Stomach lining. Coloured scanning electron micrograph (SEM) of the lining of the stomach, known as the mucosa. The stomach is a muscular sac involved in storage and digestion of food. Here, the inner mucosa layer of the central region of the stomach is shown. The surface consists of simple columnar cells that secrete mucus (upper left). The mucus protects the stomach lining from digestive acids and enzymes that act on food in the stomach. Glands producing the acids and enzymes open into the stomach through gastric pits (dark holes). Magnification: x700 when printed 10 centimetres wide.