THE ASSOCIATION CENTERS OF FLECHSIG 667 



they are scattered widely over the surface of the cerebrum, the microscopical 

 structure is much the same type in all of them. 



The same thing is taught by the mode of development of the myelin sub- 

 stance in the white matter of the brain. It is well known, through the work of 

 Flechsig, that the fibers running to certain areas of the cortex receive their 

 medullary sheaths much earlier than those running to other areas. In fact, at 

 a time when the myelin substance in some convolutions is almost entirely com- 

 plete, in others it is just beginning, and in still others has reached a medium 

 stage of development. At certain stages, therefore, we can distinguish medul- 



FIG. 298. Myelogenetic areas of the human brain, inner surface, after Flechsig. 



lated, nonmedullated and half-medullated convolutions which present uniform 

 structures throughout, and in all individuals of approximately the same age 

 have practically the same relative position. Flechsig distinguishes thirty-six 

 such " myelogenetic " areas. They are numbered according to the order in which 

 they receive their myelin substance, and are divided chronologically into three 

 groups. 



Those areas which are mostly myelinated at birth at term are called by 

 Flechsig primordial regions: here belong Nos. 1 to 10, Figs. 297 and 298. 

 Anatomically they are distinguished mainly by their great richness in paths to 

 and from the subcortical centers (projection fibers). By comparison of Figs. 

 297 and 298 with Figs. 293 and 294, it may be seen that these areas embrace the 

 points of entrance into the cortex of all sensory pathways as well as the points 

 of exit of all the motor pathways. It is not known at present whether certain 

 of these primordial regions (for example, No. 10, Fig. 297) are connected with 

 peripheral end organs or not. 



The second group of cortical areas includes the intermediary regions (Nos. 

 11 to 30) in which the myelination begins before the end of the first month 

 after birth. To the third group, the terminal regions, belong those areas which 

 are myelinated after the first month (31 to 36). In these as well as the most 



