Umbilicus

The umbilicus, also called the navel, is essentially a scar representing the former attachment of the umbilical cord in the fetus. The colloquial term for navel is belly button. It can form a depression, a type of umbilicus commonly referred to as an innie, or a protrusion, referred to as an outie. The underlying abdominal muscle layer also presents a concavity; its thinness at this point contributes to a relative structural weakness that makes it susceptible to hernia.

Mollusks

The umbilicus of a shell is the axially aligned hollow cone within the whorls of a coiled gastropod shell, where no columella has been formed. This is the hole around which the inner surface of the shell is coiled.

In species with a wide, open umbilicus, such as the Heath Snail (Helicella itala), the spiral of the whorls can be perceived from the posterior end of the shell.

The umbilicus can vary from very narrow and punctured (e.g. Trichia unidentata) to wide and shallow (such as the deep and wide depression in the Rounded Snail (Discus rotundatus). Shells with a conspicuous umbilicus are always orthostylic, i.e. with a poorly developed columella.

Sometimes there is a dimple or funnel-shaped depression, known as the umbilical region or the the umbilical field, next to or at the basal hollow of the columella, when the walls of successive whorls are not closely wound against each other.

Fashion

One fashion is clothing that leaves part of the lower abdomen with the belly button bare. It is much more common for women than for men. Displaying a bare navel has been a taboo in Western society, as the depression of the navel is taken to be an erotic visual echo. (This is in contrast to, for example, the saris traditionally worn by women in India, which may expose the navel.) Thus, fashions of display have usually been confined to women and have been perceived as part of liberation and youth movements.

A common though fortunately harmless problem among clothes wearers is the collection of belly button fluff in the navel. This has, surprisingly enough, been the subject of serious scientific research.