The circular folds are large valvular flaps projecting into the lumen of the small intestine. Unlike the folds of the stomach, they are permanent and not obliterated when the intestine is distended. They are not found at the commencement of the duodenum, but begin to appear about 2.5 centimeters beyond the pylorus. In the lower part of the descending portion, below the point where the bile and pancreatic ducts enter the intestine, they are very large and closely approximated. In the horizontal and ascending portions of the duodenum and upper half of the jejunum they are large and numerous, but from this point down to the middle of the ileum, they diminish considerably in size. In the lower part of the ileum, they almost entirely disappear. The circular folds slow the passage of food along the intestines and afford an increased surface for absorption. They are covered with small finger-like projections called villi. The villi are covered with microvilli which absorb fats and nutrients.