The
coccyx , commonly referred to as the
tailbone, is the final segment of the human vertebral column. Comprising three to five separate or fused
vertebraA vertebra is an individual bone in the flexible column that defines vertebrate animals, e.g. humans. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis. From there, vertebra continue into...
e (the coccygeal vertebrae) below the
sacrumThe sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
, it is attached to the sacrum by a fibrocartilaginous
jointA joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
, the
sacrococcygeal symphysisThe sacrococcygeal symphysis is an amphiarthrodial joint, formed between the oval surface at the apex of the sacrum, and the base of the coccyx....
, which permits limited movement between the sacrum and the coccyx.
The term
coccyx comes originally from the
Greek languageGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
and means "cuckoo", referring to the curved shape of a
cuckooThe cuckoos are a family, Cuculidae, of near passerine birds. The order Cuculiformes, in addition to the cuckoos, also includes the turacos . Some zoologists and taxonomists have also included the unique Hoatzin in the Cuculiformes, but its taxonomy remains in dispute...
's beak when viewed from the side.
In humans and other tailless primates (e.g.
The
coccyx , commonly referred to as the
tailbone, is the final segment of the human vertebral column. Comprising three to five separate or fused
vertebraA vertebra is an individual bone in the flexible column that defines vertebrate animals, e.g. humans. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis. From there, vertebra continue into...
e (the coccygeal vertebrae) below the
sacrumThe sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
, it is attached to the sacrum by a fibrocartilaginous
jointA joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
, the
sacrococcygeal symphysisThe sacrococcygeal symphysis is an amphiarthrodial joint, formed between the oval surface at the apex of the sacrum, and the base of the coccyx....
, which permits limited movement between the sacrum and the coccyx.
The term
coccyx comes originally from the
Greek languageGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
and means "cuckoo", referring to the curved shape of a
cuckooThe cuckoos are a family, Cuculidae, of near passerine birds. The order Cuculiformes, in addition to the cuckoos, also includes the turacos . Some zoologists and taxonomists have also included the unique Hoatzin in the Cuculiformes, but its taxonomy remains in dispute...
's beak when viewed from the side.
Function
In humans and other tailless primates (e.g. great apes) since
NakalipithecusNakalipithecus nakayamai is a prehistoric great ape species that lived in today's Kenya region early in the Late Miocene, 10 million years ago . It is the type species of the new genus Nakalipithecus...
(a
MioceneThe Miocene is a geological epoch of the Neogene period and extends from about 23.03 to 5.33 million years before the present . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the...
hominoid), the coccyx is the remnant of a vestigial tail, but still not entirely useless; it is an important attachment for various muscles, tendons and ligaments which makes it necessary for physicians and patients to pay special attention to these attachments when considering surgical removal of the coccyx. Additionally, it is also part of the weight-bearing tripod structure which act as a support for a
sittingSitting is a rest position supported by the buttocks or thighs where the torso is more or less upright. There are several ways for humans to sit.- Sitting on the floor :The most common way of sitting on the floor involves bending the knees...
person. When a person sits leaning forward, the ischial tuberosities and
inferior rami of the ischiumThe Inferior Ramus of the ischium is the thin, flattened part of the ischium, which ascends from the superior ramus, and joins the inferior ramus of the pubis—the junction being indicated in the adult by a raised line....
take most of the weight, but as the sitting person leans backward, more weight is transferred to the coccyx.
The anterior side of the coccyx serves for the attachment of a group of muscles important for many functions of the
pelvic floorThe pelvic floor or pelvic diaphragm is composed of muscle fibers of the levator ani, the coccygeus, and associated connective tissue which span the area underneath the pelvis. The pelvic diaphragm is a muscular partition formed by the levatores ani and coccygei, with which may be included the...
(i.e.
defecationDefecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus. Humans usually defecate from three times a week, up to three times a day...
, continence, etc): The levator ani muscle, which include
coccygeusThe Coccygeus is a muscle of the pelvic wall , located posterior to levator ani and anterior to the sacrospinous ligament....
,
iliococcygeusThe Iliococcygeus arises from the ischial spine and from the posterior part of the tendinous arch of the pelvic fascia, and is attached to the coccyx and anococcygeal raphé; it is usually thin, and may fail entirely, or be largely replaced by fibrous tissue...
, and
pubococcygeusThe pubococcygeus muscle or PC muscle is a hammock-like muscle, found in both sexes, that stretches from the pubic bone to the coccyx forming the floor of the pelvic cavity and supporting the pelvic organs. It is part of the levator ani group of muscles.-Function:It controls urine flow and...
. Through the
anococcygeal raphéThe fibers of the Levator ani pass downward and backward to the middle line of the floor of the pelvis; the most posterior are inserted into the side of the last two segments of the coccyx; those placed more anteriorly unite with the muscle of the opposite side, in a median fibrous raphé, the...
, the coccyx supports the position of the
anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as bones; food...
. Attached to the posterior side is
gluteus maximusThe gluteus maximus is the largest and most superficial of the three gluteal muscles. It makes up a large portion of the shape and appearance of the buttocks....
which extend the
thighIn humans the thigh is the area between the pelvis and the knee. Anatomically, it is part of the lower limb.The single bone in the thigh is called the femur...
during
ambulationWalking is the main form of animal locomotion on land, distinguished from running and crawling. When carried out in shallow waters, it is usually described as wading and when performed over a steeply rising object or an obstacle it becomes scrambling or climbing...
.
Many important
ligamentIn anatomy, the term ligament is used to denote three different types of structures:# Fibrous tissue that connects bones to other bones. They are sometimes called "articular ligaments", "fibrous ligaments", or "true ligaments"....
s attach to the coccyx: The
anteriorThe 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....
and
posterior sacrococcygeal ligamentThe posterior sacrococcygeal ligament or dorsal sacrococcygeal ligament is a ligament which stretches from the sacrum to the coccyx and thus dorsally across the sacrococcygeal symphysis shared by these two bones....
s are the continuations of the
anteriorThe anterior longitudinal ligament is a ligament that runs down the anterior surface of the spine. It traverses all of the vertebral bodies and intervertebral discs....
and
posterior longitudinal ligamentThe posterior longitudinal ligament is situated within the vertebral canal, and extends along the posterior surfaces of the bodies of the vertebræ, from the body of the axis, where it is continuous with the membrana tectoria, to the sacrum....
s that stretches along the entire spine. Additionally, the
lateral sacrococcygeal ligamentIn 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....
s complete the foramina for the
last sacral nerveThe sacral spinal nerve 5 is a spinal nerve of the sacral segment..It originates from the spinal column from below the 5th body of the sacrum S5 supplies the Coccygeus muscle...
. And, lastly, some fibers of the
sacrospinousThe sacrospinous ligament is thin, triangular ligament attached by its apex to the ischial spine, and medially, by its broad base, to the lateral margins of the sacrum and coccyx, in front of the sacrotuberous ligament with which its fibers are intermingled.- Relations :Together with the...
and
sacrotuberous ligamentThe sacrotuberous 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....
s (arising from the
spine of the ischiumFrom the posterior border of the body of the Ischium there extends backward a thin and pointed triangular eminence, the ischial spine, more or less elongated in different subjects.It can serve as a landmark in pudendal anesthesia.-Surfaces:...
and the ischial tuberosity respectively) also attach to the coccyx.
An extension of the
pia materThe pia mater is the delicate innermost layer of the meninges—the membranes surrounding the brain and spinal cord....
, the
filum terminaleThe filum terminale is a delicate filament, about 20 cm. in length, prolonged downward from the apex of the conus medullaris.It consists of two parts, an upper and a lower....
, extends from the
apex- Biology :* Apex of the heart, the lowest superficial part of the heart* Apex , the tip of the spire of the shell of a gastropod* Apex is the Apical meristem or its remnant on a flower* Apex predator, a predator which sits at the top of a food chain...
of the
conusThe conus medullaris is the terminal end of the spinal cord. It occurs near lumbar vertebral levels 1 and 2 . After the spinal cord tapers out, the spinal nerves continue as dangling nerve roots. This terminal nerve root tail is referred to as the filum terminale...
, and inserts on the
coccyxThe coccyx , commonly referred to as the tailbone, is the final segment of the human vertebral column. Comprising three to five separate or fused vertebrae below the sacrum, it is attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement...
.
Structure
The coccyx is usually formed of four rudimentary
vertebraA vertebra is an individual bone in the flexible column that defines vertebrate animals, e.g. humans. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis. From there, vertebra continue into...
e (sometimes five or three). It articulates superiorly with the
sacrumThe sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
. 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 is most prominent and noticeable on the first coccygeal segment. All the segments are destitute of
pediclesThe pedicles are two short, thick processes, which project backward, one on either side, from the upper part of the body, at the junction of its posterior and lateral surfaces. They connect the body of the spinal vertebra to the arch....
,
laminaeThe laminæ are two broad plates, extending dorsally and medially from the pedicles, fusing to complete the roof of the vertebral arch.Their upper borders and the lower parts of their anterior surfaces are rough for the attachment of the ligamenta flava....
and spinous processes. The first is the largest; it resembles the lowest sacral vertebra, and often exists as a separate piece; the last three diminish in size from above downward.
Most anatomy books wrongly state that the coccyx is normally fused in adults. In fact it has been shown that the coccyx may consist of up to five separate bony segments, the most common configuration being two or three segments. Only about 5 percent of the population have a coccyx in one piece, separate from the sacrum, as described in anatomy books.
Surfaces
The anterior surface is slightly concave and marked with three transverse grooves that indicate the junctions of the different segments. It gives attachment to the
anterior sacrococcygeal ligamentThe 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....
and the
Levatores aniThe Levator ani is a broad, thin muscle, situated on the side of the pelvis.It is attached to the inner surface of the side of the lesser pelvis, and unites with its fellow of the opposite side to form the greater part of the floor of the pelvic cavity....
and supports part of the
rectumThe rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
.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 rudimentary articular processes of the coccygeal vertebrae. Of these, the superior pair are large, and are called the coccygeal cornua; they project upward, 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 nerveThe sacral spinal nerve 5 is a spinal nerve of the sacral segment..It originates from the spinal column from below the 5th body of the sacrum S5 supplies the Coccygeus muscle...
.
Borders
The lateral borders are thin and exhibit a series of small eminences, which represent the transverse processes of the coccygeal vertebrae. Of these, the first is the largest; it is flattened from before backward, and often ascends 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 above downward, and are often wanting. The borders of the coccyx are narrow, and give attachment on either side to the
sacrotuberousThe sacrotuberous 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....
and
sacrospinous ligamentThe sacrospinous ligament is thin, triangular ligament attached by its apex to the ischial spine, and medially, by its broad base, to the lateral margins of the sacrum and coccyx, in front of the sacrotuberous ligament with which its fibers are intermingled.- Relations :Together with the...
s, to the
coccygeusThe Coccygeus is a muscle of the pelvic wall , located posterior to levator ani and anterior to the sacrospinous ligament....
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 Sphincter ani externus. It may be
bifidSpina bifida is a developmental birth defect caused by the incomplete closure of the embryonic neural tube. Some vertebrae overlying the spinal cord are not fully formed and remain unfused and open. If the opening is large enough, this allows a portion of the spinal cord to stick out through the...
.
Sacrococcygeal and intercoccygeal joints
The joints are variable and may be: (1) synovial joints; (2) thin discs of
fibrocartilageWhite fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions; to the former of these constituents it owes its flexibility and toughness, and to the latter its elasticity...
; (3) intermediate between these two; (4) ossified.
Pathology
Injuring the coccyx can give rise to a condition called
coccydyniaCoccydynia is a medical term meaning pain in the coccyx or tailbone area, usually brought on by sitting.-Synonyms:Coccydynia is also known as coccygodynia, coccygeal pain, coccyx pain, coccaglia or buttache...
.
A number of
tumorA tumor or tumour is the name for a swelling or lesion formed by an abnormal growth of cells . Tumor is not synonymous with cancer. A tumor can be benign, pre-malignant or malignant, whereas cancer is by definition malignant....
s are known to involve the coccyx; of these, the most common is
sacrococcygeal teratomaSacrococcygeal teratoma is a teratoma located at the base of the coccyx . It is thought to be a derivative of the primitive streak.-Natural history:...
. Both coccydynia and coccygeal tumors may require surgical removal of the coccyx (
coccygectomyCoccygectomy is surgery to remove the coccyx, e.g. for patients with coccyx pain . Typically, surgery is reserved for patients with cancer or those whose tailbone pain has failed to respond to nonsurgical treatment .In humans,...
). One complication of cocygectomy is a coccygeal hernia. Fortunately, most cases of coccyx pain respond well to nonsurgical treatment, such as medications given by local injection (often done under fluoroscopic guidance).
See also
- Coccydynia (coccyx pain, tailbone pain)
Coccydynia is a medical term meaning pain in the coccyx or tailbone area, usually brought on by sitting.-Synonyms:Coccydynia is also known as coccygodynia, coccygeal pain, coccyx pain, coccaglia or buttache...
- Bone terminology
- Terms for anatomical location
- 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 abdominal portion...
- Human vestigial
External links
- "The Female Perineum: Osteology"