Small intestine lining. Light micrograph of a section through the healthy lining of a human duodenum, the uppermost part of the small intestine. The lining is folded into numerous villi (two seen), which project from the surface. These increase the surface area for the digestion of food. Underlying these is a thin layer of smooth muscle, known as the muscularis mucosae (across bottom), which secrets mucus. The functions of the duodenum are the digestion of food and the absorption of digestive products. It also secrets an alkaline mucus to neutralise acid, and two hormones that stimulate the gall bladder. Magnification unknown.