Upon closer examination of the stomach, the muscular folds called rugae that make up the stomach lining are clearly visible. The rugae gradually smooth out as the stomach fills, permitting stomach distension. A cross section of the stomach lining reveals that in between the rugae are gastric pits, which are the openings of the gastric glands. The gastric glands are lined with different types of cells that contribute various components to gastric juice. Chief cells and parietal cells make up the gastric gland. Pepsinogen is secreted by the chief cells and the parietal cells secrete hydrochloric acid. Hydrochloric acid converts pepsinogen into pepsin. Pepsin is the key component to gastric juice and is a potent digestive enzyme. By splitting protein molecules into even smaller parts, pepsin begins the digestion of proteins.