The coccyx (: coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the vertebral column in all ,[Nakatsukasa 2004, Acquisition of bipedalism (See Fig. 5 entitled First 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, Nakatsukasa et al. Dec 2007)] the coccyx is the remnant of a vestigial tail. In animals with bony tails, it is known as 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
The coccyx is formed of three, four or five rudimentary
. 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
. 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 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 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
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 from 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 muscle 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.
[
]
Attachments
The anterior side of the coccyx has attachments to the levator ani muscle,
coccygeus muscle, iliococcygeus, and pubococcygeus, anococcygeal raphe. Attached to the posterior side is the gluteus maximus, which extends the
thigh at the hip joint.
The
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.
Some fibers of the sacrospinous and sacrotuberous ligaments (arising from the
Ischial spine and the ischial tuberosity respectively) also attach to the coccyx.
An extension of the
pia mater, the
filum terminale, extends from the apex of the
conus medullaris, 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
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[Heum Dai Kwon et al., Coccygodynia and Coccygectomy. Korean Journal of Spine, 9, 4 (2012), 326-333.] 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,
coccyx fracture.
A number of
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 word κόκκυξ
[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 Herophilus 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 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 who wrote: os cuculi, a similitudine rostri cuculi avis[, , ] and cuculus, the Latin name for the cuckoo.
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[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 ==
See also
Further reading
External links
