The large intestine is also called the colon, and the part near the anus is called the rectum, which is connected to the sigmoid colon, descending colon, transverse colon, and cecum in that order. The cecum is connected to the terminal ileum (part of the small intestine) through the ileocecal orifice, which is also collectively known as the ileocecal part. The wall of the gastrointestinal tract is like the wall of a house with several layers: the mucosal layer near the intestinal cavity is like the stucco layer of the wall, the submucosal layer is the gap under the stucco layer, and under it is the intrinsic muscle layer, which is the support structure of the digestive tract and is similar to the brick layer of the wall. It is easy to understand inflammatory bowel disease by grasping these structures: ulceration is a bad wall stucco layer, and because it is a chronic diffuse inflammation of the mucosal layer, the stucco layer of the diseased intestinal wall does not see normal mucosa and is mostly a continuous lesion, and the intrinsic muscle layer of the intestinal wall is not obviously involved, thus the ulceration is usually not deep and does not cause perforation, fistula or stricture or obstruction. The ulceration is like the destruction of the stucco layer of the wall, and as the lesion becomes more severe the closer it is to the anus, the necrotic exudation of the mucosal layer is manifested as diarrhea, bloody stools or mucus like snot in the stool. Crohn’s disease is a lesion of the whole intestinal wall, which means that the whole wall is more broken, so the ulcer is deep and easily penetrates the intestinal wall causing perforation, fistula, thickening of the whole intestinal wall or scar formation easily causes stricture or obstruction.