A belly button, also called a navel, is a mark or scar on the abdomen, in the place where the umbilical cord was attached to the body during fetal development. When the umbilical cord is removed at birth, the scar can develop in such a way as to protrude, or it may point inward, or it may show neither pattern. Identical twins can be differentiated by their navels, if in no other way, since each belly button is unique. All mammals which receive nourishment from a placenta during prenatal development have a belly button, although it is much more visible on humans than on any other species.
Though the exact appearance of each person's belly button is unique, its placement is almost always identical, in terms of proportion. For example, it is located in the part of the abdomen that is exactly between the third and fourth lumbar vertebrae of the spine. Its exact height varies somewhat from person to person, however.
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This exactness in proportional location is due to the function that the umbilical cord performs before birth, namely that of transporting blood between the baby and the placenta. It houses a vein and two arteries which lead to corresponding vessels in the baby's body that in turn lead to the heart. These are necessary since the baby is unable to obtain oxygen and nutrients any other way. After birth, when the umbilical cord is clamped and cut off, the lungs begin to oxygenate the blood, and the internal blood vessels that formerly carried blood to and from the umbilical cord gradually disappear.
The belly button serves as a convenient focal point on the abdomen. Many fashion trends have reflected this fact. It has been common, for instance, for females to pierce the navel as a place to wear jewelry. Trends like this one are usually accompanied by clothing trends which expose the part of the abdomen where the navel is. These trends are usually present only in women's fashion, with very few exceptions.
In some cultures outside of the Western world, an exposed belly button is considered taboo. This is likely linked to the fact that the exposed navel is considered by most to be mildly provocative. Many cultures, ancient and modern, have regarded the navel in this way. Even in the Old Testament of the Bible, the Song of Solomon contains a reference to the navel as one of the many attractive and beautiful features of Solomon's lover.