192 



Dr. H. Head. 



or cold, irrespective of the temperature to which the part has been pre- 

 viously exposed ([9], p. 292). 



How completely these changes in the form of the response are due to 

 recovery of control by a dominant system over the activity of one or more 

 primitive functions was shown by the behaviour of my hand when exposed 

 to the injurious effects of cold. The higher forms of sensibility are liable to 

 be unfavourably affected by the general action of external cold, especially, 

 before they have been completely restored. At a time when almost the 

 whole of the back of my hand had so far recovered that referred sensation 

 could no longer be produced, the palm was placed upon ice ; radiation and 

 reference reappeared as vividly as of old, and the hand was thrown back for 

 a time into its previous condition. The newly recovered activity of the 

 high-grade system was disturbed by the cold, and impulses from the more 

 primitive mechanism, previously inhibited, now passed on to reach con- 

 sciousness uncontrolled. 



This control may be exerted even by impulses from the healthy skin in 

 the neighbourhood, if adjacent normal and abnormal parts are stimulated 

 simultaneously. When a cold tube was placed so that it fell just within 

 that part of the dorsum of the hand which was in a purely protopathic 

 condition, a vivid referred sensation was always experienced in the thumb. 

 But when the base of the tube fell partly within the affected area and 

 partly over the adjacent normal skin, reference to the thumb was abolished. 

 The only sensation produced was one of coldness around that part of the 

 back of the hand in contact with the tube. 



If this massive and ill-defined response were due to structural changes 

 inherent in the act of recovery, it is difficult to see why it is not always 

 present, whenever nerve fibres conducting painful and thermal impulses 

 undergo regeneration. Occasionally it happens, by a fortunate chance, that 

 after division of cutaneous nerves some part of the denervated area retains 

 its discriminative sensibility to the lighter forms of tactile stimuli, though 

 insensitive to prick, heat, and cold. With the restoration of these aspects of 

 sensation, pain-, heat-, and cold-spots make their appearance ; but under 

 such conditions, the response is not unduly vivid, diffuse, or referred to 

 remote parts. Eecovery within an area already endowed with the more 

 discriminative forms of sensibihty is not accompanied by the phenomena we 

 have attributed to loss of control ([9], p. 287). If over-reaction were due to 

 irritation of " pain-fibres " in the course of regeneration, it should be inde- 

 pendent of the coincident presence or absence of other sensory activities ; 

 but this is not the case. 



Moreover, it is important to remember that tliis over-reaction is limited to 



