The stomach connects with the duodenum via the pylorus - the funnel-shaped final part of the stomach. It is positioned between the body of the stomach and the duodenal bulb. Therefore, the pylorus extends from the angular notch to the gastroduodenal junction.

The pylorus is subdivided into two smaller portions named the pyloric antrum and the pyloric canal. The pyloric antrum is the proximal part, and it is wider, while the pyloric canal is the distal, narrower portion. The pyloric canal ends with an opening known as the pyloric orifice that is surrounded and controlled by the pyloric sphincter. The pyloric orifice and pyloric sphincter mark the lower end of the stomach and the entrance into the duodenum.

As mentioned previously, the pyloric sphincter is an anatomical sphincter. Internally, it is thickened by an enlarged circular muscle layer. Unlike the lower esophageal sphincter, the pyloric sphincter usually stays partially open to allow the free flow of water into the duodenum. It closes when the food bolus reaches the stomach, and a peristaltic wave produced by the stomach nears the distal antrum, allowing for digestion to occur. Therefore, the opening and closing of the pyloric sphincter are controlled by peristaltic waves.

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 of the stomach. They extend from the left side of the cardiac opening in the inferior direction along the anterior wall and posterior wall.
The middle layer is made of circular muscular layer of the stomach. Next to the pyloric opening, the circular fibers become thicker and create the pyloric sphincter.
The outer layer is composed of longitudinal muscular layer of the stomach. These fibers are primarily located along the greater curvature 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.

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Pylorus and muscular layer of stomach