648 THE NERVOUS SYSTEM 



ASSOCIATION CENTERS OF THE CEREBRAL CORTEX. 



The theory of localization of the function of different parts of the cerebral 

 cortex has received substantial support from the study of the motor and the 

 sensory areas in man and the mammals. But when the exploration of the 

 cortex is complete and the motor and sensory areas are bounded as definitely 

 as may be, there still remain great areas in which stimulation is apparently 

 non-effective so far as our present means of interpretation reveal. Trau- 

 matic and pathological lesions produce no sensory or motor disturbances. 

 The areas of this class which are most extensive, i.e., which cover the greatest 

 amount of cortex, are three in number the frontal lobe, the parietal lobe, 

 and a large part of the temporal lobe below the superior temporal convolution. 



Flechsig has made a study of the development of the human brain, paying 

 especial attention to the progressive development of the great tracts of fibers. 

 He has shown that the tracts appear in a certain order of sequence, also that 

 the myelinization takes place progressively. 



These observations give justification to the assumption that effective 

 functionization is attained with the acquirement of the myelin sheath. He 

 showed a close correspondence in time between the development of the 

 tracts and the manifestation of functions known to involve the tracts in 

 question. The great somesthetic area and its tracts are first to develop; 

 then tracts to the occipital or visual center, to the auditory and other sensory 

 centers, and, finally, to those great areas whose functions remain obscure. 



Basing his deductions on the facts of development, on the isolated cases of 

 lesion and disease, and on the careful comparative studies of the brains of 

 certain men of unusual intellectual powers, whose personal characteristics 

 and intellectual genius are known, Flechsig has advanced the hypothesis 

 that the areas of the cortex not concerned directly with motor or sensory 

 functions are association areas. 



The Association Centers of Flechsig. The great association centers 

 are the frontal, parietal, and temporal, figure 405. These regions of the 

 cortex are apparently not directly connected with tracts of the brain stem 

 and cord, but they are richly connected with the areas that do have connec- 

 tion with the cord. Short association fibers connect neighboring convolu- 

 tions within the centers, fibers which are chiefly the axones of the poly- 

 morphous cells of the fourth layer of the cortex. Long association fibers 

 run from one center to another, such as the cingulum, superior and inferior 

 longitudinal fasciculi, etc. The longer connectives run from association 

 to association centers, and from association to sensory and motor centers. 

 Flechsig believes that the sensory centers are not connected directly with 

 each other, but only indirectly through the association areas. 



Cases of injury and of disease of the human brain in the association areas 

 are not numerous, but such as there are tend to confirm Flechsig's hypothesis 

 that the function of these areas is that of the higher psychic activity. 



