532 



PHYSIOLOGY 



CHAP. 



Like the rest of the brain, the cerebral hemispheres consist of 

 white and grey matter. The former occupies the internal part, 

 where it forms the so-called medullary centre ; the second forms 

 the superficial layer, known as the grey cerebral cortex. 



The white matter of the cerebral hemisphere consists of 

 medullated fibres, which are generally smaller than those of 

 the cord. They may be grouped into three principal systems 

 according to their course : 



(a) Comrnissural or transverse fibres, which unite the two 

 hemispheres ; 



(&) Projection fibres, that run from the brain -stem to the 

 hemispheres, or vice versa ; 



Fio. 265. Median longitudinal section through adult brain. The posterior parts of the thalamus, 

 cerebral peduncles, etc., have been removed, so as to expose the inner surface of the temporal 

 lobe. 



(c) Association or arcuate fibres, that unite neighbouring or 

 remote parts of the cortex of the same hemisphere. 



The cerebral cortex varies between 2 and 4 mm. in thickness, 

 according to the region and to age. On examination with the 

 naked eye in a vertical section, it is seen, not to be uniform, but 

 to consist of a series of parallel layers, alternately white and grey, 

 the number of which varies in different regions (Baillarger, 1840) 

 (Fig. 267). This variation in the colour shows that the structure 

 of the cortex is not uniform, as is also confirmed by microscopical 

 examination. 



II. The form and arrangement of the nerve-cells vary with 

 the varying depth of a convolution ; there are different more or 

 less well-defined layers, which are not always distinct, and do not 



