Unmodifiability of Locomotor Coordination in Amphibia, Demonstrated by the Reverse Functioning of Mutually Exchanged Right and Left Limbs

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According to the so-called resonance principle, a spinal limb center produces motor impulses specific for each individual muscle, to which the motor nerves, specified appropriately by their muscles, respond selectively. A reflex pattern is an orderly timed (coordinated) sequence of such specific impulses. Left and right half of a spinal center act separately. Corresponding left and right muscles are of identical specificity. A supernumerary muscle, through its specified nerve, picks up the messages emitted for the normal muscle of the same name in the district; therefore, a supernumerary limb grafted into the district of a normal limb acts always simultaneously and identically with the latter. If the supernumerary and the nearby normal limb are of opposite laterality (e. g., a right leg grafted near a left leg), their movements are mirror images of each other, and if the supernumerary limb can reach the ground, its movements necessarily counteract the effects of the normal limb. The animals never “learn” to remedy this impediment, at least not in the cases where the normal limb remains functional. There the question arose as to whether, if the normal limbs were removed and limbs of opposite laterality were grafted in their places, the function of the latter would still be reversed as it is in the presence of the normal limbs, or whether some adjustment in the emission pattern of the reflex might not make the grafts serviceable to the body. In a number of salamanders (Amblystoma punctatum larv.), fully developed right and left fore limbs, including the shoulder girdles, were exchanged by dorso-dorsal transplantation, i. e., palms down-ward and fore arms pointing caudad. After reinnervation had taken place, the grafts moved in perfect coordination, but always in exactly the reverse sense from what the animal obviously intended.