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The navel, also called a bellybutton or umbilicus, 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. more...

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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 occurrence of an outie versus an innie is a matter of some dispute. Many sources say that the difference is random. Other reasons for the occurrence 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 as the legs and torso appear in sound proportion.

Read more at Wikipedia.org