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
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different de ...
surrounding the margin of the
anus.
Anatomy
The external anal sphincter measures about 8 to 10 cm in length, from its anterior to its posterior extremity, and is about 2.5 cm opposite the anus, the sphincter muscle retracts on
defecating
Defecation (or defaecation) follows digestion, and is a necessary process by which organisms eliminate a solid, semisolid, or liquid waste material known as feces from the digestive tract via the anus. The act has a variety of names ranging f ...
.
It consists of two layers: ''superficial'' and ''deep''.
* The superficial layer, constitutes the main portion of the muscle, and arises from a narrow tendinous band, the
anococcygeal raphe, which stretches from the tip of the
coccyx
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 ...
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 perineum, joining with the
superficial transverse perineal muscle
The transverse perineal muscles (transversus perinei) are the superficial and the deep transverse perineal muscles.
Superficial transverse perineal ...
, 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 ...
, and the
bulbospongiosus muscle
The bulbospongiosus muscle (bulbocavernosus in older texts) 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. In fema ...
also known as the bulbocavernosus.
* The deeper layer forms a complete sphincter to the
anal canal. Its fibers surround the canal, closely applied to the
internal anal sphincter, and in front blend with the other muscles at the central point of 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 ...
.
In a considerable proportion of cases the fibers
decussate in front of the anus, and are continuous with the superficial transverse perineal muscle.
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.
Actions
(1) Like other muscles, it is always in a state of tonic contraction, and having no
antagonistic muscle it keeps the
anal canal and
orifice
An orifice is any opening, mouth, hole or vent, as in a pipe, a plate, or a body
* Body orifice, any opening in the body of a human or animal
*Orifice plate, a restriction used to measure flow or to control pressure or flow, sometimes given specia ...
shut.
(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
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 ...
, it helps to fix the central point of 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 ...
, so that the
bulbospongiosus muscle
The bulbospongiosus muscle (bulbocavernosus in older texts) 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. In fema ...
may act from this fixed point.
Additional images
File:illu_intestine.jpg, Intestines
File:Human anus-en.svg, Anatomy of the human anus.
File:1116 Muscle of the Male Perineum.png, Muscles of male perineum.
File:1116 Muscle of the Female Perineum.png, Muscles of the female perineum.
File:Gray1156.png, Sagittal (vertical) section of bladder, penis, and urethra.
See also
*
Internal anal sphincter
*
Puborectalis 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 the ...
References
External links
* - "The Male Perineum and the Penis: The External Anal Sphincter"
* ()
* ()
{{Portal bar, Anatomy
Perineum
Anus
Muscular system