Intestinal lining. Light micrograph of a section through the wall of the human jejunum, part of the small intestine. The lining has many folds, known as plicae circulares or Kerckring's valves, in this region. The folds are covered by smaller projections called villi. Both the plicae and villi serve to increase the surface area for the absorption of nutrients from the gut lumen. The innermost layer of the gut lining is the mucosa or mucous membrane (purple). Beneath this is connective tissue of the submucosa layer (pink, within the plicae), and at the base is smooth muscle (pink, across bottom). Magnification: x7 when printed 10cm wide.