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The internal anal sphincter, IAS, or sphincter ani internus is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal. It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum.
The inferior rectal nerves (inferior anal nerves, inferior hemorrhoidal nerve) usually branch from the pudendal nerve but occasionally arises directly from the sacral plexus; they cross the ischiorectal fossa along with the inferior rectal artery and veins, toward the anal canal and the lower end of the rectum, and is distributed to the sphincter ani externus (external anal sphincter, EAS) and ...
The external anal sphincter (or sphincter ani externus) is an oval tube of skeletal muscle fibers. [1] Distally, it is adherent to the skin surrounding the margin of the anus. [2] It exhibits a resting state of tonical contraction [1] and also contracts during the bulbospongiosus reflex. [3] [4] [5] [6]
The anal verge refers to the distal end of the anal canal, a transitional zone between the epithelium of the anal canal and the perianal skin. It should not be confused with the pectinate line between the upper and lower zones within the anal canal. The anal gland secretes lymphal discharge and built-up fecal matter from the colon lining.
The conjoint longitudinal muscle is a muscle layer in the wall of the anal canal between the internal anal sphincter (deep to CLM) and external anal sphincter (superficial to CLM). It is continuous proximally with the longitudinal (outer) smooth muscle layer of the rectum. It receives autonomic innervation in common with the internal anal ...
The middle fibers are inserted into the side of the rectum, blending with the fibers of the sphincter muscles; lastly in the male, the anterior fibers descend upon the side of the prostate to unite beneath it with the muscle of the opposite side, joining with the fibers of the external anal sphincter and transverse perineal muscles, at the ...
The anocutaneous line is slightly below the pectinate line and a landmark for the intermuscular border between internal and external anal sphincter muscles. The anocutaneous line represents the transition point from non-keratinized stratified squamous epithelium of the anal canal to keratinized stratified squamous epithelium of the anus and ...
The anococcygeal body (anococcygeal ligament or anococcygeal raphe) is a fibrous median raphe in the floor of the pelvis, which extends between the coccyx and the margin of the anus. It is composed of fibers of the levator ani muscle that unite with the muscle of the opposite side, muscle fibres from external anal sphincter , and fibrous ...