The stomach is connected to the duodenum by the pylorus, where prominent bands of connective tissue partially separate gastric from duodenal muscle fibers.

The pyloric sphincter marks the lower end of the stomach and the entrance into the duodenum. This sphincter is thickened internally by an enlarged circular muscle layer. Unlike the lower esophageal sphincter, the pylorus is usually open; it closes when a peristaltic wave nears the distal antrum.

Muscle layers of the stomach

Most of the pylorus has two layers of extrinsic smooth muscles - the circular and longitudinal (like the rest of the gastrointestinal tract), with the circular layer being especially developed in this part. The antrum part of the pylorus and the more proximal stomach has an additional oblique layer, which is incomplete and located on the submucosal (internal) side of the circular muscle.

The three muscular layers are important for the physical digestion of chyme:

• Longitudinal layer
• Circular layer
• Oblique layer

Along the digestive tract, the oblique layer is only present in the stomach.

Now identify the following structures inside the stomach:

• Rugae - folds of mucosa in the empty stomach that disappear as they are stretched when the stomach fills; they are covered with a protective mucous layer, preventing the gastric acid from eroding stomach walls;
• Gastric canal - a groove formed by longitudinal rugae internal to the lesser curvature, involved in stomach emptying, visible radiographically, and a frequent site of peptic ulcer.