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Diagram of the spinal cord reflex apparatus. Some of the connections of a single afferent neuron from the skin (d.r.2) are indicated: d.r.2, dorsal root from second spinal ganglion; m, muscles; sp.g.1 to sp.g.4, spinal ganglia; v.r.1' to v.r.4, ventral roots. (After Herrick.)