The external anal sphincter (or sphincter ani externus) is an oval tube of
skeletal muscle fibers.
Distally, it is 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 ...
surrounding the margin of the
anus
In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...
.
It exhibits a resting state of tonical contraction
and also contracts during the
bulbospongiosus reflex.
Anatomy
The external anal sphincter is far more substantial than the internal anal sphincter. The proximal portion of external anal sphincter overlaps the internal anal sphincter (which terminates distally a little distance proximal to the anal orifice) superficially; where the two overlap, they are separated by the intervening
conjoint longitudinal muscle.
Structure
Historically, the sphincter was described as consisting of three parts (deep, superficial, and subcutaneous). This is not supported by current anatomical knowledge. Some sources still describe it in two layers, deep (or proximal) and superficial (or distal or subcutaneous).
Some of the muscles fibres decussate at the anterior midline and posterior midline, so forming an anterior commissure and posterior commissure.
Attachments
The muscle attaches anteriorly onto the
perineal body
The perineum (: perineums or perinea) in placental mammals is the space between the anus and the genitals. The human perineum is between the anus and scrotum in the male or between the anus and vulva in the female. The perineum is the regio ...
, and posteriorly onto the anococcygeal ligament.
Innervation
The sphincter receives innervation from the bilaterally paired
inferior anal nerve (each a branch of the
pudendal nerve
The pudendal nerve is the main nerve of the perineum. It is a Mixed nerve, mixed (motor and sensory) nerve and also conveys Sympathetic nervous system, sympathetic Autonomic nervous system, autonomic fibers. It carries sensation from the exter ...
which is derived from ventral rami of S2-S4). It may also receive additional motor innervation from the
nerve to levator ani.
Histology
The sphincter consists mostly of
slow twitch fibers that allow extended continuous contraction.
Gallery
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"
* ()
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{{Portal bar, Anatomy
Perineum
Anus
Muscular system