1. Stretches of regenerating rabbit's nerve in which no connexion with the periphery is allowed contain numerous and small nerve fibres.

2. Both cutaneous sensory and motor nerve fibres become large when allowed to reach their endorgans, but the motor fibres attain greater diameters than the cutaneous sensory fibres.

3. Motor fibres regenerating without opportunity of reaching muscle fibres become little if any larger than sensory fibres when both are unable to reach their end-organs. It is, therefore, probable that the peripheral connexion is of great importance in determining fibre diameter.

4. The fibres of the central stump of a nerve regenerating unconnected with the periphery remain small and have relatively very thick myelin sheaths.

5. The diameter of nerve fibres regenerated in the region below a crushed point on a nerve is not influenced by the length of nerve to be regenerated distally.

6. Increase of diameter and medullation proceeds centrifugally from a lesion, more distant points showing a lesser degree of maturation at any given time.

7. Fibres regenerated in a grafted piece of posterior tibial nerve were only slightly smaller than those in a grafted piece of peroneal nerve. Connexion with the periphery is, therefore, able to produce an increase in fibre diameter which over-rides any constriction of fibre growth produced by the slightly smaller Schwann tubes.

Leverhulme Research Scholar, Royal College of Surgeons of England.