(iii) Sub-mucosa. It extends internally into circular folds to increase the area of absorption. It is made up of loose connective tissue, blood capillaries, lymphatic vessels and nerves.

(iv) The mucosa is the innermost lining made up of the columnar epithelial cells that have minute finger-shaped projections called villi. The core of each villus is made up of a connective tissue tunica propria containing blood capillaries and lymph vessels. The epithelial cells of the mucosa (except for the goblet cells) show a striated border of numerous microvilli. At intervals between the villi are seen two types of glands, the crypts of Lieberkuhn and the Brunner’s glands, respectively. Thus the internal surface area of the intestine is greatly increased due to muscular folds, villi and microvilli, which are essential for absorption.

The crypts of Lieberkuhn are the simple tubular glands which extend into the tunica propria as far as the indistinct muscularis mucosa. They secrete digestive enzymes.

The Brunner’s glands are large bunches of grape-like structures, extending even into the sub-mucosa. They secrete an alkaline watery fluid containing mucin and some enzymes. These glands are restricted only to the duodenum. The secretion of crypts and Brunner’s glands constitutes the intestinal juice or saccus entericus.

Besides these glands, the lymphoid tissue is also widely distributed throughout the mucosa of the small intestine as the lymph nodules. In the ileum the nodules are gathered in groups forming Payer’s patches.