The pylorus is the junction between the stomach and the duodenum (Latin pylores = gatekeeper). It contains a sphincter muscle, the pyloric sphincter, within a fold of mucous membrane which closes off the junction during digestion of food in the stomach. The pyloric sphincter surrounds the pyloric orifice, the opening of the stomach into the duodenum. At the location of the sphincter, the pylorus is marked by a slight annular groove on its surface called the pyloric constriction. The part of the stomach that immediately precedes the pylorus is a nearly cylindrical tube called the pyloric canal. The pyloric antrum is a variable dilation at the lower border of the stomach, immediately to the left of the pyloric canal and continuous with the body of the stomach.