Unmyelinated fibres in the sacral 3 and caudal 1 ventral roots of the cat.

1. 29% of the axons in the S3 and Ca 1 caudal ventral roots of the cat are unmyelinated. 2. Approximately one half of these unmyelinated axons survive in the proximal stump of a sectioned ventral root and are regarded as unmyelinated preganglionic efferents. 3. At least 27% of the preganglionic efferents in segments S3 and Ca 1 are unmyelinated. 4. The other half of the ventral root unmyelinated axons arise from dorsal root ganglion cells and survive in the distal stump of a sectioned ventral root. 5. The data in 4, combined with the receptive field data in our companion paper, show that these unmyelinated fibres are sensory. 6. Since 15% of the ventral root axons are sensory, the Law of Bell and Magendie cannot be regarded as an accurate description of the organization of these ventral roots.