Umbilicus
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- For the plant of this name, see Umbilicus rupestris.
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The umbilicus (commonly called a navel, or belly or tummy button), is essentially a scar caused at birth by the removal of the umbilical cord from a newborn baby. The scar can appear as a depression (sometimes colloquially referred to as an "innie") or as a protrusion (referred to as an "outie").
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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.
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
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Fashion sometimes exploit the belly button through clothing that leaves part of the lower abdomen 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. Therefore the trend of exposing the navel has usually been confined to women, and have been perceived as part of liberation and youth movements. This is contrasted to, for example, saris traditionally worn by women in India, which may typically expose the navel. See also halfshirt.
Along with the acceptance of navel display in Western society, piercing of the belly button is becoming common amongst young women.
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.
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 represents the belly button of the Venus, the Goddess of Love in Roman mythology (Aphrodite in Greek mythology).