Stomach lining. Coloured light micrograph of the lining of the stomach, known as the mucosa. The stomach is a muscular sac involved in storage and digestion of food. The lumen is at top (white). The surface of the mucosa consists of simple columnar cells that secrete mucus. The mucus protects the stomach lining from digestive acids and enzymes that act on food in the stomach. Beneath the columnar cells are gastric pits (pink circles at lower centre), the glands that make the acids and enzymes needed to digest food. Magnification: x400 when printed 10cm wide.