Navel - Structure. 1  The umbilicus is used to visually separate the abdomen into quadrants. [3] 2  The umbilicus is a prominent mark on the abdomen, with its position being relatively consistent among humans. 3  Parts of the navel include the umbilical tip, which is the center of the navel often described as a button shape. The periumbilical skin surrounds it. Navels consisting of the umbilical tip protruding past the periumbilical skin have been nicknamed outies.