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Dr. H. Head. 



evolution, whilst one who has never experienced any previous discomfort 

 may develop undue susceptibility to rotatory stimuli. Such persons are no 

 V longer normal, not because the response is a pathological one, but because 



for a time, at any rate, they have been reduced to a lower level of functional 

 efficiency. 



If we responded constantly and inevitably to all impulses from peripheral 

 end-organs, we should be automata, the victims of indiscriminate reactions. 

 But, with the gradual evolution of its functions, the central nervous system 

 acquires ever increasing control over afferent impressions produced by the 

 action of external forces. Such development takes place not only in the 

 race, but in the life of the individual ; and it is to this that we owe our 

 power of acquiring new reactions and facilities. 



During this evolution of function, the power of responding in a more 

 primitive manner is not lost. The mechanism of the older and more 

 immediate reaction remains intact ; but its activity is held in check, ready 

 to become manifest should the nocuous stimulus become unduly intense. 

 All normal persons become giddy if a rotatory stimulus is sufficiently severe, 

 or if it comes into action suddenly, especially if it is greater in one ear than 

 in the other. In the same way, everyone, however healthy, is liable to 

 suffer from pain during the passage of a renal calculus. 



But in states of disease this automatic control is likely to break down, less 

 from the violence of the stimulus than from causes which lower physiological 

 vitality. Of these, infective and toxfemic states are often potent on the 

 physical side. Psychical conditions, especially those associated with neural 

 exhaustion, whether temperamental or acquired, may also lead to profound 

 loss of capacity to dominate impulses derived from peripheral stimulation. 



Conclusions. 



(1) Hughlings Jackson laid down the rule that destructive lesions do not 

 cause positive effects, but induce a negative condition, which permits positive 

 symptoms to appear. I have attempted to apply this law to a number of 

 cases where organic destruction of some part of the nervous system is 

 followed, not only by loss of function, but by some form of over-activity. 



(2) Such abnormal reactions cannot be attributed to " irritation." It must 

 not be supposed that we deny the occurrence of true irritative phenomena. 

 These can be studied most easily in cases of injury to peripheral nerves, 

 where they form an instructive contrast to the manifestations of protopathic 

 sensibility. The pain and tenderness, due to definite irritation of nerve- 

 fibres, occupy the whole territory supplied by the affected trunk ; but a 

 comparatively small number of the conditions which follow a destructive 



