Small intestine. Light micrograph of a transverse section through the human small intestine, showing its folded wall with Ivilli. i 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 Ivilli i (purple) are seen. IVilli i 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. Magnification: unknown.