The stomach connects with the duodenum via the pylorus - the final portion of the stomach. 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.

Muscular layer of stomach

The muscular layer lies between the submucosa and serosa, and it is made of smooth muscle cells. Overall, the muscular layer is composed of three sublayers:

• The inner layer is formed by the oblique muscle fibers. They extend from the left side of the cardiac opening in the inferior direction along the anterior and posterior walls.
• The middle layer is made of circular muscle fibers. Next to the pyloric opening, the circular fibers become thicker and create the pyloric sphincter. 
• The outer layer is composed of longitudinal muscle fibers. These fibers are primarily located along the greater and lesser curvature. The anterior and posterior walls of the stomach have only small amounts of longitudinal fibers.

Almost all parts of the stomach have all three muscular layers. These layers are present in the cardia, fundus, body and antrum of the pylorus, and they are essential structures for the digestion of chyme. 

Most of the pylorus has only two layers - circular and longitudinal. Moreover, the pyloric canal has a very well-developed circular layer. 

Note: The oblique muscular layer is only present in the stomach.

Now identify the following structures found within the stomach:

• Rugae - gastric mucosal folds that go in various directions; when the stomach is empty, the mucosa appears wrinkled, while during distention of the stomach, the gastric folds disappear; in constant overeating, the folds can stay distended even when the stomach is empty;
• Gastric canal - a narrow longitudinal passage for saliva and liquids; formed by mucosal rugae that go in the longitudinal direction along the lesser curvature; it extends from the cardia to the pylorus.