The external opening of the rectum.

It is a continuance of the rectum and the end of the gastrointestinal system.

It has two sphincters that control the exit of feces from the body during defecation.

Control of the exit of feces is the primary function of the anus.

The two sphincters are the internal anal sphincter and the external anal sphincter, surrounding the top and bottom of the anus, respectively.

These sphincters are circular muscles that normally maintain constriction of the orifice and which relaxes as required by normal functioning.

The inner sphincter is involuntary.

The outer sphincter is voluntary in function.

It is located behind the perineum.

It can be the site of potential infections and other conditions, including cancer.

It is surrounded by folds called anal valves, which converge at the pectinate line.

The pectinate line separates the internal from the external anus, and

It is the point of transition between the hindgut and the ectoderm in the embryo.

Below the pectinate line, the mucosa of the internal anus becomes skin.

The pectinate line serves as the division between the internal and external anus.

It is surrounded by muscles, and passes through the pelvic floor.

Its blood supply comes from the inferior rectal artery.

Its innervatiion is from the inferior rectal nerves, which branch from the pudendal nerve.

Pseudostratified columnar epithelium of the gastrointestinal tract transitions to stratified squamous epithelium at the pectinate line.

The squamous epithelium accumulates sebaceous and apocrine glands.

During puberty, testosterone triggers androgenic hair growth on the body, and pubic hair begins to appear around the anus.

As the rectum fills with stool intra-rectal pressure builds, pushing the feces against the walls of the anal canal.

Abdominal and pelvic floor muscles create additional intra-abdominal pressure further increasing intra-rectal pressure.

An involuntary muscle, the internal anal sphincter, reacts to the pressure by relaxing, allowing the feces to enter the canal.

When feces are pushed into the anal canal peristaltic waves push the feces out of the rectum.

Relaxation of both the internal and external anal sphincters allows the feces to exit from the anus.

The levator ani muscles pull the anus up over the exiting feces.

Pathological aspects of anal disease include: Birth defects, including imperforation, stenosis, anal fissures, hemorrhoids, abscesses, sexually transmitted infections,

anal warts, intraepithelial neoplasia, carcinoma, Pruritus ani, trauma, damaged anal sphincter caused by surgery, or by rough/abrupt penetration in anal sex.

Abnormalities of the anus can lead to flatus, fecal incontinence, chronic constipation and, megacolon.

Has a relatively high concentration of nerve endings and can be an erogenous zone, and the pudendal nerve that branches to supply the external anal sphincter also branches to the dorsal nerve of the clitoris and the dorsal nerve of the penis.

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