Polydactylism is the occurrence of more than the normal number of digits on either the hands or feet. Usually there is but one extra finger or toe, but a far greater number have been reported; for example, one case with thirteen fingers on each hand and twelve toes on each foot, a number of cases with ten fingers and various others varying in the number of supernumerary digits down to one. All sorts of combinations may exist as to whether a single hand or foot is affected or a foot and a hand, etc.
Such a member may be rudimentary, in which case it usually amounts to little more than a fleshy tumor growing out from the side of the finger or toe to which it is attached, or else a well-developed digit having completely developed phalanges and muscles, articulating with the head or side of the metacarpal bone or
BRANDEIS JW. POLYDACTYLISM AS A HEREDITARY CHARACTER. JAMA. 1915;LXIV(20):1640-1642. doi:10.1001/jama.1915.02570460016005