Sphincter ani internus muscle
Encyclopedia
The Sphincter ani internus (or internal anal sphincter) 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
.
It is about 5 mm thick, and is formed by an aggregation of the involuntary circular fibers of the intestine
. Its lower border is about 6 mm from the orifice of the anus
.
. Parasympathetic fibers from the pelvic segments of the spinal cord supply the internal sphincter. It is not innervated by the pudendal nerve
, which carries somatic (motor and sensory)fibers that provide the innervation to the external anal sphincter.
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....
; its inferior border is in contact with, but quite separate from, the Sphincter ani externus
Sphincter ani externus muscle
The Sphincter ani externus is a flat plane of muscular fibers, elliptical in shape and intimately adherent to the integument surrounding the margin of the anus.-Anatomy:...
.
It is about 5 mm thick, and is formed by an aggregation of the involuntary circular fibers of the intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
. Its lower border is about 6 mm from the orifice of the anus
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,...
.
Actions
Its action is entirely involuntary. It helps the Sphincter ani externus to occlude the anal aperture and aids in the expulsion of the fecesFeces
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
. Parasympathetic fibers from the pelvic segments of the spinal cord supply the internal sphincter. It is not innervated by the pudendal nerve
Pudendal nerve
The pudendal nerve is a sensory and somatic nerve in the pelvic region which is a large branch of the sacral plexus that innervates the external genitalia of both sexes, as well as sphincters for the bladder and the rectum...
, which carries somatic (motor and sensory)fibers that provide the innervation to the external anal sphincter.