The patient has a small opening behind the anus, generally consider the following two causes: 1, anal fissure: the most common in the clinic may be considered anal fissure, anal fissure is the most common disease of the rectum in the clinic, mostly due to the local formation of ulcers, causing severe pain, especially in defecation, the pain may be more intense. 2, constipation: may be due to constipation, constipation, dry, due to defecation when Excessive force, caused by the anal tear, causing bleeding, pain, must be paid great attention to, sometimes serious bleeding may be more serious. Therefore, when the back of the anus is cracked open small mouth and symptoms appear, you must go to the hospital in time to find the cause through the doctor’s rectal examination, anoscopy and other examinations, and then choose different treatment methods according to the cause.