Colon lining. Fluorescence confocal light micrograph of the lining of a mouse colon (large intestine). The colon starts at the small intestine and ends at the rectum. The lumen (interior) of the intestine is at right. The surface (red) of the colon is highly folded, which provides a larger surface area for the absorption of water, the colon's main function. Within the folds are absorptive cells and mucus secreting cells, known as goblet cells. The mucus helps to lubricate the passage of digested food along the colon. Immediately to the left of the folds is a layer of muscle (blue), which contracts to push food along the colon. To the left of that is the submucosal layer (also blue), which consists of fibrous connective tissue and blood vessels.