x THE FORE-BRAIN 617 



number of parts, each representing a special centre of psycho- 

 physical activity, which are fairly easy to localise, although their 

 limits are not clearly marked, and overlap. 



According to Flechsig the myelogenetic cortical fields may be 

 grouped either from their anatomical structure i.e. as the pro- 

 jection or the association fibres predominate they are either 

 sensory and motor centres, or association centres ; or from the 

 embryological standpoint i.e. from the date of their myelinatioii 

 they may be classed as primary, intermediary, and terminal regions. 



Fleehsig's sensory and motor centres which possess mainly 

 centripetal and centrifugal projection fibres, are those which we 

 have already discussed ; they are marked in Figs. 299, 300, by the 

 zones of red dots. The association centres, in which the arcuate 

 fibres that unite different points on the cortex predominate over 

 the projection fibres, are contained in the pre-frontal, the extensive 

 temporo-parieto-occipital, and the insular regions (convolutions of 

 the island of Reil). As we have seen, Flechsig's association areas 

 include the whole of the inexcitable cortex. 



In his latest embryological studies (1904) Flechsig divides the 

 cerebral cortex into thirty-six elementary myelogenetic fields. 

 The greater part of these medullary areas myelinate before birth, 

 and represent primary fields which are the most important 

 anatomically and physiologically, because the foetus at term 

 already receives stimuli from without, and is beginning to elaborate 

 them as the intellect develops. During the first month of extra- 

 uterine life foetal post-maturity as it is termed by Flechsig the 

 process of myelination extends to the intermediate fields. At the 

 commencement of the second month myelination of the terminal 

 fields sets in, and may be completed, as far as the main nerve- 

 fibres but not their collaterals are concerned, at the close of the 

 fourth month of extra-uterine life. 



For this text-book Figs. 304, 305 will suffice to give an idea of 

 the final results reached by Flechsig in his division of the cerebral 

 cortex into thirty-six different areas of myelination ; the functional 

 significance of only a few has been determined. 



Certain objections were raised against Flechsig's theory by 

 Dejerine, 0. Vogt, Sachs, v. Monakow, Hitzig and others, but these 

 have neither confuted the observations on which it is based nor 

 diminished its importance. Dejerine was the first to argue that 

 the whole of the cerebral cortex, including probably the island of 

 Reil, possesses projection fibres that pass through the capsule. 

 The projection fibres from the association centres seem, however, 

 to be few in number, and it has not been demonstrated that all 

 projection fibres subserve sensory and motor conduction : it may 

 be their function to associate the cortical fields with the sub- 

 cortical centres, since we have no ground for denying psychical 

 ideative functions to the latter, and for attributing these 



