CHAPTER VII 

 THE THALAMUS 



Under normal circumstances the thalamic element in 

 contact sensibility rarely^ if ever^ reaches consciousness, 

 which is dominated by discriminative sensations of touch, and 

 it only becomes a conscious factor in sensation when the 

 influence of the cortex is removed. Thus, we believe that the 

 essential organ of the optic thalamus is the centre of conscious- 

 ness for certain elements of sensation. It responds to all stimu- 

 li capable of evoking either pleasure or discomfort, or con- 

 sciousness of a change in state, 



— Henry Head 



TWO dominant physiological factors have co- 

 operated in the phylogenetic differentiation 

 of the cerebral hemispheres. The first is the 

 inflow of nervous impulses from the olfactory organs 

 in front; the second is the ingrowth from lower sen- 

 sory centers of nerve fibers conducting various other 

 systems of sensory nervous impulses. The configura- 

 tion of both the thalamus and the cerebral hemi- 

 spheres of the several vertebrate types has been de- 

 termined largely by the interaction of these functional 

 influences which are discharged into them from op- 

 posite directions. 



Primitively, the vertebrate brain was probably a 

 cylindrical tube with walls unequally thickened in 

 different regions, comparable with that seen in adult 

 Amphioxus or in an early embryonic stage of any 



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