Usually a circular muscle, that is around an orifice of the body, which controls the flow through that orifice.
A muscle, usually circular, that surrounds an opening in the body, which restricts, or controls the flow of fluids through the opening.
There are approximately 40 such muscles.
'musculus sphincter ani externus' is the Latin term for the sphincter muscle, at the opening of the rectum.
'musculus sphincter ani internus' is the Latin for the sphincter muscle located within the rectum canal.
There are TWO (2) Sphincter muscles involved with the anus, and one with the penis.
One for the anus, is classed as 'involuntary' in that the body manages the opening, or restriction, automatically.
Sphincter ani internus ( internal anus sphincter ) helps in the flow of feces.
It is located about 6mm from the opening of the anus.
The other sphincter, is the one most commonly referred to in discussing anal penetration, or rimming.
This particular 'sphincter' is classed as 'voluntary' and is noted as Sphincter ani externus ( external anus sphincter )
This measures about 8 to 10cm and forms 2 fibrous bands, that encircle the opening of the rectum.
It is different in that one's 'will' can affect the control of this muscle. It can help tighten the closure, or add to the relaxation of the muscle.
Practice (Associated Acts):
Within the penis, is another sphincter, known as the urethral sphincter.
It determines when to expel urine or not.
Sphincters can also be listed as being functional or anatomical.
Anatomical are usually localized, with a more pronounced circular thickening of tissue to aid in their opening/closing.
Functional do not have this localized thickening, and instead rely indirectly on muscle contraction around or within the object.
Used in literary works, describing a scene of penetration, at how the object being inserted is being held, or attempted to be restricted from entering the rectum.