Science Fair Project Encyclopedia
- For the plant of this name, see Umbilicus rupestris.
The umbilicus, also called the navel, is essentially a scar representing the former attachment of the umbilical cord in the fetus. Colloquial terms for navel are belly button and tummy 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 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 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.
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. See also halfshirt.
Along with the acceptance of navel display in Western society, piercing of the belly button is becoming common amongst young women.
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.
The umbilicus is an important landmark on the abdomen, as its position is relatively consistent between people. The skin around the waist at the level of the umbilicus is supplied by the tenth thoracic spinal nerve (T10 dermatome).
As well as the visible depression on a person's belly, the underlying abdominal muscle layers also present a concavity; thinness at this point contributes to a relative structural weakness that makes it susceptible to hernia.
The umbilicus is also used as a landmark to describe the four quadrants of the abdomen - upper left, upper right, lower left, lower right. The navel comes in the center of the circle enclosing the spread-eagle figure in Leonardo da Vinci's famous drawing on human proportions, the Vitruvian Man. 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.
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