290 INTRODUCTION TO NEUROLOGY 



in general. Bolton believes that the granular layer (layer IV of Fig. 127) 

 marks an important boundary between functionally different cortical 

 mechanisms. The infragranular portion of the cortex is thought to be 

 concerned especially with the performance of the simpler sensori-motor 

 reactions, particularly those of the instinctive type, while the supragranular 

 layers serve the higher associations manifested by the capacity to learn by 

 individual experience and to develop the intellectual life. 



The infragranular layers mature earlier in the development of the brain, 

 and they are the last to suffer degeneration in the destruction of cortical 

 cells in the acute dementias or insanities. The supragranular layers 

 (notably the pyramidal neurons of Brodmann's third layer, Fig. 127) ma- 

 ture later than any other layers. They are thinner in lower animals and in 

 feeble-minded and imbecile men than in the normal man, and they are the 

 first to show degenerative changes in dementia. 



This doctrine is controverted by some other neurologists, but the evi- 

 dence seems to show that the supragranular pyramidal neurons are physio- 

 logically the most important elements in the higher associative processes of 

 the cortex. In this connection it is significant that the granular and infra- 

 granular layers are thicker in the projection centers, while in the association 

 centers the supragranular layers of pyramidal cells are thicker. But all of 

 the layers in each region are very intimately related, the processes of 

 most of the cells of the deeper layers extending throughout the thickness of 

 the more superficial layers (see Figs. 123, 124, 125) to reach the most super- 

 ficial layer, and in the present state of our knowledge a functional differ- 

 ence between the layers cannot be said to have been, established, save in 

 very general terms. 



It must be borne in mind that the most significant parts of 

 the human cerebral cortex are the association centers. These 

 alone are greatly enlarged in the human brain as compared with 

 those of the higher apes. In the latter animals the projection 

 centers are fully as large as those of man, the much smaller 

 brain weight being chiefly due to the relatively poor develop- 

 ment of the association centers. 



The data which we have summarized in the preceding pages 

 have led to the most contradictory theories as to the exact 

 mode of functioning of the association centers. Neurologists 

 have been prone, even up to the present time, to fall into the 

 error of attempting to find specific centers for particular mental 

 functions or faculties. But the evidence at present available 

 gives small promise of success in the search for such centers. 

 It is, in fact, theoretically improbable that such discoveries will 

 ever be made, for psychology today recognizes no such mosaic 

 of discrete mental faculties as would be implied in such a doc- 

 trine. 



The facts of cerebral localization as clinically and experi- 



