Sphincter ani externus muscle
Encyclopedia
The Sphincter ani externus (external anal sphincter) is a flat plane of muscular fibers, elliptical in shape and intimately adherent to the integument surrounding the margin of the anus
.
It consists of two strata, superficial and deep.
In a considerable proportion of cases the fibers decussate in front of the anus, and are continuous with the Transversi perinæi superficiales.
Posteriorly, they are not attached to the coccyx
, but are continuous with those of the opposite side behind the anal canal.
The upper edge of the muscle is ill-defined, since fibers are given off from it to join the Levator ani
.
(1) It is, like other muscles, always in a state of tonic contraction, and having no antagonistic muscle
it keeps the anal canal
and orifice closed.
(2) It can be put into a condition of greater contraction under the influence of the will, so as more firmly to occlude the anal aperture, in expiratory efforts unconnected with defecation
.
(3) Taking its fixed point at the coccyx
, it helps to fix the central point of the perineum
, so that the Bulbocavernosus
may act from this fixed point.
is a paradoxical contraction of the external anal sphincter, when the intent is relaxation, which can result in obstructed defecation and constipation
. Abnormal function of this muscle is also seen in anorectal malformation and after certain surgeries, including coccygectomy
.
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of 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,...
.
Anatomy
It measures about 8 to 10 cm in length, from its anterior to its posterior extremity, and is about 2.5 cm opposite the anus, when defecation occurs the sphincter muscle retracts.It consists of two strata, superficial and deep.
- The superficial, constituting the main portion of the muscle, arises from a narrow tendinous band, the anococcygeal raphe, which stretches from the tip of the coccyxCoccyxThe coccyx , commonly referred to as the tailbone, is the final segment of the 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 between...
to the posterior margin of the anus; it forms two flattened planes of muscular tissue, which encircle the anus and meet in front to be inserted into the central tendinous point of the perineumPerineal bodyThe perineal body is a pyramidal fibromuscular mass in the middle line of the perineum at the junction between the urogenital triangle and the anal triangle . It is found in both males and females...
, joining with the Transversus perinæi superficialis, the Levator aniLevator 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 the BulbocavernosusBulbospongiosus muscleBulbospongiosus is one of the superficial muscles of the perineum. It has a slightly different origin, insertion and function in males and females. In males, it covers the bulb of the penis...
.
- The deeper portion forms a complete sphincter to the anal canalAnal canalThe anal canal is the terminal part of the large intestine.It is situated between the rectum and anus, below the level of the pelvic diaphragm. It lies in the anal triangle of perineum in between the right and left ischioanal fossa....
. Its fibers surround the canal, closely applied to the Sphincter ani internus muscleSphincter ani internus muscleThe Sphincter ani internus is a muscular ring that surrounds about 2.5 cm of the anal canal; its inferior border is in contact with, but quite separate from, the Sphincter ani externus....
, and in front blend with the other muscles at the central point of the perineumPerineumIn human anatomy, the perineum is a region of the body including the perineal body and surrounding structures...
.
In a considerable proportion of cases the fibers decussate in front of the anus, and are continuous with the Transversi perinæi superficiales.
Posteriorly, they are not attached to the coccyx
Coccyx
The coccyx , commonly referred to as the tailbone, is the final segment of the 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 between...
, but are continuous with those of the opposite side behind the anal canal.
The upper edge of the muscle is ill-defined, since fibers are given off from it to join the Levator ani
Levator ani
The 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....
.
Actions
The action of this muscle is peculiar.(1) It is, like other muscles, always in a state of tonic contraction, and having no antagonistic muscle
Antagonist (muscle)
Most muscles work in pairs, and when a muscle works it needs to have an agonist and an antagonist, unless the muscle's natural state is opposite to that which is produced by the muscle, example Sphincter ani externus muscle....
it keeps the anal canal
Anal canal
The anal canal is the terminal part of the large intestine.It is situated between the rectum and anus, below the level of the pelvic diaphragm. It lies in the anal triangle of perineum in between the right and left ischioanal fossa....
and orifice closed.
(2) It can be put into a condition of greater contraction under the influence of the will, so as more firmly to occlude the anal aperture, in expiratory efforts unconnected with defecation
Defecation
Defecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum...
.
(3) Taking its fixed point at the coccyx
Coccyx
The coccyx , commonly referred to as the tailbone, is the final segment of the 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 between...
, it helps to fix the central point of the perineum
Perineum
In human anatomy, the perineum is a region of the body including the perineal body and surrounding structures...
, so that the Bulbocavernosus
Bulbospongiosus muscle
Bulbospongiosus is one of the superficial muscles of the perineum. It has a slightly different origin, insertion and function in males and females. In males, it covers the bulb of the penis...
may act from this fixed point.
Pathology
AnismusAnismus
Anismus is a malfunction of the external anal sphincter and puborectalis muscle during defecation. Normal defecation involves relaxation of both of these muscles...
is a paradoxical contraction of the external anal sphincter, when the intent is relaxation, which can result in obstructed defecation and constipation
Constipation
Constipation refers to bowel movements that are infrequent or hard to pass. Constipation is a common cause of painful defecation...
. Abnormal function of this muscle is also seen in anorectal malformation and after certain surgeries, including coccygectomy
Coccygectomy
Coccygectomy is a surgical procedure during which the coccyx, is removed. It can be performed for many reasons for instance in patients with coccyx pain , however it is typically reserved for patients with malignant cancer or for patients whose tailbone pain has failed to respond to nonsurgical...
.