The entire small intestine has circular folds of mucous membrane, also called the valves of Kerckring and plicae circulares. The majority extend transversely around the cylinder of the small intestine for about one-half or two-thirds of its circumference, but some form complete circles, and others have a spiral direction; the latter usually extend a little more than once around the bowel, but occasionally two or three times. The larger folds are about 1 cm. in depth at their broadest part; but the greater number are smaller. The larger and smaller folds alternate with each other.
Distribution
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 comparativethinness of this portion of the intestine, as compared with the duodenum and jejunum.
Difference from other gastrointestinal folds
Unlike the gastric folds in the stomach, they are permanent, and are not obliterated when the intestine is distended. The spaces between circular folds are smaller than the haustra of the colon, and, in contrast to haustra, circular folds reach around the whole circumference of the intestine. These differences can assist in distinguishing the small intestine from the colon on an abdominal x-ray.
Function
The circular folds slow the passage of the partly digested food along the intestines, and afford an increased surface for absorption. They are covered with small finger-like projections called villi. Each villus, in turn, is covered with microvilli. The microvilli absorb fats and nutrients from the chyme.
