Circular folds

The circular folds are large valvular flaps projecting into the lumen of the bowel.

They are not found at the commencement of the duodenum, but begin to appear about 2.5 or 5 cm. beyond the pylorus.

In the lower part of the descending portion, below the point where the bile and pancreatic ducts enter the small 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; hence the comparative thinness of this portion of the intestine, as compared with the duodenum and jejunum.