The esophageal muscles line the esophagus just above the point where it joins the stomach. The circular muscle fibers in the esophageal muscle walls are thickened. These fibers are usually contracted, and function to close the entrance to the stomach. In this way, they help prevent regurgitation of the stomach contents into the esophagus. When peristaltic waves reach the stomach, the muscle fibers that guard its entrance relax and allow the food to enter.