A hernia at the umbilicus (See hernia.). Hernias are a protrusion of tissue through an abnormal opening. Normally, the abdominal wall is formed by two sheets of muscle that run along both sides of the abdomen. When these sheets of muscle incompletely fuse, it permits part of the bowel to protrude up through the muscle sheets at the level of the umbilicus or belly button. As the infant grows, the muscle layers increase and gradually fuse eliminating the hernia. This is one of the few hernias which often resolves on its own. Occasionally, surgical correction is required.