TeLEXCEPHALOX — NOX-OLFACTORY PORTIOXS 109 



rats have proved, however, that a large part of their ability 

 to learn by trial and error has a subcortical basis (Lashley). 



The gray matter of the neopallium has all taken up a 

 superficial position, forming the cerebral cortex {cortex cerebri), 

 while between this and the lateral ventricle or the basal 

 ganglia, as the case may be, is a thick layer of white matter, 

 the medullary centre of the hemisphere, made up of intrinsic 

 association fibres, interhemispheric commissural fibres, and 

 projection fibres. The proportion of gray matter to white 

 matter is much greater in the rat and other small animals 

 than in man and larger forms generally.^ 



The intrinsic association fibres are of various lengths and 

 serve to connect the different parts of the cerebral cortex 

 of the same hemisphere. The longer ones are largely grouped 

 into rather definite tracts, though these do not show in 

 ordinary sections. The projection fibres include all the 

 ascending and descending fibres which run between the 

 cerebral cortex and lower parts of the brain. These all pass 

 through the internal capsule and spread out at its upper 

 extremity to run directly to the various parts of the cortex, 

 this spreading out of the fibres forming the corona radiata. 



One prominent tract which is made up of both association 

 and projection fibres connecting with the cortex of the medial 

 surface of the hemisphere above the corpus callosum (inter- 

 hemispheric cortex) and with some related parts in front of 

 it is the cingtdum (Pis. X\TII. -XXIII. ). This is a longitudinal 

 band which curves round the genu of the corpus callosum, 

 runs straight back directly dorsal to the truncus, and extends 

 round the splenium (vide infra). Many of the axons from 

 the interhemispheric cortex bifurcate and send anteriorly- 

 and posteriorly-directed branches through this tract. 



The interhemispheric commissural fibres form an immense 

 transverse mass running straight across betw^een the two 

 hemispheres and spreading out in each, the corpus callosum. 



iSee de Vries, Anat. Anz., Bd. 37, p. 387. 



