The anus is that part of the intestinal tract that passes through the muscular canal of the pelvis and anal sphincters. It is the final orifice through which stool passes out of the body. In adults, the anus is 4 to 5 centimeters long. The lower half of the anal canal has sensitive nerve endings. There are blood vessels under the lining, and in its mid portion there are numerous tiny, anal glands. This article describes four disorders that cause anal pain and irritation,Anal fissure - An anal fissure, also called an anorectal fissure, is a linear split or tear in the lining (anoderm) of the lower anal canal. Most anal fissures happen when a large, hard stool overstretches the anal opening and tears the delicate anoderm. Less often, anal fissures develop because of prolonged diarrhea, inflammatory bowel disease or sexually transmitted diseases involving the anorectal area. Acute (short-term) anal fissures are usually superficial and shallow, but chronic (long-term) anal fissures may extend deeper through the anoderm to expose the surface of underlying muscle.Anal abscess - An anal abscess is a swollen, painful collection of pus near the anus. Most anal abscesses are not related to other health problems and arise spontaneously, for reasons that are unclear. They originate in a tiny anal gland, which enlarges to create a site of infection under the skin. In the United States, more than half of all anal abscesses occur in young adults between the ages of 20 and 40, and men are affected more often than women. Most anal abscesses are located near the opening of the anus but rarely can occur deeper or higher in the anal canal, closer to the lower colon or pelvic organs.Anal fistula - An anal fistula is an abnormal narrow tunnel-like passageway, which is the remnant of an old anal abscess after it has drained. It connects the mid portion of the anal canal (at the anal gland) to the surface of the skin. After an anal abscess has drained (either spontaneously or when lanced by a physician), an anal fistula will develop at least half of the time. Sometimes the opening of the fistula at the skin surface constantly discharges pus or bloody fluid. In other cases, the opening of the fistula closes temporarily, causing the old anal abscess to flare up again as a painful pocket of pus.Hemorrhoids - Hemorrhoids do not ordinarily cause pain. Nevertheless, sometime the blood vessels in a small hemorrhoid at the edge of the anal orifice can clot off (thrombosis). This may be triggered by a period of constipation of diarrhea. When thrombosis occurs, the external hemorrhoid becomes swollen, hard, and painful, sometimes with bloody discharge.Anus