In specific places along the tract, like the esophageal sphincter, the circular layer thickens, forming these sphincters that keep food from passing from one part of the gastrointestinal tract to another.

在消化道的一些特定部位，例如食管括约肌，内环肌层增厚，形成括约肌，可以防止食物从一个部位反流至上一个部位。