OPTIC THALAMI. 



643 



FIG. 245. 



Optic Thalarni. The evidence concerning these ganglia is 

 far from being as satisfactory or conclusive as that relating to the 

 corpus striatum. 



Anatomically the matter is equally 

 clear ; they are the ganglia of the sensory 

 tracts, since the tegmentum or sensory 

 parts of the crura pass directly into them. 

 They form, in fact, the only routes by 

 which the impulses giving rise to the dif- 

 ferent kinds of sensory impressions can 

 arrive at the cerebral cortex. But the 

 evidence we can obtain by the physio- 

 logical examination of sensory impres- 

 sions is very indistinct in comparison 

 with the obvious results we find when 

 motor tracts are excited ; indeed, in the 

 complete absence of all motion, it is dif- 

 ficult to know whether an animal feels 

 or not, as we have no signs to show 

 whether the stimulus takes effect. 

 Further, it is difficult, as has been already 

 seen, to stimulate any sensory tract with- 

 out the impulse being reflected to its 

 motor neighbors, so a muscular move- 

 ment often results from stimulation of 

 a group of cells purely sensory in func- 

 tion. 



When we take into consideration the 

 foregoing points, and the fact that it is 

 difficult, if not impossible, to destroy a 

 portion of brain substance without irri- 

 tating it and the neighboring structures, 



we cannot be surprised that experimenters have arrived at very 

 contradictory results, both by stimulating and destroying the 

 optic thalami. Some find that electric stimulation causes muscu- 

 lar movements ; others find that it does not. Some authorities 

 state that destruction of the optic thalami interrupts only the in- 



Section through the 

 gray matter of the brain 

 of man, showing several 

 layers of cells into which 

 fine fibres pass from below. 



