STUDIES OF CEREBRAL FUNCTION IN LEARNING. VII 11 



Eecovery from brain injury in the rat is rapid, and with 

 this type of operation all traces of shock and discomfort 

 from the wound disappear in three to five days. When the 

 lesion lies outside of the visual area and is not too extensive, 

 perfect reactions in the discrimination box may be obtained 

 within twelve hours after operation, sometimes immediately 

 on recovery from anaesthesia. A seven-day interval between 

 operation and retention tests was therefore judged sufficient 

 to allow for recovery from the immediate effects of the opera- 

 tion. This was further controlled by a group of animals with 

 a fourteen-day interval. 



For recording and computing the extent of lesions the for- 

 mal diagram reproduced in the plates was used. This was 

 constructed from serial sections of normal brains cut in 

 transverse, horizontal, and sagittal section. From these, in- 

 ternal structures were projected to the surface and indicated 

 by dotted lines. The rat's cortex shows no superficial mark- 

 ings to which the position of a lesion can be referred, so that 

 in reconstruction of lesions from serial sections it is necessary 

 to use internal structures for identification of the level of the 

 sections. The principal structures used for later identifica- 

 tion of levels were the anterior margin of the striatum (level 

 6), and genu (8), anterior commissure (11), posterior margin 

 of the chiasma (13), origin of the tractus thalamo-mamillaris 

 (15), anterior descending margin of the hippocampus (18), 

 anterior end of aqueduct (20), anterior margin of inferior 

 colliculi (23), caudal pole of the hemisphere (30). There is 

 considerable individual variation in the relative positions of 

 these structures in various brains and the reconstructions are 

 subject to error on this account. To avoid this as far as 

 possible, the following methods were used. 



At necropsy the brains were removed and the extent and 

 position of the visible lesion measured and transferred with 

 proportional dividers to a printed diagram. The brain was 

 then hardened, cut in transverse section, and stained with 

 carbothionin. Camera sketches were made at intervals of 

 250 [i throughout the lesion. The levels of these sections were 



