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PHYSIOLOGY OF THE CEREBRUM 



anatomy has long since demonstrated various systems of fibers by which the 

 two hemispheres and different regions of the same hemisphere are joined 

 together. And quite recently our knowledge of the subject has been enriched 

 so materially by the investigations of Flechsig that future researches in this 

 field will have a much safer point of departure than hitherto. 



A. ANATOMICAL 



From what we have learned in preceding sections we know that only about 

 one third of the entire surface of the cerebrum is in direct connection with 

 tracts which mediate sensory impressions and arouse mental mechanisms. 

 We know, moreover, both from the anatomical structure of the brain and 

 from the fairly certain localization of cortical motor and sensory areas, that 

 the remaining parts of the cerebral cortex have nothing whatever to do with 

 afferent or efferent tracts. They serve to connect and associate the impulses 

 delivered by the sensory nerves, to originate the resulting motor impulses, and 



FIG. 297. The myelogenetic areas of the human brain, outer surface, after Flechsig. 



to elaborate perceptions into higher mental processes ; in short, these parts are 

 to be regarded as the organs of our purely psychical activities. We shall speak 

 of them in the following pages as the association centers. 



We find sufficient grounds for this view both in the clinical observations 

 which have been made and in the results of anatomical investigation. The 

 microscopical structure of these parts of the cortex alone indicates that they 

 are of different character from the other cortical areas. While the cortical 

 areas of the nerves of special sense possess in each case something which 

 clearly recalls the distribution of nerves in the particular peripheral organ to 

 which each corresponds, the association centers have more in common. Although 



