There is no muscle layers around the navel. It is like a volcano on the surface of the ground. Because it is hollow, scurf can be gathered there easily.
In Japan, many people say that if you clean scurf of the navel, you harm the abdomen. However, it is not good idea to accumulate a lot of scurf at the navel. The navel is an important acupuncture point called Shenque, CV-8, which is used for the severe sickness.
Try to wash the navel lightly when you take a shower to keep the flow of qi around the navel smoothly.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.