Stomach. Cutaway illustration of the human stomach. The stomach is a hollow bag-like organ functioning in the storage and partial digestion of food. It is connected to the oesophagus (at top) and the duodenum, the first part of the of the small intestine (at bottom left). When empty the wall of the stomach is thrown into numerous folds or rugae, as seen here. These enable the stomach to expand to its capacity of about 1.5 litres. The wall is made of layers of muscle lined by special glandular cells that secrete gastric juice. A ring of muscle at the stomach's lower end (bottom left), the pyloric sphincter, can close the opening to the duodenum.