74 



ATLAS OF NERVE CELLS 



cortex into relation with the lower levels of the nervous system, the various parts of which 



have been already studied. They are shown in Fig. n. 



There are (2) association fibres, which pass from one part of the cortex to other parts, 



thus bringing various regions of the cortex into mutual relation. They are shown in Fig. 12. 



There are (3) commissural fibres, which 

 pass by way of the corpus callosum from 

 the cortex of one hemisphere of the brain 

 to the opposite hemisphere, to terminate 

 in the cortex. 



It is not at all impossible, in the light 

 of recent discoveries, that a neuraxon from 

 a nerve cell may become a projection fibre 

 and give off collaterals, which may become 

 association or even commissural fibres. It is 

 certain that the human brain is superior 

 to that of all other brains in the number 

 of its association fibres which bring about 

 a perfect interaction of all its parts. The 

 termination of these association fibres ap- 

 pears to be in all of the various layers 

 of the cortex, thus bringing any cell into 

 relation with many cells of different situa- 

 tion and type. 



It has been stated already that the 

 cortex is the terminal station of a very 

 large number of fibres which reach it from 

 other parts in the projection, association, 

 and commissural tracts. These fibres are not 



Pol. P. 



w. 



shown in the diagram (Fig. 10), for the sake 



FIG. 13. (Andriezen). Cortex of human brain showing the nerve fibre sys- 

 tems and plexuses (combined Weigert's and Golgi's methods). 



,*, clear zone (free of nerve fibre). M P, molecular plexus (Exner's), Qf dearnesS) but they are s ' nO wn in Fig. 1 3 

 in the molecular layer. A sir, ambiguous cell stratum. Subm P, sub-molec- 



ular plexus. Gt PP, great pyramidal plexus. Pol P, polymorphic plexus. from a design by AndrieZCn ' which demon- 

 ic, white substance. 



strates their complexity. They enter the 



cortex from the white matter and terminate in brush-like expansions about the cells of all 

 the various layers. At some layers the intricate meshes of these fibres are very thick. The 



1 Brain, Vol. XVII. p. 629; 1894. 



