342 NERVOUS SYSTEM OF VERTEBRATES. 



Fibers of the cortex. — Four categories of fibers are connected 

 with the neopaUium. (i) Afferent or exogenous fibers, coming 

 to the cortex from other parts of the brain. Such fibers come from 

 the sensory nuclei in the thalamus and perhaps from other sensory 

 centers, such as the corpus quadrigeminum anterior and the nuclei 

 of the VIII nerve. Such fibers therefore bring sensory impulses 

 to the cortex, in which they end by widely spread arborizations. 

 In addition to these fibers, collateral fibers of unknown significance 

 rise into the cortex from the white matter. (2) Intracortical 

 fibers. This group includes a great variety of fibers which serve 

 to spread impulses in the cortex or to bring into relation more or 

 less distant areas. There may be mentioned : (a) short neurites 



Fig. 173. — Scheme of long association tracts in the hemisphere. From Cajal 

 (Nouvelles idees, etc.). a,b,c, pyramidal ceUs; d, terminal arborization; e, collat- 

 erals of the fibers of association. 



connecting the superficial layers with the deeper; {b) short neurites 

 which connect more or less distant elements in the same or 

 adjacent layers; (c) neurites which connect distant parts of the 

 plexiform zone; {d) homolateral fibers of association which con- 

 nect various parts of the cortex in the same hemisphere. (3) Fibers 

 0} the corpus callosum. These connect the cortex of one hemis- 

 phere with that of another and are in part direct neurites of 

 pyramidal cells and in part collaterals of fibers of association 

 or of projection of one hemisphere which cross to the other. 

 The place and manner of ending of the callosal fibers in man 

 is not fully known, but it is thought that they are distributed 

 to the motor and association areas. In some mammals they 



