Stomach lining. Coloured scanning electron micrograph (SEM) of the inner lining of the stomach (gastric mucosa). The large folds are known as rugae, and are present when the stomach is empty and contracted, but flatten out when full. The small indents are gastric pits. These contain cells that secrete enzymes, mucous and hydrochloric acid into the stomach. Magnification: x10 when printed 10 centimetres wide.