How long the anus is usually refers to how long the anal canal is. The anal canal is the extreme end of the digestive tract, connected to the rectum above the dentate line, and the extreme end of the lower end is the anal verge, which is generally 3-4 cm, and this is called the anatomical anal canal. Some scholars also extend the upper boundary of the anal canal to 1.5 cm above the dentate line, the plane of the rectal loop of the anal canal, which is called the external scientific anal canal, but it is not used much. In men, the anal canal is adjacent to the urethra and prostate in front, and in women, it is adjacent to the uterus and vagina, followed by the coccyx and surrounded by the internal and external sphincters. The dentate line between the anal canal and the rectum is of great importance. Above the dentate line is the upper and lower rectal artery supply, and below it is the anal artery supply. Therefore, the internal hemorrhoids are basically formed above the dentate line and the external hemorrhoids are formed below the dentate line. The pain of internal hemorrhoids is not serious, while the pain of external hemorrhoids is more serious.