128 K. S. LASHLEY AND S. I. FRANZ 



In every one of the animals clear evidence of some degree of 

 habit-formation was obtained. Whether or not the possible 

 complexity of habit was limited by the destruction in these 

 cases was not definitely determined by the experiments. No 

 attempt was made to train the animals in more difficult tasks 

 and until experiments can be carried out on a more extensive 

 scale we can conclude only that somewhat more than the anterior 

 half of the cerebral cortex in the white rat (fig. 14) is unneces- 

 sary for the formation of simple habits. 



FIG. 14. THE TOTAL EXTENT OF THE COMBINED LESIONS IN THE ANIMALS 

 WHICH LEARNED THE SIMPLE MAZE AFTER OPERATION. SERIES V 



Discussion 



To the destruction of what structural elements may the 

 loss of the plane-box habit be ascribed, and to what the reten- 

 tion in the cases of partial destruction? The lesions in many 

 cases involve the motor area and the loss of the habitual reac- 

 tion might be considered as primarily the result of the motor 

 disturbances arising from injury to this region. However, such 

 facts as we have at hand concerning the motor area in the rat 

 do not support this view. 



The cyto- and myelino-architecture of the br.ain of the rat has 

 not been worked out but Isenschmid (7) has given a full account 

 of the arrangement and composition of the cell-layers of the 

 different regions of the cortex of the mouse and the similarity 

 of the brains of the two species is probably sufficient to 

 justify the application of his results to the rat. He finds 

 three principle areas, fronto-medial, dorso-lateral, and sub-occi- 

 pital, distinguished by the relative thickness of the cell-layers 

 and the size of the elements, and within these distinguishes a 

 number of subordinate areas. Those which have come within 

 the scope of the present experiments are shown in figure 15. 



