THE CENTRAL NERVOUS SYSTEM. 249 



the gray matter, and after their entrance into the white matter, 

 these neurites give off collaterals, which branch and end in terminal 

 bulbous expansions without breaking up into a set of teleneurites. 



The irregular cells of the fourth layer (Fig. 230, C 9 D) do not 

 send their dendrites into the molecular layer, but distribute them 

 within the deeper layers of the gray matter. Their neurities, like 

 those of the pyramidal cells, enter the white matter, where they 

 may or may not bifurcate. 



Besides the cells in the deeper layers of the gray matter hitherto 

 described, those layers contain cells with short neurites, which are 

 divisible into two classes : first, spindle-shaped or stellate cells, 

 sending their neurites into the molecular layer (Fig. 231, c) or into 

 the second layer of the gray matter (Fig. 231, 6) ; second, poly- 

 morphic cells with radiating dendrites and copiously branching 

 neurites, both of which are distributed within a short distance of the 

 cell. These cells are believed to distribute nervous impulses to the 

 neurons in their vicinity. 



The gray matter of the cortex also receives centripetal neurites 

 from the white matter, which give off numerous collaterals and ter- 

 minate in the molecular layer. 



The white matter of the cerebrum contains fibres that may be 

 divided into four groups : first, centrifugal or " projection " fibres ; 

 second, " commissure-fibres," which bring the two sides of the brain 

 into coordination (these lie in the corpus callosum and in the ante- 

 rior commissure) ; third, " association-fibres," which coordinate the 

 different regions of the cerebral cortex on the same side ; fourth, 

 centripetal fibres, reaching the cortex from the peripheral nervous 

 system or cord. 



The centrifugal or projection-fibres arise from all parts of the 

 cortex, springing from the pyramidal and, perhaps, also from the 

 irregular cells. Many of these fibres give off a collateral, which 

 passes into the corpus callosum, to be distributed in the cortex of 

 the opposite side, commissural collaterals, and then pass on to the 

 corpus striatum, to the gray matter of which further collaterals 

 may be given off, after which the main neurite probably passes into 

 the pyramidal tracts of the cord through the cerebral crus (Fig. 

 232, a). 



The commissure-fibres (Fig. 232, 6, c) also arise from the pyram- 

 idal cells of the cortex, mostly from the smaller variety, and pass 

 into the corpus callosum or the anterior commissure, to be dis- 



