Supernumerary body parts are most commonly acongenital disorder involving the growth of an additional part of thebody and a deviation from thebody plan. Body parts may be easily visible or hidden away, such asinternal organs.
Many additional body parts form by the same process asconjoined twins: thezygote begins to split but fails to completely separate. This condition may also be a symptom of repeated occurrences of continuousinbreeding in a genetic line.
Specific types of additional body parts include:
Vestigial structures areanatomical structures oforganisms in aspecies which are considered to have lost much or all of their original function throughevolution.[8] These body parts can be classed as additional to the required functioning of the body. Inhuman anatomy, thevermiform appendix is sometimes classed as a vestigial remnant.
Prosthesis is an artificial extension that replaces a body part,[9] andcybernetics is the study of computer technology in relation to organisms, which can include replacement or additional body parts.[10]
Body integrity identity disorder (BIID) is apsychiatric disorder in which a person thinks that they have one or more additionallimbs than they should, despite having two arms and two legs.[11] People with this condition often wish toamputate what they see as additional body parts.
Aphantom limb is the sensation that a missing limb is still attached to the body. This is almost universal in amputees in the first month following an amputation.[12]
Asupernumerary phantom limb is the sensation of having an extra limb or body part despite no such limb actually existing. It is an uncommon syndrome, usually due to some kind of brain injuries in thesomatosensory cortex or in some parts of the right hemisphere of the brain, usually due to astroke in the brain.[13]
Achimera is an animal orplant that has two or more different populations of genetically distinctcells that originated in differentzygotes that have merged. Anatomical structures are typically mixed depending on which cells are prevalent in different body parts. For example, plants can have two different types of flowers.
Amosaic is a genetic anomaly similar in nature and effects to a chimera: genetically different populations of cells within one organism, originated from some propagated mutation of a single cell rather than from outside sources.
InHindu mythology, additional limbs and heads are considered a sign of power.
InGreek mythology,Artemis, the goddess of fertility, was sometimes represented as having numerous breasts. This was particularly notable in the cult of Artemis (Diana) celebrated in the ancient city ofEphesus, in modern Turkey.
InJapanese mythology, the god of the sea and storms,Susanoo, is sometimes depicted as having a third arm.
InHungarian mythology, having six fingers on a hand (polydactyly) is held to be the sign of innate supernatural power (seetáltos).
InChristian mythology,Goliath has brothers who have six fingers and six toes.