420 



THE CENTRAL NERVOUS SYSTEM. 



- d 



during their passage through the basal nuclei ; (2) the commissural 



fibers, which, according to 

 the original definition, pass 

 througli the corpus callo- 

 sum and anterior commis- 

 sure, thus joining corre- 

 sponding parts of the two 

 hemispheres ; (3) the asso- 

 ciation fibers, which con- 

 nect different parts of the 

 gray substance of the same 

 hemispheres ; and (4) the 

 centripetal or terminal 

 fibers i.e., the terminal 

 arborizations of those neu- 

 raxes, the cells of which 

 lie in some other region of 

 the same or opposite hemi- 

 sphere, or even in some 

 more distant portion of the 

 nervous system. The pro- 

 jection fibers originate from 

 the pyramidal cells, some 

 of them perhaps from the 

 polymorphous cells. The 

 commissural fibers are also 

 derived from the pyramidal 

 cells, and lie somewhat 

 deeper in the white sub- 

 stance than the association 

 fibers. With the exception 

 of those which join the 



cunei and those which lie in the anterior commissure, all the 

 commissural fibers are situated in the corpus callosum. They 

 give off during their passage through the hemispheres large num- 

 bers of collaterals, which penetrate at various points into the gray 

 substance and end there in terminal filaments. In this respect 

 their arborization is contrary to the old definition of these fibers, and 

 the latter must be completed by the statement that, besides joining 

 symmetric points of the two hemispheres, they also, by means of 

 their collaterals, may connect other areas of the gray substance 

 with the peripheral regions supplied by their end-tufts (Ramon y 

 Cajal, 93). The association fibers have their origin also in the 

 pyramidal cells. In the medullary substance their neuraxes divide 

 T-shaped, and after a longer or shorter course penetrate into the 

 gray substance of the same hemisphere, where they end as ter- 

 minal fibers. A few collaterals are, however, previously given off. 



Fig. 336. Schematic diagram of the cerebral 

 cortex : a, Molecular layer with superficial (tan- 

 gential) fibers ; b, striation of Bechtereff-Kaes ; c, 

 layer of small pyramidal cells; d, stripe of Bail- 

 larger; e, radial bundles of the medullary sub- 

 stance ; y, layer of polymorphous cells. 



