FURTHER STUDIES UPON THE NERVE SUPPLY AND FUNCTION OF SUPERNUMERARY GRAFTED LIMBS

1. When a supernumerary anterior limb disc is grafted (with normal orientation) caudal to the intact rudiment, so as to center ventral to the seventh or eighth somites (Figs. 2, 3), the fifth nerve, which normally supplies the orthotopic limb, may grow caudally to the graft. This has been found to occur in seven out of forty-eight cases. The entire nerve may be taken over by the graft (Fig. 7) or it may bifurcate so as to send one branch to the normal and the other to the supernumerary limb (Figs. 8, 9). When the graft is supplied by the fifth nerve its activities are coördinated with those of the normal limbs. The movements of the muscles in the grafted appendage are synchronous with those in the homologous muscles of the normal limb of the same side. 2. When the supernumerary limb disc is grafted to the same position, but with inverted orientation (dorso-ventral, Figs. 4, 5), there is a greatly reduced number of cases in which the fifth (brachial) nerve supplies the graft. In fact in only one case in fifty-eight was this condition obtained. 3. The greater number of cases with brachial nerve contribution (fifth nerve) to the supernumerary limb in the experiments with normally oriented (dorso-dorsal) grafts, as compared with those which have been inverted (dorso-ventral), is interpreted as due to the fact that a stronger attraction is exerted upon the fifth nerve. The bulk of the cells in the limb disc is concentrated in the antero-dorsal quadrant (Fig. 2). When the graft is inverted, the antero-dorsal quadrant occupies a caudo-ventral position and the distance between the outgrowing fifth nerve and the concentrated mass of limb cells is thus considerably increased (cf. Figs. 4, 5 with 2 and 3). Any attractive influence of the graft upon the fifth nerve, therefore, cannot be realized by reason of the distance factor. 4. More evidence has accumulated in the present experiments to bear out former conclusions (Detwiler, 1925, Detwiler and Carpenter, 1929) that when grafted anterior limbs exhibit movements which are coördinated with those of normal intact anterior limbs, they have some connection with the central brachial reflex correlation mechanism.