Small intestine. Cutaway illustration of the layers of the small intestine. The intestine lumen is at lower centre (round). The large folds in the intestine wall are plica circulares. They are covered in finger-like villi (red). These increase the surface area for digestion and absorption of food. The core of each plica circulare and the layer beneath them (purple spots) is the sub- mucosa containing blood vessels. Beyond are the circular and longitudinal muscle layers (red), responsible for the peristaltic movements that push the food along. Entering the intestine at right are blood vessels (red) and nerves (yellow). The outer surface (pink) is the tough serosa.