Anus

"The anus is the end of the large intestine, below the rectum, through which stool (solid waste) leaves the body. The anus is formed partly from the outer skin layers of the body and partly from the intestine. Two ring-like muscles, called sphincter muscles, open and close the anal opening to let stool pass out of the body. The anal canal, the part of the anus between the rectum and the anal opening, is about 1½ inches long.
The skin around the outside of the anus is called the perianal area. Tumors in this area are skin tumors, not anal cancer."

Anal Fissure

An anal fissure is a cut or tear occuring in the anus (the opening through which stool passes out of the body) that extends upwards into the anal canal. Fissures are a common condition of the anus and anal canal and are responsible for 6-15% of the visits to a colon and rectal (colorectal) surgeon. They affect men and women equally and both the young and the old. Fissures usually cause pain during bowel movements that often is severe. Anal fissure is the most common cause of rectal bleeding in infancy. Anal fissures are caused by trauma to the anus and anal canal. The cause of the trauma usually is a bowel movement, and many patients can remember the exact bowel movement during which their pain began. The fissure may be caused by a hard stool or repeated episodes of diarrhea. Occasionally, the insertion of a rectal thermometer, enema tip, endoscope, or ultrasound probe (for examining the prostate gland) can result in sufficient trauma to produce a fissure.