THE FUNCTIONS OF THE CEREBRAL CORTEX 287 



a definite sequence such that the fibers belonging to particular functional 

 systems mature at the same time. (2) A second law states that in the 

 'cerebral cortex there are two great functional groups of fibers which mature 

 at different times. One of these groups contains the projection fibers, which 

 mature early, chiefly before birth; the other group contains the association 

 fibers, which mature after birth. These groups are further subdivided into 

 subsidiary functional systems, each of which connects with a definite region 

 of the cerebral cortex, so that it is possible to map the cortical areas in ac- 

 cordance with the sequence of development of the related myelinated fibers. 

 There are, accordingly, two groups of cortical areas in this scheme: the 

 projection centers whose fibers mature early and the association centers 

 whose fibers mature late. 



Figures 134 and 135 illustrate the arrangement of these areas, the prim- 

 ary areas (projection centers) being marked by double cross-hatching and 

 the association centers by single cross-hatching or unshaded areas. The 

 numbers printed on the charts indicate the approximate order in which 

 the corresponding parts acquire their myelinated fibers. It will be noticed 

 that Flechsig's projection areas do not correspond exactly with those deter- 

 mined by the physiological method and by the histological study of the adult 

 cortex (Figs. 130, 131, 132, 133). 



On the basis of his studies, Flechsig elaborated a highly speculative 

 theory of the significance of the association centers, which has been criticized 

 as a return to the old attempt to localize particular mental functions in 

 definite cortical areas. These criticisms are not wholly justified; never- 

 theless it is even yet premature to attempt so detailed an analysis of the 

 cortical mechanisms of psychic processes as Flechsig has elaborated. His 

 observations on the facts of myelogeny, moreover, have not been confirmed 

 by more recent students of the question (Monakow, Vogt, Dejerine, and 

 others), though it seems to be established that the sensory and motor 

 projection centers in general acquire myelinated fibers earlier than other 

 parts of the cerebral cortex. (This entire question is critically reviewed 

 by Brodmann in Lewandowsky's Handbuch der Neurologic, Band 2, pp. 

 234-244.) The only conclusion at present possible is that the factors which 

 operate in determining the sequence of myelination of the nerve-fibers of 

 the brain are exceedingly complex, and it is impossible from the facts at 

 present known to formulate the laws of the myelogenetic development of 

 the brain. 



Attention should be called here to the fact that there are many 

 different kinds of projection fibers, that is, fibers connecting the 

 cerebral cortex with the underlying structures of the brain stem 

 and spinal cord. Most of these projection fibers, except those 

 of the olfactory system, pass through the corona radiata and 

 internal capsule of the corpus striatum. The most important 

 of these projection systems are the great sensory radiations which 

 discharge their nervous impulses into the cortical centers of 

 vision, hearing, touch, and smell, as already described (the 

 exact course of the gustatory projection fibers has not been de- 

 termined), and the great motor system of the pyramidal tract 



