PRENTISS: 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 vestigzal origin of the extra 
digits, but there ts 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 well 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 1, 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 projects distad to the proximal limit of the 
metacarpus. (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 nto the region of the metacarpus. (3) In the 
polydactyle manus one case was described in which only the distal 
