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Dr. H. Head. 



palm which we know to be supplied with " pain-fibres " from the injured 

 trunk. 



When, however, excessive reaction to prick is really due to irritation of a 

 peripheral nerve, as in some cases of causalgia (fig. 3), the extent of the 

 cutaneous tenderness is not limited by the loss of tactile sensibility, which 

 is often slight ; it spreads across the palm on to all those portions innervated 

 from the injured nerve, and can be abolished by transecting it completely 

 ([9], pp. 160 and 212). 



I have attributed these morbid conditions, where gross loss of function 

 is accompanied by unduly vivid reactions, to removal of some influence 

 from above on the activities of lower levels of the nervous system ; this 

 doctrine has been widely opposed. Many hold that, when a complex 

 function is broken up, the resultant positive manifestations are purely 

 adventitious, and throw no light on the inherent reactions of the lower 

 centres. Morbid changes lead to disordered action, of which "irritation" 

 and excessive responses form an integral part. 



But there can be no doubt that the postural rigidity of the decerebrate 

 animal and the spasticity in hemiplegia express the activity of centres, 

 which have remained unaffected by cerebral destruction. In the same way, 

 the excessive responses to affective stimuli, which appear when the optic 

 thalamus has been released from cortical control, and the mass-reflex 

 obtained from the lower end of the spinal cord demonstrate the mode of 

 reaction of these centres, when they are no longer dominated from above. 



I have included in the same category the condition we have called 

 " protopathic " sensibility,* which may appear after complete division of 



* The terms " protopathic " and " epicritic " are applicable solely to forms of cutaneous 

 sensibility, revealed by destructive lesions of the peripheral nervous system, and should 

 not be employed for the products of dissociation at higher levels. All afferent impulses 

 undergo regrouping, according to their potential sensory qualities, at the first synaptic 

 junction. For instance, those due to painful impressions from the surface of the body 

 are combined with equivalent impulses, produced by excessive pressure on deep end- 

 organs, and travel up the same specific tract as far as the optic thalamus. In the same 

 vfc-ay the products of thermal stimulation, arising in the " protopathic " or " epicritic " 

 mechanism of the skin, are sorted and re-grouped according to their power of arousing 

 sensations of heat or cold. The optic thalamus forms the centre for qualitative and 

 affective modes of sensation, whilst the so-called " sensory " cortex is occupied with its 

 spacial and discriminative aspects. When the thalamus is freed from control, the 

 abnormal half of the body reacts excessively to stimuli capable of evoking a response, 

 whether pleasant or unpleasant, and in this way resembles a part in a condition of 

 protopathic sensibility. But thalamic over -action is produced by the disintegration of 

 a highly complex group of sensory functions, whilst protopathic sensibility is due to the 

 uncontrolled action on normal centres of a set of impulses, heavily charged with feeling- 

 tone and capable of inducing impulsive reactions. 



