Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The circular folds (valves of Kerckring) (also, plicae circulares / valvulae conniventes) are large valvular flaps projecting into the lumen of the bowel. They are composed of reduplications of the mucous membrane, the two layers of the fold being bound together by submucous tissue; unlike the folds in the stomach, they are permanent, and are not obliterated when the intestine is distended.