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Polycephaly
Polycephaly is the condition of having more than one head, and so is an example of a supernumerary body part. The term is derived from the Greek stems ''poly-'' (Greek: "πολύ") meaning "multiple" and ''kephalē-'' (Greek: "κεφάλη") meaning "head". A variation is an animal born with two faces on a single head, a condition known as diprosopus. In medical terms these are all congenital cephalic disorders.
There are many occurrences of multi-headed animals, in both reality and mythology. In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed.
Bicephalic or tricephalic (two- and three-headed respectively) animals are the only type of multi-headed creatures seen in the real world and form by the same process as conjoined twins: they all result from the fusion of monozygotic twin embryos. One extreme example of this is the condition of craniopagus parasiticus, whereby a fully developed body has a parasitic twin head joined at the skull.
==Occurrences==
Two-headed people and animals, though rare, have long been known to exist and documented. The "Scottish brothers" were conjoined twins, allegedly dicephalic, born 1460 (dates vary). (Human conjoined twins, not all of the dicephalic type, have been documented since 945.)
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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