276 ORIGINAL ARTICLES AND CLINICAL CASES 



The experiments have not dealt with the lenticular nucleus and the 

 possibility that this forms a link in the conditioned-reflex arc remains. 

 The data previously presented on the effects of injuries to the striate 

 nuclei [10] indicate that the caudate nucleus and the stimulable cortex 

 together in the rat are equivalent in function to the stimulable cortex 

 of higher forms, since their combined destruction results in relatively 

 irrecoverable paretic symptoms. On embryological grounds there is 

 less evidence for assuming homologous function between the lenticular 

 nucleus and the stimulable cortex than between the latter and the 

 caudate nucleus. In some of the earlier experiments of this series, 

 Nos. 7 and 8 [10], the cerebral cortex and underlying association 

 fibres were almost completely transected at the level of the anterior 

 thalamic nuclei ; in the present experiments the greater part of the 

 internal capsule has been destroyed without abolishing visual discrimi- 

 nation. The cortical relations of the lenticular nucleus are not well 

 established, but any occipito-lenticular fibres which may exist were 

 almost certainly destroyed by these operations. It seems very improb- 

 able, therefore, that the lenticular nucleus has any more important 

 place in the conditioned-reflex arc than has the caudate nucleus. 



The evidence presented in this series of papers all points to the 

 conclusion that in the rat the area striata on the dorsal convexity of the 

 occipital pole is the only cerebral structure which functions in the habit 

 of discrimination of light intensities. The habit may be formed or 

 retained after the destruction of any other part of the cerebrum (with 

 the possible exception of the ectorhinal and inferior temporal regions, 

 which have not been explored but are almost certainly not functional in 

 this habit). This means that the conditioned-reflex arcs which mediate 

 the habit must pass to and from the cortex by way of the occipito- 

 thalamic fibres, that the reintegrations occur within the limits of the 

 visual area, and that long transcortical association fibres do not function 

 in the formation or retention of the habit. 



Are the conditions similar for other types of habit ? There are no 

 significant data for more complex habits. The poor vision of the rat 

 makes the formation of complex visual habits so slow that I have not 

 been able to carry out similar experiments with them. I have not 

 yet located an auditory area. Certain types of latch-box habits are 

 abolished by injury to the stimulable area [12]. The effective lesions 

 here do not correspond in extent to the stimulable area, but only to the 

 excitable region for the neck and fore limbs, which are used no more 

 than the trunk and hind limbs in the manipulation of the latch. It is 



