Craniopagus parasiticus is an extremely rare condition in which the two heads are joined directly together, and one twin (known as the autosite) has a functioning torso, while the other (known as the parasite) has only a vestigial torso. The parasite is supported by blood supplied from the autosite head. This threatens the life of the autosite by placing an additional burden on the autosite's vital organs. Operations to separate the two heads have been performed in the hope of saving the autosite.