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Inner Oblique layer
The inner oblique layer of the muscularis externa is a collection of muscle fibers that move, mash and physically break down food. The food is collected by the inner oblique layer in a ball formation before it is moved by another layer of the muscularis externa. The inner oblique layer ensures that food does not travel backward in the intestine.
Middle Circular Layer
The muscularis externa's middle circular layer has a complicated role. The middle circular layer is similar to the inner oblique layer in that it augments its function. Connected to this layer, the pylorus is surrounded by a thick muscular wall. It constricts and releases and makes up the pyloric sphincter. This controls the movement of chyme into the duodenum. It aligned with the longitudinal axis of the stomach.
Longitudinal Layer
The longitudinal layer of the muscularis externa is also comprised of muscular fibers. It is responsible for the coordination of the direction the other layers. It directs the contractions in peristalsis. Peristalsis is the overall function of the GI tract where the food is physically moved through the intestines. The longitudinal layer moves the balled up food made by the inner circular layer down this GI tract.
Auerbach's Plexus
Auerbach's plexus got its name from a German anatomist who studied in the mid 1800s. He was the first describer of the function between the muscle layers of the muscularis externa. Auerbach's plexus moves involuntarily. It is made up of U-shaped collections of nerve fibers that loop over the fornix and run toward the anterior and posterior gastric walls. These nerve fibers, along with ganglia, signal the muscle to begin the process of peristalsis.