Abstract
When in a salamander a supernumerary limb is transplanted into the neighborhood of a normal limb so that it receives its innervation from some part of the limb plexus, every movement of the transplanted limb is an exact duplication of the simultaneous movement of the nearby normal limb. This phenomenon of “homologous function,” discovered by the author, has led to the conception of a “resonance” principle of nervous control.∗ Alternative explanations that had proved to be inadequate were (a) a strict selectivity controlling the establishment of nervous connections between the centers and the extra limbs, and (b) secondary central adjustments in the sense of “learning” or “conditioning”, effected under the control of sensory messages reaching the centers from the extra limbs.
The former possibility has been ruled out by ample evidence. The latter possibility was contradicted by much circumstantial evidence, but a direct experimental test was wanting. Therefore, experiments were undertaken to decide whether or not homologous function would be found in limbs completely deprived of their sensory innervation so that the centers would fail to receive the necessary peripheral data prerequisite for the supposed conditioning or readjustment.
The experiments, performed on 10 axolotls of about 5 to 7 cm. in length, differed from the original ones in that, prior to the transplantation of the extra limb, the afferent nerve supply of the normal limb area into which it was to be transplanted had been thoroughly destroyed. The left fore limb was deafferented by extirpating the corresponding spinal ganglia 2, 3, 4 and 5, including their dorsal roots. I have reported 1 that, contrary to a widespread belief, the motor functions of limbs in amphibia are not noticeably affected by deafferentation, and that, despite the absence of tactile and proprioceptive control, the limbs continue to move in perfect coordination.
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