176 BRAINS OF RATS AND MEN 



of especial interest from the standpoint of localiza- 

 tion of function. This cortex contains specific fields 

 each of which more or less definitely and uniformly 

 evokes movements of particular systems of muscles 

 upon electric stimulation. The question arises wheth- 

 er this electrically excitable area and the fibers of the 

 pyramidal tract which descend from it always act as 

 the final common path for learned reactions or for 

 any cortical participation in such reactions. 



So far as the rat is concerned, Lashley's experi- 

 ments answer this question definitely in the negative. 

 The brightness-discrimination habit, which is defi- 

 nitely abolished by destruction of the occipital cortex, 

 is wholly unaffected by destruction of the entire mo- 

 tor area provided the occipital cortex is left intact. 

 The fibers descending from the cortex which are essen- 

 tial for the performance of this habit in rats deprived 

 of the frontal lobes (and presumably in normal rats) 

 evidently leave the cortex from the visual area or from 

 cortical tissue immediately surrounding this area — 

 not from the so-called motor area. We have inde- 

 pendent anatomical evidence of such fibers, descend- 

 ing from the visual area to the thalamus. 



In the case of habits of kinesthetic type, the 

 simpler ones are, in the rat, entirely unaffected by 

 destruction of the entire frontal third of the cortex 

 (or of any other third of it). More complicated habits 

 of this type show some impairment or in some cases 

 total loss after destruction of the entire frontal cortex. 



