FRANK FOX / SCIENCE PHOTO LIBRARY FRANK FOX / SCIENCE PHOTO LIBRARY
Small intestine. Light micrograph of a transverse section through the human small intestine, showing its folded wall with villi. At top is the inner lumen of the intestine. At bottom are outer layers of muscles of the intestinal wall (pink). The inner intestinal wall contains folds, on which villi (purple) are seen. Villi are projections (0.5 to 1 millimetres long) which greatly increase the effective absorptive and secretory surface of the mucosa (mucus membrane) which lines the small intestine. The human small intestine is about 6 metres in length and requires a large surface area for the absorption of food which has been digested by enzymes.
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