The |stomach| is lined with a ~mucous~ layer which protects the |stomach| from the digestive juices secreted from its walls. These juices include hydrochloric acid and the ~enzymes~ pepsin, rennin, and lipase which help digest the |carbohydrates|, |proteins| and |fats| in food. Occasionally, a portion of this lining wears thin and the digestive juices do aggravate the |stomach| lining. This condition is known as an ~ulcer~ and is a frequent occurrence which may affect the |stomach|, the lower |esophagus|, or the |duodenum|. When empty, or nearly empty, the |stomach| contracts to form folds, or rugae, in its ~mucous~ lining. Though it was once thought that the |contractions| of the |stomach| in the absence of food was what caused the sensation of hunger, it is now known that the primary impulse of hunger is due to low glucose levels in the bloodstream. However, the |contraction| of the |stomach| can often be felt, and this and the "grumbling" heard as food is passed through the lower digestive tract also serve to remind one of the sensation of hunger.
