Light micrograph of a section through a human duodenum, the beginning of the small intestine, which runs from the stomach to the large intestine. It is where digestion begins and nutrients are absorbed into the blood. The interior (lumen, top) is lined with villi, which are folds in the intestinal surface that greatly increase the surface area for absorption. Also within the surface are crypts of Lieberkuhn (red-lined ovoids), which secrete enzymes into the lumen that help to digest the food. Underlying this layer is the muscularis mucosae (thin violet line) and a layer of connective tissue (blue) with Brunner's glands (light pink) embedded in it. These glands secrete an alkaline mucous that protects the duodenum from the acidic stomach contents entering it. Across bottom are layers of smooth muscle and then circular and longitudinal muscles that contract and relax to move food through the duodenum. Magnification: x20 when printed at 15 centimetres wide.