CEREBRAL FUNCTION IN THE RAT 91 



were not operated upon until they had several times tripped the 

 plane and gone to the food without excess movements. This 

 required from 25 to 45 trials. A part were overtrained for a 

 time about equal to that required for learning, receiving a total 

 of from 75 to 115 trials. 



Operation without injury to the brain 



Experiment 17. Two trephine holes were made in the skull 

 of an old female just back of the fronto-parietal suture and 

 these were enlarged with bone forceps to a diameter of about 

 5 mm. on either side. The scalp was then sewn up without 

 further injury to the brain than was necessary in making the 

 openings. In training the animal had been given 100 trials on 

 the inclined box. Her average time per trial was for the first 

 five trials; to plane, 1025 seconds; to door, 26 seconds. For the 

 last five trials the averages were: to plane, 9.4 seconds; to door, 

 2.0 seconds. 



Retention was tested on the second day after the operation. 

 The average time per trial was: to plane, 46 seconds; to door, 

 9.2 seconds. Her reactions were unhesitating. The path fol- 

 lowed from the door to the plane was direct and the plane was 

 the only object in the restraining cage which excited more than 

 a momentary reaction. In every trial the animal sprung the 

 plane by standing beside it and pushing down upon its outer 

 end with her fore feet. The slight increase in time over the last 

 trials preceding the operation is due to a slower rate of movement 

 and not to any increase of exploratory movements. 



Autopsy showed small adhesions of the convexity of the hemi- 

 spheres at the region of the trephine openings. The extent of 

 the lesions is shown in plate III, figure 17. 



After opening of the skull and exposure of an area of the cortex 

 as great as that involved in the greater number of the experiments 

 here described, this animal showed perfect retention. 



