PKENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 289 



most highly developed when the extra digits are functional, and often 

 to an abnormal degree. 



Much greater stress can be laid on the innervation of the polydactyle 

 manus, for the structural conditions are singularly uniform throughout 

 this polydactyle series. In all cases the supernumerary parts are inner- 

 vated by an independent nerve arising from the radial side of the median 

 trunk, and at about the position where the nerve of the pollex is nor- 

 mally given off in pentadactyle animals. When two extra digits are 

 present in the manus, this branch bifurcates and supplies both. Thus 

 modifications exist in the skeletal, muscular, and nervous organs of the 

 polydactyle manus; they point towards the vestigial origin of the extra 

 digits, but there is little evidence of reversion in other parts of the manus. 



Gegenbaur's third objection, that the pollex is absent in the embryo 

 and in all adult Artiodactyla, is well taken. For if these are facts, rever- 

 sion would have to produce a digit of which there is no fundament in the 

 embryo, and reproduce an organ characteristic of only extremely remote 

 ancestors. But Scott ('95) has shown in his work on the American 

 Anthracotheridae, that Ancodus brachyrhynchus has the pollex Avell de- 

 veloped. We do not, therefore, have to go back further than the Suinae 

 to find a pentadactyle form. As to the absence of the fundament of the 

 pollex in the pig embryo, I have confirmed Rosenberg's ('73) results by 

 examining the carpus of a large number of embryos in various stages of 

 development. For this material I am indebted to Prof. E. L. Mark. 

 There was absolutely no evidence of a pollex-fundament other than the 

 trapezium. This element is generally regarded as being simply the 

 carpal element of digit i, for it develops as a single cartilage. We 

 know, however, that the scaphoid and unciform bones develop in the 

 same way, yet that each represents two carpals fused. A careful study 

 of the trapezium in the embryo, in -the normal adult and in the poly- 

 dactyle pig, furnishes some evidence in support of the view that the so- 

 called trapezium represents a rudiment of the pollex as well as a carpal 

 element. (1) In the earliest stages of its development, the cartilage 

 which is to form the trapezium has the pointed distal end characteristic 

 of its adult condition, and jyrojects distad to the proximal limit of the 

 metacarims. (2) In the normal adult carpus the trapezium has always 

 the form of an elongated cone. Its distal end is free, and pointed, 

 instead of truncated, as we should expect if we had to do with only a 

 carpal element. Furthermore, its free end projects farther distad than 

 the other carpal bones and into the region of the nietacarpus. (3) In the 

 polydactyle manus one case was described in which only the distal 



