Your belly button is a one-of-a-kind scar, and birds have one too
The navel is the scar left where the umbilical cord once attached, so no two are quite alike. Doctors sort them into innies and outies with a surprising list of sub-shapes, surgeons use them to hide incisions, and even an egg-laying dinosaur has been found preserved with one.
Anatomically the navel is a reliable landmark. It sits at roughly the level where the third and fourth lumbar vertebrae meet, though in different people it can lie anywhere from the third to the fifth, and clinicians use it to divide the abdomen into four quadrants. The band of skin around the waist at that height is served by the tenth thoracic spinal nerve. At the centre is the cord remnant, often a small protruding knot, ringed by a firm fibrous collar and then ordinary skin. Behind it runs the urachus, a thick fibrous cord that leads down to the bladder.
Classification schemes get detailed. An outie is any navel whose tip pushes out past the surrounding skin; variants include the split type, cleft like a coffee bean, and the circlet, dimpled in the middle like a doughnut. An innie is any concave navel, which may be round, vertical, oval, T-shaped or horizontal, the last almost hidden by natural skin folds. A rare swirl form also exists. Outies are sometimes confused with umbilical hernias, but they are harmless and differently shaped. A sudden change in shape, by contrast, can signal fluid build-up in the abdomen, and newborns can develop an infection of the area called omphalitis.
Because a small cut there leaves little visible trace, the navel is a favoured entry point for procedures such as gall bladder surgery. It is not a human monopoly. Every placental mammal has one, just less obvious, and live-bearing animals tend to have round navels while egg-layers such as birds carry a vertical line; a fossil Psittacosaurus shows the egg-layer version.
Cultures read it very differently. Western societies long treated a bare midriff as indecent, and some places banned female navel exposure, whereas Indian women have shown it with saris for generations. Early Jomon artisans in northern Japan pressed an oversized clay navel onto figurines because it marked where life began, and qigong treats it as the body's chief energy centre.
Source: Navel
