It is a slightly moveable joint ...
, which permits limited movement between the sacrum and the coccyx.
Structure
![Gray100](https://upload.wikimedia.org/wikipedia/commons/9/91/Gray100.png)
The coccyx is formed of three, four or five rudimentary
vertebra
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...
e. It articulates superiorly with the
sacrum. In each of the first three segments may be traced a rudimentary body and
articular
The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals.
Anatomy
In most vertebrates, the articular bone is connected to two oth ...
and
transverse process
The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
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
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
es. The first segment is the largest; it resembles the lowest
sacral vertebra
The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, 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 ...
, 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 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
The anterior sacrococcygeal ligament or ventral sacrococcygeal ligament consists of a few irregular fibers, which descend from the anterior surface of the sacrum to the front of the coccyx, blending with the periosteum.Morris (2005), p 59
This sh ...
and the
levatores 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 t ...
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 cornu
The coccyx ( : coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the vertebral column in all apes, and analogous structures in certain other mammals such as horses. In tailless primates (e.g. humans and othe ...
a they project
caudally, and articulate with the cornua of the sacrum, and on either side complete the
for the transmission of the
posterior division
The dorsal ramus of spinal nerve (or posterior ramus of spinal nerve, or posterior primary division) is the posterior division of a spinal nerve. The dorsal ramus (Latin for branch, plural ''rami'' ) is the dorsal branch of a spinal nerve that fo ...
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 low ...
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 s ...
s, to the
coccygeus
The coccygeus muscle or ischiococcygeus is a muscle of the pelvic floor, located posterior to levator ani and anterior to the sacrospinous ligament.
Structure
The coccygeus muscle is posterior to levator ani and anterior to the sacrospinous li ...
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
The external anal sphincter (or sphincter ani externus ) is a flat plane of skeletal muscle fibers, elliptical in shape and intimately adherent to the skin surrounding the margin of the anus.
Anatomy
The external anal sphincter measures about 8 ...
; 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
The perineum in humans is the space between the anus and scrotum in the male, or between the anus and the vulva in the female. The perineum is the region of the body between the pubic symphysis (pubic arch) and the coccyx (tail bone), includi ...
, located in the
intergluteal cleft
The intergluteal cleft or just gluteal cleft, also known by a number of synonyms, including natal cleft, butt crack, and cluneal cleft, is the groove between the buttocks that runs from just below the sacrum to the perineum, so named because it f ...
that runs from just below the sacrum to the
perineum
The perineum in humans is the space between the anus and scrotum in the male, or between the anus and the vulva in the female. The perineum is the region of the body between the pubic symphysis (pubic arch) and the coccyx (tail bone), includi ...
. 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 composed of muscle fibers of the levator ani, the coccygeus muscle, and associated connective tissue which span the area underneath the pelvis. The pelvic diaphragm is a muscular partition formed by the lev ...
, next to the coccyx. The
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 ...
has its
origin
Origin(s) or The Origin may refer to:
Arts, entertainment, and media
Comics and manga
* ''Origin'' (comics), a Wolverine comic book mini-series published by Marvel Comics in 2002
* ''The Origin'' (Buffy comic), a 1999 ''Buffy the Vampire Sl ...
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 togethe ...
, 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
A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articula ...
; (2) thin discs of
fibrocartilage; (3) intermediate between these two; (4) ossified.
Attachments
The anterior side of the coccyx has attachments to the
levator ani muscle
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 t ...
,
coccygeus
The coccygeus muscle or ischiococcygeus is a muscle of the pelvic floor, located posterior to levator ani and anterior to the sacrospinous ligament.
Structure
The coccygeus muscle is posterior to levator ani and anterior to the sacrospinous li ...
,
iliococcygeus
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 t ...
, and
pubococcygeus
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 t ...
,
anococcygeal raphe
The anococcygeal body (anococcygeal ligament, or anococcygeal raphe) is a fibrous median raphe in the floor of the pelvis, which extends between the coccyx and the margin of the anus. It is composed of fibers of the levator ani muscle that unite ...
. Attached to the posterior side is the
gluteus maximus, which extends the
thigh
In human anatomy, the thigh is the area between the hip ( pelvis) and the knee. Anatomically, it is part of the lower limb.
The single bone in the thigh is called the femur. This bone is very thick and strong (due to the high proportion of ...
at the hip joint.
The
ligaments attached to the coccyx include the
anterior and
posterior sacrococcygeal ligament
The posterior sacrococcygeal ligament or dorsal sacrococcygeal ligamentOMD: Definition is a ligament which stretches from the sacrum to the coccyx and thus dorsally across the sacrococcygeal symphysis shared by these two bones.
This ligament is di ...
s which are the continuations of the
anterior and
posterior longitudinal ligament
The posterior longitudinal ligament is a ligament connecting the posterior surfaces of the vertebral bodies of all of the vertebrae. It weakly prevents hyperflexion of the vertebral column. It also prevents posterior spinal disc herniation, altho ...
s that extend along the entire spine.
The
lateral sacrococcygeal ligament
In the human body, the lateral sacrococcygeal ligaments is a pair of ligaments stretching from the lower lateral angles of the sacrum to the transverse processes of the first coccygeal vertebra.
Together with the anterior, posterior, and inter ...
s complete the
for the
last sacral nerve.
Some fibers of the
sacrospinous and
sacrotuberous ligaments (arising from the
spine of the ischium
The ischial spine is part of the posterior border of the body of the ischium bone of the pelvis. It is a thin and pointed triangular eminence, more or less elongated in different subjects.
Structure
The pudendal nerve travels close to the isch ...
and the
ischial tuberosity
The ischial tuberosity (or tuberosity of the ischium, tuber ischiadicum), also known colloquially as the sit bones or sitz bones, or as a pair the sitting bones, is a large swelling posteriorly on the superior ramus of the ischium. It marks th ...
respectively) also attach to the coccyx.
An extension of the
pia mater
Pia mater ( or ),[Entry "pia mater"](_blank)
in
filum terminale
The filum terminale ("terminal thread") is a delicate strand of fibrous tissue, about 20 cm in length, proceeding downward from the apex of the conus medullaris. It is one of the modifications of pia mater. It gives longitudinal support to th ...
, extends from the apex of the
conus, and inserts on the coccyx.
Function
The coccyx is not entirely useless in humans,
based on the fact that the coccyx 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
Coccygectomy is a surgical procedure in which the coccyx or tailbone is removed. It is considered a required treatment for sacrococcygeal teratoma and other germ cell tumors arising from the coccyx. Coccygectomy is the treatment of last resort for ...
procedures performed annually to treat
coccydynia
Coccydynia is a medical term meaning pain in the coccyx or tailbone area, often brought on by a fall onto the coccyx or by persistent irritation usually from sitting.
Synonyms
Coccydynia is also known as coccygodynia, coccygeal pain, coccyx pain, ...
. 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
Perineal hernia is a hernia involving the perineum ( pelvic floor). The hernia may contain fluid, fat, any part of the intestine, the rectum, or the bladder. It is known to occur in humans, dogs, and other mammals, and often appears as a sudden ...
.
Clinical significance
Injuring the coccyx can give rise to a painful condition called
coccydynia
Coccydynia is a medical term meaning pain in the coccyx or tailbone area, often brought on by a fall onto the coccyx or by persistent irritation usually from sitting.
Synonyms
Coccydynia is also known as coccygodynia, coccygeal pain, coccyx pain, ...
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
Sacrococcygeal teratoma (SCT) is a type of tumor known as a teratoma that develops at the base of the coccyx (tailbone) and is thought to be primarily derived from remnants of the primitive streak. Sacrococcygeal teratomas are benign 75% of the ...
. Both coccydynia and coccygeal tumors may require surgical removal of the coccyx (
coccygectomy
Coccygectomy is a surgical procedure in which the coccyx or tailbone is removed. It is considered a required treatment for sacrococcygeal teratoma and other germ cell tumors arising from the coccyx. Coccygectomy is the treatment of last resort for ...
). One very rare complication of coccygectomy is a type of
perineal hernia
Perineal hernia is a hernia involving the perineum ( pelvic floor). The hernia may contain fluid, fat, any part of the intestine, the rectum, or the bladder. It is known to occur in humans, dogs, and other mammals, and often appears as a sudden ...
known as a coccygeal hernia.
History
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 ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...
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.] ''kokkyx'' "cuckoo";
[.] the latter is attested in the writings of the Greek
physician
A physician (American English), medical practitioner (Commonwealth English), medical doctor, or simply doctor, is a health professional who practices medicine, which is concerned with promoting, maintaining or restoring health through th ...
Herophilus
Herophilos (; grc-gre, Ἡρόφιλος; 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 ...
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 sometimes separ ...
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
Andreas Vesalius (Latinized from Andries van Wezel) () was a 16th-century anatomist, physician, and author of one of the most influential books on human anatomy, ''De Humani Corporis Fabrica Libri Septem'' (''On the fabric of the human body'' ' ...
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
Jean Riolan (the Younger) (15 February 1577 or 1580 – 19 February 1657) was a French anatomist who was an influential member of the Medical Faculty of Paris. His father, Jean Riolan (the Elder) (1539–1605) was also a noted French anatomist. Ri ...
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). The latter is not considered as potential candidate.
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,
German
German(s) may refer to:
* Germany (of or related to)
** Germania (historical use)
* Germans, citizens of Germany, people of German ancestry, or native speakers of the German language
** For citizens of Germany, see also German nationality law
**Ge ...
and
Dutch
Dutch commonly refers to:
* Something of, from, or related to the Netherlands
* Dutch people ()
* Dutch language ()
Dutch may also refer to:
Places
* Dutch, West Virginia, a community in the United States
* Pennsylvania Dutch Country
People E ...
terms ''tailbone'', ''l'os de la queue'',
''Schwanzbein''
and ''staartbeen''.
[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
''Terminologia Anatomica'' is the international standard for human anatomical terminology. It is developed by the Federative International Programme on Anatomical Terminology, a program of the International Federation of Associations of Anatomi ...
'',
[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 ...
.
See also
*
Bone terminology
*
Ganglion impar
The pelvic portion of each sympathetic trunk is situated in front of the sacrum, medial to the anterior sacral foramina. It consists of four or five small sacral ganglia, connected together by interganglionic cords, and continuous above with the ab ...
*
Human vestigiality
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, an ...
*
Perineum
The perineum in humans is the space between the anus and scrotum in the male, or between the anus and the vulva in the female. The perineum is the region of the body between the pubic symphysis (pubic arch) and the coccyx (tail bone), includi ...
*
Pygostyle
Pygostyle describes a skeletal condition in which the final few caudal vertebrae are fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these. The pygostyle is the main compone ...
References
Further reading
*
*
External links
* – "The Female Perineum: Osteology"
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