An anal fissure is a natural crack or tear in the skin of the anal canal. Anal fissures may be noticed by bright red anal bleeding on the toilet paper, sometimes in the toilet. If acute they may cause severe periodic pain after defecation. Anal fissures usually extend from the anal opening and are usually located posteriorly in the midline, probably because of the relatively unsupported nature of the anal wall in that location. Fissure depth may be superficial or sometimes down to the underlying sphincter muscle.