110 Anatomy of the Nervous System 



In a median sagittal section, this structure appears as a 

 rather flat arch, the main part of which is the body of the 

 corpus callosum {trunctis corporis callosi), while the posterior 

 edge shows a rounded thickening, the splenium, and the 

 anterior part, also thickened, turns sharply down as the 

 genu, tapering rapidly to a point, the rostrum, directed 

 postero-ventrally (PL XXYI.)- ^^ man, the rostrum becomes 

 secondarily connected with the lamina terminalis by a rostral 

 lamina, but in the rat, this is absent and the rostrum is not a 

 conspicuous part. The area of the corpus callosum in a 

 median sagittal section, where the corpus itself is cut exactly 

 transversely, gives the only exact indication of its real size. 

 In the adult rat, the area of such a section of the corpus 

 callosum is found to be 3.29% of the computed area of the 

 entire brain, while in man, similar measurements give a ratio 

 of 4.44% (Suitsu). The fibres of the corpus callosum connect 

 with all parts of the neocortex, though all parts are not equally 

 richly supplied with them. Callosal fibres arising from a 

 given area in one hemisphere are distributed to many different 

 parts of the opposite one. 



The cortex of the neopallium, or neocortex, is greatly 

 convoluted in man and other large mammals (gyrencephalous 

 species), but in the rat, as in all mammals with small brains 

 the cortex is smooth (lissencephalous). The degree of con- 

 volution varies more or less with the gross size of the hemi- 

 sphere. This is due to the peculiar fact that increase in 

 volume of the neocortex takes place almost entirely by two- 

 dimensional growth — growth in area, while thickness increases 

 relatively little. Thus we find that the average thickness of 

 the cortex of the adult albino rat is fully 1.9 mm. and that 

 of the wild rat about 8% more (Sugita) while that of man 

 is about 3 mm., though the total weight of the human brain 

 is seven hundred times that of the rat brain. The actual 

 thickness of the cortex varies at different points. In a general 

 way, it diminishes from the frontal to the occipital pole, and 



