uctures resemble the thumb of a normal hand ( b , d ). The sixth finger or supernumerary finger has three phalanges and a saddle carpometacarpal joint ( e ). It has two extrinsic flexor tendons and a normal extensor apparatus not dissimilar to that of a tri-phalangeal digit. Interestingly, there are muscles whose origin is the second metacarpal and whose insertion is to the proximal phalanx of the finger ( b , c ), similar to the muscles of a normal thumb with spherical range of motion The combinatorics of polydactyly’s genetics have been analyzed in seminal nineteenth century works 12 and the genes responsible for polydactyly have been identified recently 13 . However, the neuromechanics and functionality of polydactyly hands raise many questions that have never been investigated: First, is the movement of the additional finger actuated by other fingers’ muscles, or does it have its own dedicated muscles and nerves? Second, how independent is the extra finger from the other fingers? Does its movement accompany the movement of common fingers, like in the little and ring fingers 14 , or does it move independently from other fingers like the thumb? Third, hand movements are already among the most complex movements humans can perform, requiring a large area of the sensory and motor cortices to control them 14 – 16 . Therefore, how could the cortex control a hand with several additional dof? Fourth, what is the perceived body representation of polydactyly hands? Fifth, and most importantly, are the supernumerary fingers (SF) functional, and can they provide advantages in terms of additional manipulation abilities? The present case study examines these questions on two subjects with preaxial polydactyly with an SF between thumb and index finger. The results reveal dedicated muscles, nerves and neural resources that offer these polydactyly augmented manipulation abilities. Results The SF is actuated by dedicated muscles and nerves We first examined the anatomy of the six-