CEREBRAL LOCALIZATION 225 



It is the special function of the optic thalamus, 

 or rather of its mesial nuclei, to impart emotional 

 tone, pain or pleasure, to the sensation. These are 

 thalamic, not cortical, in their appeal to conscious- 

 ness. Fibres from all parts of the cortex converge 

 on the lateral nucleus of the optic thalamus, and tend 

 to control and inhibit excessive pain or pleasure 

 arising from the impulses received from the spinal 

 cord. When this lateral nucleus is destroyed, and 

 only the mesial part of the thalamus left intact, 

 stimuli are much more painful or (in the case, for 

 instance, of stroking or of warmth) more pleasurable 

 than on the normal side. Sometimes music produces 

 a remarkable emotional effect in the affected limbs, 

 especially if it is solemn or majestic. 



There are of course other signs of involvement of 

 the optic thalamus, such as hemianaesthesia, athetosis, 

 and transient hemiplegia. 



FUNCTIONS OF THE FRONTAL CORTEX. 



It is well known that the great motor centres are 

 limited to the ascending frontal or precentral con- 

 volution. This has been abundantly proved by 

 many methods : by the study of paralysis following 

 localized lesions in man, or removals in man or 

 apes ; by electrical stimulation in man and apes ; 

 and histologically, by the limitation to this region 

 of the giant pyramidal or Betz cells, which are the 

 only cells to undergo chromatolysis when the pyra- 

 midal tracts are destroyed in the spinal cord. 



It often becomes of great importance to the surgeon 

 to know whether a tumour causing hemiplegia is 



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