126 K. S. LASHLEY 



habits having a definite structural basis. Cerebral in jury may 

 destroy a very great number of these, instead of the few which 

 such an animal as the rat has formed, and the apparent loss of 

 function will therefore be greater. Further, the rate of learning 

 in the human adult depends largely upon the number and com- 

 plexity of the habits already organized. When the latter are 

 abolished the entire system must be built up slowly de novo 

 before anything like an approach to adult performance is attained. 

 The recent studies of reeducation in hemiplegia, aphasia, and 

 apraxia show that the loss from cerebral lesions is never neces- 

 sarily permanent in man and that an unlimited though slowly 

 acquired vicarious functioning is possible. The difference seems 

 to be one of degree rather than of kind. The rat loses less than 

 higher forms after cerebral injury because he has less to lose and 

 he seems to recover more rapidly chiefly because a little improve- 

 ment brings him relatively nearer the standard of comparison 

 (normality) than does the same amount of improvement in man. 



Summary 



1. Destruction of the frontal pole, including the motor area, 

 is probably followed by an increase in general activity. Injuries 

 to other parts of the cerebrum have no dynamogenic effect. 



2. Learning at normal rate of either kinaesthetic- or visuo- 

 motor habits is possible after the destruction of any given part 

 of the cerebral cortex of the rat. 



3. Destruction of cerebral nuclei also has no effect upon the 

 rate of learning. 



4. Within limits from 100 to 50 per cent of the cortex there is 

 no relation between the amount of cerebral material functional 

 and the rate of formation of complex habits. 



5. In normal animals the habit of brightness discrimination is 

 mediated by the occipital pole of the cerebrum (area striata) and 

 by no other part of the cerebral cortex. 



6. Extensive lesions to one corpus striatum are accompanied 

 by paralytic symptoms and it is probable that the stimulable cor- 

 tex and the corpora striata have alternative motor functions. 



