188 



Dr. H. Head. 



cerebri and the essential organ of the thalamus itself, each of which subserves 

 a complementary aspect of sensation. Any sensory loss which may be 

 produced by a lesion of the optic thalamus differs in no way from that caused 

 by interference with afferent impulses, either as they enter this organ, or as 

 they pass to the cortex by way of the internal capsule. But to these familiar 

 defects may be added another factor, a tendency to react excessively to all 

 potentially affective stimuli. The prick of a pin, painful pressure, excessive 

 heat and cold, all produce more distress than on the normal side. At the 

 same time, pleasurable warmth may evoke an unusually vivid response, and 

 general affective states induce a more profound reaction over the disordered 

 half of the body. 



It was customary in the past to explain the pains and discomfort, which 

 occur under such conditions, as due to " irritation." The lesion was supposed 

 to " irritate " some part of the sensory path in the region of the optic thalamus 

 and thus to produce pains and " hyperalgesia." But this conventional hypo- 

 thesis is insufficient to explain the facts observed during the clinical course of 

 those cases. 



In the large majority of instances, the lesion proved to be of vascular origin 

 and consisted of a haemorrhage or softening. Now, such disorders of the 

 central nervous system usually arise more or less suddenly ; provided the 

 destruction is not progressive, they tend to produce the maximum loss of 

 function at the time of, or shortly after, their onset. Subsequent progress on 

 the part of the patient is always accompanied by a certain amount of recovery. 

 But in this group of thalamic cases the pains and over-reaction come on as a 

 rule during the period of restoration of function, frequently a considerable 

 time after the " stroke " has occurred, and they usually last unaltered for 

 years. Moreover, the response to pleasurable stimuli may also become 

 excessive, a condition entirely incompatible with the existence of an irritative 

 process capable of evoking pain and discomfort. 



Here, again, a gross organic lesion is followed, not only by loss of function, 

 but also by positive over-action. This is due to the release of the optic 

 thalamus and neighbouring parts from the control normally exercised by the 

 activity of the cortex cerebri. On the afferent side, the essential organ 

 of the thalamus is the centre for pain, heat, cold, and other affective aspects 

 of sensation. When, therefore, it is freed from the restraining influence of 

 the cortex, every stimulus capable of acting on this centre produces an 

 excessive effect on the abnormal half of the body. Disintegration with release 

 of function and not " irritation " is the cause of thalamic over-reaction. 



From the earliest days of our work on the afferent nervous system we 

 recognised that, when the skin was deprived of certain aspects of sensibility, 



