functions than to determine what parts are utterly cut off from the sensory receptive centres. Loss of sensation represents the negative aspect of the picture, whilst residual sensibility corresponds to the functions of adjacent but intact nerve structures.
5. The Negative and Positive Aspects of a Lesion of the Nervous System.
Fifty years ago Hughlings Jackson pointed out that most lesions of the nervous system produced both negative and positive effects; there is not only a loss of function, expressing the destructive activity of the process, but positive symptoms appear owing to release of lower centres from control. This law was accepted as an explanation of certain individual conditions, such as the spasticity accompanying hemiplegia, but was not generally appplied to the phenomena of disease.
From the earliest days of our work on the peripheral nervous system we recognised that, when the skin was deprived of certain aspects of sensibility, the response to those that remained might become peculiarly vivid. Reaction to a prick was abnormal and excessive; the patient complained that it was more painful, although measurements showed that sensibility to this form of stimulation was considerably lessened. This is not a "hyperalgesia," but a more primitive mode of reaction, normally held in check by coincident activity of a higher sensory mechanism, which has been set free to exert a more powerful influence on the ultimate afferent centres.
This conception has been combated by certain critics mainly on the ground that the conditions under which our observations were made were "pathological." To many physiologists a phenomenon which can be labelled "pathological" is banned to the limbo of medicine, with which they refuse to have any concern. We, on the other hand, contend that these dissociations of function give the clue to the complex activities of the nervous system.
The final act of sensation can be decomposed by changing its physiological components. The form assumed by such dissociation may resemble nothing that has previously existed in the phylogenetic history of man; or the change in function may approximate to the character of some more primitive normal activity. This is the case with high-grade protopathic sensibility and with sensations from the glans penis where a normal part of the body responds to sensory stimulation exactly hke an organ endowed with deep and protopathic sensibility only.
There is not a section of this work where Jackson's law of the positive and negative consequences of a lesion does not illuminate the phenomena under discussion. But clinicians are reluctant to abandon their conceptions of "irritation" and "hyperæsthesia"; they assume that a part of the body which reacts excessively to stimulation must be in a condition of increased sensitiveness. They cannot be persuaded to apply the doctrine of relaxed control to the problems of sensation, although they accept it as an explanation of certain exaggerated motor activities.
