The stomach also adds a significant volume of digestive juices to the meal. Like salivary secretion, the stomach actually readies itself to receive the meal before it is actually taken in, during the so-called cephalic phase that can be influenced by food preferences. The gastric secretions arise from glands in the wall of the stomach that drain into its lumen, and also from the surface cells that secrete primarily mucus and bicarbonate to protect the stomach from digesting itself, as well as substances known as trefoil peptides that stabilize the mucus-bicarbonate layer. The glandular secretions of the stomach differ in different regions of the organ. The most characteristic secretions derive from the glands in the fundus or body of the stomach. These contain two distinctive cell types from which the gastric secretions arise: the parietal cells, which secrete hydrochloric acid and intrinsic factor; and the chief cells, which produce pepsinogens and gastric lipase. The acid secreted by parietal cells serves to sterilize the meal and also to begin the hydrolysis of dietary macromolecules. Intrinsic factor is impoant for the later absorption ofvitamin B12 or cobalamin. Pepsinogen is the precursor of pepsin, which initiates protein digestion. Lipase similarly begins the digestion of dietary fats.REF: GANONG&;S REVIEW OF MEDICAL PHYSIOLOGY, KIM BARRETT, HEDDWEN BROOKS, SCOTT BOITANO, SUSAN BARMANTWENTY THIRD EDITIONPAGE NO:432