THE EFFECTS OF DE‐NERVATION OF A CUTANEOUS AREA

1. An account is given of the results of severance of the nerves supplying the skin of the palmar aspect of the little finger, and of the phenomena observed during recovery of sensation. 2. When the severance is effected by cutting, recovery is incomplete even eighteen months or more after section; when by crushing, complete recovery is manifested within three or four months. The explanation is that when the nerve is cut across, the newly growing axis‐cylinders fail to find their way into the original neurolemmal sheaths, and reach their terminations with difficulty or, in the case of tactile nerves, probably not at all. Whereas, when the nerve is merely crushed, the neurolemmal sheaths are not severed, the newly growing axis‐cylinders pass as single fibres along their original course, and are directly guided to their terminal organs. 3. Pain‐sensations are those which are first experienced in the course of recovery. The reason is ( a ) that the fibres which subserve pain have no special terminal organs, but end by the branching of naked axis‐cylinders; ( b ) that when a mixed nerve is stimulated ( e.g . the ulnar at the elbow), the effect of stimulating the “pain‐fibres” masks that of stimulating the fibres which subserve special cutaneous sensations, such as warmth, cold, and touch, and the sense of pain is the only one which is discerned by the sensorium. 4. The pain which is produced by stimulating the growing fibres in their course towards or within the de‐nervated area into which they are extending is peculiar in two respects: ( a ) it is referred peripherally; ( b ) it is more intense than that caused by stimulation of normal nerve fibres. The explanation of the peripheral reference is simple, since this is what always happens when a nerve is stimulated in its course or when a cut nerve is stimulated central to the point of severance. The explanation of the increased sensitivity of the newly growing fibres seems to be that their environment is abnormal. In the case of a cut nerve the growing axis‐cylinders have no neurolemmal sheaths, which seem to be essential for the proper protection of the fibres; and since they are slow in acquiring these sheaths or may never do so, the increase of sensitivity (or intensification of reaction to stimuli) may last an indefinite time. In the case of a crushed nerve the neurolemmal sheaths are present from the first, but as a result of the severance of the axis‐cylinders by crushing, the lemmal cells have undergone proliferation, and the relationship of the growing axis‐cylinder to its sheath is at first abnormal. After a time the normal relationship is recovered and the increased sensitivity disappears. 5. It is unnecessary to assume that a new kind of nerve‐fibre subserving a new kind of sensation has become manifested in the process of recovery, as has been concluded by Head , who has given the name “protopathic” to the hypothetical new system of nerve‐fibres. The protopathic fibres of Head are in fact nothing other than the pain‐fibres, which react to stimuli in the peculiar manner mentioned in 4. 6. A cutaneous area which is not wholly de‐nervated but receives accessory fibres from neighbouring nerves exhibits phenomena which are different from those shown by a completely de‐nervated area. The existence of accessory fibres complicates the study of the effects of section of the main nerve and of recovery of sensation in the area. Previous observers who have experimented upon the effect of section of cutaneous nerves have not taken into sufficient consideration this material fact of the existence of such accessory fibres. This probably accounts for many discrepancies in their results, and is certainly the reason why the effects recorded are different from those obtained from an area which receives no accessory fibres, and has therefore been completely de‐nervated. 7. In illustration of this, experiments are recorded of severance of nerves to cutaneous areas of the forearm, which are unquestionably supplied by accessory fibres, and which are therefore only partially de‐nervated, and the results have been compared with those on the little finger the cutaneous areas of which were wholly de‐nervated. 8. An account is given of the sensations immediately following nerve‐severance, which are referable to the trauma caused by the operation and to the formation and contraction of scar‐tissue. 9. Observations are also recorded on the effect of severance of the nerve of a cutaneous area on the vascular condition of the area, and upon the growth of the nail.