642 



MANUAL OF PHYSIOLOGY. 



FIG. 245. 





Optic Thalami. 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 tegmen- 

 tum 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 different kinds of sen- 

 sory impressions can arrive at the cere- 

 bral cortex. But the evidence we can 

 obtain by the physiological examination 

 of sensory impressions 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 difficult 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 without the impulse being reflected 

 to its motor neighbors, so a muscular 

 movement often results from stimulation 

 of a group of cells purely sensory in 

 function. 



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 experiment- 

 ers have arrived at very contradictory results, both by stimulating 

 and destroying the optic thalami. Some find that electric stimu- 

 lation causes muscular movements ; others find that it does not. 

 Some authorities state that destruction of the optic thalami inter- 



Section through the gray 

 matter of the brain of man, 

 showing several layers of cells 

 into which fine fibres pass 

 from below. 



