34 K. S. LASHLEY 



thalamic nucleus, some injury to the optic radiations, but 

 scarcely any in the optic nuclei. Real injury to the optic 

 paths and nuclei of the thalamus was questionable. 



No. 100. There were slight injuries in the region of the 

 nucleus habenulae on each side. On the left there were slight 

 injuries to the optic radiations, but other visual structures 

 were uninjured. The right lateral ventricle was much en- 

 larged, with almost total destruction of the whole external 

 capsule of the right hemisphere. An old cyst indicated that 

 much of the destruction was due to infection. This is the 

 only case reported in which there was evidence of infection 

 of the cerebral substance. 



No. 101. On the right there were extensive lesions in the 

 pulvinar and lateral geniculate nucleus, extending caudad to 

 include part of the superior colliculus. The optic radiations 

 were almost completely destroyed. On the left the thalamus 

 was not injured. 



No. 102. The left pulvinar and optic radiations were se- 

 verely injured. The right thalamus was untouched. 



There are no data on the exact limits of the visual nuclei 

 in the rat's thalamus, so that only a rough estimate of the 

 extent of the lesion can be made. In nos. 101 and 102 the 

 injuries were restricted to one side, but there involved a great 

 part of the optic path. In other cases the injuries were slight. 



The mean number of trials in the retention tests of the 

 group with which these animals were trained was 44.6, with 

 13.7 errors. They all, therefore, exceeded the average of 

 the group. The mean trials required by cases with equal 

 cortical lesions (25 per cent or more). was 88.3, with 27.5 

 errors. The maximum was 110 trials, 'with 40 errors. All 

 except no. 99, therefore, required more practice for relearn- 

 ing than did cases of similar cortical injury without lesions 

 to the thalamus. 



We cannot draw conclusions from so few cases, but there 

 is here a suggestion that thalamic lesions disturb retention 

 to a greater extent than do cortical injuries and to a degree 

 proportional to the extent of the lesion. 



