Human anus (from Latin ānus, meaning "ring" or "circle") is the external opening of the rectum located within the intergluteal cleft (the groove between the buttocks). It is the terminal segment of the gastrointestinal tract.
Anatomy
The anus directly continues from the rectum and passes through the pelvic floor. It is surrounded by two muscular rings called sphincters:
- Internal anal sphincter – an involuntary (smooth) muscle that maintains constriction at rest.
- External anal sphincter – a voluntary (skeletal) muscle that allows conscious control over defecation.
The interior of the anus is lined with folds called anal valves, which converge at the pectinate line — the embryological transition point between hindgut and ectoderm. Above this line, the lining is pseudostratified columnar epithelium; below it, the lining transitions to stratified squamous epithelium (skin). The anus receives blood from the inferior rectal artery and innervation from the inferior rectal nerves (branches of the pudendal nerve).
Function
The primary function of the anus is defecation — the expulsion of feces from the body. As the rectum fills, intra-rectal pressure increases, triggering relaxation of the internal anal sphincter. Voluntary relaxation of the external anal sphincter, combined with contractions of abdominal and pelvic floor muscles, allows feces to exit. The anus also facilitates the passage of flatus (intestinal gas).
Clinical significance
The anus is susceptible to several medical conditions:
- Hemorrhoids – swollen blood vessels of the internal or external anal venous plexuses.
- Anal fissures – tears in the mucosal lining, often causing severe pain during defecation.
- Anal abscesses and fistulas – infections of the anal glands, sometimes associated with Crohn's disease.
- Pruritus ani – chronic itching, often due to fecal exposure, infections, or skin conditions.
- Anal cancer (anal carcinoma) and anal intraepithelial neoplasia.
- Fecal incontinence – may result from sphincter damage due to surgery, childbirth, or other trauma.
Society and culture
The anus contains a high concentration of nerve endings and is considered an erogenous zone. Anal intercourse is a form of sexual activity practiced by some individuals; it carries risks of sexually transmitted infections and tissue injury if not performed with appropriate precautions (e.g., lubrication and condom use). Attitudes toward anal sex vary widely across cultures and legal systems.
Hygienic practices after defecation include wiping with toilet paper or washing with water (e.g., using a bidet), with methods differing across cultures. Cosmetic practices such as anal bleaching, hair removal, and perineum sunning are also observed in some contexts.
The anus is known by numerous slang terms, including asshole, butthole, arsehole, bumhole, cornhole, and bunghole.
