The stomach is a J - shaped muscular sack that lies immediately beneath the left side of the diaphragm - the muscle between the chest and the abdomen. It has three muscle layers - an outer longitudinal one, a middle circular one and an inner oblique one, and it is lined by cells, called oxyntic cells, that secrete digestive juices including hydrochloric acid. Its function is to digest food both by using these juices, and mechanically by muscular contraction which breaks down large portions into smaller ones.