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. The surface consists of simple columnar cells that secrete mucus (grey). 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 not known.