Umbilicus

For the plant genus of this name, see Umbilicus (genus).

For the Japanese game company, see Navel (Company).

For the citrus fruit see Navel orange.

An umbilicus which appears as a depression in the abdomen is referred to as an "innie".

The umbilicus (commonly called a navel, or belly button) is a scar on the abdomen, caused when the umbilical cord is removed from a newborn baby. All placental mammals have a navel. While it is fairly conspicuous in humans, in most mammals it appears only as a thin hairless line.

In humans, the scar can appear as a depression (sometimes colloquially referred to as an "innie") or as a protrusion ("outie"). Although they can easily be separated into 'innie' and 'outie' categories, navels vary quite drastically in terms of size, shape, depth and overall look, between people. The height at which they are located on the abdomen also varies.

As navels are essentially scars, and not in any way defined by genetics, they are often an easy way to distinguish between two otherwise identical twins.

Contents

1 Human anatomy
2 Fashion
3 Mollusks
4 Other meanings
5 See also

Human anatomy

The umbilicus is an important landmark on the abdomen, since its position is relatively consistent among humans. The skin around the waist at the level of the umbilicus is supported by the tenth thoracic spinal nerve (T10 dermatome).

As well as the visible depression on a person's stomach, the underlying abdominal muscle layers also present a concavity; thinness at this point contributes to a relative structural weakness, making it susceptible to hernia.

During pregnancy, the uterus presses the navel of the pregnant woman outward. It usually retracts after birth.

The umbilicus is also used to visually separate the abdomen into quadrants. The navel comes in the center of the circle enclosing the spread-eagle figure in Leonardo da Vinci's Vitruvian Man, his famous drawing on human proportions. This illustrates the principle that in the shift between the spread-eagle pose and the straight pose, the apparent center of the figure seems to move, but in reality, the navel of the figure, which is the true center of gravity, remains motionless.

Fashion

Fashion sometimes exploits the navel through clothing that leaves part of the lower abdomen (i.e. the midriff) bare, and is much more common for women than for men. Displaying a bare navel has been and still is a taboo in certain Western societies, since the depression of the navel is taken to be an erotic allusion. For example, in the 1960s, Barbara Eden was not allowed to show her navel on I Dream of Jeannie.

The modern trend of exposing the navel has usually been confined to women (aside from a male belly button shirt fad in the 1980s fashion). This is contrasted to, for example, saris traditionally worn by women in India, which may sometimes expose the navel. See also halfshirt, bellyshirt.

Along with the acceptance of navel display in Western society, navel piercing is becoming common amongst young women. Short shirts to expose navels are also often worn to expose lower back tattoos or stomach/navel tattoos, which are popular among young women.

In recent years a trend has developed among young women, usually beginning in the early teen years, whereby women will expose their navel, much as women in earlier eras accented an exposed ankle.

It is common for belly button fluff to collect in the navel as a result of wearing clothes. This has, surprisingly enough, been the subject of serious scientific research.

Navels can be the focus of sexual fetishism, especially amongst heterosexual males.

Mollusks

In 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, as found in 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 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.

Other meanings

The word "navel", or its equivalent in other languages, has been used sometimes for the center for something, e.g. "nave" for the hub of a wheel.
Tortellini might represent the belly button of Venus, the Goddess of Love in Roman mythology (Aphrodite in Greek mythology).