Lining of the stomach, consisting of an inner epithelium, a middle LAMINA PROPRIA, and an outer MUSCULARIS MUCOSAE. The surface cells produce mucus that protects the stomach from attack by digestive acid and enzymes. When the epithelium invaginates into the LAMINA PROPRIA at various region of the stomach (cardia; gastric fundus; and pylorus), different tubular gastric glands are formed. These glands consist of cells that secrete mucus, enzymes, hydrochloric acid, or hormones.