190 



Dr. H. Head. 



(over-reaction) and peripheral reference are matters of great theoretical 

 interest. To the subject of them they present a series of sensations 

 entirely new and entirely outside his previous sensory experience, and yet 

 they are surprisingly definite and unmistakable. No one who has not 

 experienced them can appreciate the intense vividness with which they 

 present themselves to the subject ; and the investigator with no direct 

 knowledge of them is likely even to be wearied by the importance their 

 brilliance makes them assume in the subject's mind." 



It has long been known that certain minute spots in the normal skin 

 are especially sensitive to heat, to cold and to pain, although there was 

 reason to believe that they were not the cause of all its sensory functions 

 (Blix, Donaldson, von Frey). We were able to show that this punctate 

 mechanism was responsible for the peculiar mode of reaction present during 

 the first stage of recovery. For, whenever a part of the affected area 

 became sensitive to heat, to cold, or to pain, one or more of these specific 

 spots were discovered within it. At this stage, sensibility to contact was 

 due solely to end-organs at the base of the hairs, and, if my hand was 

 carefully shaved, I no longer responded to cotton- wool moved lightly .over 

 the surface. 



All the organs of this punctate mechanism possess certain characteristics 

 in common. They regain their function with great rapidity after the 

 peripheral nerve has been successfully united and, for many months, the 

 form assumed by the reaction of the skin to contact, to pain, to heat and 

 to cold, shows that they alone are responsible for any sensibility it may 

 possess. The response to a stimulus capable of exciting one of these organs 

 is not strictly graduated by its intensity, but is arranged more nearly on 

 an " all or nothing " principle.* So long as the stimulus is effective, it 

 matters little how cold or how hot it may be ; organs of this class indicate 

 its quality, not its intensity. 



A natural consequence of this mode of reaction is the overwhelming 

 importance of the extent of surface covered by the stimulus rather than its 

 intensity. For it is obvious that, if one cold-spot can be excited to vigorous 



* In the protopathic response there is undoubtedly a " more or less-ness," which is of 

 the same nature as that denoted by the term intensity ; it is not, therefore, strictly an 

 " all or nothing " reaction. But variations in the intensity of the stimulus are of less 

 importance to the sensation evoked than the extent of surface or number of end-organs 

 it affects. So much is this the case that in the experiments on my hand, a cold object 

 covering several cold spots, but of higher temperature, caused a colder sensation than an 

 ice-cold rod applied to one spot only. Weber's law, and other expressions of exact 

 quantitative relations between stimulus and sensation, do not hold for states of proto- 

 pathic sensibility ( [9] pp 306-311). It is the business, therefore, .of the central nervous 

 system to replace these impulsive responses by more discriminative sensations. 



