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."

Proctocolitis

Proctocolitis is a general term for inflammation of the rectum and colon. Proctocolitis has many possible causes. Common infectious causes of proctocolitis include Chlamydia trachomatis, Neisseria gonorrhoeae, HSV, and Campylobacter species. It can also be idiopathic (see colitis), vascular (as in ischemic colitis), or autoimmune (as in inflammatory bowel disease).