The stomach is a muscular bag that is lined with a thick layer of mucus to prevent acid and digestive enzymes from damaging the stomach wall. It has two special muscles. One is at the entrance of the stomach and opens to allow food in then closes to stop the contents of the stomach being pushed back up the oesophagus. If this doesn't happen then one may suffer from heartburn which is when the stomach acid gets up into the oesophagus. There is also an opening at the bottom of the stomach. It closes to hold the food in the stomach long enough for digestion to occur, then opens to allow food to enter the small intestine.