The umbilicus (commonly called a navel, or bellybutton) 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. As navels are essentially scars, and not in any way defined by genetics, they are often an easy way to distinguish between identical twins.
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).
About 90% of the people have a depression, or an innie, with the other 10% being outies. The reason for the occurance of an outie versus an innie is a matter of some dispute. Many sources say that the difference is random. Other reasons for the occurance of an outie are extra skin left from the umbilical cord or umbilical hernias, although a child with an umbilical hernia will not necessarily develop an outie.
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 at this point.
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.
The height at which navels are located on the abdomen is variable. An ideal proportion of navel height versus body height is said to be based on the Golden Section, also known as the Divine Proportion by philosophers and artists. (This is a geometric proportion in which a line is divided so that the ratio of the length of the longer line segment to the length of the entire line is equal to the ratio of the length of the shorter line segment to the length of the longer line segment.) This Golden Section ratio has a numerical value of .618. In other words, an ideal navel height is about 62% of the body height and, is said to exhibit special beauty.
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 cultures, since the sheath-like appearance of the navel (especially of an 'innie'), has erotic overtones. 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). While the West has been prudish about midriff-baring dresses till the eighties, Indian women have traditionally worn saris which usually expose the navel as the blouse/choli that is worn with it is intentionally kept short. In Indian culture, exposure of the navel is not considered a taboo and, has in fact, been accepted as a graceful identifying mark of married woman since ages. A dimpled navel is considered a special asset of any prospective bride, especially amongst South Indian girls. Another Indian community that takes navels in stride are the Rajasthanis whose women leave the midriff exposed while wearing short cholis with traditional gypsy skirts. Paradoxcally, these women cover their heads with a 'chador' and even cover their faces in front of strangers, which lends credence to the belief that navel-baring in India has a symbolic, almost mystical association with birth and life and, its display is meant to emphasise the centrality of Nature in the nurture role. Also see halfshirt, bellyshirt, blouse, choli.
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 lint to collect in the navel as a result of wearing clothes. In Punjabi language the word for navel, ie 'dhunni' signifies a small lint factory!
While navels can be the focus of sexual fetishism, especially amongst heterosexual males, it is a fact that navels arouse more than just interest, as never has one gone unnoticed. While cleavage of the breasts is certainly meant to be an alluring display of feminine charms, a navel is considered, in equal parts, an innocent emblem of feminity as much as Britney Spears' "not so innocent either".
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
See also