112 K. S. LASHLEY AND S. I. FRANZ 



failed. What is the relation of these differences of behavior to 

 the extent of the cerebral destructions? The figures of the 

 brains are grouped on plate III for comparison. The first fact 

 evident from the figures is that in every case where there was a 

 clear retention of the habit the destruction of the anterior pole 

 of one or both hemispheres was incomplete. In animals which 

 showed a loss of the habit there is apparently a greater destruc- 

 tion of cortex, so that in most if not all cases the whole of both 

 frontal poles is involved. 



It is impossible to say how sharp is the distinction between 

 these two groups when the extents of the lesions are compared. 

 The maximum extent of the lesion has not been determined 

 accurately in any case and the figures are based almost wholly 

 upon the gross lesions. It is probable, however, that these are 

 at least indices of the true extent of injury. 



Among the animals which showed no retention there was a 

 practically complete destruction of the frontal poles of both 

 hemispheres in experiments 27, 30 and 32, with almost complete 

 destruction in experiment 28. In two of the remaining a part 

 of the mesial cortex was preserved ; the extent of the lesion was 

 not determined in the third. One animal which showed doubt- 

 ful retention (experiment 25) was also found to have a complete 

 destruction of the frontal region of both hemispheres. The 

 only evidence for retention in this case was the solution of ,the 

 problem in less than the normal learning time. There was no 

 recognizable reaction to the plane which persisted after the 

 operation and the successful movements were made as though 

 by chance. The long average time (50 seconds) required by 

 the animal to reach the food^ after tripping the plane gives fur- 

 ther evidence that there was little or no retention of the habit. 

 Allowing for the questionable nature of the retention in this 

 last animal, we are justified in concluding that the complete 

 destruction of the frontal region of the rat's brain results in 

 the loss of the complex inclined-plane habit. 



After partial destruction of the frontal pole five animals re- 

 tained the habit, one gave questionable evidence of retention 

 and two showed no retention of the habit. Of the latter, one 



