The stomach contains three layers of smooth muscle that allow it to contract to mix and propel food though the digestive tract. Contained in the muscularis layer of the stomach, the three layers of smooth muscle fibers run in three different directions. These three layers of smooth muscle are required because smooth muscle can contract only in one direction.
The outermost longitudinal layer of the mucularis lies along the length of the stomach. It parts into two bands at the center of the stomach, and it rejoins near the pylorus at the bottom of the stomach. The middle circular layer, which is the thickest and strongest, wraps around the entire stomach, including the pylorus, where it forms the pyloric sphincter. Usually in a constricted state, the movement of this muscle propels the stomach's contents, called chyme, into the duodenum, or the top of the small intestine. Unique to the stomach, the innermost muscle layer lies in an oblique direction, and it is strongest near the top of the stomach at the fundus. Together, the three muscle layers work to create peristalsis, the churning motion that breaks down food and mixes it with the stomach acids and enzymes required for digestion.