Interior of the descending portion of the duodenum, showing the Major duodenal papilla .
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The entire small intestine has circular folds of mucous membrane, also called the valves of Kerkring 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.
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.
Difference from other gastrointestinal folds
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.
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 (singular, villus). Each villus, in turn, is covered with microvilli. The microvilli absorb fats and nutrients from the chyme.
- Anatomy photo:39:12-0302 at the SUNY Downstate Medical Center - "Intestines and Pancreas: The Jejunum and the Ileum"